FTM 141: How long is too long for Structural Bolts

Is there a maximum allowable length for a structural bolt?

Is there a maximum allowable length for a 1 1/2 inch a490 bolt.

This edition of the Fastener Training Minute with Carmen Vertullo was originally published June 13, 2019 as “How long is too long for Structural Bolts” during episode 141 of Fully Threaded Radio.

The structural bolt business is a risky business to be in.

I often advise people to not dabble in this business: you have to be either all in or all out. But occasionally there are some simple answers to some of the questions relating to structural bolts, and one of them came to me recently where one of our aim test lab clients said this: I have a customer asking “Is there a maximum allowable length for a 1 1/2 inch a490 bolt. They’re asking for 1 1/2 x 18 inches but I’m not sure if this would be considered too long to fall under the A490 specification”. Well,  we are all used to structural bolts having very specific dimensional requirements that should not or cannot be violated, and those dimensional requirements come from the ASTM standard called B18.2.6. ASTM B18.2.6 does not allow us to deviate very much. However the other standards that cover structural bolts have something to add to this.

Well, the question related to when we are asked to provide a structural bolt that does not meet the strict dimensional requirements of ASME B18.2.6, what are we to do? It used to be that it had to meet the spec or you could not sell it. Both the older standards that cover structural Bolts from ASTM, the performance standards,  ASTM A325 and ASME A490 had ways that we could deviate from the standard and the new spec that replaces those f3125 also allows us to deviate from the standard.
There are two ways that we might deviate from the standards. One way is by having an extra long thread, or more thread length than the structural bolt standards would typically provide.

It’s okay to do this, but when we do, we have to add an additional mark to the head of the bolt. And that worked be a “T”. When we put a “T” on the head of the bolt, we are indicating that we have longer thread.

If we’re going to do something else to the bolt, for example to make it extra long, or to have some other dimensional deviation, we simply need to add an “S” to the marking on the head. What that does is it lets an inspector who looks at the product know that if they see something odd, say too many threads sticking out the nut (which would normally indicate the possibility that the nut has driven down against the shank instead of against the steel). they would realize that this was a longer threaded bolt, or a special bolt, though it may not necessarily meet the requirements of the dimensional specs.

So those are sort of get-out-of-jail-free cards that are worth a lot that people don’t know about. So don’t hesitate when, you have to use alternate dimensions or fully threaded or longer threaded link structural bolts to tell your customer how you can do that. These specifications come from supplements in the F3125 specification called “S1”, The Fully Threaded Bolt or Longer than Normal Threads, and The Alternate Dimensions supplement called “S2”, So remember the “S” and the “T” from F3125. You can help your customers get the structural bolts they need.

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