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I have recently had reason to use the solid modelling features of Robot and I noticed some unsettling and potentially dangerous behaviour in the way it handles meshing.
I am using it to perform a torsional stability assessment of an existing, trapezoidal box girder bridge with horizontal curvature under a temporary confition involving crane outrigger loads. I generated the geometry in Revit using a sweep/model in place and used a Dyanmo scrip to export the resulting solid in .sat format. The Dynamo script also split the solid into 4 parts, one of which was at an internal supporting pier and the second split was down the centre line such that I could apply self weight to one side of the box separate to the other as opposed applying the self-weight to the box in its entiriety. The reason for this is that it allowed me to apply different partial safety factors to the self-weight of each size of the box depending on whether it was contributing or resisting torsional overturning as per AS 5400/AS 1170 stability requirements.
The model appeared to mesh correctly however, upon checking the quantities, I noticed that the volumne of concrete reported in the Robot model was substantially larger than that determined in Dynamo (see screenshots below).
Note above that Dynamo reports the volume as 233 cubic metres while Robot is reporting it as 396 cubic metres.
Upon further investigation, I discovered that Robot had double meshed parts of the structure as per the screenshots below.
As the screenshots above show, the curved span's mesh extends beyond the split at the internal support onto the straighter span while the straigher span has its own, spearate mesh which explains why Robot reports nearly twice the volumne as Dynamo.
I haven't solved the model because I deemed it unreliable upon making this discovery however, the concern is given that the self-weight of the box is critical to resisting the overturning, then over-estimating the self-weight could well product model results that are not convervative.
To confirm the findings, I have simplified the model to single spans consisting only of one solid each (i.e. no longitudinal split) and the meshed volume in Robot is consistent with Dynamo.
Has anyone else noticed this behaviour and if so, is they anything that can be done to remedy it?
Anthony
I tried the following 2 ways, but neither of them works.
1. I opened dynamo file in the Revit built-in Dynamo, it shows the structural analysis package is not available
2. I opened dynamo file in the Robot built-in Dynamo, it shows the Generativedesign analysis package is not available
The dynamo file shows only 1 solid is selected,
while in your 1st image, there are 4 solid
I am following this thread but I still cannot understand why not using shell panels to model the bridge box section.
Is there a special reason for using solids?
Rafael Medeiros
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Thanks for your interest. I have resorted to just doing that as I know it works. I used solids mostly for the purpose of testing out just how good or bad Robot handled solids and becuase I could generate the model from Revit with a relatively simple Dynamo script using just a model in place sweep and a simple extraction to .sat format. It was essentially a litmus test solid modelling in Robot and I believe based on what I have seen is that like many niche features in Robot..... technically speaking it satisfies the minimum definition of having solid modelling capabilities but it can't be trusted for anything other that the simplest, most basic situations.
Below is a screenshot of the model using plate/shell elements and it is giving me good quality answers.
I would still appreciate some hints as to why Robot is behaving so oddly because the quality of the mesh on the solid model is actually better (when modelled as a single, solid object) and there are situations where I would like to be able to use it like this.
Sorry , I cannot help you with solid modelling or even about generating a sat model in Revit to export it to Robot.
But I have done several bridges with box sections ( steel , concrete or composite)using bars or FEs panels.
As you already noticed , results are good for both structure types although more complicated to obtain the force results on the panel models.
But for testing ,every experiment is valid.
I even already modelled a full FE bridge rotated 90 degrees to the vertical so I could assign a fake core wall with stories , so I could have the the core wall force diagrams for for my " vertical" bridge.
Please ,post a comparison between your 2 types of bridge , solid and shell models as soon as you get them right.
Rafael Medeiros
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one possible reason is that you're trying to create an object from revit/dynamo that's a combination of a straight part and a curve. try creating two separate objects. a simple curve can be meshed without being duplicated.
Best Regards
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