column ties spacings

[said the noob]

Quesetion: if column vertical bars has clear distance between bars at 175mm between one tied bar and non-tied bar exceeding code, what would be the cause of concern here? is the bar going to buckle once we are >150mm? we had noticed this one a site visit

code states: ties shall be arranged so that every corner and alternate longitudinal bar shall have lateral support provided by the corner of a tie having an included angle of not more than 135 degrees, and no bar shall be farther than 150mm clear on either side from such a laterally supported bar.

 

[jayrod12]

That's in plan view.

For spacing vertically the CSA code, which is extremely close to the ACI code says something along the lines of the smallest dimension of the column or 300, or a multiple of the bar diameter. I don't have it handy at the moment.

 

[KootK]

Yes, I believe that the concern is vertical bar buckling.

 

[said the noob]

i guess we now have to disregard the bars >150mm clear from the column capacity and assumed to have buckled? Would 25mm be a real concern or am I overthinking this.

 

[MIStructE_IRE]

Can’t say I’d be losing any sleep over 25mm...

 

[Deker]

I wouldn't be too worried about bar buckling as that is generally dealt with by limiting the vertical spacing of the ties. While the unsupported bar will have to rely on the flexural stiffness of the ties to restrain buckling, I think the provision you cited is really meant to enforce moderate levels of confinement as I have shown below. As long as the distance between tied bars complies with code, I would let it go. Note that my interpretation of the code is that the center-to-center distance between tied bars cannot exceed 2 x (6 in + db).

And, if the distance between tied bars is held constant, an interesting consequence of the misplaced unsupported bar is that the ties will be more effective in restraining it from buckling since the bracing force will be applied closer to a supported bar.

 

[said the noob]

Thank you sure appreciate the explanations

 

[Hokie93]

The lateral support requirements for longitudinal column bars was discussed in an article ("Longitudinal Bar Spacing and Intermediate Ties") by Degagne, Erdogmus, and Savage in the May 2016 issue of Concrete International magazine. The article is available free of charge to ACI members. The authors present the case that the current provisions (unchanged since ACI 318-63) are apparently more based on the experience and opinion of the committee members at that time (1963) than conclusive data.

 

[Trenno]

The authors present the case that the current provisions (unchanged since ACI 318-63) are apparently more based on the experience and opinion of the committee members at that time (1963) than conclusive data.

It's interesting - I wonder how many of these code provisions are based upon experience/opinion dating back over 50 years.

Do these provisions ever get updated in line modern research? Or do typical engineers just see that as the academics hijacking the codes and making unnecessary/complicated updates?

"If it ain't broken, don't fix it?"

 

[Settingsun]

Trenno, I'm sure there is a lot of 'if it ain't broke' especially at the 'lower' end of concrete design. By that, I mean structures that aren't pushing the bounds: low concrete strength, no unusual geometry, typical building loads. There's enough work keeping up with the rapid increase in concrete strength - almost every part of the various codes has to be extended. Plus giving frameworks for modern analysis methods. Then there's the issues that have historically been actual problems such as deflection, durability, shear (in the past) etc.

In this particular instance, where does the research come from to relax the tie requirements? I believe columns are cheap as it is so no one would sponsor; research is (has been) focused on increased/better confinement for high strength mixes and seismic.