Large Concrete Column, < 1% Reinforcing
Large Concrete Column, < 1% Reinforcing
(OP)
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?


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?








RE: Large Concrete Column, < 1% Reinforcing
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.
RE: Large Concrete Column, < 1% Reinforcing
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.
RE: Large Concrete Column, < 1% Reinforcing
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.
RE: Large Concrete Column, < 1% Reinforcing
RE: Large Concrete Column, < 1% Reinforcing
That is a LOT of spirals!
RE: Large Concrete Column, < 1% Reinforcing
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.
RE: Large Concrete Column, < 1% Reinforcing
RE: Large Concrete Column, < 1% Reinforcing
RE: Large Concrete Column, < 1% Reinforcing
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.
RE: Large Concrete Column, < 1% Reinforcing
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. ;-)
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.]
RE: Large Concrete Column, < 1% Reinforcing
RE: Large Concrete Column, < 1% Reinforcing
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.
RE: Large Concrete Column, < 1% Reinforcing
RE: Large Concrete Column, < 1% Reinforcing
http://www.cement.org/for-concrete-books-learning/...
RE: Large Concrete Column, < 1% Reinforcing
AASHTO 5.7.4.2 still refers to 5.10.11.3 (et. al.), which still stipulates a minimum of 0.01Ag.
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?
RE: Large Concrete Column, < 1% Reinforcing
RE: Large Concrete Column, < 1% Reinforcing
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.
RE: Large Concrete Column, < 1% Reinforcing
Minimum reinforcement is meant to provide for forcing multiple closer spaced cracks to distribute the strain over a much longer length of the bar.
RE: Large Concrete Column, < 1% Reinforcing