Trail Bridges
A native timber bridge with accompanying log ladder steps, built on the Caney Bottom trail.
The Pisgah Ranger District is a land of picturesque waterfalls, rivers, and streams, all of which pose challenges from a trail maintenance perspective. In some cases, crossing a river or stream is inevitable and requires a bridge. The district trail system currently has a very large number of bridges, displaying a wide range in quality and condition. Bridges are one of the most time-consuming and challenging structures to build. Rigging is often required to move very heavy components into place. Maintenance, repair, and eventual replacement of bridges requires a lot of time and resources, as well.
For these reasons, it is important to build bridges only when truly necessary, and then to maximize stability and longevity. Many bridges currently standing on the District have not been built to high standards, and should be rebuilt using better methods when they are eventually replaced.
Due to the higher complexity and material needs for bridges, all bridge projects should be proposed to the District and approved prior to construction.
Key Points:
- Build bridges only where necessary
- Sustainable Materials
- Abutments
- Sills
- Stringers
- Handrails/Curbing
- Gallery
Build Bridges Only Where Necessary
Building a bridge is a big commitment. Not only is it a major project involving a lot of resources, time, and energy, but it creates a structure that will eventually need to be repaired and replaced down the line. As bridges fall into disrepair, they can become safety liabilities. There are currently many bridges throughout the District awaiting replacement. Adding new bridges where they may not be needed creates more work in the long run, when there is already a backlog of maintenance and bridge replacement.
Consider carefully whether a particular crossing really needs a bridge, or whether the crossing is mild enough that users can safely cross without a structure. Likewise, when replacing an existing bridge that is at the end of its lifespan, ask, “Does this really need to be replaced or could we just remove it and leave it as a shallow foot crossing?"
It is useful to consider a trail’s class specification. Bridges may be necessary on more developed trails that see a lot of traffic from less experienced users. The more rugged and remote a trail gets, the less we should want to construct a bridge if we can avoid it.
Seek to build the simplest bridge appropriate to trail class and designed use.
Note that bridges with very long spans or otherwise significant complexity may require a formal engineering process through the USFS. As stated above, all bridge projects must be referred to District personnel for design review and approval.
Sustainable Materials
When building with native timber, choose black locust whenever possible. It is by far the most rot-resistant wood available in the region. In some cases, where bridge sites are too remote to bring in material and locust is nowhere to be found, it may be worth considering other tree species like white oak or chestnut oak which are relatively durable.
Peel the bark from all logs used in bridge construction. This is extremely important, as leaving the bark on will trap more water and insects and speed the rotting process.
When building with dimensional lumber, be sure to select pressure-treated lumber.
Abutments
- A strong foundation to all bridges is essential. On sites where the stream banks are unstable, construct abutments as the first piece of the foundation.
- An easy way to build an abutment is to construct a simple box using black locust logs. Be sure the box is level and stable. Fill the box with stone. The abutment on each side of the crossing provides a level and stable pad on which to build the bridge.
- Use a line level to make sure abutments on either side of the crossing are level with each other, otherwise the bridge may end up sloped.
- Be sure to use the most durable material available when building abutments, as they have the most ground contact out of any bridge component.
Sills
- Bridges should generally have sills as part of the construction. Sills are the logs on each end of the bridge that the long stringers actually sit on. Sills are typically perpendicular to the stringer(s).
- Sills are important because they raise the main structure of the bridge above soil contact, which discourages rot and increase the lifespan of the bridge.
- If necessary, a rotten sill can be replaced without having to replace the whole bridge.
Stringers
- A stringer is the long piece (or pieces) that spans the length of the bridge, across the stream.
- When building with native timber, seek out stringers that are straight, sound, and sufficiently thick. The walking surface of a bridge needs to be at least 10” wide, and the full log diameter should be at least 18” wide according to the USFS Trail Construction and Maintenance Notebook.
- Stringers should have a level, flat walking surface of at least 10” wide. This can be created by using a chainsaw to rip along the length of the stringer, making a series of kerf cuts and knocking them out, or using an Alaskan mill. Use a chalk line and level to help create this flat, level walking surface.
- Stringers should be notched into the sills on both ends, and secured with spikes, timberlocks, or rebar.
- Stringers should be placed so that the crown of the wood is facing up, as this creates a more durable structure (in other words, the log or board is bowed upward). Use a peavy or cant hook to rotate the log to this orientation before notching it into the sills.
Handrails/Curbing
- Handrails are generally required on USFS bridges, unless a design analysis by USFS staff determines that they are not necessary. Wide bridges with relatively small drops typically are exempt from handrail requirements.
- If using native wood, select rot-resistant species and all bark should be peeled.
- Attach handrails securely to the bridge itself, using bolts or timberlocks.
- Do not attach handrails to nearby trees. Do not wrap metal or wire around railings to attach them.
- Handrails need to be sturdy enough to support a person’s weight.
- Handrails should not impede users more than necessary - consider angling rails outward on narrow bridges to provide sufficient walking space. Consider users with bikes on bike-accessible trails.
- Curbing is generally not needed if a handrail is used, and is a major cause of premature rot on lumber structures in the PRD. If curbing is deemed necessary, elevate it on 4”x4” blocks to allow plenty of space for air and water to flow underneath.
Gallery
Basic foundational components of a simple, well-constructed bridge.
Avoid the use of wire or metal strips to attach bridge components! This results in a sloppy appearance and can be easily avoided by using proper fasteners like screws or timberlocks.