What prevents an I-joist from moving in the joist hanger?

The correct answer is a backing block. In the context of I-joists installed within joist hangers, a backing block serves as an essential component that helps secure the I-joist in place. The backing block is typically a solid piece of material that fits snugly against the sides or bottom of the I-joist, effectively preventing lateral movement and providing stability within the joist hanger. This is crucial for maintaining the structural integrity of the floor system and ensuring that the load is distributed evenly.

The other components, while important in their own right for different reasons, do not perform this specific function. The crown refers to the natural curvature of a wood member, which is relevant to how the joist should be installed but does not prevent movement within the hanger. A rim board is used at the edge of the floor system to provide a nailing surface but doesn't actively hold the I-joist in place. Bridging serves to provide lateral support and increase stiffness between joists but is not designed to prevent an individual I-joist from shifting within its hanger.