A single simply-supported span is loaded according to AASHTO LFD's live load specifications using influence lines.
Model Setup
This model consists of a single 200-foot span divided into 10 members of equal length. The left end at (0,0,0) is fully supported in translation and left free in rotation. The right end at (200,0,0) is supported in z-translation only. Universal Restraints are used to support all joints in y-translation and x- and z-rotation, reducing the model to two dimensions in the X-Z plane.
A lane, named "LL Lane," is created along the length of the span. A unit load will march along the lane to generate the influence line coefficients. To create the lane, you can use the Draw > From Selected Members > Lane command.
The standard moving load database is connected to the project.
To create the influence line coefficients, a moving load case must be set up. In the Load Cases Explorer a load case named "Influence Load" is created. In the case, a single moving load is applied with the following specification:
| Lane Loaded | Load Pattern | Applied Load Direction | Load Factor | Direction of Travel |
|---|---|---|---|---|
| LL Lane | INFLUENCELOAD | Global -Z | 1.00 | Forward |
The Linear Moving Load/Influence analysis is run on the model.
Result Case Setup
After the analysis, influence-based cases are created for the different types of AASHTO LFD loading. Using command three times, separate cases are created:
LL Lane/HL-93 Design Truck: This case specifies the loading of a HL-93 Design Truck, which is the HS20-44 truck with the rear axle variably positioned between 14 and 30 feet behind the previous axle. The vehicle is defined in the standard moving load database and so is available because the database was connected earlier.