Foundation: Prestressed Concrete Pile Design

[tomtumx]

@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.

[lambert]

Anyway my problem is about Slenderness of a pile. My superior had instructed me to investigate the buckling load capacity of a pile having an unsupported length of 19.21 meters. Loads and other design data are follows;

Factored Axial Load= 388.42*1.2 + 324*1.6 = 984kN
Compressive strength of pile = 41.4 mPa
Factored Dead Load/Over Total Factored Load can Conservatively taken as 0.6 (this is the term Bdns using ACI code)
Hence, the Stiffness is Equal to 0.25*EcIg
Euler's Formula is Equal to pi^2 EI/(klu)^2
k is determined by the design Engineer equal to 0.75.

The pile is 450mmx450mm in dimension with 10-12.7mm standard prestressing strand in circular pattern with steel grade of 1862 mPa having a clear cover of 100mm for marine structure and 10mm spiral diameter.

We know that the capacity is very high considering this as short column, but taking into account the slenderness of the pile, it seems that the capacity will abruptly change depending on the Effective Length (klu) of the pile. Now, do I need to reduce the computed buckling strength by using a factor of safety of two or less for the buckling strength when I follow the Strength Design set by ACI code for Slender Columns???

Any comments and suggestions is highly appreciated.


Regards,
Lambert

[tomtumx]

@lambert,

1) It's clear that the things you're asking have nothing to do with the posted worksheet. I think you should post your question in the "Problem" room.

2) Section 5.9.3 of PCI Design Handbook 7th ed. might give you some idea. Also section 13.8 of Prestressed Concrete by Prof.Naaman (2004) should help clarify the differences between RC and PC long columns' stiffness. Note that the book is based on ACI 2002 provisions, which were revised a lot on 2008, eg. the slenderness limit of 100 for 2nd order analysis being removed.

[lambert]

Friend,

Can you share the link to download Prestressed Concrete by Prof. Naaman (2004) as you mentioned above???


Regards,
Lambert

[BennyP]

You realy exagerate. Kowheng posted a spreadsheet you can download and check by your self.
You use this thread (spreadsheet links for use of all users) for your personal questions. I regrete for not stopping you before but i thougt you ask about the spreadsheet only.
After receiving the answer you continue to use this thread for learning every thing you can learn in your university or to ask in the right section in forum.

tomtumx told you today you ask questions not related to the posted spreadsheet and now you use the thread for books requests.

You received until now 4 warnings (1 for wrong section) and nothing help.
Now you have a new warning and your posting privileges will be suspended for 2 weeks.

Use this time for learning the rules too.