教学方

Olivier L. Burdet

Dr

脚本

Hello. With this video, we start dealing with a very important topic : slabs which are a very popular type of load-carrying systems nowadays. In this video, we will see what types of slabs exist and we will classify them according to the type of their supports, either linear, either point-type. Here, on this picture taken about ten years ago in Europe, we can see two buildings under construction whose load-bearing systems are fundamentally different. These two buildings rest on columns, on the left, they are square, on the right, they are circular, but that is not a significant difference for us, neither for the structure by the way. We have vertical columns for both systems. We can also see that the diameter of the column increases according to the story where it is located, which is logical, the largest columns being in the lower part. Then, if we look on the left, we have in a classical manner, as we saw in the previous video, beams, which probably have a shape like that, a kind of inverted T, and this was made to facilitate the setting up of the concrete boards were placed on top of them thereafter, as it can be seen on the left part. Here, we have a system, again, with precast concrete boards; we can see that they have a certain width, and there are several which are visible. There is the same thing on the upper story: that is again a classic precast construction. On the right, on the opposite, we only have one support, which is directly the slab which carries everything through its own thickness. Here, on the upper floor, we can notice that the ground floor's slab is much thicker than the upper levels' slabs: that is probably due to its function -maybe that is a shop, nay a public place such as a restaurant or something like that- and that is logical that the loads are larger. On the right, we have what we call a floor slab, while on the left, we really have a slab on linear supports, the linear supports being the yellow beams which we have previously identified. We are going to see that in more details here with examples of slabs on linear supports; once again, on the left, the system which we have seen several times, that is to say a systems with one or several main beams in one direction, which carry secondary beams in the other direction, which, themselves carry boards in the third direction. All this is a system, and they are slabs on linear supports for example, on bi-directional springers, here, but we can also find other solutions: the linearly borne slabs, with beams in only one direction -beams or springers, that is the same thing- and then a slab on a wall, a very classic example, at least in Switzerland, for accommodation buildings, where we have concrete or masonry walls on which we have relatively thin slabs which simply carry loads between these walls. On the right, we have the case of an industrial building, where the springers, in both directions, are constituted of steel elements with hemstiches enabling to make pass a lot of installations underneath the slab. Another possibility is to have point-type supports under the slab, the clearest case being the case of the floor slab which I showed you before. We only have one column: here, it has a small plate on top of it, and then the slab is directly supported on this column. However, there are internal forces concentrations on top of these columns -I will talk about it in one of the next videos- and a solution to avoid this concentration, or at least to more efficiently spread the internal forces, is what we call a mushroom: here, it has a shape which is quite close to the one of a mushroom, but that is sometimes more schematic. The advantage here is that the surface on which the slab leans is much larger, so the internal forces are more widely distributed. We can get a solution similar to the one of the mushroom with an additional thickness of the floor slab, that is to say that the slab is locally thicker, because the internal forces on top of the column are quite significant. Finally, we have what we call a cassette slab, or else an voided slab or else a ribbed slab, they are all synonyms, but as we can see it on the picture, that is a slab in which we have a beam grid, except that on top of the columns, the slab is solid, because the internal forces are larger. That is a quite frequently used solution. In this introductory video, I have shown you how we can classify slabs, distinguishing between slabs on linear supports, that is to say walls or beams, and slabs on point supports, which are directly carried by columns.