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Video 1
I am going to discuss a new module which is about column base. In case of RCC structure we use to tell this as a footing or foundation and in case of steel structure we will say this as a column base that means column is placed on certain base through which the loadfrom super structure is going to transferred to the soil through this sub structure. So column base is basically consisting of column and base plate and an anchoring assembly.So this column base are two types one is called slab base and other is called gusset base. So in case of slab base when the load is coming concentrically and magnitude of load is comparatively less in such cases generally we use this slab base. Whereas in case of gusset base the load is generally eccentric in nature and magnitude of load is comparatively higher so for such cases we generally prefer gusset base. So today we will discuss the first lecture about the slab base.Now in the column base the main function of the column base is to transfer the loads from column to its foundation and for that we need a spreading area through which the concentrated load from the column will be spread to the soil through some medium which iscalled base plate that means column will be rested on the base plate so the concentrated load coming from the column will be distributed over the base plate and then base plate again will be rested on the concrete block that concrete block will be again rested on the soil. So stage by stage the load will be distributed from super structure to the soil, right.And the slab base is basically consist of a thick plate of steel material and this thick plate is connected with the column through some angle which is called cleat angle and that connections are made either by bolt or by weld connections. And slab base basically suitable for light loaded column only and it will be economical if we use for light loaded column and that base plate will be either welded to the stanchion or will be joint to the column throughcleat angle, right.
And design of column base plate requires consideration of bearing pressure on the supporting material and bending of the plate.
Now let us see the general assembly of the slab base. So say for example an I section istransferring concentric load from the super structure and it may transfer some moment as well, then we provide a base plate of steel material and this base plate is connected with the column with cleat angle, okay. So if it is concentrically loaded then the minimum number of bolts are provided to connect the cleat angle with the column otherwise we have to design and here we provide anchor bolts.Now below the base plate the load is again spread to the concrete base which is called concrete pedestal. So load coming from base plate is again transferred to the concrete pedestal where different grade of concrete may be used depending on the magnitude of loadand requirement of the thickness and requirement of the dimension of the block and dimension of the plate. Now anchor bolt will be provided to connect the base plate with the concrete pedestal.So anchor bolts also will be designed as per the requirement and if we see the same in top view it will look like this (as shown in the figure) and if we see the plan of the base plate the base plate will be like this (as shown in the figure).So we can provide nominal number of cleat angle bolt which will connect the cleat angle with the column. Then we need to know what will be the size of the base plate that means length, width and thickness.
So length of the base plate we have to know and width can be found from the grade of concrete means how much area we need for the base plate so that the area can be transferred to the concrete pedestal without any failure. Again the dimension of the concrete pedestal depends on the soil properties so we need to know the bearing pressure of the soil and according to that we have to find out what will be the length and width of the concrete pedestal, so that the load is distributed and transferred to the soil properly.So this is how the system are made and in this way we have to design the base plate due to this concentric load and moment and this moment is due to this uniform pressure from the concrete to the base plate. So here also uniform pressure will come from the concrete to the base plate because of the concentric load. So the maximum bending moment will be occurring at the edge of the column. So at the edge maximum bending moment will occur, sowe need to know what will be the projection in both the direction.Let us say a and b are the projection in both the axis and because of this projection the bending stress will develop, bending moment will be developed at the edge and we have to calculate the bending stress and then we have to find out its influence so that we can find out the thickness of the plate accordingly.
So to find out the thickness of the plate what we can do we can find out what is the bending moment coming so if we see the column is rested on the base plate and the base plate is having uniformly distributed load coming from the concrete theoretical ConsiderationsTaking 1 unit length strip of slab projection along xx-axisMaximum Bending Moment, Mx=w ×1×a× a2¿ w a22(1)w is the intensity of bearing pressure from concrete below the base plate.Taking 1 unit length strip of slab projection along yy-axisMaximum Bending Moment, M y=w ×1×b× b 2¿ w b22(2)If poison ratio (μ) is adopted as 0.3The effective moment along xx-axis ¿ w a22−0.3 w b22¿ w2(a2−0.3 b2) (3
The moment capacity of plate is,Mp=1.2f yZeWhere Ze=¿ elastic section modulus of the base plate.
¿1× t26t=¿ thickness of the base plate.∴Mp=1.2×f y×1× t62 =1.2f y t62 (4)From Eqs. (3) and (4) we get,1.2f y t62 = w2 (a2−0.3b2)Applying partial safety factor,1.2f yγm0t26=w2(a2 −0.3 b2 )or t2=2.5w (a2−0.3b2) γm0f yOr t=√2.5w (a2−0.3b2) γfmy0So this thickness of the base plate also is available in the code. So this formula you can find out in the IS: 800-2007. So this is how we can find out length and width of the base plate from the pressure requirement means from total load we can find out what is the bearing pressure of the concrete.
Now we will see what are the steps required to design a base plate.Step-1: Assume a suitable grade of concrete. Considering uniform stress at the bottom ofplate, the bearing strength of concrete is determined as 0.45f ck .Step-2: The area of a slab base may be computed byA= P0.45 f ckWhere P is the factored axial load on column.Step-3: Select the size of base plate. For economy as far as possible keep the projections aand b equal.Now if this a and b are different than along the larger projection the maximum moment willoccur. So to make economy we will try to keep the projection length a and b as much aspossible equal, right.Step-4: The intensity of pressure w, from the concrete pedestal is determined byw=PArea of base plate providedSo from that we can find out the intensity of pressure from the concrete pedestal.
Video 2
Now we will see what are the steps required to design a base plate.Step-1: Assume a suitable grade of concrete. Considering uniform stress at the bottom of plate, the bearing strength of concrete is determined as 0.45f ck .Step-2: The area of a slab base may be computed byA= P0.45 f ckWhere P is the factored axial load on column.Step-3: Select the size of base plate. For economy as far as possible keep the projections a and b equal.Now if this a and b are different than along the larger projection the maximum moment will occur. So to make economy we will try to keep the projection length a and b as much as possible equal, right.Step-4: The intensity of pressure w, from the concrete pedestal is determined byw=PArea of base plate providedSo from that we can find out the intensity of pressure from the concrete pedestal.
Step-5: The minimum thickness required as per cl. 7.4.3.1 IS 800:2007t=√2.5w (a2−0.3b2) γfmy0 >tfWhere, tf=¿ flange thickness of compression memberWhen only the effective area of the base plate is used, c2 may be used in the aboveequation instead of (a2−0.3b2)
So the thickness of the base plate atleast has to be greater than the thickness of flange of the compression member that also we need to check, right.Step-6: Provide nominal two or four 20-mm holding down bolts.Step-7: Check the weld length connecting the base plate with the column (this check is required only for fillet welds)So this is how one can find out the detail of the base plate, right.Now what grade of concrete we are going to provide depending on that we will find out the length and breadth of base plate. So once we find the length and breadth of base plate then we can find the thickness of the base plate and thickness of the base plate can be found on the basis of the moment and that moment has been calculated from the projection. So we will see what are the projection is there and at the column phase what is the maximum moment isgoing to develop on that basis we will find out the minimum thickness of the base plate required and that expression also is available in the code in 7.4.3.1 clause we can find out thatformula, okay.
So once dimension of the base plate is decided, after that we will provide the nominal number of bolts to hold the column in position with the base plate through the cleat angle and cleat angle size also will be nominal and then through anchor bolt also we will fix the base plate with the concrete, right. So this is how step by step one can design a base plate. So in next lecture we will go through one example to see how the base plate is designed, thank you.
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