15th July 2012
Building Drifts in ETABS
Building Drift in ETABS Drift is a very complex topic in structural engineering. It involves too many factors to arrive at a suitable decision. It involves engineering judgment, the phenomenon fresh engineers might not feel. 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. Please keep in mind, this article is not about the building drift as far as structural science is concerned, rather this topic of drift is related to ETABS software. First of all you must be familiar with the term story drift. For convenience, I am quoting here the definitions from UBC97 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 Now what for story drift limits? What is the maximum permissible value? Well it depends upon the type of drift. Is it seismic or wind? For seismic, I will refer to UBC97 code which in section 1630.10.2 talks about drift limits for earthquake.
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Now in simple words, the maximum limit for seismic drift is: 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. Now delta M = Max inelastic response displacement = 0.7R delta S where R = from Table 16N 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
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So we can understand that the limit for wind drift is "on the order of l/600 to l/400" for "common usage". This is common thing, however, in reality 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 ht. The concept of drift limits is same throughout all the governing codes, and the typical limits of story height by some number is same, but obviously you have to take care of the process of calculating the wind force or seismic forces. You should not calculate wind force from one code and apply limits of another code. 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 always 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. In seismic drift we do not convert it into service seismic force. Seismic drift is checked against the direct load case of EQx, EQy etc in ETABS. For wind drift, we need to convert 50 year wind to service wind force. It has been recommended by ASCE commentary CC.1.2
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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. So in a nutshell we create following load combinations in ETABS to check our drift: DRIFTWx1 = D+0.5L+0.7Wx
DRIFTWx2 = D+0.5L0.7Wx DRIFTWy1 = D+0.5L+0.7Wy DRIFTWy2 = D+0.5L0.7Wy For seismic drift, as discussed earlier, we do not need any combination, drift will be checked just on EQx and EQy load cases only. 3) How to check in ETABS Now we have obtained both the actual drift and the drift limit, but how can we do this in ETABS easily? Well, after creating the drift combinations as discussed in step 2, we need to do as below: For seismic drift goto File>Print Tables>Summary Report
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Select the file name
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Scroll down to the end of the page, you will find out a section about drifts, similar to this one:
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It displays the max drift for each lateral load case for each story. As we want the drift for wind to be on drift load combinations and not on wind load cases, so we will not compare this wind drift without limits. In this table we are going to check just the drift values of our ETABS model for individual seismic load cases; EQx and EQy. As you noticed, this table shows us values in fraction format. For example 1/105 that becomes 0.009523809524. This 1/105 value is story drift divided by story ht. It means delta S / story ht. Now this value is delta S. First we need to convert it to delta M by multiplying it 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). So 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 0.7x3.5/0.025 = 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 ht is being satisfied. This way you can quickly check and compare seismic drifts.
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Now for the wind drifts, goto Display>Show tables, select Point displacements>Story drifts and then select only drift combinations for results. Click on and then copy the table to EXCEL.
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To save time you can right click on EXCEL taskbar and select maximum and minimum. Then just select the column H or I and see the maximum value that should be less than H/400 to H600 limit (0.0025 t0 0.00167). Again the values reported in ETABS are divided by story ht.
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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 values. 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 for example the story drift table in wind captures 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. ranawaseem.com Posted 15th July 2012 by Rana Waseem Labels: Drift, ETABS Drift Limits, Seismic Drift, Building Drifts, Wind Drift Combination, ETABS how to check drift 13
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wiki December 6, 2012 at 9:21 AM 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 Reply
Rana Waseem
December 6, 2012 at 2:28 PM
Wikki! I didnt understand your point.. According to code, the DRIFT calculated using DELTA m should be less than 0.025 story height...
Reply
wiki December 8, 2012 at 8:44 AM 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" Reply
Rana Waseem
December 8, 2012 at 9:14 AM
Dear wiki! Its mentioned in the referenced code that drift should be calculated using delta M and drift limit using delta M should be less than 0.025...If my understanding to the code is wrong then please explain to me...we should not use delta s...delta s is the value we get from ETABS. We have to make it delta M Reply Replies thanvi February 25, 2013 at 3:26 AM 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.
wiki May 19, 2013 at 9:10 AM 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. Reply
Samer Kamel February 4, 2013 at 10:34 PM 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 Reply
Rana Waseem
February 25, 2013 at 3:43 AM
yes samer kamel, we do use value of delta m while calculating separation gap... separation gap is [ (delta M1)² + (delta M2)² ] ^ 1/2 Reply
Charles Maxino May 28, 2013 at 6:02 PM 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. Reply Replies Rana Waseem
May 28, 2013 at 11:57 PM
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 drift 2) 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.. Reply
Charles Maxino May 28, 2013 at 10:54 PM 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. Reply Replies Rana Waseem
May 28, 2013 at 11:58 PM
yeah the provisions should be followed strictly..good practice actually 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..
Charles Maxino June 1, 2013 at 4:32 AM 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. Reply
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