Governing Stresses in Z-Purlin Lap Joints

Twenty-eight stiffened Z-section beams were transversely loaded to failure and stresses in the lap joints were investigated. Spans of 2.438 m (8 ft) were used. The tested sections were of web depth-to-thickness ratios ( h/t ) ranging from 79 to 131, and were selected from those commonly used by the metal building industry. The tests included a single-section beam of each Z-section and lapped beams with a lap-to-span ratio ranging from 0.25 to 1.0. The results indicated that the lapping process significantly enhanced the load capacity and stiffness of the beams for lap-to-span ratios up to 0.5. For higher ratios, little or no enhancement was noted. Failure was encountered in the single-section zone for beams with lap-to-span ratios less than 0.5 and in the lap zone for higher ratios. The analyses of the stresses in the lap joints indicated that the failure mechanism was governed by bending. Correlations were developed to predict the relative stiffness of the lap zone. The predicted ultimate moments using these correlations were in close agreement with the test results, and the 1986 American Iron and Steel Institute (AISI) design specifications were found to be adequate for predicting the strength of single Z-sections.