MattJM
Structural
- Aug 28, 2007
- 28
I have a colleague working on a small building where the architect has raised/stepped half of one of the floors by about 2.5 feet. This has created (2) diaphragms where there was one. The engineer decided to model the structure in RAM with an additional storey 2.5 feet above the original level. The location where the two diaphragms diverge/split is along a braced frame line.
Suddenly she was getting P-Delta issues. My assessment was that:
A) Since there actually are (2) diaphragms, and each one needs to deliver its forces to an appropriate LFRS. That is, we needed to stiffen the structure at one diaphragm which originally had few frames. This is a basic engineering issue I think we solved.
B) The P-Delta analysis errors (which force the analysis to stop if the p-delta option is chosen) arise from the fact that the diaphragms are delivering shears and associated deflections across a small height difference of column at the braced frame line. These effects are REAL and MUST be dealt with by appropriate stiffness or load transfer to limit the deltas in displacements.
C) All the other stability factors calculated for the higher diaphragm, EXCEPT at the interface of the diaphragms, are calculated based on the artificially low storey height. I'd like to calculate these using the true storey height at this half of the building. Q: Is this appropriate, and or possible in RAM SS?
D) As a final note, I thought we might deal with this by adjusting the top of column elevation for the higher part but retaining the same story designation, and deal with the artificially sloping members at the interface. We would still need to model a diaphragm split here, and still need the original model to capture the effects in (B) above. Is that heading in the right direction?
Thanks.
Suddenly she was getting P-Delta issues. My assessment was that:
A) Since there actually are (2) diaphragms, and each one needs to deliver its forces to an appropriate LFRS. That is, we needed to stiffen the structure at one diaphragm which originally had few frames. This is a basic engineering issue I think we solved.
B) The P-Delta analysis errors (which force the analysis to stop if the p-delta option is chosen) arise from the fact that the diaphragms are delivering shears and associated deflections across a small height difference of column at the braced frame line. These effects are REAL and MUST be dealt with by appropriate stiffness or load transfer to limit the deltas in displacements.
C) All the other stability factors calculated for the higher diaphragm, EXCEPT at the interface of the diaphragms, are calculated based on the artificially low storey height. I'd like to calculate these using the true storey height at this half of the building. Q: Is this appropriate, and or possible in RAM SS?
D) As a final note, I thought we might deal with this by adjusting the top of column elevation for the higher part but retaining the same story designation, and deal with the artificially sloping members at the interface. We would still need to model a diaphragm split here, and still need the original model to capture the effects in (B) above. Is that heading in the right direction?
Thanks.
