Large Concrete Column, < 1% Reinforcing

[justhumm]

I am looking at the proposed replacement of an existing bridge pier column. The cross section I'm looking at is large (approx. 10-ft x 14-ft). Even when using reduced section/strength properties similar to ACI (318-08, 10.8.4), initial analyses indicate that the section is adequate with only 33% reinforcing steel.

Looking at a rough sketch of the cross section, it doesn't seem like it should be enough steel, but looking at the size and spacing of the bars, it seems like it would be excessive. So part of me is starting to question if using the classical stress-strain relations for bending would still apply for a column of this size.

Does anybody know of any references or specifications that discuss bending of large concrete columns and minimum steel requirements?

 

[KootK]

So part of me is starting to question if using the classical stress-strain relations for bending would still apply for a column of this size.

Depends. Is this thing 12' high 120' high?

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.

 

[justhumm]

The height I'm looking at is 45-ft +/-.

I don't have the KL/r & limits in front of me, but having seen the existing columns (built pre-1% in early 1930's) with my own eyes, I can't imagine they would be classified as slender.

I think this is the biggest column section that I've ever come across and (design codes aside) I'm having a hard time wrapping my head around the behavior. All insight would be appreciated.

 

[KootK]

Height to width of 4.5 isn't so far off as to toss Bernoulli assumptions in my opinion. I imagine this is only a design issue if you're trying to get a highly ductile response from the pier under seismic. The original pier was probably designed using working stress design and may be just architecturally proportioned even at that.

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.

 

[wannabeSE]

I don't no much about bridges or huge concrete columns. But, has any consideration been given for the ties? Does AASHTO or other bridge codes have different requirements than ACI 318? If not, it might be worth considering an equivalent circular section using spirals or circular ties. Last year, I saw and article,Seismic Performance of Concrete Columns with Innovative Seven- and Eleven-Spiral Reinforcement , in ACI Structural Journal about columns with overlapping (interlocking) spirals (I just looked at it quickly because I don't do bridges).

 

[Ingenuity]

Seismic Performance of Concrete Columns with Innovative Seven- and Eleven-Spiral Reinforcement

That is a LOT of spirals!

 

[JedClampett]

It's been a while, but in the power industry, we used to use sections that big to make up the legs of turbine pedestals. Maybe you can look up some designs from that area.
And why limit yourself to #8 bars? If there was ever a case for #11's, this is it. About one half the number of bars.

 

[hokie66]

I would hesitate to use less than 0.5% Ag longitudinal reinforcement. I agree with larger bars. And due to the size of these elements, they should be considered mass concrete, with appropriate measures taken.

 

[bridgebuster]

When was the bridge built? I ran across something similar recently, although in my case it was an under-reinforced wall pier, and I mean under reinforced by current standards. Nevertheless, if you look at early 20th C text books on substructure design, the idea was to keep the resultant within the middle third thereby eliminating bending. As a side note, AASHTO didn't address minimum reinforcement in wall piers until the LRFD spec.

 

[KootK]

Additional thoughts:

1) I've seen some building "super columns" where some of the reinforcement was moved to an inner cage to make things a little less crazy looking. Not sure if that flies in bridges.

2) My understanding of the min reinforcement requirements in columns is that it's there in homage to the expectation that concrete creep will ultimately shift more stress to the rebar than initially assumed in conventional design. If your axial stresses are so low that you're unlikely to have meaningful creep, perhaps there's justification for less reinforcement. Still, it would be a pretty bold designer move to step out on the code minimum requirement.

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.

 

[justhumm]

@bridgebuster
Construction was sometime in the late 1920's to early 1930's. So there's proof that the (old and likely "under-reinforced") design works. I'm just interested in proving it to myself and the project manager. ;-)

...And why limit yourself to #8 bars? If there was ever a case for #11's, this is it...

..if you're trying to get a highly ductile response from the pier under seismic...

Ductility shouldn't be a design issue. It's a reasonably low seismic zone. My first thought in trying to stay away from #11's is just constructability.

I had blankly written off the idea of an inner rebar cage a while back, but the idea is warming up to me.

[CAVEAT: I haven't finished my first cup of coffee yet.]

 

[BridgeEI]

If you're using AASHTO...check out article 5.7.4.2. It has a minimum area of reinforcing for columns. When I'm dealing with columns of that size, i am almost always forced to use #11s so I can obtain that area required and just to get spacing between the bars. Occasionally I have had to do multiple cages of rebar but each DOT has their own preference regarding that.

 

[bridgebuster]

@justhumm: As my old squad would say "bridge is still standing." If you have time, go to Google books and download "Bridge Engineering" by Waddell (1916) or "Foundations of Bridges and Buildings" by Jacoby (1914) it'll give you some insight on how our ancestors did things; not much theory. It seems they were more concerned with global stability.

I recently reviewed plans for the rehabilitation of a two-span bridge built in 1940. The pier is 150" long, 3'-6" wide, and 20' tall. The total load on the pier is over 9,000k, most of which is the load from a bridge supported by this one. The reinforcement ratio (vertical bars) is essentially zero; although it is reinforced horizontally per Waddell's text book. Still standing. One caveat: I worked for the designer of the bridge throughout the 80's; he was a brilliant engineer. One day when I have time I'm going to perform a design of the pier to see if it works, in theory.

 

[sponton]

I agree with KootK regarding creep, it makes sense considering how massive the thing is and the proably very large sustained load it carries.

 

[hokie66]

Some guidelines about mass concrete placement:

http://www.cement.org/for-concrete-...logy/concrete-design-production/mass-concrete

 

[justhumm]

...check out article 5.7.4.2. It has a minimum area of reinforcing for columns...

AASHTO 5.7.4.2 still refers to 5.10.11.3 (et. al.), which still stipulates a minimum of 0.01Ag.

...building "super columns" where some of the reinforcement was moved to an inner cage to make things a little less crazy looking...

And just thinking about it, an inner layer of bars could also make tie placement a little easier. Though I'm not aware of any widespread DOT uses of that approach. Does anyone know of some "standard" DOT column details that use two layers of rebar?

 

[justhumm]

...

 

[BridgeEI]

No standard details I know of. If I were going to use two layers I would have several pairs of stirrups that overlapped with each other and the rebar on the inside layer would line up with the outside layer. I don't have access to a scanner at the moment, but the stirrups would overlap in the 14' dimension and be full height in the short dimension of the column.

In the sketch you shown I don't know if your confinement ties would be adequate. I don't remember what AASHTO says right off hand. I was thinking the ties had to have 135^ hooks.

 

[Agent666]

Minimum reinforcement requirements are also to ensure when the concrete cracks, that the moment strength is higher than the cracking moment. Otherwise there can be a drop-off in strength, this really bad if you rely on any ductility as you get a single crack which concentrates the strain over a very short length and can cause fracture of the bars.

Minimum reinforcement is meant to provide for forcing multiple closer spaced cracks to distribute the strain over a much longer length of the bar.

 

[WARose]

Something of that size just kind of begs for just treating a perimeter width as effective only......while the interior is just fill (for structural design purposes). I don't know if AASHTO allows that, but as jed alluded to above, it's a fairly common thing to see in the power business. You would have to be sure not to anchor/sit anything anything in a unreinforced zone.