05-21-2012, 05:11 AM
@lambert,
1) The worksheet posted here is the form I have usually seen as submitted by pile manufacturers, not the one made by a structural designer of the project involved. It therefore only checks for bending moment due to handling (2 pt lifting here). The allowable tensile stress and axial load are taken from PCI Journal (with an additional 0.80 reduction to the allowable tensile stress):
Recommended Practice for Design, Manufacture and Installation of Prestressed Concrete Piling, PCI Journal, March-April 1993, pp.14-41.
You have to check and might have to modify the calculations to suit your needs.
For example, refer to posted pic, to me it is unusual to have the assumed total prestress loss of 20% applied directly to the breaking strength of the prestressing steel (Total fsi of 121034.6) and get the effective prestress of 53.5 ksc, which seems to be higher than the AASHTO's min. prestress of around 49 ksc.
The calculations I have seen usually assumes 20% loss against the initial prestress of 70% of the breaking strength (used for checking handling from the casting yard to stock yard). The effective prestress is therefore 0.56 times the breaking strength or total loss of 64% against the breaking strength. Other persons might have other figures in mind.
2) For the case you've just posted, you as a designer have to consider the surrounding soil with the loaded piles and perhaps the structures the piles are supporting. There might be many load cases to consider and you can't use this kind of worksheet. People around me use FB-Pier to model the soil-pile-pilecap interaction, and use Response 2000 to check the P-M interaction of the prestressed pile.
1) The worksheet posted here is the form I have usually seen as submitted by pile manufacturers, not the one made by a structural designer of the project involved. It therefore only checks for bending moment due to handling (2 pt lifting here). The allowable tensile stress and axial load are taken from PCI Journal (with an additional 0.80 reduction to the allowable tensile stress):
Recommended Practice for Design, Manufacture and Installation of Prestressed Concrete Piling, PCI Journal, March-April 1993, pp.14-41.
You have to check and might have to modify the calculations to suit your needs.
For example, refer to posted pic, to me it is unusual to have the assumed total prestress loss of 20% applied directly to the breaking strength of the prestressing steel (Total fsi of 121034.6) and get the effective prestress of 53.5 ksc, which seems to be higher than the AASHTO's min. prestress of around 49 ksc.
The calculations I have seen usually assumes 20% loss against the initial prestress of 70% of the breaking strength (used for checking handling from the casting yard to stock yard). The effective prestress is therefore 0.56 times the breaking strength or total loss of 64% against the breaking strength. Other persons might have other figures in mind.
2) For the case you've just posted, you as a designer have to consider the surrounding soil with the loaded piles and perhaps the structures the piles are supporting. There might be many load cases to consider and you can't use this kind of worksheet. People around me use FB-Pier to model the soil-pile-pilecap interaction, and use Response 2000 to check the P-M interaction of the prestressed pile.