In this article, I have tried to explain what is building drift, allowable limits, ways and means to check in ETABS models and to control the excessive drift.
First of all you must be familiar with the term story drift. For convenience, I am quoting here the definitions from UBC-97 code:-
STORY DRIFT is the lateral displacement of one level relative
to the level above or below.
STORY DRIFT RATIO is the story drift divided by the story
1) Maximum Limits
For seismic, I will refer to UBC-97 code which in section 1630.10.2 talks about drift limits for earthquake.
delta M shall not exceed 0.025 x story ht (if building seismic period is less than 0.7)
delta M shall not exceed 0.020 x story ht (if building seismic period is equal or greater than 0.7)
Important to note here is that it talks about SEISMIC drift so SEISMIC building period not the WIND period.
delta M = Max inelastic response displacement = 0.7R delta S
where R = from Table 16-N
delta S = displacement from static, elastic analysis
this value is read from ETABS.
you multiply this value by 0.7R to get delta M
This was all about seismic drift, but for wind drift code is mute. I will refer you to ASCE 2005 commentary CC.1.2
So we can understand that the limit for wind drift is “on the order of l/600 to l/400” for “common usage”. This figure can be up or down depending upon the ductility of cladding material and finishes. However for common usage value of l/400 is thought to be well satisfactory. Here l means story height.
2) Load Combinations
Once the drift limit has been determined separately for seismic and wind forces, now is the need to check the actual drift vs the limit. Determination of actual drift depends on the load combination and the period of recurrence. If not properly calculated, this may dramatically increase or decrease the accepted drift values in model.
Seismic force E is already factored so that’s the reason its factor is always 1.0 in load combinations of ACI/ASCE code. The recurrence period for seismic force is 50 years. Seismic drift is checked for seismic strength load combinations.
For wind drift, we need to convert 50 year wind to service wind force. It has been recommended by ASCE commentary CC.1.2
To convert 50 year service wind force to 10 year service wind force it is multiplied by 0.7, as the equation says, and other gravity loads; D and 0.5L are also added.
3) How to check in ETABS
For seismic drift go to File>Print Tables>Summary Report
Select the file name
Scroll down till the end of page, you will find out a section about drifts;
It displays max drifts for each lateral load case (not for the combinations), for each story. However, for simplicity, let’s proceed with drift from lateral load cases.
As you noticed, this table shows us values in fraction format. For example 1/105 that becomes 0.00952. This 1/105 value is story drift divided by story height. It means delta S / story height.
Now this value is delta S. First we need to convert it to delta M by multiplying with 0.7R. Assume R here is 3.5 so
delta M = 0.7 x 3.5 x 1/105 = 7/300 = 0.023333 which is less than 0.025 so safe ( if T<0.7).
Instead of calculating every time by 0.7R we can check these limits in other way. If our limit is 0.025 then the limit we get is 0.025/R/0.7. Assume R=3.5. Now the values in ETABS are inverse so our limit is 0.7×3.5/0.025 = 98.
In ETABS the drift is reported as 1/x where x is some number. Now as long as x (some number) is greater than 98 our limit of 0.025 x story height is being satisfied. This way you can quickly check and compare seismic drifts.
To check drift on load combinations, export drift tables to EXCEL to check maximum/minimum drift values.
4) Controlling Excessive Drift Values
Sometimes you may face problem of excessively large values in drift tables in ETABS. Well we are not going to talk about different measures and modeling techniques to control the drift. We are going to talk about large numbers in drift tables. Sometimes it happens that a point or node is free in the model or is connected to a NULL line or very flexible section. Drift tables capture the maximum displaced points. Obviously the displacement of several meters in tables is not what we are looking for. Drift values (relative) may be still okay for these points, but it requires you to check the displacement values too before checking directly the drift. Unlock the model and remove all free points, check for any discontinuity and modify your models to remove all the errors.