Seismic Loads on Substation Take-off Structure

[RadMilk]

I am a civil/structural engineer in the power delivery space. I've been designing substation structures and foundations on the east coast (wind controlled places) for most of career. I have an upcoming project that will be my first in California. I'm familiar with seismic design (and usually do this for non-line supporting substation structures, even though it doesn't typically control) and the California-specific requirements in general but my question is regarding the tapered tube structure by which the T-Line leaves the substation, I usually call this the "take-off structure".

We often mount other, heavier equipment (insulators, metering equipment, disconnect switches, etc.) to these structures in addition to the T-Lines. T-Line loads are governed by NESC so I usually consider NESC 250B, C, and D for my T-line loads and will couple that with extreme wind or ice + wind loads from ASCE 7 or 113 (excluding seismic) on any other equipment mounted to the structure. This has never been an issue on past projects because wind has controlled in basically all cases.

California has General Order 95 which gives an additional T-Line load case to consider but it's very similar to NESC requirements (does not include seismic). Talking to my supervisor and other engineers at my firm the consensus is that seismic loads aren't typically considered on T-Line structures, regardless of what other heavy equipment might be on them or whether it's in a high seismic zone. Can any west coast folks in the power delivery/transmission space weigh in on this? Just looking for a second opinion because it seems odd to neglect seismic on this one structure in a high seismic zone.

Thanks in advance!

 

[Lomarandil]

It's been a while since Ive designed one (we called them 'dead ends'). I seem to recall including load cases with seismic load (pseudostatic). But I don't recall them ever controlling for the deadend.

It's hard to imagine seismic activity really generating line forces, which far and away dominate the behavior of these. My guess is that is why.

 

[dauwerda]

I have experience designing these (for a fabricator) all over the US. I would say that 95% of the designs ignore seismic. When seismic is required to be included (either by the utility or the consulting firm doing the work) it is generally applied with tensions from an "everyday" load case (which are much lower than the other typical weather conditions considered), so I have never seen it control a design anyway.

The only time I have ever seen seismic loads impact a design is when it was also required that the horizontal deflection of the vertical elements be limited to H/100 under seismic load (which ASCE 113 specifically states not to consider deflection for seismic).

Based on my experience, I think you would be doing everyone a favor by not including a seismic load case, it just adds more work for essentially meaningless results. If you want to show that you at least considered it, just put a note on the drawing stating such.