How Much Bracing Do I Need?

Q: I am in the process of building a shop ( 53' x 103') that is all previously used steel (53'bar joists, 18" and 24"-I-beams) The building is excellent and functional in all aspects and I have the steel up. My question is on the diagonal bracing of the structure. At the present I have two 30' long 3\4" diameter rods at each corner on 45 deg. angles to prevent racking in two directions. Could a consulting engineer look at what I have and tell me if there is a weakness?

A: Yes. Your building must resist a 70 mph minimum wind load on both faces. In a typical environment, this amounts to (1.13)(12.6)(1.3)(1.0) = 18.5 psf. If your site is located in an open field, use 23.75 psf.  If you are surrounded by buildings, use 10.97 psf.

For a 20 foot tall building using the intermediate wind load, this equals a force of 185 pounds per foot at the roof level. For forces perpendicular to the 103 foot side, each 53 foot wall must resist a force of 9527 pounds. A 3/4" threaded rod will resist (0.309)(20000)(1.33) = 8219 pounds. Your rods are installed at 45 degree angles, therefore the maximum horizontal force these rods can resist is (8219)/(Square Roof of 2) = 5811 pounds. In this example, there is an overstress of approximately 64%. Use your roof height(s) and calculate the force on your walls...then see if you need additional bracing.

Remember to use rod bracing in crossed pairs to resist forces in alternate directions and to provide at least 50% more bracing than you calculate you require to account for initial tensioning

Scott McVicker, S.E.