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iv63 (Structural)
5 Jun 12 19:13
After evaluating the existing vessel foundation we found out that vertical dowels between pedestal and mat are NOT adequate. As the most cost effective repair we are thinking of drilling (32)~1-1/8”Ø x 90” deep holes between existing dowels and anchor bolts and setting #8 rebars with HILTI HIT HY 150 ADHESIVE (please see attached sketch). Our main concern is if drilling new holes will disturb/weaken existing anchor bolts. Your comments/suggestions would be appreciated.
Thank you,
iv
ishvaaag (Structural)
5 Jun 12 19:51
It seems to me that the hilti aditaments falling entirely within the shearing surfaces of the already installed bolts, will add scarcely any strength if anything if such surface controls the design. If the design was steel controlled, then it could be otherwise. But if my suspicions stand, I would entirely discard the reinforcement scheme and inmediately would be searching for more positive attachment to the foundation.
delagina (Structural)
5 Jun 12 20:50
why did you come up with that conclusion? is the foundation failing/cracking? or is this one of those retrofit projects where you add loads in the vertical vessel and deemed foundation not adequate. i'm curious coz 1% steel for pedestal is usually more than adequate.
iv63 (Structural)
5 Jun 12 22:08
This is the retrofit project with new loads added to the vessel. I am using 4M/(D*N)-0.9W/N to get tension force in one dowel please see attached calcs.
delagina - where did you get 1% of steel?
delagina (Structural)
5 Jun 12 22:55
nevermind, i was thinking about pedestal for column/equipment, where you use 0.5% or 1% for rebar reinforcement.
JStephen (Mechanical)
5 Jun 12 23:12
Perhaps consider drilling the new dowels outside the existing pedestal, then pouring additional concrete around the pedestal.
iv63 (Structural)
8 Jun 12 12:27
JStephen - In this case new dowels are too far from A.B. to transfer tension from anchor bolts. How to achieve bond between old and new concrete?
After further “brainstorming” we are leaning toward octagonal “ring” plate (does this new plate need to be welded to the vessel base ring?) on top of existing pedestal anchored to the mat with all threaded rods (or rods with threads on each end) located just outside of the pedestal (see attached sketch). This seems to be the most cost effective. However other ideas (“outside the box”) are very welcomed.
Thank you all for your interest.
iv
dhengr (Structural)
8 Jun 12 16:34
Iv63:
Your latest design is an improvement in one respect and not so good in another. You have saved 6 miles of drilling, some probably through existing rebars, and gallons of epoxy. I trust that you will pour a new conc. wall, as tension rod cover, with some horiz. reinf’g. hoops. The thing I don’t like about your latest design is that you are trying to strengthen or take some of the tension loading from the tank column shell and existing A.B’s., which are very stiff attachments on very stiff chairs. But, the way you are doing this is through plate bending, with a varying canti. length to boot, a very flexible (soft) attachment means. This will not pick up as much load as you wish it would before the original A.B’s. start to overstress. They must stretch too much to make the plate flex enough to load the new tension rods, and then tension in the rods will still be very low. I trust that the tank column and the A.B. chairs are still o.k. with the new loads and A.B. tensions.

