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Building Drifts In Etabs


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  • 10 months later...

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 Icr walls= 0.5EcIg, Beams = 0.5EcI g & for Columns 0.5 - 0.7 EcIg.
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  • 1 year later...
  • 2 months later...
  • 7 months later...

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?

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  • 4 months later...
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.

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  • 2 months later...
  • 2 months later...
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.

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"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.

 

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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.

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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.

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  • 3 months later...
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?? 

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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

 

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  • 8 months later...
  • 1 year later...

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.

 

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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 by UmarMakhzumi
Updated to add more clarity.
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  • 2 years later...
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?

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