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Re: Hexagonal Foundation Pedestal for Vessels, Stacks, Etc.

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Bill,

I've designed several dozen of these foundations so I think I can help you out.

I don't know the limits of your scope of work so I'll go through the whole process.

Presumably you are given the vessel, therefore you have the base plate diameter and all of the loading conditions you need for design. These should include
1.  Number of anchor bolts
2.  Empty weight.
3.  Normal operating weight
4. Weight full of water (if the project specifications call for it ever to be tested full of water, if not you should specify that the design does not include provision for hydrotesting the complete vessel)
5.  All climatic and sight specific design information.

Be nothing but conservative in your design. The vessel is probably worth 20 times as much as the foundation and the plant piping is probably worth a similar amount.

The pedestal diameter will be the diameter of the vessel base plate plus about 6" to include an allowance for slope of the base plate grout and/or reinforcing and cover outside of the anchor bolts.

You will have to calculate your vertical, horizontal (base shear). and overturning loads at the base plate, the top of footing (after you've sized the footing), and at the underside of footing.

You have to size the footing by trial and error. More often than not the governing lode case is that the factor of safety against overturning must be greater than a specified value (2.0 in Canada, 1.5 in many other locations. See your job specs.). The basic equation is
        F.S. = R/e > (2, or 1.5, as specified)
where F.S. = Actual factor of safety
R = Radius of the octagonal footing (half the diameter across the flats) e = Eccentricity of the load case (overturning moment)/(weight of foundation = empty vessel) This may take a few trials; I usually expect at least 3; since it seems to be your first time you should expect more.

The next step is to check soil stress for all of your other load cases. Then choose a footing thickness.

Finally, design reinforcing and anchor bolts. You need to design mats for the top and bottom of the footing in the normal way. I've always used #5@12 on all large surfaces and nothing through the mass of the concrete, although I do know of others who have put reinforcing through the mass on 12" centers both ways (I also think it's a waste). I would use at least two #5 around the pedestal at the top.

As far as anchor bolts are concerned I am not much of a fan of pull-out anchor cones. The bolts really need to be anchored to the footing. I've seen vessels with 20 to 36 bolts 2" + in diameter. I think you may have to lap the bolts with main vertical reinforcing in the pedestal or even extend the bolts to the bottom of the footing and have plate washers as well.

        I haven't said anything about seismic detailing.

        Have fun.

Regards,

Daryl
___________________________________________________________________________

On 30/09/2014 1:13 PM, Bill Polhemus wrote:
In almost every instance I've seen lately, the anchor bolts are able to be anchored into the concert by embedment alone. You make sure there's enough "meat" in the pedestal for the pullout "cone". The bolts are snugged to the foundation to make sure there's always compression between the base plate and the concrete surface (grout pad), so the critical limit states are nearly always the bearing at the anchorage (usually a Hex or Heavy Hex bolt head) and tensile capacity of the bolt.

I typically put pretension in the bolts of 50% to 70% Fu, using "squirter" DTI washers. The resulting maximum additional tension from wind or EQ
overturning is not much per bolt then.

There are typically "a lot" of bolts but not always. For instance, the one I'm looking at now requires only (8) 1" Dia. Bolts. It's a 12' Dia. X 32'
vertical vessel, and the ASCE 7-10 wind speed is 140 mph.

William L. Polhemus, Jr. P.E.

5


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