Tension & Torque in Bridge Engineering: The Forces That Shape Spans

Bridges are machines that manage forces. Two critical ones:

Tension (Pulling Force)

Acts on members being stretched. Key examples:

• Suspension Bridges: Main cables carry entire deck weight + live load in pure tension (up to 200,000+ kN). Anchorages resist cable pull.
• Cable-Stayed Bridges: Inclined stays in tension transfer deck loads to towers.
• Tied-Arches: Bottom chord (tie) in tension balances arch thrust.
• Truss Bottom Chords: In simply supported trusses, bottom chords are in tension.

Design requires high tensile strength (steel wire, strand, rod), fatigue resistance, and corrosion protection.

Torque (Twisting Moment)

Rotational force about a longitudinal axis. Key examples:

• Curved/ Skewed Bridges: Eccentric loads induce torsion in girders/box sections.
• Arch Bridges: Arch rib experiences combined compression + torsion if loading asymmetric.
• Truss Web Members: Diagonals/verticals transfer shear via axial forces, but connections see torque.
• Deck Slabs: Overhangs cantilever → torsion in edge girders.

Torsional stiffness (closed box sections, diaphragms) is crucial to prevent distortional buckling or excessive twist.

Interaction

Real bridges experience combined tension, compression, bending, shear, and torque. Finite-element analysis resolves these interactions for safe, efficient designs.