Beam bridges are one of the simplest support structures that create a span over open space in use today. The stringer design incorporates a pier or abutment at each end with the deck resting on simple supports which travel across each side. Extended bridges using this design may use multiple piers to support the weight of the passenger traffic that occurs above.
The simplest designs in this bridge category use stone slaps, wooden planks, or even logs that go from one end of the span to the other. When engineers create a beam bridge using modern components, it will usually entail reinforced concrete and steel to create a sturdy foundation for the deck. Any concrete elements built into the bridge may be post-tensioned, pre-stressed, or reinforced. Plate girder, box girder, or a standard girder bridge design fits into this category.
Construction types include I-beams, trusses, or girders. Engineers can design them to be half-through or braced across the top to create a through bridge.
The Feiyunjiang Bridge in China is almost 10,000 feet long, over 100 feet wide, and it uses the simply supported structure of a beam bridge to create the necessary span.
List of the Advantages of Beam Bridges
1. It does not take long to build a beam bridge.
Since most beam bridges today are a combination of steel and reinforced concrete, a building project does not take long to complete. Shorter spans for heavy transportation needs can go up in about 6 months. If you prefer something more rustic, a simple tree across a stream can create a bridge in less than a day. Longer expanses like the Feiyunjiang Bridge can be finished in about 50% of the time required for more complex designs. If you need to have a continuous span available immediately, then this is the design to use.
2. No moments are transferred with the beam bridge design.
Thrust cannot be accommodated from the beam bridge design because there are no arches in place that work to transfer moments. That means engineers must incorporate innovative design elements to ensure that the span is safe and secure for continued use. That is why you will see designs with bow-string arches and lenticular trusses added to the structure to contain the horizontal forces that the bridge must endure.
3. Beam bridges are useful in almost any location.
Although you will find that some beam bridges work better than others because of their placement location, this design is useful in a variety of situations. It is the most common bridge used along highway networks because it can quickly and effectively provide enough support while offering room for traffic to flow beneath. You can find them in use with railroads, cycling trails, and more. If you encounter a bridge while walking along a nature path, there is an excellent chance that it follows this design. This proven versatility is why it is such a popular option for communities around the world.
4. You can use multiple types of material to build a beam bridge.
Most bridges that incorporate the beam design tend to use steel and reinforced concrete because it is an inexpensive combination. Some of the elements of the bridge can be made on-site, while pre-made components can be shipped to the construction location as needed too. You can use stone slabs to create a bridge like this if your support columns will support the weight. Wooden bridges use this design in natural settings to reduce the amount of labor needed to haul materials to the location. As long as the material is strong enough to support the weight of the expected traffic, then you can use it create this bridge.
5. There are multiple design options available for this bridge.
Most beam bridges use a side-by-side beam design that permits a deck to rest on top of the supports. You would have the piers, then the underneath supports, and then the deck. This placement works well with its simplicity to create a bridge that works almost anywhere. Designers can use H-beams, girders, or trusses to manage the horizontal stresses that the structure will encounter over time. Viaducts can even be used to support the span, such as the one in the Feiyunjiang Bridge example.
You can even build aggregate bridges that incorporate multiple segments over a span to build a useful addition to the transportation infrastructure. The Virginia Dare Memorial Bridge crosses the Croatan Sound between Roanoke Island and Manns Harbor in North Carolina. It uses 2,000 pilings and 88 concrete columns to span a 3/4-mile of wetlands, and then another segment crosses 4.5 miles over the sound. The actual deck uses 7,250 tons of epoxy-coated reinforcement steel, giving it an expected lifespan of 100 years.
6. The cost of a beam bridge is minimal for most communities.
Unless exotic materials are used in the composition of a beam bridge, it is one of the most affordable building options that is available today. Since you can cut down a tree for the cost of a saw or an ax to have to span a creek, this advantage can range from a few dollars for the simple to the $90 million expense spent on the Virginia Dare Memorial Bridge. When you need to connect one spot to another over a span of any type, this advantage tends to be why it is the first choice considered by most communities.
List of the Disadvantages of Beam Bridges
1. There are no built-in supports for beam bridges.
The Lake Pontchartrain Causeway is a fixed-link of two parallel bridges that cross the lake in southern Louisiana. The northern terminus of the structure is located in Mandeville, while the southern end is based in Metairie. Since 1969, this bridge (which is almost 24 miles long) has been listed as the longest continuous bridge over water in the world. The weight of the desk is supported by over 9,500 concrete pilings.
Although beam bridges can be built in such a way that they can support quite a lot of length, most of them are used for short spans because there are no built-in supports for the structure. Only the piers work to provide relief from the stresses that the passenger traffic creates. Most of them are 250 feet or less in total size.
2. Span limitations exist for beam bridges.
You can only cross a specific amount of space when using the beam bridge design since it requires ongoing pillar support. Although you can use natural materials and this option has a high resistance to bending forces, you will rarely find a single span of more than 250 feet with this option. You can go for an unlimited length if you use multiple spans together with thousands of pilings, but that work may not be feasible in communities that experience severe weather events frequently.
If you chain multiple beams to create a longer span, then you will increase the construction time of the project as well. The work will still go faster than other design options, but it is an advantage with severe restrictions as you contemplate a longer length.
