Hi, I am reading the Specs for a theater that is currently being built, and one sentence caught my attention: "Design the wire rope grid so that when wire rope assemblies are torqued to factory recommendations, the wire rope shall not deflect more than 2 inches under a maximum concentrated load of 300 pounds." So my question is, how do one approach the load calculations for a wire tension grid? Do you treat it as a walking surface (say, a deck) and calculate load per square inch (ft), or do you isolate every single cable and calculate point loads for it? In the latter case, how do you approach the fact that wires are interwoven on 2" centers? The same with the deflection: do you calculate it for a single cable, or for a certain area? By the way, I am of course not designing a wire tension grid, I am just curious about how it works. Thanks a lot!
Well, they are usually designed for a osf load - I've seen from 15 to 40 psf - and deflection - same two criteria for any deck. I'd measure psf by using a 1 X 1 sq of ply and load it. For design I believe the forces are simply calculated as vectors. I'm not sure how to calculate deflection but again, vectors. A large challenge is the frame, with all that tension. It's the weaving that matters. I worked weaving one once, and discovered that with lines (second set) only half way across, it didn't matter a few feet back. In other words, you could have a lot of breaks and not know it, just like a contained hole in fabric - a little ways away the fabric holds fine.
Thank you very much Bill! The concept of a wire tension grid is a complete mistery to me. Aren't all these interwoven wires just tensioned catenaries? If so, isn't it true that as the vertical angle increases, the tension in the wire grows exponencially? The tension in the cable would be 28 times the load at 89° and 280 times the load at 89.9°?
Yup. It's a tensile fabric calculation (stretched grid method) rather than a beam deflection calculation. Interweaving of the grid wires makes for some complex modelling. In practice, most systems are constructed of rigid panels of tension grids to simplify design and installation, so grids of grids.
If you have an panel engineered to meet the deflection specification, any smaller panel constructed with the same wire, spacing, and tension would also meet the deflection specification. The smaller the panels, the easier it is to keep the absolute deflection low. @Yaro you're right that the greater the deflection, the less tension required to hold the load; however, the math isn't quite as simple due to the fabric providing support in all directions.
In the end, I'd always worry more about the roof or other structure failing before the twg. They have a lot of redundancy.