A common question people often ask is, “What makes a bridge strong?” The strength of a bridge is its essence, and the top priority of any bridge is safety. A pedestrian bridge must be strong enough to support people, cyclists, animals, and light vehicles, with multiple degrees of certainty.
Thankfully, the aesthetics and efficiency of a bridge design don’t have to suffer under safety goals. Through the use of tried-and-true bridge shapes, engineers can optimize a bridge design with cost in mind. The two most common design styles used are beam (stringer) and truss for pedestrian bridges.
Continue reading to learn what makes beam and truss bridges effective and determine the best material to use for your next bridge project. Then read our previous blog article about The Importance of a Pedestrian Bridge Weight.
What Makes a Bridge Strong: Beam Bridges
Beam bridges, or stringer bridges, have a simple structural form and a side curb or handrail that doesn’t provide structural support. In the pedestrian bridge realm, stringers are typically less than 30 feet and only one span. Still, you can liken them to most bridges we see on highways.
Stringer bridges consist of a set of horizontal beams that directly support the walking deck and curb/rail system above. In addition, these main structural beams will support one another at each end by abutments (foundation structures).
What makes a beam/stringer bridge strong?
The key strength element in these bridges, and almost all bridges in the world, is the geometry of the main support beams. The weight of the entire bridge, along with all the people, animals (e.g., horses or pets), and vehicles using it above, will be transferred down to the abutments, which are in turn supported by the ground. To safely transfer this weight, the main support beams will heavily dictate the design. To use the smallest beams and reduce the number of beams required, engineers almost always use an “I-Beam.”
The hundreds of years of use and empirical data have solidified this beam geometry as the industry standard for safety and strength. Now there is a simple exercise you can use to understand why.
If you take a ruler that’s lying flat on its side where you can read the numbers and then bend the ruler ends towards each other, you will find it very easy to do. You may even break a wooden ruler. Now, if you take that same ruler and flip it along its thin edge and attempt the same feat, you will likely hurt your hand before you can bend it at all.
When you flip the ruler on its side, the moment of inertia of the ruler’s shape about the axis you are trying to bend about has increased exponentially. You have just found the strong axis of your ruler beam, and the standard “I-Beam” shape creates a similar strong axis. Instead of grabbing the ruler and bending, imagine it’s suspended across two desks (abutments) at each end. Pressing down on the center of the ruler bridge will cause it to bend in the same way as in your hands. However, if you have the ruler standing on its side, it will stand strong without bending.
The weight of the deck and pedestrians will act similarly to you pressing down on the ruler. An “I-Beam” takes the “lowercase L-shape” of the ruler and improves it even more by adding flanges at the top and bottom of the beam. These flanges provide a larger area for the beams to bear on and increase the bending capacity around the strong AND weak axis (which is essential for windy days!).
I-Beam mechanics are the simple yet effective solution for what makes a bridge strong and efficient.
What Makes a Bridge Strong: Truss Bridges
Often a design calls for longer bridges that need to be even stronger. This is when you’ll use a truss bridge. These bridges are load-bearing superstructures composed of connected elements, called trusses. Typically, two horizontal “chord” members are connected by smaller members on each side of the bridge. This gives us two trusses per bridge, with the walking deck connected to each bottom chord. The top chord is placed several feet above the bottom chord for each truss and connected with vertical and diagonal rods, creating an array of triangles.
When selecting the chord member for a pedestrian bridge truss, we again call upon the trusted I-Beam. By connecting two beams with a triangular frame in the middle, we effectively create one large beam several feet high.
Remember the ruler we tried to bend on its side earlier? Now imagine trying to bend one two or three times wider! By adding interior support abutments or bents, we can create multiple span bridges that effectively have no limit in length. The top truss chord can also act as a handrail with pedestrian bridges, adding aesthetic appeal.
Types of Bridge Materials Available
When you’re ready to select the type of material for your bridge, you should compare each material’s weight, cost, and lifespan.
Fiber Reinforced Polymer (FRP)
FRP is corrosion-resistant material with a lifespan of 100+ years. As a result, this material works exceptionally well for remote crossings since it requires minimal maintenance. In addition, FRP’s lightweight components allow builders to hand carry or airlift them to the bridge construction site with ease.
The FRP bridge component’s weight is approximately 125 pounds per cubic foot, a quarter of steel’s weight. FRP components have a stronger flexible strength than wood, and pound-for-pound is more robust than steel. FRP bridges are becoming a popular choice for new or renovation pedestrian bridge projects. You have most likely seen one at your local greenway. Their popularity makes sense when you consider their cost, convenience, style, and positive environmental impact.
At Areté, we fabricate the FRP bridge pieces ourselves, cutting out the middleman and allowing us to build a mechanical camber into our truss design. This camber extends the serviceability of the truss beyond similar competitor FRP bridge designs. Most color options come standard with a UV light protective coating. We aim to provide our clients with an excellent product that exceeds expectations and safety requirements.
Steel provides a long lifespan and low maintenance. However, due to its heavy weight, it’s difficult to transport and construct in remote areas. The steel bridge components weigh approximately 500 pounds per cubic foot. Therefore, during construction, you will need to use heavy equipment to move the components to the bridge site and during the construction process. This will add additional costs to the project.
A timber bridge is a rare case where you can’t use the “I-Beam” shape. A wood bridge’s lifespan is approximately 30 years but will require frequent maintenance during that time. Although wood is lighter than steel, it still is difficult to transport and construct in remote areas. In addition, timber decking can be slippery and rot out in sections adding additional maintenance over the bridge’s lifespan.
Is It Time to Start Your Pedestrian Bridge Design Process?
When you’re ready to move forward on your truss or beam pedestrian bridge project, contact the Areté team. We’ll work with you throughout the process from design to delivery with easy-to-follow assembly instructions.