Hello. In this video, we are going to see which kinds of loads can act on structures. Gravitation is the first of the loads, which is the result from the presence of the structure on Earth. Downfalls, seen in the clouds, come in the form of snow, but also of water, are also a source of loads for our structures. We can see here, on the bottom, circulations of currents, and winds are also a source of loads for our structures, and finally, well, the Earth is subjected to tectonic movements which manifest themselves by earthquakes, and earthquakes are also a big source of loads. Finally, on Earth, we are there, the humans, either individually or in groups, or with means of transportation, and therefore people are sources of loads. Let's have a look on three examples of structures. On the building on the left, we are very certainly going to have snow loads in the winter time. If rainwater falls onto the building, it will be drained away, and generally this will not induce any other loads. Of course, we will have gravity loads induced by the self-weight of the structure. The weight of the structure is thus going to be an important load; another important gravitational load is going to be induced by the structure's users, people. Wind causes important loads on a structure like this skyscraper. Generally, wind loads increase as a function of the distance to the ground, increasing with increasing height. Wind loads generally act approximately horizontally. Obviously, we do not know in which direction because wind can blow in all directions, but it mostly acts horizontally on the facades. We are going to deal with soil's movements linked to the earthquakes, they are alternating movements, that is why I drew a double-headed arrow. Earthquakes induce on the structures effects which are similar to the ones of the wind. Unlike what we often think, it is the ground which moves but the higher parts of the building will be subjected to very, very substantial loads. And then obviously, for all these forces to be in equilibrium, we obviously have the action of the ground on the structure. It is necessary for equilibrium and it is also something which is going to vary. If now we look at a dam. Well, behind this dam, there is water which pushes, according to the hydrostatic pressure which linearly increases, then we have the water pressure. This dam has also obviously a big weight, it is a big mass of concrete, then it has also a self-weight. In addition, in the presence of people, it is possible to have people moving on and inside the dam, or the influence of snow are much less significant for this kind of structure which, however, will be sensitive to the effect of earthquakes and then, on which the action of the ground is obviously necessary. Finally, we have a pedestrian footbridge, and we have as loads the self-weight of the structure, probably a little bit of wind load, this is a footbridge which is relatively close to the seashore, and then obviously, we can observee it, the weight of people. Of course, we will also have to take into account earthquakes and we will have the action of the ground which will act on the structure. A pressure which is often mentioned when I ask students what kinds of forces act on structures, namely the atmospheric pressure. It is important to understand how it is different from the water pressure that I have shown before. The water pressure acts because on one side of our dam there is water whereas there is none on the other side. Thus water pushes on one side, and on the other side, there is nothing to push. If we had water on both sides, well, the water pressure would not have any effects because the water pressure would act on both sides. It is like if I put my hand in the water, I sketch my hand in this way, well, I will have a water pressure which acts in the same way everywhere. On the contrary, if there is water on only one side of my hand, then I will feel the water pressure. It is similar with the atmospheric pressure. A body is subjected to the atmospheric pressure above, below, on the left, and on the right, and this pressure offsets itself in such a way that we do not feel the atmospheric pressure, and structures, generally, do not feel the atmospheric pressure either. The case where it could be felt it is if we had a structure in which we would wish to create a vacuum, because when we have a vacuum, we precisely do not have the atmospheric pressure anymore. It is rare enough. It should be noted that the atmospheric pressure is a very significant pressure, since it corresponds to the pressure of 760 milimeters of mercury, or if we refer to water, of more than 10 meters of water pressure. And 10 meters of water pressure corresponds to a pressure of 100 kiloNewtons per square meter, which is huge. But fortunately, we do not have to take account of it for our structures. We have two big families of loads, on the one hand the loads which are permanent, which are always there. So, what is always there ? Well, obviously, the self-weight, and then what we call permanent loads, for example, in a building, it would be all the things which are always there, like the partition walls, the floor coatings, the equipments, but which do not have a structural function. For example, if we put a layer of marble on the floor, it is not a component which contributes to the strength of the structure, but it has a weight. The action of the ground is also a load which is of an essentially permanent nature. Among the variable actions, that is to say which are sometimes there, but which are not always there, we have the effect of people, and this also applies to the effect of automobile traffic, or of rail traffic, either there is a locomotive on the bridge, either there is nothing, the wind effect, the water pressure effect, for my dam, maybe the water is there, the dam is full, but maybe the water is not there, and the dam is not full, it is then a load which is of a variable nature, and obviously, the seismic load is a load which is only very occasionally there , during an earthquake, otherwise there is no seismic load. We will see thereafter that it is important to make the distinction between these two kinds of loads because we do not deal with them in the same way regarding safety. In this lecture, we have seen various types of loads which act on structures, some loads are of a gravitational nature, some loads are of a climatic nature, like wind, snow, which becomes gravitational loads. We have seen the effect of a pressure, knowing that the atmospheric pressure is not a load which acts on our structures, but the water pressure behind a wall is one of them. We have seen seismic loads and the actions of the ground on a structure, and in the end we have seen that it is important to distinguish between permanent loads and variable loads.
