Building Drifts In Etabs

[WR1]

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 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
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 UBC-97 code which in section 1630.10.2 talks about drift limits for earthquake.

 

 

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 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 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

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+0.5L-0.7Wx

DRIFTWy1 = D+0.5L+0.7Wy

DRIFTWy2 = D+0.5L-0.7Wy

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

Select the file name

Scroll down to the end of the page, you will find out a section about drifts, similar to this one:-

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.

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.

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.

http://4.bp.blogspot.com/-9qv8XKHgL8Q/UALNKflmVsI/AAAAAAAAAEQ/AwKBYWt2iys/s320/image022-773193.jpg

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.

 

 

[Ayesha]

Nicely explained.

[WR1]

thanks

[UmarMakhzumi]

Good article; I will throw my two cents.

Seismic drift values are much larger than wind values. UBC uses maximum inelastic response displacements rather than the design level displacements to verify the performance of the building. As stated above, the seismic drift limits are 2% & 2.5% of the story height for long and short -period buildings. So, for a floor to floor height of 12 feet the max. allowable inelastic drift value would be 2% of 12 feet= 0.02*12*12inches=2.88 inches. For wind for a 12 story height, drift would be L/400=12*12/400 =0.36 inches, A comparison of both wind and seismic drift limits shows that earthquake inelastic displacements are quiet large compared to wind displacements. That is why proper detailing is emphasied in seismic design.

Moreover, when calculating ΔS for seismic, make sure:

• you have included accidental torsion in your analysis.
• use strength design load combinations: 1.2D + 1.0E + 0.5L & 0.9D + 1.0E
• You are using cracked section properties for reinforced concrete buildings. Typical values are I_cr walls= 0.5E_cI_g, Beams = 0.5E_cI _g & for Columns 0.5 - 0.7 E_cI_g.

[EngrUzair]

Aoa.

 

Good work, moderators.

 

Rana!

 

Probably, link to original website requires updation too.

 

BTW... When is your book completing?

[WR1]

lol...not even started...but...that does not mean it wont!

[khalid]

Very well explained, looking forward more from you.

 

Thank you so much!

[RASHEED]

Thanks rana waseem bhai for nicely explaining the topic and it has cleared all my doubts about drifts.

 

thanks again..

[Waqas Haider]

Till today i have been checking drift only for top story roof. I use to check the displacement of maximum displaced point of the structure at top roof and use to take it Delta.s.... Then i convert it into Delta.M by  0.7xRxDel.S...... and then compare it with Building height from base to top roof x 0.025 or 0.02 depending on T.... This what i was told by my boss at very 1st job. Since then i tried to find out through UBC but could not understand and assumed the same. But i think it should not make much difference to check interstory drift by 0.025 story ht or drift of top with respect to base and 0.025 building height from base.... Whats ur opinion?

[Ayesha]

Stay with what code says..

[Hussain Abid]

bundles of thanx for this DRIFT article beautifully explained

 

[WR1]

On 10/6/2015 at 11:38 PM, Engr Waqas said:

Till today i have been checking drift only for top story roof. I use to check the displacement of maximum displaced point of the structure at top roof and use to take it Delta.s.... Then i convert it into Delta.M by  0.7xRxDel.S...... and then compare it with Building height from base to top roof x 0.025 or 0.02 depending on T.... This what i was told by my boss at very 1st job. Since then i tried to find out through UBC but could not understand and assumed the same. But i think it should not make much difference to check interstory drift by 0.025 story ht or drift of top with respect to base and 0.025 building height from base.... Whats ur opinion?

Inter-story drift limitation is intended to limit the damage. Abs disp of max point of upper stories wrt to base is to check for seismic separation. Both checks are must.

[Engr Sami]

very well explained bro...!!!

jazakallah...

[Waqas Haider]

On 5/28/2013 at 9:05 PM, UmarMakhzumi said:

use strength design load combinations: 1.2D + 1.0E + 0.5L & 0.9D + 1.0E

Kindly explain for what purpose? Because for seismic drift, @Rana Waseem sir has explained only seismic CASE should be used. instead of Ex or Ey with any combination.

[UmarMakhzumi]

"Ever since strength-level (as opposed to service-level) design earthquake forces were introduced in the 1997 Uniform Building Code (UBC), as indicated by a load factor of 1.0 on E in strength design load combinations, it has been required that drift computations be done directly under those strength-level forces. The drift limits were adjusted accordingly. You never reduce the strength-level design earthquake forces to service-level forces for the purposes of drift computation. This is true of the 1997 UBC, all editions of the IBC, and all editions of ASCE 7 since 1993." 

