It’s not a trick question! Of course, there is considerable overlap between vehicle and pedestrian bridges, but there are also important technical and safety differences to be aware of, which we’ll dig into in this blog.
How a Bridge is Used Impacts Design
It’s really not possible to divide bridge use strictly into vehicle or pedestrian. After all, many vehicle bridges include separate lanes for walkers and/or cyclists, and many pedestrian bridges provide periodic access for utility vehicles, golf carts, and even horses.
A bridge’s users determine how a bridge must be designed in terms of loading and width, even if some types of users are only occasional. That in turn can change the construction timeframe and budget of the bridge project.
Loading Requirements
All bridges are subject to many of the same types of loading forces, such as live and static loads and loads from snow. However, because vehicle bridges accommodate more loading weight than pedestrian bridges by default, loading requirements may be dealt with differently in pedestrian bridge design.
It can be helpful to think of primary and secondary users (i.e., pedestrian is a secondary user on a vehicle bridge, while a car or truck is a secondary user on a pedestrian bridge). But even if a user is secondary or only occasional, appropriate safety measures must still be included in the design.
The LRFD Guide Specification for the Design of Pedestrian Bridges Loading requirements for pedestrian bridges from the American Association of State Highway and Transportation Officials (AASHTO) is the standard reference guide for construction. The guide requires that pedestrian bridges be designed for 90 pounds per square foot of pedestrian live load (that is, the amount of weight it can safely bear from moving people). Further, if vehicles or horses will occasionally cross a bridge, their weight must be accounted for in the design as well.
It’s common to include loading support for maintenance vehicles (such as a golf cart or utility vehicle), but users should not assume all pedestrian bridges are designed to support heavier vehicles as a matter of course.
The width of a pedestrian bridge also dictates different loading requirements since wider bridges allow for traffic from larger vehicles. Any bridge seven or more feet wide must be designed to support vehicles up to 10,000 pounds (AASHTO calls this an H-5 truck), and any bridge 10 feet wide must support vehicles up to 20,000 pounds (an H-10 truck).
It’s important to note that these requirements apply whether or not regular vehicle traffic is anticipated – if a bridge meets the noted widths, it must also meet the proper vehicle loading requirements.
Loading requirements are also affected by siting and environmental factors like:
- Wind (i.e., forces from wind gusts)
- Seismic movements
- Snow and ice
- Equestrian traffic
- Forces from moving or rising water
- Benches or other structural elements
To learn more about loading requirements, click here.
Safety Railings and Curbs
Requirements for safety railings and guardrails differ between vehicle and pedestrian bridges. They depend on whether the bridge will be used only by one or the other, or if it will be mixed use. Vehicle bridges will always require some kind of guardrail or barrier to prevent cars and trucks from driving off the edge. With pedestrian bridges, however, the need for safety railings varies based on use and location. For example, the International Building Code (IBC) and AASHTO considerations for trail bridge safety railings include:
- Urban and high-risk areas
- Rural and moderate-risk areas
- Remote and low-risk areas
In some cases, it may also be acceptable to substitute a curb for a railing on boardwalks and trail bridges if a site analysis indicates low risk. This includes determining if hazards are the same or greater with a curb instead of a railing, and if installing a railing on only one side would be acceptable. Factors like drop off height, typical anticipated use, and volume of traffic also come into play in these design decisions.
Materials
Many of the same materials are used for vehicle and pedestrian bridges. For example, steel is often used for structural members, trusses, and railings in both. Other materials are used in different ways, depending on the primary users of the bridge.
Wood. Due to its lower strength and poor durability, wood and treated lumber are generally not used for vehicle bridges. One exception might be decorative elements, such as a roof on a covered bridge. Pedestrian bridges can be constructed with treated lumber, especially for railings or decking; however, it requires more frequent maintenance and replacement than other material choices.
Concrete. Concrete is a strong, durable, economical material for vehicle bridges. Both cast-in-place (CIP) and precast concrete can be used for structural members and foundations. Most pedestrian bridges, especially those in remote areas, benefit from lighter weight materials. For that reason, concrete is generally reserved for abutments and occasionally foundations (although helical piles are usually the preferred choice for foundations with most pedestrian bridge and soil types).
Fiber Reinforced Polymer (FRP). This material is both lightweight and durable, and can be fabricated into a range of shapes for easy installation. FRP decking can be used for both vehicle and pedestrian bridges; however, structural or railing applications are only suitable for pedestrian bridges.
Click here to compare and contrast pedestrian bridge materials.
Maintenance
While no bridge can be built and ignored forever, many pedestrian bridges can be designed with essentially no maintenance needs. Materials like FRP, weathering steel, and aluminum are highly durable as well as weather and corrosion resistant. They require no painting and will not rot, in many cases lasting for decades. In contrast, most typical vehicle bridges will need periodic inspection, repair to damaged or crumbling concrete, and periodic roadbed resurfacing.
With so many differences between these two types of bridges, it makes sense to choose a vendor with pedestrian bridge expertise. At Axcess, we have the know-how and experience to make sure your pedestrian bridge is functional and beautiful while meeting all appropriate construction requirements. Please contact us to get started!
