Tuesday, November 20, 2012

Concrete Piles - Structural Design.

(1) Prestressed concrete piles. Allowable concrete stresses should follow the basic criteria of ACI 318-89, except the strength performance factor Φpf, will be 0.7 for all failure modes and the load factors for both dead and live loads FDL = FLL will be 1.9. The specified load and performanee factors provide an FS = 2.7 for all combinations of dead and live loads.

(a) The computed axial strength of the pile shall be limitod to 80 pereent of pure axial strength or the pile shall be designed for a minimum eccentricity of 10 percent of the pile width.

(b) Driving stresses should be limited as follows:
Compressive stresses: 0.85f’c effective prestress
Tensile stresses: effective prestress

(c) Cracking control in prestressed piles is achieved by limiting concrete compressive and tensile stresses under service conditions to values indicated in Table 2-4.

(d) Permissible stresses in Ihe prestressing steel tendons should be in accordance with ACI 318-89.

(e) Minimum effective prestress of 700 psi compression is required for piles greater than 50 feet in length. Excessively long or short piles may necessitate deviation from the minimum effective stress requirement.

(f) Strength interaction diagrams for prestressed concrete piles may be developed using computer program CPGC (WES Instruction Report ITL-90-2).

(2) Reinforced concrete piles. These piles will be designed for strength in accordance with ACI 318-89, except that the axial compression strength of the pile shall be limited to 80 percent of the ultimate axial strength or the pile shall be designed for a minimum eccentricity equal to 10 percent of the pile width. Strength interaction diagrams for reinforced concrete piles may be developed using the Corps computer program CASTR (U.S. Army Engineer Waterways Experiment Station Instruction Report ITL-87-2).

(3) Cast-in-place and Mandrel-driven piles. For a cast-inplace pile, the casing is top driven without the aid of a mandrel, typically using casing with wall thickness ranging from 9 gauge to 1/4 inch.

(a) Casing must be of sufficieni thickness to withsand stresses due to the driving operation and to maintain the pile cross section. Casing thickness for mandrel-driven piles is nonnally 14 gauge.

(b) Cast-in-place and mandrel-driven piles should be designed for service conditions and stresses limited to those values listed in Table 2-5.

(c) Allowable compressive stresses are reduced from those recommended by ACI 543R-74 as explained for prestressed concrete piles.

(d) Cast-in-place and mandrel-driven piles will be used only when full embedment and full lateral support are assurecl and for loads that produce zero or small end moments so that compression always controls. Steel casing will be 14 gauge (US. Standard) or thicker be seamless or have spirally welded seams, have a minimurn yield strength of 30 ksi, be 16 inches or less in diameter, not be exposed to a detrimental corrosive environment, and not be designed to carry any working load. Items not specifically addressed will be in accordance with ACI 543R-74.

Table 2-5 Cast-in-Place and Mandrel-driven Piles, Allowable Concrete Stresses.

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