Is Your footbridge designed to fail?

ByKevin Post

Is Your footbridge designed to fail?

When designing trails within parks or green spaces, rivers and creeks become an issue. How do you cross them? A small babbling brook might be able to be jumped across or crossed by hopping between a few rocks to continue along the natural footpath. This style of water crossing is not that accessible, nor are these natural forest footpaths. Trying to walk across this type of water crossing can be hard for a person with limited mobility, an issue of our increasingly aging population. The next option is much more expensive, a bridge must be installed to cross a little creek to a larger river.

The main materials for a bridge are wood, metal, and concrete. Each of these materials has its own design consideration; cost, longevity, maintenance, accessibility, access to the build site, just to name a few.

In 2012-2013, I was tasked with reviewing bridge repairs suggested from the structural engineer’s bridge reports. I noticed that some of the oldest bridges (around 100 years old), required less work than some of the bridges 90 years younger. Were these 10 to 15-year-old bridges designed to fail? I’m not sure but I offer these few tips to help ensure the longevity of your assets.

The first thing to know about rust on bridges is a bridge is like a very big, low voltage battery or electrical circuit. My landscape architect’s very basic understanding, as taught from an electrical engineer tasked with protecting gas lines:

  • Water and salts, make an electrolyte, which allows for the current to move through.
  • Sun, wind, moving water all create electric potential within the circuit/bridge.
  • The foundations create “Ground” and can complete the circuit.
  • Lesser “noble” metals protect more “noble” metals will.
    • Protection: oxidizing or rusting gives off an ion to join with oxygen to maintain the electric potential equilibrium.
    • Weathering steel will rust faster than stainless steel,
    • Stainless steel will oxidize faster than silver.
    • If you have weathering steel fasteners on a stainless steel structure, the fasteners will oxidize, rust and fail at a much quicker rate since it is trying to protect the stainless steel – as known as- equalizing the electric potential.
  • If you think about bridge design in this fashion, small items requested during the tender process and good maintenance can help to ensure the longevity of the bridge.

One of the more common bridge types I was reviewing was made from weathering steel (Core-ten). These young weathering steel bridges (10 to 15 years old) had few structural failures and required replacement. The following Design and Maintenance tips are based on my observations and research on weathering steel (a product that was originally designed for train boxcars for use in hot and dry South-Western USA)

Foundation Connection of Weathering Steel Bridge

Foundation connection of weathering steel Bridge covered with leaves, dirt and debris and has stainless steel fasteners

Weathering Steel Bridge

Weathering Steel Bridge with wood deck with overgrown vegetation surrounding bridge – designed to fail?

Failed structure of Weathering Steel Bridge

Structural failures of support members on the underside of weathering steel bridge with wood deck. Too close to water and vegetation, plus the wood holds moisture, which will keep the weathering steel rusting (bright orange)

Design Tips:

  1. Weathering steel (core-ten) must be placed in a full sun area that will allow it to dry completely each day.
    This repetitive wetting-drying cycle forms a very uniform, thick protective layer of rust with a constant colour.
  2. Constant moisture on the bridge will prevent the steel from forming a protective layer of rust and ironically will continue to rust (darker or brighter points or streaks on the surface)
  3. Design the weathering steel bridge, with a minimum 2 meters (6′ feet) above the water.
  4. Ensure the bridge is electrically broken from the ground (foundations) and into smaller units.
  5. Electrically insulate all connection points with dielectric insulating bolt sleeves and washers (PVC, rubber, ceramic, etc).
  6. Ensure the materials to be used are compliant with your de-icing products and needs (if used)

Maintenance Tips:

  1. Keep all vegetation cleared around the bridge to ensure the wind can dry the bridge throughout the day.
  2. Ensure leaves, dirt, and debris are cleared from the bridge, especially the bridge connections.

If the above recommendations cannot be maintained, it might be a better idea to use another material, such as wood or concrete if you don’t want to replace your bridge very 10 to 15 years at over $10,000 for a small 6-meter long bridge. In future blogs, we will look at other materials and design considerations.

If you need help designing your trails, locating your water crossings and types, creating tenders for bridge replacement or help to create maintenance procedures, please give us a call or email, we would be happy to help.

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Sincerely,
Kevin Post
Beyond The POST

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