API650 - Unstiffened Shell height & Wind Girders 1

[kvbalu]

Does API650 allow for taking credit in the Design of Wind girders, when Wind speeds are less than 120 mph? Add-4 has introduced the Velocity consideration in the calculations in 3.9.6, 3.9.7. Wind velocity in my region is 87 mph. The resultant would be comparatively less no. of wind girders and smaller sections, since the ratio (V/120) would be reduced at lower speeds.

 

[IFRs]

Section 3.2 does not list a minimum wind speed so unless you can find it elsewhere or a statement that you have to choose the maximum of the wind load options in 3.2.1.f you can reduce the wind loads according to the formulae

 

[JoeTank]

kvbalu,
Just a reminder about wind speeds. The previous edition of API-650 used a maximum sustained wind speed of 100 mph. The current version uses 120 mph which is the "maximum three-second gust". You should be using the maximum three-second gust wind velocity in the newest version. Roughly, the three-second gust is a bout 20% larger than the max sustaned wind speed.

Joe Tank

 

[LeonEarle]

Guys,

We asked about the pressure resulting from wind and I have attached the answer below. When we received this response from API, we immediately followed up with a clarification request addressing minimum wind velocities as the velocity is used other places. We have not received a response from API on this question yet.

John,


36 psf is the minimum.


Best regards,
Gordon
API
Gordon Robertson
1220 L Street NW
Washington, DC 20005
P-202-682-8190
robertsong@api.org

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From: John Walters [mailto:Jwalters@ptctanks.com]
Sent: Thursday, October 26, 2006 12:09 PM
To: Gordon Robertson
Subject: FW: API 650, Tenth Edition, Addenda 4, Paragraph 3.9.7.1 Note d.


Background Section; The purpose of this inquiry is to obtain an interpretation of the standard.

Main Section;

When other factors are specified by the purchaser that are less than the factors in items a-c, is the total load on the shell to be modified by increasing H1 by the ration of 36 psf to the modified total pressure, or is 36 psf the minimum load to be applied?

 

[kvbalu]

Refer my query on minimum wind velocities. I took it up with Mr Gordon and the reply is reporduced.

"
If an API standard is silent on an issue, sound engineering takes precedent.



Best regards,
Gordon

API
Gordon Robertson
1220 L Street NW
Washington , DC 20005
P-202-682-8190
robertsong@api.org

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From: Balu K V
Sent: Saturday, February 10, 2007 1:37 AM
To: Gordon Robertson
Subject: Technical Inquiry to API650 Sec 3.9.7.1

Dear Mr. Gordon,

Background
The Basic Design Wind speed (3 sec gust, 50 year return period) in the region being considered for Tank construction is 87 miles per hour.

Question

Can credit be taken for lesser Wind velocities in the design of Intermediate Wind Girders and calculation of H1 in section 3.9.7.1 of API650 ? Can the Velocity pressure be recalculated for the changed Wind speed ?

If the answer to both above is "NO", the clause could be modified to include the following - "Minimum Wind Pressure for Design of Shells and Wind Girders shall be 36 lbf/sq.ft. No credit shall be allowed for regions where Wind Speeds are less than 120 mph".

I hope API will issue the clarification. "

 

[JStephen]

I agree with IFRs above, that the standard as written pretty clearly allows the use of lower wind speeds and pressures than 120 mph.

 

[LeonEarle]

I can not agree with JStephen. The standard "clearly" describes what is to be done when factors EXCEED the stated values. It does NOT clearly define what is to be done when lesser values are applicable. That's why we questioned API directly on this issue. Their response seems pretty clear though "36 psf is the minimum". I believe their response is in error since most of the continental United States has a wind velocity of only 90 MPH. That's why we asked the follow-up question regarding wind velocity. Unfortunately, no response from API at this time. it is also disappointing that the Technical Interpretations page at API on the internet has not been updated since June of 2006.

 

[JoeTank]

LeonEarle,
As I read the current edition and addenda, the adjustsments for differeng wind velocities are now built into each equation. These adjustments are applicable for all wind velocities, not just when it exceeds 120mph (3 sec gust). If you are disappointed with API's maintenance of the inquiries on the website, I suggest that you let them know about that. It would be useful for all of us for them to maintain it in a timely manner.

Joe Tank

 

[LeonEarle]

JoeTank,

I have done exactly that and have copied Mr. Gordon directly on this thread requesting that a clear statement be made regarding this issue.

 

[JoeTank]

LeonEarle... a star to you!

Joe Tank

 

[JStephen]

As the standard is currently written (Addendum 4), the wind pressure does not directly enter into the shell stability or wind girder equations. However, the equations do in each case have the (120/V) term. Under the definition of V in 3.9.7.1, it says "See 3.2.1f". Under section 3.2.1f, it states that the wind speed shall be 120 mph, or the 3-second gust speed from ASCE 7, or the speed specified by the purchaser. I don't see anywhere that it specifies that the 120 is a minimum, and doing so would effectively remove the "or" clause in most cases. Even the formula for velocity pressure in 3.9.7.1 includes the same notation for V. It seems to me that the interpretation mentioned would actually contradict the standard as written.

 

[LeonEarle]

JStephens, I agree completely with your logical conclusion. I was astounded when we received the interpretation that 36 psf wind pressure was the minimum to be considered. That of course is why we followed with a request for interpretation regarding the wind velocity itself. This inquiry has not been answered yet. When reading 3.9.7.1(d), we are directed to modify the load on the shell resulting from wind to accound for ther factors specified by the purchaser "that are greater than the factors in items a-c". The value for H1 is then to be decreased by the ration of 36 psf to the modified total pressure. As a result, the pressure resulting from the wind velocity, importance factor, and exposure category is germain to the stability of the shell and, as a result, any wind stiffening that might be required.

 

[kvbalu]

Its time API clarified on Wind load calculations with respect to minimum wind velocities as well as on the confusion prevailing on the Moment calculations (another thread elsewhere).

 

[IFRs]

Good engineering judgment would suggest that realistic loads be used to evaluate the stability of a structure and that there also be lower limits on the loads applied to ensure a minimum quality of construction and account for the differences between calculations and real life. For ASTs, API is one of the best entities out there to judge where the lower limits should be. I'd wait to see what they come up with.

 

[JoeTank]

IFR, kvbalu,
It looks to me like API has indeed stated that the wind velocity for design is now a valiable to be determined by the purchaser. The purchaser now has the privilaege of specifying a design wind speed may be higher or lower than 120mph. If the purchaser is silent, then the manufacturer simply reports what speed was selected for design. The second approach again gives the purchaser a chance to accept the reported design wind speed or change the contract accordingly. Does anyone know if APi is relooking at the wind speed again?

Joe Tank

 

[LeonEarle]

JoeTank, It sounds like you may be looking at API 650, Tenth Edition, Addenda 3. In addenda 3, the designer was in deed asked to report to the purchaser what wind the shell was good for if the purchaser did not specify a wind velocity. In addenda 4, it essentially allows 3 options. One is use 120 MPH. The second is to use the wind velocity shown in ASCE 7. The third is a wind velocity specified by the purchaser. When we questioned the meaning of paragrpha 3.9.7.1(d), we truly expected that API would advise that factors that result in a wind pressure less than 36 psf would be handled just like wind velocity (multiply the right side of the equasion by the ratio of constand devided by variable.

 

[JoeTank]

LeonEarle,
No, I'm looking at Add 4. The last sentence of 3.2.1 f requires that the manufacturer report the wind speed used for design to the purchaser.

Joe Tank