Allowable load on a horizontal fabric surface

[dmoench01]

Hello,

I am looking for little direction on calculating the allowable load for a fabric surface. I work for a fabric building company and we have typical rigid frames at 20' o.c. but have fabric on our roofs and walls, instead of steel panels. The fabric is attached at every truss. I have the specificiatios of the fabric so I know the tensile and tear strengths. The problem I have is determining what is the maximum load (snow, live, wind or combination thereof) that can be applied to the fabric before failure of the fabric is imminent. Originally I used equations from the CE Reference Manual(7th edition)to try and estimate what the force would be. Specifically I tried to use the sagging cables equations. Equation 41.60 gives a close approximation but it makes no sense. The more deflection I have the greater the load that can be applied so I'm thinking this is a dead end.

Any ideas on how to estimate this better? The fabric suppliers have no testing that I can find and I have asked them. Our buildings have been going into some high snow load areas (over 100psf) so this is a concern.

Thanks

 

[FishTheStructure]

That sounds correct - As the deflection increases, more snow (or rain) could drift into the low point causing the loading to increase.
If the material is "stiff" enough, several iterations of this calculation should converge on a solution where the loading due to a certain deflection results in the same deflection.
If the material is not "stiff" enough, or if the material response is not elastic, this approach will not converge to a solution which means that the fabric is not suitable to resist the intended loading.


Great spirits have always encountered violent opposition from mediocre minds - Albert Einstein

 

[BAretired]

Unlike metal panels, fabric is presumed to have no bending strength. In order to carry heavy snow loads, the fabric must have a sag built into each span so that it can operate as a membrane. As you have noted, deflection will increase the sag, so using the installed sag for calculations is conservative.

Also, the fastenings and structural members supporting the fabric must be capable of resisting membrane forces from the fabric. Design of these structures is rather a specialized type of engineering. You probably need to do a considerable amount of study to develop confidence in your design approach.

BA