Bolted bracket support bolted to existing spirally reinforced column

[ajk1]

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.

 

[KootK]

It should be build-able and I'm fine with connection to the cover concrete. It's the bearing seat that bothers me. The beam is pretty shallow (12"?) so I'm assuming that it's more of a slab band / continuous drop type of member. In it's new form, the beam will be simply supported at the column. As a result, properly developing your bottom steel over the support will be important.

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.

 

[ajk1]

To Kootk:

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?

 

[KootK]

I can make it as thick as need be for stiffness (and strength) ...can be 1" or 1½" thick.

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.

the span of the 12" thick slab band is only 14 feet

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.

I have still to check the punching shear stress.

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 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)

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.

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.

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.

 

[ajk1]

I pretty well agree with much of what you raise, except I am not sure I would be entirely comfortable chipping into an existing column that supports a 12 storey building. But I will give it more thought. It would be a neat solution.

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...

 

[ajk1]

Another option is to pour a 4"± thick concrete addition to the west and north sides of the existing column (this concrete would be "L" shaped on plan), and supported on the pile cap. It can be tied into the existing column with shallow anchors to stop buckling, and reinforced vertically. This seems to address all concerns, and avoids the need for spray fireproofing, avoids concerns about conflict of anchors with existing column reinf., blends with the original concrete structure, is not excessively costly and lets me sleep at night.

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"...

 

[KootK]

Seems like a perfect application for this technology really: Link

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.

 

[ajk1]

Not sure what you mean - do you mean the shear collar? If so, it may be problematic in my case because the ramp runs by one face of the column.

 

[KootK]

Yup, the shear collar. Maybe I'd need to see a plan view to understand why the ramp would cause problems. At first blush, it would seem that the collar could be installed below the flat slab and the extra space up to the underside of the ramp slab just filled with more concrete. Or would that interfere with headroom under the ramp?

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.

 

[ajk1]

I have pretty well decided to do what I have shown on the attached. I know it is not as elegant as what you are suggesting, but it is totally conventional, and I can sleep at night. I hope you don't point out something fatally wrong about it! Yes I am using adhesive to anchor the 10M ties, but the forces are so small that I don't think they can screw it up that badly.

 

[KootK]

Looks good ajk1. Usually simple is best. Some recommendations:

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.

 

[ajk1]

To Kootk - You have brightened my day! The idea of using self consolidating concrete is a great idea. I should have thought of it. Yes the column is still braced on at least 2 sides, and on the 3rd side by a 14x20 beam at this level, and just below that by a 24x18" beam that supports the ramp slab.

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.

 

[ajk1]

ok, thanks.