Good day,
I have completed a model in STAAD Pro utilising the floor diaphragm command to account for the effects of eccentricity, but there appears to be an anomaly in the centre of rigidity report. All the nodes in my model have been placed in the positive (+ve) x and z axes but my centre of rigidly is reported as being in the negative (-ve) z axis. I have also observed that the value being reported is different for the ground and first floor although the structural systems are the same for both levels, which is incorrect. Can anyone provide some assistance for this issue. I have attached the screenshots as well as the file to illustrate this query.
Model 1.STD
Regards,
Michael
This might help. It was written for Ram Frame but the principles are the same. https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/8604/ram-frame---center-of-rigidity
Hi Seth, thanks for your reply. However, I have used this command in previous models without these strange observations. I also tried placing the command after different load cases and the results remain the same. If someone could edit the model I believe that would assist greatly.
For a structure like this, if we consider the walls to have in-plane stiffness only, then the theoretical Center of Rigidity is at the projected intersection of the skewed wall ant the N-S wall, i.e. way up to the north. Load applied to the structure in the east direction anywhere south of that hinge point will always result in some counter-clockwise rotation.
The real behavior is a bit more complicated since some of the walls intersect and there is some out of plane stiffness in the walls since they are two-stories tall, but I do expect a north-shifted (-z) CR as you have.
Hi Seth, I believe I understand your stand point in relation to the software's analysis process,however, I don't think a purely theoretical approximation would result in the centre of rigidity (CR) being outside of the floor diaphragm for this case. This would also imply that the walls along the x-axis would have a greater impact on the z-axis component of the CR than the walls along the z-axis, which is unlikely even considering the wall intersections and non-parallel wall. Can you provide any additional links as to how STAAD Pro or RAM calculates the CR to help me understand this?
My advice is to apply a lateral load through the CR and witness the diaphragm deflections, The structure might deflect in both directions, but it should not rotate. Also keep in mind the CR location is not something directly used in any kind of code check, I consider it to be just useful information.