CIP RC/PT Girder generates a slight variation of a plane frame model for analysis. This is done to account for the effect of skewed supports on the analysis. Skewed supports couple bending moment and torsion in the superstructure and, therefore, the superstructure is modeled using a plane-frame bending element with a torsional degree-of-freedom (DOF). CIP RC/PT Girder automatically calculates the torsional moment of inertia of the superstructure and uses it in the computation of the torsional stiffness coefficient. All transverse displacements are constrained.

The connection specification in the Bridge Component Layout dialog box defines the type of force release between the superstructure and abutments and piers, or between adjacent span segments at span hinges. The connection is modeled using a zero-length, two-node "super-element" that contains two support-bearing elements spaced transversely along the skew axis of the support. In this way, the superstructure "feels" the presence of the skewed support.

The stiffness values of the support-bearing elements are assigned appropriate values depending on the specified connection. For example, a pin connection is realized by assigning large values to all translational stiffness terms for the support bearings. The vertical stiffness term for each support-bearing element is always a large value in order to maintain continuity of torsion. In essence, this assumes that the superstructure is wide enough to always transmit torsion into the substructure or across a span hinge.

The effect of modeling the connection this way is that, when there is a skew angle at a support, an off-diagonal or coupling stiffness term between the bending and torsional diagonal stiffness term is produced. A large skew angle results in a strong coupling between bending and torsion in the superstructure. Box girder bridges have closed, thin-walled cross sections and are torsionally stiff. Therefore, the overall stiffness, i.e., the three-dimensional stiffness, of the substructure is included in the plane-frame model of the bridge.

The columns in the substructure are modeled as space-frame bending elements (6 degrees-of-freedom per node), which are then statically condensed down to a single point of connection at the location where the pier attaches to the bottom of the superstructure connection. Therefore, the effects of skewed multicolumn bents and foundation spring constants (which are oriented in the coordinate system of the pier) are accounted for in the modified plane-frame superstructure model.

The structural analysis capabilities of CIP RC/PT Girder are based on the services of a finite element engine. In order to make use of those services, CIP RC/PT Girder uses its internal representation of the box girder and loadings and constructs or generates an appropriate finite element (FE) model. Once the FE model is constructed, CIP RC/PT Girder performs various operations, such as form stiffness, add loads and solve for displacements. The following describes the generation of the finite element model for a box girder or an individual girder: