Structural Modeling

As discussed earlier, Precast/Prestressed Girder uses two structural models for analysis of the various load components. The particular model used for a given load component depends on whether the load acts on the bare precast (i.e., non-composite) or the composite beam.

Single Span Beam Model

The first model is a two-dimensional, simple span beam with the section properties of the precast beam only. It is used for static loads acting on the bare precast (i.e., non-composite). The length of the beam in this model depends on which span of the structure is being considered. This model is analyzed for beam self-weight, diaphragms, cast-in-place deck and haunch weight, and any other dead loads acting on the precast section alone.

Simple Span Models used for Analysis of Beam Self-Weight, Superimposed Dead Load on Precast, and Deck Weight

Continuous Beam Model

The second model is a continuous, two-dimensional, linear structure with roller supports at the piers and is used for loads acting on the composite beam. The section properties of the total composite cross-section are used for each span. This model is analyzed by the stiffness method for both dead load acting on the composite cross-section, supplemental self-weight, and moving live load. Truck and lane loads are moved across the entire length of the structure, facing both left and right, with variable wheel spacing. The analysis is carried out in accordance with LRFD Art. 3.6.1.3.1, which states, "The axles which do not contribute to the extreme force effects under consideration shall be neglected."

Continuous Beam Model used for Analyzing Superimposed Dead Load on Composite and Live Load

This model is also used for supplemental topping weight and superimposed dead load on supplemental when supplemental topping is used.

After analyzing the bridge for all static and moving loads including DC, DW, pedestrian load, lane load, design truck load, tandem load, double truck, double tandem, and permit loads, the different factors are multiplied and the combinations and envelopes of results are created for Service I, Service II, Strength I, and Strength II limit states. Although you have the option to select which limit state is used, at least one Service and one Strength limit state must be selected. The results of analysis using both models are combined to obtain final moment and shear envelopes for which the beams are designed.

Precast/Prestressed Girder v7.0.0 introduced a new model: multiple spans modeled as non-continuous. This is an extension of the simple span model specified above, with the added flexibility of dealing with a series of simple span bridges within the same data file. Precast/Prestressed Girder internally introduces hinges at all internal support location such that moments and shears get modeled for simple span for all dead and live loads on composite.