Old anchor bolts for a new vessel built to new siesmic code

[jharris3]

A tower that is being built to the new California seismic Codes (ASCE 7-10) and ASME VIII. I use Compress software to perform my calcs because the design is conservative. I realize that there are FEA methods to reduce the size of anything with the right amount of effort, however, I don't want to discuss all the various ways to improve the model.

The vessel is a "replace-in-kind" job and the client wishes to re-use the old foundation with the old anchor bolts. California Building Code (CBC) (according to the client CE) allows the tower to be placed on the old foundation apparently without modification to the foundation if the weight of the new vessel is within 10% of the old vessel. I read this part of the CBC and I agree in part with the clients interpretation of the Code.

However, the old anchor bolts are 1.5" dia and according to the calculations from Compress the new anchor bolts would be ~3.0" dia. I checked the design of the original anchor bolts by simulating the original Code of construction on the tower and it was close enough to accept that that the original design was correct at the time.

Now the client will not replace the anchor bolts with the larger size and will reject the vessel if the base ring is cut to fit the 3.0" anchor bolts rather than the 1.5".

Are the anchor bolts considered part of the new design and therefore required to be larger? Or since they are permanently secured to the foundation are they grandfathered as part of the foundation according to CBC?

I can't separate the anchor bolt calculation from my Compress output without removing all support calculations of the base ring and it wouldn't be appropriate to do so. However, the only way I make a 1.5" anchor bolt pass to the current siesmic loads(to grandfather them) is to increase its allowable stress far above reason.

Sorry for the long post but I wanted to see what CE's, structurals, and mechanicals had to say about this pickle.

 

[desertfox]

hi jharris3

From a legal standpoint I would say that you have to use the new regulations and anchor bolts.
Whilst I am no structural engineer I am thinking what happens when the structure comes to be inspected or a worse case situation that the structure fails.
Is the client willing to put in writing that he will accept full responsibilty for the tower to be replaced using the old anchor bolts while also being aware its not to current regulations?

desertfox

 

[unclesyd]

What is the number of the 1.5" and the required 3" bolts?

What are the respective anchor bolt materials?

Are you using chairs?

Can you post some information on the type and physical dimensions of the tower and details of the base ring.

This sounds way of as we don't have seismic but have wind loads that were changed from 90 mph to 120 mph and now 150 mph. There we some minor changes required in the anchor bolting arrangement, nothing as drastic as your OP. At one time Compress calculated for using few large bolts versus using many small bolts.

 

[jharris3]

The wind loads on the tower were the governing load at the time of construction and hence the 1.5" anchor bolts. If I remember right there were about 16 of them. I don't know the material anymore but it would have been whatever was common practice in the '70's for PV's.

The client was willing to accept the higher risk rather than replace the foundation and they did put it in writing.

The original base ring was a single plate with chairs..the new vsl has a double base ring. Interesting to note that the new base ring uses thinner materials but larger anchor bolts.

I would have used more smaller anchor bolts but I was restricted to the number existing in the foundation.

For all of you who want to tinker with the design:
The old vessel was actually a little heavier than the new vessel (thank you ASME for the 3.5:1 SF).
The diameter of the vessel is approximately 5' and it is about 75' tall with trays and 4-5' of liquid old-up in the bottom.
The site class coefficients are: Ss 1.5; S1 .6; D soil; I 1.0.
The vessel was 9/16 thick 516-70 plus a 1/8" ss cladding. It processed MEA.

Now my concern is mostly how the CBC has been interpreted. It allows the 10% rule for weight while also requiring that the load imposed on any component be within a certain percentage of the original.

The pickle is that the weight of the new vessel is less than old and the old has been working fine for around 40 years.

Also, since the only thing that has changed is the seismic Code the client makes the case that the actual loads on the anchor bolts haven't changed. In other words the way the weight of the vessel was used to calculate loads on the anchor bolts has changed not the weight of the vessel.

The client understandably wants to use the CBC to their advantage on this point and avoid replacing the foundation...possibly leading to the replacment of the foundation on several in-service vsls.

What I would like to know from the greybeards out there in PV's and the CE's and structurals especially is whether or not the CBC has been applied as intended?

I talked to a PE once about this and he thought that it was a bit lenient to interpret the CBC that way but he also said that if the foundation isn't damaged then the concrete should be stronger after all these years than a new foundation would be (not incl any additional steel added to the foundation)

-Thank you for your comments its a pleasure to discuss this with you.

jharris3

 

[unclesyd]

Just for curosity I'll check Monday with my bolting buddy about materials used for larger anchor bolts. I know that we had some made from AISI 1035 and 4130 both heat treated to around Rc28. In one instance there are anchor bolts made from B7 stock. We still used 2H nuts.
I will also make an inquiry on some recent work on similar size columns being replaced that were initially installed in the 60's and 70's. Off hand I know that some anchor bolts are under 2"

 

[MJCronin]

With regard to the total installed tower cost, isn't a new foundation a very minor part ?

