Arch Bridges: Körfez Bridge

Turkey is a modern country with a sophisticated highway system. The bridges on the Trans-European Motorway are designed to the latest AASHTO specifications and they performed well during the 1999 Kocaeli Earthquake.  Still, Turkey is a country with a long past and occasionally you come upon a bridge from another era.

Körfez is a little town on the eastern shore of the Sea of Marmara near Istanbul.  It was here that I saw a closed spandrel, stone masonry arch with cutwaters and openings at the piers.  The deck rises to a peak above the large, central arch. It looks like the bridge was built just downstream of a weir.  The river is contained within stone walls of the same vintage as the bridge.  Its still in use as a pedestrian crossing over the river.  I wonder how long ago this bridge was built?

Arch Bridges: Körfez Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Arch Bridges: Dirty Devil River Bridge

At milepost 45 on State Route 95, an open spandrel, steel deck arch bridge crosses the Dirty Devil River. A couple of miles further down State Route 95, a through arch bridge crosses the Colorado River at Hite (shown on February 11th).

The bridge over the Dirty Devil River is perhaps less spectacular than the landscape that surrounds it. I wonder how difficult it is to maintain bridges in Utah? This desert used to be an ocean, and the Dirty Devil River adds 150,000 tons of salt to the Colorado River every year. Still, there isn't much rain or snow in the high desert, and bridges should age slowly.

There are also a number of stone arch bridges at Natural Bridges National Monument a couple of miles farther down State Route 95.

Arch Bridges: Dirty Devil River Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution-Noncommercial 3.0 United States License.

Arch Bridges: Navajo Bridges

In the nineteenth century, travelers in eastern Utah had a 600 mile (960 km) detour to get across the Colorado River into western Arizona.  In 1873 John Lee started a ferry service at the mouth of Glen Canyon. However, In the 1920's it was decided to build a bridge five miles downstream from Lee's Ferry, across Marble Canyon. The bridge was constructed with tie-backs supporting the two sides until they met in the middle. The area was extremely desolate and when the ferry sank killing three workers, people had to drive 800 miles to get across the river until the bridge was completed in 1929.

It's a steel, deck arch with laced spandrel columns. It has a span length of 616 ft (188 m), an arch rise of 90 ft (27.4 m), its 467 ft (142 m) above the river and has a width of 18 ft (5.5 m). However, the bridge couldn't carry heavy trucks, it made a kink on U.S. Route 89A that caused many accidents, and it was too narrow to carry two lanes of traffic.  In the 1990's it was decided to build a second crossing to address these problems, and to make the old bridge a pedestrian crossing.  The new Navajo Bridge is another steel, deck arch, but longer, wider, and able to carry heavier loads.

This photo looks west from the Navajo Nation across Marble Canyon at the old Navajo Bridge. More information about these bridges is available at:

http://www.nps.gov/glca/historyculture/navajobridge.htm

Arch Bridges: Navajo Bridges by Mark Yashinsky is licensed under a Creative Commons Attribution-Noncommercial 3.0 United States License.

Arch Bridges: The Rainbow Bridge

The Niagara is one of the few rivers in the Northern Hemisphere that flows northward.  Lake Erie is 326 ft (99 m) higher in elevation than Lake Ontario.  About half of this change in elevation occurs at Niagara Falls, 17 miles (27 km) from the river's mouth.

Downstream from the falls is the Rainbow Bridge, a 950 ft (290 m) long deck arch that was opened in 1941.  Two cross-braced steel box girder arch ribs support spandrel columns, floor beams, steel stringers, and a concrete deck.  The arches start 50 ft (15 m) above the water's surface to avoid the ice flows that destroyed the previous bridge at this site.  Nets were provided under the bridge during construction, which caught workers and prevented casualties. 

A history of all the bridges crossing the Niagara River can be obtained at: http://www/iaw.com/~falls/bridges.html

Arch Bridges: The Rainbow Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution-Noncommercial 3.0 United States License.

Arch Bridges: The Peace Bridge

The Niagara River is the international boundary between the United States and Canada. There are currently five railway and/or highway bridges that serve as border crossings along the river.

The Peace Bridge crosses the Niagara just downstream from its mouth at Lake Erie. It is a six span bridge (five deck arch spans and a through truss span over Black Rock Canal), that is 5800 ft (1768 m) long, and carries I-190 traffic between Buffalo, New York and Fort Erie, Ontario.

