Seismic Design of Electrical Panels (ASCE 7-22)

[Blackfairy]

Hi, I have an electrical panel that is attached to the ground and is just elevated by some HSS so the weight of the panel is greater than the HSS. According to ASCE 7-22 13.2.9, if the effective seismic weight of the nonstructural component (panel) is more than 20% of the combined nonstructural and the supporting structure (HSS), then it should be design as per Chapter 12 (building Structures) and if not then Chapter 13 applies (nonstructural).

I just want to verify what forces to use to design the anchorage of this to the ground. Is it Chapter 12 which is the typical Cs x W formula or Chapter 13 using the Fp formula?

 

[driftLimiter]

Chapter 15. I am using Ch 13 for elements actually attached to a building structure.

If the HSS is just on grade level I would look towards rigid non-building structures, if it is similar toa frame I would look to Table 15.4-1

 

[Blackfairy]

Chapter 15. I am using Ch 13 for elements actually attached to a building structure.

If the HSS is just on grade level I would look towards rigid non-building structures, if it is similar toa frame I would look to Table 15.4-1

It is kind of a frame system. It is like 3 panels supported by like 4 HSS columns (panels in between) and of course some horizontal steel c-channels.

Will this be considered Steel ordinary moment frames under Table 15.4-1? Or should I go table 15.4-2 and go categorize it as "All other self-supporting structures..."?

 

[driftLimiter]

I cant really say without see the geometry. The limitations you will run into are going to be related to detailing. The 'Structural System and Structural Height, hn, limits' will guide you towards what systems are allowed and what level of detailing is required. What your describing sounds like it would be best detailed according to AISC 360. I would strive for that and rearrange the structural elements as necessary to fall into that category.

You will see the penalty of doing this is taking lower and lower R factors.

As long as you can rationalize the moment connection of the channel to the HSS column and meet the required loads with R=1 then the Steel ordinary moment frame with unlimited height seems like a good path forward.

 

[TLHS]

Honestly, I have a spreadsheet that does both at once to avoid arguing with checkers.

That being said, Chapter 13 is for non-structural components and their supports. If your frame is reasonably small in scope and is just there to support your panels, then it's the support. The supporting structure is the ground. They've updated ASCE-7 to further clarify this. Table 13.6-1 has coefficients in ASCE-7-22 for support frames and there are clauses about them. I haven't used these new values, but a quick glance at the coefficients has me expecting that it's going to spit out values higher than treating it like a non-building structure.

The percentage thing is intended to make sure you aren't doing something that screws up the implied model. The non-structural component formulas are assuming that you've got a small weight component inside a heavy building. The building is the source of the applied acceleration and the component doesn't significantly effect the motion of the building. Once you get to a certain size of equipment, that's no longer the case. The big issue being if there's dynamic interaction and multiple modes happening because of it.

This is not the situation with a piece of equipment and a small scale support, where they'll generally be in a single mode together. Basically, you've gone out the other side of it, but also the consequences aren't a huge deal when you're just looking at the support. If you use the 25% thing for purpose made supports, then you start getting weird situations like every pipe support is a non-building structure.

It's still a judgement call on when to switch to non-building structures, because obviously you're going to treat a large tank four stories up on braced legs as a non-building structure of some sort not a non-structural component on a support but honestly you're doing some sort of first principles evaluation whenever you're looking at that kind of thing anyway.