Hello,
1) If after P-delta analysis i got an output that the problem is divergence for a particular load case, does this mean the structure has collapsed. And if so, do I need to resize the members or remodel the whole or part of the structure to make it more stiff?
2) If i combine two analysis, say: Buckling analysis and P-delta analysis with CHANGE COMMAND in-between them, the results of displacements, Moments, that will be displayed in the post-processing belongs to which of the analysis?
3) If bucking analysis shows that the buckling factor for the first mode is less than 1, then i move on to perform p-delta analysis and the analysis converges, does this mean if i proceed to design with the moments and forces in the post-processing i have done the right thing as long as those members pass? Does this mean the p-delta analysis is to give us much more ideal results (forces and moments) for a structure which is closer to buckling so that we can design it with these results rather than the results of linear analysis?
Thanks
Here is my response-
Answer to Q-1. This indicates that the stiffness of the structure should be increased. Yes, you need to increase the section size.
Answer to Q-2. You need to create two seperate files for these two type of analysis. Say, after Load case 1 you have buckling analysis command followed by a change command and after load case 2, you have p-dealta analysis command. Results of load case 1 will be buckling analysis and 2 will be of P-delata analysis. Please note that if you perform a buckling analysis,the post processing results are not meaningful rather you should look into the buckling factors and buckling mode shapes.
Answer to Q-3. If you get buckling factor < 1, this indicates that the loads are higher than that f buckling loads for the structur. In this case, you need to increase stiffnss of the stuctue before you go for 2nd order analysis.
Thanks for the reply
1) Since you said the results in postprocessing are not useful for buckling analysis, is this the same for P-delta analysis?
2) If after getting buckling factor to be less than one and then I increase the size of the member or stiffen the structure by modifying its geometry resulting in getting a buckling factor greater than one, why should I perform a p-delta analysis for the structure and not normal linear analysis since the buckling analysis results shows the structure will not buckle?
3) If i don't assign analysis command to individual load case, but after defining and assigning all load cases then I add a combination of buckling analysis and P-delta analysis, which of these two analysis results will be displayed in the post-processing tables?
Or are results from such simultaneous analysis not reliable and therefore not encouraged ?
Answer to Q-1. P-delta post-processing results are meaningful and it considers 2nd order anlysis effect.
Answer to Q-2. Please note that P-delta analysis needs to be perforned to capture 2nd order effect for combined vertical and horizonal loading.
Answer to Q-3. In such a case, whichever is the 1st type of analysis command, the program will honour that. As I mentioned earlier, buckling analyss is performed to check if the stucture can withstand the loading applied to be without buckling. On the other hand, for the design purpose, P-delta analysis needs to be carried out.