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.

 

[AskTooMuch]

I thought the proper way is actually to replace the anchor bolts. Existing will be core drilled and replaced with post tensioned bolts if I recall right.

 

[AskTooMuch]

https://structuraltechnologies.com/...or-bolt-restoration-strategies-are-available/

 

[Klitor]

AskTooMuch - second method can be done on piled rafts also (excavation on the edges under pile cap is needed).

 

[MotorCity]

How do you remove cast in place anchor rods?

 

[harrieng]

Thank you all for the responses. Unfortunately, we had considered these options, but the configuration and size of the vessel and anchor bolts prohibits this. I was hoping someone had experience with multiple anchor bolt circles to allow for enough post installation A.B.'s.

 

[dik]

I would suggest a new anchor plate and anchor bolts... cutting the old ones off flush. It's difficult work. Drilling the existing ones out by coring is not easy. Can you weld to the top of the existing ones? I always spec tha anchor rods be weldable.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[KootK]

I was hoping someone had experience with multiple anchor bolt circles to allow for enough post installation A.B.'s.

I haven't done this personally but I'd be willing to if it were set up right. Without the chairs and stiffeners, I see the real difficulty being in finding a way to avoid, or deal with, prying actions in the base plate. With the the right chair & stiffeners setup, however, that concern goes away. I'll try to comment further if you post a cross sectional sketch of what you've got in mind.

Would you prestress the anchor rods?

 

[TLHS]

As long as your load distribution between the bolts makes sense for your expected failure mechanism, I don't see why it wouldn't work. Your plate should probably be nice and stiff, too.

What is the problem you're having re: the post installed anchor bolts? I've done a bunch of work where I've upgraded old vertical vessel foundations for current code seismics and wind, which sounds like a similar problem. I've often managed to get post installed anchors to be designed such that the steel governs rather than anything to do with the concrete anchorage. What's the problem you're hitting?

Also, what are you using as the basis for your anchorage forces? Design basis is often that the neutral axis is at the geometric center. Depending on the vessel design, the skirt, and everything else, you can often start pushing that neutral axis further over so that you put less of the base plate in compression, pull more anchor bolts into the tension zone and increase the moment arm of your tension rods. You have to be comfortable that your primary failure mechanism is ductile, though. Depending on the geometry, you can start bringing the forces down significantly like this.

What's the governing load condition? It's presumably wind, but there are some other things to look at if it's seismics.

 

[harrieng]

Sorry, I have been out of touch on this. The issue is that the vessel is approximately 300ft tall and wind is the governing load. The original design was in the 1950's and most likely would not meet today's standards. Concrete pull out is the governing problem with anchor bolts spaced approximately 18" on center around the bolt circle of the new anchor base plate. We just recently found another supplier of post installed anchors that have a higher capacity and possibly increase our anchor bolt spacing. Our existing rebar spacing is 6" O.C. with #11 bar, which is also concerning about cutting through the bar. We've looked at moving our anchors out to reduce load, but that hasn't helped enough to make a difference. Thanks for any feedback.

 

[WinelandV]

At 300' tall, instead of messing around with the anchorage, could you fab additional collars to attach guy wires to? I assume you've got at least 5 levels of guy wires helping hold this vessel up.

Please note that is a "v" (as in Violin) not a "y".

 

[harrieng]

No guy wires. That's not allowed the way the process piping is on the vessel.

 

[JoshPlumSE]

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.

Vessels are very similar to the types of monopoles used in cell towers / communication. I mention this only because I have an excellent reference for the base plate design of these monopoles. There are a number of different ways to analyze these bases. This reference might give you a better way that results in less demand on your anchors.

https://www.towernx.com/downloads/Technical_Manual_MP_BasePL.pdf

 

[dik]

Is the foundation adequate for the new design loads?

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[harrieng]

Yes, the foundation is adequate.

 

[harrieng]

We keep getting concrete breakout failure due to the ACI 318-14 reductions for post installed anchor bolts

 

[JStructsteel]

Can you space the supports out on the footing by adding new ones each side of existing and then dividing the load to each? So if you had 4 legs before, now you have 8, but working in pairs of 2.

 

[harrieng]

It's a skirt ring support, so we have to create a new ring further out. But even adding more anchor bolts and a larger bolt circle, the anchor bolts have too much break out area overlap.

 

[dig1]

harrieng,

We did a 2nd bolt circle on shorter vertical vessels twice w/ post-installed anchors; this was about 10 years ago. It barely worked code wise only because we made the inner bolt circle (inside the skirt) in far enough and there was enough circumferential spacing between the existing bolts so we had enough space to make it work. These were difficult projects and I appreciate your situation and know exactly the situation you are dealing with.

On the last project we did the 70 bolts were spaced about 11-1/2" chord length, 2" diameter (the bolts were necked down to about 1-1/4" just above the top of the base plate), and the skirt was tapered vertically in. There was no room to put a anchor bolt circle on the exterior and inside was not an option either due to equipment inside the skirt. In this case the only solution was on a very limited number of anchor bolts at a time to cut the top plate, base plate, remove the gussets and a portion of the skirt, mill down the grout and concrete to a point where we could thread the bolts, add a coupler, and anchor rod extension, repair concrete/grout, CP weld in a new section of base plate and skirt, top plate, and new gussets. This was heavily reviewed with the contractor and owner's corporate engineering and ultimately agreed this was the best solution given the constraints this had to be done on the run.

Probably not an option for you but another scenario we have evaluated in the past (but could never make work) is to install a new outwardly tapered skirt with base ring. There would be essentially a girder at the new skirt to shell or existing skirt junction to handle the horizontal thrust. If you could get the taper out far enough you may be able to get the spacing of the bolts far enough to make post-installed bolts work; however, with a 300 ft tall vessel I suspect it would still be difficult.

 

[JStephen]

That skirt option or adding big knee braces were ideas that had occurred to me. In either case, you change the load location on the concrete appreciably as well, so you can't just figure an exact replacement of bolts.