My original post is referring to use strength level load combos in the light of justification provided. In addition to that, using dead and live loads with earthquake is a good idea as they cover any lateral deflection due to gravity load (mostly observed in non-symmetric frames) which wouldn't be accounted if only earthquake load case is used. 

Thanks.

 

[Waqas Haider]

45 minutes ago, UmarMakhzumi said:

"Ever since strength-level (as opposed to service-level) design earthquake forces were introduced in the 1997 Uniform Building Code (UBC), as indicated by a load factor of 1.0 on E in strength design load combinations, it has been required that drift computations be done directly under those strength-level forces. The drift limits were adjusted accordingly. You never reduce the strength-level design earthquake forces to service-level forces for the purposes of drift computation. This is true of the 1997 UBC, all editions of the IBC, and all editions of ASCE 7 since 1993."

Is it from some document? Kindly give reference. I would like to study more about it. thanks.

[UmarMakhzumi]

For the exact quote, Google SK Gosh Associates and start of this quote. The statement reiterates the basis of strength design earthquake design being followed. Read the codes that you are using. They all refer to strength level earthquake forces.

Thanks.

[Ahmad Shabaneh]

On 5/28/2013 at 7:05 PM, UmarMakhzumi said:

Good article; I will throw my two cents.

Seismic drift values are much larger than wind values. UBC uses maximum inelastic response displacements rather than the design level displacements to verify the performance of the building. As stated above, the seismic drift limits are 2% & 2.5% of the story height for long and short -period buildings. So, for a floor to floor height of 12 feet the max. allowable inelastic drift value would be 2% of 12 feet= 0.02*12*12inches=2.88 inches. For wind for a 12 story height, drift would be L/400=12*12/400 =0.36 inches

So for a 3m story height the maximum displacement of the upper story relative to the lower story is 0.02*3m = 0.06m ?? I think it is a large value or I misunderstand what you meant?? 

[UmarMakhzumi]

On 4/5/2017 at 0:51 AM, Ahmad Shabaneh said:

So for a 3m story height the maximum displacement of the upper story relative to the lower story is 0.02*3m = 0.06m ?? I think it is a large value or I misunderstand what you meant?? 

The 2% and 2.5% limits are from UBC Section 1630.10.2. Excerpt from UBC is also posted in the OP.

Thanks

 

[abbaskhan2294]

For serviceability purpose we never allow drift or deflection greater than 1 in. 

[Ayesha]

On 6/23/2019 at 3:40 AM, abbaskhan2294 said:

For serviceability purpose we never allow drift or deflection greater than 1 in. 

Based on what?

[abbaskhan2294]

3 hours ago, Ayesha said:

Based on what?

Based on experience by professional engineers. You can search for literature related to serviceability of structures regarding this query.

[JL07]

Hello @UmarMakhzumi,

I have read your posts on this present page and also on the page on this link:

 

I understand that the cases/combinations used to calculate the seismic drift is code specific, and that the seismic drift should be calculated using strength level earthquakes forces. 

Concerning the UBC 97 and the ASCE 7-10, do those codes require the drift to be calculated with just the seismic load cases or with the strength design load combination 1.2D + 1.0E + 0.5L &0.9D + 1.0 E ? I understand that it is a good idea to consider the dead and live loads, however is it a requirement of the codes or is it just better practice ? This part is not clear for me since the strength level seismic forces the codes refer to are the base shear.

Thank you very much for you time.

 

[UmarMakhzumi]

HI @JL07,

I am working on a US job right now and we do include the live load, anchor load with dead and seismic.

I am not sure how exactly the code words it out but its standard practice in the company I work for reasons explained above. Seismic LC's (for drift) are without redundancy factor and deflections from LCs are amplified by multiplying the deflection to Cd/Ie.

Hope that helps.

Thank you.

Edited June 21, 2021 by UmarMakhzumi
Updated to add more clarity.

[Osama Anwar]

On 6/17/2021 at 9:54 AM, UmarMakhzumi said:

I am not sure how exactly the code words it out but its standard practice in the company I work for reasons explained above.

Is it not a standard practice in Pakistani Design Offices to use only earthquake forces for the calculation of drift? "Standard practice in the company" don't justify it as standard practice in Pakistani companies is to use EQ only?