Anchor Chair Reinforcing Pad 1

[jramosjr]

When designing the anchor chairs in the API 650 tank that I'm currently designing, the stress in the shell (around the anchor chair location) exceeds the allowable stress (25,000 psi, per Table 3-21b). Options to decrease the stress are to increase the shell thickness, add more bolts or increase the chair height. Could I use a repad around the anchor chair location instead? Has anyone done this before? What is a good reference for the design of these repads? I'm currently using Steel Plate engineering Data Vol. 2, but it does not address this issue.

Jorge L. Ramos, Jr., MSCE, PE
Alonso & Carus Iron Works, Inc. (

http://www.alonsocarus.com)

Euro-American Steel Co., Inc. (

http://www.euroamericansteel.com)

 

[doct9960]

jramosjr,

The allowable stresses on Table 3-21 are for the anchor bolts, not the stresses on the tank shell. Check API 650 para. 3.12.6 or 3.12.7 for evaluation of localized shell stresses.

 

[jramosjr]

doct9960,

Thanks for the reply. However, the last column on Table 3-21b clearly states that the allowable shell stress at the anchor attachment is 25,000 psi when seismic loads are considered. This is the stress occurring just above the top of the chair. I'll check with ASME Section VIII Div. 2, Appendix 4, as noted in paragraph 3.12.7. May be my interpretaion is wrong.

Jorge L. Ramos, Jr., MSCE, PE
Alonso & Carus Iron Works, Inc. (

http://www.alonsocarus.com)

Euro-American Steel Co., Inc. (

http://www.euroamericansteel.com)

 

[TankDude]

I have used reinforcing plates for anchor chairs in the past for this very problem. It was a much more affordable option than increasing the shell thickness. You might also try increasing the chair height, though most references will tell you not to exceed 3x your top plate width (I think). In my opinion the re-pads serve the purpose to help dissipate the local stresses in the shell during a seismic event. Make sure you maintain max/min spacing rules.

 

[JStephen]

Have done that on rare occasions- when bolt is designed for full yield, and as-built or as-corroded shell was very thin.

Consider making the chairs wider as well as higher. If the tank isn't built yet, maybe make the bottom ring thicker just for this purpose.

 

[doct9960]

jorge,

You are right about the last column (allowable shell stress) on Table 3-21. That column was added on Addendum 4, December 2005.

I was looking at an older API 650 std, Addendum 3. Silly me. Sorry about that.

 

[jramosjr]

What reference/book can I use to design the repad?

Jorge L. Ramos, Jr., MSCE, PE
Alonso & Carus Iron Works, Inc. (

http://www.alonsocarus.com)

Euro-American Steel Co., Inc. (

http://www.euroamericansteel.com)