Commercial design criteria

[Sifu]

I had a conversation with a structural engineer a few days ago that illustrates a question I have had for a while. In this case, I had provided a comment that the ground snow load on his plan was not correct. He called and wanted to know what it should be. I said well, that would be as per ASCE 7 and the specific case study. He said, but it's listed on the AHJ website, and I said, no, it shouldn't be for commercial construction, that the only design criteria should be for residential. So he pulled up the website while we were on the phone and said "yeah, I see that now" and then asked about the CS I had.

Engineers using the residential design criteria for commercial construction is very common, but at least in my area, they are different. I am in a CS area, and the case study I have has the loads higher across the board. I also use the ATC website, and the ASCE 7 hazard tool website to verify loads when they don't look right. So for all the engineers out there, how do you arrive at a given design criteria for a project? Are you seeing AHJ adopted criteria for commercial construction? (not sure I ever have) Are you using the residential design criteria? Are you using the same resources I have access to?

 

[Sifu]

Just too funny. Five minutes after posting I got the following email. It just seems to me that I shouldn't be answering these questions. They should be answered by the IBC or ASCE 7...right?

 

[mtlogcabin]

Why would it not be the same? The AHJ fills in the requirements for Table R301.2(1) why not use the most restrictive for all projects?

 

[Sifu]

Why would it not be the same? The AHJ fills in the requirements for Table R301.2(1) why not use the most restrictive for all projects?

For one thing, risk category plays a role for commercial projects. But I wish they did have a table similar to the IRC for commercial, though I don't recall ever seeing one, but I have seen one AHJ that includes the commercial design criteria in their adopting ordinance (only one). I would like to think some critical thinking has gone into the IRC table adopted by the AHJ's, but I'm not confident in that in all cases. Where I am, the snow load is governed by case study, and wind is in or near a special wind region (which is a whole 'nother conversation). Both would require a little extra work when filling out the table. But when the IRC table, as adopted by an AHJ is different than the design loads assigned by ASCE 7, a snow case study or a wind study what do you do? As mentioned, I have only seen one AHJ that "adopted" design criteria for IBC regulated structures. The rest just let IBC ch. 16 govern, but when case studies and special wind regions come into the conversation it becomes murky.

I would think that if IRC t301.2(1) is meant to be used for commercial structures, it or something similar would be in the IBC.

Case in point: I received a commercial plan with a 30psf ground snow load. The case study assigns it as 35psf, the ASCE assigns it as 36psf. The IRC table uses 30psf. So which dictates for a commercial building? Or do I ignore the documents and assume the engineer did due diligence?

I have always assumed that the structural engineers were referencing the correct case studies, and wind analysis, or at least ASCE 7 in their design. But the continued questions such as the one I posted leave me wondering.