Concrete: 9. Concrete Beam Reinforcing Concepts

This video illustrates the reinforcing of a concrete beam. Before we start with the reinforcing of the concrete beam, I'd like to discuss briefly the shear and bending moment diagrams of a continuous concrete beam that runs on top of multiple supports. This is not the topic of the video, but still I want the concept of the shear moment diagram to enter this conversation. So, the maximum shear in a beam, be it continuous, be it simply supported, be it cantilevered, the maximum shear occurs at the support. In this case, the maximum shear is occurring here, here, here, and here at the supports. Yes, there is a maximum shear here also, but it's less in value than the ones the middle supports. The concrete and the stirrups are in charge of shear. So to take care of this much shear, the concrete section, depending on how big it is, how deep and how wide, the concrete section provides a certain amount of resistance to shear. And if that's not enough, the stirrups have to kick in and provide some additional shear based on the location along the span. So the concrete and the stirrups are in charge of shear. And on the other hand, the rebar and the concrete are in charge of the bending moment. So the maximum shear occurs at the support. And the maximum bending moment occurs between the supports. So there's a maximum bending moment positive here, here, and here. There's a maximum negative moment on top of the supports. So whereas maximum shear is critical at the supports, it's really the absolute value of shear that is important. positive or the negative is less consequential in this discussion. But that's not true in the case of bending moment, because a positive bending moment means the beam is bending like that, versus a negative bending moment, the beam is bending like that. And that's significant, because I need to know where the compression is, because the concrete can handle the compression. And I need to know where the tension is because that's where I need to place the rebar. So in a positive moment, it looks like the bottom is in tension and I need to place rebar on the bottom. But in a negative moment situation, it looks like the tension is on the top. And so I need to place rebar on the top. So at the point of inflection, that is right here, the bending moment is zero, which means there is no compression, there is no tension. Every time the moment diagram crosses the zero line, it's changing from positive values to negative values or reverse from negative to positive. But at the point of inflection, the bending moment is zero. So that's in concrete, in reinforced concrete, that's where the rebar is flipped from top to bottom or bottom to top. Okay, so that's the conversation I wanted to have on shear and bending moment diagram with respect to reinforcing in concrete. In essence, the shear diagram tells me how to space my stirrups, And the bending moment diagram tells me how many rebars of what size I need and where I need to place them on the top or bottom of the beam. Okay, so let's get into the actual reinforcing of this concrete beam. And how about we not even look at the shear and bending moment diagram. Instead, why don't we look at, let me pick a good color, why don't we look at the deflected shape. Remember, the deflected shape and the bending moment diagram are related to one another because bending moment causes deflection. So I'm going to draw an exaggerated deflected shape for this concrete beam. And from this shape, I'm going to go in. I'm not going to look at the bending moment diagram or the shear diagram. I'm just focused on this deflected shape. And I'm going to pull out my blue pen to represent tension. And I'm going to recognize that on the bottom of this beam here, the deflected shape is stretching. Every time between the supports, the bottom is stretching. And, of course, that is indicative of a positive bending moment. That's what it means. The bottom is in tension. So let me erase these. But that's what that means. So we have tension on the bottom between the supports. But then over the supports, it looks like the tension is on the top from this deflected shape. And then on the overhang, it looks like it's on the top as well. So the blue dot is representing rebar. Wherever we have a blue dot, we're going to need rebar. Wherever we have a red dot, that's compression, and the concrete is capable of handling the compression on its own. So between the supports, the bending moment is positive, which means the top is in compression, the bottom is in tension. The concrete handles the compression, the rebar has to do the tension. And that's what happens between the supports. But then on the overhang, the bottom goes in compression. And over the support, the bottom goes in compression, the top goes into tension. And this is telling me I need to flip my rebar to the top on the overhang and over the columns. And then I need to place the rebar on the bottom between the supports. And this point, the point of inflection, is where I will flip my rebar from bottom to top or top to bottom. So let me go ahead and place my rebar. Here's the depth of the beam. Here's the depth of the beam. exaggerated the depth so I can place the rebar and I left the columns very skinny. Very good. So looking at this blue dot, it's on the bottom of the deflected shape, it's on the bottom of the beam, which is telling me I need rebar from point of inflection to point of inflection. Let me increase the pin weight a little bit on this one. So I need rebar here. I need rebar on the bottom. I need rebar on the bottom. Now the blue dot here is on the top and I need to place some rebar right there because there is a negative moment because the top of the beam is in tension versus between the supports the bottom is in tension. So it looks like I need another piece of rebar here, and on the overhang, I need rebar over there. So on the other overhang, I need rebar