High Pitched Roof Joist Designations
Can SJI load tables be used to select roof joists on high pitched roofs? How does the specifying engineer denote these pitched joists and ensure they are designed for the required roof loads?
In the November 2022 SEU session, Ben Pitchford, PE, from New Millennium Building Systems, and Doug Hoffman, PE, from Vulcraft, presented Roof Design with Special Profile Joists. Ben and Doug identified important unique considerations when specifying steel joists and Joist Girders with pitched chords or radiused chords. They described the factors that impact special profile steel joists and gave tips to ensure designers can avoid additional RFIs during their projects.
During the presentation, Ben discussed the proper way to specify joists with a pitched top chord slope greater than ½:12. These joists can no longer be specified using an SJI load table designation, but rather they should be specified using a LOAD/LOAD designation. SJI does provide some load tables for LOAD/LOAD designated joists.
Ben explained the following design example to clarify how to designate these types of joists:
Ben previously discussed that double-pitched joist designation depth is taken at the ridge, as opposed to single pitch joists whose designation depth is at mid-span. Once the depth is established from the roof geometry, the type of joist is selected, and then the LOAD/LOAD designation is calculated using the joist spacing. The designation is displayed as the TOTAL LOAD / LIVE LOAD. Thus, with a 6 ft spacing and a total load of 40 psf and a live load of 20 psf, the correct designation for this double-pitched top chord joist is 102DLH240/120.
Ben also noted that structural engineers have the option to designate a joist as “SP” to indicate a special joist which should be designed by the joist manufacturer. These joists, as shown in the example below, can have varied shapes and slopes and the specifying engineer should provide a complete load diagram for the joist manufacturer to avoid RFIs during the project.
Engineers have multiple options to use joists and Joist Girders on projects with sloped roofs, and properly specifying these joists will eliminate confusion during the construction administration process. The Steel Joist Institute can provide further guidance through their Resource Library on their website which covers all subjects relating to steel joists.