I have separated this topic due to the fact that there are discussions that the load to be used to check drift is by using the seismic load case only and not load combinations involving gravity loads.
Now it may differ from one company to another but for us, and me personally, I include the gravity loads instead of just the lateral load.
Why? The answer is because of Pdelta.
The provisions above states that the displacements for drift should include the displacements due to gravity loads which will not be captured otherwise if we use lateral load only. And they also show the interdependency of the Pdelta and the displacements to the resulting drift values.
And in case you were wondering for the longest time already why do we need to include Pdelta in drift check, it is this: Pdelta analysis, in the simplest term that I can possibly convey is a nonlinear analysis where the additional lateral displacement due to gravity loads AFTER the swaying induced by seismic loading is considered and the answer iterated until it converges, that is the loads and deformations are in equilibrium. Without Pdelta, the sum of displacements is just a simple addition of individual load case (deflection due to dead + deflection due to dead live + deflection due to dead seismic) instead of a more complex Pdelta analysis which is more realistic (go ahead and try to validate this using ETABS!)
So the list of load combinations below by UBC97 and how are they can be derived are as follows:
By the way, the EQ loads above can either be static or dynamic. As a side note, some believe that static earthquake should not be used for checking the drift. Why? Because it is deemed to be unrealistic (using the approximate period and the approximate vertical distribution of story shear) compared to the response spectrum where it is likely to be “next to the real thing”.
Now you may argue that you can omit the others on the basis that the other load combinations are sure to be much more higher than the others. I wont argue. But for me, since it involves seismic loads that can change in direction (load reversals), I don’t think we can really define a maximum value globally such that it will always remain true locally. I don’t want to take the chance of neglecting something which can turn out to be the most critical load combination.
And of course, having discussed the Pdelta above, don’t forget to turn on the Pdelta analysis(!)
And just in case that you were wondering why the reduced dead load multiplier when we can get the maximum reactions and drifts by using the 1.20 load factor, remember that dead loads are stabilizing loads that keep a structure in place. There are some portions in structures which are highly susceptible to high lateral displacement especially the less loaded ones that will sway like crazy during a seismic event. Yes dude, we need to account for that!
The post Load Combinations For Seismic Drift Check appeared first on Civil Engineering Community.
source https://www.civilax.com/load-combinations-for-seismic-drift-check/
