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 ASCE 113 Second Edition

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Trevor Hopf's profile image
Trevor Hopf posted 04-01-2025 05:42 PM

Hello all, while designing structures per ASCE 113 Second Edition we ran into a double equation for a seismic force. There are two different equations to calculate a seismic force. One is stated to be used for structures (Equation 3-11), while the other is stated to be used for "rigid bus conductors, insulators, appurtenances, and equipment that is not subject to seismic qualifications in accordance with IEEE 693 (2018). The seismic demand as determined in this section should be used to design the component and the anchorage to the support structure" (Equation 3-36).

The question is when designing the structure that has an insulator and rigid bus on it should we use equation 3-11 that is also used on the structure or would we need to separate the two and apply equation 3-11 to the structure and equation 3-36 to the insulator for a combine effect?


George Watson's profile image
George Watson posted 04-02-2025 02:35 PM

I'll let others on the 113 committee weigh in, but the Example A.1 goes over a bus support structure that is a frame in one direction and a cantilever in the other. We don't have seismic loads in my part of Texas so I will defer to the people on shaky ground. :)  

Equation 3-11 and 3-12 are for the ELF in section 3.1.7.6.2. My general interpretation of the ELF is that it is a quick and possibly conservative seismic load that you apply to the structure and run a geometric non-linear static analysis and combine that seismic ELF load with other concurrent loads that you may have. An extreme wind with a seismic event may not occur often (if ever) plus a SCF event but you would combine those load cases and design the members and select the insulator and rigid bus size along with the anchor bolts and foundation design. 

With today's FEM computer programs, you can model the bus, insulators, beams, and columns and run a dynamic analysis for the seismic and maybe another for the SCF function and design the structure to 9 digit accuracy. You then draw the structure and foundation plans and give it to a construction crew that may follow the drawings.

HTH