Bolted, Riveted and Welded Joints

Machines are assembled from parts held by joints. Bolted joints can be taken apart; riveted and welded joints are permanent. GATE asks for their strength and for loads on the most stressed bolt or weld.

Bolted joints

• Tensile stress area of a bolt carries the axial load: σ = P/A_t.
• A bolt is preloaded (initial tension F_i) to keep the joint tight. When an external load P is applied, the bolt takes only part of it: F_bolt = F_i + C P, with joint constant C = k_b/(k_b + k_m) (bolt and member stiffnesses).
• The joint separates when the member's share (1 − C)P exceeds F_i.

Riveted joints

A riveted (or bolted) lap joint can fail by:

1. Tearing of the plate between rivets: P_t = (p − d) t σ_t.
2. Shearing of the rivets: P_s = n (π/4) d² τ (double for double shear).
3. Crushing of the plate or rivet: P_c = n d t σ_c.

Efficiency η = least of P_t, P_s, P_c divided by the strength of the solid plate p t σ_t.

Eccentrically loaded joints

When a load P acts at an eccentricity e from the centroid of a group of n bolts:

• Direct (primary) shear on each bolt: P/n.
• Secondary shear from the moment Pe: on a bolt at distance r from the centroid, F = Pe r/Σr².
• Add the two as vectors; the most loaded bolt is usually the one farthest from the centroid on the side of the load.

Welded joints

• Butt weld in tension: σ = P/(t L).
• Fillet weld: the weakest section is the throat, t = s cos 45° = 0.707 s (s = leg size).
• Parallel (shear) fillet weld: P = 0.707 s L τ.
• Transverse fillet welds are also designed on the throat area.

Worked example

A plate is joined by two parallel fillet welds, each 100 mm long with 10 mm legs; τallow = 80 MPa. Find the safe load.

• Throat area = 2 × 0.707 × 10 × 100 = 1414 mm².
• P = 1414 × 80 = 113.1 kN.

Practice