Anthony Ragone

Prof. Joseph A. Betz, R.A.

ARC 253

1/27/2010

Bridge Research

ARCH BRIDGE

Arch bridges are one of the oldest types of bridges and have great natural strength. Instead of pushing straight down, the weight of an arch bridge is carried outward along the curve of the arch to the supports at each end. These supports, called the abutments, carry the load and keep the ends of the bridge from spreading out. Arch bridges work by transferring the weight of the bridge and its loads partially into a horizontal thrust restrained by the abutments at either side. A viaduct (a long bridge) may be made from a series of arches, although other more economical structures are typically used today.

Try It!

How do the abutments support an arch bridge?

Cut a strip of cardboard that's about one inch by 11 inches. Gently bend the strip so that it has a curve. Position the cardboard on a table so that it resembles an arch. Press down on the center of the arch. What happens to the ends of the cardboard?

Next, place a stack of books at each end of the arch. Press again. Now what happens? Notice how the stacks of books act as abutments, keeping the ends of the arch from spreading apart.

When supporting its own weight and the weight of crossing traffic, every part of the arch is under compression. For this reason, arch bridges must be made of materials that are strong under compression.

The Pont du Gard aqueduct

The Romans used stones. One of the most famous examples of their handiwork is the Pont du Gard aqueduct near Nîmes, France. Built before the birth of Christ, the bridge is held together by mortar only in its top tier; the stones in the rest of the structure stay together by the sheer force of their own weight.

The New River bridge

Today materials like steel and pre-stressed concrete have made it possible to build longer and more elegant arches, including a spectacular 1700 foot span in New River Gorge, West Virginia. (More typically, modern arch bridges span between 200-800 feet.)

Centering under construction

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Constructing an arch bridge can be tricky, since the structure is completely unstable until the two spans meet in the middle. One technique is to build elaborate scaffolding, or "centering," below the spans to support them until they meet. A newer method supports the spans using cables anchored to the

Arch construction using cable supports

ground on either side of the bridge. In situations where there is an active water or road way below, this method allows contractors to build without disrupting traffic.

The Natchez Trace bridge

One of the most revolutionary arch bridges in recent years is the Natchez Trace Bridge in Franklin, Tennessee, which was opened to traffic in 1994. It's the first American arch bridge to be constructed from segments of pre-cast concrete, a highly economical material. Two graceful arches support the roadway above. Usually arch bridges employ vertical supports called "spandrels" to distribute the weight of the roadway to the arch below, but the Natchez Trace Bridge was designed without spandrels to create a more open and aesthetically pleasing appearance. As a result, most of the live load is resting on the crowns of the two arches, which have been slightly flattened to better carry it. Already the winner of many awards, the bridge is expected to influence bridge design for years to come.

Beam Bridges or Girder Bridge

Beam bridges are the simplest kind of bridge today. They are a direct descendant of the log bridge, now more commonly made from shallow steel 'I' beams, box girders, reinforced concrete, or post-tensioned concrete. It is frequently used in pedestrian bridges and for highway overpasses and flyovers. As is its ancestor, this bridge is in structural terms the simplest of the many bridge types. Like most bridges that are characterized by how they are supported, beam bridges consist of one horizontal beam with 2 supports usually on either end.

Beam bridge

A beam or "girder" bridge is the simplest and most inexpensive kind of bridge. According to Craig Finley of Finley/McNary Engineering, "they're basically the vanillas of the bridge world."

In its most basic form, a beam bridge consists of a horizontal beam that is supported at each end by piers. The weight of the beam pushes straight down on the piers.

The beam itself must be strong so that it doesn't bend under its own weight and the added weight of crossing traffic. When a load pushes down on the beam, the beam's top edge is pushed together (compression) while the bottom edge is stretched (tension).

Try It!

What happens when a load pushes down on a beam bridge?

Take a flat eraser or a small sponge and slice a shallow notch across the top and bottom. Create a beam bridge by supporting each end of the eraser (or sponge) with a stack of books. Press down on the center of the bridge. What happens to the top and bottom notches? Notice how the top notch squeezes together in compression, while the bottom notch spreads apart under tension.

