Cable-Stayed vs. Suspension Bridges: Design and Span Analysis

Introduction

Among the different types of bridges, a comparison between the suspension bridge and the cable-stayed bridge is particularly feasible because of their structural similarities. There are other types of bridges that are widely used — for example, the arch bridge, which has been found in use since prehistoric times. The arch sustains heavy loads, and arches are designed to support more weight than what may pass over them at any given time, ensuring the bridge does not collapse. Being the oldest and most common bridge form, the arch bridge remains prevalent worldwide. However, the focus of this discussion is the suspension bridge and the cable-stayed bridge.

The principal difference between the two types is rooted in their origins. The suspension bridge was constructed by ancient communities using rope and bamboo. Its principles involve cables and towers at each end anchored to strong plates, with the cables and towers supporting the deck halfway across. The suspension bridge features a curved tension member; cables carry only tension loads and cause compression in the towers. Among the advantages of suspension bridges is their relatively light weight, which allows them to span very large distances without additional reinforcement or support (Arch, Suspension, Beam, Truss, Cable-stayed — Five Bridge Types).

The cable-stayed bridge, by contrast, is a comparatively modern design and is economical to construct. Unlike suspension bridges, cable-stayed bridges are not suited for very long spans. They are best applied to moderate spans and may use a single pylon or tower. The weight of one side of the bridge balances the weight of the other side. These two bridge types are not interchangeable: the suspension bridge becomes uneconomical at shorter spans, while the cable-stayed bridge may not be appropriate for spanning huge distances. Nevertheless, a comparison of the two may narrow down their respective scopes and could be useful in developing a hybrid design, since both rely on the principle of suspension.

This paper attempts a comparison between both types — not with a view to advocate for one over the other, but to explore whether it is possible to fuse their excellent features and cover very long spans that neither type can achieve independently. In analyzing both bridge types, consideration is also given to general bridge categories and their uses. The topography of a crossing, the required clearance (for example, over a river prone to flooding, or across a large valley), and the intended duration of use all help determine the appropriate bridge type. Suspension bridges are the type recommended when very long spans are required (Vikctor, 2007, p. 11). This investigation therefore proceeds to the history and background of each bridge system.

A bridge is defined as a structure that provides passage over an obstacle without closing off what lies beneath. This distinguishes a bridge from an embankment or bund, which fills in the obstacle so that the way continues at grade. The six basic forms of a bridge are: arch bridges (dating from antiquity), cantilever bridges, beam bridges, truss bridges, suspension bridges, and cable-stayed bridges.

In the case of the beam bridge, a beam is placed between the points to be crossed and bears the load by flexure. The truss bridge is similar to the beam bridge, also taking its load by bending. The arch bridge uses the compression generated at each end of the arch to bear the load passing above it (Dayaratnam, 2000, p. 20). The cantilever bridge has three spans, with the end spans resting on supports at either side and the centre span supported by a channel in the middle; the tower or stay is located at the midpoint of the span. The suspended bridge is similar, but the stays are at the ends.

Background: Bridge Types and Structural Principles

The curved cables of the suspension bridge conform to gravity and carry load from one end to the other, transferring forces to the ground via towers at each end. In the cable-stayed bridge, by contrast, nearly straight cables carry vertical load points from a central tower, transferring the load through vertical compression of the tower on either side. The tensile forces of the cables create a horizontal compression on the deck (Dayaratnam, 2000, p. 21).

Examining the history of suspension bridges, their origins trace to India and China, where old bridges built in the high Himalayan ranges still exist. Modern suspension bridges are concentrated in the United States, China, Japan, and Europe. Notable examples include the Brooklyn Bridge, with a span of 486 meters and the first steel wire cable in the world, and the George Washington Bridge, with a span of 1,067 meters. The technical challenges posed by suspension bridges came into sharp focus after several disasters — most notably the collapse of the Tacoma Narrows Bridge at Puget Sound, Washington, caused by a gale reaching 68 km/h. This event compelled engineers to account for aerodynamic effects as a major consideration in suspension bridge design (Vikctor, 2007, p. 14).

The study of bridge aerodynamics, combined with economic pressures and the availability of new materials such as concrete and tensile steel, prompted experimentation with alternative bridge forms — among them the cable-stayed bridge, though the concept itself dates to antiquity. Cable-stayed bridges are specified as most suitable for spans of approximately 200 to 900 meters, filling an intermediate range where box girder bridges are inadequate and suspension bridges would be unnecessary (Vikctor, 2007, p. 17).

Brown (2001, p. 103) has provided a detailed historical analysis of bridges with particular emphasis on span bridges. The theory of suspension bridges has been addressed by Sir Alfred Pugsley (1957, p. 25). Historical analysis of bridges reveals some interesting observations: much of the research and engineering innovation was driven by the growth of railways and the need to create bridges strong enough to withstand the motion of trains and their loads. Two centuries ago, engineers were chiefly concerned with load-bearing capacity and span length.

For example, Cowper (1847, p. 5), writing about railway bridges in an engineering journal, proposed a suspension bridge for railway use constructed from iron — one that could not only bear traffic but could be economical in construction for long spans. His paper addressed how to prevent the deformation of a suspension bridge's shape under passing loads. Cowper observed that even small loads could alter the shape of the chain (catenary), prompting the formulation of calculations for load-bearing capacity. He further noted that as a load moves across the bridge, the curve distorts — when the load is at one end and has not yet reached the middle, the curve at the midpoint takes a parabolic shape. Accordingly, suspension bridges were modified following Cowper's theory to include an arch in the bridge length that matched the chain curves, a feature known as the "inverted arch bridge" (Cowper, 1847, p. 7).

In many ways the suspension bridge is superior for very long spans, and for hundreds of years it has demonstrated this superiority by being consistently selected as the model for major crossings. Kawada (2010, p. 29) traced the origin of the suspension bridge to the development of chain manufacture for ships. The flat eye-bar chain was used in early suspended bridges and emerged directly from shipbuilding technology.

Engineers such as Finley and his contemporaries in Britain conducted research on tower-cable interactions, with the Menai Bridge serving as a focal point. Their concern centered on designing a tower capable of withstanding the pull of the cable. Navier approached this problem with mathematical applications and categorized the various tower and cable systems and their possible combinations. He classified a slim vertical tower supported by a central cable and a separate anchor cable as one type, and a massive tower supporting a single continuous cable that could slide back and forth over the tower as another. Navier's formulas detailed the effects of cable sag and concentrated loads relative to bridge span and size (Kranakis, 1997, p. 136).