Tension Controlled Columns

[Yousef ZAA]

Hi All,
I have run columns on vertex program and in the report in pure axial case ( Eccenricity only 15+0.03h) i found my column in transition phase and at top stations at Tension Controlled range. My question is it normal having tension controlled column and how can i relate it to ductility of the column i imagine that having a tension controlled column minimize the amount of ductility of column!

 

[KootK]

It's quite common for top floor columns to be tension controlled. You get a high moment to axial load ratio there so flexure dominates. Tension controlled columns are actually more ductile, not less.

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.

 

[MotorCity]

You could have tension in your column due to axial tension (perhaps an uplift situation) or from flexural tension (in the case of an exterior column with transverse wind load). If you want to quantify ductility, one way is to calculate the change in length over a given distance.

 

[Yousef ZAA]

im having a problem figuring out whether the column will be less ductile. because i believe that column when overlaoded brittle behavior will control and when phi become 0.9 i imagine that sudden failure is likely to happen because im attracting load by increasing the capacity, please verify if my approach is wrong, Mr Kootk.

 

[Teguci]

For a reinforced concrete element, ductile behavior will occur if the reinforcing exceeds its yield point before the concrete crushes. This behavior is associated with the geometry of the cross section and not the loading. It does not include considerations of stability failure that will occur if the column is allowed to buckle due to slenderness.

 

[KootK]

please verify if my approach is wrong, Mr Kootk.

I believe that your approach is wrong Yousef. Backwards actually. The more that your section is tension controlled (phi = 0.9), the more ductile you can expect your column to be. It's is compression controlled sections that exhibit more limited ductility.

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.

 

[Yousef ZAA]

ok, then the question do we prefer ductile columns,why not designing all columns tension controlled from the beginning!

 

[Teguci]

a non-ductile concrete element occur when you have a lot of rebar at a small depth, d. For most columns, where bending is minor compared to axial compression, you want to put a lot of rebar into the element and the architect wants you to keep the d down. In order to make that same column ductile, you have to increase the columns size or spend client's money on expensive high compression concrete.

 

[KootK]

Because:

1) While we might prefer more ductile columns, we have concluded that they are not necessary for many applications. The glaring exception would be high seismic moment frame columns where we limit axial load levels for just this reason.

2) For serious axial loadin, a tension controlled column is going to be a big column. While structural engineers may be fine with that, the architects that pay our bills often are not.


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.

 

[Yousef ZAA]

ok, i know that but the fact that my column is susceptible to only pure axial and the only moment occurring from the moment magnification min. Ecc. and yet tension Controlled, that made me little messy.

 

[ALEXZ0622]

Yousef ZAA:

In my understanding was correct, you mean your tension-controlled situation was caused by something like 1.2D+1.6L Load Combo with minimum eccentricity of 15%+0.03h? If that was the case, that's a really strange one, cause if you drew the strain profile, you at least have 0.005 for the rebars, and 0.003 for the extreme compressive layer of concrete (for the case you are having the max axial load), then only 37.5% the cross section can provide compressive strength. However, if that was the case, just do some hand calc. to verify, then go for it in your design code.

For the code aspect, I'm using U.S. ACI318-14, which does not have a word related to minimum eccentricty throughout the whole book (at least I can't find it, plz give me a message if you knew).
The minimum requirement for columns are as followed:
1. In the section strength chapter, you can find that the voting committee limited the max axial strength to 0.8-0.85 to account for accidental eccentricity at the eccentricity-to-depth ratio of 0.10-0.05;
I did find something about the min eccentricity in ACI, but that would back to ACI318-02 and 08, it's some kind of changed to the equation I mentioned above;
2. 1% min Longitudinal reinf.;
3. at least 4 corner longitudinal reinf. for rectangular ties;

 

[Yousef ZAA]

Zondervan,
Yes this is my case and to be more specific : I have 450 x 450 precast column with (6 phi25) reinforcement, I've run it on vertex, and the case that i have pure axial + Min. Ecc.(already computed by vertex,as ACI318-11 code 15 + 0.03h)and in the report in 1.4 DL Combo, in the bottom stations was in transition phase and in the upper station (Roof) was Tension controlled and in 1.2 Dl + 1.6 LL was Comp. Contr. in the bottom and T Contr. in upper stations and in cases that i have little moments due to asymmetrical Loading the whole Col. was T Controlled, the question is that something that i can live with it, and the column being ductile as beam if Ten.Contr. is that something good regarding integrity, by the way the building in Seismic Category A is that fact will make the ductile column acceptable ( I mean to not worry about weak beam Strong Column, i mean the case whether the plastic hinge be in col. instead of beam).i Know that i put a lot of questions in this reply sorry for that, please verify if anything not clear, Regards.

 

[ALEXZ0622]

Yousef ZAA:

I see, this min eccentricity is actually the Moment Magnification Method, and the min 1st-order factored moment. So first you would need to check if you had a slender column, use:
1) k*lu/r<22, or
2) k*lu/r<=34+12*(M1/M2) and k*lu/r<40;
you can find this around that Chapter 10 in ACI318-11
Then if you do not, then you can just neglect this moment, since this moment magnification method is meant to account for P-little delta effect.

Since I never designed precast column, neither did I use Vertex, so I'm not sure if it checked it.

Even if you have this slender column, and all the loads are just what you have right now, you can check the wind load you are using (Since SDC=A, just consider wind at this time), if you are using MRI=700 yrs, you can change this wind load by ASCE commentary C to which ever suits your building's service life, then check if you will have this kind of deflection along your column, if no, just provide enough reinforcement, and that's it. If yes, and you are really concerning this, you can ask the supervisor or the owner, depending on the budget, you will have an answer.

But be honest, if you are not in the coast region, I won't expect this tension controlled situation happen, not mention the plastic behavior, I don't think it will happen based on this min eccentricity case. And you just can't make sure the plastic hinge occurs at where you expected, which is still an issue even you have all the detail design based on SDC=E or F.