Multi Girder Bridges#
Introduction#
Multi girder bridges are commonly used for fast and easy assembly on site. Prefabricated beams - mostly steel or pre- or post-tensioned concrete - are placed on the bearings or moulded into the cross beams according to the design layout. The concrete slab is poured on top to ensure a solid foundation for the stability of the beams and finishing layers. In case the distance between the beam top flanges is larger than the flange itself - additional formwork is required.
Design challenges#
Multi girder bridges are generally assembled with straight beams, even though the deck might be curved. The bridge deck is design based on the axis’s curvature. However, the beams follow a polygonal interpolation of the curve. The axis represents no longer a valid input in terms of the beam geometry. Bridge types such as hollow cast or double t-beam - where superstructure components following the axis curvature - follow the similar approach.
In order to achieve a high level of detail, beams are created as generic families. These families give you great flexibility in geometry creation and Level of Detail. Beams are arranged in spans according to the given layout rule such as maximum distance or fixed number.
Modeling objectives#
Logical workflow
Generic design
Adjustable
Variety of bridge types.
Definitions#
Girder Layout – Group of Girder Spans placed on an axis. General arrangement of the girders between given stations.
Girder Span – Group of lines, that indicate the primary position of Beams between two stations (path start/end). Girder Span is created according to the Girder Reference or Center Curve with additional properties and layout rule.
Beam (Family) – 3D element placed at the start and the end of each girder. The beams are connected to girder spans and updated after applying changes to the girder layout. Find out more here
Girder Reference – Line(s) with Subcategory “SOFiSTiK_Girder_Reference” in Adaptive Generic Family.
This line indicates the distribution range and vertical position of Beams in each span. The Girder Reference can be a part of Bridge Profile Families (also used for the Superstructure tool) or created as a separate chain of lines.
You can parametrize position and dimensions of Girder Reference using variables or reference to the axis. Find out more here
Center Curve - virtual curve defined between the start and end station (parameter). Geometry, distribution and vertical positioning of the Girder Lines can be referenced to Center Curve.
Workflow#
Create an Axis
Create Girder Layout according to Girder Reference, placements and layout rule.
Create Beams on Girder Layout.
Create Deck using Superstructure command.
Create additional elements such as Cross Member for further detailing of the structure.
Tip
Gap between the Beam elements and the deck can filled with additional Beam elements (haunch) or by further parametrization of Deck profile with Shape Handle Points assigned to Girder Lines.
Families#
Beams#
Adaptive generic families with two adaptive points.
Adaptive Points ‘1’ and ‘2’ are placed at the start and at the end of the girder span. Additional Offsets in X/Y/Z directions (see pic) can be defined in the Beam(s) Dialog. You can override offset parameters for each beam individually using the ‘Offset Override’ command.
Find further information to the recommended workflows for Beams customization here.
Girder Reference Family#
An Adaptive Generic Family with one single Adaptive point and at least one line marked as Girder Reference (highlighted in orange). See also Girder Reference definition above.
You need at least one family loaded in your project to be able to define a girder layout. All available Girder Reference Families will be listed in Girder Layout Dialog.
Use already provided Girder Reference Families or create your own one(s). Use Mark and Mark Invisible commands to define selected lines in your family as Girder Reference.
Tip
Use Mark tool for lines that are part of the Bridge Profile.
Tip
Use Mark Invisible tool for lines that are not part of the Bridge Profile (used also as Profile for Superstructure command), but are defining the distribution range of the Girder Span. Open or multiple polylines in the Bridge Profile Family can cause problems while generating the deck as superstructure element.
Parametrization#
You can define the geometry of a Girder Reference or Beam family with a constant value or assign a variable to the specific parameters. Using the same variable you can create a link between two components. Adjustments of the variable will trigger changes in both elements.
Examples#
Curved bridge with straight concrete beams#
Girder Layout
In this example we will focus on the definition of chorded beams along the curved alignment.
We use Center Curve definition (mode) to define the girder positioning, symmetric transversal range (10m) and layout rule of Fixed Number (4). It means that we will distribute 4 girders parallel to the Center Curve in the range (domain) of 10m.
In Properties, we set Interpolation to Linear to achieve straight geometry in horizontal plane. Additionally, we can select option ‘Void Cut’ for ‘Rotations’ to close the gaps/overlays of the girder lines between the Spans.
For the vertical profile, we select Girder Reference to determine the position of Start and End of the Girders. Girder Reference Family will be evaluated (parametrized) and Girders will be placed according to the position of Girder Reference Line. With this method we can easily accommodate the superelevation of the deck in the positioning of the Girders.
To define the camber of the Girders, we need to set Interpolation to ‘Conform to Axis’ and override the input with the Camber value e.g. 10 cm.
Now we can place Beam Families at the Girder Layout. We can use Families with 3 Adaptive Points to accommodate for the camber curvature.
Formwork slabs
We use Cross-Member Array command to place the formwork slabs between the Girders. Geometry and Position of these slab elements is defined be the Shape Handle Points (SHP), which are following Girder Lines. Don’t forget to define Local Station, in case you want to evaluate the position of SHP at the beginning and the end of the element.
Tip
You can use Infra Browser to isolate Axis and Girder Lines in View. Selecting corresponding lines for the Shape Handle Points will be much easier.
Concrete Deck
We have a similar procedure for the definition of the curved concrete slab. We use Superstructure command to extrude the Bridge Profiles along the axis. Bridge Profile is created in such a way, that the upper lines are controlled by the Adaptive Point (which follows the Axis) and bottom lines are controlled by the Shape Handle Points (which follow the Girder Lines).
