Wednesday, January 9, 2013

Design Decisions - Sizing up of the Design - Reinforced Concrete Pads and Strips..

Design decisions
The decision to reinforce a concrete foundation of this type usually follows the realization that the ground conditions are variable and/or deep trench fill is uneconomic.

Reference to (Table 10.2 Foundation selection to suit varying site conditions) on choice of foundation types will assist in this decision.

Sizing up of the design
The depth and width of the reinforced concrete strips are determined in a similar way to that adopted for unreinforced strips and trench fill. The depth to the underside of the footing is determined by the ground conditions and the level of suitable sub-strata, taking into account the need to be below the effect of any critical frost heave or swelling and shrinkage of sub-strata.

The fabric reinforced strip is used generally where there is both relatively poor ground and smallish loads or where some slight movements are expected from differential  settlement or subsidence.

More heavily reinforced strips, using bars and not fabric, are used where ground conditions are more critical and/or loading more excessive (see Fig. 11.21).

Fig. 11.21 Section through reinforced strip.

For particularly heavy loads and/or poor ground, beam strips are often used.

For axially loaded strip foundations, the breadth of the strip required is:
where P is the superstructure load/unit run and na is the net allowable bearing pressure.

The thickness of the foundation should be determined by designing for the cantilever action of the strip taking into account the bending, shear and bond stresses to be accommodated and allowing for the longitudinal moments and forces (see Fig. 11.20 Reinforced concrete strip design conditions.).

For strip footings a generous thickness for bending is  necessary in order to maintain the shear and bond stresses within permitted limits and in order to produce an economic balance for the ratio of concrete to reinforcement.

The detailed design of a reinforced concrete strip is covered in Design Example 3 which follows, but in general the  calculated foundation thickness required for shear and bending compression is rounded up to the nearest 50 mm as the economic thickness for the strip foundation.

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