Jump to content
  • Welcome to SEFP!

    Welcome!

    Welcome to our community forums, full of great discussions about Structural Engineering. Please register to become a part of our thriving group or login if you are already registered.

Building Drifts In Etabs


WR1
 Share

Recommended Posts

  • 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.
Link to comment
Share on other sites

  • 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?

Link to comment
Share on other sites

  • 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.

Link to comment
Share on other sites

  • 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.

Link to comment
Share on other sites

"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.

 

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

  • 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?? 

Link to comment
Share on other sites

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

 

Link to comment
Share on other sites

  • 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.

 

Link to comment
Share on other sites

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.
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
  • Our picks

    • Hi there,
      I am interested in performing "Performance Based Design" for a 20 story building. 
      I'll be performing "Non-Linear Static Pushover Analysis" for my model. Until now, I have decided to go with "Displacement Co-efficient method". I will be using ETABS 2017 for performing Pushover Analysis. While assigning plastic hinges, I have an option of using ASCE 41-17 (Seismic Evaluation and Retrofit of Existing buildings". I would like to know what would be a better estimate for relative distances for plastic hinges in case of beams, columns. Any input concerning assignment of hinges to beams, columns and shear walls is highly appreciated. Normally it's taken 0.05 and 0.95 or 0.1 and 0.9. What's your opinion on this?
      Secondly, it would be great if someone can recommend me a book or some good source to understand how to characterize building using performance levels. Any sort of help is appreciated.
      I have recently graduated and joined a structural design firm, so kindly guide me, considering me a beginner.

       
      • 2 replies
    • *SEFP Consistent Design*<br style="background-color:#ffffff; color:#272a34; font-size:14px; text-align:start">*Pile Design*<br style="background-color:#ffffff; color:#272a34; font-size:14px; text-align:start">*Doc No: 10-00-CD-0007*<br style="background-color:#ffffff; color:#272a34; font-size:14px; text-align:start">*Date: April 16, 2018*

      1.1. FUNCTION OF JOINT

      Beam-column joint must transfer the forces, such as moment, shear and torsion, transferred by the beam to the column so that the structure can maintain its integrity to carry loads for which it is designed.

      Another function of the beam-column joint is to help the structure to dissipate seismic forces so that it can behave in a ductile manner.

      1.2.WHY DO WE CARE

      During an extreme seismic event, the code-based structure is expected to maintain its load-carrying capacity for gravity loads even after the structure deforms into inelastic range so that it does not pose any life safety hazard. Hence, the joint can go through significant degradation of strength and stiffness, and if it fails in shear, or anchorage, the life-safety objective of code cannot be achieved.

      1.3.CONSEQUENCES OF FAILURE


      1.4.THINGS TO CONSIDER FOR BEAM COLUMN JOINT

      Longitudinal bars of beams, or slab, must be able to develop their yield stress, so that the beam/slab can transfer moment to joint. It means that longitudinal bars must have adequate development length for hooked bars. This implies that the size of the column must be such that bars can develop their tensile forces. If bars can transfer moment, they can also transfer shear as far as monolithic construction is concerned.


      The shear strength of the joint must enable the transfer of moment and shear through it.



      The joint should be Constructible: Congestion of reinforcement is the main concern.

      1.5.DESIGN SHEAR FOR BEAM COLUMN JOINT

      The design shear for beam-column joint depends upon the relative strength of beam and column at the joint.

       
      • 4 replies
    • *Comments/Observations regarding modelling in ETABS*

      *Doc No: 10-00-CD-0006*

      *Date: May 06, 2017*

      Some of the observations made during extraction of results from ETABS (v 9.7.4), for design of reinforced concrete members, are being share in this article.,

      1) Minimum Eccentricity

      ETABS always considers the minimum eccentricity for selecting the design moment of columns irrespective of the probable behavior of the column, whether short or long column. See section 10.10.6.5 and its commentary of ACI 318-08 which deals with minimum eccentricity of long columns. You should always check the design moments that ETABS uses for columns if you want to bring down the cost of construction.

      2) Unbraced/ Braced Preference

      ETABS always performs analysis of frame as if it is un-braced. You should investigate if the storey under consideration is braced, or un-braced (10.10.5.2), and decide appropriate design moments of columns.