The bridge was built in 1927 and its name celebrates 100 years of peace between the United States and Canada. In 1997 the owners began making plans for a parallel structure, a cable-stayed bridge, but (as often occurs) legal and economic challenges have delayed construction.

Arch Bridges: The Peace Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution-Noncommercial 3.0 United States License.

Arch Bridges: South Grand Island Bridges

The Niagara River is only 36 miles (58 km) long between Lake Erie and Lake Ontario, but it has a number of interesting crossings. Just south of Niagara Falls is Grand Island with a pair of toll bridges at each end that carry I-190 traffic across the river.

The South Grand Island Bridges are 3400 ft (1026 m) long truss bridges with through arches for their center spans. The arches are pinned at the foundations and support the superstructures with steel cables at the middle of the arches.
The first bridge was built in 1935 and carried one lane of traffic in each direction. A parallel structure was built in 1962 so that each bridge could carry two lanes of traffic in a single direction. Its odd that the 1962 structure uses the same old-fashioned, laced members as the older bridge. In fact, I had to look closely to determine that the bridge on the left was the newer structure.

Both the 1935 and the 1962 structures received poor ratings during recent inspections and are scheduled for deck replacements and substructure repairs later this year. Transportation planners say a third lane will be needed in each direction by 2020, which means the North and South Grand Island Bridges may need to be replaced (for $500 million).

There are a lot of truss arches around the world, but they are slowly being replaced with other types of bridges. No one builds trusses anymore. Steel truss bridges need continual priming and repainting to prevent corrosion. However, both of these bridges look well maintained. The harsh winters and use of de-icing agents over the years may account for the deterioration of the deck and other concrete members. Once the reinforcement begins to corrode, there is little one can do.

Arch Bridges: South Grand Island Bridges by Mark Yashinsky is licensed under a Creative Commons Attribution-Noncommercial 3.0 United States License.

Arch Bridges: The Anshun Bridge (1)

Stone arch bridges have been built in China for over  2.000 years.
Long, open spandrel stone arch spans have been built since 600 AD (such as the Zhaozhou Bridge). Shapes like 'horse's hoof,' 'egg-shaped,' 'pot bottom,' and 'pointed,' refer to variations of catenary, parabolic, circular, and elliptical arches used in China.  An excellent history of arch structures can be found in 'Chinese Bridges' by Ronald G. Knapp.

The Anshun (Peaceful and Favorable) Bridge crosses the Jinjiang River in Chengdu, Sichuan Province, China.  Marco Polo mentions it as one of four Chinese bridges that he admired during his travels.  That bridge was destroyed during flooding in 1980's. Li Chuncheng, the mayor of Chengdu in 2003, had the bridge rebuilt with a popular restaurant on top as part of an effort to revitalize downtown, which was highly successful.  The new bridge is a three span, closed spandrel stone arch, with stone dragons seated on cutwaters below round openings to reduce water pressure during floods.

Walking along the river, I saw couples practicing ballroom dancing (or strolling arm in arm), children catching cicadas with branches, groups practicing Tai Chi Chuan, old men smoking pipes, and young men fishing between the many different arch bridges crossing the Jinjiang River.

Arch Bridges: The Anshun Bridge (1) by Mark Yashinsky is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 United States License.

Arch Bridges: Cumbria Pedestrian Bridge

Hiking through the Lake District in Northwestern England, we noticed slate being used for walls, footpaths, houses, and in today's bridge. Actually, slate is an interesting material for an arch bridge. The thin slabs are put face to face into an arch-shape and their self-weight pushes them into compression against the abutments.  There was no mortar so the slate is wedged in with the thicker ends on top. No mortar, no spandrels, and no deck: this is a very minimalist bridge.  Based on the amount of wear on the top surface, it must be hundreds of years old.

Arch Bridges: Cumbria Pedestrian Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Arch Bridges: Bixby Creek Bridge

Another example of an open spandrel concrete arch bridge is Bixby Creek, in Monterey County, California. This is one of the most photographed bridges in the world.  The parabolic shape of the arch, the tall spandrel columns, the architectural piers, and the rugged coastal setting all contribute to an intense aesthetic experience.

This bridge was built in 1932 when California was opening State Route 1 along the coast by building bridges across seven wide canyons near Big Sur. 

The photograph was taken on a weekend when we (Caltrans and Lawrence Livermore Labs) were attaching instruments on the bridge to obtain its natural periods of vibration. The periods (and mode shapes) would be checked against a computer model of the bridge to make sure it was okay.  All the bridges along the coast were being retrofitted (by Buckland and Taylor) and this was an independent check of the retrofit strategy (similar to the one discussed for the Buena Vista Bridge).

