INF10: Influence Surface

Influence surfaces are used for a vehicular live load analysis of a bridge deck. Influence surfaces are used for live load analysis for decks modeled as a surface of plate elements.

Model Setup

In this model, a 200 ft long deck 40 feet wide is modeled as a 10-by-4 plate mesh. The joints along the start of the deck are supported in translation, and the joints along the end of the deck are supported in z-translation only.

A lane is defined on the deck surface. The lane is given a width of 40 feet in Input Data > Geometry > Lanes. The path of the lane is defined along the edge of the strip of plates down the center of the deck.

The influence surface coefficients are set up in the same way as for influence lines. A load case is given a moving load with the INFLUENCELOAD load pattern. But because many more coefficients are generated for surfaces, a position increment of 2 feet rather than the default 1 foot is used.

The moving load analysis is used to generate the influence surface coefficients.

Result Case Setup

Following the analysis, an influence-based result case is created to simulate the application of a HL-93 Design Truck. With influence surfaces, it is possible to use new two-dimensional load patterns that model vehicles with actual width. Each wheel, rather than each axle, is modeled as a separate load. The load pattern HL-93 Design Truck T/C in the standard load pattern database models the truck as 6 feet wide, with six wheels, following the AASHTO code.

Accessing Results

See the earlier sample INF01: AASHTO LFD Simple Span for an overview of accessing influence-based results.

Using graphical influence coefficients view, the position of the vehicle that leads to the most extreme effects can be viewed. The figure below shows the position of the two-dimensional vehicle on the deck.