Unusual Basering Anchor Chair Design

[fabineer]

All,

My companies customer has requested that we evaluate the attached basering design. A pipe is welded along the bolt circle and integral with the skirt. A hole is cut above the skirt to allow the nut to be placed and tightened on the stud. The vessel is rather tall and located in an area where the design wind speed is to be 160 mph so it is important to verify the design's acceptability.

Has anyone seen a basering detail like this before and if so, how did/would you evaluate it? I'm sure that most likely, a good FEA will be required to get an accurate evaluation if the customer will not budge on the design but I thought I'd at least post the question before then.

 

[marty007]

I can't remember where, but I've seen something similar before. I'm sure I'm buggering up the terminology, but I remember it as a centered bolt chair. The advantage of this design is that you do not have to deal with any bending moments generated due to the fact that the bolts are not acting in-line with the skirt. The downside is that it can be more complicated to fabricate.

As for analysis techniques, I'm not sure where you'd need to look for these. In some sense it appears to be fairly straightforward. First I would check the shear stress between the pipe and the shell to ensure adequate welding for uplift. Next, I'd check the compressive stress on the pipe due to the bolt loads. I might also do a quick FEA as a reality check.

Cheers,

 

[doct9960]

I have seen that skirt base design in the Pressure Vessel Design Handbook by Bednar.

The anchor bolt and base ring analysis is pretty straightforward. There are available vessel software that can perform the base ring analysis. No need for FEA.

I would be more concerned with the skirt opening (7" x 1'-0") that are spaced approximately 1'-10" apart. Again, there are commercial vessel software that are available to do the skirt opening analysis. Or you could do the analysis the old fashion way... manual calculations.

 

[racookpe1978]

An 8 inch high 4 inch dia "pipe" (not structural steel grade??) is close to the limit on compression buckling strength. It would be better at 6 inch high, but of structural grade "round steel". XX looks about right for wall thickness. Weld is ????

How are they going to mount the anchor bolt to the top of the cut-off 4 inch dia "pipe"? "Just a washer" ain't gonna fly.

 

[racookpe1978]

On the other hand, if they are welding the "pipe" to the base ring all around, there will be additional resistance to compression (buckling) forces.

 

[fabineer]

Thanks for the replies. I did find this type of design in the Bednar PV Design Handbook. It's a relatively straight forward calculation concerning how to check the basering thickness and weld sizing required. Thanks Doct9960!

To check for buckling between the openings above the pipe, would an AISC column buckling calculation treating the skirt sections between openings as separate rectangular beams be applicable for this design?

 

[dhengr]

Fabineer:
Attach a few pages of the Bednar book’s solution so we can see what you’re doing, and what he suggests. I think I’ve seen and used some of those books, but don’t have copies of them at the moment. You should have the 4" pipe bearing/column stand a little higher (8.5") than the skirt notch cutout (at say 8.25"). You probably don’t want the exact vert. fit-up problem btwn. the two, and you certainly don’t want the nut-n-washer bearing directly on a high skirt only. The 4" pipe should be saw cut or machined and aligned in fit-up to assure good bearing conditions, with min. secondary bearing effects. Otherwise, the detail you’ve shown doesn’t look too difficult to design and analyze.

Regarding the skirt cut-outs and buckling around and btwn. them, you have to look at plate buckling issues, not directly/really covered by AISC column buckling criteria and calcs. I would assume that Bednar’s book or one of your other PV design books, or the ASME Stds. would cover skirt design, both compression and buckling. You could reinforce the cut-out opening if you had to, to stiffen the edge. I can’t see your calcs. from here, so I don’t know that for sure. And, you haven’t shown a sketch so we have the whole picture, so we have no idea of sizes, thicknesses, loads, forces, etc. What wind forces do you have at each A.B.? What pretension do they want on each A.B. and are they properly embedded? It’s important for someone to coordinate this interface point.

 

[doct9960]

fabineer,

Go two pages back from where you found the skirt base design in Bednar's book. The book mentioned a way of checking if reinforcement is required at a skirt opening.

Openings in skirts need to be reinforced by replacing the area taken out of the skirt. You can add a collar/sleeve, repad, or structural angles around the opening for reinforcement. For reinforcement of openings subject to compressive stress, you could use the method in paragraph 4.5.17 in ASME Section VIII, Division 2.