Engineers are regularly faced with the dilemma of spending the extra time it takes to do complex hand calculations for a moving scissor lift to answer the contractor’s question: How much weight can the scissor lift carry? Can I save some time and just combine these wheel loads? Or can I simplify the calculations and combine the wheel loads to determine the maximum allowable point load on the deck?
In the June 2024 SEU Session, Joshua Canova, PE, from New Millenium, and Gerald McKenzie, PE, SE, from Vulcraft, on behalf of the Steel Deck Institute, presented Concentrated and Non-Uniform Loading on Steel Deck. Josh and Gerald explained the procedure for developing “simple” load diagrams based on recognized load distribution and explained deck limit states under concentrated loads. They also identified current and expanded SDI design approaches for concentrated loads.
After walking the audience through the detailed calculations establishing the capacity of steel deck to support a moving scissor lift load, Josh addressed the most common question posed by structural engineers looking to save time and money, ”Can scissor lift wheel loads be grouped into one load?” Josh examined the results from combining these wheel loads and used the SDI Floor Deck Design Manual’s tables for Maximum Nominal Scissor Lift Wheel Loads to address this question. As expected, grouping the wheel loads compared to addressing them individually would yield a conservative solution. But how big of a tradeoff does this short cut yield? As you can see in the slide below, the capacity of the example floor deck, considering individual wheel loads, will yield an allowable scissor lift load of 6216 pounds. However, combining the loads into one results in an allowable scissor lift load of 3100 pounds. This effectively reduces the allowed scissor lift load by more than half. While this short cut may save calculation time, it may add to increases in construction time and labor due to the reduced load allowed on the scissor lift.
While the hand calculations may be cumbersome, SDI provides useful tables within the Floor Deck Design Manual, Second Edition for common steel deck configurations which can be used to determine the Maximum Nominal Scissor Lift Wheel Load. These tables have been developed by moving a scissor lift one inch at a time across the length of a given deck configuration, turned 90 degrees and then moved back one inch at a time and determining the maximum capacity of the deck limit states. The tables are based on conservative conditions and assumptions to provide the lower bounds that any SDI member company should be able to meet; they are not necessarily accurate when it comes to maximum possible capacities for a specific manufacturer but provide accurate capacities based on the given assumptions and confidence in what any SDI member company could supply. To purchase a copy of the FDDM2 as well as other technical guides related to steel deck design, visit the Steel Deck Institute (sdi.org). Engineers are able to use these tables to both reduce their calculation time and provide a more accurate estimate of the capacity of the steel deck to resist the scissor lift wheel loads.