Composite Floors

7/29/2019 Composite Floors

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Areas from where cost can be reduced are:-

1) Reduce plinth area by using thinner wall concept.Ex.15 cms thick solid concrete block

wall.

2) Use locally available material in an innovative form like soil cement blocks in place ofburnt brick.

3) Use energy efficiency materials which consumes less energy like concrete block in place

of burnt brick.

4) Use environmentally friendly materials which are substitute for conventional building

components like use R.C.C. Door and window frames in place of wooden frames.

5) Preplan every component of a house and rationalize the design procedure for reducing the

size of the component in the building.

6) By planning each and every component of a house the wastage of materials due to

demolition of the unplanned component of the house can be avoided.

7) Each component of the house shall be checked whether if its necessary, if it is not

necessary, then that component should not be used.

Cost reduction through adhoc methods

Foundation

Normally the foundation cost comes to about 10 to 15% of the total building and usuallyfoundation depth of 3 to 4 ft. is adopted for single or double store building and also the

concrete bed of 6(15 Cms.) is used for the foundation which could be avoided.

It is recommended to adopt a foundation depth of 2 ft.(0.6m) for normal soil like gravely soil,

red soils etc., and use the uncoursed rubble masonry with the bond stones and good packing.

Similarly the foundation width is rationalized to 2 ft.(0.6m).To avoid cracks formation in

foundation the masonry shall be thoroughly packed with cement mortar of 1:8 boulders and

bond stones at regular intervals.

It is further suggested adopt arch foundation in ordinary soil for effecting reduction in

construction cost up to 40%.This kind of foundation will help in bridging the loose pockets of

soil which occurs along the foundation.

In the case black cotton and other soft soils it is recommend to use under ream pilefoundation which saves about 20 to 25% in cost over the conventional method of

construction.

PlinthIt is suggested to adopt 1 ft. height above ground level for the plinth and may be constructed

with a cement mortar of 1:6. The plinth slab of 4 to 6 which is normally adopted can be

avoided and in its place brick on edge can be used for reducing the cost. By adopting this

procedure the cost of plinth foundation can be reduced by about 35 to 50%.It is necessary to

take precaution of providing impervious blanket like concrete slabs or stone slabs all round

the building for enabling to reduce erosion of soil and thereby avoiding exposure of

foundation surface and crack formation.


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WallingWall thickness of 6 to 9 is recommended for adoption in the construction of walls all-round

the building and 41/2 for inside walls. It is suggested to use burnt bricks which are

immersed in water for 24 hours and then shall be used for the walls

Rat

trap bond wallIt is a cavity wall construction with added advantage of thermal comfort and reduction in the

quantity of bricks required for masonry work. By adopting this method of bonding of brick

masonry compared to traditional English or Flemish bond masonry, it is possible to reduce in

the material cost of bricks by 25% and about 10to 15% in the masonry cost. By adopting rat-

trap bond method one can create aesthetically pleasing wall surface and plastering can be

avoided.

Concrete block wallingIn view of high energy consumption by burnt brick it is suggested to use concrete block

(block hollow and solid) which consumes about only 1/3 of the energy of the burnt bricks in

its production. By using concrete block masonry the wall thickness can be reduced from 20cms to 15 Cms. Concrete block masonry saves mortar consumption, speedy construction of

wall resulting in higher output of labour, plastering can be avoided thereby an overall saving

of 10 to 25% can be achieved.

Soil cement block technologyIt is an alternative method of construction of walls using soil cement blocks in place of burnt

bricks masonry. It is an energy efficient method of construction where soil mixed with 5%

and above cement and pressed in hand operated machine and cured well and then used in the

masonry. This masonry doesnt require plastering on both sides of the wall. The overall

economy that could be achieved with the soil cement technology is about 15 to 20%

compared to conventional method of construction.

Doors and windowsIt is suggested not to use wood for doors and windows and in its place concrete or steel

section frames shall be used for achieving saving in cost up to 30 to 40%.Similiarly for

shutters commercially available block boards, fibre or wooden practical boards etc., shall be

used for reducing the cost by about 25%.By adopting brick jelly work and precast

components effective ventilation could be provided to the building and also the construction

cost could be saved up to 50% over the window components.

