 Today, we will going to see what is mean by column base and how to design column base, particularly we will see how to design slab base. So, at the end of this session students will be able to understand what is mean by column base and the design of slab base. So, first of all we will see why do we require column base. Here we have one I section which is a column and we have to transfer this load from this I section to the concrete footing. So, for this we are having the load is very much higher and the bearing pressure of concrete is very much less which is of the order of 0.45 fck and here we are having the maximum load. So, for the small area this will going to be fail by punching. So, we have to increase the area and for that we will use base plate. Basically, we have to increase the area and to increase the bearing area will go for base plate. So, the concrete footing will going to remain safe with the help by using the base plate. Basically, there are two types of column bases. One is a slab base and another one is our gusset base. Its suitability is slab base is used for the light load and gusset base is used for the heavy loads. The bearing pressure here we get is a uniform bearing pressure here we do not get a uniform bearing pressure. The full load is transferred by the bearing concrete that is full load is transferred to the concrete by bearing action and here 50 percent load is transferred by the gusset plate and 50 percent load is transferred by the bearing action. We have to design this gusset plate and gusset angle here we will provide a nominal plate angle for the bolted connection or if we use welded connection we will weld this section. For light loads we will use the slab base for heavy loads with movement or eccentric load we will use the gusset plate. Load is transferred by 100 percent bearing load is transferred by 50 percent gusset plate and 50 percent by the bearing action. We are particularly see what is mean by slab base we will see its design steps and after that we will see the gusset plate base. So we will see some theoretical background it is slab base it is assumed that the maximum bending movement occurs at the edge of the column. Here we will see this is the edge the maximum bending movement will occur at this edge of the column. As the slab base slab tends to bend simultaneously about the two principle axis of the slab the stress caused by bending about one axis is influenced by the stress due to bending about the other axis and due to this effect we will use an poison ratio of 0.3 to account this effect. Now we will see the bending movement how to calculate the bending movement. First of all we will take a 1 mm strip of the slab projection along the xx axis. Along the xx axis means parallel to xx axis this is our xx axis this is our xx axis. Movement is load multiplied by length it is a udl multiplied by l by 2 we will get the movement so w a square by 2 and along the yy axis means parallel to yy axis we will get the maximum bending movement will be equal to w into length w into b acting at a distance of l by 2 which is b by 2 which is equal to w b square by 2. Here we get the next movement mx coming w a square by 2 major about xx axis and considering the Poisson effect we will subtract some value its effect on another axis w a square by 2 minus 0.3 w b square by 2 this is our Poisson's ratio and taking w by 2 common we will get this value w by 2 into a square minus 0.3 b square. Now the movement capacity of the plate mp will be equal to 1.2 fi into ze where ze is the elastic section modulus of the plate. So for 1 mm strip the elastic section modulus will be equal to 1 into t square by 6 will put up some all the values which is equal to 1.2 fi into t square by 6 where t is the thickness. Now previously we get this value w by 2 a square minus 0.3 b square and we have considered the movement of the plate that is the movement capacity of the plate which is equal to 1.2 fi into t square by 6 will equate both these and we will find the value of t but before that we will apply a partial safety factor for material which is gamma m0 and then we will equate these two values to find the value of t. Here cutting all the values it is divided by 2 which is equal to 3 then we will it is going to in the numerator it will come into denominator so t square will be equal to 6 by 2 into w divided by 1.2 a square minus 0.3 b square after solving this we get t square is equal to 2.5 w into a square minus 0.3 b square gamma m0 divided by fi now t will be equal to under root of this section we will get under root 2.5 w into a square minus 0.3 b square into gamma m0 divided by fi this is partial safety factor for material this is the yield strength this is the smaller projection this is the longer projection we will see each and every term this is now we will see the design procedure. First of all we will assume a suitable grade of concrete whatever we are using we will take that grade of concrete and depending upon the grade of concrete we will find its bearing strength that is the bearing strength of concrete which is given by 0.45 fck a reduced value of 0.45 fck is used we have the maximum value as 0.6 fck which is recommended by the code but code says we have to use a reduced value which is equal to 0.45 fck we get the value of stress now to find out the area what is mean by area is nothing but load divided by stress where p is the factor load of the column the load which is coming on the column and the bearing strength of concrete which is the stress available stress or the permissible bearing strength of the concrete we will get the required area of the base plate we will see each and every term here where l is this one l l is the length of the base plate b is the width of the base plate d is the depth of the column this a is the longer projection of the base plate and b is the smaller projection of the base plate we have got the value of area now to find out the sites we will assume the square base plate and the length and width of the square base plate is calculated by this equation and to achieve the economics and designers feel that projection should be equal so they maintain a which is equal to b if you put a equal to b we get some value we can find out the value of a and b by this equation d is the depth of column b is the smaller projection bf is the width of the flange a is the longer projection and area we will assume a equal to b and we will find out the projection we get this from column we will see design steps now the intensity of pressure we can find out by this equation w is equal to p by a 1 where a 1 is nothing but area of the base plate provided in mm square if you put this value in mm square we get the value of w intensity of the pressure from concrete under the slab base in Newton per mm square we have got we will compare it with the bearing capacity of concrete if it is less then it is ok after that we will find out the thickness now the minimum thickness of the base minimum thickness t of the slab base is calculated from the equation number 6 it should not be less than the thickness of the column flange what is mean what is equation number 6 we will see this equation number 6 we will calculate the thickness of the base plate with the help of this equation and we will check it with the thickness of flange if it is less then it is ok and for the slab base will provide 2 to 4 numbers of holding down bolts of 20 mm diameter when the base is subjected only to the axial compressive force 2 bolts will be enough if it is not subjected to the axial compressive force we will provide 4 holding down bolts and welded if it is subjected to some movement then welded joint we have to design for the column and the base plate if the column and base have machine end and perfect bearing is ensured the axial load is assumed to be transferred directly and welding is designed for moments only however if the perfect bearing cannot be ensured the column should be welded to the base plate for all the forces that is we have to design the welding if it is not if it is subjected to perfect bearing then we have to just provide the nominal welding. Next it is assumed that the bearing pressure are uniformly distributed all over the contact surfaces the bottom of the column and the supporting surface of the base are machined so that the complete compressive load is transferred to the plate by direct bearing this is the base plate. Next we will see some review question you can pause the slide and answer these questions the thickness of the base plate is determined from option A is the flexural strength of the plate. Next shear strength of the plate see bearing strength of the concrete pedestal next is punching criteria you can find out the thickness on which basis. Next question the minimum thickness of a rectangular slab base is calculated just now we have seen one equation for the calculating the thickness of a rectangular slab base you you just see which equation we have used now these are the review answers answers of the review question the thickness of the base plate is determined from the flexural strength of the plate we have considered the bending about two axis we have considered the poison effect on another axis that is w a square by 2 minus 0.3 w b square by 2 that is the flexural strength of the plate and minimum thickness of an rectangular slab base we have calculated t is equal to under 2.5 w gamma I am not upon f y into a square minus 0.3 b square these are the standard terms f y is the yield stress of the steel w is the uniform pressure a is the moment and a and b are the cantilever projection smaller smaller and longer projections these are my references thank you