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Module 1: Bolt Connections

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Introduction to Bolt Connections

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Video 1
In the previous lecture various aspects of different connections which includes rivetconnections, bolt connections and welded connections are discussed. As rivet connection isbecoming absolute nowadays, we have not concentrated on rivet that much. We concentratedonly on the bolt connections in details. In case of bolt connections when we are going todesign we need to know certain terminology that we have already discussed. This lecturefocused on the bolt connections in particular where the different pattern of bolt connections isused. For different pattern of bolt connections, how the strength is going to vary, that will bediscussed. In case of bolt connections or in case of weld connections also we have different type ofarrangements and depending upon the arrangement of bolts and plates, we designed the bolt.Again depending upon the mode of load transmission, nature and location of load, wedesigned the bolt connection. Now coming to depending upon arrangement of bolts we have two type of joint we can make,one is lap joint another is butt joint. So in case of lap joint we have seen earlier that we canmake a single bolting, chain bolting and staggered bolting. And in case of butt joint similarlywe can make single bolting, chain bolting and staggered bolting. So coming to the different bolting pattern first let us come to the chain bolting. In case of lapjoint the two plates are overlapped together and are joined by different ways of connectionlike bolt, rivet or welding. Now in case of chain bolting the bolt will be in same line and alsoin same row and column as shown in fig. 1. The failure pattern for this type of bolting will bediscussed later. Now in case of zig-zag bolting, the position bolts are in a zig-zag way as shown in fig. 2. Forthis type bolting the failure may occur in various direction or path. So we have to calculatewhich will be the weakest section and according to that we have to calculate the loweststrength of the connection and accordingly we will design the connection. In case of Diamond bolting, the number of bolts are increased towards the center as shown infig. 3. Similar type of failure as zig-zag bolting may occur in this case also. We generallyprefer diamond bolting because it is much more efficient. Depending upon arrangement of bolts and plates there are various type of bolt connections asshown in fig. 4. In single bolted lap joint and triple bolted lap joint two plates are jointedtogether by one and three bolts respectively without any cover plate. But in case of singlebolted single cover butt joint single bolt per member and one cover plate is used, where inSingle bolted lap joint Triple bolted lap jointSingle bolted single cover butt joint Single bolted double cover butt jointDouble bolted double cover butt jointsingle bolted double cover butt joint double cover is used. Again in double bolted doublecover joint 2 bolt per member and double cover is used. Now depending upon the mode of load transmission we can define a bolt by single shear,double shear, or multiple shear as shown in fig. 5. When the two plates are joint togetherthrough bolt then only one shear plane is there that is why this is called single shear and ifthree plates are connected through bolt, there will be 2 shear plane, hence it is called doubleshear. When the number of shear planes is more than two then it is called multiple shear. Now depending upon nature and location of load we also can find out different type ofconnections like direct shear connection, eccentric connection, pure moment connection andmoment shear connection. In case of direct shear connection, we can see that two plates are overlapped together andconnected through bolt as shown in fig. 6, where bolts may fail due to shear so we havedesign accordingly. In fig. 7 a typical eccentric connection is shown where, beam and column are connectedthrough gusset plate. Now if we have a load P and eccentricity e as shown in the fig. 7 thensuch type of connection exerts moment because this is an eccentric connection. So foreccentric connection here we can see that the bolt will experience stresses due to the directload P the moment P×e. In pure moment connection the loading is symmetric as shown in fig. 8. So here only momentwill be developed in the connection, so this is called pure moment connection. In case of moment shear connection, both moment and shear are developed due to thearrangement of the connection as shown in fig. 9.
