Tips & tricks
After running a Global check of
the sturcutre, the result table on the bottom of the screen has a feature to
select specific load combinations for the next Analysis.
To do this, selection of desired
load combinations is needed. (As on all tables in ConSteel, multiple selection of different
load combinations can be performed easily using Ctrl+select, or Shift+select
features of Windows) With a right click on the selected combinations, a dialog
will pop up that says „Select only
these combination for the Analysis” (picture below)
This feature of ConSteel can be
very useful when working on a model with a lot of loadcases (wind in different directions, with and
without internal pressure, snow,seismic etc.), hundreds of load combinations
can be generated by the EN, and many of these combinations are irrelevant.
Running an Analysis on all of the load combinations can take a lot of time, what
could be saved, if the relevant load combinations are calculated only. To
find them, it is necessary to know, that in ConSteel two types of load
combinations can be used for calculation, and it can be decided on the
„Analysis parameters” dialog which one to use:
-1. Calculate load combinations: direct calculation of previously
created load combinations (can be used for first, second order, buckling, and
vibration analysis too, but the running time is slower because of the huge
amount of possible load combinations)
-2. Calculate simplified combinations by superposition: calculation
of load combinations by superposition of the results of load cases (can be used
for first order analysis only, but the running time is faster since only the
load cases are calculated directly during the analysis, and load combination
results are calculated by postprocess)
Fastest way of finding the relevant load combinations:
1, The first time when analysis is being performed use
the Calculate simplified combinations by
superposition option.
2, Under the global checks tab, design should be performed,
and on the results table, selection of the load combinations with significant utilization
should be selected. The limit value for significant utilization (accordingly the number of the selected load combinations which are
applied for analysis) is the choice of the designing engineer, but it is
generally true, that where first order utilization is low, stability problems
will not appear.
3, The next run of analysis can be performed using the Calculate load combination option only for the previously selected
combinations, parameters can be extended (second order, buckling analysis…)
To demonstrate the speed of this process, a simple
example will be shown using the previously shown technique of finding
relevant load combinations versus when running
all of the combinations:
The model that is being used, a simple structure, a steel
hall with 20x40 m layout dimensions, a 7 m spine and 5 m corner height.
Beside the Deadload and installations on the structure, Windload,
snow, and exeptional snow loadcases were generated. There were:
-27 load cases defined
-which are produced 143 load combinations
in persistent and transient design situations
The running time of the whole design process on a laptop, using both the Calculate simplified combinations by superposition and Calculate load combination option:
-Analysis (using Calculate simplified combinations by superposition for all load combinations) 16 seconds
-Global check (first order, for all load combinations) 84 seconds
-Selection of load combinations with utilization higher than 30% for the analysis --13 load combinations all together (instead of the original 143)
-Analysis (using the Calculate load combination option on previously selected load combinations) 28 seconds
-Global check (second order and buckling) 25 seconds
Total time: 2 minutes 33 seconds
The running time of the whole design process on a laptop, using only the Calculate load combination option on the analysis parameters panel:
-Analysis (using Calculate load combination option for all load combinations) 3:58 minutes
-Global check (second order and buckling) 3:26 minutes
Total time: 7 minutes 24 seconds
So after all, on a relatively small model like we used, this design method can save a lot of time during the analysis, which can speed up the design process of a structure.
Even so, at the very end of designing a structure, it is advisable to run the analysis using the Calculate load combinations option for all load combinations.