huangyhg

wind design of a permeable banner/signage
hello,
i am new writing on this forum but i have read many of the problems/questiosn posted by others.
i have been asked to design a 100' long x 45' tall banner that will be attached to a structure on the top and two sides. a wire will run along the bottom to hold the banner in place. i have been told the material that is going to be used is 70% permeable. i would like to know if anyone can point me in the direction of some refernece material can assist me in finding the wind loads, finding the tension in the fabric and the reactions that are imparted on the structure. i would like to consider the stiffness of the cable and the banner material. all help is welcomed even if it describes how to analyze the problem.
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to be banner of that size, it must be on good height like 100 or 200 feet above ground. wind speed at this height are more unpredictable. secondly 70% permeability is not directly propostional to 70% reduction. we need to see how good of wind fearing it does. as force vortex shedding and flutter might increase the load. go on internet and try to find leeward wind patterns for opening similar to your banner.
old nbe-88 almost equal to even older code mv-103 (30 years rulig between both them) said that for loose banners (i understand they were thinking on vertical poles) the total force should be computed based on a 25% of the total area of the banner. (one should assume on the total wind force for a rectangular setup, was not expressed). that was in point 5.5
then in point 5.6 went on factors due to the slenderness of the buildings. it was 1 till slenderness 5, 1.25 till slenderness 10, and 1.5 for slendernesses up to 60. may interpolate, said.
hence by this interpretation of such standard, taking 37.5% of the total area at the total wind forces (pressure plus suction) for an ordinary building at such height should be conservative.
might be even overly so. anyway wind wortex shedding -that for a hung banner (standard) would make its supporting cable fling up and down- is unpredictable as wind itself and even such evaluation might turn sometimes unconservative for any kind of wind, pulsed or not, that puts the cable plus banner thing in near resonance.
on the other hand, the waves on the banners of big size may have small wavelengths respect their dimension, this diminishing 1st mode enhancement of the vibration. for most cases, the estimate above should be conservative.
respect construction ... i think that discrete support of the banner along the cable must be made at small distances, if not continuous. otherwise it may turn be taking big stresses and ripping off.
i would not count on the permeability at all. wind drag on semi-open fabrics can be almost the same as a solid fabric.

at some point in a high wind the fabric will rip/shred so you will have a limiting force approximately equal to the tensile strength of the fabric
i have designed these by a combination of both jae and cvg's advice. get the client to chase up the manufacturer who should be able to give you tear strengths.
when they replace the fabric will the new fabric still be permeable? probably not.
initially i would design ignoring permability. if the material fails then i would look at the problem and try to condider the permability issue but as dgkhan said there may not be a 70% reduction in loading becaue of the permability.

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