Vessel Tower Anchor Bolts

[harrieng]

I am working on a vessel tower project for an existing vessel that has corroded anchor bolts. It is impossible to replace the anchor bolts, so we are considering post-installation of anchor bolts and adding to the baseplate/chairs. The pile cap is very large and available for additional anchor bolts. The problem is post-installation of anchor bolts per ACI-318-14 has several reduction factors so we can't get enough anchor bolts in one bolt circle to handle the load. Has anyone ever used multiple anchor bolt circles for a vessel support? Any ideas are appreciated.

 

[harrieng]

dig1,
Yes, we can install an outer bolt circle and can reduce our loads some due to the additional moment arm, but it's just not enough. One thing we are considering to get around the ACI 318-14 Paragraph 17 is to do on site testing of the post installed anchors to get empirical results. I'm not sure if anyone has ever done this for a vessel tower like this before or not.

 

[dig1]

harrieng,

We required a pull out test in the pile cap to verify the single angle capacity; a group of anchors was not tested to see if a greater group capacity was available than the literature/code rating.

 

[harrieng]

dig1,
Thanks. I think that's the way we will have to approach this.

 

[HTURKAK]

harrieng,

Although there are some questions regarding the intact of foundation, fitness for service of the tower which is 70 yrs old , if the problem is adding new anchor bolts , i will suggest the use of anchor bolts with dia 50-65 mm rather than post installed anchors with dia LT 25 mm etc available at market.

The new holes would be drilled with core drilling with dia 100 mm or more and the anchor bolts having end plate dia 90 mm etc would be epoxy anchored .

The anchor bolts should be connected to process tower with anchor chairs . The top plate of anchor chairs could be continuous as necessary..

 

[TLHS]

Old tall vessels failing wind isn't unusual when you use the modern formulations that include the effects of slender items and resonance.

You've said that they fail breakout because of reductions for post installed anchors? What problem are you actually hitting? If it's just the K value being 80% of the factor for cast in, what's stopping you from going a little bit deeper?

FYI, I have installed Hilti epoxy anchors significantly deeper than shown on the typical tables for anchor rods in situations like yours. I've done it to make sure my breakout cone would intersect with the perimeter vertical bars in the foundation pedestal. Deeper embeds are not all that unusual on the rebar side of things, and when you're attacking from the top you can use the fancy stand drills that make things a lot easier. You can also go larger diameter than Hilti has documentation for, but you probably want to build some conservativeness into that.

I would personally be figuring out my design load, finding the smallest anchor rods that hit that, and then embedding them in such a way that the anchor rod steel governs and I have stretch.

Also, I mentioned this before, have you looked at shifting the neutral axis at all? It can make a big difference on these types of structures that are predominantly in bending as long as you're comfortable that the skirt and similar areas will hold up to the compression load being concentrated heavily on one side and your bolts are failing in steel tension.

I'm much more comfortable getting analytically aggressive in this kind of situation as long as I can show that I'm meeting or exceeding the existing installation. It's been there for decades, so as long as I can find a justifiable way to make the numbers work I'd be pretty comfortable using as much of the toolbox on the load generation side as I can to bring the demand numbers down.