Welcome To Presentation on Footing Design

Presented by

Name

Lamia Sultana Efa

ID No

10.01.03.054

FOOTINGSWhat is Footing ?

• Footings are structural members used to support columns and walls and to transmit and distribute their loads to the soil.

Different types of Footings:

1. Spread or isolated footing:

A spread footing is provided tosupport an individual column.It is circular, square orrectangular ofuniform thickness.

2. Continuous or strip footing:

The footing beneath a wall is known as a strip orcontinuous footing. It isprovided for a raw of columnswhich are so closely spacedthat spread footings areoverlap or nearly touch eachother.

3. Combined footing:

A Combined footing supports two columns. It is provided when two columns are close together, causing overlap of adjacent isolated footings and where soil bearing capacity is low, causing overlap of adjacent isolated footings.

4. Strap or cantilever footing

A strap footing consists oftwo isolated footingsconnected with a structuralstrap or a lever. The strapconnect two footings such thatthey behave as one unit.

• 1.Determine the structural loads and member sizes at the foundation level;

• 2.Collect all the geotechnical data; set the proposed footings on the geotechnical profile;

• 3. Determine the depth and location of all foundation elements,• 4.Determine the bearing capacity,• 5.Determine possible total and differential settlements; check

effects at 2B depths;• 6.Select the concrete strength (and possibly the mix),

The Design Procedure.

The Design Procedure:

• 7.Select the steel grade,• 8.Determine the required footing dimensions,• 9.Determine the footing thickness, T (or D in some textbooks),• 10.Determine the size, number and spacing of the reinforcing

bars,• 11.Design the connection between the superstructure and the

foundation, and• 12.Check uplift and stability against sliding and overturning of

the structure-soil system.

Ground investigation for footings

The recommended minimum depth of investigation, za, for spread foundations supporting high-rise structures and civil engineering projects is the greater of:

za ≥ 3bF and za ≥ 6m, where bF is the foundation’s breadth.

For raft foundations:za≥ 1.5bB, where bB is the breadth of the raft.The depth za may be reduced to 2m if the

foundation is built on competent strata† with ‘distinct’ (i.e. known) geology.

With ‘indistinct’ geology, at least one borehole should go to at least 5m. If

bedrock is encountered, it becomes the reference level for za.

Fig : Recommended depth of investigationfor spread foundations

Design situations and limit statesExamples of some ultimate limit states that footings must be designed to withstand

Design considerations for footingsEuro code 7 lists a number of things that must be considered when choosing

the depth of a spread foundation

Footings subject to vertical actions

• For a spread foundation subject to vertical actions, Euro code 7 requires the design vertical action Vd acting on the foundation to be less than or equal to the design bearing resistance Rd of the ground beneath it:

Vd ≤ Rd

Vd should include the self-weight of the foundation and any backfill on it.• This equation is merely a re-statement of the inequality:• Ed ≤ Rd.• Rather than work in terms of forces, engineers more commonly consider

pressures and stresses, so we will rewrite this equation as:• qEd ≤ qRd

• where qEd is the design bearing pressure on the ground (an action effect), and qRd is the corresponding design resistance.

Figure 1 shows a footing carrying characteristic vertical actions VGK (permanent) and VQK (variable) imposed on it by the super-structure. The

characteristic self weights of the footing and of the backfill upon it are both permanent actions (WGK). The following sub-sections explain how qEd and qRd

are obtained from VGK, VQK, WGK, and ground

Figure 1. Vertical actions on a spread foundation

Effects of actions

The characteristic bearing pressure qEK shown in Figure 1 is given by:

Where Vrep is a representative vertical action; VGK, VQK, and WGK are as defined above; A' is the footing’s effective area and ψi is the combination factor applicable to the ith variable action.

If we assume that only one variable action is applied to the footing, this equation simplifies to:

since ψ = 1.0 for the leading variable action (i = 1).The design bearing pressure qEd beneath the footing is then:

Where γG and γQ are partial factors on permanent and variable actions, respectively.

Eccentric loading and effective foundation area

• The ability of a spread foundation to carry forces reduces dramatically when those forces are applied eccentrically from the centre of the foundation.

• To prevent contact with the ground being lost at the footing’s edges, it is customary to keep the total action within the foundation’s ‘middle-third’. In other words, the eccentricity of the action from the centre of the footing is kept within the following limits:

where B and L are the footing’s breadth and length, respectively; and eB and eL are eccentricities in the direction of B and L (see Figure 2)

Figure 137. Effective area of spread foundation

Different types of load:

• Dead load• Live load• Earth pressure• Water pressure• Earthquake loads• Wind loads• Snow loads

PROCEDURE (ACCORDING TO SIGHT) Step 1 : Plan the design

Step 2 : Start digging

Step 3 : Frame the footing

Step 4 : Set the column

HOW TO BUILD A Column FOOTING

• Step 1 : Measure the footing• Step 2 : Grading• Step 3 : Concrete calculation• Step 4 : Pouring a footing• Step 5 : Setting column

MATERIALS A)Sand B)CementC)Brick chips & stone chipsD)Reinforcement bar

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