Example_ Pile Cap Design

12/3/13

Exampl e: Pil e cap desi g n. | Buil der ' s Eng i neer  

Examp Exa mple: le: Pile cap design. 0

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A pile cap is required to transfer the load from a 400 mm × 400 mm column to four 600 mm diameter pi diameter  pilles es,, as shown in Fig. 14.30. Pile caps can be designed either by the truss analogy or by  bendi  bendin ng theory theory (see BS BS 8110: Part 1: 3.11.4.1(5)). In In thi this example bending bending theory will be used. For a pile ca p with with closel closely y spaced pil piles, in in additi addition on to bendi bendin ng and bond str esschecks ess checks,, a check should be made on the local shear stress stress at  at the face of the column, and a  beam shear  check   check  for  for shear across the width of the pile cap. For  more wide widelly spaced spa ced piles piles (spaci (spac ing > 3 × diameter), diameter), a  punch  punchiing shear shear check check shoul should d also also be carri carried out. 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

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12/3/13

Example: Pile cap design. | Builder's Engineer  

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 www.abuildersengineer.com/2013/02/example-pile-cap-design.html

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12/3/13

Example: Pile cap design. | Builder's Engineer  

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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12/3/13

Example: Pile cap design. | Builder's Engineer  

Fig. 14.30 Pile cap design  example.

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