How to Check Seismic Drift In ETABS

For this post, let us assume first the following to be true:

We know the reason why we are checking drift (excluding the fact that it is required by all relevant codes)
We have basic background regarding Pdelta moments
We know how to compute static base shear based on the code and understand at least the basics on the dynamic forces our building is subjected into
We know what’s seismic mass
Checked all of the above? Cool.

Now let’s say the model is complete: we have applied all related gravity loads, defined seismic parameters well, and we’re now able to set up all relevant load combinations.

Before we proceed, we need to apply first the correct property modifiers as per code. A quick explanation of this is that before being subjected to seismic forces, under gravity loads, the beams, columns and walls will experience cracking. And cracking will reduce the moments of inertia of the members.

So which property modifiers are we going to apply in the model?

Based on ACI 8.8.2 we need to consider either/or of the following: section properties defined in section 10.10.4.1(a) through (c) or half of stiffness values based on gross section properties,

The code permits to use either one of 8.8.2 (a) or 8.8.2 (b). If you’re to ask me, it never hurts to check both conditions considering that at least you have prior knowledge to the behavior of the 2 different models with different reduced stiffnesses which is very valuable when making a decision when evaluating the current structural system’s performance during seismic activity. By experience however, the resulting drift from either is never far from the other but it is worth checking and noting the difference.

Also, if you have some torsional irregularities, you will need to settle them now and provide the adjusted eccentricities prior to checking the drift. Remember that the larger the eccentricity, the larger the forces on the extremities which subsequently increases the drift.

So both models ready now?

Let’s use UBC97 which is still applicable to a lot of projects even in the middle east.

The elastic deformation that UBC 1630.9.1 refers to as “Delta s” is the elastic horizontal displacement caused by the seismic shear and when checking drift, this should include the effects of gravity loads. By the way, use the ultimate load combinations when determining drift.

But of course we need to convert this elastic horizontal displacement to the maximum inelastic response displacement defined as Delta m before we compare it with the allowable values of either 0.025 or 0.02 times the story height depending on the fundamental period.

And since we’re talking about drift, note that drift is a unitless quantity taken from dividing the relative displacement of a certain story with the story below to the storey height.

Still with me? Great.

The good news is, ETABS will compute these displacements for us including the drift which we can just extract from the tables. The computed drifts will include both drifts in the orthogonal axes X and Y. You just need to go the tables and find the Story Drift data and open it with Excel.

We just need to add a few items to the extracted table to arrive at the correct values in order to interpret the results correctly. Don’t worry though, I will try my best to discuss the table below:

Screen grabbed based on the outputs of an actual project of mine.

Columns A-H are what you will get from ETABS.
Column A is the story level where the drift occurs.
Column B is the load combination. Be sure to choose the correct load combinations.
Drifts will be resolved in the 2 principal directions in ETABS which are the global X and Y in column C.
Column D is the resulting drift which is the Delta s defined above.
Columns E to H are the point label and the point coordinates where we can locate the aforementioned drifts.
Column I is optional. Column I is just the same as Column D.
Column J is the maximum inelastic response displacement or the Delta M which in UBC is 0.7*R*Delta M, R is the “numerical coefficient of the inherent overstrength and global ductility capacity of lateral-force-resisting systems” and is 5.5 in our case (Building frame system with concrete shear walls)
Column K is the allowable limit of drift. Since our period is less than 0.7 sec, the maximum allowable is 0.025
Column L is also optional. This just compares the results of column J and column K. But this is helpful in filtering the floors where the drift failed.
Quite easy right? Don’t worry if you are not yet confident of the results. That’s what more senior engineers are for, which is to make sure that the model makes sense. But since you now know how to compute the drift yourself, you are on your way to seniorhood so just keep at it.