Your Next Bridge: Steel or Concrete?

From Transportation Alert Issue 13

One-third of America’s bridges need to be repaired or replaced, according to the Federal Highway Administra­tion. Most of these bridges are short-spans owned by county and municipal agencies. For agencies that need to replace bridges, there are essentially two choices: concrete or steel.

The use of prestressed concrete bridges has grown drastically since they were first introduced in the United States in 1949. They represent over half of all bridges built today. But don’t rule out steel yet. Steel is practical and economical in many situations.

Here is some input to help you decide which material to use on your next bridge project.

Ability of Owner to Construct. One cost saving tool growing in popularity among agencies is the use of their own forces to build all or part of their bridges. Agencies considering this option usually want a material with which their forces are experienced or familiar. Ease of construction is also a factor. Prestressed concrete box girder bridges are a popular choice simply because local crews may not be comfortable with field splices, drilling, bolts, scaffolding, and concrete deck formwork needed for steel bridges.

However, advances in steel have made it easier to install, particularly for short-spans. With light weight steel and a design that focuses on fewer components, such as cross frames and stiffeners, the labor needed to construct a steel bridge can be significantly decreased.

Availability of Materials. Precast or prestressed concrete beams can generally be fabricated with short lead times. The materials are readily available and are not as vulnerable to market volatility.

Steel beams require longer lead times to order. Depending on the location of the steel plant, delivery time can be longer and transportation costs higher. But steel can more readily be custom fabricated to meet curved and skewed design requirements, providing an advantage over concrete.

Repairs. Steel bridges are generally easier to repair than their concrete counterparts. When hit by over-height vehicles, steel will deform at the impact area, without catastrophic failure. Sections of steel beams can be replaced or, if the damage is not severe, heat straightening can be used to make repairs. Concrete bridges, however, can require extensive and time-consuming repairs when impact damages occur.

Upgrades. In these days of steadily increasing traffic, keep in mind that it is much easier to add travel lanes or sidewalks to a steel bridge. It is also more feasible to upgrade them to carry heavier loads.

Seismics. Steel bridges can be less susceptible to seismic damage. The American Iron and Steel Institute reports that when the 1994 Northridge earthquake in California destroyed the surrounding transportation infrastructure, “96 percent of all the existing steel bridges were completely undamaged, even though they were designed using 1940’s technology.”

Navigation. If navigation comes into play, span length can be a major consideration when selecting your bridge material. Steel bridges can span longer distances more economically, which means fewer piers and a reduction in foundation costs. This provides more room for navigation and less opportunity for pier damage. Steel girders can be fabricated for continuous spans with varying depths (shallower at mid-span and deepest at the supports). In areas where clearance is critical, such as over railroads or on truck routes, steel may be a better choice.

Aesthetics. Both concrete and steel can be designed to be visually appealing, but examine your site to determine which material best complements the surroundings. For highly visible projects, you may want to solicit the opinions of stakeholders. You can show them photo simulations of different options and involve them in the decision making process.

Initial and Life-cycle Costs. The Transportation Research Board notes that “many steel structures in service today were designed and constructed over a century ago and are still safe and serviceable.” A steel bridge that is designed with a deck or wearing surface and replaceable components can “remain in service indefinitely with proper maintenance.”

When all is said and done, life cycle cost, function, and safety are the biggest factors in selecting whether your bridge should be steel or concrete. Use the input from your design team, maintenance crews, and concrete and steel suppliers to make an informed decision that provides an optimum cost/benefit ratio for a structure that will serve the taxpayers and the traveling public for years to come.

For more information, contact our bridge manager, Joseph Logan, P.E., at (585) 334-1310, ext. 251 or jlogan@fisherassoc.com.