Bridges
Updated September 8, 2024
Austin area bridges across the Colorado River
Austin has several options when it comes to crossing the Colorado River. From west to east, these are the Pennybacker Bridge (over Lake Austin), the MoPac Bridge (including its hike and bike additions), the Lamar Boulevard Bridge, the Pfluger Pedestrian Bridge, the Union Pacific rail bridge (for trains only), the South First Street Bridge, the Congress Avenue Bridge (with its bats), and the I-35 Bridge. Next is the Longhorn dam itself, with Pleasant Valley Road across its top. Further east, on the way to the airport, are the Montopolis Bridge (no longer used for vehicles) and the 183 bridges.
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Pennybacker Bridge (360 Bridge): Through-arch bridge across the Colorado River (Lake Austin) west of downtown. Opened in 1982, it stands 100 feet above the water below, with a central span reaching 600 feet across.
James D. Pfluger Pedestrian Bridge: Originally built in 2001, the bridge moved pedestrian traffic from the Lamar Boulevard Bridge. The bridge spans some 700 feet.
South First Street Bridge (Drake Bridge): Completed in 1954 and widened in 1990, this beam bridge is 812 feet long in total.
I-35 Bridge: Another of our relatively unimpressive sets of bridges, the I-35 bridge is the eastern-most bridge across Lady Bird Lake.
(Longhorn Dam Bridge Project): Planned completion in 2026, this will be a “wishbone” shaped pedestrian and bicycle bridge across Lady Bird lake just west of Longhorn Dam.
(Pleasant Valley Road across Longhorn Dam): While not a bridge, Pleasant Valley Road does cross the Colorado River in East Austin via the Longhorn Dam. 183 Bridge: Next to the Montopolis Bridge, 183 crosses the Colorado River over several parallel beam bridges. These mark the furthest east crossings of the Colorado River in the general Austin area.
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Loop 1/MoPac Expressway Bridge and Roberta Crenshaw Pedestrian Walkway: The main MoPac bridges were completed in the early 1980s, the bicycle and pedestrian additions opened in 2004.
Lamar Boulevard Bridge: Completed in 1942, the bridge is an example of an open spandrel deck arch. There are 6 primary spans, and the total bridge is 659 feet long.
Railroad Bridge (Colorado River/Union Pacific): Different sources offer different information about when this bridge was built, but it is suggested that the piers were from an earlier bridge (1902 or 1906) the current trestle bridge was completed around 1936. This bridge is well known in Austin for its changing graffiti.
Congress Avenue Bridge (Ann W. Richards Congress Avenue Bridge): Our infamous bat bridge, the current Congress Avenue Bridge was built in 1910 and revised in 1980. That revision created additional space underneath that has attracted the largest urban bat colony in the world. From the spring through the fall, nearly every night more than a million Mexican Free-Tailed Bats (Tadarida brasiliensis) stream out in search of insects.
Montopolis Bridge: Completed in 1938, the Montopolis Bridge is a Parker Through Truss bridge that, since 2018, has been closed to vehicle traffic and converted to a pedestrian and biking bridge and is now part of the 183 Trail. The bridge sits at a traditional crossing of the Colorado River, and is part of the El Camino Real de los Tejas.
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What are Bridges?
Bridges are structures that cross a gap or obstacle to allow ease of movement. While we normally think of bridges crossing rivers and streams, many cross other roads, mountain valleys, or even part of the ocean. Bridges are a link, a way to connect. Thus bridges are also a metaphor for connections: between people, between cultures, between nations, and between humanity and God.
