Bolted connections


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Explains the various aspects of the design of bolted connections.

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Bolted connections

  1. 1. BOLTED CONNECTIONS1 Dr. N. Subramanian
  2. 2. Why Connection Failure Should be Avoided?  A connection failure may be 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
  3. 3. 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 foundation3 Dr. N. Subramanian
  4. 4. Classification of Connections (cont.)  Connection geometry: Single web angle, single plate, double web angle, top and seat angles (with and without stiffeners), end plates, or header plate, welded connections using plates and angles, etc.  Type of force transferred across the structural connection: Shear connections, shear and moment connection or simply moment connection, tension or compression, tension or compression with shear.4 Dr. N. Subramanian
  5. 5. Classification Based on Joint Rigidity  Rigid: That develop the full moment capacity of connecting members and retain the original angle between the members under any joint rotation. Rotational movement of the joint will be very small  Simple: No moment transfer is assumed between the connected parts and hence assumed as hinged (pinned). Rotational movement of the joint will be large.  Semi-Rigid: May not have sufficient rigidity to hold the original angles between the members and develop less than the full moment capacity of the connected members. In reality all the connections will be semi-rigid only.5 Dr. N. Subramanian
  6. 6. Examples of Rigid Connections6 Dr. N. Subramanian
  7. 7. Examples of Pinned Connections7 Dr. N. Subramanian
  8. 8. Rivets and Riveted Connections Riveting not used now due to: The necessity of preheating the rivets prior to driving Labour costs associated with large riveting crews. Cost involved in careful inspection and removal of poorly installed rivets High level of noise associated with driving rivets8 Dr. N. Subramanian
  9. 9. 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 connections9 Dr. N. Subramanian
  10. 10. Black or Ordinary Bolt and Nut Source: AISC10 Dr. N. Subramanian
  11. 11. 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 1.5mm to 2mm more than the bolt diameter and then tightened through the nuts.11 Dr. N. Subramanian
  12. 12. Tensile Properties of Fasteners For grade 4.6 bolts, nuts of grade 4 are used and for grade 8.8, nuts of grade 8 or 10 are used. In property class 4.6, the number 4 indicates 1/100th the nominal ultimate tensile strength in N/mm2 and the number 6 indicates the ratio of yield stress to ultimate stress, expressed as a percentage. Thus the ultimate tensile strength of class 4.6 bolt is 400 N/mm2 and yield strength is 0.6 times 400, which is 240 N/mm212 Dr. N. Subramanian
  13. 13. Dimensions of Grade 4.6- Hexagon Head Bolts (IS 1364) Sizes in Brackets not preferred.13 Dr. N. Subramanian
  14. 14. High-Strength Bolts (IS 3757) Made from bars of medium carbon steel. Bolts of property class 8.8 and 10.9 are commonly used.14 Dr. N. Subramanian
  15. 15. High-Strength Bolts (cont.)  The material of the bolts do not have a well defined yield point.  Instead of using yield stress, a so- called 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.15 Dr. N. Subramanian
  16. 16. High-Strength Bolts (cont.)  They are identified by manufacturer’s identification symbol and property class identification symbol 8 S or 8.8 S or 10 S or 10.9 S which will be embossed on the Source: heads of these bolts. Class 10.9 bolts should not be hot-dip galvanized16 Dr. N. Subramanian
  17. 17. 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 connections17 Dr. N. Subramanian
  18. 18. Bolt Tightening Techniques  When slip resistant connections are not required, high strength bolts are tightened to a ‘snug-tight’ using an ordinary spud wrench.  When slip resistant connections are desired with HDFC bolts, three methods are used:  Turn-of-the-nut tightening (part–turn method) –Cheap, more reliable, and common method.  Direct tension indicator tightening,  Calibrated wrench tightening (torque control method).18 Dr. N. Subramanian
  19. 19. Bolt tightening using impact wrench Source: AISC19 Dr. N. Subramanian
  20. 20. Turn-of-the nut Tightening In the American practice, 1/3 turn of the nut is prescribed for bolt length less than 4 d, 1/2 turn of the nut for bolt lengths from 4 to 8 d or 200 mm and 2/3 turn of the nut for bolt lengths greater than 8 d or 200 mm, where d is the diameter of bolt (Salmon and Johnson, 1996). The bolts are tensioned using 1/8 turn increments.20 Dr. N. Subramanian
  21. 21. Behaviour of bolt-Turn-of- the-nut Method21 Dr. N. Subramanian In this method the bolt deformation is a critical factor
  22. 22. Direct Tension Indicator Tightening  There are two types of proprietary load – indication devices.  The first type of device indicates the load by producing a measurable change in gap between the nut and the gripped material.22 Dr. N. Subramanian
  23. 23. Direct Tension Indicator Tightening (cont)  In the second type, the bolt is tightened by turning a nut, which has a protruding nib; the tightening is continued till the nib shears off.23 Dr. N. Subramanian
  24. 24. 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.24 Dr. N. Subramanian
  25. 25. Advantages of Bolted connections  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 loading25 Dr. N. Subramanian
  26. 26. Load-Deformation Behaviour of Different Types of Fasteners26 Dr. N. Subramanian
  27. 27. 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.  Holes larger than usual can be provided to ease erection and take care of lack-of-fit. However note that the type of hole will govern the strength of the connection.  Noiseless fabrication, since the bolts are tightened with wrenches.27 Dr. N. Subramanian
  28. 28. 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.28 Dr. N. Subramanian
  29. 29. Drawbacks of HDFC 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.29 Dr. N. Subramanian
  30. 30. 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 1.25d or (d+8) mm in diameter, where d is the nominal bolt diameter in mm.  Slotted hole [provided to accommodate movements) should not exceed 1.33d in length (for short slotted hole) and 2.5 d in length (for long slotted hole).30 Dr. N. Subramanian
  31. 31. 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.31 Dr. N. Subramanian
  32. 32. Bolt Dia, Pitch & Edge Distances as per IS 80032 Dr. N. Subramanian
  33. 33. Gauge Distances for bolts as per SP-133 Dr. N. Subramanian
  34. 34. 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)34 Dr. N. Subramanian
  35. 35. THANK YOU!35 Dr. N. Subramanian