Canopy next to a building

[Lion06]

I have a canopy that is directly adjacent to a building. The canopy is free standing and there is less than 6" clear between the two. The building has two elevated stories and roof (approximately 36' building height). The canopy is about 9'-6" high.

What would you use for wind loads here? If I use ASCE 7-05 6.5.13 for open buildings, the uplift is rather small (about 15psf). If I look at it as a stepped roof as in Fig 6-12, the uplift is around 27psf.

It won't matter for the actual canopy design, but will make a pretty big difference for the footings.

What have others done for this situation?

 

[ToadJones]

Is the canopy the drive thru type with a sinlge line of cantilevered columns?
Just trying to picture it.

I ususally go with the worst case and weigh my options.
not to undermine your concerns in the least, but an old professor I had used to say "why do engineers love concrete? Because this $!&! is cheap!!!"

 

[Lion06]

It has only two columns. It covers a building entrance. One column on each side of the entrance. The columns are set roughly 4 ft from the building and the "roof" members cant both toward the building and away from it off of the column.

I agree about the concrete being cheap, but I also would hate to totally overdesigning it needlessly.

 

[ToadJones]

Also, a few years ago we got a call to look at the type of canopy I described with a cantilevered roof and columns.
The steel had just been erected and there were already excessive deflections on the roof. The deflections wound up being a culmination of some footing settlement, steel deflections and poor fabrication. It was a disaster.

 

[ToadJones]

I guess I am just trying to offer some advice as to what I have seen. I have only been involved in one project like this and I'm sure the architect and the original structural engineer would have loved a chance at a re-do.

The canopy I am describing was curved in plan about 100 feet long, stood about 20' tall and had a roof structure that was cantilevered about 20' as well.

 

[Ron]

SEIT...I've done hundreds of these. I treat them as independent, open structures unless they are physically attached. Even a 6-inch gap is a large opening to dissipate wind influence. While there probably is some difference in influence, it would take a wind tunnel model to determine. I just finished one in a 150 mph wind area.

I have also treated some of them as overhangs, depending on the configuration. In most of the applications I deal with, the difference in roof height is a bit much to consider it a stepped roof.

When designing for snow drift, unless there is a separation of several feet, I do consider snow drift against the adjacent building.

 

[ron9876]

I would also calculate the worst case with "h" based on the main building height. In my opinion the building is there and it shouldn't be ignored. If the results of the design aren't crazy I would go with it. The problem comes if the results are too heavy.

The minimum condition that I would consider would be to use the positive wind pressure on the building wall applied as an uplift on the underside of the canopy. I would also add some amount of uplift to the top.

 

[Lion06]

Thanks, guys. I'm going to go with the higher (25 psf) wind. It only makes the footings go from 3'-6" square to 4'-0" square.

 

[ToadJones]

Good call, that extra 6" sure makes your pillow a little softer at night!!