Steel canopy and wind load to BS6299

[Vinny7]

I am looking at a grandstand project. The seating etc. are all ground bearing with a cantilevered steel canopy roof over. the canopy cantilevers about 9.5m and it has been proposed to support this using 2No. columns at 1.5m ctrs. (back span 1.5m for rafter) (See attched sketch detail)

In checking the wind load for the canopy I am getting a pressure coefficient of +0.7 / -1.5 (duo-pitched canopy). As the roof is lightweight this is giving me a large uplift force.

My question is how can I reduce the uplift force as the dead weight of concrete required to resist it is quite large? (I have already tried using combined bases but they are about 20m3 of concrete each, it may be this is the better solution.

Can anyone see any better of more economical ways of designing this?

 

[tngolfer]

You may look into a deep foundation system that would provide uplift capacities.

On a side note: If your sketch is to scale, your cantilevered beam does not look near deep enough to withstand deflection issues. It may work for strength but it would make people really nervous if that roof starts "galloping" in the wind. A 9.5m/30' cantilever is a long way out there no matter what your back span is. Some sort of space truss may be a better idea.

 

[Vinny7]

I agree, this is just the sketch we recieved from the client, it will have to change.

Any other ideas on resisting the uplift?

 

[patswfc]

Is your uplift coefficient of -1.5 correct? Could it be
-1.4 or even -1.2 (probably not -1.2). This will reduce uplift slightly.

Also are you using the directional method to calc q? If not then you should as this will also help reduce uplift.

Any chance of adding extra dead load to the roof structure to reduce uplift forces? Obviously this will also increase downward forces, so choose an appropriate dead load value.
I'm not sure if your client would like this though.

Other than these basic ideas, which you have probably considered already, i cant think of anything else. As golfer said you could use piles to take the uplift, rather than massive concrete footings, but this would depend on cost.

 

[Vinny7]

Patswfc,

I have calculated the wind load more accurately and the co-eff has reduced. I have also used the reduction factors in the code as the canopy the 5 bays (I assume this was correct?).

Have you designed any canopies using this code, it states the wind load co-efficients apply for all directions, therefore I have not looked at the wind loading at 90Deg to the front. I have however added a 90Deg wind load to the facsia and included it in the loadcases for the front and a windward and leeward load the the solid wall at the rear, would this sound correct?

I also assume the canopy has no internal pressure co-efficients (not theated as a building with dominant openings)?

Any thoughts on the points above?

 

[tngolfer]

You are correct that you would have no internal coefficients. I'm not familiar with the BS codes, but in IBC/ASCE 7 the coefficients listed in the tables and graphs combine the pressure coefficients for the windward and leeward faces and give those combined effects as one value. Be sure to read the footnotes below the table to make you aren't overlooking something.