Shear Reinforcement In Etabs

[Waqas Haider]

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?? 

[EngrUzair]

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.

[Waqas Haider]

Oh, i was having in mind that Av is 0.045.... i was very confused dats y. Thanx Uzair. 

[Juli]

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. 

[EngrUzair]

 

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.

 

[Juli]

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.. 

[EngrUzair]

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.

[Hussain Abid]

nicely explained uzair!! well done

[WR1]

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.

[Hussain Abid]

can any 1 explain the same for longitudinal reinforcment plzz