Bolts may be used in place of rivets for structure not subjected to vibrations. The following types of bolts are used in structures:
- Hexagonal black bolts are commonly used in steel works.
- 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.
(i) 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.
(ii) 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 joint.
- The butt joint may have a single row or staggered or chain riveting.
Failure of Riveted/Bolted Joints
- By Tearing of Plate between rivets
Strength of tearing per pitch length
Pt = (p – d') t x ft
where, ff = Permissible tensile stress in plates
t = Thickness of plate
d' = Dia of hole (gross dia of rivet)
p = Pitch
- Strength of rivet in single shear
- Strength of rivet in double shear
where, fs = allowable shear stress in rivets
d' = dia of hole.
- Failure due to bearing of crushing of rivet of plates
Strength of rivet in bearing
where, fb = bearing strength of rivet.
Efficiency of Joints
Where, Ps = Strength of joint in shear
Pb = Strength of joint in bearing
Pt = Strength of joint in tearing
P = Strength of plate in tearing when no deduction has been made for rivet holes
= p. t. ft
- Rivet value
- Number of rivet,
IS 800: 1984 Recommendation
Maximum permissible stress in rivets & bolts
- Rivet diameter, Pitch
Where t = thickness of thinner outside plate
Max Permissible Deflections
- Max permissible horizontal and vertical deflection
- Max permissible deflection when supported elements are susceptible to cracking
- Max permissible deflection when supported elements are not susceptible to cracking
Arrangement of Rivets
(a) Chain Riveting
(b) Diamond Riveting
(c) Staggered Riveting
Where, FDi = Direct force in ith rivet.
FTi = Force in ith rivet due to torsional moment
ri = Distance of ith rivet from CG
Ai = Area of ith rivet
FDi = Always acts in the direction of applied load P.
FTi = Always acts perpendicular to the line joining CG of rivet group and the rivet under consideration.
Fri = Resultant force in ith rivet.
Minimum size of weld
It depends upon thickness of thicker plate
Max clear spacing between 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
(b) to make stress distribution uniform
(c) if b1 > 16t use end fillet weld.
- Permissible Stresses
(a) Tensions and compression on section through the throat of butt weld = 150 N/mm2
(b) Shear on section through the throat of 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 weld,
Where t = thickness of weld throat and d = length of 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 = 0.67fy
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
(i) 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 weld,
- Horizontal shear stress due to bending at extreme fibre,
- Resultant stress,
- The value of pr should not exceed the permissible shear stress pq (= 108 MPa) in the weld.
- For design of this connection, the depth of weld may be estimated approximately by
(ii) Load lying in the plane of the weld: Consider a bracket connection to the flange of a column by a fillet weld as shown in figure
- Vertical shear stress at weld,
the length of weld and t = thickness of the throat
- Torsional stress due to moment, at any point in the weld,
T = torsional moment = W x e
r = distance of the point from cg of weld section
Ip = polar moment of inertia of the weld group = lx + ly
- 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.
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