FAQ #24
How are live loading effects determined in a system project if lanes are defined?
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Each lane is loaded separately, and max effects determined for the loading in that lane. The vehicle is moved from the left abutment to the right abutment, then turned around and moved from the right abutment to the left abutment. During the transit the second axle of the vehicle is placed on each span twenthieth point in the lane. At each location the coefficient of the appropriate influence surface is determined at each wheel of the vehicle, and the products of wheel load, impact, and influence ordinate summed for the entire vehicle. Max effects are retained during the transit.
After the lane has been loaded with the appropriate vehicle, or vehicles, the lane loading is applied as a line load midway between the two wheels on the axle and patch loaded, using signs of the coefficients of the influence surface, for both max and min effects.
After all lanes are loaded in this manner, the separate lane effects are sorted and combined for a total, using the appropriate multiple presence lane factors. For HS loading 100 percent of the two highest effect lanes is compared with 90 percent of the three highest effect lanes and 75 percent of all lanes, producing the max and min effects that would be expected for a particular loading. If HL93 loading, 120 percent of the highest effect single lane is compared with 100 percent of the two highest effect lanes, etc.
If lanes are floated between boundaries defined by curb lines (road width data defines the farside curb line), the entire set of lanes is moved as a unit laterally to an optimal location for each girder so that the lane boundary closest to a girder for which effects are being determined is collinear with that girder. Curb lines as range limits may prevent an exact colinear placement. Whether or not lanes are floated, the truck is always placed to the side of the lane which is closest to the girder for which effects are being determined. Wheels will not encroach on the buffer between the wheel and lane boundary which has been determined by AASHTO (two feet) or established by the user with data.