Pre-stressed concrete is an ideal material for beam bridge construction; the concrete withstands the forces of compression well and the steel rods imbedded within resist the forces of tension. Pre-stressed concrete also tends to be one of the least expensive materials in construction. But even the best materials can't compensate for the beam bridge's biggest limitation: its length.

The farther apart its supports, the weaker a beam bridge gets. As a result, beam bridges rarely span more than 250 feet. This doesn't mean beam bridges aren't used to cross great distances—it only means that they must be daisy-chained together, creating what's known in the bridge world as a "continuous span."

Lake Ponchartrain Causeway, Louisiana

In fact, the world's longest bridge is a continuous span beam bridge. Almost 24 miles long, the Lake Ponchartrain Causeway consists of two, two-lane sections that run parallel to one another. The Southbound Lane, completed in 1956, is made up of 2243 separate spans, while the Northbound Lane, completed in 1969, is pieced together from 1500 longer spans. Seven cross-over lanes connect the two main sections and function as pull-over bays in emergencies. Although impressive, the Lake Ponchartrain Causeway bridge underscores the drawback of continuous spans: they are not well suited for locations that require unobstructed clearance below.

Suspension Bridge

Aesthetic, light, and strong, suspension bridges can span distances from 2,000 to 7,000 feet—far longer than any other kind of bridge. They also tend to be the most expensive to build. True to its name, a suspension bridge suspends the roadway from huge main cables, which extend from one end of the bridge to the other. These cables rest on top of high towers and are secured at each end by anchorages.

Suspension bridge
Golden Gate Bridge, San Francisco, CA

Suspension bridge anchorage

The towers enable the main cables to be draped over long distances. Most of the weight of the bridge is carried by the cables to the anchorages, which are imbedded in either solid rock or massive concrete blocks. Inside the anchorages, the cables are spread over a large area to evenly distribute the load and to prevent the cables from breaking free.


Try It!

What are the anchorages for?

Tie two loops of string around the tops of two hard cover books of similar size. Tie a third piece of string to each loop so that it hangs loosely between the books. Press down on the center string. What happens?

Next, stand two books about 10 inches apart. Put a stack of heavy books on one end of string to secure it to the table. Then pass the string over each book (letting some string hang loosely between the books). Place a second stack of books on the other end of the string. Press again on the center of the string. What happens? Notice how the anchorages (stacks of books) help to stabilize the bridge.

Akashi bridge cable assembly

Some of the earliest suspension bridge cables were made from twisted grass. In the early nineteenth century, suspension bridges used iron chains for cables. Today, the cables are made of thousands of individual steel wires bound tightly together. Steel, which is very strong under tension, is an ideal material for cables; a single steel wire, only 0.1 inch thick, can support over half a ton without breaking.

The Humber bridge

Currently, the Humber bridge in England has world's longest center span—measuring 4,624 feet. But this record won't stand for long. In 1998, the Japanese will unveil the $7.6 billion Akashi Kaikyo Bridge, linking the islands of Honshu and Shikoku. The bridge's center section stretches a staggering 6,527 feet. To keep the structure stable, engineers have added pendulum-like devices on the towers to keep them from swaying and a stabilizing fin beneath the center deck to resist typhoon-strength winds. Because suspension bridges are light and flexible, wind is always a serious concern—as the residents of Tacoma, Washington can surely attest.

At the time it opened for traffic in 1940, the Tacoma Narrows Bridge was the third longest suspension bridge in the world. It was promptly nicknamed "Galloping Gertie," due to its behavior in wind. Not only did the deck sway sideways, but vertical undulations also appeared in quite moderate winds. Drivers of cars reported that vehicles ahead of them would completely disappear and reappear from view several times as they crossed the bridge. Attempts were made to stabilize the structure with cables and hydraulic buffers, but they were unsuccessful. On November 7, 1940, only four months after it opened, the Tacoma Narrows Bridge collapsed in a wind of 42 mph—even though the structure was designed to withstand winds of up to 120 mph.

