Bridge Types Explained: Beam, Arch, Suspension, and Cable-Stayed

Every bridge is an answer to the same question: how do you carry loads across a gap? The answer depends on span length, site conditions, available materials, budget, and aesthetics. Over centuries of engineering practice, four structural families have emerged as the dominant solutions. Each works by moving loads to the supports through a different internal mechanism. Understanding those mechanisms reveals why certain forms dominate at certain spans.

Beam Bridges: Bending as the Primary Action

The simplest bridge is a beam spanning between two supports. Load applied to the span creates a bending moment that peaks at midspan for a simply supported beam under uniform load: M_max = wL²/8, where w is load per unit length and L is the span. Shear forces peak at the supports. The beam must resist both, with the top flange or fiber in compression and the bottom in tension.

For short spans up to roughly 100 feet (30 m), precast prestressed concrete I-girders or steel plate girders offer the most economical solution. The depth required to control deflection and bending stress typically falls between L/20 and L/25, meaning a 100-foot span needs a girder roughly 48 to 60 inches deep.

Continuous beam bridges change this picture dramatically. When a beam continues over one or more interior supports instead of ending there, negative moments develop at those supports. The maximum positive moment at midspan is reduced compared to a simply supported beam of the same span. For a two-span continuous beam of equal spans under uniform load, the maximum positive moment in each span is approximately 0.070wL², compared to 0.125wL² for simply supported. This reduction of roughly 44 percent allows shallower, lighter girders for the same span. Continuous bridges also distribute live loads more efficiently and perform better under seismic and dynamic loading. The trade-off is that settlement of interior supports introduces secondary moments, and thermal expansion must be managed across the continuous deck.

Through trusses and deck trusses extend beam bridge logic to longer spans by replacing the solid web of a plate girder with an open framework of axially loaded members. A through truss has its deck at the bottom chord level, so traffic passes through the structural frame. A deck truss carries traffic on top. Trusses can economically reach spans of 200 to 500 feet (60 to 150 m).

Arch Bridges: Converting Bending into Compression

An arch solves the long-span problem by using its curved geometry to carry load primarily through axial compression rather than bending. When a distributed load acts on an arch with the correct parabolic shape, the internal force is purely axial compression along every cross-section. Real arches experience some bending because the load is not perfectly uniform and the shape is not perfectly matched to the thrust line, but bending is greatly reduced compared to an equivalent beam.

The thrust line is the path of the resultant compressive force through the arch. As long as the thrust line stays within the kern of the cross-section (roughly the middle third for rectangular sections), no tensile stress develops anywhere in the arch. Stone masonry arches exploited this principle for centuries because stone handles compression well but crumbles under tension.

The price of arch efficiency is horizontal thrust. At the base of each arch leg, called the springing point, significant horizontal reactions develop in addition to vertical ones. These horizontal thrusts must be absorbed by the abutments (which can be massive in large arches) or by a tie rod connecting the two springing points at deck level. The tied arch, also called a bowstring arch, eliminates the need for large abutments by carrying the horizontal thrust internally as tension in the tie. This form works well over water where founding large thrust-resisting abutments in rock or soil is difficult.

Concrete arch bridges can economically reach spans of 500 to 1,000 feet (150 to 300 m). Steel arch bridges, such as the New River Gorge Bridge in West Virginia with a main span of 1,700 feet (518 m), push even further. The arch works best when the site provides strong abutment rock to absorb thrust and when the rise-to-span ratio can be kept between 1:4 and 1:6 for good structural efficiency.

Suspension Bridges: Cables in Catenary Tension

Suspension bridges carry deck loads by hanging them from cables. The main cables arc overhead between two towers and anchor at massive anchorages at each end of the bridge. Vertical hangers (suspenders) connect the main cable to the stiffening girder or truss at the deck level. The entire deck load is transmitted upward through the hangers into the main cable, which carries it in tension to the towers and then to the anchorages.

Under uniform load, the ideal cable shape is a parabola. Under self-weight alone, it takes a catenary form, which is slightly different mathematically but similar in appearance. In practice, bridge designers size the cable sag to span ratio carefully: a shallow cable (small sag) produces very large cable tensions and large horizontal forces at the anchorages, while a deep cable (large sag) requires tall towers but carries load more efficiently. Typical sag-to-span ratios range from about 1:8 to 1:12.

The towers are tall columns that transfer the cable loads into the foundations. They work primarily in compression under vertical loads, but must also resist bending from unbalanced loads on one side versus the other. The anchorages must resist the enormous horizontal cable pull; in rock sites, cables are often anchored into drilled tunnels. In soft ground, gravity anchorages weighing hundreds of thousands of tons are necessary.

Suspension bridges dominate the longest span range. The current record holder, the 1915 Canakkale Bridge in Turkey, spans 6,637 feet (2,023 m). The Golden Gate Bridge spans 4,200 feet (1,280 m). No other bridge form competes at these scales because only a cable in pure tension can carry such spans without prohibitive material weight. The stiffening girder at deck level is essential: without it, moving loads would cause the cable to change shape continuously, creating unacceptable deflections and aerodynamic instability.

Cable-Stayed Bridges: Direct Load Path to the Tower

Cable-stayed bridges look superficially similar to suspension bridges, but the structural behavior is fundamentally different. Instead of a continuous main cable carried over towers, individual stay cables connect directly from the deck to the tower at various heights. Each cable is a straight line carrying direct tension from its deck attachment point to the tower anchorage.

This geometry creates two components of force at the deck connection. The vertical component supports the deck against gravity. The horizontal component pushes the deck inward toward the tower, putting the deck in significant axial compression. The deck of a cable-stayed bridge is therefore a compression member as well as a flexural one, which has important design implications. Box girder sections are preferred for their high axial and torsional stiffness.

The tower receives cable forces from both sides. When the bridge is loaded symmetrically, the horizontal cable components from both sides largely cancel, and the tower carries primarily vertical compression. Under asymmetric loading, net horizontal forces develop at the tower head that must be resisted by bending in the tower shaft. Towers are typically H-shaped, A-shaped, or single-plane diamond configurations, each with different aerodynamic and structural characteristics.

The direct cable load path from deck to tower gives cable-stayed bridges an economic advantage over suspension bridges in the 500 to 3,000 foot (150 to 900 m) span range. They use less cable material, require no massive anchorages (each side of the bridge balances the other), and can be erected efficiently by extending from the towers outward in a balanced cantilever sequence. The Russky Island Bridge in Russia, with a main span of 3,622 feet (1,104 m), is currently the world's longest cable-stayed span, demonstrating that this form continues to push into territory once reserved for suspension bridges alone.