Design of stirrups for column 5

[fratrowie]

hello pals,

im designing a column which confused me very much, some of my collegue says that desingning stirrups for column does not have great importance as long as you will provide.


i dont a agree, though im trying hear to find some answer.


from the books i have read that if the actual shear Vc, is greater than 0.85Vc/2, the procedure of designing the beam stirrups may be used.. the problem is how can we find the actual Vc where Vs is unknown?


thanks for reply in advance, God Bless...



fratrowie

 

[Lion06]

Designing stirrups is absolutely important. The degree of importance has to do with whether it is a lateral column or a gravity-only column. Either way.... it is important! The ACI chapter on shear and torsion gives criteria for this. My interpretation is that if 0.75Vc/2 > Vu then you can use the code minimum (which is a set of 3 criteria I can't recall off the top of my head - least column dimension, some multiple of tie diameter spacing, and some multiple of column bar diameter spacing).
If 0.75Vc/2 < Vu then you have to design the stirrups for strength just like you would a beam (as you mention above).
I don't understand your final question of how do you find the actual Vc if Vs is unknown. Please elaborate.
Are you in a seismic area?
In any event, designing stirrups IS IMPORTANT!!

 

[mitchelon]

It is always really important as MYerges stated. You do need to provide us with more information. Are you in a seismic area (in addition to shear strength, stirrups are extremely important for concrete confinement and longitudinal steel stability)? Is the building symmetrical (this is important due to torsional forces)? Are these columns part of the lateral resisting system?

 

[henri2]

Check PCA Notes on ACI 318-05...Chapter 12 (Shear)---Section 11.5.7 (Design of Shear Reinforcement) to solve for Av. Also as mentioned, do not forget to take SDC category into account for minimum tie area and spacing requirements.

 

[fratrowie]

MYerges because from the book i read it says that the actual shear is Vn = Vc + Vs, i completely confused which is which the actual shear and the ultimate shear, if its so that the actual shear is Vn then the Vs should be known to find the Vn... im in a siesmic zone, the structures is symmetrical.

thanks for enlighten me with this, and to you metchilon and henry2..


and oppss... ultimate shear is the shear due to lateral force times half of the column height in a particular level?


fratrowie%-)

 

[GalileoG]

That equation just means that your shear resistance comes from the steel stirrups and from the concrete itself. If you're designing the column, then you know how much resistance you need, and you also know the resistance that the concrete will offer you, and therefore you can use that equation to calculate the either the a) stirrup spacing or b) stirrup size (you will have to assume one to find the other.) Sorry if this is not the answer to your question.

 

[ash060]

Stirrups (ties) in concrete columns are not usually critical. Concrete shear capacity increases with the amount of axial load on the column (check ACI chapter 11). The concrete can handle the load most of the time and ties are required per the code and they act a shear reinforcing as well. If you have large lateral loads bump down you ties spacing to d/2 and see what capacity you have by just doing that

 

[JAE]

ash060 - stirrup ties in concrete columns may not be critical for shear, but for axial load they are very important. The ties serve to inhibit the vertical steel from buckling/exploding outward. Without them in a building I wouldn't walk into it.

 

[ash060]

Jae what you say is correct about the ties being needed for confinement, but I was just saying that they are not really needed for shear in typical cases. Also the ACI code does not give the concrete a higher allowable stress due to confining effects of the ties.

 

[JAE]

Also the ACI code does not give the concrete a higher allowable stress due to confining effects of the ties.

Hmm...well, please read 9.3.2.2 where using spiral reinforcing instead of standard ties lowers the [&Phi;] factor and thus increases the column capacity. Also, ACI doesn't use "allowable" stresses except in the appendix.

I agree with you that shear isn't usually a big issue in most columns.

 

[WillisV]

I have always wondered what the original basis of the ACI tie spacing requirements for columns really is. Yes I know they restrain buckling of the longitudinal steel - but what testing and research was done to come up with the actual "48 tie bar" diameter etc. If I have a spacing in the field that is 49 tie bar diameters is my column going to fail? What about 53? 70? It would be great to see some data on this...

 

[JAE]

WillisV - that's a good question - I believe I once read an article on it many years ago and the jist of the research was based upon the typical column compressive tests using various spacings. Normally, a research effort such as this will plot out the various levels of capacity and try to pin down a 90% probability against failure - setting the ties at some set spacing to achieve that.

The ACI commentary in chapter 7 has this statement:

[red]"Limited tests on full-size, axially-loaded, tied columns containing full-length bars (without splices) showed no appreciable difference between ultimate strengths of columns with full tie requirements and no ties at all."[/red]

The reference is to this article:

[blue]Pfister, J. F., "Influence of Ties on the Behavior of Reinforced Concrete Columns.", ACI Journal Proceedings, V. 61, No. 5, May 1964, pp. 521-537.[/blue]

 

[WillisV]

Exactly - the code limts are arbitrary.