Views: 222 Author: Astin Publish Time: 2025-03-19 Origin: Site
Content Menu
● Structural Overview of Over Truss Bridges
● Structural Overview of Suspension Bridges
● Load Distribution Mechanisms
● Material Efficiency and Construction
>> Case 1: Forth Bridge (Over Truss)
>> Case 2: Golden Gate Bridge (Suspension)
>> 1. Which bridge type handles heavier loads?
>> 2. How do environmental factors affect each design?
>> 3. Which is more cost-effective for a 300-meter river crossing?
>> 4. Can suspension bridges support railways?
>> 5. What is the lifespan comparison?
Over truss bridges and suspension bridges represent two distinct engineering philosophies for spanning distances, each excelling in specific scenarios. This article examines their structural principles, load distribution mechanisms, material efficiency, span capabilities, and real-world applications to highlight their unique advantages and limitations. By comparing these designs, engineers and planners can better determine which solution aligns with project requirements.
An over truss bridge positions its truss framework above the deck, creating an arch-like superstructure that encapsulates the roadway. Key components include:
- Top Chord: The upper horizontal member, which resists compressive forces.
- Bottom Chord: The lower horizontal member under tension.
- Web Members: Diagonal and vertical steel elements forming triangular units to distribute loads.
- Deck Supports: Floor beams and stringers that transfer live loads to the truss system.
This design leverages triangulation to convert bending moments into axial forces (tension/compression), enabling efficient load distribution. Over truss bridges are commonly used for railroads and roadways with spans ranging from 50 to 150 meters.
A suspension bridge suspends its deck from vertical cables anchored to massive main cables draped between towers. Critical elements include:
- Main Cables: High-strength steel ropes that transfer deck loads to the towers and anchorages.
- Towers: Vertical structures (often steel or concrete) supporting the main cables.
- Suspenders: Vertical cables connecting the main cables to the deck.
- Anchorages: Reinforced blocks or rock formations securing the main cables.
Suspension bridges excel in long-span applications (2,000–7,000 feet) and prioritize flexibility over rigidity, allowing controlled movement under dynamic loads.
- Compression and Tension: Live loads (e.g., vehicles) induce compression in the top chord and tension in the bottom chord. Diagonal web members alternate between these forces, dispersing stress across multiple components.
- Shear Resistance: Triangular units minimize bending by converting shear forces into axial stresses.
- Cable Tension: The deck's weight and live loads create tension in the main cables, which transfer forces to the anchorages.
- Tower Compression: Towers bear vertical compressive loads from the cables.
- Deck Stiffening: Auxiliary truss systems beneath the deck reduce oscillations caused by wind or traffic.
- Key Difference: Over truss bridges localize stress within rigid truss members, while suspension bridges rely on cable tension and tower compression for load dissipation.
| Factor | Over Truss Bridge | Suspension Bridge |
|---|---|---|
| Primary Materials | Steel, concrete, or composites | High-strength steel cables, concrete towers |
| Material Usage | 20–40% less material than solid girders | High steel consumption for cables and towers |
| Construction Time | Modular assembly; faster for medium spans | Longer due to cable spinning and anchorage work |
| Cost | Lower initial cost for spans < 150 meters | Higher initial and maintenance costs |
Over truss bridges, like the Ikitsuki Bridge in Japan, demonstrate cost-effectiveness for medium spans. Suspension bridges, such as the Golden Gate Bridge, require significant upfront investment but dominate ultra-long-span projects.
- Over Truss: Optimized for 50–400 meters. The Quebec Bridge (549 meters) is a notable exception using cantilevered truss design.
- Suspension: Ideal for 610–2,134 meters. The Akashi Kaikyō Bridge (1,991 meters) holds the record for the longest central span.
- Trade-off: While suspension bridges achieve unparalleled spans, their flexibility makes them less suitable for heavy rail traffic compared to rigid truss designs.
- Pros: Minimal movement under load; resistant to wind-induced oscillations.
- Cons: Prone to corrosion at joints; frequent inspections required for welded connections.
- Pros: Deck segments can be replaced without full closure.
