Considering new pedestrian or light vehicle bridges? Knowing your options will give you the steps you need to make the project a success.
- The first thing to decide is whether to go with a prefabricated or built-on-site option for the bridge. Prefabricated bridges offer the advantage of quick delivery and turnaround time but are often more expensive because of transportation costs and overhead. They are ideally suited for projects where proximity and location dictates limited lay-down room for materials or the remoteness of a project means that extended construction time for travel will drive up the project cost. One often forgotten aspect of prefabricated options though is the need to bring a structural engineer on board who can design the foundations or abutments at each end of the bridge. Quite often, a prefabricated supplier will not include these in their design number. Growth in the prefabricated bridge industry have brought about some very aesthetic options for these structures and the stigma of a prefabricated bridge not being good looking structures is often no longer the case
- The second consideration is to determine what the anticipated use of the structure is going to be. Typical pedestrian bridges require a uniform live load of 85 psf, however, if the bridge is to serve as an emergency egress path, then the required live load by code is 100 psf up to 125 psf. Similarly, if there is to be a need to have small utility vehicles (ATV’s, Razor’s, etc.), maintenance vehicles or even single car/pickup trucks use the structure, than how those loads need to be anticipated from the start. Probably the biggest mistake made by designers in this regard is “wish-listing” this requirement and not understanding the cost impacts of such decisions. Typically, the higher pedestrian live loads will grant enough conservation in the design to accommodate small vehicles with little to no additional cost added to the project. However, design to accommodate larger vehicles is likely going to have trickle down cost impacts to the decking, substructure, abutments and foundations, which can significantly increase the cost in some cases. One thing many designers miss in their designs though is preventing access of unintended vehicles to the structure. This can be accomplished through signage, but the only truly effective way to do this is through vehicle deterrents like bollards, landscaping, etc.
- The final consideration is the overall aesthetic appeal of the structure. While every project is unique, the common thread between all of them is that they must appeal to the end user. Whether the bridge serves as a connector for a remote section of a rail-to-trail project or a main architectural feature of new office building or stadium; every one of them has to enhance the experience of the end user. Will it be a concrete or wood deck? Does the design of the guardrails need to accommodate landscape viewing? Do the guardrails need to match some element of a nearby building or structure? What color should the exposed materials be to best complement the surrounding area? Considering these things early in the design process is critical to laying the ground work for a successful project that meets your expectations.
Allegheny Design Services
The design of a pedestrian or light vehicle bridge can sometimes seem to be overwhelming. It’s important though to have a consultant to work with who can help guide you through your next project in a way that gives you the best chance for success.
Click here to see ways how Allegheny Design Services‘ Sports and Recreation has delivered high quality pedestrian and light vehicle bridges to its clients.
Mike works as the Senior Structural Engineer for ADS. His responsibilities include project management and structural design. He graduated from the University of Pittsburgh in 2005 with a bachelor’s degree in civil engineering and graduated with a master’s of business administration from West Virginia University in December of 2014. He has extensive engineering experience as a field engineer, a civil engineer and a structural engineer. He has served as a past branch president for the American Society of Civil Engineers and chairman of the Richmond Joint Engineers’ Council. He is a current member of the ASCE 7-16 Seismic Committee and is active in many other professional committees and civic organizations.