Wind load on canopy structures - Canadian Code 1

[GalileoG]

I understand that ASCE7 addresses wind load on canopy structures (ie. gas station canopy,) but I have not been able to find any useful material on the NBCC 2005 commentaries for such structures. I am curious as to how one would determine wind loads on such structures in accordance to the Canadian code? The behaviour of canopies under wind load is complex, surely there must be something in the code that can guide me.

(anticipating BAretired's guidance)

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[BAretired]

Clansman,

I'm not sure that I can offer any real guidance in this matter. For wind pressure above the canopy, I would likely use Fig. 1-7, User's Guide - NBC 2005. For pressure on the underside of canopy, I would use an internal pressure coefficient, C_pi +/-0.7 although I would not have a great deal of confidence in that, so I might just bump it up to +/-1.0 because I believe that much higher uplift pressures are possible in the case of open structures.

The algebraic sum of the upper and lower pressures would determine the design pressure. For tiedown, I would apply a healthy safety factor, indicative of the uncertainty of the design parameters.

BA

 

[SkiisAndBikes]

I too am soon to be designing my first canopy structure using Canadian Codes. A timely question from Clansman that I was about to post.

Would the design wind pressures on the underside of the canopy structure vary depending on whether they were adjacent to a building or stand alone with no building nearby?

In my case, the freestanding canopy will be adjacent to a building but not attached to the building structure for lateral support. In fact, the canopy will abut the glass curtainwall of the building so lateral deflections must be tightly controlled, combined with a reasonable amount of separation to avoid the canopy hitting the glass. Of course, the architect wants as small a gap as possible! No cross bracing permitted so I am looking at moment connections at the top. Any design tips appreciated.

 

[BAretired]

S&B,

Would the design wind pressures on the underside of the canopy structure vary depending on whether they were adjacent to a building or stand alone with no building nearby?

The answer to that is a resounding YES! If the canopy is built in the middle of the bald prairie, wind can blow right through it. If the ceiling is flat, there may be no pressure at all on the underside.

The factor of +-0.7 applies to Category 3 buildings which include sheds with one or more sides open. This would be similar to a canopy adjacent to or attached to another building.

Keep in mind that you don't know what the owner will do in the future. The canopy could be open at design time but closed in the future. It's best to be conservative and take Category 3.

Watch for stress reversals in canopy design. A beam or joist carrying snow load has tensile stress in the bottom flange/chord requiring no lateral bracing. Under wind load, the bottom flange/chord may require substantial lateral bracing to prevent buckling.

BA

 

[GalileoG]

Thank you BAretired, that was very helpful (as always.)

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.