Lintals and ChajjasThe traditional R.C.C. lintels which are costly can be replaced by brick arches for small spans

and save construction cost up to 30 to 40% over the traditional method of construction. By

adopting arches of different shapes a good architectural pleasing appearance can be given to

the external wall surfaces of the brick masonry.

RoofingNormally 5(12.5 cms) thick R.C.C. slabs is used for roofing of residential buildings. By

adopting rationally designed insitu construction practices like filler slab and precast elements

the construction cost of roofing can be reduced by about 20 to 25%.

Filler slabsThey are normal RCC slabs where bottom half (tension) concrete portions are replaced by


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filler materials such as bricks, tiles, cellular concrete blocks, etc.These filler materials are so

placed as not to compromise structural strength, result in replacing unwanted and

nonfunctional tension concrete, thus resulting in economy. These are safe, sound and provide

aesthetically pleasing pattern ceilings and also need no plaster.

For more on filler materials checkFiller Materials Used in Concrete

Jack arch roof/floorThey are easy to construct, save on cement and steel, are more appropriate in hot climates.

These can be constructed using compressed earth blocks also as alternative to bricks for

further economy.

Ferrocement channel/shell unitProvide an economic solution to RCC slab by providing 30 to 40% cost reduction on

floor/roof unit over RCC slabs without compromising the strength. These being precast,

construction is speedy, economical due to avoidance of shuttering and facilitate quality

control.

Finishing WorkThe cost of finishing items like sanitary, electricity, painting etc., varies depending upon the

type and quality of products used in the building and its cost reduction is left to the individual

choice and liking.

ConclusionThe above list of suggestion for reducing construction cost is of general nature and it varies

depending upon the nature of the building to be constructed, budget of the owner,

geographical location where the house is to be constructed, availability of the building

material, good construction management practices etc. However it is necessary that good

planning and design methods shall be adopted by utilizing the services of an experienced

engineer or an architect for supervising the work, thereby achieving overall cost effectiveness

to the extent of 25% in actual practice.

posted inBuilding,Civil Engineering Information|44 Comments

PONDING CONSIDERATIONS IN BUILDINGS

Flat roofs on which water may accumulate may require analysis to ensure that they are stable

under ponding conditions. A flat roof may be considered stable and an analysis does not needto be made if both of the following two equations are satisfied:

C p + 0.9 C s 0.25

I d 25S4 / 10 6

Where C p = 32 L s L4

p / 107 I p

C s = 32 SL4

s / 107

s

L p = length, ft (m), of primary member or girder
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L s = length, ft (m), of secondary member or purlin

S = spacing, ft (m), of secondary members

I p = moment of inertia of primary member, in4

(mm4

)

I s = moment of inertia of secondary member, in4

(mm4

)

I d = moment of inertia of steel deck supported on secondary members, in4

/ft (mm4

/m)

For trusses and other open-web members, I s should be decreased 15 percent. The total bending stress due to dead loads, gravity live loads,

and ponding should not exceed 0.80F y , where F y is the minimum specified yield stress for the steel.

posted inBuilding|0 Comments

NUMBER OF CONNECTORS REQUIRED FOR

BUILDING CONSTRUCTION

The total number of connectors to resist V h is computed from V h / q, where q is the

allowable shear for one connector, kip (kN). Values of q for connectors in buildings are given

in structural design guides.

The required number of shear connectors may be spaced uniformly between the sections of

maximum and zero moment. Shear connectors should have at least 1 in (25.4 mm) of

concrete cover in all directions; and unless studs are located directly over the web, stud

diameters may not exceed 2.5 times the beam-flange thickness.

