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Module 1: Column Bases

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Introduction to the Design of Gusset Base

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Video 1
This lecture will focus on gusset base. So in last few lectures we have discussed about the slab base. Now in case of gusset base the column is connected to the base plate through the gusset plate. So here the load will be transferred from column to base plate not only directly through bearing but also through the gusset plate, so the load is going to be distributed to the base plate through bearing and through gusset plate and this gusset base is used when load is heavy that means the compressive load which are coming into the column is heavy or if the moment is acting on the column then gusset base will be suitable.
So it will be clear to us if we go through one diagram of the gusset base then we will be able to know how it is connected and as I told the load is transferred to the base plate partly through bearing and partly through the gusset plate. So if this is a column which are resting on the base plate now in case of slab base we are resting the column directly to the base plate.So here we are providing additional gusset plate on the column and this gusset plate is again connected with the gusset angle. So the load whatever is coming whether it is load or moment, or both are transferred to the base plate through gusset plate and through bearing. So in both the ways it is transferring the load and if we see here the because of the gusset and stiffness bear the base slab against bending therefore a thinner base plate can be provided. The base plate is undergoing the pressure coming from the concrete pedestal because of the reaction. Now again this gusset angle is also connected to the column by the use of bolt, so here we can provide certain number of bolts the height of the gusset plate should be sufficient to take care the number of bolts because here number of bolts should be used because of the load is used the concentrate load when it becomes use then only we use this type of gusset base. And the critical section at which the maximum moment will develop is the maximumoverhang portion, so this we can say C the maximum overhang portion. So now this column may face for complete bearing or may face for incomplete bearing. If it is facing complete bearing then generally 50 percent of the load are transferred to the base plate directly and 50 percent load are transferred through the gusset plate and if it is not complete bearing then we assume that whole load is transferred through the gusset plate. So this is how we are going todesign. Now if we see the side view then we can see how the gusset plates are connected. So here if we see that we are basically seeing the side view so bolt will be connected here with the angle in both the side and also bolt will be connected to the gusset plate which is connected here. Say if we provide bolts in two rows then at each flange in this case we are providing 8 number of bolts in each flange. So basic objective of providing gusset plate is to carry the load from the column to the base plate through this gusset plate that means the load is going to disperse not only through the bearing of the system but also through the gusset plate we are providing certain amount of load. So here if we see the maximum moment whatever coming will be calculated in this position and the thickness of the base plate can be found from the flexural strength of the base plate.For unit width of plate, bending moment at the critical section M=w c22Where, w=¿ intensity of pressure from concrete under the slab base.c=¿ The portion of the base plate acting as a cantilever Now here another thing we have to remember that upto this portion the thickness is becoming the thickness of base plate plus thickness of the gusset angle. So basically when we are going to provide the gusset plate then the thickness of base plate is going to be reduced.
The design bending strength at the critical section Where, t is the aggregate thickness of base plate and cleat angle for bolted gusset base and thickness of the base plate for welded gusset base, at the critical section.However this is true if we are going to provide gusset angle and gusset angle when it is bolted connections but for welded connections we do not need to provide gusset angle.
Video 2
Now we will go through step by step design procedure for the gusset base. So designing a gusset base means we have to design the base plate base plate means the dimension of the bases which includes thickness, length and breadth and once it is decided then we have to decide what will be the number of bolts required for the connection of the gusset plate with the column sections. Another thing is that when we are going to provide a gusset plate we have to know what will be the plate thickness, what will be the gusset plate thickness and what should be the gusset plate size that means length and width. So these are the few things which we need to know.So in the steps when we are going to discuss we will try to address all this issue. Then the dimension of gusset angle and the number of bolts required for connecting the system or the weld length for connecting the systems will be calculated. Step-1: Assume a suitable grade of concrete. Depending upon the grade of concrete used, thebearing strength of concrete is determined by 0.45 f ck. Step-2: The area of a slab base may be computed by A= P 0.45 f ck Where P is the factored load on column.
Step-3: The size of the gusset plate is assumed. • The gusset plate should not be less than 16 mm in thickness for bolted gusset base.• The gusset/cleat angle is chosen so as to accommodate two rows of bolts in the vertical leg and one row of bolts in the horizontal leg. Therefore, an unequal angle section is provided. • The thickness of cleat angle is kept approximately equal to the thickness of gussetplate. • The length of the gusset material is normally kept equal to the length of the base plate, parallel to the flange of the column.• For welded gusset plate, cleat angles will not be required. Step-4: The dimension of base plate parallel to the web For bolted connection: L=¿ depth of section + 2 (thickness of gusset plate + leglength of angle + overhang) For welded connection: L=¿depth of section + 2× (thickness of gusset plate + overhang) • The dimension of base plate parallel to flange, B= A L Step-5: The intensity of pressure w, from the concrete pedestal is determined by w= PArea of base plate provided Step-6: The thickness of the base plate is computed by flexural strength at the critical section. Step-7: Holding down bolts 2 or 4 in numbers and of 20-mm diameter are usually provided. The design bending strength at the critical section Md=1.2 f y γm0 Z e ¿1.2× f y γm 0 ×(1×6t2)=0.2 fγymt02 Now, M=Md⇒ w c2 2 =0.2 f yt2 γm 0 ⇒ t=c√2.5 wfγym 0 Where, t is the aggregate thickness of base plate and cleat angle for bolted gusset base and thickness of the base plate for welded gusset base, at the critical section. Now following these steps we can go through this example, okay and this example will be working out in next lecture. I hope you have understood the process means how to find out the dimension of the base plate and the gusset plate and the cleat angle or the gusset angle. Sothese are the main members which need to be designed for the gusset base. So this things will be clear if we go through one example then it will be much more clear.