Superstructure Dialog Box

The Superstructure dialog box allows you to view and modify the analytical model in CIP RC/PT Girder.

To open this dialog box, click the Superstructure button on the Model tab. The analytical model is based on physical description of the bridge entered on Geometry tab. The Superstructure dialog box displays the model span distances and locations along spans to hinge points. Support bearings and type of superstructure connection used is also displayed. The user can modify the actual span distances determined from use of the physical description of the bridge to be used for analysis. Analytical models can be based on the properties of either the full bridge or of a single girder (interior or exterior) within the cross-section. When the analysis is based upon an individual girder, LEAP®CIP RC/PT Girder will adjust the substructure properties based upon the proportional ratio of the width of the selected girder to the width of the full cross-section.

Modifying the Superstructure

Perform the following steps to modify the superstructure:

Select the girder section you want to modify from the drop-down list located in the Girder field.
Select one of the components from the grid by clicking anywhere in its row. A blue arrow displays on the left side of the grid to mark the selection.
Enter the column values in the grid for each section of the structure you want to modify. A definition for each parameter is available below.

Grid Definitions

Setting

Description

Component

This noneditable field lists the component type as defined in the Bridge Component Layout dialog box.

Ref Name

Enter the component reference name. For more information regarding the naming of components see the Technical Discussion for the Bridge Component Layout dialog box.

Span Distance

Enter the analytical distance from this component to the previous support component.

Hinge Distance

Enter the analytical distance from this hinge to the previous support component.

Bearing

Enter the component bearing. For more information regarding the bearing of components see the Technical Discussion for the Bridge Component Layout dialog box.

Connection

Select the connection type from the drop-down list. For more information regarding the specification of component connections see the Technical Discussion for the Bridge Component Layout dialog box.

Setting	Description
Free	This keyword connection releases all forces (including vertical shear) and moments. This has the effect of decoupling the superstructure from the substructure, or between adjacent span segments at a span hinge.
Roller	This keyword connection releases the longitudinal shear force and the moment about the axis of the support.
Pin	This keyword connection releases moment about the axis of the support.
Fix	This keyword connection does not release any forces or moments.
Spring	Selecting a spring connection opens the Connector Spring Stiffness dialog box.

Area

Enter the constant area to be used for this span distance. Note that this value will over-ride the actual interpolated properties computed from using the sections defined on the Geometry tab.

Izz

Enter the constant moment-of-inertia to be used for this span distance. Note that this value will over-ride the actual interpolated properties computed from using the sections defined on the Geometry tab.

E-Modulus, E28

Enter the concrete modulus to be used for this span distance. Note that this value will over-ride the modulus defined on the Concrete Materials dialog box.

Unit Weight

Enter the concrete unit weight to be used for this span distance. Note that this value will over-ride the unit weight defined on the Concrete Materials dialog box.

Technical Discussion

Generally it is not necessary to modify the superstructure data. Following are a few examples of situations where modifying the superstructure analysis data may be desirable:

The analytical model span lengths should be different than that determined by using the bridge layout.
When performing a girder analysis and the Automatic Substructure Distribution method (see discussion below) is not appropriate. A more accurate substructure stiffness can be described by creating an appropriate Pier and specifying the reference name of the Pier for the appropriate component.
To quickly compare the results of a simpler model by specifying prismatic cross-section properties.

Note: any modifications made to the analysis model on the Superstructure dialog will be lost when changes are made to the physical description of the bridge on the Geometry tab.

When performing an Optional Girder Analysis the model is based upon distances between supports along the centerline of the selected girder. The individual cross-sectional properties of the girder are used for analysis. Typically the substructure (pier and columns as shown on the plans) is defined for the entire bridge and Automatic Substructure Distribution is selected (default) to proportionally reduce the substructure stiffness for the analysis of the individual girder. The Automatic Substructure Distribution reduces the substructure stiffness defined for the full bridge in the proportion of girder width to overall bridge width.

CIP RC/PT Girder normally includes the effects of support skew through the use of a modified plane frame analytical model. CIP RC/PT Girder provides an alternative method to account for the increased shear distributed to exterior girders near the obtuse corner of skewed supports. When the Apply Skew Correction to Ext and Int Girders checkbox is selected the following occurs:

The supports of the analytical model are adjusted to be normal (i.e. no skew) to the bridge centerline. This only affects the model support skews. The physical pier skews entered on the Layout dialog are not affected. The supports are adjusted in this way so that the effects of support skew are not considered both through the modified analytical model and also by factoring.
An additional Shear Check (Ext Gir) included, which provides factored shear results for the respective girders only.
Shear in the exterior framing into an obtuse corner are magnified linearly from 1.0 at the midspan of the beam (POI 0.5L) to the specified maximum (LRFD Table 4.6.2.2.3c-1) shear factor at the beam end.
The Range of Applicability specified in LRFD Table 4.6.2.2.3c-1 are ignored.