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.

Slab deflection in ETABS


Omar Khalid
 Share

Recommended Posts

Hello,

While I was modeling a 20 story building with symmetrical concrete slab sections and loads in each story, I noticed that the deflection of the slab increases the more I go higher in elevations despite having the same sections, dimensions and loads.

I know that floors with symmetrical shapes and loads must endure the same stresses so it has the same reinforcement.

Can someone explain this behavior or explains why it occurs if I was wrong ?

Thanks in advance 

Link to comment
Share on other sites

Regardless the combination I choose to look for settlements. it's always increasing the higher I go. I've seen people modeling on youtube having the same problem except they don't talk about it like it's a normal thing. does it have anything related to vertical settlements of columns and walls ?

You can try it by yourself on etabs to check I'm right or not.

Link to comment
Share on other sites

You shouldn't have it. If you want to investigate it please provide what combinations are governing.

Here is what you should do:

1) Do a gravity only analysis use load factor of 1 to see what results you are getting.

2) Do a lateral only analysis to see what results you are getting without any gravity load.

3) Compare results and check model for errors.

Just because other people are getting the same problem, doesn't mean the problem is there. We need to define a basis. I don''t use ETABS so I can't do checks on my own. If you want my input, please provide the results as requested above.

Link to comment
Share on other sites

1 hour ago, Ayesha said:

You shouldn't have it. If you want to investigate it please provide what combinations are governing.

Here is what you should do:

1) Do a gravity only analysis use load factor of 1 to see what results you are getting.

2) Do a lateral only analysis to see what results you are getting without any gravity load.

3) Compare results and check model for errors.

Just because other people are getting the same problem, doesn't mean the problem is there. We need to define a basis. I don''t use ETABS so I can't do checks on my own. If you want my input, please provide the results as requested above.

1) due to dead load only (own weight) the building tends to undergo a displacement in the x axis (horizontal displacement) the maximum displacement is 1 cm at the roof. Cause I have a stair wall with 2 elevator cores at one side of the building.

2) No earthquake or wind loads being applied yet. Only dead and live and slab deflections still increases when you go up.

3) No errors in my model.

Link to comment
Share on other sites

35 minutes ago, miqureshi77 said:

this deflection shows in Etabs is global deflection, this will increase due to axial and lateral deformation of the members globally,

if you check deflection individuallly with respect to each floor value than u will find the same deflection

Can you tell me how to check it ?

Link to comment
Share on other sites

3 hours ago, Omar Khalid said:

1) due to dead load only (own weight) the building tends to undergo a displacement in the x axis (horizontal displacement) the maximum displacement is 1 cm at the roof. Cause I have a stair wall with 2 elevator cores at one side of the building.

That is due to unsymmetrical stiffness of vertical members. This however isn't the subject of our discussion.

3 hours ago, Omar Khalid said:

2) No earthquake or wind loads being applied yet. Only dead and live and slab deflections still increases when you go up.

Are you using any live load reduction factors? What is the difference in vertical deflection of the slab in 2 consecutive floors and what is the location of that maximum vertical deflection?

 

Edited by Ayesha
Link to comment
Share on other sites

Maximum deflection in the first floor is 8.4 mm
2nd floor is 9.4 mm

3rd is 10.6 mm

4th is 11.65 mm

5th 12.6 mm
etc the 20th 20.5 mm

 

These are maximum deflections in each slab (same place) with dead load only (self weight only)

Link to comment
Share on other sites

1 hour ago, Omar Khalid said:

These are maximum deflections in each slab (same place) with dead load only (self weight only)

Omar, when you are reporting maximum deflections I assume that you mean maximum vertical deflections and it is relative not absolute. I still can't get my head around the fact that you have so much deflection difference even for gravity load case. 

I will look further in this. I don't have an answer on top of my head. I also request other forum members to share there thoughts about this question.

Edited by Ayesha
Link to comment
Share on other sites

If your model is correct, this increasing value of vertical displacement is due to the elastic shortening of the vertical supporting elements ( columns and cores). These elements are mainly loaded with compression stresses, which causes the reduction of its height and consequently, the increase of the slab deflection (vertical displacement).   

Link to comment
Share on other sites

11 hours ago, Ayesha said:

Omar, when you are reporting maximum deflections I assume that you mean maximum vertical deflections and it is relative not absolute. I still can't get my head around the fact that you have so much deflection difference even for gravity load case. 

I will look further in this. I don't have an answer on top of my head. I also request other forum members to share there thoughts about this question.

No. It's maximum vertical displacement absolute not relative

Link to comment
Share on other sites

If the building is regular as you said with same column sizes, axial shortening should be same provided loading and gemoetry all are same and symmetrical. Gravity deflection on same combination should be same at each level. And by deflection i mean z deflection not x.

Link to comment
Share on other sites

4 hours ago, W. Alkady said:

If your model is correct, this increasing value of vertical displacement is due to the elastic shortening of the vertical supporting elements ( columns and cores). These elements are mainly loaded with compression stresses, which causes the reduction of its height and consequently, the increase of the slab deflection (vertical displacement).   

That's what I was thinking, but the maximim reinforcement of a column at the base is 1.3-2 % which indicates which indicates small loads

 

I will try to screen shot it when I get home

Link to comment
Share on other sites

2 minutes ago, Rana said:

If the building is regular as you said with same column sizes, axial shortening should be same provided loading and gemoetry all are same and symmetrical. Gravity deflection on same combination should be same at each level. And by deflection i mean z deflection not x.

 It's not entirely symmetrical I have 4 L shaped shear walls at the corners. Same thickness and nearly close lengths

 

the columns are symmetrical right and left sides with the same dimensions but not symmetrical in the Y axis

That's why I have a displacement of 1 cm at the roof in the y axis

 

but that shouldn't cause vertical displacement

 

note that the first floor has 8.4 mm vertical deflection while when I export the story to safe it gives me 2 mm deflection

Link to comment
Share on other sites

Hey guys, I think I have found what's wrong but don't know how to fix it.

I drew the slab on SAFE and I have obtained the beams moments and slab deflections and they were very far away from ETABS results. However, there is an option to check it for columns and walls in SAFE it's called :

''Automatic rigid-zone area over column''  When I uncheck it, it gives nearly the exact deflection and beams moment in ETABS

This option means the following from CSI america's website :

''Rigid zones are used to model the physical overlap between columns/walls and slab, which prevents deformation of the slab at the column location. This also will have the effect of producing maximum design moments at face of columns/walls instead of center line of columns/walls.  This in turn will produce less reinforcement and hence a more economical design, and allowed by most codes(i.e ACI-318). Deformations also will be more realistic in this case.''

So I assume applying a rigid zone is more realistic but somehow it's not assigned in ETABS and I don't know how to assign it in ETABS.

What are your thoughts ?

Link to comment
Share on other sites

Hey man, hold on. Your building isnt symmetrical. Also the theres difference between axial stiffness of cols vs walls. This will effect z direction deflections from bottom to top stories.

2nd thing..safe..in safe deflections will be different than in etabs. This has been discussed no of times here and you can search.

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.