Superstructure

The superstructure is modeled as a series of straight modified plane-frame elements with 4 DOFs per node as described above. Span lengths for the box girder and individual girders are determined from the horizontal layout, the bearings of the supports and the specified physical stationings of the supports.

Section properties for each element are obtained from the physical description of the superstructure. Cross section dimensions at needed locations are obtained by using the specified type of variation, e.g., constant, linear or parabolic, and then computing the section properties.

The number of linear segments per element is determined by the type of cross section variation over the element: for a parabolic variation, 5 segments (plus short end segments) are used over the element; otherwise a single segment is used (plus the short end segments). Each segment is assigned the following cross sectional properties:

Area
Torsional moment of inertia
Bending moment of inertia
Modulus - For the lump sum option, the modulus is Ec-28; for one of the time-dependent loss types, the modulus is calculated by first calculating the age of concrete for the specified f'_ci and then using the time variation of modulus for the selected time-dependent loss type.
Poisson's Ratio
Deformation due to shear is not considered, i.e., the shear area is zero.

Post-tensioning is transformed in the time analysis interval added by CIP RC/PT Girderbetween the Initial and Final analysis cases. Rebar is transformed in the Initial case, if specified. Dead loads (DC), superimposed dead loads (DW), and PT loadings are creepable loads when using a time-dependent loss type. Viewing the responses on the superstructure due to these loadings between the Initial and Final cases will show the effects due to creep. Viewing the responses on the superstructure due to PT will also show the effect of relaxation.