PREFACE

The object of minor project in degree course is to correlate the theory with the

practical aspect and to make students familiar with the practical difficulties which

arises during working on the field so that they can face challenges boldly while

actually working in the field. To become a perfect engineer one must be familiar

with the site environment and must be aware of the basic problems. This project

gives the student an experience of the field which he has to face in latter time.

Particularly for a Civil Engineer, such project is very important.

As per the scheme of Rajasthan Technical University, Kota, towards completion of

Four Year degree course of B.Tech. In Civil Engg. A student have project-1 during

7th Semester Examination. This report is based on the work of “TYPES OF

FOUNDATION ACCORDING TO SOIL”. The Project was under the supervision of

Mr.Manoj saini.This report contains the details of the execution work which I have

seen during my project. The contents of this report are theoretical as well as practical

as per my site experience.

RAJENDRA KUMAR KUMAWAT

ACKNOWLEDGEMENT

I would like to express my deepest gratitude to my guide and motivator Mr. Manoj

Saini , Civil Engineering Department, Sri Balaji college of engineering and

technology, Jaipur for his valuable guidance, sympathy and co-operation for

providing necessary facilities and sources during the entire period of this project.

I wish to convey my sincere gratitude to Col. P. M. Meena , H.O.D, and all the

faculties of Civil Engineering Department, SBCET Jaipur who have enlightened me

during my project.

I am also thankful to the Geotechnical Engineering Laboratory, SBCET Jaipur for

helping me during the experiments.

Date:08-11-2011 RAJENDRA KUMAR KUMAWAT

CERTIFICATE

This is to certify that the entitled, “TYPES OF FOUNDATION ACCORDING TO

SOIL” submitted by RAJENDRA KUMAR KUMAWAT in partial fulfilments for the

requirements for the degree of Bachelor of Technology 2011-12 in Civil Engineering

at SRI BALAJI COLLEGE OF ENGINEERING AND TECHNOLOGY, JAIPUR is an

authentic work carried out by him under my supervision and guidance.

To the best of my knowledge, the matter embodied in this report has not been

submitted to any other University / Institute for the award of any Certificate.

Date: 08-11-2011 Mr. Manoj Saini

Civil engg.Department

SBCET, Jaipur

SLAB DESIGNReading AssignmentChapter 9 of Text and, Chapter 13 of ACI318-02IntroductionACI318 Code provides two design procedures for slab systems:13.6.1 Direct Design Method (DDM) For slab systems with or without beams loaded onlyby gravity loads and having a fairly regular layout meeting the following conditions:13.6.1.1 There must be three or more spans in each directions.13.6.1.2 Panels should be rectangular and the long span be no more than twice the short span.13.6.1.3 Successive span lengths center-to-center of supports in each direction shall not differby more than 1/3 of the longer span.13.6.1.4 Columns must be near the corners of each panel with an offset from the generalcolumn line of no more 10% of the span in each direction.13.6.1.5 The live load should not exceed 3 time the dead load in each direction. All loadsshall be due gravity only and uniformly distributed over an entire panel.13.6.1.6 If there are beams, there must be beams in both directions, and the relative stiffnessof the beam in the two directions must be related as follows:

For slab systems loaded by horizontal loads and uniformly distributed gravity loads, or notmeeting the requirement of the section 13.6.2, the Equivalent Frame Method (EFM) of Sect. 13.7of ACI code may be used. Although Sect. 13.7 of the ACI code implies that the EFM may besatisfactory in cases with lateral as well horizontal loads, the Commentary cautions thatadditional factors may need to be considered. The method is probably adequate when lateralloads are small, but serious questions may be raised when major loads must be considered inaddition to the vertical loads.The direct design method gives rules for the determination of the total static design

moment and its distribution between negative and positive moment sections. The EFM definesan equivalent frame for use in structural analysis to determine the negative and positive momen acting on the slab system. Both methods use the same procedure to divide the moments so foundbetween the middle strip and column strips of the slab and the beams (if any).Section 13.3.1 of the Code could be viewed as an escape clause from the specific requirementsof the code. It states: “A slab may be designed by any procedure satisfying conditions forequilibrium and geometrical compatibility if shown that the design strength at every section is atleast equal to the required strength considering Secs. 9.2 and 9.3 (of the ACI code), and that allserviceability conditions, including specified limits on deflections, are met.” The methods ofelastic theory moment analysis such as the Finite Difference procedure satisfies this clause. Thelimit design methods, for example the yield line theory alone do not satisfy these requirements,since although the strength provisions are satisfied, the serviceability conditions may not besatisfied without separate checks of the crack widths and deflections at service load levels.The thickness of a floor slab must be determined early in design because the weight of theslab is an important part of the dead load of the structure. The minimum thickness can bedetermined by many factors:• Shear strength of beamless slabs (usually a controlling factor); slab mustbe thick enough to provide adequate shear strength• Flexural moment requirement (less often a governing factor)• Fire resistance requirements• Deflection control (most common thickness limitations)Section 9.5.3 of ACI gives a set of equations and other guides to slab thickness, and indicatesthat slabs which are equal to or thicker than the computed limits should have deflections withinacceptable range at service load levels.ACI code direct design method and equivalent methods can be conveniently discussed in termsof a number of steps used in design. The determination of the total design moment in concerned3with the safety (strength) of the structure. The remaining steps are intended to distribute the totaldesign moment so as to lead to a serviceable structure in which no crack widths are excessive, noreinforcement yields until a reasonable overload is reached, and in which deflections remain

within acceptable limits. These steps are discussed as we go along.Equivalent frame method may be used in those cases where:• slab layout is irregular and those not comply with the restrictions statedpreviously• where horizontal loading is applied to the structure• where partial loading patterns are significant because of the nature of theloading• high live load/dead load ratios.4Design ProcedureThe basic design procedure of a two-way slab system has five steps.1. Determine moments at critical sections in each direction, normally the negativemoments at supports and positive moment near mid-span.2. Distribute moments transverse at critical sections to column and middle-strip and ifbeams are used in the column strip, distribute column strip moments between slaband beam.3. Determine the area of steel required in the slab at critical sections for column andmiddle strips.4. Select reinforcing bars for the slab and concentrate bars near the column, ifnecessary5. Design beams if any, using procedures you learned in CIVL 4135.Positive and Negative Distribution of MomentsFor interior spans, the total static moment is apportioned between critical positive and negativebending sections as (See ACI 318-02 Sect. 13.6.3):Panel Moment Mo100% Static MomentNegative Moment Monegative Mu = 0.65 MoPositive Moment Mopositive Mu = 0.35 MoAs was shown, the critical section for negative bending moment is taken at the face ofrectangular supports, or at the face of an equivalent square support.For the Case of End SpanThe apportionment of Mo among three critical sections (interior negative, positive, and exteriornegative) depends on1. Flexural restraint provided for slab by the exterior column or the exterior wall.2. Presence or absence of beams on the column lines.

Lateral Distribution of MomentsHere we will study the various parameters affecting moment distribution across width of a crosssection.Having distributed the moment Mo to the positive and negative moment sections as justdescribed, we still need to distribute these design moments across the width of the criticalsections. For design purposes, we consider the moments to be constant within the bounds of a

middle or column strip unless there is a beam present on the column line. In the latter case,because of its greater stiffness, the beam will tend to take a larger share of the column-stripmoment than the adjacent slab. For an interior panel surrounded by similar panels supporting thesame distributed loads, the stiffness of the supporting beams, relative to slab stiffness is thecontrolling factor.The distribution of total negative or positive moment between slab middle strip, columnstrip, and beams depends on:• the ratio of l2/l1,• the relative stiffness of beam and the slab,• the degree of torsional restraint provided by the edge beam.The beam relative stiffness in direction 1 is:11cb bcs sa E IE I=whereEcbIb1 = Flexural rigidity of beam in direction 1EcsIs = Flexural rigidity of slabs of width l2= bh3/12 where b = width between panel centerlines on each side of beam.similarly22cb bcs sa E IE I=in general0 < a < ∞a = ∞ → Supported by wallsa=0 → no beamsfor beam supported slabsa < 4 or 5l2h8Note:Values of a are ordinarily calculated using uncracked gross section moments of inertia for bothslab and beam.Beams cross section to be considered in calculating Ib1 and Ib2 are shown below. (see ACI sect.

13.2.4)The relative restraint provided by the torsional resistance of the effective transverse edgebeam is reflected by parameter tβ such as:2cbtcs sE CE Iβ =whereEcb = Muduls of Elasticity of Beam ConcreteC = Torsional Constant of the Cross–sectionThe constant C is calculated by dividing the section into its rectangles, each having smallerdimension x and larger dimensionACI Two-Slabs Depth Limitation• Serviceability of a floor system can be maintained through deflectioncontrol and crack control• Deflection is a function of the stiffness of the slab as a measure of itsthickness, a minimum thickness has to be provided irrespective of theflexural thickness requirement.• Table 9.5(c) of ACI gives the minimum thickness of slabs without interiorbeams.• Table 9.5(b) of ACI gives the maximum permissible computed deflectionsto safeguard against plaster cracking and to maintain aesthetic appearance.• Could determine deflection analytically and check against limits• Or alternatively, deflection control can be achieved indirectly to more-orlessarbitrary limitations on minimum slab thickness developed fromreview of test data and study of the observed deflections of actualstructures. This is given by ACI.For am greater than 0.2 but not greater than 2.0, the thickness shall not be less than

DESIGN AND ANALYSIS PROCEDURE- DIRECT DESIGNMETHODOperational StepsFigure 11.9 gives a logic flowchart for the following operational steps.1. Determine whether the slab geometry and loading allow the use of the direct designmethod as listed in DDM.2. Select slab thickness to satisfy deflection and shear requirements. Such calculationsrequire a knowledge of the supporting beam or column dimensions A reasonablevalue of such a dimension of columns or beams would be 8 to 15% of the average ofthe long and short span dimensions, namely (l1 +l2)/2. For shear check, the criticalsection is at a distance d/2 from the face of the'! support. If the thickness shown fordeflection is not adequate to carry the shear, use one or more of the following:(a) Increase the column dimension.

(b) Increase concrete strength.(c) Increase slab thickness.(d) Use special shear reinforcement.(e) Use drop panels or column capitals to improve shear strength.3. Divide the structure into equivalent design frames bound by centerlines of panels oneach side of a line of columns.4. Compute the total statical factored moment5. Select the distribution factors of the negative and positive moments to the exteriorand interior columns and spans and calculate the respective factored moments.6. Distribute the factored equivalent frame moments from step 4 to the column andmiddle strips.7. Determine whether the trial slab thickness chosen is adequate for moment-sheartransfer in the case of flat plates at the interior column junction computing thatportion of the moment transferred by shear and the properties of the critical shearsection at distance d/2 from column face.8. Design the flexural reinforcement to resist the factored moments in step 6.9. Select the size and spacing of the reinforcement to fulfill the requirements for crackcontrol, bar development lengths, and shrinkage and temperature stresses.

