When you are asked to consider replacing or adding a recreation bridge there are many options to choose from. Here are a few basics to help you navigate the task.
Should you get professional help?
Eventually every bridge should be designed by a registered Professional Engineer. The point at which you start consulting with a bridge professional will be determined by your experience, the complexity of the crossing, regulatory requirements or available resources within your organization. The bridge consultant may be a local engineering firm or a national supplier of prefabricated kits. The choice of a prefabricated kit will reduce your overall expense, but may still require some site specific help for surveying, hydraulic analysis and foundation design. Reputable manufacturers can help guide you through the process.
Review the site
The site will dictate the overall length of the bridge and the amount of foundations allowed. In general if the bridge can be broken up into multiple spans it will be less expensive. Think of a boardwalk versus a clear span. Typically if the bridge is low to the ground and can be built with inexpensive foundations, short spans will be the choice. As the foundations become more expensive the spans should be longer to balance the overall cost.
Often there are features of the site dictating longer spans. Regulations may require you stay out of the water and above certain flood event levels. Grade separation crossings require spanning a road, trail or railroad. Site topography will determine if you should consider approach spans leading up to the main span. There may be restrictions on using fill in the approach area, or the height of the bridge may make it more economical to add approach spans rather than build tall retaining walls. Remember there may be clearance requirements for navigation or traffic under a grade separation crossing.
Site access will become important when choosing the bridge style. While many prefabricated designs provide for shop quality control and fast installation, they often require large cranes for installation. If access is limited, field assembled kits may be a better option. Some designs allow for deck-down construction where the bridge can be built as installation progresses. This helps minimize disturbance of the site.
Understand the purpose
Once the length is determined you should review the primary purpose of the bridge. Whether the bridge is for a back country trail, multi-use trail or specific recreation (ie. snowmobile, equestrian, golf course) will influence the width of the bridge. Back country trails are often limited to hikers only with the bridge only four to six feet wide. Most multi-use trails start at eight feet wide, but may expand to fourteen depending on the mix of bike and pedestrian traffic. Guidelines are available for multi-use applications. Your funding source may also have specific requirements for travel lanes and clear zones. Golf course bridges will vary depending on the need for two-way traffic or accommodating specific maintenance equipment. Snowmobile and ski bridges are often controlled by the grooming equipment used to maintain the trail.
The secondary purpose is often for maintenance and emergency access. Once the bridge width exceeds six feet you should assume a vehicle will pass over the bridge. Even if maintenance or emergency vehicle access is not required, the public may take a joy ride. Once the bridge span length approaches 30 feet, the pedestrian load controls the design of the main structure and the vehicle load only affects the deck design. Accommodating moderate vehicle loads is not cost prohibitive. Bridges can be protected with use of permanent or removal bollards if you would like to control access.
Other loads associated with secondary purpose may include utilities. Modest electrical and communications conduits have little effect, but larger water, sewer or gas lines should be identified at the early design phase as their loads can become significant.
Style and material choices
Once the required geometry is determined, the style of bridge can be considered. Shorter spans will often be built from treated wood materials. Longer spans are more effectively built with steel designs. There are some pre-cast concrete options, but they are typically better suited to highway applications. Composite fiberglass materials are gaining popularity when site access dictates very light components.
Like I tell my friends, bridges don’t just appear. They take thoughtful planning and often professional help. Hopefully this helps you prepare for what is ahead.