Pile caps can be designed either by the truss analogy or by bending theory (see BS 8110: Part 1: 126.96.36.199(5)). In this example bending theory will be used.
For a pile cap with closely spaced piles, in addition to bending and bond stress checks, a check should be made on the local shear stress at the face of the column, and a beam shear check for shear across the width of the pile cap. For more widely spaced piles (spacing > 3 × diameter), a punching shear check should also be carried out.
Local shear check
The ultimate column load is Pu = 6400 kN.
Length of column perimeter is u = 2(400 + 400) = 1600 mm.
The shear stress at the face of the column is
Bending shear check
In accordance with BS 8110: Part 1: 188.8.131.52, shear is checked across a section 20% of the diameter of the pile (i.e. D/5) inside the face of the pile. This is section A–A in Fig. 14.30.
The shear force across this section – ignoring the self-weight of the pile cap, which is small in comparison – is given by
The corresponding shear stress is given by vu = Vu/bvd, where bv is the breadth of section for reinforcement design.
In accordance with BS 8110: Part 1: 184.108.40.206, this must not exceed (2d/av)vc where av is deﬁned in Fig. 14.30 and vc is the design concrete shear stress from BS 8110: Part 1: Table 3.8. Thus
c is 0.4 N/mm2, giving
The necessary depth for the pile cap is
h = d + 25(diameter bar) + 75(cover)
= 846 + 100
= 946 mm ⇒use h = 950 mm
|Fig. 14.30 Pile cap design example.|