Allowable Stress in Shell at Anchor Attachment

[Tomtation]

Question 1) Shouldn't the heading of the right-most column of Table 5-21b read "Allowable Shell Stress at Anchor Attachment" (as Table 5-21a reads)?

Question 2) The allowable shell stress at anchor attachment is given in Table 5-21b as 25,000 psi which is lower than the values listed for many of the materials listed in Table 5-2. Is this correct?

Question 3) Shouldn't the allowable stress for the combined loads of Design Pressure + Wind be higher? For instance, the ASME code Section VIII-div.1 allows a 20% increase in allowable stress when combining wind or seismic loading with other loads.

 

[JoeTank]

The 25ksi allowable is for the additional anchor stresses only. The increase factor is therefore much higher than ASME

Joe Tank

 

[Tomtation]

Do you mean to say that I should add the values from Table 5-21 to the Allowable Design Stress values from Table 5-2?

 

[JoeTank]

The intent is to calculate the anchor-induced stresses and compare them to the 25ksi allowable stress. The hoop and local stresses are not additive for the comparison.

Joe Tank

 

[JStephen]

As far as I know, the shell stress being calculated is in the vertical direction, not the horizontal.

 

[gdubyuh]

API-650 5.12.6 refers to AISI E-1, Volume II, Part VII "Anchor Bolt Chairs" for anchor chair design. Within that specification, under the section 'Chair Height' there is some discussion of the stress imposed on the shell due to the eccentricity of the anchor bolt to the shell. Here is some of the commentary...

"Chair height must be high enough to distribute anchor bolt load to shell.. without overstressing it."

"Except for the case where a continous ring is used at the top of the chairs, maximum stress occurs in the vertical direction, and is a combination of bending plus direct stress."

"Maximum recommended stress is 25 KSI. This is a local stress occurring just above the top of the chair. Since it diminishes rapidly away from the chair, a higher than normal stress is justified, but an increase for temporary loads, such as earthquake or wind is not recommended."

This is based on work of P.P. Bjilaard, "Stresses from Local Loadings in Cylindrical Pressure Vessels", ASME Transactions, Vol. 77, No. 6, 1955

 

[Tomtation]

Joe Tank,

If the calculated stress due to anchoring is up around 25,000 psi and if the additional stress due to pressure, static head, etc, is also up near the maximum allowable stress per Table 5-2, the case could arise where the stress is well above the yield point of the material. I'm having trouble following your logic.

 

[JoeTank]

It's not my logic. That's the way API-650 is applied. I suggest you address this question directly to API. Yes, one could have hoop stresses of about 25ksi and vertical stresses of about 25ksi due to anchors. And yes, that's kinda high, but they are not directly additive.

Joe Tank