Monday, November 19, 2012

Steel Piles - Structural Design.

Pile shoes should be used when driving in dense sand, gravel, cobble-boulder zones and when driving
piles to refusal on a hard layer such as bedrock. Bending is usually minirnal in the lower part of the pile.

(1) The upper portion of a pile may be subject to bending and buckling as well as axial load. A higher allowable stress may be used when the pile experiences damage because the pile enters the inelastic range. This will cause some strain hardening of the steel and increase the pile load capacity. Since damage in the upper region is usually apparent during driving, a higher allowable stress is permitted.

(2) The upper portion of a pile is designed as a beam-column with consideration of lateral support. Allowable stresses for fully supported piles of A-36 steel of minimurn yield strength fy = 36 kips per square inch (ksi) are given in Table 2-3.

(a) Limits to combined bending and axial compression in the upper pile are given by

Table 2-3 Allowable Stresses for Fully Supported Piles. (English Units)

A noncompact section can develop yield stress in compression before local buckling occurs, but it will not resist inelastic local buckling at sirain levels required for a fully plastie stress distribution. A compact section can develop a fully plastic stress distribution and possess rotation capacity of about 3 before local buckling occurs. Moment rotation capacity is (θup) - 1 where θu is the rotation at the factored load state and θp, is the idealized rotation corresponding to elastic theory when the moment equals the plastic moment. Refer to the ASIC Manual of Steel Construction (1989) for further information on noncompact and comp act sections.

(b) Allowable stresses for laterally unsupported piles should be 5/6 of those for beam columns giveri by the AISC Manual of Steel Constmction.

(c) A computer piogram for the analysis of beam columns under lateral loading, may be used to compute combined stresses, taking into account all the relevant parameters.

(d) Cross scctions of pipe piles may be dctcrmincd from Appcndix B.

(3) Allowable driving stresses are limited to 0.85fy, Figure 2-2.

Figure 2-2.  Limits to pile driving stresses

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