Bolted bracket support bolted to existing spirally reinforced column
Bolted bracket support bolted to existing spirally reinforced column
(OP)
Does anyone have any comment on my proposed new bracket fastening to an existing square but spirally reinforced column, to support a parking garage beam that has to be broken out and re-poured due to severe corrosion of rebar? Please see attachment. Is it practical? Is there any concern about bolting into the "over concrete" that is not reinforced? I have included in my calculations all the Hilti recommended reduction factors for edge distance and centre-to-centre spacing. The alternative is to put a one storey steel post down to the caisson cap.






RE: Bolted bracket support bolted to existing spirally reinforced column
As proposed, I think that flexibility in the bearing plate will result in an effective bearing width closer to 3" than 6". I'd prefer to see something beefier in that regard like a more conventional stiffened seat assembly with triangular stiffeners etc.
With such a shallow beam, I'd expect quite a bit of rotation up against the column once you make the beam simply supported. That might open up a sizeable crack against the column face. It may be something to consider in your durability detailing.
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: Bolted bracket support bolted to existing spirally reinforced column
Good to hear from you. Very perceptive of you -- it is indeed a 12" deep slab band, although it is labeled and scheduled as a beam (with no stirrups) on the old original drawings.
I have not yet actually designed the cap plate...the thickness was just a bad guess...I can make it as thick as need be for stiffness (and strength) ...can be 1" or 1½" thick.
The slab band is actually continuous past the east face of the column on the east side of the column, the span of the 12" thick slab band is only 14 feet, and I am going to add north-south top bars on the east side of the column (can't do too much on the west side because there is and east-west ramp that starts up at the south side of the column.
I have still to check the punching shear stress.
I was thinking that you might say that there was no guarantee that the column spiral was where it is supposed to be (due to construction tolerances) and they might hit it when drilling for the anchor bolts (although I have tried to allow some clearance in selecting the anchor bolt embedment length).
A 3½"x3½" HSS steel post down to a short concrete pedestal poured down to the top of caisson cap and extending 4" above the slab on grade would be a more foolproof option, although it would jut slightly into the drive aisle and probably a little more expensive than the bracket.
What do you think?
RE: Bolted bracket support bolted to existing spirally reinforced column
Even if it were 4" thick, I still wouldn't like it. In my opinion, the main issue is that the plate is cantilevered from a weak axis connection to some pretty flimsy channels.
That's much shorter than I had imagined. Even at 12" deep, it should be relatively stiff and lightly loaded which alleviates some of my concerns.
This confuses me a bit. Beam shear will be a one way check according to your detail. Are you doing a different kind of connection around the other three sides that would justify punching shear checks?
I see what you mean. Now that I think of it, I'd prefer the use of drill and epoxy bolts here for two reasons. Firstly, wedge bolts are going to encourage spalling of that thick concrete cover in the corners. Secondly, with a drill and epoxy bolt, it's a bit easier to just stop it a bit short if your run into something. Maybe add a few extra bolts to provide this kind of reserve.
I'm sure it would work but I like what you've proposed better. I'm not sure how you'd deal with punching shear at the top of the post and I'm sure that you'd save some $$$ not messing with the slab on grade.
Now that I understand the column reinforcing arrangement here, I'm wondering if you could just chip out a concrete bearing ledge instead of messing around with the steel bracket. You could make a nice wide ledge out of the corners of the column without even messing with the spiral ties. And, as we've discussed previously, punching shear stresses are largely concentrated at column corners anyhow so it would fit the theory well to do it this way.
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: Bolted bracket support bolted to existing spirally reinforced column
I am not keen on adhesive anchors because of bad experience when we have load tested them on other projects and many of them failed at well below half the load that they should have been able to resist. Our experience is that contractors just won't take the care and time to install them properly, including cleaning the hole to the meticulous detail required by the manufacturer. I respect that you must have had different experience with adhesive anchors or you would not recommend them, but the only way I would use them is if I am present when the contractor is installing them.
I am not sure if it is one way shear or two way shear...I will check them both.
Costs are not that much of a factor on this half million dollar repair project, so I don't see the cost of breaking thru the slab on grade at one column as being such a big deal...the main question is whether the post will encroach on anything.
I just realized that the slab has been chopped away from not only the north side of the column, but also from the west side of the column. So that may require brackets on both the north side and on the west side, which may make bolted brackets unworkable.
Thanks for raising all these points and for your thoughtful comments and recommendations. It is very helpful and very much appreciated. As I work my way thru the calculations, I will undoubtedly revise my thinking further...
RE: Bolted bracket support bolted to existing spirally reinforced column
Support by chipping into the existing column as you suggest would be neater, but I am not sure that I am comfortable about doing that. Probably need to chip in at least 4"...
RE: Bolted bracket support bolted to existing spirally reinforced column
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: Bolted bracket support bolted to existing spirally reinforced column
RE: Bolted bracket support bolted to existing spirally reinforced column
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: Bolted bracket support bolted to existing spirally reinforced column
RE: Bolted bracket support bolted to existing spirally reinforced column
1) You'll probably want a self consolidating, low shrinkage mix. If the new bits on the side of the column shrink up to much, you'll end up with load being transferred through the dowels.
2) I'd put the uppermost horizontal bars inside the verticals since that bit of concrete is kind of cantilevering off from the nearest hairpin.
3) Hopefully the column is still braced by some monolithically cast concrete on one or more sides. I wouldn't want to have to rely on the repair for that in tension.
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: Bolted bracket support bolted to existing spirally reinforced column
A special thanks to your positive contribution to this project and many helpful and well taken points.
I did not intend to divulge my name though.
RE: Bolted bracket support bolted to existing spirally reinforced column