Bridges

Bridges carry roads, railways and people across obstacles. Learn bridge types (beam, truss, arch, cable-stayed, suspension) and their span ranges, loads (vehicle, pedestrian, wind, thermal), moving-load analysis, bearings and expansion joints, cable mechanics and inspection.

Bridges are the most visible civil structures — from a 10 m culvert to spans exceeding 2 km. The span, site, loads and construction method decide which form is most efficient.

Bridge engineering adds moving loads, fatigue, dynamics, durability in harsh exposure and complex construction staging to ordinary structural design.

An analogy

Every bridge form moves load to the supports in its own way: beams bend, arches squeeze, cables pull, trusses share tension and compression among members.

Bridge types and spans

Slab/beam bridges (up to ~50 m; precast girders), box girders (40–300 m), trusses (50–500 m), arches (50–550 m; compression), cable-stayed (150–1100 m; deck hung from towers by straight cables), suspension (500–2000 m+; main cables with hangers).

Loads

Dead load (deck, surfacing), vehicle loading models (IRC Class A/70R, Eurocode LM1, AASHTO HL-93) with dynamic allowance, pedestrian crowds, braking, centrifugal forces, wind, temperature, creep/shrinkage, earthquake, and collision (ships, vehicles).

Moving loads and influence lines

Vehicles are moved across the bridge to find maximum moments and shears — using influence lines or the absolute-maximum-moment theorem. Lane positions and multiple-presence factors determine design effects for each girder.

Bearings, joints, dynamics and inspection

Bearings (elastomeric, pot, sliding) allow rotations and thermal movements; expansion joints bridge gaps. Long spans are sensitive to wind (flutter — Tacoma Narrows 1940) and pedestrian-induced vibration (London Millennium Bridge 2000). Regular inspection, load rating and structural health monitoring keep aging bridges safe.

Formulas

• Suspension cable (UDL): H = wL²/(8d), T_max = √(H² + (wL/2)²)
• Absolute max moment (moving loads): under an axle when mid-span bisects that axle and the resultant
• Simple beam frequency: f₁ = (π/2L²) √(EI/m)
• Thermal movement: ΔL = α L ΔT

Worked example: Absolute maximum moment

Given: Span 20 m; Two 100 kN axles, 4 m apart

Find: Maximum moment.

• Resultant 200 kN midway between axles
• Place axle 1 at 9 m, axle 2 at 13 m (mid-span bisects 9 and 11)
• R_A = (100 × 11 + 100 × 7)/20 = 90 kN
• M under axle 1 = 90 × 9 = 810 kN·m (vs 800 with axles symmetric)

Answer: 810 kN·m

Worked example: Suspension cable

Given: Main span 1000 m, sag 100 m; Deck load 200 kN/m (per cable)

Find: Horizontal and maximum tension.

• H = 200 × 1000²/(8 × 100) = 250 000 kN
• T_max = √(250 000² + 100 000²) = 269 000 kN

Answer: H ≈ 250 MN, T_max ≈ 269 MN

Common mistakes

• Wrong: Placing moving loads only at mid-span. — Right: Use influence lines or the absolute-max theorem to find the critical position.
• Wrong: Ignoring dynamics for light footbridges. — Right: Check vertical (≈ 1.6–2.4 Hz) and lateral (≈ 0.5–1.2 Hz) pedestrian frequency ranges.

Applications

• Highway & rail: Precast girder viaducts, box girders, steel trusses.
• Long-span crossings: Cable-stayed and suspension bridges over rivers and straits.
• Footbridges: Lightweight, often dynamically sensitive, architectural structures.

Key takeaways

• Choose bridge type by span, site and construction.
• Moving loads need influence lines/critical positions.
• Cables: H = wL²/8d.
• Check dynamics, bearings, durability and inspection.

Going further

• Aeroelastic stability: Wind-tunnel tests and CFD check flutter and vortex-induced vibration of long-span decks.
• Construction staging: Balanced cantilever and incremental launching require stage-by-stage analysis.
• Bridge health monitoring: Sensors track strain, acceleration and cable forces for predictive maintenance.