abuildersengineer.com A pile cap is required to transfer the load from a 400 mm ×
400 mm column to four 600 mm diameter piles, as shown in Fig. 14.30.
Pile caps can be designed either by the truss analogy or by
bending theory (see BS 8110: Part 1: 3.11.4.1(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: 3.11.4.3, 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: 3.11.4.4, 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
For grade C35 concrete, from BS 8110: Part 1: Table 3.8,
assuming six T25 bars, the minimum value of
vc is 0.4 N/mm2, giving
Thus, provided the average effective depth exceeds d = 846 mm (the local shear check), minimum
reinforcement to satisfy bond and
bending tension requirements will be adequate in this instance.
The necessary depth for the pile cap is
h = d +
25(diameter bar) + 75(cover)
= 846 + 100 =
946 mm ⇒use h = 950 mm
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| Fig. 14.30 Pile cap design example. |