Structural Fasteners (Rivets, Welds & Bolts) - Notes | Study Design of Steel Structures - Civil Engineering (CE)

Riveting

The size of the rivet is the diameter of the shank.

• Gross dia of rivet or dia of hole d' = d + 1.5 mm for d ≤ 25 mm
  and d' = d + 2.0 mm for d ≥ 25 mm
  where d = Nominal dia of rivet
  d' = Gross dia of rivet or dia of hole…
• Unwin's formula  
  where, d_mm = dia of rivet in mm
  t_mm = thickness of the plate in mm.

Bolted Joints

Bolts may be used in place of rivets for structure not subjected to vibrations. The following types of bolts are used in structures:

Black bolts

• Hexagonal black bolts are commonly used in steelworks.
• They are made from low or medium carbon steels.
• They are designated as black bolts M x d x l where d = diameter and l = length of the bolts.

Precision and Semi Precision Bolts

• They are also known as close tolerance bolts.
• Sometimes to prevent excessive slip, close tolerance bolts are provided in holes of 0.15 to 0.2 mm oversize. This may cause difficulty in alignment and delay in the progress of work.
• Types of Riveted and Bolted Joints.

There are two types of riveted or bolted joints:

1. Lap joint

• The lap joint is that in which the plates to be connected overlap each other.
• The lap joint may have single-row, staggered or chain riveting.

2. Butt joint

• The butt joint is that in which the plates to be connected butt against each other, and the connection is made by providing a cover plate on one or both sides of the joint.

• The butt joint may have a single row or staggered or chain riveting.

Failure of Riveted/Bolted Joints

1. By Tearing of Plate between rivets
   Strength of tearing per pitch length
   P_t = (p – d') t x f_t
   where f_f = Permissible tensile stress in plates
   t = Thickness of plate
   d' = Dia of hole (gross dia of rivet)
   p = Pitch
2. Strength of rivet in single shear 
3. Strength of rivet in double shear
   where f_s = allowable shear stress in rivets
   d' = dia of the hole.
4. Failure due to bearing of crushing of rivet of plates
   Strength of rivet in bearing P_b = f_b.d'².t
   where, f_b = bearing strength of rivet.

Efficiency of Joints ^(η) 

Where, Ps = Strength of joint in shear
  P_b = Strength of joint in bearing
  P_t = Strength of joint in tearing
P = Strength of plate in tearing when no deduction has been made for rivet holes
= p. t. f_t 

• Rivet value
• Number of rivets,

IS 800: 1984 Recommendation

Maximum permissible stress in rivets & bolts

• Rivet diameter, Pitch

Where t = thickness of the thinner outside plate

Permissible Stresses

Max Permissible Deflections

• Max permissible horizontal and vertical deflection
• Max permissible deflection when supported elements are susceptible to cracking image015
• Max permissible deflection when supported elements are not susceptible to cracking  

Arrangement of Rivets

1. Chain Riveting
2. Diamond Riveting
3. Staggered Riveting
   
   Where, F_Di = Direct force in i^th rivet.
   F_Ti = Force in i^th rivet due to torsional moment
   r_i = Distance of i^th rivet from CG
   A_i = Area of ith rivet
   F_Di = Always acts in the direction of applied load P.
   F_Ti = Always acts perpendicular to the line joining CG of rivet group and the rivet under consideration.
   F_ri = Resultant force in i^th rivet.
   

Minimum size of weld

It depends upon the thickness of the thicker plate.

Max clear spacing between the effective length of weld in compression zone = 12t or 200 mm (minimum). In tension zone = 16 t or 200 mm (minimum)

• Slot weld
• Slide fillet weld(i)
  (ii)  to make stress distribution uniform
  (iii) if b1 > 16t, use end fillet weld.

Welded Connection

• Permissible Stresses
  (i) Tensions and compression on the section through the throat of butt weld = 150 N/mm²
  (ii) Shear on the section through the throat of the butt of fillet weld =108 N/mm² ≅ 100 N/mm²
  Throat thickness t = k x size of weld
• Butt-welded Joint Loaded Eccentrically
  Let the thickness of weld throat = t, and length of weld = d
• Shear stress at the weld,
  Where t = thickness of weld throat and d = length of the weld.
• Tensile or compressive stress due to bending at extreme fibre,
  
  For the safety of joint the interaction equation.
• Equivalency Method
   (based on max distortion energy theory)
  Permissible bending stress for flanged section = 165 N/mm² = 0.67f_y
  For solid section  permissible bending stress is 185 N/mm²

Fillet-Welded Joint Loaded Eccentrically
There can be two cases:

• Load not lying in the plane of the weld
• Load lying in the plane of the weld

1. Load not lying in the plane of the weld:

• Let thickness of weld throat = t and total length of weld = 2 x d
• Vertical shear stress at the weld,
• Horizontal shear stress due to bending at extreme fibre,
  
• Resultant stress,
• The value of p_r should not exceed the permissible shear stress pq (= 108 MPa) in the weld.

• For the design of this connection, the depth of weld may be estimated approximately by,
  

2. Load lying in the plane of the weld: Consider a bracket connected to the flange of a column by a fillet weld as shown in figure

• Vertical shear stress at the weld,
  where,  = the length of weld and t = thickness of the throat
• Torsional stress due to moment, at any point in the weld,  
  where, T = torsional moment = W x e
  r = distance of the point from cg of weld section
  I_p = polar moment of inertia of the weld group = l_x + l_y
• The resultant stress,
• For safety,  permissible stress in fillet weld, i.e. 108 MPa.
• The resultant stress pr will be maximum at a point where r is maximum and q is minimum.