ASCE 113 - 100/40/40 rule

[ChiEngr]

Hello,

I am designing a substation support structure, and I was wondering if anybody had any guidance or experience with orthogonal combination of seismic loads when using the Equivalent Lateral Force procedure. Per ASCE 113, Section 3.1.7.8, it states a series of design load combinations using the 100/40/40 rule. I am using the ELF and have never used the 100/40/40 rule unless using a dynamic analysis procedure. Is this a potential mistake in ASCE 113, or does the orthogonal combination of seismic load vectors apply even for the ELF?

 

[transmissiontowers]

I was the Chair of the committee that wrote the Guide and just came across this post. (I don't get on here very much). The intent of the ELF was a simple load check since most of our substation structures are rarely controlled by seismic. We had over 30 engineers that did the revision from the 1st edition and I know about zero seismic. We don't have to deal with seismic in my part of Texas.

I have contacted the seismic gurus on the committee to get a more definitive answer. In my simple understanding, the section 3.1.7.8 was really meant to combine the orthogonal components of a full-blown dynamic analysis. I read it over and it does imply you take the ELF from 3.1.7.6.2 into Eqn 3-23 but it may be a case where we meant the dynamic analysis procedure from 3.1.7.6.3 was to be used. Taking the ELF and running it thru the SRSS or CQC equations may be a big overkill for a load case on a bus support structure.

We read over the text about 7000 times and thought we caught all the errors. The peer review committee did not say anything about this section. If I find a better answer, I'll post another reply.

 

[ChiEngr]

Thank you for your response. If you do end up getting more information, I would be happy to hear and learn.