General: Lintels

Let's talk about lintels that are used to span an opening in a load-bearing wall. First of all, a header is the term that is used to span an opening in wood frame construction, and it's usually made of two 2x with a half-inch gap in between. Sometimes they put OSB or plywood in that half-inch, and they span the opening with it. We're not calling it a lintel because a lintel usually carries a lot more load or weight of masonry or something above it. Versus the header, it's just carrying either OSB or carrying sheetrock or something like that with some crippled studs. So it's not very heavy, so we call it a header. In masonry, you can use a lintel block and reinforce it with some rebars here. and then pour some concrete, and you can span an opening that way. Or it could be a precast concrete lintel that is lifted into place, and then continue with masonry above it. If the opening is large, then typically a steel member is used. If it's 10 feet or more, a white flange is preferred. Otherwise, you can use, for brick, you can use an L or an angle and an inverted T. So, what kind of load does the lintel carry? It's not carrying all of these. That's not what it carries. So, I'd like to show you this animation very quickly. Here we have some dowels coming out of the foundation. and then there's going to be a CMU wall running on top of it. Oops. So, here are the dowels. Then they bring some rebars and splice them to the dowels so that they're in the cells of the CMU. And then they put a studded plate in that corner cell and they put a lintel to span the opening and then they continue with the masonry or CMU wall and what happens here is called corbeling. And it's that triangular pattern of blocks or bricks or whatever they are that is sitting on the lintel. Any other CMU other than the red ones is not sitting on the lintel but rather going off on the side to the walls. Let's go back to paper and pencil here. And let's see, this is the image we had, and what's happening here is all of these blocks are sitting on the lintel, and it makes a triangular formation. And this block here is going this way. The red blocks are going on the lintel and then going down here, so they must put some major reinforcing here. And remember, the studded plate is sitting in that corner so we can weld the steel beam to it. Excellent. This wall needs to be wide enough. Otherwise, there's a kick because this is like an arch and it wants to kick laterally. So if you have only one CMU, I'm afraid the wall might topple because of lateral thrust. So it needs to have enough section modulus, moment of inertia, etc. to take care of that lateral force. Very good. So essentially this lintel here is carrying the weight, the dead load of those red blocks. Excellent. Very good. So here is a picture that a candidate sent me that I think is so wonderful. there was this truck backing up and it hit the lintel and the lintel collapsed and we can see very clearly the pattern, the triangular pattern we're talking about depending on the type of bond you have. In CMU, you have a running bond where these are making a 45 degree versus a third brick overlap, then that would be a different angle. But if this opening were 10 feet, then this height would be 5 feet, this would be 5 feet, this would be 5 feet. And so the total area of blocks that are sitting on the lintel would be base times height over 2. The base is 10 feet. The height is 5 feet divided by 2. So that's how much is that? 50 over 2, 25 square feet of elevation. And how much do these blocks weigh? I don't know. Let's say they weigh 100 pounds per square foot. Then this lintel is carrying 100 pounds per square foot times an elevation worth 25 square feet. That's 2,500 pounds. And that would be big W. That would be big W, a uniform, or not a uniform, but a uniformly increasing load worth 2,500 pounds. Very good. So looking at the steel manual and at the symmetrical charts that I gave in a previous video, all I need is big W, for example, and that turned out to be 2,500 pounds. So if you want the maximum moment, it's telling me it's at the middle. I don't need to draw diagrams. I just want a formula. So it's WL over 6, where W in this example was 2,500 pounds, L was 10 feet. Multiply them, divide by 6, you have the maximum moment. So WL over 6 is the maximum moment for a uniformly increasing or a triangular load. We won't see this load except with lintels. Okay, hope this helps.