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Module 1: Weld Connections

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Video 1
Hello, i am going to discuss about the weld connections. Before the design ofweld connections, I will discuss little bit about the advantages and disadvantages of weldconnections. As we have seen, in case of bolt connections, there are certain advantages anddisadvantages and according to the requirement in the field, we have to choose a certain type ofconnections. There are two type of bolt. One is bearing type of bolt and another is high frictiongrip bolt. The two type of bolt connections have been designed and certain number of exampleshave been worked out. Now in this lecture I will first discuss about the weld connections, weld isbasically a process of joining metals, metal pieces by the application of heat and with or withoutpressure.
Different process of weld connections are there, i.e. forge welding, thermit welding, gas welding,resistance welding and electric-arc welding. Nowadays the electric arc welding is a most populartype of welding
Now coming to weld connections, let us see first the certain type of advantages, weld connectiondoes not need any hole in the plate. So there should not be any reduction of area that means thestructure members are will be more effective to take the load. Second is, in weld connection fillerplate, gusset plate, connecting angles etc. are not used that means the total weight of the jointwill be less in such cases. Now weld joint should be economical as less materials are required. So these are the certain advantages. Another advantages is that efficiency of weld joint is morethan that of the riveted or bolted joint, because when bolt connection or riveted connections areused we create hole and because of creation of hole the net effective area of the plate is going tobe reduced and this area under tension will be less and therefore, the capacity or strength of thebolt joint will be less compared to the weld joint. Therefore, the efficiency of weld joint will becomparatively more than the efficiency of the bolt or rivet joint.Now another advantage is that, the weld joint look better than the bulky riveted and bolted joint.If we want to make a particular shape of the joint, we can make by the weld joint, but in case ofbolted joint or riveted joint it will look bulky, because of extra bolt and bolt or rivet will beadded. 
Another advantage is the speed of fabrication. In case of weld joints, speed of fabrication will bemuch faster than the riveted joint and complete rigid joint can be achieved through weld process.In case of bolted joint complete rigid joint we cannot achieve, but in this case we can make it.Then alternation and addition of the existing structure is easy as compared to riveted joint,because rivet joint is permanent in nature, it is difficult to add or alter the existing structure.Now another important advantage is that no noise is produced during the welding process as incase of riveting process and also, the welding process requires less work space in comparison torivet and any shape of joint. So these are certain advantages we obtain in weld joint.
Weld material are brittle in nature and as a result the fatigue strength is less Now let us come to the disadvantages of weld joints. Weld joints are basically brittle in natureand therefore, it is means its fatigue strength is comparatively less. This is one disadvantages andanother disadvantage is that due to uneven heating and cooling of the member during thewelding the members may distort resulting additional stresses. So distortion may come andbecause of distortion the additional stress may come into picture. Then another disadvantage is,we need skilled labor and electricity for welding say for example, if we want to construct astructure at the remote place where electricity is not there, in that case, it would be difficult to goby the weld joint. So this is another disadvantage. Then there is no provision for expansion andcontraction in welded connection and therefore, there is a possibility of cracks. So cracks maydevelop because of this and inspection of weld work is more difficult and costlier than rivetingwork and another disadvantage is the defects like internal air pockets, slag inclusion, incompletepenetration are difficult to detect.
Now coming to type of weld, there are three types of weld we have; one is fillet weld. Fillet weldis used when two members are lapped together.
Suppose two members are there and these are overlapped, when we are going to join this weneed to join at the overlap portion and if we see in that plan we will see that these are say, theseare welded. So this type of welding is called fillet weld means when 2 members are lappedtogether, 2 members are to be joined, in that case fillet member we can use and in case of filletweld or I should say that in case of 2 members joined in a different plane then fillet weld can beused. Butt welds we can use when the two members are joined in same plane, suppose this is a member and this is another member, we will be joining in same plane. So in this case we canprovide butt weld and we can fill with weld material and it may have complete penetration, itmay have incomplete penetration, different type of butt welds are there. I will come into it laterand according to the process of penetration the throat thickness of the weld is defined and thestrength calculation is carried out.
