Nonstructural Component Design and Building Forces

[structeng2]

Hello,

I'm working on some equipment support/anchorage designs that are supported by steel platforms. We calculated the Fp forces based on ASCE7 Ch13 to determine the anchorage forces between the equipment and platform and then used Chapter 15 to analyze the platform structure itself (OCBF steel frame). For the seismic weight of the platform, we included the weight of the equipment.

There was a discussion in our group about how far the Fp force needs to be carried out into the structure. Chapter 13 forces are "...for their supports and attachments", but the question is how far do you follow the load path for the 'support'. (Let's assume that the equipment weight is less than 25% of the platform weight).

To highlight this, here are two extreme cases we discussed:

Case 1 - Assume we have a large building with a small piece of equipment anchored to one of the floors. It's very likely that after you get the Fp load through the anchors into the floor, the building forces control the rest of the building component/member designs after that.

Case 2 - Assume we have a smaller support frame with a decently sized piece of equipment. The Fp force may be the governing load for the equipment bolts, support beams, frame beams... maybe even the diagonal braces.

It feels like what we've seen in the industry is to just design the anchors or attachment points for Ch13, then the rest by Ch12 or Ch15. Curious what the input is from everyone here.

 

[Greenalleycat]

I don't work under ASCE but sounds as if our codes are broadly similar
We term this type of design to be 'parts and components'

Case 1 - agreed. The manufacturer designs the proprietary element for the seismic load
You design the fixings (say bolt size and epoxy fixing capacity) for the parts loads that results
In your building seismic design consider the equipment as a fixed dead load and lump the mass into your floor mass
Ignore the local effects as you assume they are not concurrent with other peak floor actions (noting that gravity loads from heavy equipment could matter!)

Case 2 - The definition of where your 'part/component' starts and stops is important here
I'm imagining that this is a suspended floor with a frame + equipment on top
The floor will have it's own overall seismic acceleration and then the differing period of the frame + equipment kicks in to locally amplify these effects
I would determine the 'parts and components' acceleration and track them through the whole frame
It's a complicated one as arguably you could get additional acceleration of the equipment onto the frame - where does that load go..?
At least in the case of the equipment mass being a smaller portion of the frame, I would just ignore it for simplicity

 

[driftLimiter]

Consider how the MLFRS forces are scaled for seismic loads. The R factor of the seismic system is used to calculate Cs which scales the Seismic Mass.

Fp forces will be considerably higher than Cs Wp. In general I would carry Fp through the anchorage, and perhaps the local elements supporting the equipment. But for the MLFRS if the Wp of the component is included in the seismic mass then it is accounted for in the base shear and the design of the MLFRS.

Thus I would agree with your final statement.