BOLTED CONNECTIONS 1 Dr N Subramanian Why Connection

BOLTED CONNECTIONS 1 Dr. N. Subramanian

Why Connection Failure Should be Avoided? A connection failure may lead to a catastrophic failure of the whole structure Normally, a connection failure is not as ductile as that of a steel member failure For achieving an economical design, it is important that connectors develop full or a little extra strength of the members, it is joining. Connection failure may be avoided by adopting a higher safety factor for the joints than the members. 2 Dr. N. Subramanian

Classification of Connections Method of fastening: rivets, bolts and welding. Connection rigidity: simple, rigid or semi-rigid. Joint resistance: Bearing connections and friction connections Fabrication location: Shop or field connections. Joint location: Beam-column, beam-to beam, column to foundation 3 Dr. N. Subramanian

Rivets and Riveted Connections Riveting not used now due to: ØThe necessity of preheating the rivets prior to driving ØLabour costs associated with large riveting crews. ØCost involved in careful inspection and removal of poorly installed rivets ØHigh level of noise associated with driving rivets 4 Dr. N. Subramanian

Types of Bolts Unfinished bolts or black bolts or C Grade bolts (IS: 1363 -1992)-bearing type connections Turned bolts - Expensive & used in Spl. jobs Precision (A-Grade)& Semi-precision (B-Grade) bolts (IS: 1364 -1992) -They are used when no slippage is permitted Ribbed bolts (Rarely used in ordinary steel structures) High strength bolts (IS: 3757 -1985 and IS: 4000 - 1992)-Friction type connections 5 Dr. N. Subramanian

Black or Ordinary Bolt and Nut Source: AISC 6 Dr. N. Subramanian

Hexagonal Head Black Bolt and Nut (IS 1363) Figures in brackets are for High-strength Bolts & Nuts Black bolts are inserted in clearance holes of about 1. 5 mm to 2 mm more than the bolt diameter and then tightened through the nuts. 7 Dr. N. Subramanian

High-Strength Bolts (cont. ) The material of the bolts do not have a well defined yield point. Instead of using yield stress, a socalled proof load is used. The proof load is the load obtained by multiplying the tensile stress area (approximately equal to 0. 8 times the shank area of bolt) by the proof stress. In IS: 800 the proof stress is taken as 0. 7 times the ultimate tensile stress of the bolt. 8 Dr. N. Subramanian

High-Strength Bolts (cont. ) They are identified Source: www. nichiasteel. co. jp 9 Dr. N. Subramanian by manufacturer’s identification symbol and property class identification symbol 8 S or 8. 8 S or 10. 9 S which will be embossed on the heads of these bolts.

High-Strength Friction Grip (HSFG) Bolts Special techniques are used for tightening the nuts to induce a specified initial tension in the bolt (called the proof-load), which causes sufficient friction between the faying faces. Such bolts are called High-Strength Friction Grip bolts (HSFG). Due to this friction, the slip in the joint is eliminated; joints with HSFG bolts are called non-slip connections or friction type connections 10 Dr. N. Subramanian

Bolt tightening using impact wrench Source: AISC 11 Dr. N. Subramanian

Calibrated Wrench tightening Wrenches are calibrated by tightening, in a hydraulic tension-measuring device, using a minimum of three bolts of the same diameter. Impact wrenches are set to stall when the prescribed bolt tension is reached. A click sound can be heard and felt when the set torque is reached. Manual torque wrenches have a torque indicating device, using which the torque required to produce the initial tension is measured. 12 Dr. N. Subramanian

Advantages of Bolted connections offer the following advantages over riveted or welded connections: Use of unskilled labour and simple tools Noiseless and quick fabrication No special equipment/process needed for installation Fast progress of work Accommodates minor discrepancies in dimensions The connection supports loads as soon as the bolts are tightened (in welds and rivets, cooling period is involved). Main drawback of black bolt is the slip of the joint when subjected to loading 13 Dr. N. Subramanian

Advantages of HSFG Bolts HSFG bolts do not allow any slip between the elements connected, especially in close tolerance holes, Thus they provide rigid connections. Because of the clamping action, load is transmitted by friction only and the bolts are not subjected to shear and bearing. Due to the smaller number of bolts, the gusset plate sizes are reduced. Deformation is minimized. Noiseless fabrication, since the bolts are tightened with wrenches. 14 Dr. N. Subramanian

Advantages of HSFG Bolts The possibility of failure at the net section under the working loads is eliminated. Since the loads causing fatigue will be within proof load, the nuts are prevented from loosening and the fatigue strength of the joint will be greater than in welded/connections. Since the load is transferred by friction, there is no stress concentration in the holes. Unlike riveted joints, few persons are required for making the connections. No heating is required and no danger of tossing of bolt. Thus safety of the workers is enhanced. Alterations, if any (e. g. replacement of defective bolt) is done easily than in welded connections. 15 Dr. N. Subramanian

Drawbacks of HSFG Bolts Bolting usually involves a significant fabrication effort to produce the bolt holes and associated plates or cleats. Special procedures are required to ensure that the clamping actions required for preloaded friction-grip joints are achieved. The connections with HSFG bolts may not be as rigid as a welded connection. HSFG bolts are about 50% higher than black bolts The percentage elongation at failure is 12% only. 16 Dr. N. Subramanian

