The function of a rim board is to provide for the transfer of loads applied from the walls, floors and roof above, including lateral forces from the diaphragm, to the wall or foundation below. Both of these load transfers must occur in order for the structure to perform properly.
TJI® joists have series/depth specific allowable design properties. The TJI® joist span tables are based on full utilization of these properties to maximize distances. Efficient design of the TJI® joist system assumes loads from walls above are supported by the rim and not the joist. This means the loading is transferred around the joist, not through the joist and thereby reducing the remaining capacity of the joist to resist it's own loading.
To ensure proper load transfer around the joist, the rim depth must closely match that of the joist. Solid sawn lumber used as rim material will not perform adequately in a TJI® framing system. Generally, TJI® depths don't match lumber depths. More importantly, the moisture content of sawn lumber yields a finished height after shrinking that will prove to be ineffective in the finished structure.
The best choice of rim board is one that is engineered itself, with depths to match the joists, produced with a low moisture content and having specific load capabilities.
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