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 re-distribution of load (and therefore soil stresses) takes place. This is particularly required in five and six pile caps.

C:\My Structures\BEng3 SED\Teaching\Foundations\Pile Caps.doc 03/11/03 1

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

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

Tension - resisted by steel reinforcement

Compression - resisted by concrete Reinforcement is designed to provide: a) Resistance to tensile 'Truss' forces in the bottom of the cap b) Resistance to vertical shear c) Resistance to punching shear d) Resistance to bursting forces in the side of the cap Tensile 'Truss' Forces

2 pile cap 4dNlFt = kN

3 pile cap d9

NlFt = kN

4 pile cap d8

NlFt = kN

5 pile cap d10

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

C:\My Structures\BEng3 SED\Teaching\Foundations\Pile Caps.doc 03/11/03 2

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) Resistance to tensile bending forces in the bottom of the cap b) Resistance to vertical shear c) Resistance to punching shear d) 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

- shear stress vc x 2da v

From Table 3.9 in BS 8110:4131

v

s

mc d

400dbA100790 /

/.

= v

Reduces to: 31

v

sc db

A100707/

.

0v = for fcu = 25N/mm2 and (m = 1.25

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.

C:\My Structures\BEng3 SED\Teaching\Foundations\Pile Caps.doc 03/11/03 3

Shear