Design of connections

Design of connections

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  Sanjivani Rural EducationSociety’s Sanjivani College of Engineering, Kopargaon-423 603 ( An Autonomous Institute Affiliated to Savitribai Phule Pune University, Pune) NAAC ‘A’ Grade, NBA Accredited, ISO 9001:2015 Certified Unit I : Design of connections By Prof V M Mahajan (Asst. Professor) Department of Civil Engineering Email Id: [email protected]
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  Connections  As weknow that, A steel structure is an assemblage of different element, which are connected, fastened to one other. So that they will act together as a single composite unit.  Connections acting as a medium for transfer of force, load or moment from one member to other.  The following types of connections: 1. Riveted connections 2. Bolted connections 3. Welded connections 4. Pins connections 5. Combination of above 2
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  Bolted connections  Abolt may be defined as a metal pin with a head at one end and a shank threaded at the other end to receive a nut.  Steel washers are provided under the bolt as well as nut for; distributing the clamping pressure of the bolted member and to prevent threaded portion of the bolt from the bearing on the connecting pieces. 3
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  Classification: 1) Based onforce transferred: a) Concentric connections- load passes through C.G. b) Eccentric- load away from the C.G. c) Moment resisting connections- joint subjected to moment-beam and column 2) Based on type of force: a) Shear connections- lap and butt joint b) Tension connections- hanger connection. c) Combined shear and tension- bracing. 3) Based on force mechanism: a) Bearing type- bolt bear against the hole to transfer the force. b) Friction type- force is transferred by friction. 4 Bolted connections
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  Types of bolts 1)Unfinished or black bolt-  Bearing type of bolt. It also called as ordinary, common, rough bolt.  Made from mild steel rods with square or hexagonal head.  Nominal diameter- 16, 20, 24, 30 and 36 are commonly used. 2) Finished or Turned bolt- shank of these bolt is formed from a hexagonal rod. 3) High strength bolts-  Made from medium carbon heat treated steel and from alloy steel.  Has high strength two or more times than ordinary bolt.  Available in size 16mm to 36mm. 5
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   Grade 4.6– M20  20mm is diameter of bolt  Ultimate tensile strength = 4x100, fub = 400 N/mm2  Yield stress = 400x0.6, fyb = 240 N/mm2  IS 1367 (part – 3) Advantages: 1) Connections can be made quickly; erection of the structure speedy. 2) Less skilled persons are required. 3) Less equipment are required. 4) Cost is less. 6 Grade of steel fub fyb 3.6 330 180 4.6 400 240 4.8 420 320 5.6 500 300 5.8 520 400 Types of bolts
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  Types of boltedjoint 1) Lap joint-  Two member are overlapped and connected together.  It may be single or double bolted lap joint.  2) Butt joint- 2) Butt joint-  Two member to be connected are placed end to end.  Additional plates are provided on either or one side is known as cover plates. 7
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  Failure of boltedjoints Failure may be in bolts or member connected: 1) Shear failure of bolt:  The maximum factored shear force in the bolt may exceed the nominal shear capacity of the bolt.  Bolt may fail in single or double shear. 2) Bearing failure of plates:  When member is subjected to shear force , slip takes place and bolt comes in contact with plates. 8
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  3) Bearing failureof bolt:  Bolt is crushed around half circumference.  It does not occur in practice except when plates are made of high strength steel. 4) Tension failure of bolt: Bolts subjected to tension may fail at the stress area. 5) Tension or tearing of plate: it occurs when bolt are stronger than plate. 6) Block shear failure: if bolts are placed at a lesser end distance than required causing plate to shear out. Block of material within the bolted area breaks. 9 Failure of bolted joints
