Using a Portable Nail Extractor to Determine Roof Nail Withdrawal Capacity of Existing Residential Structures

Abstract

Over the past four years, researchers at the University of Florida have collected nail withdrawal load data from 12 existing residential buildings to evaluate the in-service nail withdrawal strength and assess environmental effects. Tests were conducted using a portable nail extractor but the scatter in results far exceeded that of the laboratory based nail withdrawal tests. It was not clear whether coefficients of variation in the 30% to 60% range were due to large variation in existing wood strength or it was a factor of the test method. The goal of this investigation is to correlate laboratory based ultimate nail withdrawal capacity of nails tested with the portable nail extractor device using industrial-standard ASTM D1761 test protocol. 1,072 nails installed in southern yellow pine wood frame were tested. About half the nails were tested using the portable nail extractor and other half tested per ASTM D1761. The results showed that the withdrawal capacity of nails tested with the nail extractor was statistically lower than the withdrawal capacity obtained with the ASTM D1761 test protocol. An improved preparation method to obtain samples yielded lower COV (26%) in withdrawal capacities and more consistent results. The research establishes an adjustment factor of 1.60 to estimate true nail withdrawal strength installed in Southern Yellow Pine members based on the improved test preparation methods. These results and analysis will provide valuable input for refining the probabilistic models of the performance of wood residential roof structures in strong wind events.