Canopy Load Combinations

[FootNMouth]

We recently received a deferred submittal for an aluminum canopy that is cantilevering from our our structure (HSS girt), see image below.

In the Design Criteria, the signed and sealed submittal had the following notes:

[ul]
[li]"Roof maintenance not permitted during design event(s) as roof live load not combined with wind loading for gravity direction load combination analysis"[/li]
[li]"Drift snow load and wind load not applied simultaneously".[/li]
[/ul]

For the first note, I see the logic of what they are doing as why would you be on top of a canopy during a major wind event, but is this allowed by code? It does have a large impact on the design as the load combination with dead + roof live + positive wind typically controls by a wide margin (D + 0.45W + 0.75*max (Lr, S, R)). Per my calculations on this project (D + 0.45W + 0.75*max (Lr, S, R)) = 37.5 psf versus the next closest combination is 26.6 psf (D + 0.6W+)

For the 2nd note, the project is in Austin, Texas so snow load will be negligible, but I doubt this is allowed by code.

 

[GC_Hopi]

I think they should use the design combinations given in the code that was adopted by the AHJ and that you have specified. The code has done quite a bit of research on the reasonableness/statistics of the load combinations it seems cavalier to just state we don't want to do it. Reminds me of some contractors.

Also how do they intend to keep maintenance workers off during the canopy during a windy day... put a sign.

 

[lexpatrie]

Regarding snow - I think drifts are supplemental cases for snow load and only the basic snow load applies to combinations. You might have to dig into O'Rourke to confirm though. I forget the source. Texas there's a snow load in parts but it is pretty low. Some parts have 0 ground snow load.

https://sp360.asce.org/PersonifyEbusiness/Merchandise/Product-Details/productId/233136724

Regards,
Brian

 

[Aesur]

Seems pretty simple to me, rejected. As the EOR you will ultimately be liable for this should something happen. Out of curiosity, who designs the connection to your structure being that it says connection by GC. I don't believe most GC's have the ability to design said connections. Your value of 37.5 psf is in line with what I recall from previous projects with similar cases.

 

[Lomarandil]

If adjacent to a wall, won't the overhang bottom face pressure "bouncing off the wall" significantly offset any positive pressure on the top face?

No dice if it's an open structure, but it (and the negligible snow load) might make this all a moot point.

Generally though, I'm with GC_Hopi.

 

[MotorCity]

Seems to me like they are creating their own load combinations. Load combinations are usually pretty clear on the combinations of events that structures should be designed for.

I think both stipulations are ridiculous...

1.Are you expecting a maintenance worker to know (and care) that he/she is not conforming to a prescribed load combination by being on the roof when its windy?

2. Are you saying the owner is expected to clear or eliminate the snow drift on the roof before the wind blows again??
Sorry, but the wind is going to blow regardless of whether or not there is snow drift on the roof (Lets not forget that THE WIND CREATED THE DRIFT!!!)