Ties in concrete columns

[brizstruct]

We are looking at a project where the EOR has designed columns without intermediate ties as required by the code. Bars are not bundled and details are Column size 32"x32". Single #8 bars at 6" ctrs with only an outer tie. In the commentary to 318-77 there is a suggestion that ommission of intermediate ties could be allowable.

Can anyone shed any light on this

Thanks
Brizstruct

 

[JAE]

The commentary refers to a 1964 paper. Over 40 years later and ACI hasn't omitted ties. I'd be hesitant to simply blow off a code requirement for ties based on the paper.

There was a parking garage that I once worked on where the original construction had omitted some ties and the engineer at the time required the addition of some 3/8" x 2" steel straps to be placed around the square columns. They fabricated the straps a bit short and then heated them to fit around the column hot. The ties were welded to form a complete perimeter square and then as they cooled they tightened around the concrete. Probably not immensely expensive and I would sleep better knowing I'd required it.

 

[henri2]

If the 6 in o.c is in the lateral direction, as would be shown in cross-sectional plan view, then check ACI 318 Section 7.10.5.3, commentary in R7.10.5 and Figure R7.10.5. Also, the ACI detailing Manual of Standard Practice for Detailing Reinforced Concrete Structures--ACI 315 has details which address this situation. I think the CRSI Manual of Standard Practice also has these details.

 

[WillisV]

I cannot think of how you could get around such a specific ACI requirement (nor for that matter why you would want to as if they truly need #8@6 around the perimeter those bars are going to be heavily loaded and need the ties to prevent spalling of the concrete and buckling of the intermediate reinforcing) - they need the ties.

 

[UcfSE]

I agree with JAE and WillisV.

 

[brizstruct]

Thank you guys.
I believe that the ties are required. THere is a need to provide local buckling restraint and the ligs also keep the vertical alignment try.
With lapped splices there is a need to provide restraint for the bars at that point so I didn't see it being a solution.
I was wanting to see if there had been some other research done that anyone else was aware of
Cheers
Brizstruct

 

[samdamon]

You don't mention the loads on or height of your columns. Check to see if the column geomtery is within the limits of Sec 22.8 and 22.10 in ACI 318. If so, you can design assuming plain concrete. Otherwise, I agree with above posts that intermediate bent bar ties are required for the columns.

 

[boffintech]

brizstruct, just came from a job that had these same columns 32"x32" w/#8 bars at 6" ctrs. I just do the pre-pour SI. The columns had outer ties clearly shown on the drawings but I could not find any cross-ties anywhere. Having not ever seen a column this big without cross-ties I searched all over the plans for any reference cross-ties or intermediate ties and studied the column schedule and column details. Couldn't find a thing. Finally I had to break down and call the EOR and ask what the secrete code was for figuring out the columns. He explained that the design didn't require cross-ties so there were none.

Does it make a difference if it's a pier and not a column?

 

[JAE]

ACI 318 doesn't apply to drilled piers in the ground...for short columns or long columns, ACI 318 would generally apply in areas that adopt the IBC, and other codes relative to U.S. jurisdictions and others that depend on the IBC.

There is no way any concrete construction can go without these cross ties unless you qualify under "Plain Concrete" of Chapter 22. Not sure what your EOR was smoking.

 

[JAE]

I might add that ACI 7.10.5 simply refers to "compression members" and does not qualify the column nor provides any calculation that can determine if you can ignore them.

Ask your EOR how he got around 7.10.5 which expressly requires cross ties. There is no exception that I'm aware of - ask him/her where the exception is. If you get an answer I'd sure like to know where it is.

 

[brizstruct]

All,
I can't see how ties can be avoided as per the standard. The code is explicit and I am yet to find research that would suggest otherwise.
Ties provide
a) Restraint against local buckling of the bars
b) Confinement of the inner core of the concrete
c) Vertical alignment control of the bars.
I beleive that compliance with the code is necessary unless you can argue that the member complies with the Plain Concrete requirements and the reinforcement is additional. (In this case, I think you would be unemployed due to the construction cost)
I did find an interesting paper in the Canadian Journal of Civil Engineering Vol 29 No 6 "Strength and ductility of laterally confined concrete columns"
Thanks for the comments
Regards
Mark Hickey