Pedestrian bridges are common structures in parks, cities, and trails. The concept seems simple enough: the term pedestrian bridge is widely used in the bridge industry to define a bridge that moves people from one side of a gap to the other. The Merriam-Webster dictionary defines pedestrian as an “action going or performed on foot” and bridge as “a structure carrying a pathway or roadway over a depression of obstacle“.
But this basic definition falls short of conveying the entire story, and that is probably why other terms such as footbridge and trail bridge are used interchangeably with it. Ultimately, they all mean the same thing in that the bridge’s primary purpose is to allow pedestrians to walk from one side of a gap to the other.
A more technical and comprehensive definition comes from the American Association of State Highway and Transportation Officials (AASHTO) in the LRFD Guide Specification for the Design of Pedestrian Bridges. It covers some additional aspects of pedestrian bridge design such as live loading requirements, vibration, and deflection.
Loading requirements
Pedestrian bridges must accommodate “live” loads, which are objects that move across the bridge, in addition to stationary loads that remain in one place. Depending on the planned used of the bridge, live loading requirements can include horses and vehicles in addition to people. Environmental loads must also be considered.
Pedestrian Loading. Pedestrian bridges are designed for 90 pounds per square foot of pedestrian live load. To visualize this weight distribution in simple terms, assume that the bridge is completely full of people standing shoulder to shoulder.
Equestrian Loading. Pedestrian bridges that are intended to be used for equestrian traffic must be designed for a patch load (i.e. concentrated load) of 1,000 pounds over a 4-inch by 4-inch square area.
Vehicle Loading. Depending on it’s width, a pedestrian bridge may be required to accommodate vehicle loading. Pedestrian bridges must be designed for vehicle loading if they are seven or more feet wide. The exception is if they have a permanent physical barrier to prevent vehicle access. Pedestrian bridges that are seven to 10 feet wide must be designed for a 10,000 pound vehicle, while those more than 10 feet wide must be designed for a 20,000 pound vehicle.
Environmental Loading. Beyond the live loads detailed above, pedestrian bridges are designed for various environmental loadings based on the specific site conditions. These include factors like wind, ice, snow, and seismic conditions.
Motion control
When crossing a pedestrian bridge, it’s often possible to detect some motion or vibration in the structure. Bridge design must follow requirements to keep these within acceptable ranges.
Vibration. The allowable amount of vibration in a pedestrian bridge is limited by AASHTO in order to prevent user discomfort or concern. The fundamental frequency in the vertical direction is required to be greater than 3 hertz, while the horizontal direction must be greater than 1.3 hertz.
Deflection. The amount of deflection (i.e. bending) that a bridge can experience is limited to the span divided by 360. For example, a bridge that has a 40-foot span is limited to 1.3 inches of deflection when the live loads are applied.
Though it may look simple, the requirements highlighted here show there really is more to pedestrian bridge design than meets the eye. The experienced designers and engineers at Axcess, LLC work with these and other design requirements to ensure safety, functionality, and beauty for the life of your bridge. We’re ready to help make your next project a reality, so please get in touch today!