New Tacoma Narrows Bridge

The failure came as a severe shock to the engineering community. Why did a great span, more than half a mile in length and weighing tens of thousands of tons, spring to life in a relatively light wind? And how did slow, steady, and comparatively harmless motions suddenly become transformed into a catastrophic force? To answer these questions engineers began applying the science of aerodynamics to bridge designs. Technical experts still disagree on the exact cause of the bridge's destruction, but most agree the collapse had something to do with a complex phenomenon called resonance: the same force that can cause a soprano's voice to shatter a glass.

Today, wind tunnel testing of bridge designs is mandatory. As for the Tacoma Narrows bridge, reconstruction began in 1949. The new bridge is wider, has deep stiffening trusses under the roadway and even sports a slender gap down the middle—all to dampen the effect of the wind.

Cable-Stayed Bridge

Cable-stayed bridge
Clark Bridge, Alton, IL

Cable-stayed bridges may look similar to suspensions bridges—both have roadways that hang from cables and both have towers. But the two bridges support the load of the roadway in very different ways. The difference lies in how the cables are connected to the towers. In suspension bridges, the cables ride freely across the towers, transmitting the load to the anchorages at either end. In cable-stayed bridges, the cables are attached to the towers, which alone bear the load.

The cables can be attached to the roadway in a variety of ways. In a radial pattern, cables extend from several points on the road to a single point at the top of the tower. In a parallel pattern, cables are attached at different heights along the tower, running parallel to one other.

Parallel attachment pattern

Radial attachment pattern

Try It!

How do cable-stayeds work?

Stand up and hold your arms out horizontally at each side. Imagine that your arms are a bridge, and your head is a tower in the middle. In this position, your muscles are holding up your arms.

Try making cable-stayeds to support your arms. Take a piece of rope (about five feet long), and have a partner tie each end of the rope to each of your elbows. Then lay the middle of the rope on top of your head. The rope acts as a cable-stayed and holds your elbows up.

Have your partner tie a second piece of rope (about 6 feet long) to each wrist. Lay the second rope over your head. You now have two cable-stayeds. Where do you feel a pushing force, or compression? Notice how the cable-stayeds transfer the load of the bridge (your arms) to the tower (your head).

Even though cable-stayed bridges look futuristic, the idea for them goes back a long way. The first known sketch of a cable-stayed bridge appears in a book called Machinae Novae published in 1595, but it wasn't until this century that engineers began to use them. In post-World War II Europe, where steel was scarce, the design was perfect for rebuilding bombed out bridges that still had standing foundations. Cable stay bridges have begun to be erected in the United States only recently, but the response has been passionate.

For medium length spans (those between 500 and 2,800 feet), cable-stayeds are fast becoming the bridge of choice. Compared to suspension bridges, cable-stayeds require less cable, can be constructed out of identical pre-cast concrete sections, and are faster to build. The result is a cost-effective bridge that is undeniably beautiful.

Sunshine Skyway bridge

In 1988, the Sunshine Skyway bridge in Tampa, Florida won the prestigious Presidential Design Award from the National Endowment for the Arts. Painted yellow to contrast with its marine surroundings, the Sunshine Skyway is one of the first cable-stayed bridges to attach cables to the center of its roadway as opposed to the outer edges, allowing commuters an unobstructed view of the magnificent bay. Recently, in Boston, Massachusetts, a cable-stayed design was selected for a new bridge across the Charles River—even though cheaper options were proposed. City officials simply liked the way it looked.

Cantilever Bridge

A cantilever bridge is a bridge built using cantilevers, structures that project horizontally into space, supported on only one end. For small footbridges, the cantilevers may be simple beams; however, large cantilever bridges designed to handle road or rail traffic use trusses built from structural steel, or box girders built from prestressed concrete. The steel truss cantilever bridge was a major engineering breakthrough when first put into practice, as it can span distances of over 1,500 feet (460 m), and can be more easily constructed at difficult crossings by virtue of using little or no falsework.