Bridge Analysis : Modelling implications

Defining the structure

Different models are usually needed to determine global effects on the main structural members, and local effects (local bending of slabs/plates). The main bridge analysis features are designed for determining global effects. Grillage models of complete bridges are unlikely to be sufficiently refined to give a reasonable estimate of local effects, which also depend on wheel contact areas etc. These effects on local parts of bridges can be investigated using fine grillage meshes for short lengths of bridge, and using the Generate Static Vehicle tool to model critical wheel loads as patches.

Bridge loading is applied to Grid Planes in GSA, and the results from a Bridge Load Optimisation analysis will depend on the type of grid plane chosen. The describes in detail how the loads are distributed to elements and defines some of the terms used below. Some knowledge of how the load is transferred from space on to the structure will avoid some of the potential problems associated with bridge analysis.

Structure edges

When checking the paths a pass is made along the path checking where the path first crosses and where it last leaves the structure and looks for any positions between these where the path falls off the edge of the structure. The method of doing this relies on identifying the edge of the structure. If no edge can be found the checks will fail.

Note: Where a bridge deck is constructed from longitudinal beams with transverse beams that cantilever out from the main beams, the resulting comb like sides cannot be interpreted as edges of the structure. This problem can be avoided by adding dummy elements between the tips of the cantilever elements. These elements are ignored in the analysis but they allow the identification of the structure edge and panels for loading.

Tolerances

When checking which elements lie in the grid plane and thus those forming the deck the Grid plane tolerance is used. If the deck is not flat the tolerance may be adjusted to include all deck elements, but if panels are significantly out of the grid plane a solution is unlikely to be found.

When checking for the start and end chainages for bridge analysis the Grid loading edge tolerance is used. This may require adjustment if the path is reported as crossing the edge of the structure.

Grid plane span direction

There is no difference between two-way spanning and multi-way spanning unless a Grid Area Load completely covers a lot of simple rectangular panels, in which case two-way spanning will give quicker results. One way spanning, because the span direction is related to the grid plane definition axes, will only give meaningful results for straight bridges. Normally it is recommended to use multiway spanning onto a list of elements (defined with the grid plane) which only includes the longitudinal beams and end diaphragms. This will model the spreading effect of the slab without confusing global and local effects.

Influence line interval

During an Influence Analysis, values of effects are calculated at a number of regularly spaced points along the length of the structure (provided the alignment extends over the full length). By default the program will try to provide approximately 100 points. Where the structure is very long and an effect is very local (e.g. bending of a transverse element) there may not be sufficient calculation points in this local area to accurately position the load to pick up the maximum effect. If this is a concern then the number of influence calculation points can be increased in the Bridge Specification dialog.

Note: Increasing the number of points will increase the analysis time, so the number of points should be chosen to be sufficient but not excessive.