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.


Waqas Haider
 Share

Recommended Posts

Assalam o alaikum.

I have just designed a frame structure with SMRF. The out put of shear seams weird to me. Column reports design shear Av/s as 0.045. (Images are attached). but when i right click the member, it must show me the most critical case HIGHLIGHTED AUTOMATICALLY. But it highlights load combo 38 (auto-generated combos have been used) which reports Av/s as 0.038. And 0.045 value is at combo 32. Is their any logic behind it?? More over how to interpret this Av/s?? means 0.045 in kip-in units means what? How can i convert this into spacing?? 

post-1702-0-82414600-1420286521_thumb.pn

Link to comment
Share on other sites

Assalam-o-alaikum!

 

Shear reinforcement values indicated on the column member are generally the maximum values for the two shear axes, whether these are from on load combo or from two different load combos. Right-cIick on the member reinforcement diagram generally leads straight to load combo resulting in the maximum longitudinal or maximum shear  reinforcement. However, as you have indicated, I am unsure, why this does not always happen. Surely, someone else might be able to explain this.

 

Interpretation of shear reinforcement value 'Av/s', given by ETABS, is not very difficult.

 

Since your units are 'kip-in", Av (i.e. cross-sectional area of ties) is in 'sq.in.' and s (i.e., spacing) in 'inches'.

 

For your case,  Av/s = 0.045.

 

For 2-legged #3 ties, Av = 0.11x 2 = 0.22 sq. in.

 

Substitution of value of Av in above equation, results in required tie spacing,  s = 0.22 / 0.045 = 4.88 in.

 

 

Regards.

Link to comment
Share on other sites

  • 1 year later...
On 1/3/2015 at 7:58 PM, engruzair said:

Assalam-o-alaikum......

...........

.........

 

For your case,  Av/s = 0.045.

 

For 2-legged #3 ties, Av = 0.11x 2 = 0.22 sq. in.

 

Substitution of value of Av in above equation, results in required tie spacing,  s = 0.22 / 0.045 = 4.88 in.

 

 

Regards.

Very good explanation..  But it's for column tie. Now  can you please explain this term for beam shear reinforcement? For a beam, shear reinf value is shown at 3 locations, so is it also same as Av/s ? and also then which should be the distance long of 3 locations? I mean please explain from a etabs output of beam shear reinf result. I am attaching an image of 18 ft beam which etabs is showing the shear reinforcement value in sq-in at 3 locations, now detail the shear reinf for this beam and make the detailing of only shear reinforcement. Thanks in advance. 

shear.PNG

Link to comment
Share on other sites

 

1. In the following explanation, I am referring to ETABS version 9.7.4, with active units of 'kip-in' for all quantities. You are probably using ETABS 13 or later version. In these later ETABS versions, systems of adoptable units have been improved, and new options of units have also been implemented. However, in order to understand the explanation given in the following, you will need to change active system of units your model to 'kip-in' for all quantities. Otherwise you might miss some important point.  

2. In case of beams supported at both ends, shear force (& accordingly, shear reinforcement) is more near the beam ends, as compared to middle portion of the beam. For these beams, ETABS indicates shear reinforcement at 3 points along the beam _ at the start, at the middle and at the other end of beam span. If the beam is not subdivided into smaller parts, shear reinforcement value would generally be larger at both ends, as compared to that for the middle portion of the beam.(Beam in your attached image is indicating the similar results.)

3. Calculation of required stirrup spacing is similar, to that described for the ties in case of columns in my earlier post.

4. Upto which distance, what stirrup spacing is to be provided depends upon the shear force distribution along the beam, as will be clear from shear force diagram for the controlling load combination. This spacing will however subject to relevant provisions of ACI 318-08 Chapter 21, depending upon the earthquake zone or design category of the area in which your structure is located.

5. Now referring to the beam shown in your attached image, the value shown in the middle (0.1000), indicates maximum required shear reinforcement applicable to middle portion of the beam, whereas the other two values (0.1521 & 0.1351) are the maximum required shear reinforcement, to be placed at the relevant beam portions adjacent to supports (columns) on the relevant ends.  In case your lengths in 'inches', and shear reinforcement in in 'sq.in/in.' units, you may calculate required stirrup spacing, similar to column tie spacing discussed earlier, keeping in view relevant ACI requirements. 

6. For exact detailing of the 18 ft span referred beam, following further information is required:

  a. Overall Beam size (width & height)

  b. Image showing required shear reinforcement values, after changing the active units to 'kip-in' for all the structural parameters, similar to ETABS 9. It would be better & simplify the detailing a lot, if the beam is subdivided into 4-equal parts, before analysis.

  c. Earthquake zone or seismic design category of the area in which the structure is located.

  c. Structural frame system used (OMRF, IMRF, SMRF,  etc.)

  d. Minimum reinforcing bar size used for flexural reinforcement of this beam

  e. Preferable bar size for stirrups (Normally #3, or #4 bars are used).

 

Regards.

 

Link to comment
Share on other sites

Thanks a lot for your nice and clear explanation, it's very helpful and informative and I have got the point. One fact is, you have said that simplifying the detailing would be better by subdivided beam into 4-equal parts, before analysis. Can you explain this please? So do I need to subdivide the beams? I generally mesh also all the beams by selecting all the frames in Etabs 2015.  

 

Best wishes.. 

Link to comment
Share on other sites

2 hours ago, Juli said:

One fact is, you have said that simplifying the detailing would be better by subdivided beam into 4-equal parts, before analysis. Can you explain this please?

Seismic detailing of beams requires the placement of shear reinforcement at a closer spacing within a length, equal to twice the beam height, from the support face. This is close to span/4 of the beam in many practical cases. Dividing the beam in 4 segments will give you 12 shear reinforcement values in total, with 3 values for each beam segment. Without dividing the beam into segment, you are getting only 3 values for the 18 ft span beam. With 12 values, you can more easily calculate stirrup spacing by adopting a suitable value applicable to each beam  segment. Thus, it would be a lot more easy to detail the stirrups, as compared with the present situation of a single beam without segments or subdivision.

HTH

Regards.

Link to comment
Share on other sites

  • 4 months later...

When you right click for governing combination, make sure you are not checking flexural governing combo.

Max shear combo might be from another combination.

2nd point is, when more than 1 load combinations govern (for example when column has 1% reinf from all cases), right click might not give you the "governing" combos.

You have to check it;

manually

or by telling ETABS to check instead of design (for columns) and check utilization ratios

or by other mean.

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.