Wind load on sign attached to building

[flight7]

I'm calculating the wind load on a sign similar to the one shown in this photo. I have used the ASCE 7-10 provisions for a freestanding sign, but the resulting load seems conservative. I would think that the load on an isolated sign is quite a bit higher than the load on the same sign attached to a building. Any suggestions for how to calculate the wind load in this scenario?

 

[phamENG]

If the area is similar to what's in the picture, you could develop high winds between buildings that could make the loads very reasonable - even unconservative. Some big cities have special provisions for this sort of thing.

When you consider the fluid flow around the sign, in some ways this could even be slightly worse than a stand-alone sign. A sign off by itself will allow the wind to flow around it, whereas the wind will be impeded on one side in this case. I could see the imbalance in pressures creating just as much or even more load on the sign than the standard provisions.

 

[r13]

I don't understand your (OP) philosophy and how to justify it. I would argue that the stiffer the support, the more loads it attracts, as opposed to more flexible support. However, I don't have anything to backup my argument, so I follow the code at the end of the day - just be conservative.

 

[flight7]

Thanks for your opinions. I was trying to persuade myself that smaller loads would be valid, but clearly that's not a good idea.

retired13 - My intended anchorage would probably be stiffer than what is pictured, so your point about attracting more load underscores the need to avoid reducing loads.

Thanks, all!

 

[skeletron]

You need to consider the base building design for these cases. I think it is a lot more complicated than some designers think, so good for you for having the intuition that sensed some uncertainty in determine your load. Is it higher or lower than what the code says a free-standing sign takes? There are other variables at play once you hang an attachment in an urban area. Different wind patterns, pressures, or shielding effects can be possible.

I follow the code load but consider the anticipated behaviour at that design force. Determine if the design level can be affected by your unknowns after that (ie. bracket the solution).

 

[BridgeSmith]

Wind pressure on objects with irregular surroundings can be highly unpredictable. Given that, and the small material cost to be conservative, I would suggest being 'overly conservative' is prudent in this situation.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[BridgeSmith]

Also, it's typically not cost-effective to do any detailed loading calcs. If it takes an hour to shave $20 off the cost of the supports for the sign, was it really worth it?

Rod Smith, P.E., The artist formerly known as HotRod10