      3) Time Period

      ETABS has a tendency to select a time period of the building that is considerably less than the value obtained by the approximate method, Method A, of the section 1630.2.2  of UBC 97. To quote the FEMA 451 document: ''Because this formula is based on lower bound regression analysis of measured building response in California, it will generally result in periods that are lower (hence, more conservative for use in predicting base shear) than those computed from a more rigorous mathematical model". So, there is no need to use the value of time period that is lot less than Ta. One should always check the time period used by the software; ETABS can overestimate the seismic force by more than 2 times.

      Visit the forum link to read the complete article.
      Link: http://www.sepakistan.com/topic/2300-commentsobservations-regarding-modelling-in-etabs/
      • 0 replies
    • The minimum amount and spacing of reinforcement to be used in structural floors, roof slabs, and walls for control of temperature and shrinkage cracking is given in ACI 318 or in ACI 350R. The minimum-reinforcement percentage, which is between 0.18 and 0.20%, does not normally control cracks to within generally acceptable design limits. To control cracks to a more acceptable level, the percentage requirement needs to exceed about 0.60% (REFRENCE ACI COMMITE REPORT 224R-01)



       

       



       

       

      So according to above statement , should we follow 0.60%, to be on more safe side??



       
      • 12 replies
    • Dear Sir/Madam,

      This email is an invitation for the participation in the First South Asia Conference on Earthquake Engineering (SACEE-2019) which will be held on 21-22 February 2019 in Karachi, Pakistan. This conference is the inaugural event in this series of conferences which has been constituted under the auspices of South Asia Earthquake Network (SHAKE). The organisers of the conference include NED University, University of Porto, University of Fuzhou, University Roma Tre and Institution of Engineers Pakistan. The conference website can be visited at http://sacee.neduet.edu.pk/.

      Please note that world leading earthquake engineering experts have confirmed their participation in the conference. These include Prof Abdelkrim Aoudia (Italy), Prof Alper Ilki (Turkey), Dr Amod Mani Dixit (Nepal), Prof Bruno Briseghella (Italy), Prof George Mylonakis (UK), Prof Khalid Mosalam (USA), Prof Humberto Varum (Portugal) and many others. The presence of these distinguished experts allows you to exchange your work/issues with them and discuss possibility of any future collaboration. Please note that participation in the conference is strictly based on registration. Early registration in different categories at reduced rates are available till 10 December 2018. Please visit the conference website to see the details and the link for registration.

      If there are any queries, please do not hesitate to contact the Conference Secretary at the following address

      Prof. Muhammad Masood Rafi
      Conference Secretary- SACEE-2019
      Chairman
      Department of Earthquake Engineering
      NED University of Engineering & Technology Karachi, Pakistan.
      Phone: 0092-21-992-261261 Ext:2605
      Email: rafi-m@neduet.edu.pk
    • What is the Minimum reinforcement For Precast Pile  according to different codes (ACI,BS)??  Pile length is 40 times of pile least dimension . 
      • 1 reply
    • Dear members, I am working on a 10 storied rcc factory building with one basement,  where floor loads are in general 125 psf(Live) . but there are 2 warehouse in the building at ground floor & 10th floor where the Live load of stacked materials are 450psf. I have modeled it and analysed in ETABS. After analysis, seeing the floor displacement for seismic load,  i am in big shock to see the pattern. the displacement pattern suddenly increased hugely & then got normal . if the warehouse load created problem, then why it effected only Ground floor level, not the 10th floor! Please tell me how can i solve it. 
      • 1 reply
    • Asalamualaikum all,

      I have columns which are conflicting with the underground water tank as shown in figure.
       

      So I have decided to make underground water tank base slab as a footing for column. So I import etabs model to safe and just take uniform water load on base slab and point load from columns.

      This is the residential house. The BC is 2tsf. But SAFE is showing tension on the base slab and the thickness from punching is 30''. I believe that thickness is too high. What can be the error? Is this approach is correct for design base slab of ugwt to carry load of two edge columns?
      • 11 replies
    • SAFE perform iterative uplift analysis,any one having experience how to check the results of this analysis???what is the purpose and scope of this analysis???
      • 15 replies
    • Shear wall design
      AOA 

      i am facing problems in shear wall design .what are the pier and spandral ?what will be the difference when we assign pier or spandral? without assigning these the shear wall design is incomplete .

      i am taking about etabsv16

      someone have document about shear wall design plz provide it 

      thank you

       
      • 13 replies
  • Tell a friend

    Love Structural Engineering Forum Of Pakistan? Tell a friend!
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use and Guidelines.