- Cons: Cables degrade due to moisture and fatigue; anchorage inspections are complex and costly.
The Brooklyn Bridge (hybrid suspension/truss) exemplifies how combining designs mitigates weaknesses, offering both span and rigidity.
This cantilever truss bridge in Scotland spans 2.5 kilometers using a double-cantilever design. Its rigid truss framework supports heavy rail traffic while resisting North Sea winds.
With a main span of 1,280 meters, this iconic structure uses 129,000 kilometers of steel cable. Its deck truss system reduces torsional movement, ensuring stability in foggy, windy conditions.
- Composite Materials: Fiber-reinforced polymers (FRP) reduce weight and corrosion risk.
- Smart Sensors: Strain gauges monitor stress in critical joints, enabling predictive maintenance.
- Hybrid Designs: The San Francisco–Oakland Bay Bridge integrates self-anchored suspension and truss systems for seismic resilience.
- Carbon Fiber Cables: Experimental replacements for steel offer higher tensile strength and corrosion resistance.
Over truss bridges and suspension bridges serve divergent engineering needs. The former excels in cost-effective, rigid structures for medium spans, while the latter dominates long-span projects despite higher complexity. Advances in materials and hybrid designs continue to blur traditional boundaries, offering solutions like the cable-stayed truss for scenarios demanding both span and load capacity. Understanding their mechanical behaviors, environmental adaptability, and lifecycle costs remains critical for infrastructure planning.
Over truss bridges are better for concentrated heavy loads (e.g., freight trains) due to their rigid structure. Suspension bridges prioritize distributed loads (e.g., vehicles).
Over truss bridges withstand high winds but suffer in corrosive environments. Suspension bridges require dampers to mitigate wind-induced oscillations.
An over truss bridge would likely be cheaper due to simpler construction and lower material costs.
Yes, but they require reinforced deck trusses to limit flexure. The Tacoma Narrows Bridge collapse (1940) highlighted the risks of inadequate stiffening.
Over truss bridges last 80–100 years with maintenance. Suspension bridges can exceed 120 years but demand rigorous cable inspections.
[1] https://aretestructures.com/how-to-design-a-truss-bridge/
[2] https://usbridge.com/through-truss-steel-bridges-optimal-design/
[3] https://en.wikipedia.org/wiki/Suspension_bridge
[4] https://science.howstuffworks.com/engineering/civil/bridge6.htm
[5] https://aretestructures.com/beam-bridge-vs-suspension-bridge-comparison/
[6] https://civiltoday.com/construction/bridge/346-advantages-disadvantages-of-suspension-bridges
[7] https://www.britannica.com/technology/truss-bridge
[8] https://www.britannica.com/technology/suspension-bridge
[9] https://www.cedengineering.com/userfiles/S02-031%20-%20The%20Basic%20Types%20of%20Bridges.pdf
[10] https://housing.com/news/different-types-of-bridges-components-advantages-and-disadvantages/
[11] https://www.witpress.com/Secure/elibrary/papers/HPSM14/HPSM14043FU1.pdf
[12] https://en.wikipedia.org/wiki/Truss_bridge
[13] https://www.mdpi.com/2071-1050/14/11/6628
[14] https://www.tn.gov/tdot/structures-/historic-bridges/what-is-a-truss-bridge.html
[15] https://aretestructures.com/what-is-a-truss-bridge-design-and-material-considerations/
[16] https://civilfunz.com/key-members-in-a-suspension-bridge-and-their-functions/
[17] https://www.wsdot.wa.gov/TNBHistory/suspension-bridges.htm
[18] https://dozr.com/blog/suspension-bridges
[19] https://testbook.com/civil-engineering/suspension-bridge-meaning-design-and-types
[20] http://zacoeb.lecture.ub.ac.id/files/2015/03/Mg-12-Suspension-Bridge.pdf
[21] https://www.iitf.lbtu.lv/conference/proceedings2012/Papers/039_Goremikins_V.pdf
[22] https://practical.engineering/blog/2024/5/21/every-kind-of-bridge-explained-in-15-minutes
