A Note Regarding Shell Force/Stress Contours in ETABS

In viewing the forces in walls and slabs using contours, it is of great help to look at the lower left portion of the screen which shows the maximum and minimum value of forces/stresses.

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In viewing the forces in walls and slabs using contours, it is of great help to look at the lower left portion of the screen which shows the maximum and minimum value of forces/stresses. This is particularly helpful if you’re evaluating the areas that crack under tensile loading which are usually caused by lateral loads.

max min.jpg
Location in the screen of maximum-minimum force/stress in ETABS. Notice that contour averaging at node is “By Objects”

And just in case you are trying to pinpoint the exact location of that force, you have to set the Contour Averaging at Nodes to “None” to identify that specific panel. Because if you set Contour Averaging at Nodes by “By Objects” you will not see graphically where the maximum force/stress occurs. And the reason is because ETABS is averaging the forces/stresses to adjacent panels.

Say for example you’re threshold is 1200 kN and you saw a value of say 1500 kN, don’t panic, yet. That value might just be occurring on a certain panel. After all the real life wall isn’t really made up of separate panels and so averaging is completely justified.

As an example, please refer to the succeeding photos as I emphasize some points.

contour averaging by none
Notice the random-like contours if the averaging is set to “None”.
close up averaging by none
Close up of boxed portion (where the maximum value occurs). It can be seen as we hover the pointer on the edge.
close up averaging by objects
The same boxed portion where the maximum force occurs. Notice that the force is already dispersed and you could hardly see any trace that the maximum value occurs here.

Author: The Romantic Alpha

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