Volume 26, Issue 5 (September 1998)
Establishing Uplift Design Values for Metal Connectors in Light-Frame Construction
Wind loads cause uplift pressures to be applied to a roof resulting in a net uplift force on the structure. One of the most critical locations in the vertical load path is the connection between the roof and the wall, specifically between the roof rafter (or truss) and the top-plate of the wall. This connection is typically made using mechanical fasteners such as metal straps and nails. The manufacturers of these straps provide allowable capacities for their products based on ASTM test procedures for tension straps, joist hangers, or other similar products. Manufacturers' published design capacities for hurricane straps are based on the lowest value of three test criteria. The first criterion is the ultimate uplift load divided by three, the second is the load 1/8 in. (3.2 mm) deflection, and the third is the allowable design values for nails or other fasteners used to attach the connector to wood members. Recent test results suggest these values may be overly conservative in some cases and that some test criteria or procedures may not be representative of actual design conditions. This paper first reports on a testing program to evaluate uplift capacities of lightframe connections made using commonly available hardware. Based on an evaluation of the results and observations made during the tests, shortcomings of current test procedures and testing program details are described. Deficiencies, and specific needs to overcome them, are identified in the context of structural performance as well as safety.