As structures, bridges can be simple or complex, they can be boring or aesthetically exciting (and at times perhaps ugly). In studying bridges, we have to consider physics, structural engineering, geometry, geology, materials sciences, and even anthropology. A bridge must physically fit within its environment, and when well designed also fit visually. Bridges must be strong enough to carry the loads that cross them, and flexible enough to allow for vibrations, wind, and other physical forces. As we noted in the Rivers and Streams honor, many cities are built along the banks of rivers, and thus many cities are characterized by their bridges. Think of some of the famous bridges in the United States, such as the Brooklyn Bridge, or the Golden Gate Bridge. Here in Austin, our most notable bridge is the Pennybacker Bridge (sometimes called the 360 Bridge), which crosses over Lake Austin. It is a through-arch bridge, braced into the limestone walls on either side of the river, crossing in a single span, and thus leaving the river unobstructed. But as we will see, Austin has many other interesting bridges, either due to their structure, their historic importance, or their role in hosting local wildlife. 
Pennybacker Bridge over Lake Austin.
Building Bridges: Forces, Falsework, and Fun
In building bridges (including the model bridge you will make for this honor), you need to consider the forces that act upon a bridge.
Compression and Tension: These are opposite sides of the coin - compression is when forces are acting in opposition to one another, “squeezing” the object in between. If you stand on a pillow, your force and the force of the earth beneath you are compressing the pillow between you. Bridge columns are under compressive forces. Tension is when two forces are working opposite to one another, “stretching” the object between them. Two teams playing tug of war are applying tensional force to the rope. The cables that hang a bridge from its suspension cables, or from an arch, are under tensional force.
Bending: When an object bends, it is under both compressive and tensional force at the same time. Imagine you have a board with each end set on cinderblocks. When you walk to the middle of the board, it bends down toward the ground, under your weight. But if you were to look at that board from the side, the part of the board nearest the floor in the middle is feeling tensional force (it is stretching) while the part of the board touching your feet is under compressional force (it is being squeezed). This is why when a board or shelf breaks, it breaks beneath before it breaks above - the tensional force is tearing it apart along the outside of the bend, while the inside of the bend is being compressed. Torsion: Torsional force is twisting force, putting spiral stress of compression and tension on an object. Imagine a stop sign on a post during a strong wind. The wind blows agains the broad flat portion of the stop sign, but it may not blow evenly, so the sign starts to twist I the wind, and that twisting (or torsional) force is carried on into the post, stressing it. Shear: While Compressive and tensional forces are evenly matched, shear force occurs when the forces are working opposite to one another, but in parallel, causing a stress that may tear or “shear” the material. The blades of scissors exert shear force by moving in opposite directions but not being exactly opposite one another (if the blades met directly, they would be exerting compressional force).
A final force that impacts bridges is Resonance. This is a vibrational force, that creates waves or ripples in the structure. Resonance may be caused by wind blowing through the cables of a bridge, by traffic flow, or even by large numbers of people walking or marching in unison. Additional supports, diagonal bracing, or cross beams may be added to reduce the impact of resonance on a bridge design. For more on forces, visit: 
During the construction of the Margaret McDermott Bridge, you can see the falsework supporting the second arch in the background
With these forces taken into consideration in the design and materials, it is time to build a bridge. Model bridges are fairly easy to build (though try to make one of each major type to see how they act differently). For simple bridges, you can use paper or cardboard, popsicle sticks, legos, or other items around the house. Have fun with your construction. See how much weight it can support, or how far your span can cross.
One more thing. If you have ever had a chance to watch the construction of larger bridges, you may notice that sometimes there are structures built to support he bridge during construction that are later torn down. These are called falsework, and allow the bridge deck or arch to be built over time, with the falsework removed once the final structure is stable. You may also see for concrete bridges large frames usually of wood or steel set up, into which concrete is poured. These frames, which are removed after the concrete is hardened, are called formwork, and are the second temporary structure often accompanying the creation of a bridge. Scroll through the assorted bridge pictures. Can you identify what type (or types) of bridges they are?
