Wednesday, January 9, 2013

Design Example 3: Reinforced Strip Foundation.

The load-bearing wall of a single-storey building is to be supported on a wide reinforced strip foundation.

A site investigation has revealed loose-to-medium granular soils from ground level to some considerable depth. The soil is variable with a safe bearing capacity ranging from 75–125 kN/m2. Also some soft spots were identified, where the bearing capacity could not be relied upon.

The building could be supported on ground beams and piles taken down to a firm base, but in this case the solution chosen is to design a wide reinforced strip foundation  capable of spanning across a soft area of nominal width.

To minimize differential settlements and allow for the soft areas, the allowable bearing pressure will be limited to na = 50 kN/m2 throughout. Soft spots encountered during construction will be removed and replaced with lean mix concrete; additionally, the footing will be designed to span 2.5 m across anticipated depressions. This value has been derived from the guidance for local depressions given later on raft foundations. The ground floor slab is designed to be suspended, although it will be cast using the ground as permanent formwork.


If the foundations and superstructure are being designed  to limit state principles, loads should be kept as separate unfactored characteristic dead and imposed values (as above), both for foundation bearing pressure design and for serviceability checks. The loads should then be factored up for the design of individual members at the ultimate limit state as usual.

For foundations under dead and imposed loads only,  factoring up loads for reinforcement design is best done  by selecting an average partial load factor, γP, to cover both dead and imposed superstructure loads from Fig. 11.22 (this is a copy of Fig. 11.20 Reinforced concrete strip design conditions.).

Combined partial safety factor for dead + imposed loads.
Fig. 11.22 Combined partial safety factor for dead + imposed loads.

From Fig. 11.22, the combined partial safety factor for superstructure loads is γP = 1.46.

Weight of base and backfill, f = average density × depth
                                             = 20 × 0.9
                                             = 18.0 kN/m2

This is all dead load, thus the combined partial load factor for foundation loads, γF = 1.4.

Sizing of foundation width
New ground levels are similar to existing ones, thus the (weight of the) new foundation imposes no additional  surcharge, and may be ignored.

The minimum foundation width is given by

Adopt a 1.2 m wide × 350 mm deep reinforced strip foundation, using grade 35 concrete (see Fig. 11.23).

 Fig. 11.23 Reinforced strip foundation design example – loads and bearing pressures.

Reactive upwards design pressure for lateral reinforcement design
Lateral bending and shearb = 1000 mm.

Thus vu < vc , therefore no shear reinforcement is required.

Loading for spanning over depressions
Where a local depression occurs, the foundation is acting like a suspended slab. The ultimate load causing bending and shear in the foundation is the total load i.e. superstructure load + foundation load, which is given by

Longitudinal bending and shear due to depressions
Ultimate moment due to foundation spanning – assumed simply supported – over a 2.5 m local depression is

Width for reinforcement design is b = B = 1200 mm.
Thus vu < vc = 0.49 N/mm2, therefore no shear reinforcement is needed.

Depression at corner of building
The previous calculations have assumed that the depression is located under a continuous strip footing. The
depression could also occur at the corner of a building where two footings would meet at right angles. A similar calculation should then be carried out, to provide top reinforcement for both footings to cantilever at these corners.

Fig. 11.24 Reinforced strip footing design example – reinforcement.