Design of bolted connections

Design of bolted connections

1. INTRODUCTION TO BOLT CONNECTIONS 1 01-07-2020

2. TYPES OF JOINTS Depending up on arrangement of bolts and plates Depending up on the mode of load transmission Depending up on nature and location of load 2

3. TYPES OF JOINTS Depending up on arrangement of bolts and plates Mainly two types of joints subjected to axial loads, they are Lap joints: The two members to be connected are overlapped and connected together. Such joint is called lap joint. The load in the lap joint has eccentricity, as the centre of gravity of the load in one member and centre of gravity of the load in the second member are not on the same line. 3

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5. TYPES OF JOINTS Butt joint The two members to be connected are placed end to end. Additional plate/plates provided either one or both sides, called cover plates, are placed and are connected to the main plates. Two types mainly Single cover butt joint Double cover butt joint 5

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8. 8 It is more desirable to provide butt joint rather than a lap joint for snug-tight bolts because In the case of a double cover butt joint, the total shear force to be transmitted by the members is split into two parts so shear carrying capacity of a bolt in a double cover butt joint is double that of bolt in lap joint. Eccentricity of forces does not exist in butt joint and hence bending is eliminated.

9. Chain bolting zig-zag bolting and diamond bolting 9

10. Chain bolting zig-zag bolting and diamond bolting 10

11. TYPES OF JOINTS Depending up on the mode of load transmission Single shear Double shear Multiple shear 11

12. TYPES OF JOINTS Depending up on the mode of load transmission Single shear Double shear Multiple shear 12

13. 13 Single shear Double shear

14. 14 Multiple shear

15. TYPES OF JOINTS 3. Depending up on the nature and location of load Direct shear connections Eccentric connections Moment shear connections 15

16. 16 Direct shear connections Eccentric connections

17. 17 Moment shear connections

18. Failure of bolted joints Following are the failure modes of bolted joints Shear failure of bolts Bearing failure of bolts Tensile failure of bolts Shear failure of plate Bearing failure of plate Tensile failure of plate 18

19. Shear failure of bolts 19 The maximum factored shear force in the bolt may exceed the nominal shear capacity of the bolt.

20. Bearing failure of bolts 20 The bolt is crushed around half circumference. The plate may be strong in bearing and the heaviest stressed plate may press the bolt shank. Plates are made of high strength steel and Bolts are made of very low grade steel

21. Tension failure of bolts 21

22. Bearing failure of plates 22

23. When an ordinary bolt is subjected to shear forces, slip takes place and bolt comes in contact with the plates. The plate may get crushed if the plate material is weaker than the bolt material. The bolt spacing and end distance will influence the bearing strength. So adhere to the codal provisions of bolt spacing, end distance, max pitch etc. 23

24. Tension or tearing failure of plates 24 Tension failure occurs because when the bolts are stronger than the plates. Tension both the gross area and net effective area must be considered.

25. Block shear failure 25

26. Block shear failure 26 Bolts may have been placed at a lesser end distance than required causing the plates to shear out. May occur when a block of material within the bolted area brakes away from the remainder area. Occur when fewer bolts of high strength are used. Also checked by observing the specifications of end distance.

27. Design strength of ordinary black bolts ( clause 10.3 of IS 800:2007) The strength of an ordinary bolt is minimum of strength of bolts in shear, bearing and tension. ( As discussed in the failure of joints) No of bolts required = However failure of a joint can be in bolt or plate. Thus the strength of a joint will be minimum of strength of joint on the basis of strength of bolts in the joint and net tensile strength of the plate.   27

28. Shear strength of bolts ( clause 10.3.3 of IS 800:2007) The design strength of the bolt, V dsb as governed shear strength is given by V dsb = V nsb = Nominal shear capacity of a bolt =   28

29. Shear strength of bolts ( clause 10.3.3 of IS 800:2007) f ub = ultimate tensile strength of bolt n n = number of shear planes with threads intercepting the shear plane. n s = number of shear planes without threads intercepting the shear plane. A sb = nominal plain shank area of the bolt A nb = net shear area of the bolt at threads may be taken as the area corresponding to root diameter at the thread. Ɣmb = Partial safety factor according to table 5 of IS 800:2007 29

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