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Wood I-joists are available for use in roof and floor joist applications. Produced by different manufacturers, I-joists are currently not built to a common standard. The database file supplied with Sizer is for APA Performance Rated I-joists (PRI). It is intended to be modified using Database Editor to match the properties of the I-joists you are using.
You are advised to contact your I-joist manufacturer or supplier for complete design data on the particular type of I-joist to be used.
Sizer uses a simplified design methodology for them as follows:
1. I-joists tend to deflect more than typical solid sawn sections due to shear deflection. Sizer calculates shear deflection by modifying the equation that increases deflection for simple span and loading conditions so it serves as an approximation for increasing the deflection from the Sizer finite element solution to any loading situation. That is, the program multiplies the deflections by 1 + 384 EI / 5KL2
K is the shear constant value and EI is the stiffness, each stored in the database for each product and L is the member length . Shear deflection generally increases deflection by about 10%.
2. Sizer considers reduced deflections due to composite action between floor sheathing and the I-joists, by increasing the overall floor stiffness by 10%. This is done after the calculations for shear stiffness are made.
3. Sizer does not do bearing design for I-joists. Required bearing lengths are based on the supporting members only.
4. Sizer does not consider web holes, partial load combinations for web holes, web stiffeners, web reinforcement, or I-joist vibration design. Custom versions of Sizer for individual manufactures like Nordic Engineered Wood and Web Joist have these capabilities.
5. I-joists are assumed to be fully laterally supported top and bottom, and are not designed for buckling effects.
6. Unlike solid cross-sections, shear is evaluated at the support instead of a distance from the support that equals the depth of the joist.
Refer to NDS 7 for further information on I-Joist design.