DESIGN OF SLAB

Slabs are generally designed on the assumption that they consists of a number of

beams of breadth ‘one meter’.

Effective Span

The effective span of a simply supported slab shall be taken as the lesser of the

following:

1. Distance between the centers of bearings,

2. Clear span plus effective depth

Thickness of Slab

The following table gives the maximum values of the ratio of span to depth.

Type of slab Ratio of span to depth

Simply supported and spanning in one

direction

30

Continuous and spanning in one direction 35

Simply supported and spanning in two

directions

35

Continuous and spanning in two directions 40

Cantilever slabs 12

Reinforcement

Minimum reinforcement in either direction shall be 0.15 percent of total cross-

sectional area.

Main reinforcement which is based on the maximum bending moment shall not be

less than 0.15 per cent of the gross sectional area. The pitch of the main bars shall

not exceed the following:

1. Three times the effective depth of slab, and

2. 45 cm.

Distribution bars are running at right angles to the main reinforcement and the pitch

shall not exceed

1. Five times the effective depth of slab, and

2. 45 cm.

The diameter of main bars may be from 8 mm to 14 mm. for distribution bars, steel 6

mm or 8 mm are generally used.

Cover of Reinforcement:

The minimum cover to outside of main bars shall not be less than the following:

1. 15 mm and

2. Diameter of the main bar.

Steps to be followed in the design of slab

1. Assuming suitable bearings (not less than 10cm), find the span of the slab between

the centers of bearings.

2. Assume the thickness of slab (take 4 cm per metre run of the span).

3. Find the effective span which is lesser of (i) distance between centres of bearings,

and (ii) clear span and effective depth.

4. Find the dead load and the live load per square meter of the slab.

5. Determine the maximum bending moment for a one meter wide strip of the slab.

The maximum bending moment per meter width of slab,

Where, w = total load intensity per square meter of the slab.

1. Equate the balanced moment of resistance to the maximum bending moment

Find the effective depth ‘d’ from the above equation.

1. Calculate the main reinforcement per metre width

For M15 concrete, lever arm = 0.87 d

Spacing of bar =

CONTINUOUS SLAB

Suppose a slab is supported at the ends and also at intermediate points on beams,

the maximum sagging and hogging moments to which the slab is subjected to due to

uniformly distributed load, can be computed as follows:

Let = intensity of dead load per square metre

= intensity of live load per square metre.

Bending moment due to dead load and live load may be taken as follows (IS: 456 –

2000)

  At middle of

end span

Over

support

At middle of

interior

support

Over

interior

support

BM due to

dead load

Bending

moment due

to live load

Related Tags:

project report on software of rcc slab, ppt on working stress method rcc design,

cobiax .ppt, structural design of slab, calculate the no of main bar for rcc, design

example of rcc flat slab, 6 meter wide concrete slap sagging, what is meant by main

bar and distributio workn bar in structure, how to calculate thickness of rcc slab,

structural design slab, how to calculate rcc slab steel spacing , maximum cover in

reiforcement in rcc in coasta area,

DESIGN OF SLAB

Slabs are generally designed on the assumption that they consists of a number of

beams of breadth ‘one meter’.

Effective Span

The effective span of a simply supported slab shall be taken as the lesser of the

following:

1. Distance between the centers of bearings,

2. Clear span plus effective depth

Thickness of Slab

The following table gives the maximum values of the ratio of span to depth.

Type of slab Ratio of span to depth

Simply supported and spanning in one

direction

30

Continuous and spanning in one direction 35

Simply supported and spanning in two

directions

35

Continuous and spanning in two directions 40

Cantilever slabs 12

Reinforcement

Minimum reinforcement in either direction shall be 0.15 percent of total cross-

sectional area.

Main reinforcement which is based on the maximum bending moment shall not be

less than 0.15 per cent of the gross sectional area. The pitch of the main bars shall

not exceed the following:

1. Three times the effective depth of slab, and

2. 45 cm.

Distribution bars are running at right angles to the main reinforcement and the pitch

shall not exceed

1. Five times the effective depth of slab, and

2. 45 cm.

The diameter of main bars may be from 8 mm to 14 mm. for distribution bars, steel 6

mm or 8 mm are generally used.

Cover of Reinforcement:

The minimum cover to outside of main bars shall not be less than the following:

1. 15 mm and

2. Diameter of the main bar.

Steps to be followed in the design of slab

1. Assuming suitable bearings (not less than 10cm), find the span of the slab between

the centers of bearings.

2. Assume the thickness of slab (take 4 cm per metre run of the span).

3. Find the effective span which is lesser of (i) distance between centres of bearings,

and (ii) clear span and effective depth.

4. Find the dead load and the live load per square meter of the slab.

5. Determine the maximum bending moment for a one meter wide strip of the slab.

The maximum bending moment per meter width of slab,

Where, w = total load intensity per square meter of the slab.

1. Equate the balanced moment of resistance to the maximum bending moment

Find the effective depth ‘d’ from the above equation.

1. Calculate the main reinforcement per metre width

For M15 concrete, lever arm = 0.87 d

Spacing of bar =

CONTINUOUS SLAB

Suppose a slab is supported at the ends and also at intermediate points on beams,

the maximum sagging and hogging moments to which the slab is subjected to due to

uniformly distributed load, can be computed as follows:

Let = intensity of dead load per square metre

= intensity of live load per square metre.

Bending moment due to dead load and live load may be taken as follows (IS: 456 –

2000)

  At middle of

end span

Over

support

At middle of

interior

support

Over

interior

support

BM due to

dead load

Bending

moment due

to live load

Related Tags:

project report on software of rcc slab, ppt on working stress method rcc design,

cobiax .ppt, structural design of slab, calculate the no of main bar for rcc, design

example of rcc flat slab, 6 meter wide concrete slap sagging, what is meant by main

bar and distributio workn bar in structure, how to calculate thickness of rcc slab,

structural design slab, how to calculate rcc slab steel spacing , maximum cover in

reiforcement in rcc in coasta area,

Design of Foundations on Expansive Clay

General Design Guidance

SlabWorks Home

Design Home

General Design

Guidance

Code Requirements

BRAB Design

WRI Design

PTI Design

Reinforcing

Three reinforcing options exist for the design of slabs-on-grade. These are bonded reinforcing (conventional reinforcing), post-tensioning and hybrid systems combining post-tensioning and bonded reinforcing. Any of these systems can produce acceptable performance if properly design and constructed.

Design Methods

There are three generally accepted design methods available for the design of slab-on-grade foundations on expansive clay. These are:

BRAB WRI/CRSI PTI

With modification, any of the three methods can be used to design either stiffened or uniform thickness slabs using post-tension reinforcement or conventional bonded reinforcement.

The Geotechnical Report

The design engineer should obtain a geotechnical report prior to designing a foundation on expansive clay. The geotechnical report should contain: 

1. Recommendations for foundation system type. The geotechnical engineer should indicate if the site is appropriate for stiffened slab-on-grade construction. The report should differentiate between compliant slabs-on-grade (where the superstructure loads are carried by a deep foundations

such as piers and the interior thin slab simply forms a barrier between the interior and subgrade) and stiffened slabs-on-grade where is slab is free floating and superstructure distress is limited by the stiffness of the foundation.

2. Fill requirements. The geotechnical engineer should clearly indicate if fill is required for slab-on-grade construction. Fill properties and required compaction should be specified.

3. Expected movement. The geotechnical engineer should indicate the potential vertical movement at the site for both heave and settlement conditions.

4. Soil related foundation design parameters. The geotechnical report should contain all soil design parameters required to design the foundation.

5. Effect of existing vegetation. The geotechnical engineer should discuss the effect of removing existing vegetation.

General Design Guidance

When implementing one of the recognized foundation design methodologies (PTI, BRAB or WRI), there are design considerations that are not discussed in these design guidelines. Some of these design considerations include:

General Fill

In many developments, uncontrolled fill is placed over large areas, commonly resulting from the excavation of roadbeds and general grading for site drainage. This may occur several years before construction of buildings is started. The foundation engineer must ensure that uncontrolled fill has not been placed on the lot, or that the uncontrolled fill has been accounted for in the design. This could be done be removing and replacing the uncontrolled fill, or perhaps ensuring that the grade beams bear on undisturbed soil.

Select Structural Fill

Select structural fill can be used to reduce the amount of potential vertical movement. If select fill is used, care must be taken to ensure the fill is properly compacted and meets the requirements for select fill. This normally requires the services of a geotechnical testing firm. Consider testing the physical properties of the fill (such as PI and LL) after the fill is placed to ensure compliance with specifications.

mpermeable Perimeter Cap

Movement of water into and out of the soil under the slab is the primary cause of foundation movement. When the slab is built on select fill, the problem is exacerbated because the select fill frequently extends outside of the building perimeter and is relatively permeable compared to the native soils. This can result in the select fill acting as a bathtub, exposing the underlying expansive clay to large amounts of water (Figure 1).

 

Figure 1: Possible Bathtub Effect of Fill.

The solution to this problem is to install an impermeable cap of fat clay around the perimeter of the building. Ensure that this cap provides proper drainage away from the foundation (Figure 2).

Figure 2: Solution to Bathtub Effect of Fill.

Beam Bearing

Good practice indicates that the beams should bear on undisturbed native soils or compacted structural fill. Foundations that are cast on improperly compacted fill will experience differential settlement resulting from consolidation of the fill.

Corners

All three design methods treat the slab as a one-way system. However, at the foundation corners, a biaxial state of bending occurs. In slabs with widely spaced beams, the point of maximum moment at the corner may not cross a beam. Additional beams or a diagonal beam running to the corner to the first beam intersection should be considered in these areas.

Ground Penetration

The perimeter grade beam also serves as a vertical moisture barrier. The deeper the grade beam penetrates into the soil, the more effective it will be in stabilizing the soil moisture. At least a foot of penetration is generally recommended.

Drainage

Proper drainage away from the foundation is important to maintain constant soil moisture. The minimum slope that is generally accepted is 5% within 10 feet of the building perimeter. This drainage must be established using impermeable fill. The engineer should verify that local drainage is being taken care of by the civil or landscape architects. If this is not the case, proper drainage must be ensured on the structural drawings.

Tree Removal

Trees significantly alter the soil moisture balance of the soil, reducing the equilibrium soil moisture in their vicinity. If a foundation is constructed over an area where a tree was recently removed, the soil will gain moisture over time and heave after the foundation is constructed. This effect is increased if the tree is removed during a dry period and construction is started soon after the tree is removed.

Landscaping Beds

Improperly constructed planting beds can result in saturated soil around the perimeter of the building, even when the soil surface nominally has positive drainage away fro the building (Figure 3). This problem is exacerbates if the building is constructed on select fill extending under the planting beds. IN this case, improperly constructed planting beds can act to inject water directly into the select fill (Figure 1).

