Geometry Of Sections: 1c. Stress Bearing & Thermal

So far we spoke about axial stresses, axial tension and axial compression. We also spoke about perpendicular or normal stresses, which are essentially shear stresses and bending stresses. So in this video I would like to look at some other stresses beginning with bearing stresses. Let's see what this one is. So in a bearing wall, for example, we have these wood floor joists sitting on top of a wall. This one is considered a load bearing wall and there is a certain minimum bearing length required so that those joists don't fall down. So when a wall or a beam or whatever it is is perpendicular to these loads, then it is considered a load-bearing wall. If it is parallel, if there were a wall here, that one is parallel to the joist. That one is not load-bearing. In this example, we have these open web joists framing into a load-bearing wall perpendicular to the joist. then there's a load-bearing wall versus this wall, this other wall here is parallel to the joists. It's not carrying any of their load. These joists clearly are sitting on this joist girder, and the joist girder, yes, is sitting on that wall, and so we need local reinforcement here so that that portion of that CMU wall would act like a column. Very good. So bearing is basically the area above the column. In this example of wood beam notched and another wood beam notched, the area above the column is called the bearing area. And the span begins over here. But in this case, since there is these diagonal braces at the corner, at the support, the span probably begins at these points from here on out. That's where the span begins and the shear becomes critical. Looking at this image here on the right, the support has a haunch in it. And this haunch is where this upside down beam, precast concrete beam is sitting. So that area is the area of bearing. And your span of this member begins here. The span begins here. Looking at the joist itself here, there's a concrete joist sitting on top of that upside down T. that's the bearing area. So again recapping the contact area between this beam and the haunch that area over there is considered the bearing area and it has to be a certain minimum otherwise the beam falls off the haunch. The same with these joists the joist is doing that well then that's the bearing area. Very good. So once there is a clear span, that's where shear begins. But over the support, we have bearing stresses. Excellent. Sometimes when you have a beam sitting on a load bearing masonry wall or something similar, they will place a beam bearing plate and it's exactly as its name says. It takes the load from the beam and spreads it over to this red area, the area of the plate that is anchored usually in the masonry and the load is spread onto that load bearing wall. We talked about this image before and we said there is the bearing area. It's that much, it's that deep and it comes down. That is the bearing area of the concrete precast beam sitting on top of the support. With open web joists, the joist manufacturer recommends a certain amount of bearing area, which is the overlap distance pretty much that the joist has to sit on its support. And we can see over here in this image, here's the top of the beam. Here's the top of the beam, and we see a certain amount of overlap. that distance, sorry, this member here, this portion of the member is in bearing, and then the span begins, and that's where we get shear and bending towards the middle. But at the support, the shear begins here, and the bearing is whatever's over the support, bearing area. Okay, what else do we have here? Good, we talked about these. Next, for example, column base plates. The load from the column, whether it's concrete, whether it's steel, whatever it is, is spread to a base plate typically, and then from the base plate to the concrete. So here's the construction process. For a slab on grate, for example, they're going to pour a slab on grate, but they will leave a block out, as it's called, because the column has to sit on its foundation. The slab on grade is not carrying load, but the column itself is carrying roof load or floor load or something like that, so it cannot sit on the slab because it'll punch right through it. So instead, the column has its own footing. So looking at this detail, here's my steel column, and it has a base plate, there's anchor rods in the concrete and there's leveling nuts here that make the column plumb in the north south and east west direction and once all of that is set the column comes in over here in this picture the column comes in with the base plate and the anchor rods are sticking up from the foundation of the column and then they put the leveling nuts they level the column and then they put some non-shrink grout and then they can pour this hole separate from the slab it can be done either before or after very good so column base plates also in foundations, we have typically bearing stresses. So in the case of a pile or a pier in this case, a pile is driven. These are piles. They're driven by a pile driver. These guys look like they're cast. They don't look like they're drilled. They don't look like they're driven. Sorry, they are drilled. Okay, so if a pile does not reach a firm soil, then it