Determine wind forces on banners, different from flag wind forces

[dm3415]

I'm designing some flag poles for a banner that stretches across a street. I've reviewed the ANSI/NAAMM 1001-07 document which addresses wind forces on flags quite well. It does not address wind forces on a banner supported on both sides, which acts more like a parachute than the wind forces on a flag. Using the ASCE 7-05 wind forces for a solid sign seems overkill, as the banner obviously gives more than a solid sign. Does anyone have equations they can reference for this type of loading?

 

[TLHS]

If anything I'd be concerned that it could be worse than a solid sign. If you've got all four corners tied off a wind coming in square to the banner will make it form a sort of cup for the wind. The air will have to deflect through a greater angle to escape from the banner.

Think of it this way. What would act as a better parachute? A parachute that curves to form a dome or a flat plate?

If you've cut slits in the banner it would help.

Unfortunately, I haven't personally run across literature on this.

 

[dm3415]

Based on the cantenary shape a banner's support wires would take I'm finding loads do increase versus a standard rigid sign, but the laymen's documentation I've found on banner sites indicates that the shape of the banner also decreases and they (the banner industry) use a reduced size and a magical drag coefficient to decrease their loads. I'm not comfortable guessing what the fluid dynamics of the banner are with wind loads though. Basically, I need a published document if anyone knows of one?

 

[csd72]

I am not aware of any published information, we used to use the catenary method and the standard wind loads similar to what you have described. The EI information should be available for the fabric manufactirer.

I have found that much of what the sign industry specifies for this type of thing is not very accurate.

I have often also limited the loads applied to 20 to 50% above the published tearing strength of the fabric. Sometimes this gives a more economical design.

Dont forget that the supports will deflect thus increasing the banner deflection and reducing the catenary tension.

 

[dm3415]

Didn't think of the pole deflection, thanks!