Ponding analyses can be time consuming when performing this iterative process by hand, however it is required in certain situations depending on your roof slope and drainage conditions. The methods for ponding investigations in Appendix 2 of the AISC Specification can be limiting based on the requirements for flat roof construction and framing geometry. Since IBC requires a minimum roof slope, how can engineers adequately investigate roofs for ponding instability?
In the November 2020 SE University session, Mark Denavit, PhD, PE, from the University of Tennessee, Knoxville, presented New Methods for Ponding Analysis of Open Web Steel Joist Roofs. Mark’s session reviewed the main roof ponding requirements from design standards and described the limitations of the methods of ponding analysis given in Appendix 2 of the AISC Specification. Mark demonstrated how to use the SJI Roof Bay Analysis Tool to determine the stability of a simple roof system and explained the main factors influencing the ponding stability of open web steel joist roofs.
SJI’s Roof Bay Analysis Tool is an especially useful tool available as a free download at SJI’s website. Included with the download, you will receive SJI’s Technical Digest 3 which describes the new method for ponding analysis that is the basis for the spreadsheet. This tool allows the engineer to input the specifics of their framing system and checks whether the roof bay has adequate stiffness and strength to meet the ponding requirements in the code. In Version 4, the tool allows for steel framing options in addition to steel joists, and performs deck checks within the ponding analysis. Click below to listen and watch Mark run through a quick example of the various ponding specific input and the resulting output generated by this tool.
As shown in the example, for design situations where the required strength and stiffness for the ponding loads is not met, the engineer would need to return to modify the joist girder panel point loads on the worksheet ‘Joist and Joist Girders’ – under the section OPTIONAL INCREASED LOAD DATA and then re-run the analysis.
The new ponding analysis included in SJI’s Roof Bay Analysis Tool makes this iterative process quick and useful for roofs where ponding needs to be investigated. Be sure to review the basic instructions for using the Roof Bay Analysis Tool and the Ponding Analysis before getting started to be sure you are familiar with its methodology.