GUIDELINES FOR FOOTING DESIGN | REQUIREMENTS FOR FOOTING DESIGN | LCETED

 

Shallow foundation: 𝐷/𝐵 = 0.25 − 1.0

Deep foundation: 𝐷/𝐵 = 5 − 20

 

For design of footing – allowable or net safe B.C is used,𝑞_𝑎 = 𝑞_𝑢 /𝐹𝑂𝑆

 

In case of normal soils, provide a base of footing at 1.0 – 1.5 m from NGL.

 

For black cotton soils, provide a base of footing at 2.5 – 4.0 m from NGL.

 

Better to provide cover of 75 mm under all exposure conditions.

 

Minimum thickness of isolated footing = 600 mm.

 

Thickness of raft footing = 800 mm - 1500 mm (steel: 20 dia @ 150 c/c both ways at Top and Bottom.)

 

Axial load ± 20% (same footing)

 

Min. steel in each direction = 0.2% 𝑜𝑓 𝑏𝐷

 

For thick foundation (𝑡 > 1.0𝑚), provide a nominal reinforced of 360 mm2 /m in each direction on each face (bottom and top).

Levelling course, 100 mm thick is provided as (1:5:10 or 1:4:8).

 

As a thumb rule, if the spacing between two isolated footings is < 0.5𝑚, provide combined footing

 

For combined footing, 𝐿/𝐵 ≯ 1.5 (if this is the case, increase B)

 

If 𝑃1 close to property line and 𝑃2 > 𝑃1, use eccentric combined footing.

 

If 𝑃1 close to property line and (½) < (𝑃2 /𝑃1) < 1, use trapezoidal combined footing.

 

If 𝑃1 close to property line and (𝑃2 /𝑃1) < (½) and (𝐷/𝐵) > 2.5, use strap combined footing.

 

Offset of raft from exterior column = depth of raft (say 800 mm)

 

Provide extra reinforced. At bottom for 𝐿/3 around each column in raft foundation, except side columns or columns on retaining wall

The column bars be bent at 90° into the footing for a length of 300 mm. same practise should be adopted while terminating the bars at the top of building

 

Design steps:

Area of footing is calculated based on working load,

Length of footing,

Width of footing, 𝐵_𝑓 = 𝐴_𝑓/𝐿_𝑓

Projections:

Parallel to a depth of column, 𝑋 = (𝐿_𝑓 – 𝐷)/2

Parallel to a width of column, Y = (𝐵_𝑓 – 𝑏)/2

Upward footing reaction (due to 𝑃_𝑢) causing bending in footing, 𝑤_𝑢 = 𝑃_𝑢/𝐴_𝑓 < 𝑆𝐵C

 

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