For the primary superstructure members of a bridge (not including the deck), concrete (reinforced and prestressed) and structural steel are the principal candidates. Concrete and steel both have desirable attributes and shortcomings as bridge materials. In general, bridges of both materials can be designed, constructed, and maintained to ensure long life.
Claims of both steel and concrete industry associations, including references to national bridge inventory data used to support contentions of superiority of one material over the other, must be critically considered. One can find examples of both concrete and steel bridges that are old and in good condition, and conversely, relatively new and in poor condition. The trade associations do a service in countering each other’s claims.
Some advantages of concrete bridges are
• They do not require painting.
• They do not rust (but are susceptible to rebar corrosion).
• They can be formed to the desired shape (if of reinforced concrete).
• If of prestressed concrete, they may be fabricated more quickly than steel, although in some emergencies steel replacement structures have been fabricated and erected as quickly as prestressed members.
• They are not susceptible to fatigue failure (to date).
Some advantages of steel bridges are
• Lighter weight permits smaller cranes for erection.
• Lighter weight permits reduction of substructure size, number of piles, etc.
• They are more readily dismantled and reused at the same or another site.
• Use of conventional erection and construction techniques may avoid construction cost overruns and litigation sometimes experienced with segmental concrete.
• Attachments to bridge are readily made by bolting or welding.
• Components are accessible and visible for inspection.
• Members damaged by vehicular collision may be more easily repaired than concrete members.
For short — to medium-span bridges, the selection of material will depend on which bridge type and material are the most economical for the particular site. This may be known by experience with bids received over a period of time, or can be determined by taking alternative bids on projects.
Long-span bridges are often designed in both steel and concrete, or in different framing systems of the same material, so that contractors bidding on both sets of plans can make the determination of which is less costly. An increasingly common practice for bridges of all sizes is to allow the contractor to submit alternative designs, which must be designed by professional engineers and conform to the requirements of the owner.
