Straight and curved steel I-girder bridges - Computational Analysis and Design of Bridge Structures - page 202

7.2 PrInCIPle and ModelIng
of Steel I-gIrder BrIdgeS
7.2.1 analysis methods
It is always recommended to perform some kind of simplified verification of
the results of more complex analysis models by means of simpler analysis
models or hand calculation, or both. These types of checks are extremely
valuable to allow the designer an opportunity for better understanding the
behavior of the structure and validating the correctness of the more com-
plicated analysis. It is also advised to perform a number of simple check
calculations directly based on the analysis results. For instance, the simplest
check when performing an analysis is to see whether the summation of dead
load reactions equals the summation of the applied dead loads and whether
the distribution of dead load reactions among the various support points
matches the anticipated internal load distribution in the structure.
Depending on the complexity of the steel framing, the level of analysis
required can range from simple hand calculations to 3D finite element mod-
eling, which are briefly discussed here:
1. Beam charts . In the United States, there are a number of standard
beam design charts and other design aids that can be of use to the
designer. The AISC Manual includes a table of beam shear, moment,
deflection, and reaction graphs and formulas for the cases of uniform
load and point load. Although these patterns of loading are typically
too simplified to be of direct benefit to bridge engineers, these design
aids can serve a valuable purpose by providing a handy resource for
finding approximate analysis methods for use in the preliminary
design or in the checking of more complicated analyses.
2. Line girder analysis method . This method is referred to as approximate
method in the AASHTO LRFD specifications (2013). The line girder
analysis method uses load distribution factors to isolate a single girder
from the rest of the superstructure system and evaluates that girder indi-
vidually. When modeling, beam elements are lined up with the neu-
tral axis. For composite sections, there are four stages, as described
earlier, where their neutral axes and sections may change accordingly.
Figure 7.7 shows the perspective view of a composite section with its
associated neutral axis locations. The live load distribution factors can
be simply determined by some approximate formulas for both straight
bridges and curved bridges (AASHTO 2013).
3. Grid analysis method . This method is also referred to as plane
grid or 2D grillage analysis method. In this method the structure is
divided into plane grid elements (as shown in Figure 7.8) with three
DOFs at each node (vertical displacement, rotation angles about the