With heavy concentrated loads, the uniform spacing of shear connectors may not be sufficient

between a concentrated load and the nearest point of zero moment. The number of shear

connectors in this region should be at least

N 2 = N 1 [ ( M b / M m a x ) - 1] / ( b1 )

where M = moment at concentrated load, ft kip (kN-m)

M max = maximum moment in span, ft kip (kN-m)

N 1 = number of shear connectors required between M m a x and zero moment

b = S t r/ S s or S e f f/ S s , as applicable

S e f f= effective section modulus for partial composite action, in3 (mm 3 )

Shear on Connectors
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The total horizontal shear to be resisted by the shear connectors in building construction is

taken as the smaller of the values given by the following two equations:

V h = 0.85 f

c A c / 2

V h = A s F y / 2

where V h = total horizontal shear, kip (kN), between maximum positive moment and each

end of steel beams (or between point of maximum positive moment and point of

contraflexure in continuous beam)

f c = specified compressive strength of concrete at 28 days, ksi (MPa)

A c = actual area of effective concrete flange, in2 (mm 2 )

A s = area of steel beam, in2 (mm 2 )

In continuous composite construction, longitudinal reinforcing steel may be considered to act

compositely with the steel beam in negative-moment regions. In this case, the total horizontal

shear, kip (kN), between an interior support and each adjacent point of contraflexure should

be taken as

V h = A s rF y r/ 2

where A s r= area of longitudinal reinforcement at support within effective area, in2 (mm 2 );

and F y r= specified minimum yield stress of longitudinal reinforcement, ksi (MPa).


COMPOSITE CONSTRUCTION

In composite construction, steel beams and a concrete slab are connected so that they act

together to resist the load on the beam. The slab, in effect, serves as a cover plate. As a result,

a lighter steel section may be used.

Construction In Buildings

There are two basic methods of composite construction.

Method 1. The steel beam is entirely encased in the concrete. Composite action in this case

depends on the steel-concrete bond alone. Because the beam is completely braced laterally,

the allowable stress in the flanges is 0.66F y , where F y is the yield strength, ksi (MPa), of the

steel. Assuming the steel to carry the full dead load and the composite section to carry the

live load, the maximum unit stress, ksi (MPa), in the steel is

F s = ( M D / S S ) + ( M L / S t r) ? 0.66F y

where M D = dead-load moment, in-kip (kN-mm)
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M L = live-load moment, in-kip (kN-mm)

S s = section modulus of steel beam, in3 (mm 3 )

S t r= section modulus of transformed composite section, in3 (mm 3 )

An alternative, shortcut method is permitted by the AISC specification. It assumes that the

steel beam carries both live and dead loads and compensates for this by permitting

a higher stress in the steel:

fs = M D + M L / S s ? 0.76 F y

Method 2. The steel beam is connected to the concrete slab by shear connectors. Design is

based on ultimate load and is independent of the use of temporary shores to support the steel

until the concrete hardens. The maximum stress in the bottom flange is

F s = M D + M L / S t r 0.66 F y

To obtain the transformed composite section, treat the concrete above the neutral axis as an

equivalent steel area by dividing the concrete area by n, the ratio of modulus of elasticity of

steel to that of the concrete. In determination of the transformed section, only a portion of the

concrete slab over the beam may be considered effective in resisting compressive flexural

stresses (positive-moment regions). The width of slab on either side of the beam centerline

that may be considered effective should not exceed any of the following:

1. One-eighth of the beam span between centers of sup- ports

2. Half the distance to the centerline of the adjacent beam

3. The distance from beam centerline to edge of slab


FASTENERS IN BUILDINGS

The AISC specification for allowable stresses for buildings specifies allowable unit tension

and shear stresses on the cross-sectional area on the unthreaded body area of bolts and

threaded parts. (Generally, rivets should not be used in direct tension.) When wind or seismic

load are combined with gravity loads, the allowable stresses may be increased one-third.

Most building construction is done with bearing-type connections. Allowable bearing stresses

apply to both bearing-type and slip-critical connections. In buildings, the allowable bearing

stress F p , ksi (MPa), on projected area of fasteners is

F p = 1.2 F

where F u is the tensile strength of the connected part, ksi (MPa). Distance measured in theline of force to the nearest edge of the connected part (end distance) should be at least 1.5d,
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where d is the fastener diameter. The center-to-center spacing of fasteners should be at least

3d.

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