Arch Bridges: Bixby Creek Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Arch Bridges: Buena Vista (Broadway) Bridge

Since we studied a closed spandrel arch bridge yesterday I thought we might look at an open spandrel arch today.

Downtown Los Angeles was more rural in appearance when the Buena Vista Bridge (across the Los Angeles River) was built in 1910.  Certainly, this bridge was grander before its sculptural details were removed. Actually, this photo was taken while the bridge was being retrofit, which required taking most of it apart and reassembling it. Seismic retrofits for deck arch bridges require strengthening the spandrel columns and tying the deck together to carry the longitudinal forces to the abutments. There are a couple dozen reinforced concrete arch bridges in Los Angeles and Pasadena that were retrofitted in the 1990's, usually at more expense than replacing the bridges would have cost.

This structure, designed by the architect A.F. Rosenheim, is typical of open spandrel, reinforced concrete bridges built in California at the beginning of the 20th century.  Steel was expensive (and considered ugly) in California, concrete was cheap, and the ornate Beaux Arts style was applied to bridges with the idea of making new cities (like Los Angeles) more attractive. 

Arch Bridges: Buena Vista (Broadway) Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Arch Bridges: Alte Brücke

Since we've been studying arch bridges, I thought we'd look at a closed spandrel arch.  These bridges are constructed by forming one or more arches and building walls along the sides (and abutments or piers at the ends).  The interior is filled with granular material and some kind of deck or road surface is built on top.

The Alte Brücke crosses the Main River in Frankfurt, Germany.  Its name means 'Old Bridge,' which is not exactly true. Records indicate that a stone bridge has stood at this location perhaps as far back as 782 AD, but this bridge has been destroyed and rebuilt many times, with its latest reincarnation coming in 1965.  Actually, the part of the bridge in this photo may be from an older structure, but the arches in the channel have been replaced with steel girders.  Note the huge cutwaters in front of the piers. Perhaps the bridge was damaged in the past due to debris carried by a flood.  The south bridge abutment is next to an outdoor cafe with stairs leading to the river.  This pleasant location attracts many swans (and children to feed them).

Arch Bridges: Alte Brücke by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Arch Bridges: Fremont Bridge

The Fremont Bridge carries US-30 and I-405 traffic over the Williamette River in downtown Portland, Oregon. It was named after the military officer, explorer, and presidential candidate John Charles Fremont.

The bridge looks like a through arch but it is actually a tied arch, with the 902 ft (275 m) center section fabricated in California, assembled at Swan Island, floated to the bridge site, and lifted 175 ft (53 m) into place.

It is a double-deck bridge with four lanes of traffic in each direction. A commission was invited to participate in the design, which greatly increased it's cost (to $82 million). The bridge was opened to the public in 1973. At that time, it was the world's longest tied arch bridge ever built and the largest weight (at 6000 US tons) ever lifted.

A tied arch bridge is sometimes called a bowstring arch because the arch is like a bent bow with bowstring (the deck and girders) under tension. These high stresses make this type of bridge susceptible to maintenance problems due to fatigue at the connections.

Arch Bridges: Fremont Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Arch Bridges: Glen Canyon Bridge

The Glen Canyon Bridge carries US-89 across the Colorado River at Page, Arizona. The bridge was built just downstream from the Glen Canyon Dam and was completed in 1959 (the dam wasn't completed until 1966).

Like the Alexander III bridge we studied yesterday, this structure is a deck arch, although considerably different in appearance. It has a single 1027 ft (313 m) span between the faces of the canyon's walls.

The steel members were fabricated in California, assembled to ensure that everything fit, then disassembled and shipped by rail to Flagstaff, and trucked to the job site. Meanwhile, the sides of the canyon were prepared for the foundation with light blasting and drilling. A tie-back system on towers was used to assemble the arch with rivets. Spandrel columns were preassembled, lowered into position, and attached to the arch. High-strength bolts connected the deck system to the columns and leave-in-place forms were used to cast the concrete deck. The entire bridge was constructed in a single season. The U.S. Bureau of Reclamation oversaw construction by the joint venture firm of Kiewit Judson Pacific Murphy. The bridge was constructed without any fatalities and only a few injuries. More information on this project is in the February 1959 issue of Civil Engineering Magazine.