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Figure 3: Effect of Improper Landscaping

The engineer should discuss landscaping expectations with the architect/owner to ensure that the effect of landscaping on the structure is fully understood. Ideally, no planting beds will be located near the structure. However, this is rarely possible. One solution is to line the bottom of the planting beds with a moisture barrier or layer of impermeable fat clay. However, design of the landscaping is outside of the scope of services of the structural engineer, and is the responsibility of the landscape architect or owner.

Trees

Trees planted near a foundation can upset the soil moisture balance due to the water demand of mature trees, especially during drought cycles. While it may take a number of years before the tree gets large enough to cause structural damage, this will eventually occur if the tree is close enough to the slab. In general, the distance from the tree to the foundation must be at least half the height of the tree, but the required distance varies with tree species.

The engineer should consider discussing landscaping expectations with the owner and/or architect. If landscaping requirements dictates that trees must be planted near the foundation, the engineer can recommend over-designing the foundation to account for the effect of trees.

Plumbing Trenches

Plumbing trenches should be backfilled with compacted select fill in order to prevent entry of moisture under the slab through void space in the trench backfill material. Trenches should never be backfilled with sand of granular materials. Consider requiring the use of a fat clay plug at the building perimeter.

Slab Reinforcing

Overall structural performance of stiffened slabs is generally independent of the performance of the thin slab in the areas between beams. This portion of the foundation slab is generally intended only to acts as a separator between the building and the soil below. However, if thermal or shrinkage cracking is noted in these areas, many owners will perceive the foundation is in a failed state. This is particularly important if the owner anticipates the use of tile or stone finishes. Therefore, performance expectations with respect to slab cracking should be discussed with the owner and architect prior to design.

In the past, many engineers have relied on the minimum temperature and shrinkage steel requirements from ACI-318 (0.18% steel). These guidelines are intended for elevated structural slab and are not applicable to slabs-on-grade. This is discussed in the commentary to the latest version of ACI-318.

The engineer should instead refer to ACI 224 “Control of Cracking” for guidance on controlling cracking of slabs-on-grade. Generally, cracking in stiffened slabs is controlled with bonded reinforcement, and control joints are not used. According to ACI 224, 0.50%-0.60% steel is required to control cracking with steel alone. Control joints can be installed, with the control joints located mid-way between the stiffening beams. Control joints near beams will not be effective because the beams restrain the concrete from movement.

DESIGN OF SLAB-ON-GROUND FOUNDATIONSINTRODUCTION 1EARLY DEVELOPMENTS 1SOILS INVESTIGATIONS 2LABORATORY TESTING 3DETERMINING THE EFFECTIVE P.I. 5OTHER PARAMETERS 5WARNING 6LOADING CONSIDERATIONS 7SUPPORT CONSIDERATIONS 7THE SLAB DESIGN 10BEAM SPACING AND LOCATION 12SLAB REINFORCING 13BEAM REINFORCING 13SITE PREPARATION 14SLAB FORMING 15STEEL PLACEMENT 16SPECIAL CONDITIONS 19

CONCRETE PLACING 20INSPECTION 22SHRINKAGE CRACKS 23APPENDIX A NOTATIONAPPENDIX B DESIGN EXAMPLESAPPENDIX C REFERENCESTF 700-R-07INTRODUCTIONWithin the last several years there has beena lot of interest in a design procedure for thedesign of light foundations, particularly foruse under single family residences. Reportsand recommendations have been undertakenand prepared by several study groupsfor the purposes of developing design criteriaor extending the Criteria for Selectionand Design of Residential Slabs-on-Ground,BRAB Report #33. The recommendationsderived from these and other studies varyfrom extremely light to extremely heavy.It was actually the widespread use of the“post-tensioned” slab-on-ground whichinduced this interest in design procedureand in many studies of reported slab failures.Such reports have; perhaps, created anover-cautious climate concerning any movesto lighten the design requirements set forth inBRAB Report #33. Many theoretical analysesshow that no lessening of the requirements ispossible, while other studies and actual fieldinstallation indicate that considerable variancesare permissible in many areas.In the design procedure to be presentedherein, adjustments are made to the BRABprocedure which allow the use of this simpleprocedure with larger slabs and further simplifythe design engineer’s problem of designingan adequate foundation at a reasonablecost, both in terms of the engineer’s time,and cost of the installation itself.The intent of this handbook is to provide adesign procedure which could be used in anyConsulting engineer’s office to give adequatedesigns for economical construction withoutthe use of large computers, or the necessityfor site investigations so extensive asto make the use of engineered foundationseconomically prohibitive. The following procedure,with modifications, has been used forthe last 15 years in designing foundations in

the southwest with excellent results.EARLY DEVELOPMENTSIn the early 1950’s the use of the monolithicreinforced slab foundation become widespreadin the south central portion of theUnited States. For the most part there wereno consistent standards, and many differentversions of this foundation were to befound throughout the area. Each office ofthe Federal Housing Administration had adifferent version being used in its area, andthe differences in cross-section and reinforcingwere great. Engineers did not have agenerally accepted procedure to analyze theslab, and, therefore, the problem was mostlyignored.In 1955 the Federal Housing Administrationtogether with the National Academy ofScience organized a group of nationally eminentauthorities and began a several yearresearch project to develop guidelines fordesign of slab-on-ground foundations.The final report, Building Research AdvisoryBoard (BRAB) Report #33 en-titled Criteriafor Selection and Design of Residential slabon-ground, was issued in 1968 and waswidely discussed by builders. First designsto follow the BRAB Report required foundationsheavier even than the San AntonioFHA office standard LAS-22 ( Fig. 1). LAS-22 was thought to be the heaviest designever needed, but a local study showed it wasinadequate perhaps 30% of the time. Therewas naturally, great resistance to the addedcosts of design and construction required bythe BRAB Report.TF 700-R-07 • Page 1The next important contribution also occurredin 1968 when a full scale post-tensionedslab was built and tested to destruction.A subsequent report established thefeasibility of using post-tensioning in slab-ongroundconstruction and verified many of theBRAB assumptions.In 1965 the writer developed a complete,overall design system, later modified to conform,in format, to BRAB Report #33 and furtherinfluenced by the work done by H. PlattThompson, P.E. This system gained wideuse in both Austin and San Antonio because

of the lower cost which the post-tensionedslab enjoyed compared to the heavier F.H.A.San Antonio “Standard Slab”.Variations from the BRAB Report #33 weredeveloped to maintain a reasonable ratiobetween cost of the slab-on-ground and thevalue of the house it supported. The variationspresented later in this paper have beenderived empirically.SOIL INVESTIGATIONSIt is considered imperative that a soils investigationbe made on any site on which adesign is to prepared.TF 700-R-07 • Page 2RESIDENTIAL SLAB-ON-GROUND CONSTRUCTIONFEDERAL HOUSING ADMINISTRATION SAN ANTONIO~ TEXAS INSURING OFFICEEXTERIOR EXTERIOR INTERIOR ATTACHED GARAGE,BEAM BEAM BEAM CARPORT, PORCH BEAMS(MASONRY) (FRAME)ALL RESIDENTIAL SLAB-ONGROUNDCONSTRUCTIONSHALL COMPLY WITH THESEMINIMUMS. VARIATIONS AREACCEPTABLE WHERE SOILINVESTIGATION OF THEBUILDING SITE, CLIMATICRATINGS, AND ENGINEERINGANALYSIS INDICATE A SLAB OFLIGHTER OR HEAVIER DESIGNIS SUITABLE.CONCRETE: 2500 PSI MIN. COMPRESSIVE STRENGTH. LAPSOR SPLICES: MINIMUM 30 DIAMETERS.SLAB: 4” MINIMUM THICKNESS WITH W OR D9 WIRE 10”O.C. BOTH WAYS. MAXIMUM CLEAR PANEL BETWEENBEAMS IS 15 FEET.BEAMS: 10” WIDE BY 30” DEEP. (24”DEEP FOR ATTACHED GARAGE. CARPORT, OR PORCHBEAMS) REINFORCE WITH TWO #6 BARS TOP AND TWO#6 BARS BOTTOM, CONTINUOUS. SPACE ALL STIRRUPS22” O.C. ALL BEAMS SHALL PENETRATE MINIMUM 6” INTOUNDISTURBED SOIL.CORNER BARS: PROVIDE #6 CORNER BARS IN ALL CORNERSOF THE PERIMETER OR EXTERIOR BEAMS. INSTALL ONEAT TOP, OUTSIDE. AND ONE AT BOTTOM, OUTSIDE.FIGURE 1For a small site with one structure, theminimum is obviously one test boring, whichshould be made where the worst soil conditionis anticipated; ie, where fill is located,or where the worst clay is suspected. If it is

not obvious, then more than one test holeis indicated. In no case should a design beattempted without an adequate soils investigationof the site.For large sites with large structures or morethan one structure, several test holes mustbe used. In planning the investigation, planfor the worst. It is always possible to omitborings in the field, based on data as it develops.For a subdivision, there can be no fixedminimum number of borings. The work doneshould be that which is required to get theanswer. In general, locating holes about oneto every four or Five lots, if the subdivision isreasonably uniform, will be adequate. Shoulddifferent materials be encountered, additionalborings must be placed to provide morecomplete information of the underlying soils.In some cases it is necessary to drill eachlot. When a contact between a high P.I. soiland limestone is discovered, for instance,each lot which the contact crosses must bedesigned as though the entire lot were theworst soil condition.As drilling progresses, samples should betaken at 2’ intervals and at each differentsoil strata encountered, to a depth of at least15’ If it is likely that some soil will be cutfrom the lot, borings should be deepenedappropriately. Perhaps all borings shouldbe 20 feet deep to allow for any cut, but atpresent, 15’ borings are considered sufficient.Undisturbed samples should be taken,where possible, to allow evaluation of unconfinedfined strengths of the various strata. Asunconfined strength of 1 ton is usually sufficientfor single story frame houses such asthose under consideration. For commercialand multi-story, 2 tons is usually adequate toinsure against bearing capacity failure.During field investigation it is important tomake notes of existing fill, trees, thickets, oldfence lines, roads, slope of each lot, topography,seeps, sinks, rock outcrops, and anyarea which may require fill to bring it up tograde before construction. Grading and drainageplans, when available, may be helpful inidentifying some of these significant features.Note these fill lots or even suspectedfill lots in the report so that proper care may

be exercised by the insuring agency, city officials,design engineers, et. al. Uncompactedfill under the beams of an engineered slabwill almost certainly create problems. Specifythat all fill be acceptable material, properlycompacted. H.U.D. projects and subdivisionsare supposed to require that fill be placed inaccordance with “Data Sheet 79-G”. 15LABORATORY TESTINGAfter the proper field investigations have beenmade, it is necessary to run laboratory testson samples from the various strata takenin the field. It is important that all strata becorrectly identified and tested. Identificationshould be in accordance with the unified soilclassifications chart shown in Fig. 2. Suchterms as “caliche,” “fat clays,” “loam” andother colloquialisms should be avoided orused only as extra comment. Plotting liquidlimits and plasticity indices on the classificationchart will confirm field evaluations. Ifproper testing and Identification are done,some degree of uniformity can be applied toSlab-on-Ground designs.TF 700-R-07 • Page 3TTFF 770000--RR--0073 •• PPaaggee 41Figure 2* Based on the material passing the 3 -in. (75-mm) sieve.MAJOR DIVISIONSGROUPSYMBOLSTYPICALNAMESCOARSE GRAINED SOILSMore than 50% retained on No. 200 sieve*GRAVELS50% or more ofcoarse fractionretained on No. 4sieveCLEANGRAVELSGWWell-graded gravels andgravel-sand mixtures,little or no finesGPPoorly graded gravelsand gravel-sand mixtures,little or no fines

