Welded & bolted joints on the PE Mechanical exam

Mechanical attachments — fasteners and welds — are a dedicated Machine Design & Materials area and a reliable source of points. Most problems reduce to finding a stress on a known area, with the wrinkle being eccentric (off-center) loads.

What you actually need to own

• Bolts in shear: τ = P / (n·A), where n is the number of shear planes/bolts and A is the bolt area (use the shear area — body or threads, as specified).
• Bearing on the connected material: σ_b = P / (t·d), and tension on the net section.
• Bolts in tension (e.g., bolted flanges): F = P/n, plus preload effects when the problem includes them.
• Fillet welds: the strength is on the throat, not the leg. Throat = 0.707 × (leg size) for an equal-leg fillet; weld shear stress τ = P / (throat × weld length).
• Eccentric joints (the classic trap): the load produces a direct shear (P over all fasteners/weld) plus a torsional shear from the moment (P × eccentricity). The two are vectors — combine them at the most-stressed fastener or weld point.

Where people lose points

• Using the weld leg instead of the throat (0.707 factor forgotten).
• Missing the eccentricity — solving only direct shear and ignoring the moment-induced shear.
• Wrong combination of direct and torsional shear (adding magnitudes instead of vectors).
• Bolt area confusion — gross vs net vs thread (tensile-stress) area.

How to study it

Drill the basic shear/bearing/tension cases until they're trivial, then put your reps into eccentric bolt-group and weld-group problems — locating the critical point and vector-summing direct + torsional shear is exactly where the exam separates people. Connect it back to combined-loading stress analysis.

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