Remedy For Bouncy Floor Trusses
Question:
We are experiencing problems with bouncy wood floor trusses. I'm wondering what the industry standard is on deflections (live and total load). Also, do you have any ideas on how to decrease the deflection without affecting the profit margin significantly?
Answer:
One of the best ways to decrease deflection without significantly affecting truss design is to install strongback bridging.
Floor trusses are built to have a certain amount of deflection or stiffness. Structural designers describe stiffness in terms of deflection criteria. You may have heard of the terms l/360, l/480, l/600, etc. This is the deflection level the designer or customer chooses and the individual floor or roof component is designed to meet or exceed that level. The l in this equation represents span, so if you divide the span by 360, that is the expected amount of deflection the truss will experience under initial full live loading. For example: a 20 ft. truss is 240 in. 240/360 = 0.67 in. Therefore, a 20 ft. truss designed to an l/360 deflection criterion would see 0.67 in. of deflection. The same span truss designed to an l/480 deflection criterion would see 0.5 in. of deflection. The same span truss designed to an l/240 deflection criterion would see 1.0 in. of deflection.
IRC 2012/2015 table R301.7 requires at least l/240 for all structural members. l/360 is required for floor components with brittle finishes regardless of whether they are sold sawn joists, I-joists or trusses. Table 7.6-1 of the Truss Plate Institute's ANSI/TPI 1 includes specific deflection requirements for trusses. For floor trusses, the minimum deflection due to live load only is l/360. For deflection due to live load plus creep, the minimum is l/240. For deflection due to total load, the minimum is l/240. If ceramic tile is being used, the applicable standards come from The Tile Council of North America’s ANSI A108-A118-A136.1.