I am trying to understand if RAM Frame takes into account the effect of openings in concrete shear walls on their stiffness when distributing story shears in a rigid diaphragm due to wind loading. To determine this I have modelled a square building, one side of which has shear walls with very large openings and on the opposite side of which it does not have openings. It has 3 floors all with rigid diaphragms. I then evaluate the shear induced on the walls by wind load in the direction of the shear walls with openings and without openings. Unfortunately, I have been alarmed to see that the shear forces are the same on each side... if there are 3 shear walls on 3 sides of the building. However, if I add a 4th shear wall, the walls with openings take on less shear than the walls without openings as expected! It is very strange. Adding another level of confusion is that the Center of Rigidity is very clearly in the right place and is taking into account this change in stiffness for both the 4 walled system and the 3 walled system. But, unless I specify a special load case for the CR, the button does not turn on, which indicates to me that RAM Frame does not compute torsional forces about the center of rigidity. Is it computing them about the COM? Essentially, I have two questions:
1. Does RAM Frame account for the change in stiffnesses due to wall openings when distributing lateral forces due to wind in systems with rigid diaphragms?
2. How does RAM Frame compute torsion for these systems? If not about the COR, why not?
1. Yes, as you can see from the wall mesh, the openings are considered and the walls have a reduced stiffness when there are openings modeled. A wall with openings deflects more than a wall without openings under the same loads.
2. Torsion on the system is a natural product of the finite element analysis (whether the diaphragm is rigid or not). You can add a Center of Rigidity load case and then show the location of the COR, but it's optional to do so. For more see:
A point about your test, if you have a three walled open system like the image below, and assuming the out-of-plane stiffness of the walls is neglected, then there is a single load path for Y direction loads parallel. Consequently all the load goes to that wall regardless of it's stiffness.
Figure 1: 3 walled model, reactions at base
Figure 2: Reactions at base of a 4 walled model
Figure 3: COR in 3 walled model and force case considered
Figure 4: COR and load case considered for 4 walled building
Figure 5: Roof of 3 walled structures with shears in each wall displayed
Thanks for your reply. I now understand that RAM FRAME should be taking into account the change in stiffness per its documentation. However, the results from my experimental model are still very confusing and contradictory. My models (3 walled and 4 walled) are shown in Figures 1 and 2 with their respective Ry reactions at the base for the Y-load case. The walls with openings are along line 1 and should take on lower shear than that of the solid walls on line 4. However, surprisingly, in Figure 1 the walls with openings take on more load than the walls without openings. In Figure 2, with a 4th wall, for some reason the model behaves as expected and the solid walls do indeed take on more load than the walls with openings.
Figure 5 shows the resulting shears in the shear walls at the roof due to a Y-dir wind load case. It is very concerning to me as it appears that RAM Frame simply split the diaphragm load up equally among the 2 y-dir. lateral resisting systems and did not account for their differences in stiffness.
My initial thought was that shear due to torsion may have induced the perfect amount of shear to make the resulting total shears equally on both sides. However, I have tried changing building dimensions and with 3 walls, the amount of shear induced still remains the same on both sides. Additionally, Figures 3 and 4 show that the COR x coordinate remains very similar for both cases.
Another odd note is that both models have higher deflections in the shear walls with openings than the shear walls without openings, which indicates to me that RAM frame is appropriately lowering the stiffness of the wall panels with openings.
I think you are on the right track with your comment about torsion, but think of it as an equilibrium check for moments about the z axis from the middle where the forces is applied like middle like so:
As soon as you introduce a fourth wall the assumption that Fy=0 is no longer true. Another way to put it is like so - the 3 walled system has nearly zero resistance to torsion, any amount of applied torsion would result in huge diaphragm rotations.
Answer Verified By: Gwen Carris