Unless there is something unusual about the anchorage design, isn't this "chump change" ?

Unless.....of course ......you have someone who is very stubborn and reluctant to be reasonable who is representing the client...

What is the "rest of the story" ??

A new expensive tower failing because of "used anchor bolts" makes for great ironic television

 

[jharris3]

No, the foundation is not a minor part for most vessels. Consider that a new foundation would involve a new survey, new design to current Code etc. then it becomes very expensive.

Again I am not questioning that the CBC could be interpreted this way anything welded to the PV is part of the PV even if it is a non-ASME part (like the skirt or lift lugs). Likewise the anchor bolts are permanently attached to the foundation and the foundation is clearly exempted.

My question: Was the intent of the CBC to include the anchor bolts?

 

[chicopee]

It is ironic that the vessel is build to the new seismic code but the foundation is not being upgraded or rebuilt regardless of what CBC states. Study the old foundation design as it may be suitable even with the upgraded seimic code. You also have not stated the number of 3" bolts vs the number of 1-1/2" bolts.

I go with Desertfox suggestions if everything fails to change the owner's mind.

 

[jamesl]

I think ASCE uses 0.6*dead weight with seismic load as one of the load combination case , which is too conservative since we can predict the vessel weight pretty accurately. I guess that is the load combination case that got you into trouble. ASME Div 2 code(table 4.1.2) give exemption to this load case. Some vessel design guideline use 0.9 as a factor, which will help you a lot.

If you run a manual calc using shifted neutral axis method, you may be able to reduce the bolt size a little more.

If you have 16 bolts for a five feet diameter vessel, it is hard to add any more bolts.

 

[TomBarsh]

I've only quickly looked over this thread but I suspect that jamesl nails it. COMPRESS performs the ASCE 7 load combinations that combine 0.6*D with wind or seismic; these can be turned off for the reason that James suggests (see Set Mode Options). I believe that one of the PIP standards discusses this issue. There is just too much difference in bolt size to be explained any other way.

COMPRESS performs the shifted neutral axis method (from Jawad-Farr, Troitsky, etc) to determine the actual bolt tensile load. But this modified load is applied only to the anchor bolt chair and skirt loading, the anchor bolt itself is (conservatively) designed using the "traditional" P = W/N + 48M/N*bc) formula (which assumes that the neutral axis of the forces acting on the base ring/foundation is located at the vessel axis; in general this won't be the case due to the differing moduli of elasticity of the metal base ring and the concrete foundation).

Tom Barsh
Codeware

 

[jharris3]

Now those two replies make lots of sense, Thank you Tom and James!

So to sum up:

better concrete (good)

New code with worse coefficients (can't get away)

Killer load case (get rid of it!)

Conservative design method (check with a less traditional method)

I think the people writing the CBC are aware of this and that is why they allow a virtually identical structure to be placed on the same foundation with the same anchor bolts. They may not have considered pressure vessels explicitly but I think I will feel much better about doing this in the future should I be asked to do so.

 

[unclesyd]

Even though I don't' think that after recalculating you will require larger anchor bolts here is an idea.
If by chance you need to use larger anchor bolts there may be an out using mechanical anchors in the same pad and footprint. It would require a little ciphering and calculations. I believe it can be done.
I haven't been able to catch my anchor bolt guy or the record keeper at the plant but if you could get a small piece of the current bolt and check the hardness you may be lucky and have a low alloy anchor bolt with the much higher allowables.

http://www.drillcogroup.com/maxi_bolts/large_bolts/case-study.html

http://www.drillcogroup.com/maxi_bolts/engineering/load_spacing_table.html#loads_a36

 

[dig1]

I have come across this situation more than once and in general it is a replacement of a corroded vessel with the same size vessel in a more corrosion resistant alloy.

The anchorage is always an issue and what we have now done is have a meeting with the local code enforcement/building official. In the end it is up to them if they will accept the re-use of the foundation. In every case I have been involved with, we have had the original foundation loads and have been able to prove that the bolts were of adequate to resist the original loads. In some of these cases there was no seismic required in the original building code and now there is and it controls over wind.

Replacing the foundtion can be extermely expensive because many of thee facilities would have been down or running at reduced capacity for the amount of time to demo the original vessel and foundation, construct a new foundation, and then install the vessel.

In any event, contact the local building offical to make sure they are comfortable with what is being proposed.

Patrick

 

[jharris3]

I went back and checked the numbers using James, Tom, and Patrick's comments and the results were positive.

Two things to remember:
Load combinations include SF
Replaced .6D with .9D as a check and also turned off the .6D comb in Compress

I used the shifted neutral axis on the anchor bolts with loads from the above combinations to find the tensile stress in the bolts and it was less than 36000 psi (specified minimum for A-307 anchor bolts).

Finally, as Patrick pointed out the client was able to work out an agreement with the jurisdiction to allow the foundation and anchorage to be reused.

Thank You!