Why does Leap Bridge Substructure Calculate Braking Loads using the Two Design Truck Live Load?

Question

When I generate live loads I do want the program to include the two design truck load for live load reactions but it does not seem correct to use that load when calculating braking forces. They should only be based on the design truck per AASHTO LRFD 3.6.4. The values shown in the charts in the commentary C3.6.4-1 confirm the load should only be based on the design truck. 
 Also, I would be helpful to have a way to indicate what portion of the braking force should be applied to the pier being designed. If the truck load controls, the program is applying the entire load to one pier, however if the lane load controls it is only using the tributary area (I think).

Vinay mysore · Answer

This is how the program is internally calculating the Braking forces as per the user manual and then user can manually input the Truck Load and Lane load , please see the attached screen shot. 
 " Longitudinal / braking load can be generated from two locations in the program. First is the generation from Longitudinal / Braking load case dialog. Secondly it can be generated along with Live load generation. When longitudinal load is generated from its own load case, only one longitudinal load case is generated by the program. When the longitudinal load is generated along with the live load generation, program generates same number of LF/BR cases as it generates live load cases. For this situation, every LF/BR case is generated for same number of live load lanes loaded as in the live load case. For example, if program generates 5 live load cases, it will generate 5 LF/BR cases. If the first live load case generated is based on two lanes loaded simultaneously, then first LF/BR case will also have two lanes loaded: 
 As per AASHTO Standard Art. 3.9, the longitudinal load as a percent of live load is, 
 C = 5% of lane load 
 As per AASHTO LRFD Art. 3.6.4., the braking force as a percent of live load is - The braking force shall be taken as greater of: 
 C = 25% of the axle weights of the design truck/tandem 
 C = 5% of the design truck + lane load, or 
 5% of the design tandem + lane load 
 When lane load is used in computing the LF/BR force, the uniform load intensity is multiplied by a contributing length to compute the total force. This total force is then used to compute the LF/BR percent (C) above. Substructure assumes that average of the two spans adjacent to the pier as the default contributing length. However, there may be cases when a different length may be adequate. User must adjust this length in that case: 
 In LRFD mode, when a truck load is considered for generation, all axles of the truck are considered in computing the total load even if the span is too short to accommodate the entire length of the truck. 
 Program also provides an option in which the total live load values for lane and truck can be specified rather than selecting a truck or lane load. In AASHTO Standard since only lane is used, user can specify only lane load total. In AASHTO LRFD, both lane as well as truck total live load per lane have to be specified. In that case, program uses the value input as total for one loaded lane. 
 Once the percent C is computed, program then determines the total longitudinal load and then computes the bearing loads. In this computation, it uses the live load reduction factor / multiple presence factor. In computing bearing/girder and cap loads it assumes that the total longitudinal load is applied at 6 ft above the deck in the direction of stationing. For LRFD California the user can input the height above deck, the default value in this case is considered 0. By default the LF and BR load cases are reversible in Substructure so this load gets considered in both directions. 
 The total longitudinal load is distributed among all the bearings as bearing loads. The moment about the X axis is applied as cap load and the moment computed about Z axis is applied as a force couple in Y direction at exterior bearings. If there are two bearing lines, half of the force is distributed on bearings in each bearing line. For skewed bridges, Substructure resolves the load into X and Z components resulting in bearing loads in both X and Z directions of the pier. 
 The total longitudinal load is distributed among all the bearings/girder locations as bearing loads. The moment about the X axis is applied as cap load and the moment computed about Z axis is applied as a force couple in Y direction at exterior bearings. If there are two bearing lines, half of the force is distributed on bearings in each bearing line. For skewed bridges, Substructure resolves the load into X and Z components resulting in bearing loads in both X and Z directions of the pier. 
 This generation internally accounts for the pier view direction and then generates loads as per the view direction. 
 In case of skewed bridges, Substructure calculates the appropriate force and resolves it into X and Z components appropriately. The moments generated about X axis are applied as cap load whereas the moments generated about Z axis is applied as couple on the exterior bearings/girders. If the pier is non-integral and has two bearing lines then each line will resist half of the moment which is then divided by the distance between the first and the last bearing to generate the force in Y direction. "