ASCE 7-16: Ch13 vs. Ch15 Loading Requirements

[structeng2]

Hello All -

I am interested to see how other folks approach nonstructural component support design when the support system is more analogous to a structural system than a component.

Consider a 500lb sheet metal box transformer (ap = 1.0; Rp = 2.5), and the following two cases:

1. A sheet metal box transformer mounted directly to a grade-level concrete slab.
   • This is pretty clearly just a chapter 13 calculation (using Fp force).
2. The same sheet metal box is mounted on top of a 2-foot tall HSS frame (total frame weight = 250lb) with diagonal bracing similar to an OCBF, which is anchored to a grade-level concrete slab.
   • ASCE 7-16 13.6.4 would seem to indicate that both the attachment of the transformer to the HSS frame and the design of the HSS frame itself should be done using the Fp forces per Ch13.
   • ASCE 7-16 13.1.1 says that if the nonstructural component weighs > 25% of the structure, then it should be designed per ch15. (This assumes you can call the hss frame a 'structure').

How would you approach case #2?

Using Fp to design what is essentially an OCBF doesn't seem accurate since you are using seismic response factors for a sheet metal box to determine lateral forces on a braced frame. And designing the entire thing per ch15 seems to not accurately capture the response and force demands of the transformer onto the support frame.

Thanks for your input!

ref 13.1.1


ref13.6.4

 

[HTURKAK]

How would you approach case #2?

Using Fp to design what is essentially an OCBF doesn't seem accurate since you are using seismic response factors for a sheet metal box to determine lateral forces on a braced frame. And designing the entire thing per ch15 seems to not accurately capture the response and force demands of the transformer onto the support frame.

-Still my approach would be similar to case #1 assuming the component operating weight Wp would be total wt ( 750 lb ) and use expression (13.3-1)
- Although the dimensions are not stated , sheet metal box transformer together with supporting structure could be assumed rigid and clause 15.4.2 Rigid Nonbuilding Structures could be followed.
- Nonbuilding structures that have a fundamental period, T, less than 0.06 s, including their anchorages, shall be designed for the lateral force obtained
from the following:
V =0.30*SDS*W*Ie
- Notice that this expression (15.4-5) is similar to (13.3-3) Fp =0.3*SDS*Ip*Wp which is lower bound value ,
- Sharpening the pencil and performing refined period calculation ( 15.3.2 ) eventually would not effect the practical size of anchors .

 

[structeng2]

Thanks for the response. Good thought with the rigid nonbuilding structure, I had not thought about that provision. I suppose my question is more general though where the supporting structure of a nonstructural component is not at all like the component - where the ap/Rp values in Ch13 don't really make sense anymore.

I guess my thought when it comes to this type of situation is that the seismic force originates at the ground level, which excites the HSS frame, then excites the transformer. So, the response of the HSS frame is going to be more like an OCBF and should be designed per ch15 with the weight of the transformer included as part of the seismic mass.
Basically my approach has been to design the transformer attachment to the HSS frame per Ch13, then design the frame per Ch15.

 

[canwesteng]

Are you detailing the frame to be OCBF? Normally, these are detailed as steel not detailed for seismic resistance, you get an R=1.5 and it wouldn't be that different to use the Ap/Rp from chapter 13.

 

[structeng2]

I agree, for this type of structure, we would detail to AISC360 (Table 15.4-1, OCBF w/ unlimited height). But for sake of argument, say we did detail to OCBF, R = 3.25. I think the correct approach is to utilize Ch15 for the frame design, since it is responding like an OCBF system, and not a nonstructural component.