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Reasons not to add two (or more) independent structures in the same Diamonds project

Although it is very common to make different variations of the same model in the same project, there are a few situations where it’s wise not to do so.

In particular if your model contains soil layers and /or requires global imperfections and/or requires the calculation of the global buckling factor and/ or requires a modal, seismic or dynamic analysis. And there’s why:

1. Global imperfections

  • A Diamonds project contains two columns. Both columns have the same geometry. All loads are the same except for the wind loads:
  • We run a first order calculation with global imperfections.
    In the results, the bending moment My is not “mirror symmetric”. You expect to see the same absolute value in both columns. But that’s not the case.
  • Reason: Diamonds applies the global imperfections using equivalent horizontal forces (example here). It will apply those equivalent horizontal forces in different directions. Of those directions, it shows the most severe in the results, not the envelope of all the directions! If two (or more) directions result the same |absolute maximum|, Diamonds shows one of them.In this particular example: if the equivalent horizontal forces are applied in the +X-direction, they will have a favourable effect on the bending moment of the left column and an unfavourable effect on the right column. If the equivalent horizontal forces are applied in the -X-direction, they will have a favourable effect on the bending moment of the right column and an unfavourable effect on the left column.
    The effect of the equivalent horizontal forces in the +X and -X on this structure is the same but the sign is opposite. Since the absolute value is the same, Diamonds only shows the results of one direction: +X in this case.
  • Solution: create a separate Diamonds model for each structure.

2. Soil layers

  • A Diamonds project contains two plates next to each other.
    Both plates have the same geometry and are supported by the same soil layer profile.
  • In the deformations, you see that both plates affect each other. This is because of Boussinesq law More info in our webinar about Settlement calculations or in our e-learning course (only available in Dutch).
  • Solution: place the plates far enough from each other. But what is “far enough”? In one model 50m might be enough, another model might require 100m. And does the model still remain pratical? Because comparing results won’t be so easy.
    For your own convenience, create a separate Diamonds model for each structure.

3. Global buckling factor

  • Two frames have the same geometry. Both frames loaded differently.
  • We run a second order calculation.
    The lowest global buckling factor is 2,86. But from the table we cannot deduct to which frame that value applies. Oké, we could look at the first buckling mode and conclude the minimum factor is valid for the frame on the left.
    But what does it say about the global buckling behaviour of the frame on the right? Nothing!  
  • Solution: create a separate Diamonds model for each frame.

4. Modal analysis

  • A Diamonds project contains two independent beams.
    The right beam is significantly stiffer than the beam on the left.
  • We run a modal analysis requesting 10 eigenmodes.
    In the results, 9 out of the 10 eigenmodes are found to the beam on the left.
    Only eigenmode 5, is related to the beam on the right.
  • If these results are later used to describe the dynamic or seismic behaviour of these beams, how can you achieve sufficient accuracy in the right beam with only one eigenmode?
  • Solution: asking 20 eigenmodes instead of 10? That’s not really a solution, because it doesn’t guarantee that the next 10 eigenmodes will be in the beam on the right. Nor does it allow you to check if sufficient effective model mass is reached for each beam.
    The only reliable solution is to create a separate Diamonds model for each structure.

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