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11.3.11 Pre-Stressed Concrete Pre-stressed concrete piles have been widely used in deep foundation construction when the subsurface and environmental conditions are suitable for pile driving. A pre-stressed concrete pile has a configuration similar to a conventional reinforced-concrete pile except that the longitudinal reinforcing steel is replaced by pre-stressing steel. The pre-stressing steel is usually in the form of strands of high-strength wire that is placed inside a cage of spiral steel to provide lateral reinforcement.
As the term implies, pre-stressing creates an initial compressive stress in the pile so the pile is more capable of providing bending resistance. Pre-stressed piles can usually be made lighter and longer than reinforced-concrete piles of the same size. The application of a bending moment typically results in a reduction of compressive stresses rather than cracking as with conventional reinforced concrete members. Thus, there is a significant improvement in bending stiffness of the pre-stressed pile as compared to a conventionally reinforced pile. A further benefit is that the pre-load protects the pile from cracking due to tensile stresses developed during pile driving operations. The use of pre-stressing leads to a reduction in the ability of to resist
normal compressive loads, a factor that is usually not critical in laterally loaded piles. Considerable bending moment maybe applied to a pre-stressed pile before first cracking. Consequently, the pile-head deflection of the pre-stressed pile in the uncracked
state is substantially reduced, and its performance under service loads is improved. When analyzing a foundation consisting
of pre-stressed concrete piles, the designer must define a value for the level
of stress due to pre-stressing, after considering losses due to creep and other factors. The value will vary with manufacturers from region to region and will also vary with the geometry of the pile.
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