General
Chapter C of the AISC specification (2005 and later) covers stability analysis and design. This document discusses how the requirements are met in RAM Elements.Analysis and Design Using General Second-Order Elastic Analysis
RAM Elements has the ability to perform a general second-order elastic analysis per AISC 360 C2.1. It is an option in the Process tab – Analyze Model – Analysis tab. This method captures big P Delta effects but will not capture small P delta effects unless the individual members are segmented into smaller pieces in the finite element analysis. For example, imagine a column that is 15’ long. If it is modeled as a 15’ long member, small P delta effects are not captured. If it were modeled as five 3’ segments, small P delta effects are captured because there are additional nodes between the ends of the member. Currently, there is no option to automatically segment the members this way in the finite element model and it must be done manually. Please note, if you segment a member, be sure to specify the proper unbraced lengths in the steel design parameters. The unbraced lengths will always default to the physical length of the member. AISC 360 C2.2a(2) requires an analysis for ASD design to be carried out under 1.6 times the ASD load combinations and the results divided by 1.6 to obtain the required strengths. It is currently not possible to divide the results by 1.6 in RAM Elements. Therefore, you must use LRFD. AISC 360 C2.2a(3) requires notional loads to be considered in the gravity only combinations unless initial out-of-plumbness has been modeled. Currently, notional load cases do not exist in the program as a predefined load type. They need to be manually modeled as unique load cases and manually accounted for in the load combinations. For example, imagine you have a structure with dead load and live load. One would create a notional dead and live load cases in two independent directions. Let’s assume they were called NDX, NDZ, NLX and NLZ. For the typical gravity combinations 1.4 DL and 1.2 DL + 1.6 LL, you would need to create a total of four load combinations to account for the notional loads. 1.4 DL + 1.4 NDX 1.4 DL + 1.4 NDZ 1.2 DL + 1.2 NDX + 1.6 LL + 1.6 NLX 1.2 DL + 1.2 NDZ + 1.6 LL + 1.6 NLZ AISC 360 C2.2a(4) discusses effective lengths. This Wiki page discusses how effective lengths are implemented in RAM Elements. Effective Lengths in RAM Elements
A practical example for the general second-order elastic analysis is discussed here. RAM Elements AISC 360 Stability Example
Other Limitations
It is not possible to run P Delta with dynamic load cases. Second order effects must be accounted for by hand. Currently, the Direct Analysis Method discussed in AISC 360 Appendix 7 is not implemented in RAM Elements V12. Therefore, structures with a second order drift to first order drift ratio exceeding 1.5 cannot be designed in RAM Elements per AISC 360 C2.2.
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