A beam bridge is the simplest structure for a bridge span that can support modern weight requirements. It is supported by a pier or abutment on each end of the span. Support columns are added underneath the bridge to accommodate extra length as required. It is a “simply supported” design as no moments are transferred throughout the support structures.
Here are the key pros and cons of a beam bridge to consider when evaluating all design options.
List of the Pros of a Beam Bridge
1. Beam bridges are an affordable building option.
The design of a beam bridge is considered to be the most affordable of any bridge design that is currently in use. For areas that are low on resources, but require a bridge, this design is an effective option to use. On smaller spans, the bridge can be as simple as two beams with a deck built across it.
2. They can be built very quickly.
Depending on the design and materials being used for the beam bridge, the construction of it can be completed very quickly. Even if the bridge is being built to support a roadway, short spans can be completed within 6 months. Once the support piers or columns are built, the beams are then placed on top. Then the deck is connected to the beams.
3. There are multiple design options available for beam bridges.
Side-by-side beams is the most common type of beam bridge that is constructed, as described above. The beams can be I-Beams, box girders, or trusses. They can also be braced or half-through. They are not limited to a single span either. Concrete elements can be used to improve the strength of the bridge. Some, such as the Feiyunjiang Bridge in China, have viaducts that are supported spans.
4. Multiple material options are available to use for beam bridges.
If you wanted to build a simple pedestrian bridge to span a creek, you’d use wood products to create a beam bridge. Planks, stone slabs, steel, concrete, and virtually any other material can be used to create this type of bridge. Each material option has specific placement pros and cons to consider, so not every material will work for every specific bridge. It would not be cost-effective to build a small steel bridge to cross a creek, nor would it to be to build a large, wooden bridge for highway traffic.
5. They can be placed almost anywhere.
Some beam bridges perform better than others because of their placement, but all perform adequately in virtually any environment. From walking trails to interstates, this bridge design is proven to be effective.
List of the Cons of a Beam Bridge
1. There are span limitations to beam bridges.
A single span for a beam bridge is somewhat limited. With current technologies, a single span can handle about 100 meters of length for modern weight requirements. Anything longer than that will require additional spans, which can be linked together as needed to form a longer bridge. Chaining multiple beams together adds time to the project, however, which reduces some of the advantages this bridge design tends to offer.
2. Beam bridges can be susceptible to sagging.
Because there is no weight transfer occurring on the supported structures of a beam bridge, repetitive heavy weight on a specific location can cause the bridge to begin to sag. As more weight is added to the bridge, additional supports become necessary to prevent further drooping. If no maintenance occurs on the bridge, there is a possibility that it could collapse over time.
3. A beam bridge provides only basic supports.
There is nothing fancy about a beam bridge. Its goal is to get one specific job completed successfully. Although some communities may add artwork or embellishments to improve the appearance of this bridge design, there is no getting around the basic aesthetic elements of its construction. Beam bridges are not going to become tourist attractions. They are simple, cheap, and effective.
4. They weaken as they get older.
Beam bridges eventually wear out over time from the wear and tear of supporting the weight from the deck. Even with proper maintenance, the lifespan of a simple beam bridge for pedestrian traffic may be 10 years or less. For vehicular traffic, using steel and reinforced concrete, the lifespan may be 25-50 years, depending upon the maintenance schedule for the bridge. At some point, however, beam bridges will need to be replaced.
5. A beam bridge has limited placement options.
Beam bridges are generally confined to areas where vehicular traffic passes underneath it or there is a natural obstacle, such as a small river or lake, that must be crossed. If a bridge is required over a navigable waterway, the structural design of a beam bridge eliminates it as an option for that type of placement. Large ships cannot pass underneath the basic design of a beam bridge using current technologies.
6. The deck span width of a beam bridge is naturally limited by its design.
Because the deck is fully supported in this type of bridge design, the span is going to be naturally limited in size as well. That is why most beam bridges that carry traffic provide just 2 lanes of traffic support. If 4 lanes of traffic must be supported, there will usually be two bridges built instead of just one. This process may still be cheaper than installing a larger bridge that can carry more weight, but it will provide unique, challenging infrastructure questions along with it.
7. The cost advantages of a beam bridge can disappear.
Beam bridges may be the cheapest design to build from an average standpoint. That doesn’t mean it will be the cheapest option for every community. When the costs of steel and reinforced concrete are considered for small, single spans, a beam bridge can be one of the more expensive design options to build.
8. It is a design option that offers little flexibility.
A beam bridge is not designed to handle difficult atmospheric conditions. Vehicles using a beam bridge in high wind conditions could experience sideways movement while crossing because there is no protection in place. High wind forces may even increase the wear and tear on the support beams of the bridge, causing it to have a reduced lifespan compared to other bridge types. That means the ongoing maintenance costs for a beam bridge may outweigh the installation benefits over time.
These beam bridge pros and cons show that this cheap, simple bridge can be a viable option for communities that have basic obstacles or traffic flow needs to navigate. It can be a component of a major traffic network. It will not, however, be the foundation of a major traffic network because of the design limitations that are present.