Deflection of Nested Cold-Formed Steel Z-Section Beams

Stiffened Z-section beam purlins were tested to evaluate the deflection behavior of lap joints under combined bending and shear. The tested beams had a span length of 243.84 cm (8 ft), nominal web depths of 20.32 cm (8 in.) and 24.13 cm (9.5 in.), and a metal thickness ranging from 1.542 mm (0.060 in.) to 2.565 mm (0.101 in.). The corresponding web depth-to-thickness ratio $(h/t)$ ranged from 79 to 131. The experimental program included testing a single section beam of each Z section and lapped beams with lap-to-span ratio $(l/L)$ ranging from 0.25 to 1.0. The test results indicated that the lapping process enhanced the ultimate load capacity and stiffness of the beams. This enhancement was more pronounced for beams with lap-to-span ratios less than or equal to 0.5. For higher $l/L$ ratios, little or no change in the results was noted. The failure mechanism was governed by bending stresses and was caused by buckling of the compression flanges at stress levels close to the yield stress of the parent steel. A relative stiffness behavior approach was used to analyze the results. This approach compares the deflection behavior of lapped beams with that of a single section beam having the same span length and the same Z section. Based on this analysis, semiempirical equations were obtained to predict the deflection of lapped beams. Theoretical deflection equations were also derived and the results based on these equations were compared with the experimental results. It was concluded based on this comparison that the proposed correlation was adequate for predicting the deflection in the lap joint zone of nested Z beams.