Typically, trusses are spaced evenly. The spacing shown on MiTek engineering drawings is a tributary area carried by the trusses. Each truss is designed to support one-half of the roof/floor load on each side. The spacing between the trusses may be of any combination if the tributary area does not exceed the spacing shown on the engineering drawings. Occasionally trusses are installed directly in the path of a large drainpipe or other obstructions. Can a truss be moved to avoid obstructions? The answer is maybe. [For both Figures, See PDF or View in Full Issue.]
As shown in Figure 2, moving a single truss 3 inches (0-3-0) affects the tributary area of the adjacent truss, thus affecting the amount of load it carries. Instead of carrying 2 ft (2-0-0) of tributary area as designed, the adjacent truss now carries 2 feet 1-1/2 inches (2-1-8) of tributary area. Depending on how close the combined stress index (CSI) values are to 1.0 on the original truss design, increasing the on-center spacing to reflect the new tributary area could result in overstressed lumber and plate failures. In addition, in our example the ability to apply repetitive stress is lost, as per section 6.4.2.1 of ANSI/TPI 1 (National Design Standard for Metal Plate Connected Wood Truss Construction) repetitive member design values cannot be applied to trusses spaced more than 24 inches on center.
Another thing to keep in mind, maximum span rating for ¾” plywood that performs a load-carrying function in floor is 24 inches. Even if truss design remains unchanged, the sheathing might need to be reinforced.
Any truss modification should be reviewed by a qualified engineer to ensure the modification will not compromise the truss design. For additional information or if you have any questions, please contact the MiTek Engineering department.