Standard On Center Spacing
Question:
What is the recommended standard spacing for open-web wood joists? I have seen them placed 24 in. O.C. Is this acceptable for a customer who will have an exercise room with 500 pounds of free weights? I understand there are problems with bouncy floors with 24 in. spacing. Is this true?
Answer:
There is no set standard spacing, but 24 in. O.C. is common. Spacing is generally chosen such that the sheathing edges will fall at the centerline of the trusses. As a result, truss spacing usually divides evenly into a common plywood sheet (8 ft.). For example:
- (8ft. x 12in./ft.)/4 = 24 in. on center (O.C.)
- (8ft. x 12in./ft.)/5 = 19.2 in. O.C.
- (8ft. x 12in./ft.)/6 16 in. O.C.
You should let the truss designer know how the room will be used (i.e., exercise room with 500 lb. weights) so that the trusses can be designed with the appropriate loading conditions. Trusses can be designed to almost any deflection criteria. According to the Truss Plate Institute's ANSI/TPI 1, the maximum live load deflection for floor trusses shall be limited to Ls/360. This can be achieved by increasing the depth of the trusses as necessary based on the span length. If you are concerned with bouncy or vibration in your floor trusses, you should consider strongback bracing when installing the trusses. A strongback is a two inch dimensional framing member attached perpendicular to floor trusses, often through the chase opening, and placed vertically against the vertical web. It is recommended to reduce vibration and connect the trusses so they act more like one unit.