Design of Steel Structures | Introduction to Connections | Alison
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Module 1: Overview of Design of Steel Structures

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Introduction to Connections

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Video 1
This lecture focused on the connections, which are essential to assemble the steelsections/members to build a complete structure. Various connections are available for joiningmembers in case of RCC structure. Generally, we used to connect RCC members by castingthem in-situ. But in case of steel members different type of steel roll sections are available inthe market. The steel roll sections are need to join together and that can be done by applyingvarious types of connections, like rivet connections, bolt connections, weld connections andcombination of those two or three. Generally, connection between two members are required at the following joints: beam andcolumn, main beam and secondary beam, column and column, column and brackets, columnand caps, Purlins and rafters, wind braces and columns, rail and columns. In case of trussstructure, truss members are connected through gussets that means when different membersare connected at a particular point then that can be connected through gussets member. Sowhen more than two members are joining at a point we need connections. Further stiffeners in plate girders, diaphragms in plate girders, flange and web connections inplate girders, stiffener plates in column joints are also used for connecting different type ofmembers. Methods of fabrication are basically three types: rivet joints, bolt joints and weldjoints. Also we can make combine of two or three of the above means in a particular joint wecan make use of rivet and bolt, bolt or weld, or bolt and rivet connection. So requirementwise we have to choose an appropriate connection. Now let us come to the requirement of good connection. What is good connection? Basicallygood connection mean it should be such that it can easily be installed, inspected andmaintained, it should be such that there is the least possible weakening of the parts to bejoined and it should be rigid enough to avoid fluctuating stresses which may cause fatiguefailure. Now coming to rivet connections we know rivets are inserted in the plates to join together.With different plates and by adding heat we can insert the. A typical rivet joint is shown inFig. 1, where different members are connected to plate by riveting and in the parts of rivet,the upper part is called head and the lower cylindrical part is called shank. Shank hasparticular length depending on the thickness of the plates, so it can vary accordingly.Different type of heads is available and according to that different name of the defects aregiven. Now this rivet head has a particular diameter which is called rivet head diameter andthe diameter of shank is called nominal diameter or rivet diameter or shank diameter.Depending on the size of nominal diameter the strength of rivet can be calculated on the basisof the type of material used and accordingly we can calculate the rivet strength. Advantages of Riveted connections:i. Ease of riveting process.ii. Rivet connection is permanent in natureiii. Cheaper fabrication cost.iv. Low maintenance cost.v. Dissimilar metals can also be joined; even non-metallic joints are possible withriveted joints.vi. Rivet connection is possible without electricity in remote areaIn case of welded connection, we need electricity otherwise it will be difficult to join themembers but in case of rivet connections, only through application of heat we can joined. Disadvantages of Rivet Connection:i. Necessity of pre-heating the rivets prior to drivingii. Create high level of noise at the site of constructioniii. Skilled work necessary for inspection of connectioniv. Cost involved in careful inspection and removal of poorly installed rivetsv. High labor costSo because of certain disadvantages nowadays riveting connections are becoming absolute,mainly because of noise and because of generation of heat and difficulty to change theimproper insertion of the rivet. There are two types of rivets: power driven rivet and hand driven rivet. Power driven rivet iscalled hot rivet and hand driven rivet is called cold rivet. Again in power driven rivet we havetwo types, one is called power driven shop rivet and another is called power driven field rivet.Similarly, for hand driven rivets, there are two types, hand driven shop rivet and hand drivenfield rivets. Now commonly used rivets are like snap head where the head dimensions are represented bythe shank diameter. If diameter of shank is d then we can consider that the diameter of rivethead is 1.6d and the height of the rivet head is 0.7d. So with different height and diameterdifferent types of rivet heads are available. Two types of rivet are generally used as shown infig. 