DHARMSHALLOW FOUNDATIONS 647Total area provided =285 105
2..+
× 6.4 = 12.48 m^2 (O.K.)
Total DL + LL = 2440 kNLocation of c.g. from B 1 = (6.4/3)2105205
390×+..
= 2.70 mLocation of resultant DL + LL =940 5 4
2440×.
= 2.08 m from Col. Bor 2.58 m from B
e = 0.12 m
Moment of inertia of the section about longer edge=1
3 × 1.05 × 6.4(^3) +^1
12 × 1.80 × 6.4
(^3) = 131.09 m 4
M.I. about an axis through c.g. = 131.09 – 12.48 × 2.70 = 40.1 m^4
qmax =
2440
12 48
2440 0 12 2 7
. 40 1
+ ×× = 215 kN/m^2 < 225 kN/m^2 (O.K.)qmin =2440
12 482440 0 12 2 7. 40 1
− ×× = 176 kN/m^2The structural design of the footing has now to follow.
Example 15.6: A raft, 9 m × 27 m, is founded at a depth of 3 m in sand with a value of N = 25
upto great depth. Determine the total load which the raft can support. If the raft is designed as
a floating foundation, what will be the load it can support?
Assume γ = 18 kN/m^3.
Allowable soil pressure for a footing for N = 25 is 330 kN/m^2
(from Terzaghi and Peck’s charts for 40 mm settlement)
Allowable soil pressure for a raft = 2 × 330 = 660 kN/m^2
(According to Peck, Hanson and Thornburn)
Total load which the raft can support = 660 × 9 × 27 = 160,380 kN
If the raft is designed as a floating foundation,
The soil pressure = relief of stress due to excavation = 3 × 18 = 54 kN/m^2
Total load which the raft can support in that case = 54 × 9 × 27 ≈ 13,120 kN.Summary of Main Points
- Foundations are categorised as shallow foundations (Df /b ≤ 1) and deep foundations. Shallow
foundations are either footings or rafts. Footings may be spread footings which may be continu-
ous for walls, or isolated for columns; the shape of the latter being square, circular or rectangu-
lar. Strap footings and combined footings are used to support more than one column; the latter
may be rectangular or trapezoidal in shape.
A floating foundation is not a type but a concept whereby the relief of stress due to excava-
tion is made nearly equal to the structure load.