Bridges Honor
1. Define the following terms: a. Bridge. b. Aqueduct. c. Falsework d. Compression force. e. Tension force f. Bascule bridge. g. Covered bridge 2. Describe these six main types of bridge design and give two examples of each: a. Arch bridge. b. Beam bridge c. Cable-stayed bridge. d. Cantilever bridge e. Suspension bridge. f. Truss bridge 3. Discover the distance each of the following bridge designs can span: a. Arch bridge: b. Beam bridge c. Cable-stayed bridge. d. Cantilever bridge e. Suspension bridge. f. Truss bridge 4. Name the bridge design that can span the longest distance. 5. Draw or illustrate the basic design of each of the following bridge types. a. Beam bridge. b. Arch bridge c. Suspension bridge. d. Covered bridge e. Truss bridge. f. Cable-stayed bridge (fan shape) g. Cantilever bridge. h. Bascule bridge i. Cable-stayed bridge (harp shape) 6. Do the following activities: a. Watch a video about bridges. b. Make a list of at least eight notable bridges OR name two bridges that you have crossed and tell what design they are. 7. Make a video/multimedia presentation or scrapbook presentation about bridge(s) you have seen. 8. Build a bridge using materials such as craft sticks, toothpicks, yarn, thread, and glue. 9. Recite John 3:16, then tell how this verse describes Jesus’ role as a bridge between heaven and earth. |
Types of Bridges
To begin, lets talk about some of the various types of bridges. In general, bridges are categorized by the their structural components, though as we will see, many bridges are made of of several different styles.
BEAM BRIDGE Perhaps the simplest of all bridges is the Beam Bridge. A beam bridge is simply a horizontal item placed over a gap. If a tree or log is placed over a stream, it is a beam bridge. Due to their simplicity and minimal material necessary, beam bridges may be relatively cheap to construct. They are fairly common on highway flyovers and for crossing over other roads. But beam bridges have a structural limitation - the beams themselves are limited in length. Think about placing a board over a stream. A short board wont bend much in the middle when you cross. The longer the board, the more it will bend in the middle. So beam bridges are limited by the length of each span (the portion of the bridge between supports). The span of a beam bridge is usually limited to not more than 250 feet due to materials constraints. However, a bridge may have several spans (and each span may be of a different type). The longest continuous beam bridge, and the longest continuous bridge over water is right next door in Louisiana. The Second Lake Pontchartrain Causeway is 23.87 miles long. The causeway also has a bascule bridge, something we will address below. TRUSS BRIDGE
Next up are Truss Bridges. A truss bridge is basically a reinforced beam bridge, taking advantage of the strength of triangles. A triangle cannot be distorted without changing the length of its sides, so it is a very stable structure (a parallelogram, such as a rectangle or square, can be deformed without changing its sides - think of a cardboard box that is flattened by changing the angles at the corners). Trusses are rails or open walls made up of vertical, horizontal, and diagonal bracing. When a bridge has trusses on both sides, like fences or walls, but no “roof” or connecting crossbeams, it is called a Pony Truss Bridge. When the trusses are beneath the bridge deck, rather than above the deck, it is called a Deck Truss Bridge. When the trusses on each side of the bridge are connected at the top by crossbeams, it becomes a Through Truss Bridge. Truss bridges are common on railroads, and truss elements are often used to strengthen other types of bridges (including Cantilever Bridges, which we will address below). Truss bridges are also categorized by the various arrangements of vertical, horizontal, and diagonal bracing, and usually named for a particular designer or common user of that arrangement of trusses. The Montopolis Bridge in East Austin is a truss bridge, and was completed in 1938, replacing another bridge that had been destroyed in a flood a few years earlier. ARCH BRIDGE
Another way beam bridges are strengthened is through the use of arches. Arches themselves are very strong structures that shift weight to the supply pillars, and have been a feature of classical architecture around the world. If a bridge deck is carried above the arch, it is called a Deck Arch Bridge. When the deck is suspended from the arch, and the arch legs are below the deck, it is considered a through arch bridge. Our local Pennybacker Bridge is an example of a through arch bridge. A variation on through arch bridges is a tied arch bridge, where the deck or some other horizontal structure serves as the bowstring to the arch, providing tension to strengthen the arch structure. Many older bridges are arch bridges, and in Austin one notable (but often unseen) example is the West 6th Street Bridge over shoal creek, a masonry arch bridge completed in 1887, making it a year older than the State Capitol building. The current longest arch bridge in the world is the Tian’e Longtan Bridge in Guangxi, China, spanning some 1969 feet. The longest stone arch bridge (made up of several arch spans, not a single span lie the Tian’e Longtan Bridge) is the Rockville Bridge near Harrisburg, PA, coming in at 3820 feet. In Texas, the longest stone arch bridge is the Possum Kingdom Bridge found in Graford. CANTILEVER BRIDGE
