Does the Length of Building Fasteners Play a Role In Reliability?
Steel bolts dependably react to the conditions of the application. For every 30,000 psi in tensile stress a steel bolt with be stretched out by 0.001 inches multiplied by the length in inches. Does this make the length of building fasteners a factor in reliability?
Yes, it does. For example, if two bolts of the same grade and thread are used in a similar situation, the longer bolt will lose less of the residual tension and provide a more reliable fit.
How does this work? By working through the numbers this scenario becomes obvious.
Assuming all conditions are the same with the exception of fastener length, a 3-inch bolt tightened to 60,000 psi will elongate by 0.006-in. A 6-inch bolt in the same application will stretch out by 0.012 inches.
If grip relaxation is 0.002-inches in either case, the shorter bolt will lose residual tension in the amount of 1,550 pounds, while the longer bolt will find only 775 pounds of residual tension is lost. Those figures translate into the 3-inch bolt maintaining just 67 percent of its clamping force, while the longer bolt retained 83 percent of the initial force.
A longer bolt will be more dependable in applications that are subject to vibration and other cyclic loads. Because it retains a higher percentage of the all-important clamping force, the longer bolt provides more stability and reliability. It stands to reason that longer bolts are also better at resisting fatigue and loosening, making them a better choice for all structures.
There are instances were longer building fasteners are not practical. Due to structural design and workspace, many joints present a maximum fastener length. It also doesn’t make sense to use a longer bolt in areas where the excess will present a danger or clog up space. In those cases designers need to weigh the advantages of reliability with the space available.
Longer building fasteners are more reliable in the face of vibrations and cyclic load. Steel fasteners elongate under tensile stress, losing a portion of clamping force and loosening. Designers are wise to specify longer bolts where possible, in order to combat these issues and provide a more reliable fit.