2. In case of flat head, the head diameter is 2d and head height is 0.25d and where d is thenominal diameter of the rivet. The length of the shank is called rivet length. In this case weshould remember that there is two type of diameter, one is rivet diameter (nominal diameter)another is hole diameter (gross diameter). Gross diameter is little higher than the rivetdiameter, and it is sometimes 1.5 or 2 mm more than the nominal diameter. Now while designing the rivet joints we have certain assumptions we have to make, whichare as follows:i. Friction between the plates is neglected.ii. The shear stress is uniform on the cross section of the rivet.iii. The distribution of direct stress on the portion of the plates between the rivet holes isuniform.iv. Rivets in group subjected to direct loads share the load equally.v. Bending stress in the rivet is neglected.vi. Rivets fill completely the holes in which they are drivenvii. Bearing stress distribution is uniform and contact area is d × t, where d is the nonaldiameter and t is the thickness of the plate.As rivet connection is becoming absolute nowadays therefore in new code in IS:800-2007details of rivet design is not given in Limit State Method however in case of bolt and weldconnection it has been described explicitly. The design of bolt connection is generally done followed by IS 800-2007, where differenttypes of failure of bolt connections is consider and design accordingly. In clause 2.4 of IS:800-2007 it says that bolts, nuts and washers shall conform as appropriate to the followingcodes: IS 1363-1967, IS 1364-1967, IS 1367-1967, IS 3640-1967, IS 3757-1972, IS 6623-1972 and IS 6639-1972. In those codes the bolt properties like their dimension, differenttypes, strength are given. In fig 3a it is shown that different members are connected at a particular point using boltconnection. Now If we come to the parts of bolts we can see bolt has a head, shank, runout,thread and nut as shown in fig 3b. Nuts are generally tightened over the plate to connectdifferent plates at a particular point. The thread length, grip length, nominal length of the boltis also illustrated in fig. 3b.So bolt has different parts like head, nut, shank, thread, thread length, grip length andnominal length which will be required for our design. When we will be going for design ofdifferent type of bolts we will see these parameters are required, these different dimensionlike what is the nominal diameter of bolt, what is the gross diameter or hole diameter of bolt,what is the type of head whether it is hexagonal or square, like this we will come across. Now before going to use bolts we should know the advantages and disadvantages of bolts. AsI discussed earlier, there are three types of joints: bolt joint, rivet joint and weld joint, whereevery joint has certain advantages and disadvantages. So we have to look what are theadvantages and disadvantages for a particular case where we are going to joining certainmember. Based on the type of connection design and the advantages and disadvantages, wechoose an appropriate connection.Advantages of bolt connection:i. Less manpower unlike rivet connectionii. High strength bolts are much stronger than rivet. Hence, bolted connections need lessfasteners than rivet joints mean less holes in the plate resulting stronger connection.iii. Bolting operation is much fasteriv. Bolting operation is very silent in contrast to hammering noise in rivetingv. Bolting is a cold process; no risk of firevi. Bolt can be removed, replaced or retightened easily in the event of faulty bolting ordamaged bolts due to accidents/hazards. Disadvantages of bolt connection:i. Bolted connections have lesser strength in axial tension as the net area at the root ofthe threads is lessii. Under vibratory loads, the strength is reduced if the connections get loosenediii. Unfinished bolts have lesser strength because of non-uniform diameteriv. Architectural look. 
Video 2
There are different types of bolts are available and these types are classified in different way.According to material and strength we can classify this as ordinary structural bolt and highstrength steel bolt. But according to type of shank we can make three types of bolt:unfinished or black bolt, turned bolt and high strength friction grip bolt (HSFG). This is veryimportant that high strength friction grip bolt is generally use in case of high load and if weneed less number of hole, less number of bolt then we have to go for HSFG bolt. Accordingto pitch and fit of thread three types of bolt can be categorized: standard pitch bolt, fine pitchbolt and coarse pitch bolt. Then according to shape of head and nut we can make as squarebolt or hexagonal bolt. Square bolt means if the head of the bolt is square and hexagonal boltmeans if the head is hexagonal. This is a typical example of hexagonal bolt. If we see here we will see that it has six numberof sides in this case. Now we need to know some certain terminology before going to use design procedure of boltconnections, like in case of rivet and bolt some terms are used like pitch distance, gaugedistance, edge distance, end distance, bolt hole, gross diameter, nominal diameter.Two plates can be connected either by bolt or rivet connection. Now according to boltposition there are various types of bolt