The Glen Canyon Bridge is 700 ft (213 m) above the Colorado River, making it the fourth highest bridge in the U.S. We've previously studied the fifth highest bridge, New Mexico's Rio Grande Gorge Bridge. Both bridges were constructed to avoid obstacles caused by the earth's tectonic activity. The Rio Grande Gorge was formed due to the Pacific Plate's movement with respect to the stable North American Plate. The Colorado River created deep canyons by cutting through sedimentary layers that were slowly rising due to tectonic stresses.

As I mentioned, the bridge was part of the project to dam the Colorado River, a controversial project due to its negative impact on the environment. I like to think that bridges are environmentally benign, but often they are not. This bridge was built to provide equipment, material, and workers access to both sides of the dam during it's construction.

Arch Bridges: Glen Canyon Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Arch Bridges: Pont Alexander III

A third very popular type of arch bridge is the deck arch, in which the arch supports the deck from below.  The arch in this type of bridge cannot rely on the deck to resist its large axial force and so it must be carried by the substructure and foundations instead.

An example of this type of structure is the Alexander III Bridge in Paris, designed by Louis Jean Resal in 1899. As Fritz Leonhardt mentions in his excellent book 'Brucken,' the rise to span ratio of this arch is very flat (1/17), resulting in large longitudinal forces that must be resisted by the abutments. In this way, the bridge is able to cross the Seine in a single 107.5 meter span.  The arch ribs are composed of cast iron I-sections that are covered in ornamentation reflecting the tastes of La Belle Epoque.  Golden seraphim, candelabra, garlands, and coats of arms decorate the balustrades or sit on pedestals.  A tower by another great bridge engineer of the period, Gustav Eiffel can be seen in the distance.

Arch Bridges: Pont Alexander III by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Arch Bridges: Hernando de Soto Bridge

We'll continue our study of tied arch structures with the Hernando de Soto Bridge (named after the first European to see the Mississippi River).  This bridge carries I-40 over the Mississippi in Memphis, Tennessee.  It's different from the previous tied arch bridge we studied (the I-80 POC) because it's supported by two steel truss arches.  

The Mississippi River is so wide at this location that the bridge piers were built in the water. There are several steel girder approach spans and two 900' arch spans going over the river.  To cross in a single span would have required a suspension bridge, which would still have been inadequate when the Mississippi floods it's banks.

This bridge was opened to the public in 1972 and is currently getting a seismic retrofit due to it's proximity to the New Madrid Seismic Zone. The retrofit (designed by Roy Imbsen) is similar to that used for many long span bridges.  The superstructure is isolated from the substructure, in this case by large, friction pendulum bearings.  Isolation is a good solution for long span bridges because otherwise the large forces generated by the heavy superstructure are difficult to resist.  

Just as in yesterday's I-80 bridge, the axial forces in the arches are resisted in tension by the superstructure. The isolation devices are placed below the superstructure and reduce the lateral force being transfered to the substructure during an earthquake.

Arch Bridges: Hernando de Soto Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Arch Bridges: I-80 Pedestrian Overcrossing

We've been looking at arch bridges lately and so I thought we might discuss this attractive, basket-handle arch, which is located in Berkeley, California.  Designed by Mark Ketchum in 2002, this bridge carries pedestrians (and bicycles) over I-80 on a single, 95 meter span.

This is a tied arch bridge, which is similar to a simply-supported girder. The arch is like the top flange of the girder, the deck is like the bottom flange, and the cables are like the web.  The dead and live loads put the arch into compression and this force is resisted by the prestressed concrete superstructure.

Arch Bridges: I-80 Pedestrian Overcrossing by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Arch Bridges: I-94 Gateway Arch Bridges (1)

These through arch bridges are in a much different location than the bridge I discussed yesterday (in Utah's Painted Desert).  However, they are remarkably similar in form and function.

The Gateway Arch Bridges carry I-94 traffic over Telegraph Road just west of Detroit. They replace an existing four-span bridge with a single span, while maintaining the same vertical clearance and without changing the elevation of I-94.

This is another example of the versatility of arch bridges when a clear span greater than 200 feet is desired (the Gateway Arch has a clear span of 246 feet between the abutments).  This length would be difficult to achieve from a steel or concrete girder bridge without using webs of ten to twelve feet in depth (which would change the vertical clearance, the deck elevation, or both). The through arch is a great choice when a narrow superstructure depth and a long span length are required.

This is a great looking bridge. The bright blue arches with eye-shaped cross-bracing creates a powerful image. Also, you can clearly see how the ties hold up the 9" concrete deck with bright blue floor beams. The Gateway Arch Bridges are a successful combination of engineering and aesthetics.  More information about these bridges can be obtained at http://www.lusas.com/case/bridge/gateway_arch_bridge.html.

