Punishing shear questions 1

[drasticxxxx]

I studied the punishing shear according to ACI requirement and I have the following questions
1. The spacing between adjacent shear reinforcement in the first line (perimeter) of shear reinforcement shall not exceed 2d measured in a direction parallel to the column face (ACI 11.11.3.3). Can’t understand which distance he is speaking about? And this my lead me to my second question, in the attached picture does this mean I am allowed to put only one line of stirrups per directions, or shall stirrups be covered all the perimeters of the critical section (the graph is really miss leading)
2. When calculating the flexural reinforcement, I did understand that the code consider that steel in column strip as part of this reinforcement and designer has to check or add to this reinforcement if the reinforcement is not enough, however my main question ,this reinforcement I design for flexure for the sake of reinforcement where shall I Put it ,top or bottom??
3. I t seems that safe program is only designing for stirrups and not given longitudinal reinforcement? Is this the case???and what does safe mean by 9x5-10d@50 mm
Does it mean 9 lines of shear reinforcement (links) with 2d distance between line and the other, and every line has 5 legs of 10 mm diameter with spacing 50 mm
(I appreciate if anyone can give hand sketch to illustrate this, so I can get this)

 

[msquared48]

I believe that it's called "punching shear".

Mike McCann, PE, SE (WA)

 

[KootK]

1) It is a bit confusing. I interpret that clause to mean that you should have shear reinforcing (stirrup legs) around your column faces spaced at no more than 2d measured parallel to those column faces. I do not interpret the clause to limit the number of stirrup legs that you're allowed to place along the faces of your columns.

2) The flexural reinforcement should be placed near to whichever surface of the slab experiences flexural tension. Normally, when slab/column joints are not a primary part of the lateral system, flexural rebar will be located at the top of the slab. In specialized cases, such as where slab strips and columns are used as LFRS moment frames, flexural rebar may be required at the bottom of the slab.

3) SAFE is capable of designing the flexural reinforcement.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Trenno]

Punching shear shouldn't be taken too lightly, it can be quite punishing.

 

[bookowski]

1. This is a confusing statement. Not sure which aci you are using but if you look in '11 there is a section view which shows the '2d' as applying to adjacent rows of shear reinforcing (there's a similar diagram showing 2d as adjacent studrails if using rails). The 'first line' part of the statement is confusing as I don't imagine someone using a layout where the distance between adjacent rows varies - unless that is put in there for radial patterns. Strangely if you look in the CSI SAFE RC Design Manual they show the 2d meaning something different - they show 2d as the max distance to slab edge for the first rail.

3. Not sure what you mean - are you referring to the Gamma_F x Mu portion of moment that needs to be resisted within 1.5H? If so then I don't believe that safe is checking this for you in the flexural design.

4. safe isn't very good at checking punching shear in my opinion. At a minimum I've found:
- it uses a method (principal axes) which I've yet to determine is the 'correct' approach, however I have determined that ram concept, risa floor (es), pca design guide do not use. So when using safe expect to get much more conservative results, maybe by a factor of 2 or more.

http://www.eng-tips.com/viewthread.cfm?qid=392228

- it gets confused at corner, edge conditions and may check as an edge or even interior when you have a corner if you don't have a perfectly typical corner.
- doesn't deal well with openings nearby
- won't check punching if there is any change in section, i.e. a beam or drop or change in slab thickness

I've found other issues with safe as well (although I still use it...)

 

[drasticxxxx]

Thank you all.

Let me summarize what I understood
1.Safe isn't calculating flexural reinforcement

2.It is better to use Safe to check Punching not for design stirrups

3 Regarding 2d, it is really confusing

Code is saying precisely :ACI318-11

The distance between the column face
and the first line of stirrup legs that surround the
column shall not exceed d/2. The spacing between
adjacent stirrup legs in the first line of shear reinforcement
shall not exceed 2d measured in a direction
parallel to the column face. The spacing between
successive lines of shear reinforcement that surround
the column shall not exceed d/2 measured in a direction
perpendicular to the column face.

I can say that distance between stirrups should not exceed 2d(This my interpretation)

4.One last question is that when calculating Punching shear due shear and part of moment affect ,the moment should be taken form the column reaction (in column centroid) then this moment should be shifted to the centroid of critical sections and then the direction of the moment should be opposite when considered in Punching shear calculations(But practically shifting moment is not significant and it can be ignored and only filliping direction is considered )Right???

Thanks Community and sorry for the wrong spelling of Punching shear

 

[bookowski]

#3 Distance between stirrups is d/2 measured between each stirrup traveling away from the column. The 2d is between adjacent lines of stirrups. Look at the diagram in 318-11, it's relatively clear.

#4 I wouldn't ignore the moment from p x e where 'e' is the distance to centroid. It can make a sizable difference. If you are doing it by hand but using safe you need to take the column reactions from the frame output tables, don't take the moments from the punching output tables as those have already been modified per design provisions. You should take the vu from the punching output though.

 

[KootK]

1.Safe isn't calculating flexural reinforcement

SAFE definitely calculates the quantity and length of the flexural reinforcement. The only part of the flexural design that may not be covered is the lateral distribution of the flexural steel across the design strips.

2.It is better to use Safe to check Punching not for design stirrups

To my knowledge, no one hear has yet made a statement to this effect.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.