Friday, February 15, 2013

Pile Caps.

1 Introduction
The design of pile caps had at one time become a math-ematician’s delight – and a designer’s nightmare. Highly complex formulae with numerous empirical variants could result in expensive design and construction to save a couple of reinforcing bars. As in all design and construction the aim must be ‘to keep it simple’.

2 The need for pile caps – capping beams
It is frequently not possible to sit a superstructure column direct on to a pile because:

(1) It is practically impossible to drive piles in the exact position and truly vertical. Piles wander in driving and deviate from their true position. A normal specification tolerance for position is ±75 mm and for verticality not more than 1 in 75 for a vertical pile or 1 in 25 for a raking pile. A column sitting directly on a pile with
an eccentricity of 75 mm will exert bending as well as direct stresses in the pile.
(2) A single, heavily loaded column supported by a pile group will need a load spread (pile cap) to transmit the load to all the piles.
(3) A line of piles supporting a load-bearing wall will need a capping beam to allow both for tolerance of pile positioning and load spreading of the piles’ concentrated load to the wall.

3 Size and depth
Pile caps are usually of concrete but can be large slabs of rock or mats of treated timber. This discussion is limited to the more common use of concrete.

To allow for the pile deviation the pile cap should extend 100–150 mm beyond the outer face of the piles. The pile group centroid should ideally coincide with the column’s position (see Fig. 14.16).

Plan on triple pile cap.
Fig. 14.16 Plan on triple pile cap.

The depth must be adequate to resist the high shear force and punching shear and to transmit the vertical load (see Fig. 14.17). The shaded area of the pile cap plan in Fig. 14.17 is the area where the column load is directly transferred to the piles. For such a condition the shear stresses are generally small but bending moments need to be catered for.

 Load transfer from column to piles.
Fig. 14.17 Load transfer from column to piles.

Alternatively, peripheral steel as a ring tension around a cone shaped compression block may be considered to be a suitable equilibrium of forces (see Fig. 14.18), however, full tension laps must be provided for the peripheral steel.

Ring tension pile cap.
Fig. 14.18 Ring tension pile cap.

Single column loads supported on larger pile groups can create significant shear and bending in the cap which will need top and bottom reinforcement as well as shear links (see Fig. 14.19).

Pile cap, typical reinforcement.
Fig. 14.19 Pile cap, typical reinforcement.

The heads of r.c. piles should be stripped and the exposed reinforcement bonded into the pile cap for the necessary bond length. Pile caps to steel piles can be reduced in depth if punching shear is reduced by capping and/or reinforcing the head of the pile, as shown in Fig. 14.20.

Reinforced pile head.
Fig. 14.20 Reinforced pile head.

Piles for continuous capping beams supporting load-bearing walls can be alternately staggered to compensate for the eccentricity of loading due to the 75 mm out-of-line tolerance (see Fig. 14.21).

 Continuous capping beam.
Fig. 14.21 Continuous capping beam.

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