General: Axial Loads

So axial loads are basically pure compression along the axis or pure tension along the axis without any bending moments, without any shear. So and of course there's no deflection because there's no span typically. So if we look at these examples here, columns are axially loaded, usually in compression because there's a load coming down and transferring that load to the foundation, versus cables. Cables and fabrics are always going to be in tension. Arches are always going to be in compression so that if there is a load here, that load transfers in the direction of the member. So that's what happens in an arch. It goes along the length of the member and down to the foundation. Same with piles. Piles are loaded axially. And so whether they're wood steel or concrete, piles are axially loaded, so are footings. You have a column here sending a concentrated load down to a pad or a footing, and then it is spread onto the soil, and we're going to consider that axial loading. So with the exception of diagonal bracing and bridging, these guys are for lateral loads. Everybody else on this page is for gravity loads. So diagonal bracing is basically across the whole floor to floor, across the height of the floor, versus bridging is basically between members. So bridging is a shorter member compared to diagonal bracing that takes up the whole facade. But in either case, the idea of bridging is to prevent the member from twisting. So we don't want the bottom to come out. And that's why we can put horizontal bridging or diagonal bridging. and depending if this one wants to twist this way then this member goes into compression and keeps it in place basically. The bridging is to keep a deep member this is pretty deep to keep a deep member from twisting so its bottom doesn't move. Its top is braced of course with the metal deck it's just when it gets very deep and slender the bottom could twist so we use either horizontal bridging or diagonal bridging is even better versus bracing. So bracing is for wind or earthquake and what happens here is if an earthquake comes in in this direction then the mass wants to go in the opposite direction and therefore this diagonal brace will go into compression versus if the ground moves this way, then the mass wants to go in the opposite direction, and then that diagonal brace is going to go into tension, axial tension. So the load is going along the length of the member. Axially loaded members are not meant to be loaded perpendicular to the axis. That's for members that are loaded perpendicular. So for example, this brace here, that's not what it's designed for. It's not meant to be loaded in that direction. Very good. Trusses, on the other hand, are going to be either compression or tension. Typically, the bottom cord is in tension and the top cord is in compression. I should watch what I say. The top cord is in axial compression. The bottom cord is in axial tension. And we can read the truss very clearly when a member is chunky in steel. When a member is chunky, it's in compression. When a member is skinny, it's in tension. Very good. But axially, the members are loaded axially. Is there anything I missed here? No, I said everything I want to say. So tracing the loads, clearly, it's triangulated in the case of a truss. Even in this vertical truss system, this is at the road center in New York. It's a 10-story curtain wall. So any kind of wind loading would come in here and push on these pieces of glass. And then it's transferred to the spider connector and then to the truss. And now this face will go into compression and the rest will go into tension. It's just flip it 90 degrees, pretend it's spanning horizontally, and you can see that better. But all members are loaded axially, and in a truss, all joints must be pinned. Otherwise, we're going to have bending moment transferred from one member to another. We don't want that. We're in axial loading. Very good. That's just the definition and some pictures for axial loading. So as far as the math goes, actually axial loads are extremely efficient in transferring load on very little material. Unlike perpendicular loads, span, shear, moment, tension and compression are pretty efficient. So whether it's a diagonal brace or bridging or arches or footing or whatever it is, what we have here is F is equal to P over A, where P is the load. F is the allowable stress, and I need to fix that. Allowable stress either in compression or in tension. So this one depends on the material. How much can the material handle in tension and in compression or in compression before it fails? And that allows me to size or how many square inches of material are needed for this much load on this material. So load per unit area is stress. and stress depends on material, and steel is very good in tension. In compression, it's excellent, but not when it's slender, so in buckling, it's weak. Compression can handle, sorry, concrete can handle compression very easily, no tension. Wood is good in compression and tension, but not as good as concrete or steel. So the properties of the material come in to this equation, and that's how we size an axially loaded member, any one of these on this page. F equal P over A.