Design of Pile Cap

Pile Cap Design A brief introduction A pile cap is a reinforced concrete structure, which allows the load from a column to be equally shared between a group of piles:

Column

Pile cap

Piles

Typical Proportions Centre line pile spacing = 3 x pile diameter Cap depth = 2.5 x pile diameter Cap projection beyond piles = 150mm

If all piles are assumed to be equally loaded then one must accept that some redistribution of load (and therefore soil stresses) takes place. This is particularly required in five and six pile caps.

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Methods of Design The 'Truss' Analogy Pile caps for groups of two to five piles are designed using the 'Truss. analogy:

Compression - resisted by concrete Additional bursting forces are resisted by horizontal reinforcement in the sides of the pile cap

Tension - resisted by steel reinforcement

Reinforcement is designed to provide: a) b) c) d)

Resistance to tensile 'Truss' forces in the bottom of the cap Resistance to vertical shear Resistance to punching shear Resistance to bursting forces in the side of the cap

Tensile 'Truss' Forces 2 pile cap

Ft =

Nl kN 4d

3 pile cap

Ft =

Nl kN 9d

4 pile cap

Ft =

Nl kN 8d

Nl kN 10d where d= effective depth of cap l = pile c/c spacing N = factored column load

5 pile cap

Ft =

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Caps in Bending Caps for six pile groups would be designed to transfer the column load primarily by bending resistance. Reinforcement is thus designed to provide: a) b) c) d)

Resistance to tensile bending forces in the bottom of the cap Resistance to vertical shear Resistance to punching shear Resistance to bursting forces in the side of the cap - since some 'Truss' action can occur.

Anchorage (Truss and Bending) Provide a full anchorage length beyond the pile centre line, for all bars in tension ('Truss' or bending). Lacers Horizontal reinforcement must be provided in all faces of the pile cap: Minimum area ∃ 1/4 area of tensile reinforcement Pile Embedment in the Cap All piles must be cast in to the cap by at least 75mm 600mm of pile reinforcement must be anchored into the cap Shear 1. Vertical shear - check at critical section 1/5 pile diameter the inside piles. - use full width and effective depth d for shear area 2d - shear stress vc x av

0.79  100As  From Table 3.9 in BS 8110: vc = γ m  bv d

  

1/3

 400     d 

1/4

1/3

 100As   for fcu = 25N/mm2 and (m = 1.25 vc = 0.707  Reduces to:  bv d  2. Punching shear - check around column perimeter (at column face) -

shear stress

0.8 fcu or 5 N/mm2

These checks are the same as for Pad Foundations.

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