STAAD Pro Questions - Plate Shear Force and Stress Output
STAAD Pro Questions - Plate Shear Force and Stress Output
(OP)
Hello All,
First time posting on Eng-Tips, but I've been using the forum as a resource for many years. I am working on a fairly complicated finite element model using quadratic and triangular plate elements. I am at a point where I am comfortable with the model results given the geometry and loading and am now evaluating the results. I have somewhat "subdivided" my structures into separate walls and slabs and am designing the structural elements according to worst-cast reactions. I have a two-part question, the first being technical and the second being more output based:
1) Since most of my structural elements are two-way spanning, I am grabbing the worst-case Mx and My values and using them to design the structures outside of STAAD. Where I am getting hung up is designing for shear. I've done research on this area, but am still unclear about how to utilize results. The way I have typically derived shear force is to identify max SQY and SQX values x plate width/length (in direction of shear) x thickness of plate. I understand the theory of FEM in regards to force matrices, but am not a master by any means. I wanted to do a "dummy" experiment, to see how the method I described works if you vary slab thickness of the particular member I'm designing. My initial thought was that the shear force computed should be consistent for each slab thickness. In my dummy experiment, I varied the slab thickness as follows: 3', 2', 1'. What I found was that the shear force was highest for the 3' slab (19.2 kip), similar for the 2' slab (18.1 kip), and much lower for the 1' slab (12.8 kip). In my opinion, I think that the method I'm using is correct. My theory on why the shear force drops is that the model I have is three-dimensional and I am dropping the stiffness of a certain member, thus now distributing forces to the stiffer elements around it. Can someone confirm that my approach to deriving shear forces is correct? Is there a more straight-forward method for grabbing shear forces from STAAD? I've read recommendations for "Print Element Forces" and "Results Along Line", but these methods don't seem as intuitive.
2) Please see the attached, which shows the SQY plate stress distribution in one of the walls I'm designing. The way I isolated the wall was to do "View Selected" in order to get a graphical output of the results. For this load case, I found that the max SQY was 154 psi. But, the table in the upper left-hand corner is telling me that the max shear stresses are much higher. What I believe is happening is that even though I isolated this wall, STAAD is showing the max stresses elsewhere in the model. Is there any way to correct this?
Thanks much!
BLoEngineer
First time posting on Eng-Tips, but I've been using the forum as a resource for many years. I am working on a fairly complicated finite element model using quadratic and triangular plate elements. I am at a point where I am comfortable with the model results given the geometry and loading and am now evaluating the results. I have somewhat "subdivided" my structures into separate walls and slabs and am designing the structural elements according to worst-cast reactions. I have a two-part question, the first being technical and the second being more output based:
1) Since most of my structural elements are two-way spanning, I am grabbing the worst-case Mx and My values and using them to design the structures outside of STAAD. Where I am getting hung up is designing for shear. I've done research on this area, but am still unclear about how to utilize results. The way I have typically derived shear force is to identify max SQY and SQX values x plate width/length (in direction of shear) x thickness of plate. I understand the theory of FEM in regards to force matrices, but am not a master by any means. I wanted to do a "dummy" experiment, to see how the method I described works if you vary slab thickness of the particular member I'm designing. My initial thought was that the shear force computed should be consistent for each slab thickness. In my dummy experiment, I varied the slab thickness as follows: 3', 2', 1'. What I found was that the shear force was highest for the 3' slab (19.2 kip), similar for the 2' slab (18.1 kip), and much lower for the 1' slab (12.8 kip). In my opinion, I think that the method I'm using is correct. My theory on why the shear force drops is that the model I have is three-dimensional and I am dropping the stiffness of a certain member, thus now distributing forces to the stiffer elements around it. Can someone confirm that my approach to deriving shear forces is correct? Is there a more straight-forward method for grabbing shear forces from STAAD? I've read recommendations for "Print Element Forces" and "Results Along Line", but these methods don't seem as intuitive.
2) Please see the attached, which shows the SQY plate stress distribution in one of the walls I'm designing. The way I isolated the wall was to do "View Selected" in order to get a graphical output of the results. For this load case, I found that the max SQY was 154 psi. But, the table in the upper left-hand corner is telling me that the max shear stresses are much higher. What I believe is happening is that even though I isolated this wall, STAAD is showing the max stresses elsewhere in the model. Is there any way to correct this?
Thanks much!
BLoEngineer






RE: STAAD Pro Questions - Plate Shear Force and Stress Output
Also with regard to your moment design.....do not forget to include the Mxy (i.e. the twisting moment) in your computations. Most of the time I just combine the absolute values of Mx & Mxy and My & Mxy to get my design values.
RE: STAAD Pro Questions - Plate Shear Force and Stress Output
The shear stress I'm seeing does increase as the thickness decreases. This makes sense because you end up multiplying the shear stress by the slab thickness to get shear force. My point was that I saw a sharp decrease in shear force as the thickness decreased, signifying to me that the stiffness reduction in the slab was also dropping the shear.
I have read a couple forums that recommend adding the twisting moment component. Why is this? If I were doing a hand calculation on a two-way slab, I would not add any type of twisting component. I'm interested in hearing the reasoning for adding this additional contribution in.
RE: STAAD Pro Questions - Plate Shear Force and Stress Output
RE: STAAD Pro Questions - Plate Shear Force and Stress Output
You can find other useful material online.
There is usually not much you can do with STAAD's graphic interface. I am afraid you cannot do much with the second problem
RE: STAAD Pro Questions - Plate Shear Force and Stress Output
On the 2nd item regarding STAAD's visual output, I did find a Bentley forum that explains how to view stress contours for a selected view in a larger model. What you do is select all the plates in a given element that you want to isolate (a wall, for example), right click and select "New View" and select "Display View in Active Window". The stress contours will adjust to the selected plates. To get back to the global model, go to the View Tab at the top and select "Whole Structure".