I have not been modelling extensively in ETABS this recently so this query from a former 後輩 (こうはい) had my technical engine humming once more.
She sent me the table below of the modal participating mass ratios extracted from ETABS and asked what’s wrong with her model. Now let’s elaborate what’s wrong, the seemingly wrong and the nothing wrong.
#1 – Probably nothing wrong here but a lot of old timers like me would say that the first mode torsion is out of place. Ideally, the first two modes should be translation. Torsion should only come third or later.
#2 – The second mode is the translational vibration of the structure along X. Why X? The maximum percentage along the UX column 73.03%, is directed along X hence the period along X should be the period at mode 2 which is 0.885 sec. Notice also that it’s corresponding UY is almost 0 which means model rotation or application of an angle to the spectral forces is not required.
#3 – The third mode is the translational vibration of the structure along Y. You can follow the same logic as specified on #2. Period along Y = 0.756 sec. Compared to 0.0001 UX, we can assume that it is predominantly directed along Y.
#4 – Why did I highlight this? UBC 1631.5.2 states that 90% of the mass participation should be reached. In the table above, neither UX nor UY achieved this. So more modes are required to reach it? It’s either yes or no. Yes because you need to add more modes until you reach 90%. No because as you have observed, the sum is fixed from mode 3 which should tell you that something’s fishy and incorrect.
#5 – The same reason why mass participations did not reach 90% is because there is no mass added, it’s zero. This should raise a red flag now because there is nothing right or normal about it (read: something’s wrong with the model!)
There you have it. If it’s your first time to see this table or you’re not yet able to tell something about your ETABS model just by looking at this, don’t worry. You just have to do this very often and have a senior engineer who can assist you so that you’ll grow your mental structural muscle.
By the way, this is one of the sensibility checks in ETABS involving dynamic analysis, hence this is something fundamental that you must commit to heart and memory.
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