Effects of Wind Loading on 40'x20' Banner

[gmf]

I am working on the wall anchors for a large 40'w x 20'h banner which is to be suspended from the uppermost wall of an 11 storey building. The wall consist of 4 inch thick precast panels. I am trying to use 5/8" dia. through bolts spaced at 5'-0" on centre (top and bottom of banner).

As I apply my wind loading to the banner, I notice that I should be considering the effect of the resulting tension in the banner which in turn creates large shear loads in the bolts if the banner is tight to the wall (i.e. like pulling on the centre of a tight steel wire between two roof anchors. When there is very little slack in the wire, the resulting loading on the anchors perpendicluar to the applied load are large).

Am I getting to analytical in considering this. Should I just be considering straight pull out resulting from the wind on the specific anchor bolt's tributary area?

Thanks

gmf

 

[miecz]

If your banner can't span to the bolts in bending, I think you need to consider the shear force due to the tension in the banner. Limit the tension, though, to the strength of the banner. Maybe even, the strength of the banner at the connection.

 

[MiketheEngineer]

That is a real problem - the outside cables - if too tight will definitely fail the bolts in shear or the brick or whatever.

If you can keep the cable from sagging and do not preload too high - loads will not be too bad - In other words - more connections

Basic equation:

H = wL^2/8d

H = tension
w = plf on cable
L = span
d = sag

Watch units and good luck

 

[csd72]

gmf,

The catenary tensions forces will be the primary load on your anchors in this type of structure. I would expect the shear from this to be roughly 3 or four times the tension.

This is something that I have seen overlooked by many engineers and it is a good thing that you have picked it up.

Look up the catenary formulii in roarks for the typical formulii for a one way cable. You can get a rough estimate of the average loads on each side by equating the deflection at the banner center.

The hardest thing on this will be determining the EA as this information is not usually readily available. I would suggest seeing if you can obtain elongation data from the manufacturer from which you can estimate the EA at the service loads.

Keep in mind that inward pressure will blow the banner into the face of the building and therefore does not normally need to be considered.

The banner also will need to be tensioned to take the kinks out, they never get the size right. Have you allowed for future banner replacement?

 

[gmf]

Thanks everyone for the help. This has been great. I have been able to get a udl load confidently at the top and bottom of the banner using Roark's and analyzing the banner as a simple flexible (cable) beam. Now using the udl's I have obtained at the top and bottom of the banner, I am wondering if I have the same "flexible beam" effect within the banner perimeter edge (top edge for example) spanning between grommet holes, creating tension in the perimeter of the banner. I figure the tensions created in this case would cancel each other out based on adjacent spans, except at the corners of the banner where there is no adjacent span. Would this be a correct way at looking at this? Currently, I am connecting the banner grommet holes directly to the anchors above (spaced now at 2'-6") though I am wondering if I need an anchor level with the top of the banner with a horizontal tie member to account for potential tension in the banner. I have also considered attaching the banner to a steel wire running between the anchors at the top and bottom, though again, I seem to get tension issues and therefore shear issues on the anchors at each end of the steel wire (at corners of banner). I hope what I am trying to describe can be pictured.

csd72 - I have allowed for future banner replacement. In fact, the client wants the ability to replace the banner on a regular basis.

Thanks again, everyone, for the help.

gmf

 

[hokie66]

gmf,

As csd72 noted, the forces are catenary in nature. The only way to avoid shear forces on the anchors is to provide a compression frame against the wall to resist that component.

 

[csd72]

I would bring those anchors in closer and use an aluminum flat bar in a fabric sleeve rather than grommets (which are great starting points for tears).

I stress again that the fabric manufacturer will definately not get the banner size right and you should allow 3" tolerance to allow for this.