TJI®Joists are occasionally doubled to support other members or loads from above. Depending on how the load is applied, it may be necessary to connect the two joists together with a filler block. However, if the loading is applied from above through floor sheathing, through a load bearing wall, or post that rests on both joists, then no attachment between joists is required.
When a load is applied to one side of a double joist (by another joist or header, for example), then the double joist must be connected together so both joist members are effective in supporting the load. Proper connection requires the gap between joist webs be filled with a "filler block", and the connection between the two joists made through the filled area.
Refer to detail H2 in Technical Bulletin 834 for allowable loads and connections for typical joist to joist connections. Note that the filler and backer blocks are necessary only where the load is being applied to the side of the double joist member. If the double joist is supporting perpendicular framing along its length, filler and backer blocks are needed at the load point locations along the length of the double joist. Depending on the spacing of the load locations, it may be more efficient to use a continuous length of filler and backer material instead of multiple short lengths.
When supporting a perpendicular member, a triple TJI®joist may be used. However, it is recommended to avoid such applications due to the complicated attachment requirements. See Technical Bulletin on 3 ply joists TB-820 for additional information.