APA - The Engineered Wood Association
Filler blocks are used to fill the rectangular space between a pair and more of I-joists acting as a single bending member. The purpose of the blocks is to transfer load from one bending member to the next - load sharing. This is accomplished by forcing each of the joists to deflect the same amount under the applied load. Filler blocks must be placed the full length of the double I-joists. Filler blocks, however, do not have to be continuous - they can be made up of shorter lengths of lumber and/or wood structural panel.