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 Results > Influence Line/Surface Case 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.

LL Lane/AASHTO Point 18kip (moment)/Lane Load:

This case specifies the loading of a 18 kip point load, which is to be used for evaluating moments, as well as a 0.64 kip/ft lane load.

LL Lane/AASHTO Point 26kip (shear)/Lane Load:

This case specifies the loading of a 26 kip point load, which is to be used for evaluating shears, as well as a 0.64 kip/ft lane load.

Accessing Results

The three result cases show up in the Analysis Results Explorer. To access results, click the name of the case and use graphical or spreadsheet results. Moments for the second case are shown in the figure below.

LARSA will not automatically switch between the 18 and 26 kip loads depending on whether you are investigating shears or moments. The engineer is responsible for using the appropriate result case for each investigation.

You may check where LARSA decided to place the truck and point loads to produce the maximum effect for any point on the structure using the graphical influence coefficient view. An image of that is shown below. Arrows indicate the locations of the axles of the HL-93 Design Truck to produce the maximum moment at the first station of the sixth member.

Solution Comparison

Sources

LARSA 4D
Theoretical

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