over there. So at the point of inflection, I need to flip my rebar from bottom to top or top to bottom. Now anybody with good experience in concrete will tell you, you cannot stop a rebar and start it right away. You will get a crack right there immediately. So that's not a good idea. So the ACI code, the American Concrete Institute code says you have to overlap that point of inflection by a certain number of bar diameters. So I'm going to go back and inflect my, sorry, overlap my point of inflection. Sometimes we used to see rebars doing that in the past, which is bridging that point of inflection. Right now, I'm just going to add the rebar and make it overlap that point of inflection every time. Okay, so now I'm satisfied with this, and the rebar at this location would be on the bottom. I don't know how many bars I have, but they would be on the bottom. Let's say I have three rebars. But on the overhang, the rebars would go to the top. And please remember, blue rebar is for tension. And on the ARE, you need to place the rebar in the tension areas, which is what this diagram is about. Now, of course, in the real world, nobody does that. Everybody does the following. Let me erase here very quickly. So, delete this. Sorry about that. But I'd like to reinstate one thing only, which is my deflected shape, because really that tells me everything I need to know. So, there we go. I redrew that guy. Okay. So, actually, what happens in the real world is you have rebar running, continuous top and bottom. But actually what's happening is the rebar in this area is doing blue work. In this area it's doing tension. In this area it's doing tension. In this area tension. In this area tension. In this one tension. And tension. Now the truth But in these areas, the same rebar is doing compression. So we run rebars top and bottom. But for the ARE, it's the blue locations or the locations of tension reinforcing that we need to emphasize. Let me go back and put my rebars in the beam. We said here it would be on the bottom. And here it would be on the top. actually there would be some rebars on the top as well but they're doing redness or they're doing compression in this area and in this area they're on the bottom also doing compression now i don't need the red rebars i don't need the red segments of the rebar because the concrete is a big bore it can handle compression. But if I do run those bars continuous, it's more economical. It's easier to lay the rebars for the workers. And most of all, if there is a red portion of the rebar, if the rebar is doing compression here, well then I'm going to need less concrete. So it's sustainable also, because the rebar is helping take some of the compression off of the back of the concrete, so I need less concrete. So it's a smaller section, it's less dead load, it's smaller foundation, so it makes perfect sense to put the rebar also in the compression zones instead of chop it up into small pieces and put it only in the tension areas. Very good. So that's the story of the rebar, and the story of the rebar is told by the moment diagram. And wherever there is tension, we need rebar, and we're helping the concrete with red rebar, or rebar in the compression zones. Now, we said earlier that the shear diagram is responsible for the stirrups, sorry, the concrete and the stirrups are responsible for the shear. So if the shear is high at the support, I expect the stirrups to be closer at the support. So let me put some stirrups here. This is the face of the support, and I need to run my stirrups at the face of the support. So the contractor will look at the drawings, and we'll see that the stirrups are placed, let's say, every 12 inches on center. So they'll put stirrups every 12 inches on center. and then they'll go back and take another look at the drawings and the drawings will say, place the stirrups every 12 inches on center except for the first 2 feet, for example. Maybe put them at 4 inches on center, which means there's going to be a lot more stirrups in the beginning and much closer in the beginning and then less spacing towards the middle because shear is zero over here. If shear is zero means you don't need as much stirrups. The concrete can handle it on its own. So what I can do actually is take out a couple of stirrups from here if I need to, and run whatever it is I need across there, because shear is low, so I can take out the stirrups, but I cannot touch the stirrups at the support. I need all the stirrups. So let me run the rest of the stirrups in here. I'm going to tighten up at the support, and then gradually I can space out, and then tighten up at the support again, and then gradually space out. So tight at the support, and then less so towards mid-span. The same with shear studs on top of a steel beam. You place them very close at the support, and then you can space them out a lot further. So at the support, we have a lot of shear, so we'll put a lot of stirrups. And then towards the end of the overhang, we don't have as much shear, so we will reduce the spacing. I always got this question when I was teaching this topic. So I'll go ahead and answer it anyway. Do you have any stirrups over the column? Actually, that concrete over the column belongs to the column. It's not part of the beam. The beam span is from here to here. Over the column, there's compression. It's part of the column. And so the reinforcing of the column is probably going to go in there. So I have reinforcing for the column. And the reinforcing for the column is going to go up in there. So I don't need the stirrups. Okay, so let me just color the stirrups on this sectional drawing, and that's it. So here's the stirrup, and its spacing is dictated by, sorry about that, is dictated by the shear diagram. So here's the stirrup, and here's the stirrup. Very good. So that concludes this video. So here's a summary of notes for the presentation on reinforcing the concrete beam. Please read carefully.