Today, all but the H10-44 vehicles are still in included in the AASHTOspecifications. To load a structure one such truck per lane, per span is used. The truck isthen moved along the length of the span to determine the point of maximum moment. Recently, to account for higher load conditions, some states have begun using the HS25design vehicle, which represents a 25 percent increase in loading over the standardHS20-44 truck for a total gross vehicle weight of 90,000 lbs. (40,824 kg).(1) It isimportant to stress that the H and HS trucks do not represent an actual truck being usedto transport goods and materials. They are approximations used to simulate the greateststresses caused by actual trucks.Replacing the train of trucks in the 1935 circa design code are laneloading configurations,which approximate a 40,000 lb. truck, followed by a train of 30,000 lb. truck. To modelthis, a uniform distributed load is used, combined with a concentrated force. This forcevaries for moment and shear computations. Where truck loading generally governs forshort, simple spans, lane loading typically hods for long continuous span bridges. Liketruck loading, the concentrated load is moved along the span to determine the point ofmaximum moment.A reduction in the live load intensity is permitted for bridges with three or more lanes thathave maximum stress caused by fully loading each lane.(1) A 90 percent reduction ofthree land structures and 75 percent reduction for bridges with four or more lanes isallowed. The reduction is permitted given the rarity of the situation where simultaneouslyloading in multiple lanes occurs.While I have discussed the legal load limits for trucks, there is also the issue of providingfor those trucks which are overweight. In an attempt to deal with extralegal loadingconditions, the California Department of Transportation developed a live loadingconfiguration known as the permit design loads or P loads. Like their H and H...
