It is recommended that the depth of the beam should be between L/15 and L/20. However,
with a heavily loaded wall only a small degree of composite action is allowed and it may be
necessary to use a beam deeper than L/15. When considerable composite action is present,
i.e. when the bending moment is less than WL/40, the reinforcement should be calculated
for a beam having a depth equal to L/15 even if a deeper beam is to be provided for other
reasons. This is to ensure that there is sufficient steel in the beam to act as a tie across the
springing of the arch within the brickwork.
When designing pile capping beams by limit-state principles it is seldom necessary to
consider the serviceability limit-state. However, an examination of the limit-state of
cracking is necessary if the beam is to be exposed to soil or groundwater which can be
expected to be corrosive. The limit-state of deflection should be checked if the beam is to
support a wall faced with a material such as mosaic tiles, which are particularly susceptible
to cracking due to small movements.
The BREdesign method assumed that the ground floor slab is carried by the soil and is not
connected to the capping beam. If a suspended floor slab is provided the capping beam must
be designed by conventional methods.A disadvantage of the BREdesign is that it can inhibit
house owners from removing large areas of load-bearing walls in the course of making
improvements such as the addition of rear conservatories. There is a danger that the house
owner may not be aware of the basis of the design of the ground beams when proceeding
with these and similar structural alterations.
It is a good practice to provide a suspended ground floor slab in cases where piles are
provided to restrain a structure from lifting due to a swellingsoil. A ground floor slab cast
directly onto a swelling soil will lift and will, in turn, cause the lifting of internal partitions,
with the consequent distortion of any floors carried by them, and the cracking of plaster
finishes. Uplift pressures due to soil swelling against the underside of a pile capping
beam must be considered. In clay soils where mature trees or hedges have been removed
the clay may swell up to 100 mm over a long period of years. Swelling of pyritic mudstones
and shales can occur due to the growth of gypsum crystals within the laminations of
these rocks. Gypsum growth can be caused by chemical and microbiological changes
consequent on changed environmental conditions(7.10). Swelling pressures, if the upward
movement of the soils is resisted by a reinforced concrete capping beam, can be of a
magnitude which will cause the piles to fail as tension members, or which will lift the piles
out of the soil.
It is essential to insert a layer of compressible material such as Clayboard or special low
density polystyrene or to provide a void between the soil and underside of the capping beam
to reduce the uplift forces transferred to the piles (Figure 7.15). Load/deflection tests should
be made on specimens of the compressible material to ensure that the amount of compres-
sion required by the predicted degree of soil swelling does not generate a pressure on the
ground beam that is sufficient to cause structural failure of the beams or piles, or lifting of
the building. There have been a number of cases of failure and cracking of piles, ground
beams, and superstructures to low-rise buildings constructed on swelling clays in recent
years. These have been caused mainly by deficiencies in design such as inadequate tension
reinforcement and lack of proper provision for uplift on ground beams. The latter should be
of generous depth to provide for differential uplift forces caused by local tension failure in
piles in unpredictable conditions of soil swelling.
Horizontal swelling forces can also impose loads on pile capping beams due to the
restraint provided by the beam to the expansion of the mass of the soil. To avoid excessive
394 Structural design of piles and pile groups