Flexibility of flat bottom tanks for seismic 1

[CDLD]

Good morning Eng tippers,

For flat bottom vessels, is it safe to rely on the operating dead weight to counteract seismic overturning moments, when looking at anchor bolt tension?

I am concerned that the flexibility of the bottom plate will prevent the content dead weight from counteracting bolt tension.

PIP STE03350 has this equation for calculating anchor bolt tension, however I am unsure if it is equally applicable to flat bottom tanks as it is to skirted tanks.

Thank you.

 

[Geoff13]

It might be better to post this question in forum1452.

What Code is your flat bottom tank designed to? Why are you thinking of using PIP for the anchorage design instead of the applicable Code for the tank design.

If it's an API 650 tank you should use the rules therein for the anchor bolt loading. API fully addresses what dead, product and live loads are applicable for both unanchored and anchored tanks.

 

[Geoff13]

To address your question, an anchored tank will not have any uplift at the shell and thus the bottom can't pick up any product weight. The anchor bolt must resist 100% of the net uplift.

Product weight can only resist uplift in an unanchored tank. API limits this resisting product weight depending on the thickness of the bottom plate.

 

[CDLD]

The tank is designed to API 650.
I used the PIP guide just to estimate the anchor bolt tension (using applicable load cases from ASCE 7).

The anchorage will be designed according to ACI.

 

[HTURKAK]

The title of this thread ( Flexibility of flat bottom tanks for seismic ) . However ,apparently the problem is for flat bottom vessel.
If the bottom plate is flexible ( eg thin plate ) , the tank shall experience uplift to mobilize the content to resist overturning. In this case i am with Geoff13 (Structural).

Pls provide some more info. ( size , content , shell and bottom plate thk.s ) to get better responds.
..

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[CDLD]

The overturning moment is 65700 ft-kips, operating weight is 20300 kips, bolt circle is 34 ft, 60 anchor bolts.

Using the PIP equation for bolt tension, you would get:

Would it be applicable to use the equation as above? Or should I remove the dead load portion and design for 132 kips tension in the bolts?

 

[Geoff13]

Don't mix-and-match codes.

If the tank is API 650, then you must use API 650 for applicable loading, load cases, allowable stresses, etc. Neither PIP for anchor bolts nor ASCE for load cases should be used.

API 650 has rules for permitted anchor bolt stresses, so API will control the number and size of anchor bolts. ACI should only be applied to the foundation embedment length and details, however since you're located in Canada I would have expected CSA A23 as opposed to ACI.

 

[CDLD]

OK thanks Geoff, in this case I am looking at the anchorage of the bolts though (pullout, side-face blowout, concrete breakout). For this part of the design, ACI or CSA A23 would apply.

 

[Geoff13]

20,300 kips dead load is HUGE for 34' diameter tank (it would imply 350' tall for water !!). This large a value obviously includes the product weight, but as I noted above only a small portion of the product can resist seismic in an UNANCHORED tank.

Assuming there's a error in this dead load value, then the seismic moment begins to look quite high.

You say 60 anchor bolts on a 34' diameter tank. That's only 21" bolt-to-bolt. How can this be?

Please review API 650 5.11, 5.12, Annex E and Annex F (if applicable) before designing your anchor bolts.

 

[CDLD]

Geoff, the tank widens to 56 ft, there is no error in the loading.
Yes, the 20300 includes both product weight and dead weight of shell.
Yes, roughly 21" bolt spacing.

We actually have a skirted tank, so there is no tension in the anchor bolts as per my calculation above.

My question is purely academic, if this tank were not skirted and had a flat bottom, than the anchorage would have to be designed for the full overturning moment without reduction from the product weight.

 

[Geoff13]

It would have been nice if you had mentioned this was a purely academic question in your first posting.

In the case of a flat bottomed tank with your geometry, since the shell flares out at some elevation the product weight outside the 34' lower diameter would be supported by the lower shell, and would reduce the anchor bolt load. However the product inside the 34' diameter would load the bottom directly and not reduce the anchor bolt load. This would be more of an API 620 design, but certainly not an API 650 one.

Your geometry sounds like a high-density tank at a pulp mill, but it's probably something else.

 

[CDLD]

Thanks Geoff, that is helpful.

Basically, for a flat bottom tank (with shell flaring out) you still load up the shell from the product weight on the upper portion - This is interesting, I hadn't though of this.

For a standard flat bottom tank (with no flare), you ignore the product weight for bolt tension - which, in my opinion should be clarified in the PIP guide.

And finally, for a skirted tank, the product load will reduce the bolt tension, and in my case reduces the bolt tension to 0.

Good guess - you are right, its an HD tank in a pulp mill.

 

[Geoff13]

I reserve the term "flat bottom tank" for simple cylindrical vessels. I find it too confusing to call an HD tank an FBT.

Once you get away from a simple cylindrical shape you need to think through the load paths from first principles, not just grab a formula from some place.

Good luck getting PIP to change. However it's not obvious to me that they intended the guide to apply to an API 650 FBT. Since API fully addresses anchorage design they might not imagine anyone consulting their PIP for this.

HD tanks are interesting. I've designed a number of them over my career. Good luck.