ASCE 10-97 and AISC (ASD or LRFD)

[cocodrilo]

Hello,
I am trying to calculate an electrical substation and the client ask for using ASCE 10-97.
I'm not very used to American standards and I have some doubts.
My software is ROBOT Structural Analysis, that hassn't ASCE 10-97 code implemented but just AISC, either ASD or LRFD. This software also allows you to use TIA/EIA code for transmission towers that I have used in previous projects.
The problem is that when calculating with AISC (ASD) the comparison stress seem to be much lower than with other codes. It is decreased by a factor of 1.67 that I am not sure if it is correct.
I made calculations on paper for a single bar according to ASCE 10-97 and the compression comparison value, Fa, appears to be much higher that that Pn/Omc of AISC.
I also made some quick comprobations using EIA and Eurocode codes in ROBOT and comparison values result both quite similar and 40% (app. 1/1.67) higher than with AISC.
I am probably totally wrong because I am not familiar with American codes, but, could anybody tourn on some light on my doubts?
Thank you very much in advanced
Jorge

 

[transmissiontowers]

Hola Jorge;
ASCE 10-97 is done for lattice transmission towers and you should probably use LRFD because the loads on the tower should include an appropriate overload factor specified by the tower owner. The world leader in T-Line software is PLS-CADD and if you plan on doing many of these projects you should get a copy.

The capacity equations in ASCE 10 (based on lots of full scale tests) should be used to determine if the member is overloaded. I'm not familiar with ROBOT but I would use it to do the analysis and find the loads in the members, then create a spreadsheet to calculate the member capacities and compare the member loads to the member allowables. This will be a slow and tedious process but AISC LRFD code is not appropriate for T-Line lattice towers.

I'm not really familiar with the EIA/TIA but I think the latest rev G may use the LRFD method.

If you are doing substation structures you should get a copy of ASCE 113 which should answer a lot of your questions. If your structures are lattice, follow ASCE 10. If your structures are moment frames like dead end structures or buss support structures, you can use AISC LRFD code. The ASD method in AISC uses unfactored loads and reduces the allowable stress in the members. The LRFD method uses factored loads and a small reduction factor on the allowable stress based on statistics.

If you have wire loaded structures where they support electric conductors, you will need to calculate the sag and tension in the wires to apply to your structure.

Good luck and ask more if all this is confusing. I know most of the people that wrote ASCE 10 (a new version is coming out soon) and I am in the process of revising ASCE 113.

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