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This depends on the top chord and its capacity to support the additional bending moment from the axial load at the bottom of the bearing seat. Smaller angles have very low bending capacity and axial forces as low as 3 kips will need special design and manufacturing. When axial forces must be transferred between joists or joist girders, it is always recommended that tie plates, or some other form of transfer, be used, i.e., tie rods or tie angles. This ensures a direct transfer of the axial forces throughout the structure. If a number has to be placed on this, it is recommended that axial loads of 5 kips and larger have some form of axial transfer.

Because the tie plate connection is from one joist top chord to another, there is no load transfer to the column. The only force the column receives when a tie plate is used to transfer axial loads is the vertical end reaction of the joist.

Using a tie plate to transfer the axial load from joist-to-joist will not add moments to the joists. The small eccentricities make this type of connection a shear transfer. The joist manufacturer should check the joist top chord for shear lag under the axial tension force.

If the forces are small, they can be transferred through the joist girder seat.  In most case an additional means, like a knife plate is required to designed to transfer the forces to the column cap plate.

This is a function of the joist top chord’s residual capacity to absorb the additional bending moment created by the axial load’s eccentricity from the bottom of the bearing seat transferred to the top chord centroid. This varies greatly with the top chord angle size, applied loads, and bearing seat depth and a single number that can be applied to standard joist is not really feasible. Because this varies greatly between joists, the SJI and its manufacturers have always recommended that some form of axial transfer mechanism be used. i.e., tie plates, tie rods, knife plates. If you ask the various SJI manufacturers, you will get different answers to this question. This has been referred to the SJI Engineering Practice Committee who responsible for developing and interpreting the SJI’s technical publications and information for their consideration.

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