That topic keeps coming up from engineers who are starting to use #3Dseismicanalysis for buildings. Inevitably, a student asked me that very question just a couple of weeks agoโฆ
FEA users are always encouraged to first carry out a simple #handcalculation before running a complex 3D analysis. One should always have beforehand a rough idea of the results that the 3D model should produce. And, most importantly, when there are differences, ๐จ๐ง๐ ๐ฌ๐ก๐จ๐ฎ๐ฅ๐ ๐๐ ๐๐๐ฅ๐ ๐ญ๐จ ๐๐ฑ๐ฉ๐ฅ๐๐ข๐ง ๐ญ๐ก๐๐ฆ.
There are three main causes of differences in results between a hand calculation using #EquivalentLateralForces (#ELF) and a 3D spectral analysis using the #ResponseSpectrumMethod (#RSM).
๐ First, the mass considered for the calculation of the seismic force is ๐ง๐จ๐ญ ๐ญ๐ก๐ ๐ฌ๐๐ฆ๐ in the two methods. In ELF, the #totalmass of the structure is considered. In RSM, the #effectivemass is considered separately for each significant mode, and results are then combined using, for instance, the SRSS method. Consequently, the resulting total seismic force is usually lower for RSM than for ELF.
๐ Second, the fundamental period may be different depending on the model that is used, which can lead to different values of spectral acceleration. Moreover, in RSM, each mode may have a ๐๐ข๐๐๐๐ซ๐๐ง๐ญ ๐ฏ๐๐ฅ๐ฎ๐ ๐จ๐ ๐ฌ๐ฉ๐๐๐ญ๐ซ๐๐ฅ ๐๐๐๐๐ฅ๐๐ซ๐๐ญ๐ข๐จ๐ง. Please note that this point can either increase or decrease the overall effect of the seismic action.
๐ Last but not least, the typical approach for hand calculation of shear resisting systems consisting of shear walls neglects #frameeffects. In contrast, 3D models consider these effects unless specific measures are taken. As a result, the hand calculation produces only shear forces and bending moments in the shear walls; however, in the 3D analysis, #axialforces also typically appear. Consequently, a significant portion of the overall seismic overturning moment can be supported by the lever arm of these axial forces. Additionally, non shear-resisting supporting members can contribute in a similar manner
๐ Each of the three causes above can lead to significant discrepancies when comparing simplified ELF and 3D RSM approaches. The question for each of them is then if it is an effect that ๐ข๐ฌ ๐๐๐ฌ๐ข๐ซ๐๐ ๐จ๐ซ ๐ง๐จ๐ญ. If it is desired, then the difference must be accepted or, if possible, the simplified approach must be refined to better validate the 3D analysis. If it is not desired, then the 3D model must be adjusted. Please note: the #validation of the 3D model and its adjustment is a step that must be carried out for any complex analysis โ seismic or not.
