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Bearing Fixity
This section defines the translational and rotational fixities at each bearing location, controlling how forces transfer between the superstructure and substructure.
Navigate to Superstructure > Boundary Conditions > Bearing Fixity in the tree view.
Navigation
Step 2: Select Insertion Points
Click the first cell in the Insertion Point column, then click Pick to graphically select the insertion points that define boundary conditions.
Select Insertion Points
Step 3: Define Fixities
For each bearing location, set the fixities or stiffness values in the Tx, Ty, Tz, Rx, Ry, and Rz directions based on the bearing type.
To determine Tx and Ty stiffness values, compute them based on your bearing pad stiffness. This example uses a value of 10.
Fixity Values
Bearing Settings
Step 4: Set Transfer Force to Substructure
Set Transfer Force to Substructure to Yes for all bearing locations where substructure is modeled.
Yes: Creates a two-node spring between the pier cap and girder (use when substructure is modeled)
No: Creates a one-node spring at the bottom of the girder (use for abutment locations without modeled substructure)
Quick Tip: Degree of Freedom Reference
Nodes are assigned degrees of freedom (DOFs) based on the local coordinate system at each insertion point, which varies along the support line and alignment.
Tx/Ty/Tz/Rx/Ry/Rz fixities:
DOF Reference
Bearing Rotation rotates the support clockwise in the XY plane:
Bearing Rotation
Transfer Force to Substructure FE behavior:
FE Behavior
Quick Tip: Torsional Stability
Setting Rx = 0 requires constructing the diaphragm with girders to maintain torsional stability. To overcome stability issues when the girder is constructed without transverse elements (diaphragm or deck), a nonzero Rx value is necessary.
Verification
After completing these steps, your bearing fixity configuration should match the view shown below.
