文档介绍：reading material-- bridge types and layouts The development of prestressed concrete design and construction, mostly pioneered in Europe, is spreading fast into United States and all throughout the world. From 1950 to 1964, 85% of the total number of bridges built in West Germany were concrete, leaving only 15% to steel. The ratio by length of bridges was 75% for concrete and 25% for steel. It is also known by far the greater part of the concrete bridges built in Germany since 1950 were of prestressed concrete. It was further estimated that for 2185 bridges built in Germany from 1974 to 1977, 90% are concrete and 10% are steel. M ost of the prestressed portion was due to the increase in long bridges. In the United States, prestressed concrete bridges occupy 27% of the total bridge construction in 1973,reinforced concrete 14% and steel bridges 59% . In the state of California, however, prestressed concrete bridges occupied 61% of constructed bridge area in 1972. Bridge sections in the United States vary approximately in the following order: I-sections, box sections,T-sections, hollow core, and channel sections. While there are inherent advantages to each section, its availability and local economic factors often determine the adoption of one or the other. Precast bridge of short spans are generally simply supported, but they are often made continues by placing reinforcement over