The Insider's Guide to Steel Hardware Assembly

I heard this quote once about “the perfect couple,” and I think it could relate to different circumstances: “Perfect is not when two people are together. It’s when you’re both different but in a way, you complete each other.”

You’re probably scratching your head wondering how in the world assembly hardware relates to “the perfect couple.” Well, don’t worry, this isn’t a blog about 5 Ways to Save Your Relationship, but it is important to know that when it comes to assembly hardware, while they have their different roles, the right hardware needs to be paired with the right bolt.

Just like knowing if you’re compatible or not with someone, you should know which pairings are suitable when either ordering or supplying assembly hardware for steel structures. If the bolt and nut don’t match up in accordance with ASTM standards, you could run into strength problems, requirements issues or over match hardware when it is unnecessary. A low strength nut shouldn’t be paired with a high strength bolt because it would take away the strength of the bolt. In other words, the connection is only as strong as its weakest part. For example, if an A325 high strength bolt is specified for a connection, then you want to make sure the recommended nut and flat washer is supplied to match the A325 bolt’s loading capacity. The same applies if you pair a high strength nut with a low strength bolt, you’ll run into overkill.

In this situation, you end up paying for a more expensive nut when there may have been a more economical option without compromising the loading capacity of the bolt. In the ASTM A563 standard specification for carbon and alloy steel nuts, there is a good nut compatibility chart that shows which nuts are recommended for most bolts. This is also provided in our offer at the bottom of the page, which also includes locking devices and other pertinent information, like available bolt diameters and lengths. This can be helpful when sizing the bolts and selecting the proper locking device because some locking devices aren’t available in larger diameters.

**Just because the bolt you are ordering comes in that size, it doesn’t guarantee that the locking device you selected also comes in that size.

Common terms defined:

Bolt- headed and externally threaded fastener designed to be assembled with a nut.

Nut- An internally threaded product intended for use on external or male screw threads such as a headed bolt or a stud for the purpose of tightening or assembling two or more components.

Locking Device- a device used in conjunction with a fastener in order to positively lock the fastener, so that the fastener cannot work loose from vibrations. 

One of the more common bolts used is the ASTM A325 Type 1 bolt (galvanized). The ASTM A325 specification covers two types of heat treated heavy hex bolts with starting tensile strength of 120 ksi for sizes 1.0 in. and less, and 105 ksi for sizes over 1.0 to 1.5 in. These bolts are used in steel connections, and the bolts are then classified by “type” based on their chemistry.

For the A325 bolt, Type 1 is described as a medium carbon, carbon buron or medium carbon alloy steel. If your project consists of galvanized steel, then you’ll need to order an A325 Type 1 bolt that is zinc coated…and of course it doesn’t stop there. When ordering these bolts, the supplier also needs to know size (including diameter), threaded pitch, bolt length, length of threads, etc. From there, if the bolt and corresponding hardware is ordered as an assembly, the ASTM A325 specification requires a ROCAP test (Rotational Capacity Test). A ROCAP test essentially tests the capaability between the bolt and nut of certain heat lots.

And just to add, for the A325 bolts, the nuts need to be A563-DH because this nut automatically comes lubricated, which is required in the ROCAP test. (Note: You may be able to get an A194-2H nut lubricated; check with your bolt supplier.) In the instance that you need custom thread lengths, A449 bolts are suitable to use in the place of A325.

Some other commonly used bolts in the utility industry include A307, A394 and A36. These bolts are low to mid strength and are galvanized. They are suitable with A563-A hex or square nut (same grades), or a USS washer (F884). To supply a heavy hex nut or hardened flat washer with these type bolts would be over matching. You can supply higher strength nuts and washers with these bolts; however it’s not as economical and doesn’t provide any extra benefits.

For connections that require a bolt with a higher tensile strength than an A325 bolt, an A193-B7 is a suitable option.  These come as fully threaded studs, and require an A194-2H nut.

So, just as important as it is to choose the right match, it’s critical to know what hardware is suitable, as well as their pairings. That’s also where your bolt supplier or fabricator can come in and service you better.