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Statistics indicate that about 90 percent of United States bridges have maximum spans of 100 feet or less, and almost 67 percent--nearly 200,000 bridges--are in the 20- to 60-foot range. Many of these smaller structures are aging and have become either obsolete or unsafe for present-day loading. Concrete is a leading contender for replacement or new construction of the short or short span bridges because of its adaptability, durability, low maintenance, and low life-cycle cost. Whether bridges are cast in place or precast and prestressed--or a combination of the two--will depend on site conditions as well as local economic and political conditions.

PLANNING AIDS OFFERED

Recognizing the need for rapid comparison of costs of the alternative methods of building short bridges, both the Concrete Reinforcing Steel Institute (CRSI) and the Prestressed Concrete Institute (PCI) have issued publications to assist with preliminary comparison of bridge types.

"A New Look at Short Span Reinforced Concrete Bridges" presents the CRSI view that the flat slab bridge, with or without haunches, is the most economical cast-in-place choice for short spans. Simplicity of forming is an important factor. This type of bridge has an excellent performance record, and should be relatively maintenance-free for many years if properly designed and constructed.

Citing prime advantages of speed of erection without weather constraints, PCI's manual is titled "Precast Prestressed Short Span Bridges: Spans to 100 Feet." Typical engineering properties are given for a dozen types of bridge sections, including: solid and voided slabs; single and double tees; channel, multistemmed, and box sections; bulb tees and I-sections. Precast abutments, piling, wingwalls, and deck panels are shown, as well as connection, bearing, and diaphragm details for the superstructure sections.