Connections

Connections

• 1.
  Connections by: Dinesh Nath
• 2.
  Connections 1. Riveted Connections 2.Bolted Connections 3. Welded Connections 4. Eccentric connections
• 3.
  Fabrication of structures Beamand column Beam and Beam Column and column Column and caps Purlins and rafter Rails and column
• 4.
  Requirement of goodconnection • It should be rigid enough to avoid fluctuating stresses which may cause fatigue failure. • It should be such that there should be least possible weakening of the parts to be joined. • It should be such that easily installed, inspected and maintained.
• 5.
  Assumption • Friction betweenthe plates to be neglected • The shear stress is uniform on the cross section of rivet. • The distribution of direct stress on the portion of the plate between the rivet holes is uniform. • Rivets in the direct loads share the loads equally • Bending stress in the rivets is neglected. • Rivets fill completely the holes in which they are filled • Bearing stress is uniform and contact stress is uniform.
• 6.
  Types of Rivets 1.Snap 2. Pan 3. Flat Countersunk 4. Round Countersunk Rivets may be placed in cold state or red hot state. When rivets are heated before they are placed they are called Hot driven field rivets or hot driven shop rivets.
• 7.
  Types of RivetedJoints Lap joint: The two members to be connected are over lapped and connected together. a) Single riveted lap joint b) Double riveted lap joint
• 8.
  Butt joint: Thetwo members are to be connected end to end. Additional plates are provided on either one or both sides. a) Single cover butt joint b) Double cover butt joint
• 9.
  Specifications • Gross diameter(d):nominal dia(𝛟) + 1.5mm d≤25mm • Gross diameter(d) nominal dia(𝛟) + 2 mm d≥25mm Gauge: Distance between adjacent rivet lines. Pitch: Distance between centre of the two consecutive measured along a row of the rivets. Min pitch: 2.5𝛟 Max pitch: 16t or 200mm (which ever is less) in tension 12t or 200mm (Which ever is less) in compression 32t or 300mm( which ever is less) if tack riveted.
• 10.
  • Edge distance(e):Distance between centre of the rivet hole to the adjacent edge of the member. If we put e small then a) The failure of plate in tension may take place. b) The steel opposite to the hole may bulge out and crack. Maximum edge distance = 1.5d (hand cut) 1.7d (machine rolled) Maximum edge distance= 40 mm + 4t Tacking Rivets: Tacking or stitch rivets are used to make the section to act unison Permissible tensile stress in plate = 0.6 fy Permissible Stresses in rivets Type of Rivet Axial Tension (Mpa) Shear (Mpa) Bearing (Mpa) Power driven 100 100 300 Hand driven 80 80 250
• 13.
  Types of failures Platefailures : Failure of plate in Shear, Bearing and splitting can be eradicated simply by providing sufficient edge distance itself.
• 14.
  Rivet failures: • Shearfailure • Bearing failure
• 15.
  Strength of aRiveted Joint The strength of riveted joint is computed by considering the failure of rivets in shear and Bearing and of plates tearing only. • Strength of rivet in shear: The resistance of a rivet to shear depends on cross sectional area of the shank and allowable unit shear stress. a) single shear: Strength of one rivet (Ps) = 𝜋 4 d2τ 𝑎 Strength of riveted joint = n 𝜋 4 d2τ 𝑎
• 16.
  • Double shear:This mainly seen in double cover butt joint. Strength of one rivet = 2 𝜋 4 d2τ 𝑎 Strength of riveted joint = n 2 𝜋 4 d2τ 𝑎 Where: Ps – Strength of riveted joint in shear. d – Gross diameter of rivet τ 𝑎 - Allowable shear stress in rivet n – number of rivets in each side of joint.
• 17.
  Strength of rivetin bearing: projected area x permissible bearing stress. Pb = dtσb Strength of rivet joint : Pb = n dtσb Where: Pb – Strength of rivet joint in bearing in Newton's t - Thickness of thinner plate in lap joint - Thickness of main thinner plate or sum of cover plate thickness, which ever is less σb – permissible bearing stress in rivet in MPa.
• 18.
  • Rivet ValueRv= The min strength of rivet in shear or bearing is called Rivet value. Strength of plate in tearing: Depends upon the net section resisting the force. Pt = (B-nd)t 𝜎𝑡 Pt- Strength of riveted joint in tearing in Newton B – Width of the plate. 𝜎𝑡 – Permissible tensile stress in plates Strength of joint is the minimum value of Rivet value and tearing strength of plate i.e min( Rv , Pt )
• 19.
  • Efficiency ofriveted joint (𝜂): 𝑆𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑟𝑖𝑣𝑒𝑡𝑒𝑑 𝑗𝑜𝑖𝑛𝑡 𝑆𝑡𝑟𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑆𝑜𝑙𝑖𝑑 𝑝𝑙𝑎𝑡𝑒 x 100 • Where Strength of solid plate = Bt𝜎𝑡 • Unwin's formula for nominal diameter = 6.01 𝑡 • Thickness of cover plate should not be less than 5/8 times the thickness of main plate. • Field rivets permissible stresses are reduced by 10% • If Wind or earth quake loads are acting then permissible stress on rivets increased by 25 %
• 20.
  Examples: 1. Two platesof 8 mm and 10 mm thickness are joined by 16 mm power driven field rivets in a single riveting lap joint. Calculate rivet value of the joint.
• 21.
  Design criteria toremember in designing of Rivet joint. • Length of joint should be as small as possible • The number of joints must be increased gradually as we move towards the joint for uniform stress distribution in rivets • Centre line of all joints meeting at a joint should be coincide at one point only other wise the joint will twist out of position. • The arrangement should satisfy the pitch and edge requirements.