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Bridge Spans - HL-93

            The primary live loads on bridge spans are due to traffic. Vehicular live load consists of two types of vehicle groups. These are: design vehicular live load – HL-93 and permit vehicles – P loads. For both types of loads, axles that do not contribute to extreme force effects are neglected. The HL-93 load is a combination of design truck and lane load or design tandem and lane load. The higher moment produced on the unbraced span by the load combinations is the moment you design for. Same goes for the shear force. The load combination that controls the moment doesn't necessarily have to control the shear. The design truck is a 3 axel vehicle with a force of 8 kips on the first axel and 32 kips on axel 2 and 3. The first and second axels are spaced out 14 feet, and the second and third axels have a variable spacing between 14 feet to 30 feet. Whichever spacing gives the higher moment and shear is the spacing that controls. Normally the 14 foot spacing would control, but in rare cases a larger spacing will produce a higher moment. A design tandem is a 2 axel vehicle with 25 kips on each axel and a spacing of 4 feet between axels. The lane load is a uniform distributed load of 640 pounds per linear foot.
             In order to determine the maximum effects on the span by the load combinations we need to use influence lines. On the bottom of the slide you can an example with influence lines. Lucky for us, the civil engineers that came before us have a table of worst cases due to the live loading on many different span lengths. Not only do they give us the maximum moment and shear, but they tell us what load combination caused it. 'a' signifies that the design truck and lane load control and 'b' signifies the design tandem and lane load controls. There are 3 other pages of this chart on the self-study document that dr. zaghi but on husky ct. There is 1 other page of longer spans for live loading without dynamic load allowance.


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