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Module 1: Eccentric Connections

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Video 1
This lecture will be the continuation of last lecture. In last lecture, we have discussed aboutthe bolt group connection where the load is lying perpendicular to plane of the bolt group. Sofar we have covered the followings; how to find out the tensile force developing on the boltand the shear force in each bolt, how the neutral axis will be decided and on the basis of thathow the tensile force is going to vary from bolt to bolt and through interaction formula howto check the bolt group.So in this lecture we will go through one example to understand the same process whateverwe have discussed. So before going to the example, let us take an overview of designprocedure whatever we have discussed in last lecture. 
Let us say one bracket is connected to a stanchion which is loaded with certain amount offorce at a certain distance away from the structure that means it is an eccentric connection.Now we have to see first whether it is out of plane load or in plane load. If it is out of planeload then we have to design according to the design steps given.So we will start with certain assumptions means diameter of bolt which will not be given. Sowe can start with certain bolt diameter then we can find out what will be the pitch distanceand edge distance according to the bolt diameter, what will be the bolt strength due to shearand other things. Again we do not know how many number of bolts are exactly requiredbecause it is not given. So what we can do we can assume certain number of bolt groups,certain number of bolt lines, right two or three whatever it is, then per line what will be thenumber of bolt that approximately we can calculate this from the moment formula.So that we are showing here, say suppose in first step we can see that we can find out thenominal diameter of bolt and we will provide the pitch and edge distance. Accordingly wecan find out the design shear strength assuming number of bolt lines and the external momentdue to eccentricity. Then we can find out the approximate number of bolt per line from theformula given in step 4 and also we can find out the shear force coming in each bolt.
Then in next step we can find out the design tensile strength of the bolt group and also thetensile strength in the extreme bolt. Further we have to check individually that the shear forcedeveloped on a particular bolt should be less than the design shear strength of the bolt andsimilarly the tensile force develop on the bolt should be less than the tensile strength of thebolt.So individually it has to be less if it is not than either we have to increase the number of boltor we have to increase the diameter of bolt to make sure that mentioned conditions aresatisfied and once it is satisfied we have check against combined shear and tension using theinteraction formula given in step 9. If the equation is not satisfied then we have to increaseeither number of bolt or diameter of bolt and we have to recheck once again.
Video 2
Example:Design a bracket connection to transfer an end reaction of 200 kN due to factored load asshown in the figure. The end reaction from the girder acts at an eccentricity of 250 mm fromthe face of the column flange. Design bolted joint connecting the Tee-flange with the columnflange. Steel is of grade Fe 410 and bolts of grade 4.6
So what we could see that maximum tensile force develop on the extreme bolt is less than thetensile carrying capacity of the bolt. So this is ok, that means the number of bolts chosen inthis case are ok.
Now we have to check from the interaction formula that means if it acts as combined then forthe combination of this formula we have to check the following
Hence the bolt group is safe.Now what we can see here we can see few aspects that interaction formula shows that thecombined coefficient is coming 0.2 only in place of 1, right so it is much much safer with thiscombination. Also Vsb and Tb are much less compare to Vdsb and Tdb respectively,that means may be it is over designed.So in place of number of bolt 6, we can come to number of bolt 4 or 5 and we can check, tomake economy in the design we can decrease the number of bolt or the diameter of bolts andthen we can check once again if we decrease the diameter of bolt than strength capacity alsowould be reduced then also we can check.So if design is much safe means over designed than to make it economy we can redesignmeans either we have to decrease the number of bolt or we have to decrease the nominaldiameter of bolt. So if we reduce the number of bolt automatically that interaction formula itwas much less than 1, it will be near to 1, right.So this way we can think but if we see that in the interaction formula it is more than 1, thenwe have to increase the number of bolt or diameter of bolt to make it safe.