GRAVELSWITHFINESGM Silty gravels, gravelsand-silt mixturesGC Clayey gravels, gravelsand-clay mixturesSANDSMore than 50% ofcoarsefraction passesNo.4 sieveCLEANSANDSSWWell-graded sands andgravelly sands,little or no finesSPPoorly graded sandsand gravel-sand mixtures,little or no finesSANDSWITHFINESSM Silty sands, and-siltmixturesSC Clayey sands, sand-siltmixturesFINE GRAINED SOILS50% more passes No. 200 sieve*SILTS AND CLAYSLiquid limit 50%or LessMLInorganic silts, very finesands, rock flour, silty orclayey fine sandsCLInorganic clays of low tomedium plasticity, gravellyclays, sandy clays, siltyclays, lean claysOLOrganic silts andorganic silty clays oflow plasticitySILTS AND CLAYSLiquid limit greaterthan 50%

MHInorganic silts,micaceous or diatomaceous,fine sands orsilts, elastic siltsCH Inorganic clays of highplasticity, fat claysOH Organic clays of mediumto high plasticityHighly Organic Soils PT Peat, muck and otherhighly organic soilsDETERMINING THE “EFFECTIVE P.I.”The BRAB report bases its design procedureon the soil plasticity index (P.I.). This designprocedure also uses the P.I. because it is arelatively simple-pIe test which is routinelyperformed in all testing laboratories.Since the soil is not always constant withdepth, it is necessary to find the “effectiveP.I.” of the underlying 15 Feet. BRAB Report#33 suggests a weighing system (Fig.3).This seems as valid as any weighting method,as McDowell’s16,17 procedure for calculatingpotential vertical rise also indicatesthat the upper few feet is the most active.The activity then decreases with depth due toconfining pressure and protection from seasonalmoisture change, etc. Any system thatgives more attention to the surface soils isprobably satisfactory. One place where thissystem might give erroneous results wouldbe in formations which contain sand stringersor are overlaid by porous sand which wouldprovide quick, easy routes for water to reachunderlying or interbedded CH clays.Another case would be high P.I. clays overlayingrock. Using a zero (0) P.I. for theserock layers can reduce the “effective P.I.”excessively making it appear to be a veryinnocuous site, It is probably best never touse zero for a P.I. Since BRAB recognizes 15as a breaking point for Type Ill slabs, someminimum value such as 15 should alwaysbe used for those layers with little or no P.I.BRAB recognized the problem by utilizing theP.l. immediately below the slab if it was higherthan the P.I. of the lower layers. This veryconservative approach will always yeld good,safe designs, considerably overdesigned.OTHER PARAMETERS

Once the “effective P.I.’s” foreach boring are calculated,they need to be modified bysome other parameters. Theslope of the lot should beused to increase the “effectiveP.I.” Figure 4 can be used todetermine coefficients basedon slope.The degree of over-consolidationof the natural materialcan be estimated from theunconfined compressive strengths. By usingFig. 5 a coefficient for over-consolidation canbe determined.TF 700-R-07 • Page 5Slope of natural ground vs. Slope Correction CoefficientFigure 4Figure 3Other factors are known to require consideration;moisture condition at time of construction,geologic formation, percentage ofsoil passing #40 sieve, percentage passing#200sieve, all of these affect the potential volumechange of the underlying soil. The correctvalue of “effective P.I.” is that from the equation:Eff. P.I. ( ) = Effective PI x Cs x Co x Cy x Cz • • • • • CnMuch work needs to done in this area.The ultimate performance of a slab reflectshow well the soil analysis was done. Slabdesign is only as good as the soil data onwhich it is based. Some engineers saythey do not need soil data to do a design.They are either deceiving themselves orare over-designing their slabs in which casethey delude their clients and ultimately, thepurchaser of the structure. There are fewcircumstances where the engineer is justifiedin over-designing and wasting the client’smoney. There are no circumstances wherethe engineer is justified in under-designing--even at the client’s request.WARNINGIt should be recognized that there are certainconditions which neither this procedure norany other will be able to anticipate. Examplesof such problems which might cause difficulty,even to a well designed slab, would bethe location of an old fence row beneath the

foundations, a broken water pipe, improperdrainage away from the foundation, a slablocated on top of of previously existing treeor thicket, massive erosion or loss of supportdue to lack of compliance with proper sitepreparation standards, poor maintenance,or improper installation. There are numerousdocumented cases where slabs haveexhibited less than the desired results dueto one or more of these causes. Most of thecauses mentioned above can be mitigatedby proper construction and inspection. Theothers, such as old fence lines, trees, orthickets are generally unknown to the SoilsEngineer and the Design Engineer, and, inmany cases, cannot be anticipated at all. Itis felt that the present state of the art makethese conditions fall beyond those for whichthe designer can properly be consideredresponsible. The problem with this line ofreasoning is that by the time it becomesapparent there is a problem with the slab, it isnot possible in most cases to determine thatthe problem is one of those which could notbe anticipated. The owner is having difficulty,and he is seeking relief, and quite often,revenge and restitution. These cases usuallyend up being decided by a jury. This is onevery good reason for not trying to reduce thedesign standards too far and for trying to geta good standard adopted so it will be clearlydefined when the engineer has done all thathe can be reasonably expected to do.TF 700-R-07 • Page 6Unconfined Compressive Strength (qu) TSFFigure 5LOADING CONSIDERATIONSFirst look at a small slab for a single storyhouse, trussed roof construction, masonryveneer, fire place and one car garage. Whatdo the loads look like? (see Fig. 6 )1. Roof LL & DL, stud wall, brick veneer andceiling loads2. Brick chimney load3. Stud wall and brick veneer4. Wheel loads5. Floor live loads (including non bearingpartition allowances)6. Concentrated loads from beam spanninggarage doors

Loads in Fig.6 are only the loads applied tothe top of the slab. To these must be addedthe weight of the slab, edge beams, and interiorstiffening beams. (see Fig.7).To the soil underneath, these loads are notnearly so clearly defined. For the small slabsgenerally used under houses and small commercialbuildings the loads can be assumedto be uniform. When the unconfined strengthof the soil is less than 1 ton/sq. ft., settlementor bearing can be a problem and should beconsidered, but on stiff expansive clays anydistress in the slab and superstructure willbe caused by the volumetric movement ofthe soil due to moisture change. If the soildid not change, the weight of the house orsmall building would be transmitted directlythrough the slab and into the under-lying soilwhich with the exception mentioned above,can easily carry the weight since it is usuallyless than 500 lbs. per sq.ft.Since the loads are small, it seems justifiedto use the simplifying assumption of a uniformload. This has given good results onsingle story residences.SUPPORT CONDITIONSPrior to the time the BRAB report was issued,the writer had been working on the problemsome years and had developed a workingdesign procedure.The procedure involved an area of loss ofsupport, (Fig. 8 ) the diameter of which wasa function of the soil (P.I., degree of compaction,etc.) and which was allowed to moveto any position under the building. The mostcritical locations, of course, were under loadbearing walls and columns. The equationhad been adjusted to give both positive andnegative movements.TF 700-R-07 • Page 7Superstructure LoadsFigure 6Slab ConfigurationFigure 7Area of Support LossFigure 8This procedure was developed entirely fromlooking at slabs that seemed to work andthose which did not and writing an equationwhich would produce sections equal to those

which had been performing satisfactorily.Formulas had been developed which tookinto account loss in the center as shownabove, loss at edges and corners. Also, therewere provisions for inclusion of concentratedloads.This procedure designed only one or twobeams at a time. The BRAB report showedsupport conditions (see Fig.9) which allowedall beams in a given direction to be consideredat one time. This simplified the designprocedure, and, when the two design procedureswere compared, they were foundto give similar results. The BRAB procedureproduced heavier-designs, but, with minormodifications, they could be adjustedThe moment equations developed by BRABgive a maximum moment, both positive andnegative at midspan (Fig. 10). This is not asimple cantilever moment. For short slabs itis a reasonable analysis. For longer slabs itquickly becomes excessive.To eliminate the problem, several alternativeshave been discussed:a. Design all slabs, longer than a certainlength, for a maximum moment based onthat length and all slabs less than that, fortheir exact length (Fig. 11a)b. Use an effective length in the originalBRAB equations. (Fig. 11b)c. Design all slabs for both positive and negativebending based on some cantileverlength. (Fig. 12)Note that with Figure 11a there is no increasein design moment beyond the assumedmaximum length. Obviously, as the slabs getlonger, more reinforcing needs to be addedto compensate for friction losses, drag, etc.P.C.I. goes into great detail to calculatethese losses. By using an effective value for“L” as shown in Figure 11b, these losses areautomatically covered. While this was a com-TF 700-R-07 • Page 8Assumed Support ConditionsFigure 9Location of Maximum MomentFigure 10Length Modification AssumptionsFigure 11Non-supported

SupportedNon-supporteda. Cantileverb. Simple Span Beampletely empirical approach, it was easy to useand gave good results.It has been noted during previous researchconcerning slab-on-ground construction thatthe large slabs tend to reach an equilibriumin the center portion and fluctuate only withseasonal moisture change.Some routine testing during the time of thesoils investigations can reasonably definethe depth of the zone of seasonal moisturechange which, many say, is roughly equalto the horizontal distance moisture maypene-trate under a slab and cause differentialmovement or pressure. While this doesindeed give a cantilever action such as waspreviously described, the point of maximummoment is not located at a distance fromthe edge equal to the depth of the seasonalmoisture change and nato distance ofMuch work has been done trying todefine this cantilever distance. Thisdesign procedure has developed an empiricalcurve which, when used with the equationsset out later, gives good results. Again,it makes no difference whether the cantilevertheory is used or the BRAB equations areused, so long as the proper input is suppliedfor either criteria.The BRAB equations utilizing an effectivelength, as opposed to the total length, wereused for years and gave good results. Sincethe P.C.I. and P.T.I. have advocated a cantileverapproach, this procedure has beenmodified to use a cantilever (see Fig. 12)which gives the same results as the modifiedBRAB equations. Note that in cases bothpositive and negative reinforcing are supplied.There is, at this time, a great deal of discussionconcerning the relative equality ofthe positive and negative moments used indesign. It seems that a large number of engineersfeel that the positive moment is notas significant a design parameter as is thenegative moment. Numerous proposals havebeen offered for the reduction of the positivemoment. A look at the loading conditions on

most slabs will offer support to this reductiontheory, and some experimental work hasbeen undertaken by this firm to evaluate thisproposal. The results observed indicate thatsome reductions are justified and allowable.To date, no findings have been brought forth,backed by any performance data, to indicatewhat magnitude of reduction should be considered.TF 700-R-07 • Page 9

