for the output of a plate element unit, I understand in Plaxis 2D, output unit is kN*m/m, that is in/out of plane issue.
In plaxis 3D, i have a piled protection slab for gas main, the output unit of plate element is still kN*m/m, how do I interpret the total moment, just ignore the /m ?
M11 and M22 is in 2 different directions of local axis, the plate is very long along the gas main and I don't believe I need to times the length of plate to represent the total moment kN*m. So why the output is shown as kN*m/m
Dear Mikael,
I believe that you have misunderstood my answer and it is important to fully understand what we mean when we integrate the structural forces as this is fundamental when checking the results of your model. It is very critical that you review your extraction of results with a fellow colleague from the geotechnical or (even better) structure team to help you understand this concept of integrating structural forces of surface (plate) elements.
To start with, in your original question you asked about integrating the structural forces that PLAXIS gives for a surface plate element to consider/simplify it to a line element. The way to do this is by using the current width of your tunnel slice, which you specified is 0.06m.As I mentioned before, when we do this integration over a slice width, we assume that the results are constant over that width. Then our integration can become a simple multiplication of the result value and the slice width, which in this specific case is 0.06m.
The number "0.06m" is no special number but rather refers to the slice width for that specific integration of forces.
PLAXIS Output always returns the resulting bending moments in the units of kNm/m as it refers to a surface element and not a line.If one wants to consider simplifying to a line element (from a surface element) an integration of the forces over the width (perpendicular direction to the slice) is required.
In your latest post, you mention that you would change the model and the tunnel slice instead of 0.06m would now be 1.00m. The results you will see in Output will still be in the units of kNm/m but if the integration is now done over the 1.00 slice, then that is an easy calculation to make.
Note that it is important to understand that the assumption and simplification we make here is that we have a constant result over that width. Therefore, it is not as simple to multiply a width with a value and get a line element out of a surface element. That requires that the considered integration width is much (much) smaller than the full size of the tunnel, for instance, as otherwise, the accuracy of the calculation is far from sufficient.
To sum up, I hope that it is now clear that the value of 0.06m is ONLY applicable when you consider an integration over a width of 0.06m and it is NOT a generic number you need to use to consider a line element out of a surface element.
Once again, I would strongly advise discussing the way you extract and interpret your structural forces with another geotechnical or structural engineer in case of integrating structural forces of surface elements.
Hi Stefanos,
Continuing the discussion I would like to ask you:
1) I was checking the Plaxis3D Structural Verification for Plates and Shells (Plates:https://communities.bentley.com/products/geotech-analysis/w/plaxis-soilvision-wiki/45502/bending-of-plates Shells:https://communities.bentley.com/products/geotech-analysis/w/plaxis-soilvision-wiki/45505/performance-of-shell-elements) and noticed that both verifications consider the plate self-weight as zero. Using the same models I considered the self-weight of the plates and the results were not the same. Using the plate self-weight it was necessary to multiply the max moment by the length of the plate to get the correct value, similar to what you have described. Why this difference? I believe is due to the thickness (d) that I am considering in the plate properties that must be in accordance with the desirable load of the plate. But I would like to confirm this with you.
2) I am modelling a vertical tunnel shaft. I am a little bit confused with the discussion about the integration of the results. It is ALWAYS necessary to multiply the moment results by the plate length (perpendicular to the moment)? Or this is applicable only in cases that I want to simplify and turn out the plate into a line?
My model is illustrated below. This is only one panel of the shaft. This panel is 5m wide and 50m deep. The M11 value that PLAXIS is illustrating is the final value or do I need to multiply it by the length of the panel?
3) For the Plates results PLAXIS gives the M11, M11 min and M11 max. How does PLAXIS calculate the M11?
Thanks!
Hi,
I'm also very interested in the 3rd question that Renato asked about M11 min & max and how M11 is calculated? it doesn't sound like being average of the two. Is it related to thickness of the structure, e.g. showing moment in the inner and upper side of a plate element?
M11 is based on the stresses that result directly from the calculation. M11min and M11max are just the minimum and maximum value of M11 in the current plot.
With kind regards,
Dennis Waterman
Hi Dennis,
thanks for the reply. i modelled a retaining wall as plate in Plaxis 3D and as i can see the plate consists of several elements, each element had 6 nodes assigned to it. & for each node there are M11, M11 min & M11 max (the same results for M22 and other normal and shear forces). What i have difficulty to understand is how a node can have max and min bending moments?
we issued the results to client and they are asking explanation for this and if they should use M11 max or only M11?
Best Regards,
Dear Rafi,
No, each node has only an M11. M11min and M11max are the lowest and highest value of M11 in the plot, over all the nodes.
So if you have a model with 5 nodes with values of M11 of 100,120,90,160 and 110 kNm/m, then M11min = 90 and M11max = 160.