Bolt Holes Bolt holes are usually drilled. IS: 800 allows punched holes only in materials whose yield stress (fy) does not exceed 360 MPa and where thickness does not exceed (5600/fy) mm. Bolt holes are made larger than the bolt diameter to facilitate erection. Oversize holes should not exceed 1. 25 d or (d+8) mm in diameter, where d is the nominal bolt diameter in mm. Slotted hole [provided to accommodate movements) should not exceed 1. 33 d in length (for short slotted hole) and 2. 5 d in length (for long slotted hole). 17 Dr. N. Subramanian

Lap Joints 18 Dr. N. Subramanian

Butt Joints 19 Dr. N. Subramanian

Failure of Bolted Joints 20 Dr. N. Subramanian

Bearing Failure of Bolt Photo by P. S. Green (Copyright© AISC) 21 Dr. N. Subramanian

Tension Failure of Bolts Photo by J. A. Swanson and R. Leon of Georgia Institute of Technology © AISC 22 Dr. N. Subramanian

Bearing Failure of Plates Photo by J. A. Swanson and R. Leon of Georgia Institute of Technology© AISC 23 Dr. N. Subramanian

Pitch, Staggered holes & Gauge The edge distance should be sufficient for bearing capacity and to provide space for bolt head, washer and nut. A minimum spacing of 2. 5 times the nominal diameter of the fastener is specified in the code to ensure that there is sufficient space to tighten the bolts, to prevent overlapping of the washers and to provide adequate resistance to tear-out of the bolts. 24 Dr. N. Subramanian

Bolt Dia, Pitch & Edge Distances as per IS 800 25 Dr. N. Subramanian

Gauge Distances for bolts as per SP-1 26 Dr. N. Subramanian

Note on IS Rolled Sections Bolting is often poorly executed: Shank gets bent due to tapered flange To avoid it use Tapered washers (IS 5372/IS 5374) 27 Dr. N. Subramanian

Design Strength Of Black Bolts The nominal capacity, Vnsb, of a bolt in shear is given in the code as where nn = number of shear planes with threads intercepting the shear plane, ns = number of shear planes without threads intercepting the shear plane, βlj = reduction factor which allows for the overloading of end bolts that occur in long connections βlg = reduction factor that allows for the effect of large grip length, βpk = reduction factor to account for packing plates in excess of 6 mm. The factored shear force Vsb should satisfy Vsb ≤ Vnsb / γmb (γmb = 1. 25) 28 Dr. N. Subramanian

Shear Planes With and Without Threads included in the Shear Plane 29 Dr. N. Subramanian Threads Excluded from the Shear Plane

Design Strength of Black Bolts (cont. ) Asb = Nominal shank area Anb = Net tensile stress area through the threads Anb = / 4 (d - 0. 9382 p)2 ≈ 0. 78 Asb p= pitch of thread, mm Reduction Factor for Long Joints: βlj = 1. 075 – lj (200 d) with 0. 75 ≤ βlj ≤ 1. 0 Reduction Factor for Large Grip Length: βlg = 8 d / (3 d + lg); lg ≤ 8 d; βlg ≤ βlj Reduction Factor for Packing plate: βpk = (1 -0. 0125 tpk ); tpk is the thickness of the thicker packing plate in mm 30 Dr. N. Subramanian

Bolts in Tension The nominal capacity of a bolt in tension is: Tnb = 0. 90 fub Anb < fyb Asb (γm 1 / γm 0 ) where Asb = Shank area of bolt Anb = Net Tensile Stress area of bolt fyb = Yield stress of the bolt γm 1 = 1. 25; γm 0 = 1. 10 The factored tension force Tb shall satisfy Tb ≤ Tnb / γmb ; γmb = 1. 25 If any of the connecting plates is flexible, then additional prying forces must be considered. 31 Dr. N. Subramanian

Bolts in Bearing The nominal bearing strength of the bolt is : Vnpb = 2. 5 kbd t fu = Ultimate tensile stress of the plate in MPa d = nominal diameter of the bolt in mm t = summation of the thicknesses of the connected plates experiencing bearing stress in the same direction (If the bolts are countersunk, the thickness of the plate minus one half of the depth of counter sinking) kb is smaller of e/(3 do), p/(3 do)-0. 25, fub/ fu and 1. 0, where fub is the ultimate tensile stress of the bolt, e is the edge distance, p is the pitch of the fastener along bearing direction, and do is the diameter of the bolt hole. Vnpb should be multiplied by a factor 0. 7 for over size or short slotted holes and by 0. 5 for long slotted holes. 32 Dr. N. Subramanian

Bolts in Bearing (cont. ) The factor kb takes into account inadequate edge distance or pitch and also prevents bearing failure of bolts. If we adopt a minimum edge distance of 1. 5 x bolt hole diameter and a minimum pitch of 2. 5 x diameter of bolt, kb may be approximately taken as 0. 50. The bolt bearing on any plate subjected to a factored shear force Vsb, shall satisfy Vsb ≤ Vnpb / γmb ; γmb = 1. 25 33 Dr. N. Subramanian

Capacity Of Ordinary Bolts (Grade 4. 6) Based on Net Tensile Area 34 Dr. N. Subramanian

35 Dr. N. Subramanian