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Result Superposition Combination

Requires: Advanced Level

What is a Result Superposition?

VisualAnalysis has the ability to superimpose the analysis results to create a post-analysis load combination. This is useful for combining loads that cannot be combined before analysis. You will want to use this feature to combine static and dynamic results or to combine results from moving loads with other static results. This type of combination will generally consist of an envelope pair (extreme minimum and extreme maximum) results, depending on whether any of the results you include are 'extreme only' results. The extreme only result cases include the time-history envelope results, and the moving load case results.

The superposition is carried using simple algebraic summing of like results from each result case. There is no special logic or intelligence in the process. When an envelope is included, the normal results from simple result cases are added to both the extreme minimums and the extreme maximums.

How to Use

To access this feature use the Load Case Manager, Advanced tab, and select which possible result items to include. You can create this combination before you have analysis results. After analysis this combination will only contain results if all of the included result-sources actually exist.

Superposition combinations may be marked as a design load combination (ASD, LRFD, or deflection). This is the approach to take to get design checks for moving load results or for dynamic response or time history results.

When viewing Result Views showing any envelope results, use the Filter tab in Project Manager to toggle between the High Extremes and the Low Extremes.

Limitations & Details

This feature is not available if the model is nonlinear. Superposition does not apply to nonlinear analysis, so adding result cases makes no sense. For example, if a tension-only member is removed from an analysis because it goes into tension, how can you add the results from another analysis where this member may not be in tension? VisualAnalysis does not allow the combination of results that could be combined before analysis, you should use the other types of load combinations for combining static loads, for example.

VisualAnalysis allows you to combine P-Delta (2nd order analysis) results, however it will warn you that doing so is really not valid. P-Delta analysis is nonlinear, so using superposition on the results may yield incorrect results. From a practical standpoint, it may not make a big difference, as P-Delta results are usually within 10% of the linear results, yet you may be adding 'apples' with 'oranges'! Please exercise caution and double-check the answers you get.

Dynamic Response result cases contain only absolute values. When a Dynamic Response is combined with any other result cases, these positive values are added blindly during the superposition—that is, there is no intelligence in the process to determine a 'worst case' superposition. These positive values are added to both envelope extremes, if the superposition is an envelope.

Envelope result 'locations' are not retained or calculated in the superposition combination. You will not be able to determine from a result superposition where a moving load was, or the time step from a dynamic time history analysis.