Video 2
Now we will discuss about the different type of failures because when we are going to designa bolt, when we are going to find out the strength of a bolt, or strength of the connection as awhole we have to find out what are the type of failure may occur and to arrange those failureswhat are the mathematical equations are developed and how we will calculate the strength.The failure of bolt connection may occur due to the failure of bolt or the failure of the plate. The bolted joint may fail in any of the following seven ways, out of which somefailure can be checked by adherence to the specifications of edge distance. Therefore,they are not of much importance, whereas the others require due consideration. As discussed earlier, there are different types of shearing failure which may occur in boltedjoint. In single shear only one shear plane is developed in the shank portion of the boltwherein double shear two shear plane are developed in the bolt. Now we will see bearing on plate and bolt and how bearing failure happens. So if two platesare connected with bolt then due to bearing on plate it may fail that means it may fail bycrushing as shown in the above figure. So this is one type of failure which is called bearingfailure.Another type of failure may come in to picture also which is called shear tear-out of planethat means this is somewhat it will look like as shown in the above figure which may fail bycrushing due to shear tear-out of plate. So such type of failure may occur when force P is inthe direction as shown in the figure.Another failure may come which is called tension failure of plate because when two platesare under tension and we are going to join through bolt by insertion of hole then along thathole line tension failure of plate may happen. Suppose we have a bolt as shown in the figure,where the two plates may tear out as shown in the figure due to failure of plate. So this isanother sort of failure, so different modes of failure of bolt joints we are observing. And another failure is called block shear failure. When a plate is connected with another platesay for example a gusset plate and a force P is acting and if bolts are inserted as shown in thefigure then the block shear failure may occur as a hole. Now we have to remember certain things for an efficient connection using bolt that is one isthe stress concentration results in a considerable decrement in the tensile strength so we haveto try to avoid the stress concentration. Then loose fit of the joint can reduce the stiffnesswhich may result in excessive deflections. So that has to be taken care that means we have totighten the bolts properly so that the loose fit does not occur and joint does not get reducedinstantly. Then vibration cause loosening of nuts which can jeopardize the safety of the structure. Sothis has to be taken care while designing a joint using bolt. And certain criteria we have to remember for designing the joint under axially loadedmembers. First of all, the length of joint should be as small as possible to save material oncover plates and gusset plates. Say for example we have a gusset plate connected with truss member as shown in the abovefigure. So how do we connect all the members at a particular point? This type of joining ispossible; we have to join in such way that cg of each members pass through a particular pointotherwise eccentricity may develop then the moment due to eccentricity may come intopicture. The length of joint should be as less as possible to reduce the material amount ofmaterial that means we have to join through a gusset plate. Now if we provide more numberof bolts then we will not be able to reduce the length of joint that means we cannot reduce theamount of material required for gusset plate, that is why we have to keep in mind that as lessas possible the length of joint should be. Another thing is that the center line of all membersmeeting at a joint should coincide at one point only otherwise the joint will twist out ofposition. The number of bolts should be increased gradually towards the joint for uniformstress distribution in bolt. For this type of connection, we prefer diamond bolting where thenumber of bolts increased towards the center. Then the arrangement should satisfy the pitch, gauge and edge distance requirement, thatmeans what will be the pitch distance, what will be the gauge distance, what is the edgedistance all the things has to be satisfied. Next is the strength of joint reduces due to the boltholes. So the reduction in area due to bolt holes can be minimized by arranging a zig-zagform or diamond form and the arrangement can be made in such a way that the strength canbe increased.So in short while designing a connection using bolt we have to remember certain criteria oneis the length of the joint should be as less as possible. Now if we increase the diameter ofbolt, we should not require large number of bolts. We have to try to reduce the number ofbolts so that the number of holes become less so the tensile strength of the member becomeshigh as well as because of less number of bolt the length of the joint will be less and therebythe gusset plate require for joining plates will be less. So the amount of gusset plates can beminimized through this.Another thing is the members meeting at a point should be designed in such a way that cg ofall the members are coinciding at a particular point, otherwise due to eccentricity theadditional moment may generate that also has to be keep in mind. Another important thingwhich have to keep in mind that whether we are satisfying the pitch and edge distanceproperly because as per codal provisions we have to consider the minimum pitch andmaximum pitch. So the pitch distance whatever we will provide we have to be in betweenmaximum and minimum as prescribed in IS 800: 2007. So with this I would like to conclude todays lecture saying once again that when we are goingto design a bolt connection we have to keep in mind what type of connections we are going tohandle and accordingly what type of load may come and whether we are going for lap joint orbutt joint, whether we are going for eccentric connections or concentric connection we haveto see and accordingly we have to calculate the external forces and then we have to find outthe bolt strength once we get bolt strength we can find out the number of bolt and we candesign accordingly with the use of codal provisions for pitch distance, edge distance andgauge distance.In next class we will discuss the bolt strength due to different failure means different aspectslike bearing failure, shearing failure, tearing failure what will be the bolt strength that we willcalculate as per the codal provisions, thank you very much.