The lane loads are used in designs of bridge decks that consist of several lanes on multiple spans. This new load, shown in Figure 3, consists of two axles 4 feet apart with each axle carrying a load of 24,000 pounds.Ī system of lane loads was created to provide a simpler method of calculating moments and shears rather than using concentrated wheel loads shown in Figures 1 and 2. The “Alternative Military Load” (also referred to as “Interstate Load”) was created to cover axle loads from heavy military equipment. Figure 2 describes the load and load spacing for HS20-44.Īs the Interstate Highway System evolved in the 1950s, one of its goals was to transport military vehicles. The additional ‘S’ made an allowance for heavier tractor-trailers that were available at the time. The 1944 edition included the HS20 truck load and started a policy of affixing the year to loadings making HS20-44 the official designation. The front axle carried 8,000 pounds and the rear axle, 14 feet away, carried 32,000 pounds. The truck loads first used had the designation of H20 (see Figure 1), which covered a two-axle truck weighing 20 tons. The design methods in those 17 editions included the Allowable Stress Design (ASD) and Ultimate Load Factor Design (LFD). The most recent edition of the “Standard Specifications for Highway Bridges” is the 17th edition, published in 2002. The recommendations from this document are included in ASTM specifications written for underground precast concrete structures such as C478, C890, C913, C1443 and C1557. The document titled “Standard Specifications for Highway Bridges” has for many years defined the load and design requirements for underground precast (or cast-in-place) concrete structures. State Department officials volunteer their services to generate necessary specifications. The American Association of State Highway and Transportation Officials (AASHTO) was created in 1914 to provide guidelines for the design of structures within highway boundaries. How will the new loading specifications affect future designs for underground structures that are currently based on the requirements of the old loading specifications? They are beginning to see specifications for projects that require an AASHTO HL93 truck load. Precasters who manufacture underground products such as box culverts and pump chambers have for many years designed their products for AASHTO HS20-44 or Alternate Military Loading (Interstate Loading), whichever produces the worst condition on the structure.
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