Types of Bridge Structures

Why This Matters

Bridge engineering sits at the intersection of physics, materials science, and practical problem-solving—and understanding why different bridge types exist is far more valuable than memorizing their parts. You're being tested on your ability to recognize how engineers match structural systems to specific challenges: span length, load requirements, site conditions, and available materials. The principles at work here—force distribution, tension vs. compression, structural efficiency, and material behavior—appear throughout engineering mechanics and design courses.

When you encounter these bridge types, don't just picture what they look like. Ask yourself: How does this structure handle forces? What problem does this design solve better than alternatives? Whether you're analyzing a free-body diagram or evaluating a design proposal, the conceptual categories below will help you think like an engineer—and that's exactly what exams reward.

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Compression-Dominant Structures

These bridges rely primarily on compressive forces, channeling loads through materials that excel under compression—like stone, concrete, and masonry. The geometry directs weight outward and downward into abutments or piers, minimizing tensile stress.

Arch Bridges

• Curved geometry transfers loads as compression—weight pushes outward along the arch into abutments at each end, eliminating the need for tensile reinforcement in traditional designs
• Material versatility spans ancient to modern: stone arches have stood for millennia, while steel and concrete arches now achieve spans exceeding 500 meters
• Aesthetic and structural efficiency makes them iconic—the parabolic or catenary curve naturally follows the thrust line, minimizing bending moments

Viaducts

• Multiple-span elevated structures carry roads or railways across valleys, rivers, or urban obstacles using repeated arch or pier-and-beam systems
• Pier spacing balances structural efficiency against foundation costs—closer piers reduce span demands but increase substructure expense
• Urban applications allow transportation routes to pass over existing infrastructure without disrupting ground-level activity

Aqueducts

• Gravity-fed water conveyance requires precise gradient control—Roman engineers achieved slopes as gentle as 1:3000 over distances exceeding 50 kilometers
• Arch construction elevates the channel above terrain obstacles while maintaining the continuous downward slope essential for flow
• Historical engineering significance demonstrates early mastery of surveying, materials, and large-scale project management

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Tension-Dominant Structures

These bridges harness the extraordinary tensile strength of steel cables, allowing spans that compression-based designs cannot achieve. Cables in tension are structurally efficient because the entire cross-section carries load uniformly—no buckling concerns.

Suspension Bridges

• Main cables hang in a catenary curve between towers, with vertical suspenders transferring deck loads upward into the cables and then into massive anchorages
• Longest spans achievable—the Akashi Kaikyo Bridge reaches 1,991 meters, enabled by cables that work purely in tension
• Flexibility accommodates dynamic loads—the deck can move under wind and traffic, which requires careful aerodynamic design to prevent resonance (see Tacoma Narrows collapse)

Cable-Stayed Bridges

• Cables connect directly from towers to deck in a fan or harp pattern, eliminating the need for massive anchorages required by suspension bridges
• Stiffer than suspension bridges because cables are taut and shorter, reducing deck deflection under asymmetric loading
• Economical for medium-to-long spans (200–1,000 meters)—simpler construction and smaller foundations offset material costs

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Bending-Dominant Structures

These bridges resist loads primarily through bending (flexure), with the deck and supporting members experiencing both tension and compression across their cross-sections. Efficiency depends on beam depth and material placement—deeper sections resist bending with less material.

Beam Bridges

• Horizontal members span between supports with load transferred vertically downward—the simplest structural concept and easiest to analyze
• Limited to short spans (typically under 75 meters) because bending moment increases with the square of span length, requiring impractically deep beams for longer distances
• Concrete and steel dominate modern construction, often as precast elements that speed installation and ensure quality control

Girder Bridges

• Large I-shaped or box-shaped beams provide efficient bending resistance by concentrating material in flanges (top and bottom) where bending stresses peak
• Box girders offer torsional stiffness critical for curved alignments or eccentric loading—the closed cross-section resists twisting
• Segmental construction allows long girder bridges to be built piece by piece, with post-tensioning connecting segments into a continuous structure

Cantilever Bridges

• Projecting arms extend from piers and meet at midspan, creating a structure that works like two diving boards touching tips
• Eliminates midspan supports ideal for deep water or unstable soil where foundations would be difficult—the Forth Bridge famously crosses a wide estuary this way
• Balanced cantilever construction builds outward symmetrically from each pier, maintaining equilibrium throughout erection

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Triangulated Structures

Truss bridges transform bending problems into axial force problems by arranging members in triangular patterns. Triangles are inherently stable—they cannot deform without changing member lengths, so loads produce only tension or compression, never bending.

Truss Bridges

• Interconnected triangles create a rigid framework where each member carries purely axial force (tension or compression), maximizing material efficiency
• High strength-to-weight ratio enables long spans with relatively light structures—critical for railway bridges where self-weight competes with live load
• Configuration varies by application: Warren trusses use equilateral triangles for uniform load distribution; Pratt trusses optimize for gravity loads with vertical members in compression; Howe trusses reverse this for uplift resistance

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Specialized & Adaptive Structures

Some bridges prioritize functional requirements beyond static load-carrying—accommodating navigation, adapting to site constraints, or serving non-vehicular purposes.

Movable Bridges

• Mechanical systems allow passage of marine traffic by raising, rotating, or retracting portions of the deck—essential where fixed bridges would block navigation channels
• Three primary types: bascule (pivots upward like a drawbridge), swing (rotates horizontally around a central pier), and vertical lift (deck rises between towers like an elevator)
• Operational complexity introduces maintenance demands and traffic delays, making them appropriate only where navigational clearance cannot be achieved with a fixed high-level crossing

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Quick Reference Table

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Self-Check Questions

1. Tension vs. compression: Which two bridge types rely primarily on cables in tension, and what distinguishes their load paths from anchorage to deck?

2. Span selection: An engineer must cross a 600-meter estuary with poor foundation conditions at midspan. Which bridge types should she evaluate, and why would each be appropriate?

3. Compare and contrast: How do truss bridges and girder bridges differ in their approach to resisting bending moments, and what are the trade-offs in material use and fabrication complexity?

4. Force analysis: In an arch bridge, why does the curved geometry reduce or eliminate tensile stress in the arch rib? What happens if the load distribution doesn't match the arch shape?

5. Design constraints: A city needs a new crossing over a busy shipping channel. Compare the long-term implications of choosing a movable bridge versus a high-level cable-stayed bridge, considering construction cost, maintenance, and traffic flow.