Truss and beam bridges are both popular design options for pedestrians, bicyclists, equestrians, and lightweight vehicles. However, since they have similar characteristics, it can be challenging to determine which one is better suited for your project. This article will explain how a truss bridge works and compare it to a beam bridge. Then we’ll provide additional information on what to expect if you select Areté Structures as your bridge supplier and manufacturer.
How Does a Truss Bridge Work When Compared to a Beam Bridge?
Understanding how a truss bridge works compared to a beam bridge can help you determine which bridge will be the right option for your project.
A truss bridge is a load-bearing structure. Using the following design techniques, it can hold the weight from above by directing it to the foundation below. Every truss bridge contains vertical, horizontal, and diagonal members. The horizontal members, also known as chords, help reinforce the bridge to support the weight. The top chords are in compression, while the bottom chords are in tension. The diagonal (and sometimes the vertical) members will connect to the chords to transfer the compression and tension forces. Then the resultant triangular shapes are part of the truss bridge’s unique design. The trusses give the structure the strength it needs to handle the load, often with less raw materials & weight than that of a beam bridge. The bridge is supported by an abutment at each end of the truss where the load is transferred to the ground.
A beam bridge is a simple structural form which means its span is supported by an abutment or pier at each end. Each bridge contains a set of horizontal beams (typically I-beams) which are connected to abutments located at each end of the structure. During the load transfer, the forces from the walking deck are transferred down the beams then to the abutments supported by the ground.
To summarize the main points, both truss and beam pedestrian bridges are excellent options for trails, parks, and community spaces.
Truss bridges are ideal options for pedestrian bridges with:
- Longer span designs of 30 to 200 feet
- Heavier loads i.e. accommodating vehicles
- A visually appealing design
Beam bridges are ideal options for pedestrian bridges with:
- Shorter span designs of 30 feet long or less
- Moderate load conditions (i.e. accommodating lightweight vehicles)
- A great economical alternative
- Ease of installation
What to Expect with an Areté Structures Bridge?
Each supplier and manufacturer have a specific process they follow when creating and designing bridges. At Areté Structures, when our team begins designing a bridge, we take into consideration the following factors.
- Use criteria
- Total span needed
- Total width needed
- The expected design load of live,dead, and environmental loads
- Live loads include foot, bike, equestrian, or lightweight vehicle traffic
- Dead loads include the structure’s weight, and decking
- Loading due to anticipated weather conditions (snow or wind)
- Reviewing the environment for bodies of water or potential flooding
Once our team receives the above information, they will begin the planning process. Both truss and beam bridges contain specific fundamental elements included in their designs.
The Areté Structures truss bridge includes five essential parts:
- Foundation/abutments and piers
- Floor beams and outriggers
The Areté Structures beam bridge includes four essential parts:
- Foundation/abutments and piers
Preferred Material of Choice: FRP
Most Areté Structures pedestrian bridges are constructed from fiber reinforced polymer (FRP). FRP is a composite material that involves a pultrusion process, which pulls glass strands through a die that’s then embedded in a polymer. That process creates a durable product which engineers will use in structural applications.
FRP advantages include:
- Extreme lightweight properties
- Simple to transport, including remote areas
- Quick assembly using an inexperienced crew with hand tools
- 100+ year lifespan with minimal maintenance
Which Bridge is Right for You?
How Does a Truss Bridge Work?
A truss bridge is a load-bearing structure that contains multiple vertical, horizontal, and diagonal members. Two horizontal “chord” members are connected by smaller members on each side of the bridge. The members are often configured in triangular shapes to transfer forces efficiently through a structure to maximize the strength of the structure using minimal materials. The full assembly of these members is called a “truss”. There are two trusses per bridge, with the walking deck connected to each bottom chord. The trusses support the bridge and its weight over large span areas.
When selecting the chord member for a pedestrian bridge truss, typically I-Beams are used. By connecting two beams with a triangular frame in the middle, we effectively create one large beam several feet high. By adding interior support abutments or bents, multiple span bridges can be created that effectively have no limit in length. The top truss chord can also act as a handrail with pedestrian bridges, adding aesthetic appeal.
Every truss has a top chord and a bottom chord, which are the horizontal members. Top chords are in compression, bottom chords are in tension. Diagonals and/or verticals connect the top and bottom chords. These members may be in compression or tension. The configuration of the members can vary widely; however, there are several classic designs. The most common truss designs include the Pratt, the Warren, the Bowstring, and the Howe.
At Arete Structures, our most common truss bridge design is the Howe, where all the diagonal members are in compression.
Structurally, Arete Structures truss bridges contain five fundamental elements:
- Foundation/abutments and piers (for multi-span)
- Floor beams and outriggers that connect and support the two trusses
- Stringers sit on top of the floor beams and support the decking
When designing a truss bridge consider and review the design and environmental factors below.
- The use criteria
- How far the bridge needs to span (e.g. across a stream)
- Width needed (just foot traffic vs light vehicle)
- Design load (both live load and dead loads)
- Live loads include:
- Expected foot traffic
- Expected bike, equestrian, or lightweight vehicle traffic
- Dead loads include:
- Weight of the structure itself
- Decking (wood, FRP, etc.)
- Live loads include:
- Snow and wind loads (can vary greatly depending on location)
- Environmental considerations (e.g. potential for flooding)
Arete Structures pedestrian truss bridges are made from fiber reinforced polymer (FRP). We use FRP because it is a great lightweight material option for pedestrian, equestrian, and lightweight vehicle bridges. Our FRP truss bridges typically span from 25-80 feet long; they can even reach up to 100 feet in length. This type of bridge works great for:
- Remote trail location
- Community spaces
- Equestrian trails
- Nature trails
- National parks
Advantages of FRP include:
- Easier access to inaccessible lands
- Quick assembly with unskilled or an inexperienced crew
- Several customized options with various rail, curb, and decking choices
- Long lifespan with minimal maintenance