Modeling Concrete Domes and Ring Beams in RISA3D

[mike20793]

Does anyone have any experience modeling concrete domes with ring beams in RISA3D? I'm having a hard time getting the tension force in the ring beam to match the theoretical force based on shell theory. The meridinal forces are reasonably close (within 0.5%) of the theoretical, but my tension forces are way off. I'm pretty sure it has something with the way I'm restraining the roof. Basically, I modeled the circular roof with a concrete member around the outside. I restrained each node of the concrete beam in the vertical direction. I also restrained one node in the global x and z directions and another node I restrained just the z direction, which is tangential at that node. This allows the ring beam to stretch but not twist; but it is restrained against "stretching" at the pinned node. See the attached plan view of my model for clarity. Note that because it is plan view, the vertical restraints are not shown.

 

[mike20793]

Update: The plate elements with out of plane stiffness need to be used, and, if the walls are modeled, the roof needs to be pinned where it meets the walls. After further research, it appears that the theoretical ring beam force is based on the assumption of zero displacement or rotation at the edge of the dome. In reality, there is some rotation and "spreading" of the beam, so a reduction in the theoretical ring beam force is allowed. I used RISA to verify that, with the pinned conditions we mentioned above and plate elements with out of plane stiffness, the calculated force from the direct stiffness method closely matches the reduced force from the flexibility method. It makes sense that plane elements wouldn't capture out of plane effects such as rotation.

Thanks again for the help!