Threaded rods are commonly used in continuous load path systems to resist uplift forces in a structure. Whether these uplift forces are a result of overturning in a shearwall or roof uplift restraint, threaded rods can provide effective structural restraint provided all of the design parameters are considered. One such parameter is rod elongation which is a fundamental design concept calculated from a simple engineering formula.

The rod elongation that occurs increases as the load increases and can result in negative performance of the load path system if not properly considered in the engineering design. In a shearwall the stretch in the rod will cause more lateral movement (or drift) which can cause structural damage. Often in areas of the country with a potential for high seismic activity the building departments limit the amount of elongation in a system to be less then a certain amount (typically between 0.125" and 0.200") to control the amount of drift that may occur in the wall.

In an uplift system the rods are taking the uplift loads from the double top plates to the foundation. Excessive rod elongation can cause the double top plates to lift and separate from the studs causing structural problems. As such there are suggested limits on the amount of elongation for those systems as well (see "Calculation Results" note for additional clarification).

This calculator determines the amount of elongation in a rod given the design application. In addition to rod elongation, wood shrinkage is another consideration that may effect deflection in a threaded rod system. For information regarding wood shrinkage, see the see the Wood Shrinkage Calculator.