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44 60000 50000 40000 Number of Bridges Built All Bridges Structurally Deficient Bridges 30000 20000 10000 0 2002-2006 1997-2001 1992-1996 1987-1991 1982-1986 1977-1981 1972-1976 1967-1971 1962-1966 1957-1961 1952-1956 1947-1951 1942-1946 1937-1941 1932-1936 1927-1931 1922-1926 1917-1921 1912-1916 1906-1911 1905 and earlier Figure 29. Year of construction of U.S. bridges as of 2006. designed for HS20 loadings. The design load is indicated as 3.4 Application of Canadian "other or unknown" in the database for most of the remain- Experience to the United States ing bridges (23%), though essentially all highway bridges in Texas were probably designed for HS20 loadings during this This section summarizes elements of the Canadian experi- period. However, before 1960, 34% of the existing 16,527 ence, both the impacts of the size and weight limits and the bridges built in Texas at the time were designed for H15 load- process used to harmonize them, which may have relevance ings. And, in the 1960s, during the Interstate highway con- for U.S. regulators. It identifies areas in which the Canadian struction boom, 9,327 existing bridges were built in Texas truck size and weight limits have resulted in particular suc- with about 11% designed for H15 loadings. Consequently, in cesses, or problems. 2006, about 15% of the state bridge inventory presently in use was originally designed for a loading less than HS20. 3.4.1 Freight and Trucks In Canada, low-density freight moves in 5-axle tractor- Table 4. National Bridge Inventory system semitrailers below mass capacity, however, when loads of bridge-design loads as of 2006. higher density freight are traveling to or from the United States in 5-axle tractor-semitrailers, they tend to be loaded close to Design Load Count of Bridges Fraction Other, or unknown 120,497 17.66% the maximum permissible weight of 36,287 kg (80,000 lb). The H10 11,389 1.67% allowable gross weight of this vehicle is 39,500 kg (87,082 lb) H15 70,244 10.29% HS15 11,997 1.76% in the four western provinces, and 41,500 kg (91,491 lb) in the H20 53,977 7.91% six eastern provinces. Generally, if a commodity would chal- HS20 229,486 33.63% lenge the weight capacity of a 5-axle tractor-semitrailer, then HS20+Mod 64,210 9.41% Pedestrian 534 0.08% more of it can be carried in a vehicle with greater weight ca- Railroad 253 0.04% pacity, so that is how it moves. The 6-axle tractor-semitrailer, HS25 22,514 3.30% with a tridem semitrailer, has an allowable gross weight from