I think your new system wants to engage the old system right at the chairs where your design details can control the load out to the new tie rods. It appears that the existing A.B’s. and the corners of the octagonal pedestal are symmetrical w.r.t. each other. So, over a 45° arc, with a pedestal corner at the center of the arc, you might design 8 identical new chair systems which would fit over the existing chairs and maybe even under the existing A.B., and out to the new tie rods. Now, you can design this with varying stiffener (web) stiffness to account for the different canti. lengths out to the new tie rods. The top and bottom flange plates on this box section might stay the same. The existing A.B. pattern must be matched but the new tie rods will flex a bit to fit their hole pattern. Your new chair system would actually react on the outer edge of the new conc. wall, so your chair system clamps down on the old chairs. Then you might also vary the spacing of your new tie rods to optimize their action w.r.t. the varying canti. span lengths on your new chair system. Maybe these box beams should be 8 units for the long canti. spans, centered over the pedestal corners; and 8 units for the short canti. spans, each covering a 22.5° arc and engaging two existing A.B. chairs. Now you might vary the flanges and webs on these box beams and probably end up with something more easily fabed and welded (weld accessibility).
hokie66 (Structural)
8 Jun 12 18:55
I must be missing something. Those #8 bars are only ever going to have to resist the mass of the footing and earth over the footing as the footing tries to rotate. The way I figure it, about 11 kips is all that one bar will need to carry, well within its capacity.
dhengr (Structural)
8 Jun 12 19:31
Hokie:
Another problem not too well defined and explained. But, I suspect he is running into a problem with the existing A.B. shear cone pull-out and edge distance and spacing situation and the newer ACI app D. It’s a crazy thing that those A.B’s. worked all these years, not knowing that they were working under the wrong (an inferior) code criteria. And, in his first design solution, the new standing rebar, inside the pedestal, was an attempt to get some steel across the cone shear cracks. Although, if that be the case they might not have to go all the way down into the ftg. and more bars of a smaller size might have been more effective.
hokie66 (Structural)
8 Jun 12 19:57
He said in the first post that the problem was at the footing to pedestal joint. Maybe he has other issues. For instance, I wonder if he checked the footing for top bending, as those top bars in his sketch look nominal.
iv63 (Structural)
9 Jun 12 9:43
I have two issues:
1. pedestal to footing joint
2. transfer tension from anchor bolts to vertical dowels
hokie66 - how did you get 11 kips/bar? I am getting about 50 kips/bar using 4M/(D*N)-0.9W/N (please see calcs in my second post) while "allowable" is 0.9*fy*As = 0.9*40*0.79 = 28 kips. Also I have checked the footing for bottom bending only. Top of the footing is always in compression, isn't it?
Regards,
iv
hokie66 (Structural)
9 Jun 12 18:10
If the top of the footing is in compression, the vertical pedestal bars do nothing. The overturning moment creates bending in the footing, similar to a retaining wall.

My sanity check for the force on the vertical bars was just to add up the total mass of the footing and soil over the footing. I used density of 150 pcf for both concrete and soil, and got 26 x 26 x 8 x 150 = 811 kips. Dividing this by 72 bars, I get 11 kips as a working load. Therefore, the bars are capable of carrying the entire load if you try to lift the footing, and that is all that will ever be required to mobilize the footing in bending or uplift. I won't try to interrogate your program or calculations, and maybe you do have problems, but not the one you were trying to solve.
iv63 (Structural)
11 Jun 12 17:19
hokie66:
I do not follow your logic. Where does the moment from wind go? Would you, please, elaborate? Attached is my FBD.
iv
hokie66 (Structural)
11 Jun 12 18:06
The moment has to be resisted by the mass of the footing, pedestal, and soil over the footing. In order to engage the footing and soil mass, the footing would have to be subject to bending, thus my question about the footing reinforcement, particularly the top bars.

You seem to be focussing just on the actions, without consideration that the reactions may not be be adequate. My "sanity check" was just to show that the bars in the pedestal are adequate to engage the resisting mass, but I haven't looked at whether the resisting mass is sufficient. I also didn't look at the anchor bolt design, as that wasn't part of your original question, but I do suspect they may be a problem.



hokie66 (Structural)
11 Jun 12 20:29
iv63,
Let me try to be a bit clearer. I have done a quick calculation on the moment capacity of the pedestal, and I agree that the reinforcement is insufficient to resist your wind moment (actually the moment is shown on the elevation as 9500 and in your calculation as 8500, but no matter. What I was trying to emphasize is that you didn't go far enough in your analysis, as the footing is not large enough to resist the overturning, and thus augmenting the pedestal doesn't help without increasing the overturning capacity of the footing.

One suggested way of increasing the overturning capacity would be to drill ground anchors through the footing outside the pedestal, and cast a new pedestal surrounding the existing, incorporating the ground anchors.
iv63 (Structural)
12 Jun 12 8:33
Hokie:
Thank you for the clarification. Actually I do not have problem with overturning because mat is 72ft long with three more smaller vertical vessels on it. I am still looking for the most cost effective pedestal reinforcement (pedestal to footing joint and transfer tension from anchor bolts to vertical dowels).
hokie66 (Structural)
12 Jun 12 18:08
But how much of that 72' can you count on in the weak direction? You would have to depend on torsion of the footing to transmit some of the load along the length. The footing is not reinforced as a torsional element. Don't the smaller vessels also attract wind?
racookpe1978 (Nuclear)
13 Jun 12 3:20
Let me go back to the OP for a moment.

Why 90 inch deep anchor bolts?

The pull-out "cone" of each anchor bolt is going to be 45 degrees from the base of the bolt through the concrete to the surface, but his spacing is so close that the extra depth is going to overlap from bolt to bolt so there is no added effectiveness from going deeper, right?

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