Concentrated Load Design Procedures for Wood-Joist Floor Systems

Current light-frame wood construction design specifications recognize repetitive member effects in wood-joist floor systems. However, it is unclear whether these design stress increases apply equally to both uniform and concentrated load cases. To address this issue, eight floor systems were constructed and tested to determine their load-sharing properties when subjected to a concentrated load. The concentrated load was independently placed on each joist at quarter span and midspan of the floor while monitoring midspan deflections and reactions at one end of each joist. Statistical distributions were then chosen to represent percentages of load remaining at the end reactions of each joist. By comparing and combining experimental data with previous finite-element results, floor joist design equations were developed for a single joist. Factors addressing load distribution were introduced into these equations for calculating maximum moment and deflection. Additional design equations were developed to account for the “pseudo” concentrated load cases stipulated in ASCE standard 7-95, Minimum Design Loads for Buildings and Other Structures. Inherent load sharing in wood-joist floor systems was found to substantially reduce the moment and displacement in the joists directly subjected to a concentrated load.