3. Beam bridges can sometimes start to sag as they age.
The weakness of the beam bridge design is that it does not offer any weight transfer as traffic crosses on the top of the deck. If it should receive a weight that is consistent in its placement on a specific area of the bridge, then the support mechanisms can begin to buckle. The sagging continues to worsen unless maintenance interventions occur. That is why you will see some weight limits posted on small bridges in rural communities. A bridge could actually fail if it sees a reduction in support at the same time heavy vehicular traffic begins to pass on the overhead deck.
4. There is little aesthetic value to consider with a beam bridge.
The standard beam bridge looks a little like a table. You have the deck on top, the supports below, and piers that work to support the weight. Many of them stay low to the surface of the water on an extended span because of the stresses involved in such a placement. You will quickly discover that there is nothing architecturally fascinating about this design since it looks like any other roadway. If you want to create something distinctive that will become a landmark for the community, a different option other than this is more suitable.
5. The cost of steel often dictates any price advantages which may be present.
There are times when the construction of a beam bridge is more expensive than other options because of the presence of steel tariffs, import shortages, and other marketplace factors that raise the price of the construction material. Some communities may decide to wait on the final approval of a bridge since the variation in costs can be as high as 25% in some years. Since you need the strength of this metal to get the work done, you might find that the cost disadvantages of this project might outweigh the possible benefits.
6. Beam bridges can go through a significant amount of wear-and-tear in their lifetime.
Most beam bridges have an average lifespan of 50 years or less. Even when you use reinforced concrete and steel for the bridge, a span which receives heavy traffic might have a rated lifespan of just 25 years. If you create a simple bridge with natural materials for pedestrian traffic, you might receive 5-10 years of life. Ongoing maintenance can sometimes increase their usability, but the costs eventually start to add up.
Although it isn’t a beam bridge, the 1935 construction of the Deception Pass Bridge connected Whidbey Island, WA to Fidalgo Island across Pass Island. Almost 1,600 tons of steel were provided for the construction. When crews had to paint the bridge in 1983, it cost more to do that work than it did to build the bridge in the first place. Beam bridges encounter a similar circumstance.
7. Beam bridges do not offer a lot of flexibility.
The design of a beam bridge is meant to provide a safe deck to cross a span and nothing more. There are limited options available to engineers when there is a need to handle challenging atmospheric conditions at the placement site. If there are severe side winds that impact the structure consistently, then there is no protection for the traffic or the deck from this pressure. Your ongoing maintenance costs will become much higher in that situation because of the increased levels of wear-and-tear.
Flexibility is not part of the beam bridge design. Although there is an advantage in its rigidness, this issue may create some specific disadvantages at some geographic locations.
8. You can spend more on maintenance costs with beam bridges when compared to other designs.
Beam bridges are very affordable from an initial construction viewpoint. Boston has one of the most expensive roads in the world, paying $1 billion per mile to complete the construction. When Seattle began the recent update to the Alaskan Way Viaduct, the expense was over $180 million per mile. Using steel and concrete to form the structure can help to lessen the costs somewhat, but it is still more than $1 million per mile when bridges and roadway expenses come together. The cost-savings occur partially due to the fact almost all overpass bridges over a highway are beam bridges.
9. The width of the deck span is limited with a beam bridge.
Most beam bridges will support two lanes of traffic because of its size and strength limitations. That is why the Lake Pontchartrain Causeway is technically two bridges that run parallel to one another. The design is mean to hold a specific amount of traffic weight and nothing more. If you need to support four lanes of traffic, you’ll still see the double bridge design used for overpasses. The only difference is that the unit is placed side-by-side instead of running next to one another.
Although it is still cheaper to build two beam bridges than one long span of a different design that can support an equal amount of weight, there are more size and placement considerations to evaluate when choosing the beam bridge option.
10. There are limited placement options available for beam bridges.
Most beam bridges cover short spans because that limits the number of piers, pilings, and supports that are necessary to support the structure. Most of them will cross small navigable waterways, highways, or natural obstacles. Construction projects like the Lake Pontchartrain Causeway are more of the exception than the rule. Because there is no flexibility for the traffic which passes beneath the bridge, this design is typically reserved for longer spans over water where minimal boat traffic is necessary.
Your only other option is to build a steep bridge with a rigid frame like the Eshima Ohashi Bridge that connects Matsue and Sakaiminato over Nakaumi Lake in Japan.
Conclusion of the Pros and Cons of Beam Bridges
One of the most common bridge designs in the United States is called the Cortez Bridge. It is a produce offered by U.S. Bridge, who has AISC certification as an advanced bridge fabricator. It is a beam bridge that can be used with asphalt and concrete, offering a simple span that supports a concrete deck slab and stay-in-place forms. It also works with a plank and asphalt system. It is available in custom lengths of up to 80 feet and comes with painted, galvanized, or self-weathering finishes.
Beam bridges have come a long way in their design options over the years, but the primary elements of this concept remain the same. As long as you have a sturdy support system in place that allows traffic to pass by on an overhead deck, then you can create a span that will cross challenging terrain or long distances over water.
This design is easy to maintain and affordable to install, but the pros and cons of beam bridges also suggest that there are challenges to face with this design as well. By taking the time to work with your contractor to determine which type of bridge is best for your community, you’ll find it is easier to build a structure that can support the needs of everyone whenever there is a need to travel somewhere by car, bicycle, or foot.
Crystal Ayres has served as our editor-in-chief for the last five years. She is a proud veteran, wife and mother. The goal of ConnectUs is to publish compelling content that addresses some of the biggest issues the world faces. If you would like to reach out to contact Crystal, then go here to send her a message.