EVALUATING THE LOAD-CAPACITY OF GLULAM DAVIT ARMS AFTER 20 YEARS OF EXPOSURE

The development and assessment of capacities for davit arms are crucia l for uninterrupted electrical service, This paper addresses vertical loads for davit arms, but the approach can be used also for horizontal load capacity, The National Electrical Safety Code (NESC) method of d etermining and assessing allowable capacity is based on ultimate capac ity, and does not incorporate the variability of the population. Assig nment of allowable capacity based on fiber stress and engineering anal ysis is a rational alternative approach. In the example reported, glul am davit arms were removed from service in eastern Oregon after 20 yea rs. These arms were tapered/curved, glue-laminated timbers, and had be en preservative treated by the Cellon(R) process. The population was s ampled and tested to establish residual strength and load capacity. Tw o davit arm geometries, with the principal difference in the radius of curvature of the centerline, were evaluated. Statistical results show ed that the bending stiffness of the two types was not significantly d ifferent, but was lower than the original stiffness. Allowable load ca pacities were established for the two configurations, and were compare d to the original load capacities based on the grade of lumber used at the time of fabrication. Results of the NESC method indicated that th e arms no longer had the minimum overload capacity, and should be remo ved from service. The developed allowable bending stress and engineeri ng analysis provided the same conclusion. The confidence level assumed during the determination of allowable stress was shown to affect the results, and thus the potential decision for action.