Footings | Why are they used?

be it burj khalifa the pentagon or your house the weight of these structures is ultimately borne by a structural element called a footing a design mistake in the footing can result in the collapse of the whole building which is why a proper foundation design is crucial footings are the most important part of the foundation their job is to distribute the heavy load of the building concentrated on the columns to the wider surface area of soil to achieve structural stability let's analyze the importance of footings and their different types come along before understanding the design concept of the footings first you need to learn something about the soil more specifically engineers should analyze whether or not the soil at the particular location is capable of bearing a calculated amount of load if the soil is compressible under loading excavation is done until and unless you find the incompressible layer which can stand the loading you can see the plate load test on this layer is not causing much compression to the layer this layer of soil in the physics of civil engineering is called hard strata on this hard strata we can add columns to transfer the load and cover the pit with soil with this process the soil should be able to bear the entire structural load quite easily however this concentrated load on the columns will cause excessive stress in the soil due to lesser contact area if this stress exceeds the soil's bearing capacity it will undergo the punching effect which leads to imbalance in your structure and eventually cracks or even failure of the whole building to avoid these failures you can increase the contact area between the soil and columns let's introduce a plate member that is the footing at the base of the column the load on the column will now be evenly distributed on the soil through plate members this increased contact area reduces the stress on the soil and the building stands strong the main job of a structural engineer in a footing design is to find out the area and thickness of the footing these two factors are determined according to the bearing capacity of the soil present on the site and weight of the building we have already seen the importance of the footing area more the weight of the building wider should be the footing additionally if this structure is subjected to lateral forces like winds floods etc footings will help your building avoid sliding as a result of frictional resistance between the footing and the soil now the second design aspect the thickness of the footing the footing has to withstand pressure applied by the soil as well if the footings thickness is inadequate the footing on either side of the column will bend due to the soil applying uplift pressure on it this effect is called applied bending moment you can see this effect in this fea result here to prevent this result the thickness of the plate is increased there you go we have designed a stronger and safer footing for our building this type of pad or isolated footing is employed to support lightweight houses that rest on soil with medium strength this design can be further optimized economically if you observe the pressure applied from the soil on this area it is uniform however applied bending moment increases as we come to the center zone of the footing thus the need for footing thickness in the center is greater than at the edges therefore a trapezoidal footing shape has been adopted in modern structures for isolated footings however a pad type footing is used in other more complex types of footings suppose your building's columns stand exactly on the edge of the property line obviously your neighbors won't allow you to take up extra space simply to extend the footing in such a case the column is placed on the edge of the footing causing all loading transfer on the soil to become unbalanced or eccentric this footing can carry a considerable amount of load on the structure however if the load increases beyond a certain limit the structure may fail the trick to solving this problem is transferring the excess loading into another adjacent footing with good strength this transfer is accomplished by placing a connecting beam in between which acts as a strap one of the columns in this strap footing will generally have an eccentric footing moreover in some cases when the columns are too close to each other engineers purposely combine the footing in most modern heavy buildings the structures columns are provided with combined footing as a whole a single reinforced concrete slab is made for each and every column these footings are called raft or mat types and they spread the load evenly imposed by a number of columns or walls on the soil this type of footing is adopted to avoid deep foundations which would otherwise go around 60 to 200 feet deep in an area with expansive soil the raft foundation is a boon that lessens the chances of differential settlement the burj khalifa uses a combination of raft foundation and piles which forms a combination of shallow and deep foundation we'll learn more about that in the next video do remember to be a member of the lessex team thank you

Channel: Lesics

Views: 4,954,611

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Id: ZIKSAJ8B07U

Length: 5min 57sec (357 seconds)

Published: Fri Oct 22 2021