Reaction

As gravity pulls on a joist and the things it carries, the joist pushes down on its supports. Simply stated, a reaction is the force of a support pushing back on the joist. A reaction is reported in pounds and is a function only of the load supported by the joist or beam.

Allowable reaction is among the design properties listed for I-joists. A bearing or reaction failure of an I-joist typically occurs at a support when the web-flange joint gives way due to the "knifing" action of the web pushing through the flange. Web stiffeners may be installed to increase allowable reactions. To a point, longer bearing lengths allow higher I-joist reactions.

Allowable reactions for beams, on the other hand, are controlled by allowable compression perpendicular to grain. Longer bearing lengths allow for higher beam reactions. For more information see bearing length.

Open-web trusses are attached to their supports with a bearing clip, a built-in hardware connector. Trusses are designed with an assumed bearing width, which is stated in the design results. If the capacity of the bearing clip connection on the assumed bearing is exceeded, this will be noted in the design results. Please contact RedBuilt™ Technical Support for bearing widths less than the stated assumption.