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Choosing the Right Bridge

A look at structure types, costs and other issues.

photo: A rustic bridge on the Palmetto Trail
A rustic bridge on the Palmetto Trail

by Robert Steinholtz

Whether it is the Calaveras River in Stockton, the Arana Gulch in Santa Cruz or San Gabriel River in Los Angeles, a bridge will probably constitute the single most expensive item for a local bikeway and pedestrian system. Choosing the right bridge for your crossing-- and understanding the tradeoffs between bridge types-- may have a great impact on your short and long term costs.

Choosing the Right Bridge

There are three basic types of bridge structures:

  1. cast-in-place or pre-cast concrete
  2. pre-fabricated steel
  3. wood bridges

The most appropriate type of structure for your crossing depends on numerous factors, including local construction costs, aesthetics, schedule, and overall and clear span requirements.

Local construction factors play an important role in cost and feasibility. "Pre-cast concrete or steel structures can cost 15% to 18% more than cast in place structures for some crossings in northern California," says Walt LaFranchi, P.E. of Greiner Engineers in Roseville, "mostly because local contractors are already set up with the form work here. In other areas, such as Oregon and Washington, pre-cast concrete is used more often."

Prefabricated steel bridges are perhaps the most common type of pedestrian/bicycle bridge used throughout the United States. Firms such as Continental Bridge located in Alexandria, MN, have constructed and shipped more than 5,000 steel truss bridges nationwide. The greatest advantage cited by pre-fab manufacturers such as Continental or Steadfast Bridges, located in Fort Payne, AL, is the low cost and speed of fabrication, often between 8 and 10 weeks (excluding abutments and piers).

The required span of your crossing, and related factors such as height, pier restrictions, and soil conditions, can be met with either a concrete or steel truss bridge -- with clear spans of 200 and 250 feet typical. Clear spans over 400 feet would best be served by lighter-weight steel truss structures, however.

Tradeoffs

A good engineer responsible for selecting a bridge will want to look at wood, steel truss, and concrete cast in place or pre-cast options. They will hear these types of arguments from various bidders:

"Owners and specifiers know that prefabricated bridges save engineering expense, require small installation crews and are ready for use faster than conventional bridges," cites the Continental Bridge brochure.

Pre-fab bridges can carry equivalent live loads as concrete structures (up to 60 psf for structures over 50 feet long), enough to support a 10,000 pound vehicle plus 30% impact.

Long term maintenance of steel structures is often quoted by concrete bridge engineers as a liability and major consideration. Most steel bridges are made of a self-weathering steel called

CorTen, which oxidizes to form a protective surface not requiring painting. While CorTen offers low long term maintenance, it does have the appearance of rusted steel and may stain abutments. Painted steel structures do require on-going maintenance, and may cause environmental concerns over wetlands.

Design Considerations

Aside from maintenance, other considerations include aesthetics and requirements of local and state highway departments and railroad companies. A thorough knowledge of encroachment permits and lead time on easements is critical to maintaining project schedule.

A new 140-foot pedestrian bridge in Emeryville, CA, designed by Santina & Thompson Engineers of Concord, is a pre-cast concrete structure. "The City chose this type of structure early-on because of aesthetics and their concerns about long term maintenance. The bridge is close to a salt water environment and also spans over a number of mainline railroad tracks. Both were significant concerns in looking at maintenance issues," says David Wemmer, P.E., of Santina & Thompson.

Cost Comparisons

Construction costs for bikeway and pedestrian bridges are influenced by many external factors, especially the number and workload of local contractors. Bridge construction costs in northern California generally range between $65-$80 per square feet for cast in place concrete, to $70-$90 sf for prefabricated bridges. This would translate into a 200 feet long by 8 feet wide bridge costing between $100,000 and $140,000.

A prefabricated steel structure of the same size could be built and delivered for about $100,000, according to Skip Wilson of Steadfast Bridges. The cost of abutments, footings, crane and other mobilization costs would be extra. Because pre-fab bridges are priced separate from actual installation costs, we recommend that engineers find a recently built example in the region to gain a better idea of true costs.

Another design feature with large cost implications are the approach ramps. The Americans with Disabilities Act (ADA) requirement of a maximum 4.8% ramp gradient means that a bridge overcrossing 20 feet above grade will need an approach ramp over 400 feet long. For example, the approach ramps to a proposed 220 feet long bike/pedestrian bridge across I-80 in Davis represent about 70% of the total $1.5 million construction budget. This means that, in some cases, it may be less expensive to utilize elevators (pedestrian only) or pursue an undercrossing option.

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