**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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