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Module 1: Eccentric Connections

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Video 1
This lecture will be the last lecture on the module eccentric connection. In last few lectures we have seen the design procedure of eccentric connections where load was lying in the plane of joint and load is lying on the plane of perpendicular to the plane of joint.But one case we have not consider where load is lying perpendicular the plane of joint but the connection is welded connections. So in this lecture we will discuss when a welded joint is under eccentric load and load is perpendicular to that welded joint. And you know that there are two types of welded joint, fillet weld and butt weld. Therefore we have to calculate the stress develop on the fillet weld as well as butt weld. So let us consider the first case that is when a column is loaded with a load P with an eccentricity e and the connection is done by filled weld. Now we have to find out the stressesmeans two type of stresses will come into picture in such type of connection one is shear stress and another is stress due to bending and both will be perpendicular to each other. So first if we see shear stress which is due to direct load, will come in the direction of direct load. The direct shear stress in the weld = q = LoadEffective weld area (a) Fillet Weld1. The shear stress in the fillet weld,2w t t P P q l t d tWhere, P is the load and e is the eccentricity, d is the depth of bracket plate/welding depth, is total effective length of weld, is the throat thickness of the fillet weld.2. The stress due to bending, b Mf yIWhere,3t1I 2 t d12dy2 and M P e Design Steps:i. Select a suitable size of weld and then compute throat thickness and weld strength, 3uwmwfRii. Calculate the depth of weld using the following expression:3t wPedt Riii. Increase depth d to certain percentage to accommodate shear stress as well.iv. Calculate direct shear stress, q, which should be less than ��.2tPqd t v. Similarly, compute stress due to bending, fb, which should be less than23btPeft dvi. Calculate equivalent stress, fe, which should be less than ��.2 2f q f e b  vii. If the equivalent stress exceeds the design weld strength �� then length of weld should increase and above process be repeated till the checks are satisfied. So unless it is satisfying this criterion we have to go on increasing the depth of weld or sizeof weld or both to make sure that design is ok.
Video 2 
Example: (Fillet weld) Design a fillet weld to connect a 10 mm thick bracket to the flange of a column as shown in the fig Solution:There are three cases, either size of the weld is given then you have to find out the depth or depth is given then you have to find out the size of weld or both depth and size of the weld is not given then you have to find both. So in case of finding both we have to go for trial and error means we have to assume certain size of the weld from the basis of the thickness i.e. from minimum size of the weld and from maximum size of the weld, we have to find the suitable size and then we have to find the depth and then we have to check. So both cases can be done. So in this case the depth is given but size of the weld is unknown. Let s = size of weld; Throat thickness, tt = 0.707s So we can adapt a weld size of 5 mm. So what we have seen in this example that the depth of weld is given but the size of the weld was not known so designing this what we have found we have assumed certain size of weld and then we have equivalent stresses due to external forces equal to the weld strength. So by making equal of this, we can find out the size of the weld. (b) Groove Weld:In case of groove weld the weld has been done by grooving in the plate. So here if weconsider the depth of weld as d and thickness as t then we can find out the shear stress and the bending stress in a similar fashion. 1. The shear stress in the fillet weld,w e eP Pql t d t Where, P is the load and e is the eccentricity,d is the depth of bracket plate/welding depth, is total effective length of weld and �� is the effective thickness of the groove weldIn earlier case, we have considered lw as 2d because in two sides (the periphery) the weld joint has been done by fillet weld but in case of groove weld this will be only d. 2. The stress due to bending, Design Steps (Groove weld): Now coming to the design steps, so for designing a groove weld if we see in a similar way we can move forward. i. Select a suitable size of weld and then compute effective thickness and weld strength, ii. Calculate the depth of weld using the following expression:6e wPedt Riii. Increase depth d to a certain percentage to accommodate shear stress as well. iv. Calculate direct shear stress,  , which should be less than . ePqd t  v. Similarly, compute stress due to bending, ��, which should be less than26bePeft d vi. Calculate equivalent stress, which should be less than3 2 2f q f e b vii. If the equivalent stress exceeds the design weld strength then the length of the weld should be increased and above process be repeated till the checks aresatisfied. Now the same example which has been considereded earlier, will be done using the groove weld. Example: (Groove weld) Design a groove weld to connect a 10 mm thick bracket to the flange of a column as shown in the figure below. Solution: So if we use groove weld how to design the connection that we will see. Let provide a double J groove weld. Therefore, effective throat thickness = thickness of bracket plate = te = 10 mm Therefore, vertical shear stress, So the resultant stress is less than the weld strength that means the joint is safe. So in this way we can design, in this case, the length of the weld was fixed length of weld or depth of weld was fixed and the thickness is also fixed because it was a double J groove weld and so the effective thickness was also fixed. So what we need to do here, we have to find out the equivalent stress and that equivalent stress whether it is less than the strength permissiblestrength or not that we have to check, if it is ok then fine, if it is not ok then the design is not safe. However if the depth is not given then we can find out the depth also by trial and error method the way we have discussed earlier, in the same way we can find out the approximate depth and then we can check whether it is ok or not, if the equivalent stress is much less than the weld strength then we have to decrease the depth and we have to check again and ifequivalent stress is more than the permissible stress then we have to increase the depth of the weld so that we can accommodate it.