Calibration of Resistance Factor for Self-Tapping Screws in Canada

Abstract

Under the load and resistance factor design philosophy used in North America, building codes structures are designed using load and resistance factors calibrated to a target reliability level. Despite this, few attempts have been made to calibrate resistance factors for timber connections. A calibration process was carried out for the withdrawal resistance of axially loaded self-tapping screws using a database of test results from across Canada. Two load cases were considered, reflecting load combinations and statistics drawn from the 2015 National Building Code of Canada and from a proposal developed to provide greater consistency in reliability outcomes, and the reliability under each case was determined using the first-order reliability method (FORM). Given the low ductility–brittle nature of withdrawal failures, a resistance factor of φ=0.7 was recommended for a target reliability index of β=4.0. The outcomes will assist in the selection of a resistance factor for self-tapping screws for use in the Canadian timber design standard, CSA O86.