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Anchor Chair Inside Column Skirt

Anchor Chair Inside Column Skirt

Anchor Chair Inside Column Skirt

We were looking reference to install Anchor chair inside Column Skirt. For one of our Column, To increase the capacity, Column Diameter need to be increased However there is constraint of foundation. Since space is not available BCD of Anchor Bolt and Base Ring OD need to be kept as per existing Column. Foundation is adequate for revised load i.e. increased load due to increase in Dia and existing Anchor Bolt is also found adequate Hence same Anchor Bolt on same BCD can be utilized. To accommodate same (i.e. lower BCD with higher Skirt Diameter) Anchor chair need to be installed inside skirt for which we are searching for some reference. Please help.

RE: Anchor Chair Inside Column Skirt

The design philosophy and equations are the same as external anchor chair. You can check by hand calculations using formula in any design handbook. Make sure you will check local stress at the skirt to ensure enough skirt thickness.

Or use commercial program to model the chair with bottom two feet of skirt inside the chair as usual. Build a cone to connect to upper larger diameter of skirt. Program will check everything as well as local stress on the skirt due to anchor bolt. Make sure you manually increase dead weight and wind/seismic load (which shall be negligible in my opinion) to the actual vessel. Make sure you use the same dimension between anchor bolt and skirt because this is a critical dimension in the calculation.

If the stress pass, it will pass for sure when you put chair inside of the skirt because it has higher section modulus and cross-sectional area.

RE: Anchor Chair Inside Column Skirt

Doesn't seem like a good idea when you consider uplift on the skirt. Instead of the bolts resisting the uplift it now relies on the prying action, so your base ring changes from a moment connection to a pinned connection. For a start you would likely need to greatly increase the thickness of your base ring, bolt size would need to increase and likely increasing the ID of the ring will help reduce the prying action, somewhat. Could lead to cracking of the grout if not designed properly as well. I have seen standard details where the bolt circle is the same size as the skirt, and the skirt is notched out at each bolt location. This would gain a little bit of size without resorting to putting the bolts on the inside. Why not increase the skirt thickness, what is actually limiting the design that you need to increase the diameter?

RE: Anchor Chair Inside Column Skirt

Be careful with coming down with the crane, be difficult to see the bolt holes inside the skirt.


RE: Anchor Chair Inside Column Skirt

Could you provide some kind of dimensioned sketch or drawing of this unique proposed anchorage ?

I don't believe that this anchorage has ever been attempted before

Any photographs of the existing installation would help ....

As stated above by R6155, placement of the new column on the bolts will be nearly impossible.

I don't understand how the existing bolts can be deemed as acceptable for the new larger diameter column since they now must sustain both tensile AND PRYING loads in the new configuration. Are not the wind loads significantly larger ? Did the existing bolts have that much margin in their design ?

Was this new bolting configuration a "money saving" idea by your top management ?

I refuse to believe that this proposed configuration will be significantly cheaper than demolition of the old column and foundation and installation of new equipment and new foundation

Sr. Process Engineer

RE: Anchor Chair Inside Column Skirt

Not certain if this type of construction has ever actually been attempted, but at our firm we have been asked about this detail a number of times, and for the very reason(s) indicated: re-use of existing foundation and anchor bolts. Although in few cases the concern was for truck shipping clearance.

That said, again, I don't know if these were ever actually fabricated and constructed like this. And certainly the concerns discussed above are all valid.

RE: Anchor Chair Inside Column Skirt

Thanks for contribution.
I was following the same procedure as mentioned by jtseng123 knowing that this is not good idea and that's why this type of design is hardly in use. I myself don't have any reference but due to two major constraints we are doing this exercise, 1)requirement to increase capacity of existing Column, diameter of column would increase 2) There is no space to increase size of foundation, existing foundation to be used.
As mentioned in my query due to existing overdesign margin in existing Anchor Bolts and Foundation, they found adequate for revised load which is increased by almost twice. Please refer attached hand sketch for inside anchor bolt design. We are also checking with our erection team for their comments. Our structural people are saying that for some big structures such design is used for which i have demanded drawings.
Please note that this is not for cost saving at all, this we are exercising only to overcome plant constraints. We as a Mech. engineer shall keep working on new ideas smilesmile .
It was good discussion. If anybody have some more views please add.

RE: Anchor Chair Inside Column Skirt

Anchor bolts inside the skirt can produce injuries. Space inside skirt must be free of elements, as minimum as possible. Consider as confined space.


RE: Anchor Chair Inside Column Skirt

Your sketch is really ill proportioned. I don’t care that it is freehand, but it should generally be in proportion, and only you know that, at first. Otherwise, you are deceiving yourself and others, and will likely get some ill informed suggestions and answers. I did lay it out, to scale, and there is about a 12" eccentricity btwn. the BCD and the skirt o.d., and that is an awful “e” w.r.t. that shallow a chair and the uplift loading. Then, you don’t give any info on A.B. size, numbers of A.B’s., various loads, clearances around the vessel head and the skirt, the head dimensions and skirt height, etc. so it is difficult to know the magnitude of the problem, or what some sizes, thicknesses and dimensions should/could be to deal with these.

I can understand your need for this difficult solution, but someone better figure out some of the difficulties mentioned by others above so this solution has some chance of being practical and doable. The existing A.B’s. will take the lateral loads to the found., and an 8-10" ring base pl., on the skirt, with an o.d. of 1950mm should take the gravity loads (conc. bearing loads). Then the A.B’s. must also take the uplift in prying (pull out) and bending through the chair and out and up into the skirt, and that could be a real dog. I think I have a start on some details for this, but much to be worked out without knowing the loads, etc. and it’s not my first choice. Then, you have the problem of putting the vessel and skirt over the A.B’s. without being able to see them, without your head under the vessel hung from a crane. Finally, how do you get in there to tighten and inspect the A.B’s?

You might be better off to design a bending/torsion ring (box section) with t&b flanges, two vert. ring webs, the outer vert. matching the skirt o.d. and various stiffeners which fits over the existing A.B’s. and transmits loads out to a BCD of about 1950mm in the top flg. of the box section. This would match a bot. flg. on the skirt with a 1950mm BCD and an o.d. of about 2000mm. In this flg. to flg. connection there would be 3 or 4 smaller dia. nuts and bolts at each of the existing A.B. locations. These two flgs. can be match drilled in the shop. Thus, you could place the bending/torsion ring and see what you are doing while tightening the A.B’s. Then the skirt/vessel would be placed and bolted up from the outside.

RE: Anchor Chair Inside Column Skirt

Since it seems like you are proceeding down this path regardless, I am not sure the method proposed will accurately capture the base ring interaction. You should at least perform hand calculations for this or have an experienced analyst run FEA for this custom case. It is a 'deceptively' complicated design you are proposing.

I would be interested to see the final design concept if you can share it?

RE: Anchor Chair Inside Column Skirt

The only tricky thing observed here is skirt thickness due to anchor chair reaction for which we don't have any reference and even hand calculation as per B&Y I found difficult Hence I am considering same sized Anchor chair outside skirt so that Moment due to load P would be same only side would be change. At present just feasibility study is going on, nothing is finalized yet. Second option is to provide reducer cone on skirt to match BCD where analysis is easy but I like inside anchor chair option since it is new for me and it requires some brain storming.
Dhengr You are correct, Sketch was not in good shape but since I was occupied that time I have attached same editing dimensions in Pdf editor.

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