Cantilever Bridges Page 4 Advantages

¶ … Cantilever Bridges

Page 4 Advantages of Cantilever Bridges

Page 5 Top 5 Longest Cantilever Bridges

Page 6 Cable Stayed Bridges

Page 8 Advantages of Cable Stayed Bridges

Page 8 Top 5 Longest Cable Stayed Bridges

Page 8 Comparison: Cable Stayed with Cantilever

Cantilever and Cable Stayed Bridges

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 water that requires a bridge to span over it and the land on either end of the bridge.

This paper provides numerous resources to compare and contrast both styles of bridges, and will describe the advantages and disadvantages of both bridge styles. First, the paper will fully flush out the basic facts and the engineering particulars of both styles of bridges.

Cantilever Bridges

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, in fact, 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). When thinking about how a cantilever bridge is secured to one side, another comparison is with a skyscraper, one of man's largest structures. The Acrow group explains that the foundation of a huge skyscraper is the "rigid support" that keeps the skyscraper "in equilibrium."

When construction is taking place for a cantilever bridge, "…arms are first stretched from opposite ravines or shorelines" and those arms are secured to solid and rigid foundations that "counter balance each cantilever" (Acrowusa.com). When both arms -- from opposite side of the waterway -- are in place, but are not touching the opposing arm at the center of the waterway crossing, a center beam is put in place, and both arms from opposite sides of the waterway are locked into that center beam, the Acrow Bridge company explains (p. 1).

As to the load distribution on a cantilever bridge, there is a "dead load" and a "live load" in the center; both those loads "push down, creating compressive force" (Richman, 2005, p. 94). While creating that force, pressure is also place on the "side cantilevered spans" and that creates tension, Richman continues. The tension is expected and needed, 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, 94). All these forces working in sync -- in effect, providing a counterbalancing dynamic -- are absolutely vital to the steadiness and safety of the bridge.

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

When did cantilever bridges first begin being built? Ian McNeil writes in the book, an Encyclopedia of the History of Technology, that this particular design "…appears to have originated in China in pre-Christian times" (McNeil, 1990, p. 463). One example of a cantilever bridge is the bridge across the River Main in Hassfurt in Germany. Built in 1867 -- designed and engineered by Heinrich Gerber -- it is considered among the first modern cantilevers. The central span of the bridge across the River Main was 130 meters (425 feet).

Another good example of a cantilever bridge is the Forth railway bridge, McNeil explains, which was completed in 1889 in Scotland. The Forth Railway Bridge was the first "major bridge in Europe to be built of steel," McNeil points out. The Forth Railway Bridge is made of 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, 464). The steel used in the construction of the Forth Railway Bridge in Scotland was Siemens-Martin steel; about 45,000 tons of that steel went into the construction of the Forth Bridge.

An article in the website forthbridges.org.uk points out that the Forth Railway Bridge "…ranks as one of the great feats of civilization" and is a mile and a half across. The bridge was built by Tancred-Arrol, and features three double-cantilevers, each of which are connected by 105 meter (345 feet) "suspended" girder spans that "…rest on cantilever ends and are secured by man-sized pins." As mentioned earlier in this paper, counterbalance is among the keys to the building of a successful cantilever bridge, and in the case of the Forth Railway Bridge, the double cantilevers on the outside portion of the bridge carry weights "…of about 1,000 tons to counterbalance half the weight of the suspended span and live load" (forthbridges.org.uk). About 4,000 men, 54,000 tons of steel and 6,500,000 rivets were used.

The bad news regarding the construction of the Forth Railway Bridge is that 57 men were killed during the construction. Even though rescue boats were positioned under each cantilever, and 8 lives were saved that way, the loss of 57 lives in any construction project it tragic. Today, about 180 to 200 trains pass over the bridge every day (forthbridges.org.uk).

Advantages of Building Cantilever Bridges

One advantage that can be said for the construction of cantilever bridges is that they can be build from both sides of the waterway "…simultaneously," and the two sides can either meet or have a "final center span put into place to link the two extended 'diving board' spans" (Richman, 2005, p. 94). A cantilever bridge offers an advantage because: a) they can help to span "wide spaces"; b) they can be constructed without building "expensive falseworks" (that are built in order to support the bridge until it's finished, then are torn down); c) they can be built without "foundation piers" which often "disrupt the flow of the river"; and d) the cantilever bridge is noted for its strength and rigidity, which allows heavy rail transportation to cross safely without damaging the bridge or causing too much stress (Richman, 94).

Author Mark Denny, a PhD in physics at Edinburgh University, points out that cantilever bridges are built "symmetrically" so the load that acts on the supporting piers "…is always vertical, even during construction" (Denny, 2010, p. 160). The advantage of the symmetrical design is that the supports "…are simple piers that have to resist only vertical forces" because there are no "horizontal stresses upon them" (Denny, 160). In fact simple vertical loads means that it is reasonably simple for the bridge designer to take "thermal expansion and ground movement" into account (Denny, 160). As mentioned earlier in this paper, while it is being build, a cantilever bridge does not disturb wildlife in the river or lake that the bridge is spanning (Denny, 160).

The Top Five of the World's Longest Cantilever Bridges

The Forth Railway Bridge, discussed earlier in this paper, is the second longest cantilever bridge in the world, according to Jackson Durkee. It is 1,710 feet (521 meters), built in 1890. The longest, according to Durkee, is the Pont de Quebec, which is the longest to date and was built in 1917; it is 1,800 feet across (549 meters). The third longest cantilever bridge is the Minato Bridge (510 meters, 1,673 feet) in Osaka, Japan, completed in 1973. The Commodore Barry Bridge that links Bridgeport, New Jersey, with Chester, Pennsylvania, is 1,644 feet (501 meters) and was built in 1974. it's the fourth longest cantilever bridge in the world. And the fifth longest cantilever bridge is the Crescent City Connection in New Orleans; it is 480 meters (1,575 feet) across, and was finished in 1988.

