Creating Bridge Models in OpenBridge Modeler

Why 3D Bridge Modeling Matters at TxDOT

Imagine you're standing on a Texas overpass — three spans of prestressed concrete beams carrying northbound traffic over a busy intersection. Somewhere in the design process, a slight misalignment between the deck cross-slope and the superelevation shape went undetected until construction. The cost? Weeks of rework and hundreds of thousands of dollars.

OpenBridge Modeler (OBM) exists to prevent exactly that scenario. As TxDOT's standard 3D bridge modeling application, OBM allows Transportation Engineers, Engineer Assistants, Transportation Specialists, and Design Technicians to build detailed parametric bridge models that catch geometric conflicts long before steel is cut or concrete is poured. Critically, OBM is a modeling tool — not a design or analysis tool. The design and analysis capabilities live within the broader OpenBridge Designer (OBD) suite, which includes LEAP Bridge Concrete, LEAP Bridge Steel, and RM Bridge for structural analysis.

This lesson walks you through the foundational workflow for creating a concrete superstructure bridge model from scratch — the same workflow TxDOT engineers use on real projects. You'll learn how to set up a new project file, place SupportLines to define your bridge geometry, configure the deck with variable constraints, lay out beams across multiple spans, and place substructure elements. By the end, you'll understand the complete element-by-element modeling sequence that forms the backbone of every bridge project in OBM.

Setting Up Your Bridge Project

Every bridge model in OBM begins with proper project setup. When working in TxDOT's ProjectWise environment, the workspace and workset are configured automatically, but you still need to create a new OBM file with the correct seed file.

The critical step during file creation is selecting the TxDOT_BridgeSeed3d.dgn seed file. This seed provides the standard TxDOT configuration — levels, styles, and settings — that ensure your model conforms to department standards. When creating the file, set the file creation method to "No Wizard" (we'll cover the Bridge Wizard approach separately), verify the seed file path, name your file descriptively (e.g., Bridge_Model_01.dgn), and set the application to OpenBridge Modeler.

Once your file is created, you need to attach reference files. At minimum, you must attach the horizontal and vertical alignment geometry files — these define the roadway path your bridge follows. Optionally, attach the edge of pavement (EOP) and superelevation shape files if available, as these allow OBM to automatically drive deck widths and cross-slopes. Set reference attachment options to Coincident – World orientation with Full Scale 1:1 and Live Nesting set to 2.

After attaching references, two setup tasks remain. First, set the GeoCoordinate system to match your reference files — use the Search Ribbon, key in "Coordinate," select Coordinate System, and use the "From File" option to import the coordinate system from your geometry file. Second, enable terrain contours by selecting the terrain boundary, opening the Properties dialog, and changing Override Symbology to Yes with Override Template set to Terrain\Existing Contours. This makes the existing ground visible for substructure placement later.

Placing SupportLines and the Bridge Deck

With your project set up, the modeling begins. The first step is creating the bridge placeholder using the Add Bridge button. You'll enter the bridge description, NBI structure number, unit name, unit description (e.g., "215.00' Prestr. Conc. I-Beam Unit (45'-125'-45')"), and select the Unit Bridge Type as "Beam Slab (P/S or RC Concrete Girders)." After selecting the alignment, verify the bridge appears in the Project Explorer.

Next, set the terrain as the active terrain for the bridge — this is essential for substructure elements to interact correctly with the existing ground. Then place SupportLines, which define the start/end of the bridge and the locations of abutments and bents. Using the Place Multi SupportLine tool, you specify the skew angle (e.g., -10°00'00"), support line length, start and end stations, and number of support lines. For a three-span bridge, you'll place 4 support lines and configure the span lengths — for example, 45', 125', and 45' for a typical TxDOT overpass.

With SupportLines in place, you can now place the deck using the Place Deck tool. Select the appropriate TxDOT deck template, set Analytical Deck to True, choose the deck material, and identify the first and last SupportLines to define the deck limits. This opens the Variable Constraints window — a powerful feature that lets you define how the deck geometry changes along its length.

Variable constraints control deck width and cross-slope. For each variable (e.g., LT_Width_Lane1, RT_Slope_Lane1, RT_Width_Lane1), you toggle the "active" checkbox and add rows defining the values at specific locations. Constraints can be applied by SupportLine (most common), by ratio of span length, or by station. For example, setting LT_Width_Lane1 to -23 from start to end defines a 23-foot left overhang. Cross-slopes can also be driven automatically from ORD superelevation shapes using the Assign Superelevation tool, or deck widths can be controlled by edge of pavement lines using Point Controls.

Beam Layout and Placement

After the deck is placed, the next step is configuring the beam layout. The Beam Layout tool defines how beams are distributed across each span. For each span, you specify the number of beams, edge distance, and whether beam start/end values are the same. The SL Offset controls how far the beam extends past the SupportLine, and the Skew Ends setting determines whether beams follow the skew angle at that support.

For example, in a three-span bridge with spans of 45', 125', and 45', the outer spans (Span 1 and Span 3) might have 6 beams each while the center span has 8 beams to handle the longer distance. Edge distances are typically set to 3.000 feet with Equal Edge Distance enabled. A key detail: set Skew Ends to True only at the abutments (the bridge ends) and False at interior bents — this ensures beams are square at the bents where they meet, which is standard TxDOT practice.

Once the layout is configured, press Validate to check your input, then Save. If different spans share the same beam configuration, you can save time by defining one span, setting it as the Default Span, and checking "Set All To Default" to apply it across spans — or use Ctrl+C/Ctrl+V to copy between spans.

With the beam layout defined, use the Place Beam tool to physically place the beams in the model. Select Custom mode, set Use Beam Rotation to False (unless non-plumb beams are required), and click on the beam layout. For beams requiring multiple bearing pads, define the number and spacing in the Beam Layout dialog. To match BGS-generated beam reports, change the Method to "Along Skew" and set the Spacing Reference for the first and last beams to "Alignment" — this lets you enter beam positions exactly as reported by BGS.

Remember: if you need to modify the beam layout after placement, select the layout and use the "SELECT to Edit" button next to the Pattern field in the Properties dialog. When multiple units exist in a bridge, element placement can only be performed on the active unit — indicated by a green icon in the Project Explorer.

Key Takeaways

Creating a bridge model in OpenBridge Modeler follows a clear, sequential workflow that builds the structure element by element:

1. Project Setup — Create a new file with the TxDOT_BridgeSeed3d.dgn seed, attach alignment and terrain references, set the GeoCoordinate system, and enable terrain contours.

2. Bridge Placeholder — Use Add Bridge to establish the bridge identity, structure number, unit type, and alignment association.

3. SupportLines — Place multi support lines to define span lengths, skew angles, and abutment/bent locations. These form the geometric skeleton of the entire model.

4. Deck Placement — Place the deck using TxDOT templates and configure variable constraints for width and cross-slope. Leverage ORD superelevation shapes and edge of pavement lines when available.

5. Beam Layout and Placement — Configure beam distribution per span, validate the layout, and place beams with correct skew end settings.

This element-by-element approach gives you precise control over every aspect of the bridge geometry. In subsequent lessons, you'll learn to add substructure elements (abutments, bents, footings), place excavation and riprap, generate reports, and use the Bridge Wizard for faster initial setup.