Greetings from Bridging Solutions!
This installment of “A Bridge Moment” is a follow up related to the significant permanent and transient design loadings for bridges. This time we review live load which is an example of a transient load. The live load specifications for the design of bridges has evolved over the years. The current version of the Standard Specifications for Highway Bridges is written by a volunteer committee of bridge engineers under the auspices of the American Association of State Highway and Transportation Officials (AASHTO).
The 1931 version of AASHO introduced the H-20 live load designation for live load which consisted of a two-axle, single unit truck (box truck) weighing 20 tons. As shown below, the front axle is 8,000 pounds and the rear axle is 32,000 pounds. The 1944 edition of AASHTO introduced a new truck loading that was designated H20-S16-44 and commonly referred to as the HS-20 truck which is depicted below. This truck configuration includes an additional 32,000 pound axle which simulates a truck with a tractor–semi-trailer combination. The 1944 specifications also required engineers to check a lane load that consisted of a uniform load of 640 pounds per linear foot uniformly distributed in the longitudinal direction. In transverse direction, the design lane load is assumed to be uniformly distributed over a 10.0 foot width. Concentrated “rider” loads are also included with the lane load.