Another type of weld is plug weld, the plug weld is required when two members are connectedtogether having a limited length of the joint, say suppose one members is like this and anothermember is connected, right and we have limited length. Limited length means here if we see thatlength is this much say, this is l1 and we will get this is l2, so total length will be available lengthwill be here2×l1+l2 and suppose the force along the joint is so high that the required length LR ismuch more than the available length. So in this case we have to adjust the total length inbetween, so what we can do we can make a slot here and we can provide way. So in this way wecan increase the length of the weld joint by the insertion of slot. So this is how one can makeadjustment of the additional length with the insertion of slot. So these are the 3 types of weld wewill come across. Now I will quickly show some basic type of welds and their symbols, we do we use sub-symbolthat means if we use certain symbol like this, then it means this is fillet weld. Similarly if asquare butt weld is used then in the drawing we use this parallel line 2 parallel line. So thatmeans when parallel line is there that means we understand that this is a square butt.If we write V that means it is a single V butt joint where joint will be like this, again if we cometo double V butt joint it will be like an X when joint is single U butt we will make a symbol likethis. So in case of double U butt joint, its symbol will be like this. So different symbols we usefor different type of weld in the drawing sheet, so we have to know that what symbol are given inthe drawing and what does it mean. Similarly double V, double-bevel-but joint, single-j-butt joint, double-j-butt joint we can also usein terms of its symbol. Now shape of the weld when it is flat, its symbol is like this, and when convex, its symbol weuse like this and when concave we will use symbol like this. 
Video 2
So first we will discuss the fillet weld, its different parameters, then we will see the designmethodology for fillet weld joint then we will go for butt weld joint and finally we will go forslot weld joints. Therefore, in case of fillet weld, as we know that when two lapped plates are tobe joined. In case of fillet weld, we know that certain terms will be used in case of fillet weld.Say, what is size of fillet weld? How do we define the size of fillet weld? Then we will discussthen what is the throat of fillet weld and throat thickness of that fillet weld, then effective lengthof fillet weld, we know the length of fillet weld, but what is the effective length?Then another term we will use in fillet weld is end return, then overlap, then side fillet,intermittent fillet, single fillet weld and permissible stress and strength of fillet weld we have tofind out. So these terms we will be frequently used in case of fillet weld design. So we need toknow one by one about the term.
So, if we see here that the two members are joined together and by the application of heat thenthis will be the fusion zone where the members are joined. Now this is the portion, which iscalled root. So from root to toe the length is called leg and this will be the size of the weld. Thisis a weld face and we will see that some extra deposit are there means if we make a straight linefrom this toe to this, we will see this is the extra deposit which is call reinforcement, right.
The leg length of the weld, is the distance from the root of the weld to the toe of the weldmeasured along the fusion face.
Now minimum size of the weld are given in clause 10.5.2.3 of table 21 of IS 800-2007. So itdepends on the thickness of the thicker part of the member, suppose the two members are joinedtogether overlapped. So it may be of same thickness it may be of different thickness. Nowminimum size of the weld we can define on the basis that if the thickness of thinner part is up to10 mm then minimum size of the weld will be 3 mm. Similarly, from 10 to 20 mm if thickness ofthicker part is existing then minimum size of the weld will be 5 mm. Similarly 20 to 32 it will be6 mm and so on. So these details are given in table 21 of the IS 800-2007. So, we can use thatwhile designing the fillet weld.
Now few things we have to remember like when the minimum size of the fillet weld is greaterthan the thickness of the thinner particularly, the minimum size of the weld should be taken asthe thickness of the thinner part. So minimum size cannot become more than the thickness of thethinner part. Then when the thicker part is more than 50 mm thick special precaution likepreheating etc. will be taken care and as per clause 10.5.2.1. For deep penetration weld where thedepth of penetration beyond the root run is minimum of 2.4 mm, the size of the filet weld isminimum leg size plus 2.4 mm and this is about the minimum size of the fillet weld.Now the maximum size is also defined in the code that is the thickness of the thinner part minus1.5mm, it can go up to that means the thickness of the thinner part we know and the maximumsize we can become that thickness of the thinner part minus 1.5 mm. Similarly, in case of anglethe maximum size of the fillet weld be three fourth of the nominal thickness of the angle.