Arch Bridges: I-94 Gateway Arch Bridges by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Arch Bridges: The Hite (Colorado River) Bridge

Although the Colorado River runs 400 miles through Utah, there are only a couple of bridges that cross it.  The Dewey Suspension Bridge was built in 1916 but was accidentally burned down in 2008.  An unnamed bridge on US Route 191 was built in 1955 and crosses the river near Moab.  The Hite Bridge on State Route 95 was built in 1966 and crosses the Colorado River near the town of Hite.  Arch bridges are a good choice for crossing deep river canyons.  The arch supports can be founded on hard rock and can span up to 300 meters. The Hite Bridge is a steel, through arch with box girder arch ribs and I-girder cross-bracing. A through arch supports the deck from below (with spandrel columns) and above (with cables or ties). The ends of the arch ribs are pinned to reduce bending in the arches and to simplify the analysis.

The stark landscape enhances the bridge's beauty. Note the change in color from Jurassic Cedar Mesa Sandstone on the desert floor to Triassic Moenkopi Formation on the buttes. 

Lake Powell was formed by the Glen Canyon Dam about 80 miles south of Hite and a marina was built at the northern tip of Lake Powell in Hite.  However, years of drought have shrunk the lake and left the marina stranded. 

Arch Bridges: The Hite (Colorado River) Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Arch Bridges: The Lilac Road Bridge

As I've previously mentioned, California too often adopts a cookie-cutter approach to bridge design. All the more remarkable then when a truly unique and dramatic bridge gets built.

The Lilac Road Bridge is a reinforced concrete arch built high over I-15, a little north of San Diego, California. It was designed by Caltrans architect William Wells (with help from Caltrans engineers) and it was completed in 1978.

The question of the architect's role in bridge design is an interesting one. David Billington in 'The Tower and the Bridge' writes that an aesthetic experience occurs when we can see clearly how a structure resists the forces acting on it. This seems hardly the role for an architect, who is more concerned with visual themes and variations in a design. Still, it is often a collaboration of engineer and architect that creates the finest bridges.

A few days ago, this blog presented a photo of the Millenium Bridge, a collaboration between the engineer Ove Arup and the architect Norman Foster.

The Lilac Road Bridge is a very dramatic and attractive bridge. It looks like a giant scythe poised over the roadway. The way the arch joins the deck looks organic, more like the way a plant grows than something made by humans. I think that this bridge is as good as anything done by Eiffel, Maillart, Menn and the other great designers of arch bridges.

Arch Bridges: The Lilac Road Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Suspension Bridges: Akashi Kaikyo Bridge

The Akashi Kaikyo Bridge has the world's longest main span. It was originally designed to be 1990 meters long. However, during the 1995 Kobe Earthquake the towers moved apart an additional meter, requiring a slight redesign to the unbuilt superstructure. The side spans are 960 meters for a total length of 3991 meters. Construction was completed in 1998.

This bridge is superlative in every way. The painter's walkway (between the stiffening trusses) was designed to carry vehicles. Dry air is blown through the suspension cables to prevent corrosion. The 300 meter towers have tuned-mass dampers to reduce vibrations during high winds. The north anchorage is supported by a cylindrical diaphragm wall that was cast in the ground. The soil in the center was then excavated and the opening was filled with roller-compacted concrete. The towers are on steel caissons that were floated into position and dropped through the swift moving Akashi Straits.

The bridge carries traffic between the west side of Kobe and Awaji Island. It is one of two long-span bridges on the 81 kilometer long Kobe-Awaji-Naruto Expressway.

Suspension Bridges: Akashi Kaikyo Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Cable-Stayed Bridges: Tatara Bridge

Cable-stayed bridges have become the most popular structures for span lengths between 300 to 1000 meters. When I visited the Tatara Bridge in 1998, it was the longest cable-stayed bridge in the world with a main span of 890 meters. However, it will move to third place later this year with the completion of the 1018 meter Stonecutters Bridge in Hong Kong. Currently, the longest cable-stayed bridge in the world is China's Sutong Bridge with a main span length of 1088 meters. Undoubtedly, it will eventually be replaced by an even longer cable-stayed bridge. Of course, cable-stayed bridges are dwarfed by the much longer suspension bridges. Currently, the longest suspension bridge is the Akashi-Kaikyo Bridge with a main span of 1991 meters.

