Building Drifts in ETABS

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
height.
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.

27 comments

  1. salam sir in MAXIMUM LIMITS portion should DeltaM be less than .025 times story height or Dela s be less than 0.025 times story height?as in the refrence you qouted above says that story drift using DeltaM should be less than .025 times s.height.it is not the Delta M limit sir

  2. drift is delta s and limit of drift is .025.it is not the limit of delta m as u used in above calculations "delta M = 0.7 x 3.5 x 1/105 = 7/300 = 0.023333 which is less than 0.025 so safe"

  3. when calculating the distance of separation between the buildings according to UBC 1633.2.11 do we use also delta M because it gives a very large values. for ex. it give me a 20 cm between the building which is not logic

  4. Greetings Mr. Waseem! Your understanding of calculating Delta M and its limit of 0.025 for T < 0.7 is correct and Wiki is wrong. it is evident from the code as well.

  5. thanx sir rana i again read the article got what i missed,mr.thanvi i am student and trying to get the concept and also trying to clear my confusion i did not said that sir rana is wrong.

  6. wonderful article, i have some question to asks…1) is the drift per level reported by etabs already the interstory drift or is the drift relative to the base? If it is relative to the base then we still have to subtract the two adjacent drifts per floor in order to get the interstory drift. 2) Is the drift reported by etabs is the displacement at the center of mass or it is the max drift of a particular floor considering node displacement. I asked this because the specific requirement of the calculation of seismic drift as per ASCE-05 is that the drift to be considered should be at the center of mass. I have been working with Midas Gen and this drift calculation is automatic you just have to set the limits for both seismic and wind drifts and the program will just automatically calculate the drift requirement and it will show a remark of ok and fail and you also have the option to calculate the drift at the center of mass or the maximum displacement that will normally occur around the perimeter node.

  7. In my office we strictly follow all the code provisions specially the requirements for drift. For seismic drift we are not using only EQX and EQY but instead we used EQX(or EQY)+5%Ecc*Ax. The codes UBC 97 and ASCE require to include not only the translational displacement but also torsional displacement. So the correct load to check for the drift must be the static seismic load plus the 5% accidental torsion multiplied by the torsional amplification factor Ax.

  8. 1) it depends in ETABS output..you can either select total displacement relative to base and subtract the displacements for the lower stories to get inter-story drift2) again refer to step 1…all output options are there in ETABS . you can select whatever you want to report…3) like midas gen now in new etabs 2013 there is better option to control and report drifts..

  9. yeah the provisions should be followed strictly..good practiceactually in ETABS when defining load cases…5% Ecc is already included so you dont have to include it again. For irregularities and torsional amplification..yes you have to check it manually as ETABS is just an anlysis package..rest is upon engineer..

  10. There are some functionalities in Midas Gen which are not present yet in ETABS but I don't know about the latest version of ETABS which is version 2013. One of them is the automatic calculation of torsional amplification factor Ax aside from the automatic calculation of vertical and horizontal irregularities. The one function for us that is very essential is the automatic calculation of story shear force ratio I don't know if ETABS has this function already.

  11. Greetings Mr. Waseem. I got several question regarding ETABS modeling. 1) For the wind drift H/400, that H represent building height or each story height? In my ETABS modeling, the floor Height is 3.2 m. and the result of diaphragm drift for wind load is 2.35×10^-3. is it ok? 2)Beside seismic and wind drifts, what is other element to be consider to get safe design when using ETABS?

  12. ENG. Amin, referring to Eng. Waseem, explanation point no-3, the Values displayed in ETAB is Displacement/ Story Height. So, if you multiply the this ratio, with the story height in mm. You will get the actual Deflection. and then you compare the same with the Wind load drift limit specified in code as well as mentioned in above discussions, Story height / 400. If will do the drift check for wind induced drift.

  13. I have read that doc but i have a question regarding displacement and drift…… displacement is the movement of point relative to the base but drift is the relative displacement of the stories,,…. so drift and displacement are two different things… in UBC limit is for drift not for displacement… that is 0.025 X story height. how they put 0.025 equals to delta M…………. kindly elaborate this …..

    • because ETABS already gives the result which is dived by the height, so no need to multiply in the allowable drift equation…

      If you want Delta S from ETABS, then multiply height of Storey with Delta S given by ETABS and in this case you will multiply allowable drift equation with storey height. its same thing because you are comparing two things and if you multiply both with the same number (i-e Storey height) so no difference will occur

      • Dear Azeem,
        Yes you are right, its an old article and i did not update since long. Yes, drift should be checked on all load combinations including E.

  14. This article is very useful. I just want to ask that when we are calculating seismic drift then we use stiffness modifiers as per code but for wind drift which is calculated in service loads we do not require to use stiffness modifiers. Please confirm. Thanks

  15. “For seismic drift, as discussed earlier, we do not need any combination, drift will be checked just on EQx and EQy load cases only”

    From UBC 1997, it is evident that elastic displacement (Delta S) is computed on Load Combinations of Section 1612.2 not load cases… Please refer the following words from code

    1630.9.1 Determination of Delta S:- A static, elastic analysis of the lateral force-resisting system shall be prepared using the design seismic forces from Section 1630.2.1. Alternatively, dynamic analysis may be performed in accordance with Section 1631. Where Allowable Stress Design is used and where drift is being computed, the load combinations of Section 1612.2 shall be used. The mathematical model shall comply with Section 1630.1.2. The resulting deformations, denoted as 􀀀S, shall be determined at all critical locations in the structure. Calculated drift shall include translational and torsional deflections.

    Kindly correct me if I am wrong

  16. 1) Load combination for Wind drift is:
    D+0.5L+0.7W
    W is based on 50-year wind.

    2) Load combination for Seismic drift is:
    D+E
    E by using torsional amplification, vertical component & orthogonal effects.

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