Another type of bridge is a Cantilever Bridge. Cantilevers are interesting structures. Unlike beam bridges, in which the horizontal element is supported at both ends, a cantilever is only secured at one end, the other reaches out freely, allowing it to cross areas with no additional; supports. A diving board at your local pool is a simple example of a cantilever - it is secured at one end, but reaches out into space over the pool at the other. In bridge construction, particularly modern concrete bridges, cantilevers are often paired, reaching out in opposite direction from a central support beam. This provides balance, and lets the bridges span gaps further than beam bridges without requiring all of the additional structure of arches. At times, two cantilevers may reach out toward one another, and a simple beam bridge fills the gap between them without any additional support underneath, extending the span they can cross without column supports. The West 5th Street Bridge over Shoal Creek is an example of a cantilever concrete girder bridge, completed in 1931. The current longest cantilever Bridge is the Quebec Bridge in Canada, spanning 1800 feet between pylons. SUSPENSION BRIDGE
Suspension Bridges are some of the longest bridges in the world. In a suspension bridge, cables are secured at each end of the structure, and held aloft by pylons or towers. The bridge deck is then suspended from these cables by vertical cables - suspending the bridge. The Golden Gate Bridge in San Francisco and the Brooklyn Bridge in New York. In our Central Texas area, there aren’t many suspension bridges, but one close example is the Waco Suspension Bridge over the Brazos River, which now only services pedestrian traffic. The longest suspension bridge in the world is the Canakkale Bridge in Turkey, completed in 1915. Measuring a total of 15,118 feet long, its longest single span 6,637 feet (2023 meters, marking the 100th anniversary of modern Türkiye). CABLE-STAYED BRIDGE
A particularly interesting style of bridge, that may at first glance be confused with suspension bridges, is called a Cable-Stayed Bridge. Rather than having vertical support cables hanging from horizontal support cables, these bridges have support cables run directly from the (usually) vertical pylons or towers. In their simplest form ,the tower is vertical, and the support cables either all start from near the same point of the tower and stretch down to the bridge deck (fan shaped cable-stayed bridges) or the cables are (usually) evenly spaced down the tower, and then stretch out to the bridge deck (harp shaped cable-stayed bridges). While these do not span the distance of suspension bridges, their basic design allows for a lot more creativity - the support column isn’t always vertical, it isn’t always even straight. The current longest cable-stayed bridge may be the new Changtai Yangtze River Bridge, which has a longest single span of 3963 feet. This exceeds the previous record holder, the Russky Island Bridge in Russia, which had a single span of 3622 feet. In recent years, the Dallas area added two new cable-stayed bridges; the Margaret Hunt Hill Bridge, and the bike lanes of the Margaret McDermott Bridge. COVERED AND MOVEABLE BRIDGES
There are a few more types of bridges for us to consider. Covered Bridges (often beam bridges or enclosed through truss bridges) are basically framed in bridges, with walls (and often windows) and a roof. Most covered bridges were made of wood. (Click here for a list of Texas covered bridges. These are more common in the US northeast than down south here in Texas). Finally, we will address movable bridges. There are three basic types of moveable bridges:
Variations on bascule bridges are found all over the world, in cities and near ports. Some bascules are single, others are paired to allow two “halves” of the bridge to lift up. Bascules are often included within other bridges - as noted earlier there is a bascule section to the Lake Pontchartrain Causeway. Bascule bridges allow traffic over water while still allowing taller traffic in the water to pass the bridge. Examples of lift and bascule bridges not too far from Central Texas can be found in and around Galveston. |