Now, (I will come to) now I will discuss about the effective throat thickness, as I told one is sizeof the weld. So if we see if we make a diagram that this is a size of the weld and (this the) this isroot and from root to toe is a size of the weld and if size of the weld in different direction is samethen we can write S or the minimum of that. Now extra deposits happens here, which is calledreinforcement and this extra deposit we provide to increase the efficiency of the joint.Now if we give a name say suppose A, B, C. Now from root to perpendicular distance to thehypogenous will be the thickness of the weld. So if this is D then the throat thickness will be BDwhere the size will be BC or AB. Now how to find out the relation between T and S. So we cansee here that, in triangle ABD, if we write in triangle ABD,BA BC S
Now this is true for right angle triangle if it is right angle then fine we can find out this relationbut what will happen about different angle. So in that case what will happen, so that is defined incode that is in clause 10.5.3.1 of IS 800, it is told that the throat thickness will not be less than 3mm and generally not exceeding 0.7t or 1.0t under special circumstances where t is the thicknessof thinner plate?For the angle other than right-angled fillet weld the value of throat thickness is given as:T = KSWhere,T = Throat thickness of weldKA constant depends upon the angle between fusion faceS Thickness of the weld
In table 22 of IS 800-2007 the values of K is given. So in clause 10.5.3.2 we will see that thevalue of K will be 0.7 if angle between fusion face is 60 to 90 degree. Similarly for 91 to 100degree the value of K will be 0.65, 101 to 106 degree. The maximum value is 0.7. So maximumvalue of K is 0.7, when angle between fusion face is 60 to 90 degree. If it is more than that thenthe value of K will be reduced. So we have to take the value of K from this table.
Now coming to effective length. So effective length can be found out from the area of the weldfor which specified size of the effective throat thickness of the weld exist. Suppose we have alength this much then if we say, this is effective length then total length will be l + 2S, totallength and if it is effective length. So why 2S , because we assume that the strength will becarried out by the length which is called effective length, but we have to provide little more intwo side to make sure the strength is being carried out by this le. So the total length we will makelittle higher, right and this effective length should not be less than four times the size of the weld.
Now coming to the design strength of fillet weld. When we are going to calculate the strength ofthe fillet weld join we have to find out what is the permissible strength of the joint or permissibleforce of that joint. So that can be calculated from this formula which is given in the IS 800 that is
Now if we go to the design procedure we will see that first what we have to do? We have toassume a size of the weld, suppose we are going to design a weld joint, we have been given aparticular load then what we can do? If we have a certain length then we can find out the what will be the throat thickness of the weld and size of the weld, in other way also we can do, we canfix the size of the weld from the maximum and minimum criteria and the we can find out theeffective length Lw and then the total length, right.So one way we can do, we can assume size of the weld based on thickness of the member thenby equating the design strength of weld to external factor load the formula which I have given inlast slide the effective length of weld is calculated. If length exceeds 150te then we have toreduce the design capacity of the weld as prescribed in clause 10.50.7.3 and is as given below,right. So that reduction factor will be Where, lj = length of joint in the direction of force transferte = throat thickness of the weld.Then another thing is end returns of length equal to twice the size of the weld are provided ateach end of the longitudinal fillet. Let us come to that slide, this is called end return that means when the length of joint is this thenwe have to provide a end return of this value and that is 2S. So this return, this is 2S, right. So ifwe provide here, the weld in this direction and this direction and if it is extended up to the cornerthen we have to extend up to 2S more, which is called end return and another thing we have toremember that is overlap.The overlap of lap joints should not be less than four times the thickness of the thinner plate or40 mm whichever is less, this also we have to keep in mind while going to design. So this is allabout the calculation of design strength of fillet weld we have seen that how to find out thedesign strength of fillet weld Pdw which is basically Where fu is the ultimate stress of the weld metal and Lw is the effective length not the total lengthand te is the effective throat thickness. So this is how we can find out the design strength of weldconnection, thank you.