Structural Concrete - Hassoun

13.10 Footings on Piles 483
13.9 SLABS ON GROUND
A mat foundation would be used when buildings are founded on soft and irregular soil, where pile
foundation cannot be used.
A concrete slab laid directly on ground may be subjected to
1. Uniform load over its surface, producing small internal forces.
2. Nonuniform or concentrated loads, producing some moments and shearing forces. Tensile
stresses develop, and cracks will occur in some parts of the slab.
Tensile stresses are generally induced by a combination of
1. Contraction due to temperature and shrinkage, restricted by the friction between the slab and
the subgrade, causing tensile stresses.
2. Warping of the slab.
3. Loading conditions.
4. Settlement.
Contraction joints may be formed to reduce the tensile stresses in the slab. Expansion joints
may be provided in thin slabs up to a thickness of 10 in.
Basement floors in residential structures may be made of 4 to 6-in. concrete slabs reinforced
in both directions with a wire fabric reinforcement. In warehouses, slabs may be 6 to 12 in. thick,
depending on the loading on the slab. Reinforcement in both directions must be provided, usually in
the form of wire fabric reinforcement. Basement floors are designed to resist upward earth pressure
and any water pressure. If the slab rests on very stable or incompressible soils, then differential
settlement is negligible. In this case the slab thickness will be a minimum if no water table exists.
Columns in the basement will have independent footings. If there is any appreciable differential
settlement, the floor slab must be designed as a stiff raft foundation.
13.10 FOOTINGS ON PILES
When the ground consists of soft material for a great depth, and its bearing capacity is very low, it is
not advisable to place the footings directly on the soil. It may be better to transmit the loads through
piles to a deep stratum that is strong enough to bear the loads or to develop sufficient friction around
the surface of the piles.
Many different kinds of piles are used for foundations. The choice depends on ground conditions,
presence of groundwater, function of the pile, and cost. Piles may be made of concrete, steel,
or timber.
In general, a pile cap (or footing) is necessary to distribute the load from a column to the
heads of a number of piles. The cap should be of sufficient size to accommodate deviation in the
position of the pile heads. The caps are designed as beams spanning between the pile heads and
carrying concentrated loads from columns. When the column is supported by two piles, the cap
may be designed as a reinforced concrete truss of a triangular shape.
The ACI Code, Section 13.4.2.2, indicates that computations for moments and shears for
footings on piles may be based on the assumption that the reaction from any pile is concentrated
at the pile center. The base area of the footing or number of piles shall be determined from the
unfactored forces and moments.