Shade Canopy on Existing Parking Garage

[RacingAZ]

Hi, question about a steel shade canopy (with solar panels) being put on top of an existing concrete parking garage (4-story SMF). The canopy is an ordinary cantilever column system but the plan reviewer is requiring that I design the canopy under section 13 of the ASCE for seismic purposes. He is classifying the addition as a non-building structure thus requiring that I generate the seismic forces per ASCE section 13.3.1. I'm questioning why the steel canopy is being classified as a non-building structure when it's being given a use occupancy of S-2.

My opinion is that the canopy should be designed under Chapter 12 for seismic and also the existing parking garage checked for additional seismic loads under chapter 12. This is located in CA.

 

[Sawbux]

There is most likely a great disparity in stiffness between the parking garage and the sun shade canopy, so it could not be considered an extension of the existing. Table 13.5-1 gives values for Penthouses, which would be the likely candidate for the structure.

Equation 13.3-1 gives a z/h multiplying factor to account for whipping effect at higher levels of structure - it will greatly increase the seismic forces.

 

[racookpe1978]

Too bad you can't design the new canopy to be "loosely/slide fit" attachment to the existing concrete so the two are not locked together when the earthquake attacks. If the two were separately mobile (within reason) then the whipping would occur at different frequencies and at slightly different times. Likely be less loading both ways: garage to bottom of canopy, and mass of canopy (at strut attachment) to concrete deck.

 

[RacingAZ]

Thanks for the response.

I'm kind of visualizing the canopies too as like a penthouse but not quite though. I'm being told since it does not match any of the non-building types under chapter 13 or 15 tables for classification, it should be under "all other self-supporting structures" type classification. Due to the over-strength factor for this classification, the shear force would be almost 4 times for designing the base plate connections. Going into an existing concrete parking garage columns with limited concrete area, the bolt capacities would be the limiting factor. Whether they realize it or not, this would have the inadvertent effect of almost not allowing certain canopy structures to be installed on existing parking garages.

Would a more rational approach would be to design the canopy members under chapter 12 then when it's time to check the existing parking garage for the additional seismic load, this is where you'll consider chapter 13 to amplify the vertical distribution of seismic forces?

 

[Deker]

If you're determined to use chapter 12 you might see if you can satisfy the requirements of the two-stage analysis procedure in section 12.2.3.2. I think the more conventional approach is to analyze it as a nonstructural component per chapter 13, but I would consider it a cantilever element braced below its center of mass rather than a penthouse. If your anchorage forces get out of hand, you can try capping your overstrength at the maximum force that can be delivered to the system (perhaps the capacity of the base plate?). To check the existing lateral system, analyze the building per chapter 12 with the additional mass on the roof.

 

[KootK]

Regardless of the ideal code clause to follow, I feel as though I've got a pretty good handle on what the answer should be:

1) Based on the mass differential, I assume that the concrete garage shakes as though the canopy were not there.

2) I feel that the canopy will vibrate as a cantilevered column excited by a base acceleration equivalent to the acceleration of the garage roof level. This is consistent with Dekers two-stage recommendation.

3) We should be able to estimate the acceleration of the garage roof as the diaphragm seismic load there divided by the mass.

4) I'd expect this to yield some pretty high forces still so this may not be any better than the non-structural component route. Rather, it may justify the non-structural component route.

In terms of building code classification, I feel that it's essentially roof top mechanical. The fact that it supplies some shade doesn't change all that much in my estimation.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.