The proper procedures for soils investigationsand reporting have been mentionedin this report, and, assuming that the properinformation is available, an actual foundationdesign can be begun. The design procedurebegins by determining a unit weight of thebuilding including its foundation. Assumethat such weight is distributed uniformly overthe entire foundation area. Those conditionswhere concentrated loads are felt to be ofsuch magnitude that they must be considered,are not covered in this paper.As previously stated, the weight of the structureis not so significant as the support conditionsof the underlying soil material, however,the weight calculated in these proceduresis generally indicative of the amount of differentialmovement which can be toleratedby the superstructure. The heavier the unitweight, the more brittle and sensitive tomovement is the superstructure material ingeneral. Also, heavier loads generated bymulti-story buildings indicates that additionalstiffness must be supplied to the foundationbecause of the sensitivity of multi-story buildingsto differential movement. A very lightwood frame structure with wood siding andno masonry would be far less susceptible tostructural and cosmetic damage than wouldbe a heavy all-masonry or brick veneer typebuilding. Use of these increased unit weightsautomatically generates additional momentand deflection criteria to satisfy the need foradditional stiffness and strength.These criteria, incidentally, apply to residentialand small commercial construction andnot to the more monumental type structuressuch as banks, churches, and highrise building.These same design procedures could beused for these types of buildings, reducing

the allowable deflections and stresses, andincluding allowances for high concentratedloads to produce the more rigid foundationsnecessary for this type construction.In any event, assume that for this criteria thecalculated weight of the house, including thefoundation for one storybrick veneer type construction,is “w” lbs. persq. ft. (The value 200 canbe used for almost anysingle story woodframe,brick veneer type constructionand not be toofar from the actual weightof the house).With the weight of thehouse known, refer toFig. 14 which is extracteddirectly from BRAB reportto select the climatic ratingfor the city in whichthe house is to built. Thevalues for Texas rangeTF 700-R-07 • Page 10Climatic Rating (Cw) ChartFigure 14from 15 in west Texas to as high as 30 ineast Texas. This chart reflects the stability ofthe moisture content which may be expectedin the soil due to the climatic conditions whichmay vary from year to year. A very low numberindicates an arid climate which will be verylow humidity and low ground moisture exceptfor a few weeks or months of the year whena heavy rainfall will occur and the groundwill take on a considerable amount of moisturecreating a potentialfor a large volumetricchange in a short periodof time. The larger numbers,such as those ineast Texas, indicate ingeneral a more humidclimate where the moisturecontent of the soiltends to remain moreuniform the year round.Refer to the BRAB reportfor a more complete description

of this chart.The P.I. and the climateconditions now beingknown, it is possibleto select from Fig. 15the soil-climate supportindex, indicated as (1-C).The following formulas will be used to calculatethe moment, shear and deflection, usingthe equivalent lengths shown in Fig. 14 aspreviously discussed.These calculations are performed for boththe “Long” and “Short” directions. The actualvalue of L and L’ are used when they referto the width of the slab. The most critical ofthese is deflection. A slab which deflects toomuch will cause serious problems for thesuperstructure, even though the slab doesnot actually break. In general then, it is bestto solve first for “I required”.The cross. section of slab is not known, butthe value of 200 lbs ./sq.ft. is almost alwaysadequate to include the slab weight.TF 700-R-07 • PBEAM SPACING AND LOCATIONAlmost all houses, if not a basic rectangle, canbe divided into two or more rectangles. If thebuilding under consideration is a combinationof two or more rectangles, a set of calculationsmust be done for each rectangle. Therectangles are then overlaid and the heavierdesign governs the common areas as shownin Fig. 16. Obviously there will be times whengood engineering judgement is required, asall houses are not nice neat modules.On some occasions the geometry of thehouse will dictate where the beams are tobe placed. When this is the case, the beamscan be located, and the calculations carriedout for width and depth based on the knownnumber of beams in each rectangle.If the design seems excessively heavy byusing the maximum spacing’s, it is possibleto recalculate beam depths and reinforcingbased on supplying additional beams.Once the spacing and location are known, thesize of the beams can be determined by trialand error. BRAB specifies that the maximumface to face distance between beams shouldbe 15’. P.C.I. states the maximum should be

20’. Experience has shown that these arevery conservative values. They are to applyto any slab on soil with P.I. of 15 or above.This is a very rigid requirement. Perhaps amore rational approach is one such as isshown in Fig. 17.The designer can then use a chart such asthe one shown, or the various maximumsto make the first run. The number of beamsthen will be:No. = L + 1 Where S = Spacing from the chartSWith the number of beams known, a widthfor each can be selected, and a calculationmade for the moment of inertia. If desired, avery good first approximation can be madeby using the following formulas.The difference in the two equations takes intoaccount the cracked section moment of inertiavs. the gross section allowed in the prestressedslab. Anyone not wishing to use thegross section moment of inertia can use thereinforcing steel for both type slabs. Theseequations are good only with beam spacingsno greater than those shown in Fig. 17.SITE PREPARATIONOften the most overlooked part of the entireoperation is the site preparation. The propersequence should include the following:1. Site clearing2. Excavation (if any)3. Fill selection and placementInadequate attention to any of these phasescan cause foundation problems even yearsafter the slab is built.It is very important that the site be clearedof all grass, weeds, old decaying or decayedorganics, roots and trash. This material whenleft under the slab can and will continue todecay and cause settlement at later dates. Itis surprising how little settlement is requiredto cause superstructure distress. The removalof approximately six inches of top soil isusually adequate to remove grass, weeds,etc. and their roots. Trees and large bushesgenerally require grubbing to greater depthsto insure adequate removal. This site clearingshould be done prior to beginning anyrequired excavation.Excavation of on-site material can begin after

the clearing and grubbing is completed. Thisallows any acceptable on-site fill materialuncovered to be placed or stockpiled withoutcontamination. This is desirable when practicalbecause it is cost effective to handlethe material only once. When a continuous,simultaneous cut and fill operation can bearranged, it will save the owner-developerquite a bit in site preparation costs.When, for some reason, this operation cannotbe arranged, it is necessary to stockpileor waste the excavated material. Stockpilesshould be made on prepared sites. Theyshould be cleared the same as a buildingsite. Wasting should be in an area which willnot be utilized later for building and which willnot be subject to erosion or create drainageblockage.All on-site material which is not suitable forstructural fill should be wasted or removedfrom the site.Fill selection is usually governed by theexpansive qualities of the natural soil. Fillshould always be as good or better thanthe on-site material on which it is placed.Sometimes more than one type of fill may beused.In general, lot preparation in subdivisions ispoorly done. Side slope lots requiring cut andfill on each lot are usually done without anyeffort to select the best material or supply anycompaction to the fill.TF 700-R-07 • Page 14Side Slope LotsFigure 21SLAB FORMING1. Foundation forms are to be built to conformto the size and shape of the foundation,and should be tight enough to preventleaking of mortar. The bracing must bedesigned so that the concrete may bevibrated without displacement or distortionof forms.2. Beams should be formed by one of twomethods:a. Single family slabs have been traditionallydone by placing loose fillinside the forms and forming thebeams with paper sacks filled withsand or fill material. For small, lightly

loaded slabs this seems adequate.b. Large slabs such as apartments,warehouse, shopping centers, etc.,are often beamed by placing compactedfill to underslab grade and thentrenching the beams with a powertrencher. This method adds support tothe slab and, helps it resist deflectionby effectively reducing the potentialexpansion of underlying soil.Unless specified on the plans or specificationsfor single family foundations,it is assumed that the methoddescribed in “a” above will be used.Method “b” may be used if desired,but it is not required. For multi-familyfoundations or commercial work,method “b” should be required.3. After the beams are Formed, a waterproofmembrane should be placed.** Either 6mil poly or hot-mopped asphalt impregnatedfelt may be used. The waterproofingshould be lapped adequately to providea continuous sheet under the entire slab.When poly is used, care must be takento see that it does not become entangledin the reinforcing. Nailing the beam sidesto the fill just before placing helps. At theexterior beam, the poly should be cut offat the bottom inside face of the beam andnailed as shown (Fig. 22), carried up ontothe exterior form and nailed (Fig. 23), orlapped with felt and nailed (Fig. 24).TF 700-R-07 • Page 16Steel Placement at CornersFigure 26Steel Placement Interior to Exterior BeamFigure 272. After the beam steel is in place, the slab steel is placed. If it is necessary to lop slab steel,the laps in adjacent bars should be staggered at least 5’ - 0” (Fig. 28)The slab steel is run continuously from side form to side form (lapping 24 diameters mm.where splices are required), allowing 1-1/2” cover over the ends of the bars. On the edgeswhere the bars run parallel to the form, the first bar should be placed a maximum of 12”from the outside form. All slab steel should be securely tied and blocked up by chairs orconcrete briquettes. (Figures 29 & 30 )

3. To insure the lowest possible foundation cost the use of welded wire reinforcement forslab reinforcement should be investigated. Different styles of WWR can furnish the samesteel area and the following are suggested for design example:TF 700-R-03 • Page 17Stagger Laps in Slab SteelFigure 28Blocking Steel Interior BeamFigure 30Blocking Steel Exterior BeamFigure 29TF 700-R-07 • Page 18