Cable-Stayed Bridges

It has been noted in this paper that cable-stayed bridges were not built until the 20th century. Historian John a. Weeks explains that after World War II, Europe needed new bridges, big bridges -- some to replace those destroyed in the war -- in order to get the economy back into high gear. But steel was not readily available, and that meant large steel bridges were not a possibility, and "…suspension bridges were too costly, in both time and materials," so they were not on the agenda either. Given those realities, engineers designed the cable-stayed bridge, in which the roadway crossing the water (or canyon) is given support by cables that rise up to the towers (Weeks, 1996). Hence, a cable-stayed bridge is lighter weight than a typical suspension bridge.

While the cable-stayed bridge is very unlike the cantilever bridge, the cable-stayed bridge can appear to be similar to suspension bridges because both kinds of bridges have roads that are suspended from cables and both kinds of bridges have towers, according to an article in NOVA, produced by the Public Broadcasting Service. In cable-stayed bridges are attached to the towers, which "alone bear the load"; but in the design of suspension bridges the cables "…ride freely across the towers, transmitting the load to the anchorages at either end" (NOVA, 2003).

The original idea for cable-stayed bridges goes back to 1595, according to the NOVA article; in a book called Machinae Novae, published in that year, a sketch of a cable-stayed bridge is clearly presented. Apparently no engineer took that sketch to be a possible bridge solution for hundreds of years because the first cable-stayed bridges were not constructed until the twentieth century (NOVA). A very good example of a cable-stayed bridge is the Sunshine Skyway Bridge in Tampa, Florida, which was built in 1988 and won the prestigious Presidential Design Award from the National Endowment for the Arts.

Robert Lamb and Michael Morrissey explain in the Discover Company's publication, Science / How Stuff Works, that the author of Machinae Novae was Croatian inventor Faust Vrancic. Lamb and Morrissey note that a cable-stayed bridge may, "at first glance," appear to be a cousin of the suspension bridge; but even though they both have "similar towers and hanging roadways" they are not the same. Cable-stayed bridges don't need anchorages and they don't need two towers either, the authors explain.

In a cable-stayed bridge, the cables run from the roadway "…up to a single tower that alone bears the weight," and the tower has the responsibility to absorb the "compressional forces" (Lamb, et al., 2011). Cables may be connected to the roadway in several places and all spiral up to a single point in the tower; Lamb compares the fact of several cables attached at different places on the roadway below to a high point above on the tower to "…numerous fishing lines attached to a single pole" (Lamb).

There are two basic types of bridges in the cable-stay category, according to Aileen Cho writing in Engineering News-Record (Cho, 2012, p. 2). There is the "fan" type and there also is the "harp" type in the cable-stay genre, Cho writes. Of the top 10 cable-stay bridges, all use the modified fan configuration instead of the harp configuration, Cho explains, because in the harp configuration there tends to be "…increased compression in the superstructure" (p. 2). In the fan type of cable-stay bridge the cable stays "…are spaced out over the top portion of the pylon" to allow more room "…to be individually anchored near the pylon top" (Cho, p. 2). While the harp model uses cable stays "…in equal spaces over much of the height of the pylon" and hence, it offers a "pleasant aesthetic appearance" albeit the harp style is not as efficient structurally (Cho, p. 2).

Advantages of Cable-Stayed Bridges

According to Robert Lamb and Michael Morrissey, cable-stayed bridges offer all the positives that go along with suspension bridges, but for cable-stayed bridges that have spans of 500 to 2,800 feet, they cost less to build. Moreover, cable-stayed bridges do not need as much steel cable as other bridges, they are quickly to construct and they "incorporate more pre-cast concrete sections" (Lamb, 2011).

The Top Five of the World's Longest Cable-Stayed Bridges

The Sutong Bridge -- which crosses the Yangtze River in China, is 1,088 meters, Aileen Cho explains. The Stonecutters Bridge spans the Rambler Channel in Hong Kong Harbor; it is 1,018 meters and was completed in 2009. Another long Chinese bridge, the Edong Bridge, which is 926 meters long, spans the Yangtze River at the Port of Huangshi; the Edong Bridge is aesthetically beautiful with harp-like cables connecting the highway with the pylons. It was completed in 2010. The Tatara Bridge, in Japan, crosses the Inland Sea of Japan to connect the main island in Japan, Honshu, with Shikoku. It is 890 meters and was completed in 1999; at that time it was the longest cable-stayed bridge in the world. The fifth longest cable-stayed bridge in the world, as of 2012, is the Pont De Normandie Bridge in Northern France; it is 856 meters and it spans the River Seine. (Cho, 2012).

Comparing Cable-stayed Bridges with Cantilever Bridges

For one thing, a cable-stayed bridge is less costly than a cantilever bridge (Weeks, 1996). Indeed, author Mark Denny explains that cantilever bridges "…tend to be massive and therefore expensive" (Denny, 160). Denny goes on to say that cable-stayed bridges "…have much in common with cantilever bridges"; in fact, Denny asserts, a cable-stayed bridge is really "…a cantilever bridge with cables added to relieve the load" (164). Since the cables of a cable-stayed bridge are "…distributed symmetrically about each tower," and hence the weight of the deck (highway or railroad tracks) supported by the tower is also symmetrical, Denny writes (164).