7.9 The design of pile capping beams and connecting ground beams
Pile capping beams have the function of distributing the load from walls or closely spaced
columns onto rows of piles. For heavy wall loading in conjunction with transverse bending
moments the piles are placed in transverse rows surmounted by a wide capping beam
(Figure 7.14a). The piles may be placed in a staggered row for walls carrying a compressive
loading with little or no transverse bending moments (Figure 7.14b). A lightly loaded wall
can be supported by a single row of piles beneath the centre-line provided that the beam
capping the piles is restrained by tying it to transverse capping beams carrying cross walls
in the structure. Attention should be given to providing adequate restraint to transverse
movement and bending where ground beams are supported by minipiles.
A design method was proposed by the Building Research Establishment(7.9)which allows
the bending moments and shearing forces in a beam to be reduced if the beam can be shown
to be acting compositely with the brick wall built upon it. The method may be applied to the
design of pile capping beams for house foundations, and is applicable to walls having a
height of not less than 0.6 of the span of the beam. The walls must not have door or window
openings near to the supports, as this would interfere with the arching action of the
brickwork. The bending moment produced by a uniformly distributed load on a freely
supported beam is WL /8. With full composite action between beam and wall, this moment
may be reduced to a minimum of WL/100 for light loading, where Wis the total distributed
load on the brickwork (including self weight) and Lis the span of the beam. A property of
composite action is that the compression in the arch within the brickwork is directed radi-
ally towards the nearest firm supports; therefore, shearing reinforcement of the beam is the-
oretically unnecessary if the loaded lengths at the supports can be shown to be not greater
than the depth of the beam. However, the BRErecommended that all beams designed for
bending moments of greater than WL/60 should be designed to resist the shearing forces
produced by the full dead and imposed loading.
Structural design of piles and pile groups 393(a) (b)Figure 7.14Arrangement of piles in capping beams (a) Heavy wall loading with transverse
bending moments (b) Light wall loading with little or no transverse bending.