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.

Column Reinforcement From Summary In Etabs


EngrJunaid
 Share

Recommended Posts

AOA Respected Seniors !

 

At station location 0.00 In column reinforcement summary the longitudinal steel is very high as compare to other location in the same column. My friend told me to ignore the 0 location reinforcement and provide the rebar in the column according to other station locations.

here is a snapshot of the column summary, so should i ignore the location 0 reinforcement ????

 

THANKS in advance.

post-1602-0-86193300-1430719590_thumb.pn

Link to comment
Share on other sites

First thing is why etabs is not taking end offsets automatically?

 

end offset should be at the soffit of beam for the column design.

 

if etabs is not taking it automatically you can assign it manually. No need for moment design at 0 location.

Link to comment
Share on other sites

Junaid,

 

As sameer has said, by default ETABS calculates reinforcement value at three locations along column height - at location 0.00 (start of column at floor level), at mid-height , and at top face of column (i.e., at base level or soffit of upper floor beam). According to my understanding, column reinforcement is designed for the largest of load effects at these locations. 

 

IMO, you should check your model carefully.  If you have applied 'frame end offsets' properly, and these are actually being applied by the software to the model (as may be verified from the moment diagrams for the load combinations involving earthquake loading for concrete frame structures, and wind load for steel frame structures), your reinforcement location values should be as given in above (with column height taken upto soffit of upper level beam). If OK, you SHOULD NOT ignore the reinforcement value at column location 0.00 in such a case. It will, in most of the cases, be the controlling value for the column.

 

If in doubt, check the column reinforcement design manually for the suspected locations, using applicable load combinations, to see whether the reinforcement provided by the software is OK or not. You should ignore the program provided reinforcement, only when you are sure that there is really some error in program output.

 

 

Regards.

Link to comment
Share on other sites

AOA !

Thanks for your kind responses.

I rechecked my Model, the end offset length have already AUTOMATIC FROM CONNECTIVITY option ON (screenshot is attached here).

I am uploading my model. I will really appreciate your time for checking my mistakes in the model in you convenient time.

 

 

post-1602-0-00722800-1430808673_thumb.pn

3 FATA.EDB

Link to comment
Share on other sites

Dear Rana there is no end length off set issue here at location 0. or you are not familiar with ETABS. ETABS always calculate the reinforcement at 0 loaction. and as far as end lenght off set is concern for this then it will be below slab not above slab.

Link to comment
Share on other sites

@Rana,

 

My point of view, is as under:

 

1.   When we select & adopt a structural software, we believe that it works correctly at least for basic design-related calculations;  both the structural analysis i.e., computation of member forces and moments etc, and proportioning of members i.e., sizing of members and determination of reinforcement. 

 

2.   Now, if we believe that the software is performing correctly for the analysis part (which is not only most important and a harder calculation), will it be logical to say that design results of software (Which are a lot simpler and easier to calculate) are incorrect, WITHOUT verifying it manually?  In simple words, if analysis results of ETABS (or any other structural software) are acceptable to me, I will not object to its design output merely because I 'feel' that it is giving excessive reinforcement or erroneous results, UNLESS I check them manually and is prove them wrong.

 

3.   In the present case, if we are sure that all other calculations done by ETABS (including all the analysis and other design output) are OK, I see no reason for believing that the software is giving incorrect reinforcement (both in display as well as design summary) for the columns - the most critical members of the structure. 

 

4.   As such, IMHO column reinforcement (or other design results), displayed in graphic output or shown in design tables, SHOULD NOT be ignored ordinarily, without any soild reason.

 

 

Regards.

Link to comment
Share on other sites

From an analysis point the 0 location shall be significant as the joint can be treated as rigid. I am not in to softwares so can't comment what SAP or ETABS would do, but if I am asked to do an analysis, I would make the above assumption and do the job.  Also, when in doubt please be conservative. I guess there is no right or wrong here. Engineering is all about judgement and this situation is one good way to exercise it.  

 

Thanks.

Link to comment
Share on other sites

1. ETABS will never design a column or beam inside end-offsets (NEVER!) You can do joint design manually by taking moment inside the joint but I am talking here about the column design specifically.

