Commins Manufacturing Inc. builds AutoTight Tie-Down Systems for light frame construction.  

AutoTight systems compensate for shrinkage and settling. Auto-Tight complies with all codes for strength and system elongation.

Code Tight

The code requires tie-downs to have both strength and stiffness.  Designing for strength is relatively simple. Designing for deflection is difficult.  Deflection sources include: rod elongation, bearing plate crushing, Holdown deflection, shrinkage-settling, and shrinkage compensator deflection and looseness.  Leave anything out and the system won’t meet code.

Average shrinking-settling in wood frame buildings begins at 1/8” per floor (manufactured materials and kiln dried plates) and can range up to 1” per floor for green lumber [1] . Many engineers estimate shrinkage at ¼” per floor for manufactured wood and ½” per floor S-Dry lumber joists. The shrinkage must be added to the tie-down deflection or a code listed shrinkage-compensator must be used. But using a shrinkage compensator is only half the problem. Selecting a shrinkage compensator that meets your requirements can be difficult.

Twice as Tight?

Shrinkage compensators add two sources of deflection to tie-down systems. These deflection sources are D_a and D _r.   Delta a is deflection at the adjusted load. Delta r is deflection due to reversed loading. Delta r is added in full to the system deflection. The graph compares the system deflection of typical shrinkage compensators. For a more complete analysis see Elongation #11 and 12.

Two videos on our web site are useful in understanding tie-down performance.
First watch Understanding shrinkage and straps (45 seconds). (under construction)
Then watch Shrinkage Compensators, Delta R, and Shear Wall Performance (5 minutes).

Thank you for considering AutoTight. Follow this link for expanded information on Tie-Down systems including a:

            AutoTight Overview
            Review of Code Requirements,  
            Model Tie-Down Specification
            Elongation limits and Designing for Strength, Elongation and Shrinkage in 30 minutes.

        AutoTight    “Connection-Perfection”