Fabric Roof Structures

[kcprofessor]

I don't know why things happen in clusters, but I am getting a number of requests for the design of framing to support fabric roofs. These are typically light-weight fabric with steel cable around the edges and no structure to give the fabric any shape. I am not sure of the amount of prestressing force, if any, that is usually applied and the subsequent application of other roof loads such as wind uplift affect the horizontal force that must be supplied. Anybody have any insight?

 

[thaidavid40]

We design-built a lot of these types of structures, "Sprung Structures", for hangars and maintenance facilities in Afghanistan, Kyrgyzstan, and Iraq. They were all pre-engineered types, so all we had to do was provide adequate foundations for them (exclusively wind uplift/overturning driven), and we were good to go. Are you being asked to support the design of pre-engineered structures, or to design custom structures? If custom, I am very surprised that a custom structure could compete economically with a pre-engineered structure - something like a custom steel building competing with a PEMB structure. In any event, other than the code allowance for reduced roof live loads on fabric structures, I think the best place to start picking up information would be the several pre-engineered fabric structures manufacturers' websites. They have been through this rodeo a bunch of times, and they know most of the ins and outs - and the pitfalls - of these types of buildings. I will say this though: they do not hold their looks in the South (humid environment) as they get mildewy on the exterior, and look like crap. Cleaning with "Simple Green" takes care of that, and without polluting the ground around it. However, they do seem to hold up well in dryer climates, and they seem to resist UV degradation fairly well too.
Good luck,
Dave

Thaidavid

 

[kcprofessor]

Thaidavid-
Most of these structures are small and part of other projects and are architecturally driven. Can't use a pre-engineered buildings because of sometimes odd shapes, etc. So what I am looking for is a way of determining the reactions from the fabric on my structure. There are formulas out there for catenaries, but these cables are prestressed and the formulas are for essentially slack cables. Is there any difference in the behavior of a prestressed versus slack system?

 

[thaidavid40]

I wish I could help you with that specific point - I've never had to get that deep into the weeds with those systems - I was simply an end user. However, we've designed quite a few "trot line"-type fall protection systems using overhead cables. For the plain cables with no pretension loads, the math comes down to simple statics for point or distributed loads (check Roark's catenary formulas). Most of the systems though use proprietary, pretensioned components, with spring buffers on the ends. The reaction loads from a fall event are determined by the buffers' tested spring rates, so those may not be specifically analogous to your case. If you have lateral loads contributing also (which I suppose you do), then I guess you'll have to make some simplifying assumptions as to the way to analyze that, since you now have catenary loads on a cable in two orthogonal directions, pulling it into a complex curve. Unless you have some way to do the analysis in an FEM program, I think that you are going to have to approach this with a simplifying assumption as to the cable's final shape. I think that some of the higher-end FEM analysis programs out there do complex cable analyses, but I can't recommend one from experience. Maybe someone else in this forum (a suspension bridge engineer, maybe?) can recommend a good software program to get a better answer for you. The software that I regularly use doesn't have this capability.
Dave

Thaidavid