Cantilever vs. Cable-Stayed Bridges: Engineering Comparison

Introduction

There are major differences between a cantilever bridge and a cable-stayed bridge, and there are specific and interesting engineering perspectives that create the preference for one style over another, depending on the body of water a bridge must span and the land on either end of the structure.

Cantilever Bridges: Design and History

This paper draws on numerous sources to compare and contrast both styles of bridges and describes the advantages and disadvantages of each. It begins by establishing the basic facts and engineering particulars of both bridge types before moving to a direct comparison.

A cantilever bridge is designed so that it is firmly supported at one end. The way in which the cantilever bridge is secured at one end can be compared to a tree, according to the Acrow Bridge building group. Trees are "flawless cantilever models" because the roots act as the "rigid support that prevents the trunk and branches from crashing to the ground" (Acrowusa.com, 2001). Another useful comparison is with a skyscraper: the foundation of a large skyscraper is the "rigid support" that keeps the structure "in equilibrium."

When construction of a cantilever bridge takes place, "arms are first stretched from opposite ravines or shorelines" and those arms are secured to solid, rigid foundations that "counterbalance each cantilever" (Acrowusa.com). When both arms — from opposite sides of the waterway — are in place but have not yet met at the center, a center beam is installed and both arms are locked into it, the Acrow Bridge company explains.

As to load distribution, there is a "dead load" and a "live load" in the center; both loads "push down, creating compressive force" (Richman, 2005, p. 94). That force also places pressure on the "side cantilevered spans," creating tension. The tension is expected and necessary, because there is a "pulling" force moving upward; additionally, there is a compression force "on the main piers that sustain the cantilevered side spans" (Richman, p. 94). All these forces working in sync — providing a counterbalancing dynamic — are absolutely vital to the steadiness and safety of the bridge.

Cantilever bridges are engineered on the principle of counterbalances, described as weights on one end that balance weight on the opposite end (Thinkquest.org, 2002). Many cantilever bridges have at least four arms that balance each other equally; others have just two arms that "equally balance each other, almost like a perfectly balanced see-saw" (Thinkquest.org).

Ian McNeil writes in An Encyclopedia of the History of Technology that the cantilever design "appears to have originated in China in pre-Christian times" (McNeil, 1990, p. 463). One early modern example is the bridge across the River Main in Hassfurt, Germany, built in 1867 and designed by Heinrich Gerber. Its central span was 130 meters (425 feet), and it is considered among the first modern cantilever bridges.

Another important example is the Forth Railway Bridge in Scotland, completed in 1889. It was the first "major bridge in Europe to be built of steel," McNeil notes. The Forth Railway Bridge features two equal main spans of 521 meters (1,710 feet) and consists of "three double cantilever frames with short suspended trusses of 106 meters (350 feet) between the trusses" (McNeil, p. 464). Approximately 45,000 tons of Siemens-Martin steel were used in its construction.

Advantages of Building Cantilever Bridges

According to forthbridges.org.uk, the Forth Railway Bridge "ranks as one of the great feats of civilization" and stretches a mile and a half across. Built by Tancred-Arrol, it features three double-cantilevers, each connected by 105-meter (345-foot) "suspended" girder spans that "rest on cantilever ends and are secured by man-sized pins." The double cantilevers on the outside of the bridge carry weights "of about 1,000 tons to counterbalance half the weight of the suspended span and live load." Approximately 4,000 men, 54,000 tons of steel, and 6,500,000 rivets were used in construction.

Tragically, 57 men were killed during the construction of the Forth Railway Bridge. Although rescue boats were stationed under each cantilever and saved 8 lives, the loss of 57 workers remains a sobering reminder of the human cost of large-scale engineering. Today, about 180 to 200 trains cross the bridge every day (forthbridges.org.uk).

One significant advantage of cantilever bridges is that they can be built from both sides of a waterway simultaneously; the two sides can either meet at the center or have a "final center span put into place to link the two extended 'diving board' spans" (Richman, 2005, p. 94). More specifically, cantilever bridges: (a) can span wide spaces; (b) can be constructed without building expensive falseworks — temporary supports erected during construction and then torn down; (c) can be built without foundation piers that would otherwise disrupt the flow of a river; and (d) are noted for their strength and rigidity, allowing heavy rail traffic to cross safely without damaging the structure or causing excessive stress (Richman, p. 94).

Cable-Stayed Bridges: Design and History

Cable-stayed bridges were not built until the twentieth century. Historian John A. Weeks explains that after World War II, Europe urgently needed new bridges — some to replace those destroyed in the war — in order to restart its economy. Steel was not readily available, and "suspension bridges were too costly, in both time and materials," so they were not a viable option. Given these constraints, engineers designed the cable-stayed bridge, in which the roadway crossing the water or canyon is supported by cables rising up to towers (Weeks, 1996). As a result, a cable-stayed bridge is lighter in weight than a typical suspension bridge.

While the cable-stayed bridge differs significantly from the cantilever bridge, it can appear similar to a suspension bridge because both types have roadways suspended from cables and both feature towers, according to NOVA, produced by the Public Broadcasting Service. In cable-stayed bridges, the cables are attached directly to the towers, which "alone bear the load"; in suspension bridges the cables "ride freely across the towers, transmitting the load to the anchorages at either end" (NOVA, 2003).

The original concept for cable-stayed bridges dates to 1595, according to the NOVA article. In a book titled Machinae Novae, published that year by Croatian inventor Faust Vrancic, a sketch of a cable-stayed bridge is clearly presented. Despite this early concept, no engineer pursued the idea for centuries, and the first cable-stayed bridges were not constructed until the twentieth century (NOVA). A well-known example is the Sunshine Skyway Bridge in Tampa, Florida, built in 1988, which won the prestigious Presidential Design Award from the National Endowment for the Arts.

Robert Lamb and Michael Morrissey, writing for the Discovery Company's Science / How Stuff Works, note that a cable-stayed bridge may "at first glance" appear to be a cousin of the suspension bridge; but even though the two share "similar towers and hanging roadways," they are not the same. Cable-stayed bridges require neither anchorages nor two towers (Lamb et al., 2011).

In a cable-stayed bridge, cables run from the roadway "up to a single tower that alone bears the weight," and the tower absorbs the "compressional forces" (Lamb et al., 2011). Cables may be connected to the roadway at several points and all converge at a single point atop the tower; Lamb compares this arrangement to "numerous fishing lines attached to a single pole."

There are two basic types of cable-stay configuration, according to Aileen Cho writing in Engineering News-Record (Cho, 2012, p. 2): the "fan" type and the "harp" type. Of the top ten cable-stay bridges, all use the modified fan configuration rather than the harp, because the harp configuration tends to produce "increased compression in the superstructure" (p. 2). In the fan configuration, cable stays "are spaced out over the top portion of the pylon" to allow more room for each cable "to be individually anchored near the pylon top" (Cho, p. 2). The harp model spaces cable stays "in equal spaces over much of the height of the pylon," offering a "pleasant aesthetic appearance" but is not as structurally efficient (Cho, p. 2).

Conclusion

For engineers, a thorough understanding of the various bridge types offers significant practical advantages. This paper has drawn on a range of engineering and historical sources to illuminate the strengths of both cantilever and cable-stayed bridges. Based on the evidence presented, cable-stayed bridges are generally the superior choice between the two: they are less costly to build and, when utilizing the harp configuration, are considerably more visually appealing. The rapid evolution of cable-stayed bridge technology over the past half-century further confirms their place as the dominant bridge form of modern engineering.