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Resistant moment for piles

Resistant moment for piles

Resistant moment for piles

(OP)
Hi, most of my work is related to geotechnical engineering, so I would like to hear the opinion from the structural side about the differences between crack moment and ultimate moment resistance parameters. I am evaluating precast prestressed concrete piles under lateral loading. I have been checking for lateral capacities to not to exceed the crack moment of the piles. However, just curious to know if there are any implications/considerations if I do the analysis based on the ultimate moment resistance. Thanks for your input.

RE: Resistant moment for piles

Normal practice would be to consider the ultimate strength of the pile with a suitable load factor. Cracking moment is very low; a design based on cracking moment would be ultra conservative.

BA

RE: Resistant moment for piles

Probably some clarity is required here - prestressed concrete is often designed so there is no tension under service loads. The "cracking" moment, when tensile stress in the concrete in bending exceeds the modulus of rupture of the concrete, is indeed quite low. Generally, I would think you want to design your pile such that there is no tension in the concrete under service loads, and that under ultimate loads the ultimate moment capacity of the concrete is not exceeded.

RE: Resistant moment for piles

Good point, canwest. There are two conditions to check, namely service load and ultimate load. Under service load, it might be reasonable to permit a nominal tension in the concrete, but that would be an engineering judgment.

BA

RE: Resistant moment for piles

(OP)
Thanks for your replies. If I understand your comments correctly, I can use the "cracking" moment for service loads and for seismic loading, the "ultimate" moments can be used? This sounds reasonable to me. Thanks again.

RE: Resistant moment for piles

That's how I would design the piles, yes.

RE: Resistant moment for piles

(OP)
canwesteng, could you provide examples for typical service loads for buildings?

RE: Resistant moment for piles

1.0D + 1.0W, but I don't line in the US. I think the ASD load combinations from ASCE 7 might be approriate, but I wouldn't consider any type of seismic loading as a service load case for prestressed piles.

RE: Resistant moment for piles

Also, should mention that with prestressed concrete determining creep is also a concern, so there is another load case of just dead load.

RE: Resistant moment for piles

(OP)
canwesteng, thanks. This info is helpful. I will look at ASCE 7-10 for more details.

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