Live Loads
The vehicular live load is designated as HL-93 and is a combination of the design truck or design tandem along with the design lane load. The Design Truck is shown below. The design lane load is a uniform load with a magnitude of 0.64 kip/ft (9.3 kN/m). The design tandem is also a truck with a pair of 25 kip (110 kN) axles spaced 4 ft (1.2 m) apart.
A major difference between AASHTO LRFD and LFD is the new provision that explicitly states that only forces contributing to the maximum force effect need to be considered (Art. 3.6.1.3.1).
For continuous spans, in addition to the design truck and design tandem, 90% of the effect of two design trucks with a 14 ft (4300 mm) distance between the 32 kip (145 kN axles) and a minimum spacing of 50 ft (15000 mm) between the lead axle of one truck and the rear axle of another truck, combined with 90% of the effect of the design lane load shall be considered for negative moment between points of dead load contraflexure and reaction at interior supports. This type of optional loading is predefined in the program as Double Truck.
Additionally, where multiple lanes of heavier versions of vehicles are probable, consideration should be given to investigating negative moment and reactions at interior supports for pairs of the design tandem spaced from 26 ft (8000 mm) to 40 ft (12000 mm) combined with the effect of the design lane load. This loading can be invoked by selecting the Double Tandem Load. Refer to LRFD Art. C3.6.1.3.1 for additional information.
Permit Vehicle Loading
If Strength II Limit State is selected, the program considers any Permit Vehicle (PV) selected for inclusion in the Live Load envelope results (LRFD Art 3.4.1). If only one permit vehicle is selected the program considers its results in the formation of the live load results. However if multiple Permit Vehicles are selected then the program takes the envelope of these results to form the live load results. If Strength II Limit State is selected and no Permit Vehicle is selected, the program considers the HL93 (Design Lane, Design Tandem and Design Truck) loading for the Strength II Limit State as well. Optionally the user can also select the option to Use Permit Vehicle side by side with Design Loads as specified in Art 4.6.2.2.5. When this option is selected the program applies the code provisions if the following criteria are met:
- The user has selected to include Strength II Limit state in the analysis.
- The user has selected the option Use Permit Vehicle side by side with Design Loads on the Analysis Factorson the Analysis tab.
- At least one Permit vehicle type live load (PV) has been selected for analysis.
- The lever rule has not been specified for both single and multiple lane loadings.
- The special requirement for exterior girders of beam slab bridge cross sections with diaphragms specified in article 4.6.2.2.2d has not been utilized for the simplified analysis.
In the case where the single lane live load distribution factor is greater than the multi-lane live load distribution factor program computes the final force effect applied to the girder to be the force effect due to permit truck times the single lane distribution factor.