FAQ – Special Joists

Return to FAQ

What are the bridging requirements for double pitch joists?
For any joist where the center of gravity is above the support point, all rows of bridging should be bolted cross bridging.  The number of rows will vary with length, pitch and weight of joist.  A joist manufacturer can give direction on this.


What is the correct way to specify the depth of a special profile joist?
Single pitch = depth at mid-span
Double pitch (gable), bowstring, scissor, arch = depth at ridge


Can I use the bridging on special profile joists as bracing for the structure?
Bridging is only designed for stability of the joist system. Additional bracing requirements shall be designed and sized by the engineer of record. Bracing material is not supplied by joist manufacturer.


Can I use a designation from the SJI standard load table for my special profile joist?
SJI designations (e.g. 20K4, 44LH09) shall only be used for single pitch and double pitch joists if the pitch does not exceed 1/2 inch per foot.


How do I specify a special profile joist if an SJI designation is not valid?
Provide a detailed load diagram or specify the joist as ‘LOAD / LOAD’.


Should special profile joists be specified with standard SJI camber?
Standard SJI camber my not be ideal. Consider the transition between special profile joists and other areas. Specify zero camber, or specific camber requirements, as needed.


How do I resolve horizontal reactions on the structure from scissor and arch joists?
To remove this thrust force, a slip connection should be used at one end of the joist. If diaphragm forces collected in the joist top chord need to transfer to the structure, alternate which end of the joist has a slip connection.


Do I need to provide more than just the pitch or radius of the roof for special profile joists?
It is best if the structural drawings provide all necessary dimensions and joist depths. This ensures there are no questions or misunderstandings.


Can I use standard seat depths no matter the slope or pitch of my joist?
Seat depths at the high end of joists shall be increased according to SJI tables 2.2-2 & 2.2-3. This ensures proper clear bearing and working point.


What is the maximum depth and length for a special profile joist?
There are many factors that affect these limits, such as production shop capacity, job site access, and shipping limitations. Therefore, discuss options with your preferred joist manufacturer.