The Tatara Bridge was part of the project that linked the islands of Honshu and Shikoku with three expressways (the Akashi-Kaikyo Bridge was also part of this project). The project included 17 long-span bridges as well as many viaducts. The Tatara Bridge is part of the westernmost Nishiseto Expressway that includes nine long-span bridges and is 60 km in length. The Tatara Bridge has two, 220 meter tall steel towers. We rode up to the top in a special elevator that had wheels on its side to navigate the sloping bottom section of the tower. The deck carries two lanes of traffic in each direction along with pedestrian and bicycle lanes. Clearance below the deck is only 26 meters, preventing the Tatara Straits from being used as a shipping channel. The bridge has uneven side spans which resulted in using some reinforced concrete box girders to balance the weight. It was originally planned to be a suspension bridge, but was eventually changed to a cable-stayed bridge to reduce the environmental impact of excavating large anchorages into Ikuchijima and Ohmishima Islands.

Cable-Stayed Bridges: Tatara Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Cable-Stayed Bridges: Cavenagh Bridge

Singapore is a wealthy tropical island with a history and laws that encourage a comfortable and civil society. It's a city state, with a very diverse population who have learned to live together. It was a British colony and currently has a parliamentary government dominated by a single party.

Singapore has many handsome buildings, gardens, and a variety of very fine bridges crossing Marina Bay and the Singapore River. The Cavenagh Bridge was designed by the British colonial government, built in Scotland, disassembled, and rebuilt across the Singapore River in 1869. As commerce and trade increased, the bridge became overloaded and was converted to a pedestrian bridge in 1910.

The structure is a suspension, chain-stayed, and through-girder bridge. Instead of cables, it is supported by chains made of steel links that go up through the tower saddle and are anchored into the ground. The bridge and surrounding area are decorated with a variety of sculptures and historic plaques, including the Cavenagh family coat of arms.

The people of Singapore have shown their usual good judgement in preserving this important structure instead of replacing it with a new bridge when it became less economically viable.

Cable-Stayed Bridges: Cavenagh Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Los Angeles River Bridges: Burbank Boulevard Bridges

The Los Angeles River is a concrete-lined channel for most of its 50-mile length between the San Gabriel Mountains and the Pacific. At a few locations, such as just north of the Sepulveda Dam, the river is allowed to meander freely.

The Burbank Boulevard Bridges are three span structures built in 1976. They cross the Los Angeles River just north of the Dam. Their shape is organic, which is very unusual for bridges in California. The piers slope upward and meet a soffit that looks formed by a human hand.

The Dam is also striking, but it was built in 1941 and is much less organic in appearance. These bridges are sculptural, with a style reminiscent of Naum Gabo or Henry Moore. They blend in well with their environment, and are a happy change from the cookie-cutter style of most California bridges.

Los Angeles River Bridges: Burbank Boulevard Bridges by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Arch Bridges: Alsea Bay Bridge

Oregon is fortunate to have many lovely bays along its coast. It is also fortunate to have had Conde B McCullough as the chief bridge engineer when US101 was being constructed during the Great Depression. As a result, there are a dozen beautiful bridges crossing the rivers and bays along Oregon's coast.

However, when the existing Alsea Bay Bridge began to deteriorate due to the saltwater environment, an equally handsome and innovative bridge, designed by Semyon Treygor at HNTB was built in 1991. The main span is a 450 foot long reinforced concrete through-arch with long approaches supported on 'Y' shaped piers. The replacement bridge was built with a thicker concrete cover to resist the salt-water spray. All that is left of Conde McCullough's original bridge are the concrete viewing platforms at each end of the new bridge.

Arch Bridges: Alsea Bay Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.

Suspension Bridges: The Millenium Bridge

The Millennium Bridge carries pedestrians from St Paul's Cathedral to the new Tate Modern Art Museum across the River Thames in London.

A problem with pedestrian bridges is that people walking in step can create large vibrations and even resonance that can damage the bridge. This problem is sometimes solved by people walking out-of-step with each other as they cross the bridge. However, in this case, the vibrations were so bad that the bridge was closed until special devices could 'dampen' the bridge's response.

The Millennium Bridge is an unusual three span suspension bridge with the cables hanging below the deck to give pedestrians an unobstructed view of their surroundings. The spans are 81 meters, 144 meters, and 108 meters for a total length of about 333 meters. The deck is 4 meters wide and can carry about 8000 pedestrians. The designer was Ove Arup who made a name for himself designing innovative, pedestrian bridges.

Suspension Bridges: The Millenium Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution 3.0 United States License.