The two welded wire reinforcement styleswith 12” spacing for smooth wire and 16”spacing for deformed wire have been recentlydeveloped to further improve the efficiencyof welded wire reinforcement. The largerwire spacings make it possible to install thewelded wire reinforcement at the desiredlocation in the slab because it permits theworkmen to stand in the openings and raisethe welded wire reinforcement to place thesupports.All welded wire reinforcement sheets mustbe spliced at both sides and ends to developthe full design fy. For smooth welded wirereinforcement ACI 318-77 requires that thetwo outside cross wires of each sheet beoverlapped a minimum of 2 inches and thesplice length equals one spacing plus 2 incheswith a minimum length of 6 inches or 1.5Id whichever is greater. For slabs on groundthe one space + 2 inches or 6” minimum willprevail. This means that for a 12” wire spacingthe minimum side lap splice would be 14”but by spacing the 2 edge wires at 4” the lapis reduced to the minimum of 6”. In additionthe lapping of 2 wires in the splice lengthwill provide twice the required steel area.By reducing the area of the 2 edge wires by50%, the required As is provided uniformlythroughout the width of the slab. This reductionin wire size does not reduce the capacityof the splice because ASTM SpecificationA-185 provides that the weld strength shallbe not less than 35,000 times the area of thelarger wire. These tonnage saving featuresapply only to side laps but many welded

wire reinforcement manufacturers can providesheets with variable transverse wirespacings and the length of end laps can bereduced even though wire sizes cannot.The length of splice for deformed weldedwire reinforcement is determined by the sizeand spacing of the spliced wires, and onlythe outside cross wire is lapped. While thelap length cannot be changed, the size ofthe outside cross wire can be reduced withoutchanging the strength of the lap. ASTMSpecification A497 stipulates that the weldshear strength shall not be less than 35,000times the area of the larger wire. These engineeredwelded wire reinforcement styles arenot generally available for small foundationslabs, but, when numerous small buildingsor large slabs are being considered, it isprudent to check with welded wire reinforcementsuppliers because substantial savingsin cost can often be accomplished. As withrebar reinforcing, welded wire reinforcementmust be chaired or supported on brick orblocks to insure proper placement in theslab.SPECIAL CONDITIONS1. Special conditions from time to time willarise which will require modifications tobeam depths, forms, etc. Many of theseare covered by typical details which illustratewhat modifications are allowed withoutapproval from the Engineer. If aspecial condition occurs, such as a deepbeam, the Contractor needs instructionsin the typical details telling howto handle the situation. In the caseof the deep beam, deepen the beamby the required amount and relocatesteel (Fig. 32).Refer to “Note A” tosee if additional steel is required.Obviously, if the beam exceeds 72”,the engineer must be contacted foradditional information.2. When an exterior beam is deepened,some slight changes must be made to theinterior beams which intersect the deepenedbeam (Fig. 33). The bottom of theinterior beam should slope down at leastas deep as the mid-depth of the deepenedexterior beam. If the interior beam depth

is already deeper than the mid-depth ofthe deepened exterior beam, no changesare required to the interior beam.3. Extended beams to carry wing wallsshould be handled as shown in the typicaldetail (Fig. 34). It is very important thatTF 700-R-07 • Page 19NOTE “A”1. WHEN OVERALL DEPTH EXCEEDS 36”ADD 1 - #3 HORIZONTAL IN EXTERIORFACE OF BEAM FOR EACH 18”. STARTSPACING 6” FROM THE TOP.2. FOR BEAMS 36” TO 54” DEEP USE #3STIRRUPS AT MAXIMUM 24” O.C.3. FOR BEAMS 54” TO 72” DEEP USE #3STIRRUPS AT MAXIMUM I8” O.C.4. FOR BEAMS OVER 72” DEEP CONTACTENGINEER FOR SPECIFIC DETAILS.Engineered Welded Wire Reinforcement Sheet with Special Side LapsWelded Smooth Welded Wire ReinforcementFigure 31Deepened BeamFigure 32Interior to Exterior BeamFigure 33Extended Concrete BeamFigure 34the additional top reinforcing be addedas shown; otherwise the beam may bebroken off.4. Beams that continue throughdrops must be deepened by theamount of the drop, and the transitionsloped (Fig. 35). If the dropis framed as a sharp corner onthe bottom of the beam, stressconcentrations can occur whichmay cause difficulties.5. Other special conditions may arise fromtime to time but they are too numerous tobe covered here.TF 700-R-03 • Page 20Side View of Typical Beam at DropFigure 35Side View of Typical Beam at DropFigure 36Drop under Sleeve through BeamCONCRETE PLACINGOver the years the word “pouring” has cometo be used almost exclusively to describe the

function of placing concrete. Unfortunatelythat term is all too descriptive of the practicewhich has become common throughout theindustry. When placing concrete for an engineeredfoundation, it is imperative that theconcrete actually be placed, not “poured”.Residential floors need to have adequatestrength, surfaces that are hard and free ofdusting, and the cracking should be held toa minimum. The hardness and finish of thesurface will depend on how densely the surfacematerials are compacted during finishing,and the adequacy of the cement paste.Cracking however is mainly a function of thedrying shrinkage which takes place immediatelyafter placing and is generally morecontrolled by atmospheric conditions than bythe consistency of the concrete itself. Thisis a gross generalization, however, as highwater cement ratios will increase the shrinkageproblem. Good concrete for slabs-ongroundshould be made from a mix in whichthe water cement ratio is kept low and shouldcontain as much coarse aggregate as possibleat the surface. Compressive strengthsfor concrete slab-on-ground foundations aregenerally specified as a minimum of 2500PSI at 28 days. It is important to note theword minimum. The 2500 PSI should be theminimum strength, not the average strength.This 2500 PSI is a generally accepted figurein the industry, since it has become anaccepted figure for HUD/VA construction.In keeping the water cement ratio low, addmixtures can be of particular benefit. This isparticularly true with respect to air entrainment,retardants and accelerators.Calcium chloride is a common cold weatheradditive to accelerate settling and hardening.It should properly only be added to the mix inthe mixing water. It is important to emphasizethat calcium chloride is not to be used in afoundation which is pre-stressed. The useof calcium chloride in foundations with rebarreinforcing or welded wire reinforcing mustbe limited to a minimum of 2% by weight ofcement.Several operations need to completed beforebeginning the placing of concrete. Screedsshould be set inside the form area to establish

finished slab grade prior to beginningconcrete placing. This will improve the levelof the finished slab and eliminate much ofthe unevenness of the slabs currently found.Keys for joints may, on certain occasions,be used as screeds, since they need to beplaced at proper intervals in large slabs toeliminate or control the shrinkage cracking.When the concrete is delivered it should beplaced as close as possible to its final positionin the foundation. It should be spreadwith short handle, square ended shovels andnot by the use of rakes. Internal vibration atthe time of placing should be mandatory, asthis allows a stiffer mix to be used and facilitatesplacing.Screeding, tamping, and bull floating will beof course finished prior to the time the bleedwater has accumulated on the surface. Afterthe bull floating, the final finishing should notbegin until bleed water has risen and evaporated,and the water sheen has disappearedfrom the surface. At the time the concreteshall be stiff enough to sustain a man’s footpressure without indentation.With regard to final finishing operations, theaccumulation and evaporation of bleed waterwill vary considerably with weather conditionsand types of mixes. When bleed water is tooslow to evaporate, it may be pulled off with ahose, or blotted with burlap. The surface of afoundation should never be dried with whatis commonly called dusting. This is a methodwhereby dry cement and, sometimes, drycement and sand is placed on the slab to blotthe bleed water. This will cause a weak surfaceand, possibly, subsequent deteriorationof the surface.Immediately after the foundation has been finished,a curing compound should be placedto inhibit further evaporation of water fromthe concrete mix. This will tend to reducethe amount of shrinkage cracking which willoccur in the foundation.When using a liquid curing membrane, it isimportant to select a compound that will notinterfere with future bonding of floor finishes.There are several such compounds on themarket.Forms should remain on the finished concrete

slab for a minimum of 24 hours. Removal priorto that time can cause damage to the concrete.After 24 hours the forms can be carefullyremoved without damaging the concrete.TF 700-R-07 • Page 21INSPECTIONThe most general problem encountered waslack of suitable field inspection and control. Ifthe foundations are not constructed in accordwith the design drawings and specifications,then benefits to be derived from improvementsin codes or the state of the art will bediminished.” 20Before placing, the contractor should call foran inspection by some inspection agency.For FHA-VA single family construction, this ishandled by the FHA or VA. For non FHA/VAhouses it is, or should be, handled by thecity, but most cities, especially small ones, donot have enough staff.Cities should require adequate inspection,,either by their own forces, or by the designengineer who should, after all, be most familiarwith his own design.When the Engineer is not permitted to checkthe construction, one of the other inspectorsshould furnish a certificate to the Engineerthat the slab is properly installed in accordancewith the Engineer s plan. The followingis a partial list of points which should be verifiedby the inspector.TF 700-R-07 • Page 221. Check number of beams2. Measure beam width3. Measure beam depth4. Check beam spacing5. Check tightness and alignment of forms6. Check blocking under beam reinforcement7. Check compliance with fill penetration or have fill certification8. Check beams for proper number and size of reinforcing bars9. Check the slab reinforcing for proper size and spacing.10. Check to see that all slab reinforcing is adequately blocked to insureproper placement in concrete.11. Test concrete for maximum 6” slump12. Make cylinders for strength certification13. See that concrete is vibrated or rodded14. Insure adequate curing15. Check for cracking or honeycombingSHRINKAGE CRACKSAll concrete has cracks! There is not yet in

the industry the ability to produce crack-freeconcrete. What can best be done is to limit orreduce the amount and kinds of cracks.Those cracks that occur prior to the hardeningof the concrete are generally formed bythe movement of the form work, settlement ofthe concrete during setting, or plastic shrinkagecracks which occur while the concreteis still plastic. Other cracks which can occurafter the setting of concrete are shrinkagecracks due to drying of the concrete, thermalcracks due to changes in internal heatof hydration or due to external temperaturevariations, cracks due to stress concentrations,or cracks due to structural overloads.The most common cracks which are seenin foundations are plastic shrinkage crackswhich occur early after the concrete is placedand are due to the rapid drying of the freshconcrete. Even if plastic cracking does notoccur, similar type cracks can form duringthe early stages of hardening even days afterthe final finishing has taken place. While curingmembranes will not eliminate the plasticshrinkage cracks that occur prior to setting,they can be very beneficial in reducing oreliminating the shrinkage cracks which willoccur after finishing.The effects of temperature, relative humidity,and wind velocity are, in general, beyond thecontrol of the engineer or the contractor andmust be accepted as risks when the slab isplaced.It is, therefore, wise to specify the minimumsacks of concrete which will be expected togive the recommended compressive strength,utilize the minimum water content necessaryfor workability, and not permit over-wettingof concrete on the job. It cannot be said toooften that the use of internal vibration willfacilitate placing of concrete and help eliminateinternal settlement. The use of a surfacecuring membrane, placed as soon as possibleafter final finishing, will help eliminateshrinkage cracks which are caused by dryingof the hardened concrete.It is common practice in commercial work touse control joints to reduce shrinkage crackproblems. These are good procedures, butnot commonly used in single family construction.