connection, like regular bolting, zigzag bolting orplane bolting or chain bolting, diamond bolting etc. The overlapping portion of the two plates as shown in the fig. 4, is called lap distance. Nowas you can see in fig. 4 that pitch is the center to center distance of adjacent bolt measure inthe direction of stress means the force direction. Similarly, the perpendicular to the directionof stress, the center to center distance of adjacent bolt is called gauge distance. Parallel to thedirection of stress, the distance from the center of outermost bolt to the edge of the plate iscalled end distance and perpendicular to the stress, the distance if we consider is called edgedistance.Now the details of pitch distance and edge distance is discussed in clause 10.2.2 of IS 800-2007 for particular connections. According to the code the minimum pitch distance is 2.5d or2.5 times nominal diameter of the rivet or bolt. Why this minimum pitch is required becausewe need sufficient space between this rivet or bolt to tightened so that it does not overlap. Sominimum pitch is required to tighten the bolts properly and to prevent the bearing failurebetween two bolts if it is very closer than a bearing failure may occur, so to prevent thisbearing failure we need to specify a minimum pitch and code has specified this 2.5d.Let us come to maximum pitch, what is maximum pitch and why that is necessary. Maximumpitch is desirable to place bolt sufficiently close to reduce the length of connection and if wehave different members connecting at a point, we have more pitch distance than the gussetplate will be require more. So the amount of material for gusset will be more that we do notwant, that is why we will try to make pitch distance as less as possible but not less thanminimum pitch. So this maximum pitch is defined in code which is written that the pitchshould be 16t or 200 mm in tension and it should be less than 12t or 200 mm in compression. So while designing a member, say in case of a lap joint we need to provide bolts in such away that it follows the codal provision that means the limit of maximum pitch and minimumpitch has to be maintained.So minimum edge distance for rivet that is given 1.5d, where d is the nominal diameter of therivet, right and gross diameter as I told rivet has nominal diameter is termed as small d andgross diameter which is the hole diameter actually in case of rivet that is termed as D and thisD will be d +1.5 for d is less than 25 mm and it will be d + 2 mm for d is greater than or equalto 25 mm, which is given in IS 800: 1984 in the earlier code, in clause 3.6.1.1. When earliercodes were available that means 1984 which was based on Working Stress Method at thattime the code has provided the gross diameter as nominal diameter plus 1.5 for nominaldiameter less than 25 and if it is more than 25 mm than it is d + 2, that means clearance hasbeen taken as 2 mm. Now for bolt, in case of bolt the minimum and maximum edge distance and end distance aregiven in clause 10.2.4.2 and 10.2.4.3. It is stated that the minimum edge or end distance thatshould be greater than 1.7 times the hole diameter (dh) in case of sheared or hand-flame cutedges and it should be greater than 1.5 times the hole diameter in case of rolled, machineflame cut, sawn and planed edges.So for different cases the minimum edge distance is defined either 1.7 times the dh or 1.5times dh and maximum edge distance, but it should be less than 12tε, where ε = (250/f y)1/2 andt is thickness of the thinner part and maximum edge distance should not exceed this 12tε andε can be calculated from the steel property. Now another term which we have already used that will come into discussion that is bolthole, which is required to fascinate the insertion of bolts to make the connection betweensteel members and this bolt holes’ details are given in clause 10.2.1, table 19. The bolt holesgiven in code depends on the diameter of the shank that means bolt diameter or nominaldiameter. If nominal diameter is 12 to 14 then standard clearance means hole will be 1 mm,that means bolt hole is bolt diameter plus clearance of holes. You can refer to the code wherevery meticulously this table has been described. Now for different cases this hole clearancewill be different, like for standard clearance it is 1 mm, for over size it is 3 mm, for short slotit is (3 mm) 4 mm and for long slot it is 2.5 times d, where d is the diameter of the bolt.Similarly, in case of bolt diameter from 16 to 22 this standard clearance is 2 mm, over size is4 mm, short slot in case of that it is 6 mm and for all the cases it is 2.5d for long slot. Thenfor 24 mm diameter of bolt the standard clearance is considered 2, over size is 6 and for shortslot it is 8 and for long slot 2.5d. If the diameter is greater than 24 mm then the standardclearance is considered as 3 that means the whole diameter for standard is will be 24 + 3 thatis 27. Similarly, for over size it will be 8, for short slot it will be 10 and for long slot it will be2.5d. So this is how the bolt hole will be calculated. 

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