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Helical Pile Connection Bolts - SAE or ASTM?
by Wayne Thompson, PE, LEED AP
A Few Seemingly Minor Differences are in Fact Not So Minor When
It Comes to Structural Engineering Calculations
We at CTL|Thompson have encountered some confusion lately regarding the bolts being used for connections. To be honest, the issue was brought to our attention and we had to dig a little deeper. For those of you who deal with connection design on a daily basis, feel free to jump to a different article. For the rest of us…prepare to be bored.
SAE J429 bolts are not the same as ASTM A325 bolts or ASTM A490 bolts. Not Earth shattering (nor remotely interesting), but let’s continue. If you have heard of a Grade 5 bolt, it’s likely an SAE J429 Grade 5 bolt. Same thing goes for a Grade 8 bolt. A lot of people know that a Grade 5 bolt has the same material strength and material characteristics of an ASTM A325 bolt. Similarly, a Grade 8 bolt is similar in metallurgy to an ASTM A490 bolt. For those of us who do structural engineering, it sounds like the bolts can be interchanged. Not so fast.
The ASTM bolts are the ones that are most commonly used in the steel design manuals for connection design. They are manufactured to some specific, stringent criteria. In addition, they have some geometric differences. First, ASTM A325 and ASTM A490 bolts have larger hex heads, which allow for a greater bearing area on the base metal when the bolts are tightened. Second, the thread lengths are shorter than for Grade 5 and Grade 8 bolts. Therefore, there is a much greater degree of certainty that the shear plane of a bolted connection will not go through the threads.
Fastenal has addressed this issue, with some very helpful illustrations, on their website.
What does this mean to the helical pile community? Designs should be very specific as to the type of bolts used, the lengths of the threads, the location of the threads in relation to the shear planes, and any allowance for substitutions. As an engineer, my recommendation would be to use the ASTM bolts, since that is the default that all civil / structural engineers are taught in college. Plus, the prescribed thread length for those bolts is so short that there is low chance of threads being in the shear plane.
Readers with questions or comments can reach Wayne by:
Phone - 720-347-0054
Email - firstname.lastname@example.org