Again, it is important to recognize that allshrinkage cracks cannot be eliminated, giventhe present state of the art.TF 700-R-07 • Page 23NOTATIONAc = Gross Area of Concrete Cross-SectionAss = Area of Steel Reinforcing in SlabAsbb = Area of Steel Reinforcing in Bottom of BeamAstb = Area of Steel Reinforcing in Top of Beama = Depth of Stress Block ( ult. strength)bb = Width of Beam Portion of Cross-Sectionbs = Width of Slab Portion of Cross-SectionB = Total Width of all Beams of Cross-SectionCw = Climatic Ratingdb = Depth of Beam Portion of Cross-Sectionds = Depth of Slab Portion of Cross-SectionE = Modulus of Elasticity of ConcreteEc = Creep Modulus of Elasticity of Concretef’c = 28 Day Compressive Strength of Concretefy = Yield Strength of ReinforcingIg = Gross Moment of Inertia of Cross-SectionIo = Moment of Inertia of Segments of Slab Cross-SectionkI = Length Modification Factor-Long Directionks = Length Modification Factor-Short DirectionL = Total Length of Slab in Prime DirectionL’ = Total Length of Slab (width) Perpendicular to LLc = Design Cantilever Length (Ick)Ic = Cantilever Length as Soil FunctionMI = Design Moment in Long Direction in kftMs = Design Moment in Short Direction in kftNI = Number of Beams in Long DirectionNs = Number of Beams in Short DirectionPI = Plasticity IndexS = Maximum Spacing of BeamsV = Design Shear Force (Total)v = Design Shear Stress (Unit)vc = Permissible Concrete Shear Stressw = Weight per sq. ft. of House and Slabq allow = Allowable Soil Bearingqu = Unconfined Compressive Strength of Soili-c = Soil/Climatic Rating Factor allow = Allowable Deflection of Slab, in.TF 700-R-07 • Appendix A-1

TF 700-R-07 • C-2

ANALYSIS AND DESIGN OF FLAT SLABS USINGABSTRACTFlat slabs system of construction is one in which the beams used in t he conventional methods ofconstructions are done away with. The slab directly rests on the column and load from the slab is directlytransferred to the columns and then to the foundation. To support heavy loads the thickness of slab near thesupport with the column is increased and these are called drops, or columns are generally provided withenlarged heads called column heads or capitals.Absence of beam gives a plain ceiling, thus giving better architectural appearance and also lessvulnerability in case of fire than in usual cases where beams are used.Plain ceiling diffuses light better, easier to construct and requires cheaper form work.As per local conditions and availability of materials different countries have adopted different me thods fordesign of flat slabs and given their guidelines in their respective codes.The aim of this project is to try and illustrate the methods used for flat slab design using ACI-318, NZ-3101, and Eurocode2 and IS: 456 design codes.For carrying out this project an interior panel of a flat slab with dimensions 6.6 x 5.6 m and super imposedload 7.75 KN /m2 was designed using the codes given above.6IntroductionBasic definition of flat slab: In general normal frame construction utilizes columns, slabs &Beams. However it may be possible to undertake construction with out providing beams, inSuch a case the frame system would consist of slab and column without beams. These types ofSlabs are called flat slab, since their behavior resembles the bending of flat plates.Components of flat slabs:Drops: To resist the punching shear which is predominant at the contact of slab and columnSupport, the drop dimension should not be less than one -third of panel length in thatDirection.Column heads:Certain amount of negative moment is transferred from the slab to the column at he support.To resist this negative moment the area at the support needs to be increased .this is facilitatedby providing column capital/headsFlat slab with drop panel & column head7Design of flat slabs by IS: 456The term flat slab means a reinforced concrete slab with or without drops, supported generally without

beams, by columns with or without flared column heads (see Fig. 12). A flat slab may be solid slab ormay have recesses formed on the soffit so that the sof fit comprises a series of ribs in two directions.The recesses may be formed by removable or permanent filler blocks.Components of flat slab design:a) Column strip :Column strip means a design strip having a width of 0.25 I,, but not greater than 0.25 1, on each sideof the column centre-line, where I, is the span in the direction moments are being determined,measured centre to centre of supports and 1, is the -span transverse to 1,, measured centre to centre ofsupports.b) Middle strip :Middle strip means a design strip bounded on each of its opposite sides by the column strip.c) Panel:Panel means that part of a slab bounde d on-each of its four sides by the centre -line of ad) Drops :The drops when provided shall be rectangular in plan, and have a length in each direction not less thanone- third of the panel length in that direction. For exterior panels, the width of drops at right angles tothe non- continuous edge and measured from the centre -line of the columns shall be equal to one -halfthe width of drop for interior panels.Since the span is large it is desirable to provide drop.Drop dimensions along:Longer span Shorter span1 L =6.6 m , 2 L =5.6 mNot less than 1 L /3 = 2.2 m1 L =5.6 m , 2 L =6.6 mNot less than 1 L /3 = 1.866 mHence provide a drop of size 2.2 x 2.2 m i.e. in column strip width.e) column head :Where column heads are provided, that portion of a column head which lies with in the largest rightcircular cone or pyramid that has a vertex angle of 90”and can be included entirely within the outlinesof the column and the column head, shall be considered for design purposes (see Adopting the diameter of column head = 1.30 m =1300 mmf) Depth of flat slab:The thickness of the flat slab up to spans of 10 m shall be generally controlled by considerations of span( L ) to effective depth ( d ) ratios given as below:Cantilever 7; simply supported 20; Continuous 26For slabs with drops, span to effective depth ratios gi ven above shall be applied directly; otherwise the

span to effective depth ratios in accordance with above shall be multiplied by 0.9. For this purpose, thelonger span of the panel shall be considered. The minimum thickness of slab shall be 125 mm.10area. Shall be designed to-resist moments arising from loads .In an interior span, the total design moment 0 M shall be distributed in the following proportions:Negative design moment 0.65Positive design moment 0.35In an end span, the total design moment 0 M shall be distributed in the fol lowing proportions:Interior negative design moment: 1The negative moment section shall be designed to resist the larger of the two interior negativedesign moments determined for the spans framing into a common support unless an analysis ismade to distribute the unbalanced moment in accordance with the stiffness of the adjoining parts.Column strip :Negative moment at an interior support: At an interior support, the column strip shall bedesigned to resist 75 percent of the total negative moment in the panel at that support.Negative moment at an exterior support:a) At an exterior support, the column strip shall be designed to resist the t otal negative moment inthe panel at that support.b) Where the exterior support consists of a column or a wall extending for a distance equal to orgreater than three-quarters of the value of 2 l . The length of span transverse t o the directionmoments are being determined, the exterior negative moment shall be considered to be uniformlydistributed across the length 2 l .Positive moment for each span: For each span, the column strip shall be designed to r esist 60percent of the total positive moment in the panel.Moments in the middle strip:a) That portion of-the design moment not resisted by the column strip shall be assigned to theadjacent middle strips.14b) Each middle strip shall be proportione d to resist the sum of the moments assigned to its two halfmiddle strips. cl The middle strip adjacent and parallel to an edge supported by a wall shall beproportioned, to resist twice the moment assigned to half the middle strip corresponding to the fir strow of interior columns.

Stiffness calculation:let the height of the floor = 4.0 mclear height of the column = height of floor –depth of drop – thickness of slab –thickness of head.= 4000 – 140 – 285 – 300 = 3275 mmEffective height of column = 0.8 x 3275 = 2620 mm(Assuming one end hinged and other end fixed)stiffness coefficientDesign of flat slabs as per NZS: 3101DEFINITIONS:A flat slab is reinforced concrete slab directly supporting on column (without anysupport of beams).Flat slabs is divided into column strips & middle strips.Column strips is a design strip with a width on each side of a column centre lineequal to 0.25L1 or 0.25L2,whichever is less.A middle strip is a design strip bounded by 2 column strips.A panel is bounded by column, beams, or wall centre lines on all sides .DESIGN METHOD:There must a minimum 3 continuous spans in each directions.Panels shall be rectangular with a ratio of longer to shorter spans ,centre tocentre of supports ,not greater than 2.Successive span lengths, centre -to-centre of supports, in each direction shall notdiffer by more than 1/3 of the longer spans.Columns may be offsets a maximum of 10% of the span (in direction o offset)from either axis between centre lines of successive columns.All loads shall be due to gravity only and uniformly distributed over entirepanels. the live loads shall not exceeds 2 times the dead load.DESIGN PROCEDURE:First analysis the column strips & middle strips using 0.25L1/0.25l2.Drop panel is used to reduce the amount of negative moment reinforcementover the column of the flat slab, the size of drop panel shall be 1/6 of the spanlength measured from centre–to-centre of support in that direction.22Estimate the depth of flat slabs from clauses 14.2.5 & 3.3.2.2.(b)Assume fy=300MPA.Fy(MPA) Exteriors panels Interior panels300 Ln/36 Ln/40400 Ln/32 Ln/35The absolute sum for the span shall be determined in a strip bounded laterally bythe center line of the panel on each side of centre of the supports.The absolute sum of positive and average negative moments in each direction atthe ultimate limit state shall be not less than:Mo=WuL2Ln²/8;Negative & positive design moments:In an interior spansNegative moments—0.65Positive moments---0.35In end spansExterior edgeunrestrained

EURODODEIntroductionThis Eurocode gives all structural design irrespective of the material of construction.It establishes principles and requirements for safety, ser viceability and durability ofstructures The Eurocode uses a statistical approach to determine realistic values for actionsthat occur in combination with each other. Partial fa ctors for actions are given in thisEurocode, whilst partial factors for materials are prescribed in their relevant Eurocode. It isagain divided into different codes based on the materials. In thisEurocode2 gives the design of concrete structures.EUROCODE 21. Eurocode 2 is generally laid out to give advice on the basis of p henomena(e.g. bending, shear etc) rather than by member types as in BS 8110(e.g. beams, slabs, columns etc).2. Design is based on characteristic cylinder strengths not cube strengths.3. The Eurocode does not provide derived formulae (e.g. for bendi ng,only the details of the stress block are expressed). This is the traditional Europeanapproach, where the application of a Eurocode expected to be provided in atextbook or similar publication.4. Units for stress are mega pascals, MPa (1 MPa = 1 N/m m2).5.Higher strengths of concrete are covered by Eurocode 2, up toclass C90/105. However, because the characteristics of higherstrength concrete are different, some Expressions in the Eurocodeare adjusted for classes above C50/60.6. The partial factor for steel reinforcement is 1.15. However, thecharacteristic yield strength of steel that meets the requirementsof BS 4449 will be 500 MPa; so overall the effect is negligible.Eurocode 2 is applicable for ribbed reinforcement with characteristicyield strengths of 400 to 600 MPa. There is no guidance on plainbar or mild steel reinforcement in the Eurocode, but guidance is given in the backgroundpaper to the UK National Annex10.7. Minimum concrete cover is related to bond strength, durability and fire resistance. Inaddition to the minimum cover an allowance for deviations due to variations in execution(Construction) should be included. Eurocode 2 recommends that, for concrete cast againstformwork, this is taken as 10 mm, unless the construction is subject to a quality assurancesystemic which case it could be reduced to 5 mm or even 0 mm whereon -conformingmembers are rejected (e.g. in a precast yard).8. The punching shear checks are carried at 2 d from the face of thecolumn and for a rectangular column, the perimeter is rounded atthe corners.31Design of flat slabs as per EUROCODE 2A procedure for carrying out the detailed design of flat slabs is given below.

