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Module 1: Eccentric Connections

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Video 1
In last lecture we have discussed how tofind out the strength of a bolt and what will be the critical bolt and what is the maximum loadcoming to a particular bolt when the load is lying in the plane of joint. So firstly we havedecided bolt arrangement and we have found what are the total load is coming, total loadmeans the combination of direct load and eccentric load due to eccentricity. We have alsofound that what is the moment and what is the load in each bolt due to direct load and due tomoment.Then we found the resultant of the forces acting on different bolt and we found the maximumload coming on a particular bolt then we see whether it is exceeding the design strength ofthat particular bolt or not. If it is exceeding, we have to redesign and if it is not exceedingthen it is safe. In this lecture we will discuss that how to design a bolt group means if youhave given a load then what should be the number of bolt. First we have to find out numberof bolt required then the arrangement of the bolts means how many lines of bolt we canprovide and how many bolts we can give in each line. Accordingly, we have to decide thatwhat will be the pitch distance, what will be the edge distance, what will be horizontalspacing.Now it is not easy to find out the number of bolt at the very beginning because number ofbolt will be deciding on two factors one is due to direct load and another is due to moment.Now we can find out number of bolt is equal to direct load divided by bolt strength but thatbolt is going to resist the moment also.So that means the resultant we have to note unless we know the resultant force coming intoparticular bolt we will not be able to find out the design right so for doing that we have to gowith certain trial and certain approximate calculation. Firstly, we have to find out what willbe the total number of bolt approximately required and then we will make a trial arrangementthen we will see that whether with this particular number of bolt and with that arrangement,the bolt will be safe or not that means critical bolt forces is more than the bolt strength or not. If bolt forces are coming more than the strength, then the design is unsafe so we have torearrange the bolt group or we have to increase the number of bolt then we can find out right.
So if we see here that for this case we will decide load line in plane of bolted joint, so heresay for example we have given four bolt in each line and total number of bolt are 8 right. Sowith this if we have a value of P here then whether this is safe or not that we can find outfrom earlier lecture but we have the value of P then what will be the number of bolt thatmeans whether 4 or 3 or 8 how do we decide that we will try to discuss.
Now we will discuss about the design procedure then we will go through one example so thatwe will be able understand the design procedure. Firstly, we have to assume a suitablediameter of bolt depending on the availability. If we want the lesser length of the joint thenwe have to make bigger diameter otherwise we can go for small diameter also and it dependson the thickness of the member, so depending on that we will decide the diameter of bolt.Then we have to find out the strength of one bolt in shear, bearing and get bolt value Vsd.So Vsd is the bolt value correct and how do we find out this shear and bearing, this can befound whether it is double shear or single shear we have to know. Also we have to find outwhether it is HSFG bolt or whether it is bearing type bolt. Then when we will be going forbearing stress we have to know what is the edge distance, what is the pitch distance andaccordingly we have to find out the kb, the constant value of kb then we have to find out thebearing strength right.So for finding out the bearing strength we have to find out first the pitch and edge distance aswell as the type of bolt whether it is bearing type or friction type depending on that we willbe able to calculate the strength of the bolt. Once it is calculated then we can use this formula,
if P is the external direct load then P by Vsd is equal to the number of bolt. So when will usethis formula will use if moment coming on the joint is much less compare to the direct loadthat means if eccentricity is quite less and moment is coming therefore is less then we can usesimply this n is equal to P by Vsd to find approximate number of bolts.When the moment is comparatively much less than the direct load, means if the eccentricityis less than moment will be less, then we will increase the number of bolts to a certainpercentage which depends on the designers’ experience.From designer from that experience they can understand that how much percentage should beincreased to make the total number of bolts and whether it will be in two or more verticallines that also have to decided. In earlier lecture I have shown we had two vertical lines rightso it may be two or more so with a suitable spacing we have to make. So we will increase thenumber of bolt with a certain percentage to find out the total number of bolt for checking thestrength of that bolt joint right.And another case is if moment on the joint is much high compared to direct load that means ifeccentricity is high then will not use the earlier formulae we will use the following formula
where Vsd is the bolt strength and moment M is equal to P×V and p is the pitch distance andn' is the number of vertical lines.
So in this case also we will increase the number of bolts to a certain percentage (depends onthe designers’ experience) and then arrange the number of bolts per line.So in that case we have to take decision accordingly right so once we consider someapproximate number of bolts will make the arrangements of bolts then will find out theresultant force for the critical bolt. We will see in which bolt it may come more right if wecannot predict that then we have to calculate for each bolt or in few bolt we have to calculatebecause in few bolts similar forces will be developed.So we will calculate the resultant force in few bolts and will see what is the maximum forcecoming on that bolt then we will see the resultant force in the critical bolt is less than the boltvalue or not if it is less than the bolt value then the joint is safe that means if the resultantforce in the critical bolt is less than the the joint is safe that means the arrangement whateverwe have done is correct, the arrangement means in terms of number of bolts and number ofbolt line, pitch distance, edge distance and spacing between two bolt lines.So these arrangements are correct if it is not if forces in bolt is coming more than the boltvalue then we have to redesign that means we have to either increase the number of bolt orwe have to rearrange the bolt in such a way that the reaction force on the bolt is coming lessthan the bolt strength. So if it is so what will do we will either increase the number of boltsper line or we will change the bolt orientation in different positions means either number ofbolts per line the spacing and edge distance and pitch and will check whether the critical boltforce is coming less than the bolt value in this way will design the entire process.
Video 2
So this will be clear if we go through one example say let us go through this example right.Example: Design a bolted bracket connection to transfer an end reaction of 300 kN with aneccentricity of 170 mm, due to factored load as shown in the figure. The steel used is of gradeFE 410. Use 20 mm diameter bolt of grade 4.6. The thickness of bracket plate is 10 mm andthe column section is ISHB 200 @ 365.91 N/m
Solution:So this is the example we will try to solve that means you have been given 300 kN of loadacting at an eccentricity of 170 mm and this load is acting on a bracket of 10 mm thickbracket and this bracket is connected with a ISHB 200 I section column right. So now wehave to design a bearing type of bolt to withstand this 300 kN, so we do not know how manybolts will be provided here, how many bolt lines will be provided here, we do not haveanything. So we have to fast find out the approximate number and then we have to arrangewith increase of certain number of bolts and then we have to check whether it is ok or not. Sothrough this example we will learn how to do it ok.
So if we check for few bolts and if we find out the critical one then we can see whether theproposed arrangement of bolt and proposed number whether it is ok or not. This arrangementalso can be changed little means Fm value is coming here little more, that can be reducedslightly if we change the P value suitably then also Fm value slightly will be reduced butalways it is suggested that you cannot change much more. So if the design strength of bolt isquite less than the force coming on the bolt then it is better to increase the number of bolts inplace of changing the pitch distance or edge distance or the spacing because it will not changemuch.So this is what we have discussed today, to summarize once again let me conclude that designof a bolt group due to eccentric load lying in the plane of joint can be done by trial and errormethod.We can find out the approximate number of bolt either from force or from moment then wecan increase a certain number of bolt on the basis on that approximate number and then wecan arrange the bolt group means bolts in a particular way, once it is arranged then we have tocheck whether that bolts are safe or not that means the forces on critical bolt is coming lessthan the bolt strength or not. If it is not then we have to redesign we have to increase thenumber of bolt or we have to change the orientation means position of the bolts in terms ofspacing and pitch and edge or if it is safe then we can stick to that connections.