UG-23(d) Allowable Stress Increase During Hot Shutdown?

[AWDMIKE]

Section VIII, Division 1 UG-23(d) refers you to footnote 15 which states that:

UG‐23(d) permits an increase in allowable stress when earthquake or wind loading is considered in combination with other loads and pressure defined in UG-22. The 1.2 increase permitted is equivalent to a load reduction factor of 0.833. Some standards which define applicable load combinations do not permit an increase in allowable stress, however a load reduction factor (typically 0.75) is applied to multiple transient loads (e.g., wind plus live load, seismic plus live load, etc.).

It seems clear that when you combine pressure with earthquake or wind loads then you are permitted to use an allowable stress increase of 1.2, effectively reducing the load side by 1/1.2 = 0.833.

My question is if you did NOT have pressure, would you still be permitted to use an allowable stress increase of 1.2? The Code seems quite explicit that it is for the case where you have pressure with earthquake or wind loads. If the load reduction factor is intended to account for the probability that you likely wont have full internal pressure during an earthquake, then it makes sense that the answer to my question is no.

As a quick note, in my research on this subject I found this post from several years ago whereby mention was made to replace the value in a Division 2 (Table 5.3) load combination of 0.7*E with 0.833*E, however I believe that it would have been correct to say to replace the values of 0.75 with 0.833 in Division 2 (Table 5.3) load combinations seven and eight, with no changes to load combinations one through six. The value of 0.7 is a factor that reduces the nominal load in ASCE 7 to be combined with other service level loads.

http://www.eng-tips.com/viewthread.cfm?qid=324430

 

[TGS4]

Are you dealing with a Division 1 or 2 vessel?

If Division 2, then last attention to the Notes in Tables 5.3, 5.4, and 5.5 regarding including combinations where one or more of the loads may be zero.

EditIf Division 1, note that the increase in allowable stress is only for tensile stresses. In an earthquake with zero internal pressure, compressive stresses may govern - in which case the external pressure (bucking) allowable stresses would govern (and they may end up being much lower).

Make sense? Or did I misinterpret your question?

 

[racookpe1978]

The intent is also to avoid requiring you assume an earthquake strikes during a hurricane while you are operating at maximum internal pressure....

 

[AWDMIKE]

I suppose this could be for a Division 1 vessel (UG-23(d)) or a Division 2 vessel if using Part 4 (Section 4.4.2).

For my own edification, I am just trying to understand the logic behind the code. From the footnote I typed out above, I was trying to verify that I read it correctly. I do understand the statement you made TGS4 about compressive stresses governing. The Code seems to always mention both pressure and seismic/wind when referring to the 1.2 stress increase.

I don't mean to hijack my own thread, but UG-23(d) says:

(d) For the combination of earthquake loading, or wind loading with other loadings in UG-22, the wall thickness of a vessel computed by these rules shall be determined such that the general primary membrane stress shall not exceed 1.2 times the maximum allowable stress permitted in (a), (b), or (c) above.

It refers to sections (a), (b), and (c) meaning that the 1.2 increase applies to longitudinal compressive stresses (not even to circumferential compressive stresses). Am I missing something?

 

[TGS4]

AWDMIKE - you are correct that the bump in allowable stress described in UG-23(d) does indeed apply to all of internal pressure, external pressure, and axial compressive loads on a cylinder - the last sentence of UG-23(d) explicitly says so. I was incorrect, and have edited my post above accordingly.

My question is if you did NOT have pressure, would you still be permitted to use an allowable stress increase of 1.2?

After reading UG-22 and UG-23 again, I would take the view that Division 1 does not explicitly answer that question. My engineering judgement would be that a pressure=zero AND hot AND earthquake situation could be one of the design conditions covered by UG-22, and hence UG-23(d) would still be applicable. However, this is probably one of those cases where it would be good for you to search through the interpretations to see if this has been covered by an interpretation before. If not, then I would suggest that you submit an interpretation request to the Section VIII Code Committee. The details of how to do this are in VIII-1, page xxx in both the 2013 and 2015 Editions. However, the 2015 Edition requests that users submit interpretation requests to

http://cstools.asme.org/Interpretation/InterpretationForm.cfm.

Division 2 is another matter. For a Design By Rules calculation, Table 4.1.2 is applicable for the Design Load Combinations. For a Design By Analysis calculation, Tables 5.3/5.4/5.5 are applicable. In Tables 5.3/5.4/5.5, there is a note that says

Loads listed herein shall be considered to act in the combinations described above; whichever produces the most unfavorable effect in the component being considered. Effects of one or more loads not acting shall be considered.

For me, that explicitly requires the analyst to investigate a P=0 situation, while still providing credit for the lower load factor on earthquake. However, Table 4.1.2 has no such note. This seems a bit inconsistent to me. Again, I would submit an interpretation inquiry to the Section VIII Committee.

 

[AWDMIKE]

As sort of a carryover to my original question/comment, I noticed that in 2015 Section VIII, Division 2 that paragraph 4.4 DESIGN OF SHELLS UNDER EXTERNAL PRESSURE AND ALLOWABLE COMPRESSIVE STRESSES permits you to increase the allowable stress for F[sub]bha[/sub] or F[sub]ba[/sub] in several specific equations by a factor of 1.2. This increase in allowable stress is only mentioned in this section (and not in Table 4.1.2). Based on what was stated above:

However, Table 4.1.2 has no such note. This seems a bit inconsistent to me. Again, I would submit an interpretation inquiry to the Section VIII Committee.

I am not sure if you are saying that it may not have been the intent of the Code to include the 1.2 factor in section 4.4? If it is then I can submit an interpretation but it would seem strange to say "Please verify that the factor 1.2 should not be there".

 

[TGS4]

The inconsistency that I noted was with regards to General Note in Tables 5.3, 5.4 and 5.5. Table 4.1.2 has no such note.

Increasing the allowable stress in compression for combinations with seismic or wind is consistent with the load factors provided in Table 4.1.2. It's a matter of which side of the equation the factor is on - left side: 1/1.2, right side: 1.2.

 

[AWDMIKE]

As Section 4.4 contains the verbiage to include a 1.2 factor on the allowable stress, and Table 4.1.2 contains the 0.75 reduction where there are multiple transient loads, I believe this would constitute a double dip, whereby the loads are reduced and the allowable stress is increased.

I understand that the load combinations in Table 4.1.2 use the reduction of 0.75 (or 4/3 on the right side of the equation) but ASME has always used 20% increase for the shell (0.833 on the left side of the equation).

Wouldn't we be double dipping if we used both the 1.2 increase (Section 4.4) along with the load combinations in Table 4.1.2 (which have the reduction factor)???

 

[TGS4]

Good question. I'll have to think about that some.