Hello I will restrict my lecture on design of gusset plate, gusset plate is a plate which isused to connect several members meeting at a joint, gusset plates are provided when morethan one member, means, at least two members are joined.
Gusset plate if we see in the picture it looks like this, say for example in a truss member whenmore than two or more than one member are joined, here four members are joined together.Now in this figure we can see several aspects. First of all these are angle sections so its Cg ofall the members should meet at a particular point so that the eccentricity does not generate, sothat we have to keep in mind.The size of the gusset plate actually depends on the number of bolts used or weld length ofweld used to join the section. Say for example suppose for this case we are seeing that fournumber of bolts are there, right so the length of gusset plate on this direction we have todecide on the basis of the number of bolts and number of bolts are decided on the basis of thenominal diameter of bolt & magnitude of the force.
While designing we have to keep in mind that the gusset plate length should be as less aspossible so that the material for gusset plate is minimized. Another point is that as I told thatgusset plate thickness of the gusset plate should be little more than the member itself.
So if I look back to the design criteria we can see first is the shape and size of a gusset plate isdecided on the basis of direction of members meeting at the joint. Then the plate size isdecided to meet specification of pitch and edge distance.Another thing is, the gusset plates are designed actually to resist shear mainly shear and directand flexible stress acting on the critical section. So when we will be going for designing wehave to keep in mind that the gusset plates should not fail due to shear which are resistingshear and the direct tensile forces coming into the gusset plate. And I told that it is a usualpractice to provide thickness of gusset plate equal to or higher than the members that have tobe connected by the gusset plate that also we have to keep in mind.
Example:Design the gusset plate of thickness 12 mm at the joint O of a lower chord of truss asshown in the figure below. Use M20 grade 4.6 bolts.
Member OC:Here, the value of kb will be same as derived for member OB as pitch and edge are same.Bearing strength of bolts on 6 mm thick angles
We can see here, this will be the arrangement, here if you see we have considered 4 boltshere, these are 4 bolts along member OB, this is O, this is B, this is A, this is C, this is D. Soif we see that for member OB we need 4 bolts, so we could find out the length accordingly.Similarly, this length also will be 260 mm 260 mm, right because 4 bolts are required formember OC. For member AD we need 2 bolts therefore this length we calculated as 140 mm,right. So in this way the gusset plate dimensions have been decided.So length of the gusset plate in different directions can be reduced by increasing the boltdiameter and if we increase the bolt diameter the number of bolts will be reduced and as aresult we can find out lesser length of the gusset plate towards that member. So this is howone can find out the gusset plate dimensions.
Here before going to end again I am just giving importance that suppose this is a anglesection in this directions whose Cg is this and this is another angle sections whose Cg in thisdirection. So what I wanted to tell that Cg of all the members should coincide at a particularpoint, if it does not coincide then eccentricity will develop and as a result additional momentwill come into picture. To avoid that moment or eccentricity we have to make the alignmentin such a way, we have to place the angle sections in such a way that all the Cg of the sectionsare meeting a particular point, that we have to keep in mind.And if gusset plate becomes larger that means if length of gusset plate become greater thanthe weight of gusset plate will be more and if the weight of gusset plate become more thenthe structure the dead load coming to the structure will be more and it will not be economic.So the dead load due to the gusset plate can be reduced if we properly make use of number ofbolts. If we properly reduce the number of bolts so that the gusset plate size can be reducedand if it is reduced then we can reduce the weight of the gusset plate and as a result the deadweight of the structure will be less.