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  Specification for boltedjoints 1) Pitch of the bolts (P): (IS 800:2007, Cl. No. 10.2.2, page no. 73)  The distance between centers of two consecutive bolts in row, measured along the direction of load.  P shall not be less than 2.5 times diameter of bolt.  P shall not be exceed 16t or 200mm, whichever is less (tension member)  P shall not be exceed 12t or 200mm, whichever is less (compression member)  P shall not be exceed 4.5 d, for butt joint. 2) Gauge distance (g):  c/c distance of consecutive bolt of adjacent rows and measured perpendicular to direction of load.  g shall not be exceed 100 + 4t or 200mm, whichever is less.  g shall not be less than 2.5 of dh. 10
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  3) Edge distance(e): (IS 800:2007, Cl. No. 10.2.4.2, page no. 74)  It is the distance of center of bolt from adjacent edge of plate.  e shall not be less than 1.7 times diameter of hole………for hand flame edge.  e shall not be less than 1.5 times diameter of hole………for machine flame edge.  e shall not be exceed 12t t= thickness of thinner plate  e shall not be exceed 40 + 4t 4) End distance (e’ ): same as above 11  Specification for bolted joints
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  Bearing – typeconnections  It is assumed that, the load to be transferred is larger than the frictional resistance caused by tightening the bolts, consequently members slip a little over each other, placing the bolt in shear and bearing.  No. of bolts required is equal to load divided by the strength of one bolt.  Generally, a minimum two or three bolts are provided even if one bolt is required.  The strength of a bolt multiplied with the number of bolts in the joints gives the strength of joint on the basis of bolts.  However, the failure of joints can be in bolt or in plate. 1) Shearing strength of bolt’s: (IS 800:2007, Cl. No. 10.3.3, 75)  The resistance of bolt to shear is called as the nominal capacity of bolt in shear and denoted by V nsb.  It depends upon the ultimate tensile strength of bolt fub and no. of shear planes n.  The nominal shank area Asb and shear area of bolt at threads Anb in each shear plane. 12
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   The nominalshear capacity of the bolt is given by, Where, = No. of shear planes with threads. = No. of shear planes without threads (shank, plane area) a) If length of joint (i.e. distance between first row & last row of bolt) exceeds 15d, where d = diameter of bolt, then Reduction factor is multiplied, (IS 800:2007, 10.3.3.1, 75) The nominal shank area, Asb =[(π/4)x(d2)] Anb =0.78X[(π/4)x(d2)] Anb = 0.78 X Asb 13   3 bssbnnbu bsn AnAnf V   nn sn d l j jl 200 075.1  jl Bearing – type connections
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  b) If thegrip length (depth) lg exceeds 5d the shear capacity shall be reduced to (IS 800:2007, 10.3.3.2, 75) c) If packing plates in excess of 6mm are used the shear capacity should be reduced to (IS 800:2007, 10.3.3.3, 75) d) Bolt subjected to combined shear and tension (IS 800:2007, 10.3.6, 76) Vsb = Vb = factored shear force acting on bolt Vdb = design shear capacity (10.3.2) Tb = factored tensile force acting on the bolt and Tdb = design tension capacity (10.3.5) 14   caseanyindexceednotshalllandexceednotshall ld d gjlgl g gl 8 3 8     )0125.01( kpkp t 0.1 22             db b db b T T V V Bearing – type connections
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  2) Bearing strengthof bolt: (IS 800:2007, Cl. No. 10.3.4, 75) Nominal bearing strength of bolt; = diameter of hole d = diameter of bolt e, p = edge and pitch t = thickness of connected plate = ultimate tensile stress of bolt and plate respectively Bearing strength 15        0.1;;25.0 3 ; 3 u bu oo b f f d p d e ofsmallerk od ubbpn ftdkV  5.2 ubu ff , bm u b bm bpn bm bpn bp f tdk VV V   5.2 Bearing – type connections
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  3) Tensile strengthof bolt: (IS 800:2007, Cl. No. 10.3.5, 76) Nominal tensile capacity of bolt in tension, 16 om bm bsbybnbubn AfAfT    9.0 bm bnbu bm bn bd Af T Tif      9.0 Bearing – type connections