 

2. ETABS will design a column at 0 location in some cases. For example at foundation level. There is no framing so end-offset will be zero and design will be reported at 0.0

 

3. It is important to check the local axis 1-1 direction (red) in column to see which one is I end and which one is J end.

 

 

As EngrUzair said,

we should be sure if ETABS or any other software taking the input and output correctly or not.

 

 

And Junaid, If you right click on C15 in your model, you will find out that end offset i = 0 (at floor level) and end offset j = 36inch (at soffit of beam). This is what sameersaleem was suggesting actually. 

 

so basically what ETABS is doing in your case is correct. It is reporting values at 0.00 that is your slab level.

 

If you want to go a step ahead you can change the i end offset value = half of slab thickness because it is center to center modelling.

Link to comment
Share on other sites

In my openion we should not change any end length off set to pass some member or reduce the steel %age. suppose if  we agree with rana that we can provide end length off set as slab half thickness then what for all other columns that the software repoting at 0 loaction and further thing is that if you can go to elevation 3 and exturude it shows that slab will be inside the beam by default and beam is exdtuded bleow column so how we can not provide end length off set as slab half thickness.

Link to comment
Share on other sites

Sameer yes you are right for slab extrusion below beam. In this case you will not apply end offset as = half slab thickness. But I was talking about a general concept. Anyway, sometimes it is necessary to change end offsets manually. But for most of the part in ETABS end offsets are okay. For example when a beam is rested on another beam end-offset is not properly calculated. I am not sure if it is the same in ETABS 2013 or not.

 

I was talking in general for end-offsets. As I thought Junaid was talking about j end. But in this case Junaid, you dont need to change anything. You have to provide reinforcement at 0 location at i end.

 

Thanks and it was a nice discussion btw. Hope this clarifies your concern Junaid?

Link to comment
Share on other sites

@sameer, Uzair, Rana & Umar...... THAAANK you very much for such a nice and fruitful discussion.

 

But Sir rana and sameer i am confuse that why it is giving 16 in2 (about 4.5%) reinforcement on the C17 & C18 (elevation 4) due to combo 3 and 4.

Is there any mistake in my model because for a 4 story structure its very heavy reinforcemnt...which i think is not logical...

Link to comment
Share on other sites

Without checking your model I would suggest that you check on which combination you are getting 4.5% reinforcement.

 

Then check the moment diagram on that combination and make sure its okay!

 

You can always make a simple 2d model of a frame to compare the results.

 

It the column is at upper story, you will get more moment than axial force so e is more and so is the reinforcement.

Link to comment
Share on other sites

In ETABS frame section properties are assigned to line objects. However, actual structural members have finite cross-sectional dimensions. When two members such as a beam and column are connected at a point, there is some overlap of the cross-sections. In many structures, the dimensions of the members are large, and the length of the overlap can be a significant fraction of the total length of the frame element. ETABS provides the capability of defining end length offsets along the length of frame elements to account for these finite dimensions of structural elements.

  • If the Automatic from Connectivity check box is checked, ETABS will automatically calculate the offset length. ETABS bases the end offset length at the end of the beam on the maximum section dimensions of all columns that connect to that end of the beam. This is the default.

    ETABS automatically calculates offset lengths for beam and column-type frame elements. It assumes the offset length for all brace-type frame elements to be zero. (You can define your own non-zero offset lengths for brace elements if necessary.) Also, the dimensions of brace elements that frame into the ends of column and beam elements are not considered when calculating the end offset dimension for a column or a beam.

    • Beams. When ETABS automatically calculates the end offsets along the length of a beam, it bases the end offset length at an end of the beam on the maximum section dimensions of all columns that connect to that end of the beam.

    • Columns. When ETABS automatically calculates the end offsets along the length of a column, it bases the end offset length at an end of the column on the maximum section dimensions of all beams that connect to that end of the column.

    • If the Define Lengths check box is checked, enter values for the length of the offset at the start (End I) and end (End J) of the line object.

                      (From ETABS)

 

SO, In my opinion it is better to define the end length offset manually depending on size of connected beams keeping rigid zone factor as o.

 

Thanks

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.