is considered a friction pile, and that end area, that end bearing area that we see here in red is not sitting on firm soil, so it's just purely a friction pile, versus if it reached firm soil, then you get friction and end bearing. The end bearing area is the cross-sectional area, which is shown here in red, which is basically pi r square, or whatever that dimension is for this concrete pile, just square it length times width, or if it's a square, it's a side times side. Very good. So friction piles. Then there is more discussion a little bit on piles. Piles could be wood, could be concrete, or could be steel. In this image here, they're driving a steel pile, an HP pile. HP profile is for piles typically. It's driven in the ground. there's five piles over here that have already been driven and then let's look at the animation I think I can explain that better over there so here's this animation the pile driver is driving the pile the steel pile into the ground the top of the pile of course is totally mutilated so they go back and they trim them or they cut them with a torch and then they put a studded and the rebar they tie everything together and then there's they pour the concrete and leave anchor rod sticking up so that then here's a description of it this one is a cast in place cast in place a concrete footing so you have a footing here you have some dowels sticking up from the footing and these dowels are cast into the concrete sorry they are cast into the concrete footing and then the column formwork is shuttered and the column reinforcing cage is brought in and tied to the dowels. And then the column transmits its load to the footing. And from the footing, it used to be just that much area, wherever the column was. Now the area is much larger. So now we have a situation of the column sent its concentrated load to the concrete footing, and then it transmitted it as a uniform load onto the soil. Here we can see the process where the dowels are sticking up of this big concrete footing. We can see a different situation here where also there's anchor rods coming out of this concrete footing that is going to be poured. And here it is. After it's poured, there's the concrete footing, sticking out of it are four anchor rods to which a steel column and base plate will be leveled and grouted. Very good. So bearing stresses also are important in retaining walls where settlement might happen. And if this wall is settling, then there's this crack here, or maybe this part of this retaining wall is settling and this part is sitting on a little more firmer soil so it didn't go down as much and so we got a diagonal crack in the masonry. Same thing here, we have a strain gauge here. They're checking on that crack to see how often, not how often, but how quickly that hole is gaping. Anyway, other stresses that we need to talk about include thermal stresses. So here we see this steel column. Water got into the steel column during construction and then it froze and it cracked the steel. So you got to give room for thermal movement, expansion, contraction. That's why in this handrail, for example, there is this gap. It's not bad craftsmanship, but rather it allows some expansion and contraction. There is this pedestrian bridge on my daily walk a few years ago. It's 100 feet long. And if you notice here, the structure is shy of the concrete. There's a little gap here. And then there's a cover plate that is attached only on one side to the deck. So that if it moves, if the deck moves or expands 100 foot is a long dimension. So it's going to expand and contract quite a bit. And it's fastened only to the bridge. You notice that there's nothing on the other side. So that when it expands and contracts, the plate goes with it. And we can see that here, there's a little bit of a marking on the concrete. to show movement of the bridge. That's why you have an expansion joint, especially in concrete. It expands and contracts a lot. It has a high coefficient of thermal expansion contraction. So we need to separate this handrail from that handrail with a gap. And this gap separates segment one from segment two. There's a gap and there's a soft material so that if the bridge wants to compress a little bit, it compresses a soft material. If it wants to expand, it has room to expand. Very good. So let's go back and look at this other animation. Here it is. So this is a highway girder that is probably spanning six lanes, so that's quite a distance. and it's made of steel and it's going to expand and contract. So it has to be seated on some kind of connection that allows the movement. So we can see here that if the temperature drops, that roller support allows movement. If the temperature rises, it allows movement in the other direction. So it is critical in this situation to have a roller on one end to allow for a roller or a rocker, to allow for thermal movement of this beam. But on the other end, you cannot have a roller. It's got to be pinned, otherwise the whole thing is rocking. So roller on one end for long span structures, but pin on the other end. cannot have both pinned because if it wants to expand it's going to bow up and break everything so we allow it to move on one direction sorry on one side where the roller is i hope this was helpful