1. Determine design life2. Assess actions on the slab3. Determine which combinations of actions apply4. Determine loading arrangements5. Assess durability requirements and determine concrete strength6. Check cover requirements for appropriate fire resistance period7. Calculate min. cover for durability, fire and bond requirements8. Analyse structure to obtain critical moments and shear forces9. Design flexural reinforcement10 . Check for deflection11 .Check punching shear capacity12 .Check spacing of barsDetermine design lifeBased on structural design and their usage the values are given in tableDesign life(years) Examples10 Temporary structures10-30 Replaceable structural parts15-25 Agricultural and similar structures50 Buildings and other common structures120 Monumental buildings, bridges and other civilengineering structuresAssess actions on the slabThe load arrangements for flat slabs met the following requirements1. The ratio of the variable actions (Qk) to the permanent actions (Gk)does not exceed 1.25.2. The magnitude of the variable actions excluding partitions does notexceed 5 kN/m2.32Procedure for determining flexural reinforcementCarry out analysis of slab to determine design moments( M)(Where appropriate use coefficients from the below Table).End support/slab connection FirstPunching shearThe design value of the punching shear force, VEd, will usually be thesupport reaction at the ultimate limit state .1. The maximum value of shear at the column face is not limited to 5 MPa, and depends onthe concrete strength used.2. The control perimeters for rectangular columns in this have rounded corners.3. Where shear reinforcement is required the procedure is simpler; the point at which noshear reinforcement is required can be calculated directly and then used to determine theextent of the area over which shear reinforcement is required.4. It is assumed that the reinforcement will be in a radial arrangement. However, thereinforcement can be laid on a grid provided the spacing rules are followed.Procedure for determining the punching shear1. Determine value of factor β from the below fig2. Determine value of vEd,max design shear stress at face of column fromvEd,max = β VEd /(ui deff)

where ui is perimeter of columndeff = (dy + dz)/2 (dy and dzare the effective depths in orthogonal dire ctions)Determine value of vRd,max from Table 1Check vEd,max ≤ vRd,max if not redesign the slab.Determine value of vEd, (design shear stress)vEd,max = β VEd /(ui deff)where u1 is length of control perimeterDetermine concrete punching shear ca pacity(without shear reinforcement), vRD,c from where rl = (rly rlz)0.5(rly, rlz are the reinforcement ratios in two orthogonal directions for fully bonded tensionsteel, taken over a width equal to column width plus 3 d each side.)Is vEd > vRd,c if it satisfies Punching shear reinforcement not requiredotherwise35Determine area of punching shear reinforcement per perimeter from:Asw = (vEd – 0.75vRd,c)sr u1/(1.5 fywd,ef)Where sr is the radial spacing of shear reinforcementfywd,ef = 250 + 0.25 deff ≤ fywdDetermine the length of the outer perimeter where shear reinforcement not required from:uout,ef = b VEd/(vRd,c d)Check spacing of barsMin area or reinforcement1. The minimum area of longitudinal reinforcement in the main directionis As,min = 0.26 fctm bt d/fyk but not less than 0.0013b d.2. The minimum area of a link leg for vertical punching shearreinforcement is1.5Asw,min /(sr.st) ≥ 0.08fck½fyk.which can be rearranged asAsw,min ≥ (sr.st)/Fwhere sr = the spacing of the links in the radial directionst = the spacing of the links in the tangential directionF can be obtained from Table 10Max area of reinforcementOutside lap locations, the maximum area of tension or compressreinforcement should not exceed As,max = 0.4 AcMinimum spacing of reinforcementThe minimum spacing of bars should be the greater of:Bar diameterAggregate size plus 5 mm20 mmMax spacing of main reinforcementFor slabs less than 200 mm thick the following maximum spacing rules apply:1. for the principal reinforcement3h but not more than 400 mm2. for the secondary reinforcement:3.5h but not more than 450 mmThe exception is in areas with concentrated loads or areas of maximummoment where the following applies:1. for the principal reinforcement2h but not more than 250 mm

2. for the secondary reinforcement3h but not more than 400 mmWhere h is the depth of the slab.36For slabs 200 mm thick or greater reference should be made toSection 7.3.3 of the Eurocode.Spacing of punching shear reinforcementWhere punching shear reinforcement is required the following rulesshould be observed.1. It should be provided between the face of the column and kdinside the outer perimeterwhere shear reinforcement is no longer required. k is 1.5, unless the perimeter at whichreinforcement is no longer required is less than 3 d from the face of the column. In thiscase the reinforcement should be placed in the zone 0.3 d to 1.5dfrom the face of thecolumn.2. There should be at least two perimeters of shear links.3. The radial spacing of the links should not exceed 0.75 d4. The tangential spacing of the links should not exceed 1.5 d within2dof the column face.5. The tangential spacing of the links should not exceed 2d for any other perimeter.6. The distance between the face of the column and the nearest shear reinforcementshould be less than 0.5dDesign of flat slabs using ACI-318:Drop of flat slabs:Where a drop panel is used to reduce amount of negative moment reinforcement over thecolumn of a flat slab, size of drop panel shall be in accordance with the following:Drop panel shall extend in each direction from centerline of support a di stance not less thanone-sixth the span length measured from center -to center of supports in that direction.Projection of drop panel below the slab shall be at least one -quarter the slab thickness beyondthe drop.In computing required slab reinforcement, thickness of drop panel below the slab shall not beassumed greater than one-quarter the distance from edge of drop panel to edge of column orcolumn capital.Thickness of the slab :For slabs without interior beams spanning between the supports an d having a ratio of long toshort span not greater than 2, the minimum thickness shall be in accordance with theprovisions of Table below and shall not be less than the following values:(a) Slabs without drop panels as ......................... 5 in.(b) Slabs with drop panels as defined.................. 4 in.MINIMUM THICKNESS OF SLABS WITHOUT INTERIOR BEAMS

41Design stripsColumn strip is a design strip with a width on each side of a column centerl ine equal to0.25 l2 or0.25 l1, whichever is less.Middle strip is a design strip bounded by two column strips.A panel is bounded by column, beam, or wall centerlines on all sides.Column headThe upper supporting part of a column is enlarged to form the column head. The diameter or thecolumn head is made 0.20 to 0.25 of the span length.Total factored static moment for a spanTotal factored static moment for a span shall be determined in a strip bounded laterally bycenterline of panel on each side of centerline of supports.Absolute sum of positive and average negative factored moments in each direction shall not beless than.shall not be less than 0.65 1 l . Circular or regular polygon shaped supports shall betreated as square supports with the same area.2 l =When the span adjacent and parallel to an edge is being considered, the distance fromedgeto panel centerline shall be substituted for 2 l .In an interior span, total static moment 0 M shall be distributed as follows:Negative factored moment .................................0.65Positive factored moment ...................................0.3542In an end span, total factored static moment 0 M shall be distributed as follows:Negative moment sections shall be designed to resist the larger of the two interior negativefactored moments determined for spans framing into a common support unless an a nalysisis made todistribute the unbalanced moment in accordance with stiff nesses of adjoining elements.Edge beams or edges of slab shall be proportioned to resist in torsion their share of exteriornegative factored momentsFactored moments in middle strips:That portion of negative and positive factored moments not resisted by column strips shallbeproportionately assigned to corresponding half middle strips.Each middle strip shall be proportioned to resist the sum of the moments assigned to its twohalf middle strips.A middle strip adjacent to and parallel with an edge supported by a wall shall be

proportioned to resist twice the moment assigned to the half middle strip corresponding tothe first row of interior supports.43Factored moments in column strips:Column strips shall be proportioned to resist the following portions in percent of exteriornegativefactored moments:Column strips shall be proportioned to resist the following portions in percent of exteriornegativefactored moments:Modification of factored moment:Modification of negative and positive factored moments by 10 percent shall be permittedprovidedthe total static moment for a panel in the direction considered is not less than that requiredbyShear provision(punching shear):Two-way action where each of the critical sections to be investigated shall be located so thatits perimeter 0 b is a minimum but need not approach closer than d / 2 to(a) Edges or corners of columns, concentrated loads, or reaction areas, or44(b) Changes in slab thickness such as edges of capitals or drop panels.Nominal shear strength of concrete:for flat slabs c V =nominal shear strength of concretec V Shall be smallest of the following:[Where c is the ratio of long side to short side of the column, concentrated load orreaction areaand where s is 40 for interior columns, 30 for edge columns,20 for corner columns]This is distributed as follows:Negative design moment = 237 x 0. 65 = 154 ft-kips = 208.89 KNmPositive design moment = 237 x 0.35 = 83.00 ft -kips = 113.22 KNmThe column strip has a width ofBending moment for column stripNegative moment for column strip = 75 % of total negative moment in the pannel= 0.75 x 154.00 = 115.50 ft -kips = 157.66 KNmPositive moment for column strip = 60 % of total positive moment in the panel.= 0.60 x 83.00 = 49.8 ft -kips = 67.977 KNm46Bending moment for middle strip along shorter spanNegative moment for middle strip = 0.25 x 96.24= 24.06 ft-kips= 32.84 KNmpositive moment for middle strip = 0.40 x 51.891= 20.7564 ft-kips= 28.33 KNm

Bending moment for middle strip along longer spanNegative moment for middle strip = 0.25 x 154 = 38.5 ft -kip= 52.55 KNmPositive moment for middle strip = 0.40 x 83.00 = 33.2 ft -kips= 45.318 KNmMax moment (+ve or –ve ) along shorter span = 72.18 ft -kipsMax moment (+ve or –ve) along longer span = 115.50 ft -kipsmax = maximum permitted reinforcement ratioConclusions:By comparing with different codes we concluded that ACI 318, NZS 3101& euro codes aremost effective in designing of flat slabs.As per Indian code we are using cube strength but in international standards cylinderedare used which gives higher strength than cube.Drops are important criteria in increasing the shear strength of the slab.Enhance resistance to punching failure at the junct ion of concrete slab & column.By incorporating heads in slab, we are increasing rigidity of slab.In the interior span, the total design moments (Mo) are same for IS, NZS, ACI.The negative moment’s section shall be designed to resist the larger of the two interiornegative design moments for the span framing into common supports.According to Indian standard (IS 456) for RCC code has recommended characteristicstrength of concrete as 20, 25, and 30 and above 30 for high strength concrete. For designpurpose strength of concrete is taken as 2/3 of actual strength this is to compensate thedifference between cube strength and actual strength of concrete in structure. After thatwe apply factor of safety of 1.5. So in practice Indian standard actually us es 46% of totalconcrete characteristic strength. While in International practice is to take 85% of totalstrength achieved by test and then apply factor of safety which is same as Indian standardso in actual they use 57% of total strength.Pre fabricated sections to be integrated into the design for ease of construction.