Maximum Stress for top plate and chair height (AISI Steel Plate Engg. VOL 2 Anchor Chair Design)

[TheEverest]

After calculating S (for the top plate) from equation (7-1) and S(for chair height) from equation (7-3), what shall we compare them with? Do both of them need to be less than 25 ksi?
(PS: Expecting for some help)

 

[JStephen]

If it's an API-650 tank- API-650 gives you the allowable stress for the shell, in the chart that gives anchor loads U.
And API-650 specifies that anchor chair part stresses are to comply with AISC-360, the structural steel standard.
If it's not an API-650 tank, then it would depend on what it was.
One thing they don't address well is that there is an increased chair load for seismic, and no allowable is specified for that, so it's not clear if you get to increase the allowable for that condition.

 

[TheEverest]

JStephen,
Thank you for your response. According to API 650 section 5.12.6, "An acceptable procedure for anchor chair design is given in AISI Steel Plate Engineering Data, Volume 2, Part 5, Anchor Bolt Chairs.” So, I referred to this specific book and found that the maximum recommended stress is 25 ksi at the top of the top plate. So, I wanted to make sure if 25 ksi is used as maximum stress for both critical stress in top plate and stress in shell?

 

[JStephen]

That reference gives the procedure.
Allowable shell stress is per the far right-hand column in Tables 5.20a and 5.20b in API-650; see 5.12.10.
Allowable stress in the chair itself is per 5.12.9.
The last sentence of 5.12.13 gives an additional load not included in Table 5.20a and 52.0b, and it is not clear what the allowable stresses are for that condition. I currently have a request for interpretation in on that point.

 

[HTURKAK]

I have API 650 11TH ed. I copied and pasted below,


This is not additional load. This item is weight requirement for foundation to resist to uplift and overturning to provide the acceptable factor of safety.

 

[JStephen]

I was referring to the 13th edition, and 5.12.13 is a different section there.

 

[HTURKAK]

Will you please write the last sentence of the 5.12.13 (13th edition ) ?

 

[IFRs]

5.12.13 The embedment strength of the anchor in the foundation shall be sufficient to develop the specified
minimum yield strength of the anchor. Hooked anchors or end plates may be used to resist pullout. See E.6.2.1.2
restrictions for hooked anchors for Annex E tanks. When mechanical anchorage is required for seismic, the anchor
embedment or attachment to the foundation, the anchor attachment assembly and the attachment to the shell shall
be designed for anchor attachment design load. The anchor attachment design load shall be the lesser of the load
equal to the minimum specified yield strength multiplied by the nominal root area of the anchor or three times seismic
design uplift load per anchor, Tb, defined in 5.12.2.

 

[HTURKAK]

In previous editions of API 650, the failure of anchorage system was implicitly limited with the yielding of anchor bolts (which is ductile failure). That is, other failure modes ( pull out of anchor, concrete shear failure, chair failure, tearing of tank shell ...) are implicitly forbidden with limiting the allowable Shell Stress and Anchor Attachment with 170MPa. for seismic , wind and test cases.

The design approach for the previous versions was, to limit the allowable stress to 170 MPa to warrant the bolt will yield before the tank shell or chair yields. That is, after selection of anchor bolt (with the applicable load cases ) one can design the attachments, anchor chair with using the anchor bolt tension capacity.

The new version of 5.12.13

IMHO, the new version explicitly states the requirement for ductile failure with limiting the anchor attachment design load to yield strength of anchor bolt or 3 X seismic design uplift load per anchor.
The multiplication with 3 , ( which is the average of Response Modification Factors for ASD Methods ) to obtain elastic seismic force , that is to keep the anchor attachment , shell , anchor chair still elastic when anchor bolt yielded.