Steel: Bolts Terminology

Although bolts and welds are really the specialty of engineers, a little bit of an overview and some terminology would not hurt us architects. Okay, there is two grades of bolts, rather high strength bolts because there is an A307 and an A307 is not for structural steel connections but rather light duty work. So we have an A325 and an A490 grade bolts. We have to understand from the stress strain diagram that wood looks like this, concrete looks like this, steel structural shapes look like this, but then bolts look like that. So they have a lot of carbon in them and they're very strong, but of course they're brittle when you have a lot of carbon in there. And who needs a malleable bolt? We need a strong bolt. So the two grades of bolts are essentially A490 and A325 and then A490 has a lot more carbon in it. There is also a configuration called a tension control where this spline breaks off when there is the right amount of tension in the bolt to clamp the pieces of steel that are being connected. So we have something called a shear connection where there is one plane of contact versus a double shear connection where there's two planes of contact. That's basically the configuration of the joint. So looking at this example here on the right, there is one plane of shear over here. There's another plane of shear over here. So this is a double shear connection. But at the same time down here, this plate stopped here. So there's only one plane of contact. So this is a single shear connection down here. Other things to talk about is the type of connection. It could be a bearing connection or it could be a friction connection. And a friction connection is a slip critical connection. and those are used in bridges, in large infrastructure, but also in seismic zones. Slip critical means the bolt does not touch the plate or the hole of the plate versus a bearing connection is where there will be contact with the bolt and the wall of the hole. So a friction connection is so tight that the force is transferred through the feying surfaces by pure friction. In a bearing connection, there's two types. There is either a type N connection or a type X connection. And it's all about the threads. Are there threads in the plane of contact? In this case, it's straight on the shank, not on the threads. Therefore, this is a type X connection versus in this case, there is threading in the plane of shear. And it's called a type N or the threads are included versus the threads are excluded. I'd like to discuss this drawing here. So when this beam is loaded, these bolts here are in double shear because there is a plate here and there's another plate on the other side of the web of this beam. So that's similar to what we have here. It's a double shear connection. But then now looking at the angle to the web connection, there's only one plane of shear. And so that's a single shear connection versus the other one is a double shear connection. Now, if there is a pair of angles at the bottom and there is a plate here, again, if there's bolts passing through one, undo, sorry, pen size right here. So, one plane and another plane of shear, so that's a double shear connection. But then these bolts here are being pulled. So they're in axial tension. So typically bolts are in single shear or double shear. Sometimes they are in axial tension. Now, to culminate, let's look at this example here from one of the animations that is covered in another video at more length. When this beam is loaded, when this beam is loaded, And it's supported on the other end. It's not necessarily cantilevered. Then what wants to happen is this wants to pull out. And this flange wants to push in. And they're making a couple. And as this flange wants to, it wants to do that. So it is kicking in over here. It's pulling out over here. And that's called a couple. So it looks like these bolts are in tension. They're being pulled out, these two bolts, versus these two bolts are being pushed in. And anything on the web is in shear. So these bolts, these four bolts, are taking the load. Let's say the load is 20 kilopounds. Then each one of these is going to take 20 divided by 4. And these guys are in charge of preventing rotation. If these two bolts are pulling out, then this plate is trying to keep them from going. So this one is in tension. And if these two are pushing in, then this plate is pushing back. and this plate is in compression. So it's all about trying to figure out how the beam rotates or moves up down when loaded. That's what determines the type of stress in the bolt. I'm going to do another quick video to talk a little bit more about this, but this was just general introduction to terminology of bolts. A307 is not a structural bolt but A325, A490R and there's single shear, double shear and there's axial tension. There is a friction connection and it's called slip critical, not allowed to slip. If it does slip, if it's designed to slip, then it's called a bearing connection and there's two types of bearing connections on the threads, that's a type N, and on the shank, not on the threads, that's a type X.