## API 650 Addendum 4 Sec. 3.11.2 Moment Definitions

## API 650 Addendum 4 Sec. 3.11.2 Moment Definitions

(OP)

I've been asked to look into the addendum 4 and help find and understand the changes from the previous version. I would like somebody to confirm my understanding of the Moment terms as defined in Section 3.11.2 Unanchored Tanks. This is what I think:

Mpi = Moment about th shell to bottom joint from design

internal pressure. I think that this is

(int'l press.)x(cross sect. area of TK)-(Weight of TK shell + roof)x(radius of TK).

Mw = overturning moment due to horizontal plus vertical

wind pressure. I am pretty confident about this one

(wind press.)x(Height x dia.)x(Height/2) for shell.

Mdl = moment about the shell-to-bottom joint from the

weight of the shell and roof supported by the shell.

I think it is: (weight of shell+roof)x(radius)

Mf = moment about the shell-to-bottom joint from liquid

I think it is:

(weight of liq. per API definition)x(radius)

Are my assumptions correct? I appreciate any advice.

jcoots

Mpi = Moment about th shell to bottom joint from design

internal pressure. I think that this is

(int'l press.)x(cross sect. area of TK)-(Weight of TK shell + roof)x(radius of TK).

Mw = overturning moment due to horizontal plus vertical

wind pressure. I am pretty confident about this one

(wind press.)x(Height x dia.)x(Height/2) for shell.

Mdl = moment about the shell-to-bottom joint from the

weight of the shell and roof supported by the shell.

I think it is: (weight of shell+roof)x(radius)

Mf = moment about the shell-to-bottom joint from liquid

I think it is:

(weight of liq. per API definition)x(radius)

Are my assumptions correct? I appreciate any advice.

jcoots

## RE: API 650 Addendum 4 Sec. 3.11.2 Moment Definitions

Mpi = (int'l press.)x(cross sect. area of TK)x radius

Mw = (wind horizontal press.)x(Height x dia.)x(Height/2) +

(uplift wind pressure on roof)x(horizontal roof area)x radius

## RE: API 650 Addendum 4 Sec. 3.11.2 Moment Definitions

Stupid question though, when summing the moments, shouldn't we have the proper positive and negative signs in there? For instance, if Mpi and Mw are negative moments (CW), then shouldn't Mdl be positive since it acts in the opposite direction (CCW)? Or vice versa, depending on how you choose to define the positive direction of your moments.

Otherwise, your moments are going to be HUGE.

Thoughts?

KLee777

## RE: API 650 Addendum 4 Sec. 3.11.2 Moment Definitions

## RE: API 650 Addendum 4 Sec. 3.11.2 Moment Definitions

## RE: API 650 Addendum 4 Sec. 3.11.2 Moment Definitions

If I sum the moments around the point shown in Fig 3-23, with the CCW direction being positive, then Mpi and Mw would be negative, and Mdl and Mf would be positive. Makes sense from a physical standpoint, but when calculating the criteria of 3.11.2, I can't keep these signs, because then inequality doesn't make sense.

Example:

Mw = -7.7E+5 lb-ft (CW rotation about the point)

Mpi = -1.47E+5 lb-ft (CW)

Mdl = +8.93E+5 lb-ft (CCW)

Mf = assume zero for now (haven't calculated this yet since I can't find proper reference to E.2.1.1) (CCW)

Then,

0.6*Mw+Mpi = -608847 lb-ft

Mdl/1.5 = +595561 lb-ft

This "passes" the first criterion. But, if I were to choose the CW direction as being positive instead, then Mdl is no longer larger than 0.6*Mw+Mpi and it fails the criterion. Thus,

0.6*Mw+Mpi = +608847 lb-ft

Mdl/1.5 = -595561 lb-ft

1st criterion fails. Tank needs anchors.

My assumption is that when calculating the crtieria, you treat the calculated moments as *magnitudes* only, with no sign. So my tank really does need anchors after all, since 608847 > 595561, failing criteria #1. Wind and internal pressure prevail over dead load.

Do you agree?

Also, I believe the reference you gave was supposed to be 3.2.1.f.2, at least that's where it resides in Addendum 4.

Thanks,

KLee777

## RE: API 650 Addendum 4 Sec. 3.11.2 Moment Definitions

When calculating moments for overturning checks, all the moments are positive.

However, the Mw as calculated for the overturning check shouldn't be the same as the Mw used elsewhere for anchor bolt loadings- one is the moment about the center of the tank, one about the edge- but they are confused in the standard.

## RE: API 650 Addendum 4 Sec. 3.11.2 Moment Definitions

Really? Then why is Mw defined in Table 3-21b as "wind moment in ft-lbs (see 3.11)."

The Mw in 3.11 is defined about the edge, but its definition appears in the anchorage section (3.12) in Table 3-21b. So you're saying that we shouldn't be using the Mw defined about the edge for the calcs in Table 3-21b?

Not sure I understand where another Mw value was calculated about the center of the tank...although I'm not as familiar with API 650 as I wish I was, so I'm still learning.

Which Mw is used in Appendix F in the equations in F.4.2? The value calculated in 3.11? (Wow, I am starting to confuse myself.)

Thanks for the answer on the moments, by the way.

## RE: API 650 Addendum 4 Sec. 3.11.2 Moment Definitions

## RE: API 650 Addendum 4 Sec. 3.11.2 Moment Definitions

The equations giving equivalent force for a moment are based on F = (Mc/I +/- P/A) x A. For a thin ring cross section, I=pi*R^3*t, A=2*pi*R*t, and F=2M/R +/- P. This is the basis for the 4M/D terms in the equations for U.

Where you have wind moment due to a lateral load of M, and wind uplift of P, the equivalent bolt load should be 4M/D+P. But if you take Mw as M+PR, then 4Mw/D becomes 4M/D + 4PR/D = 4M/D+2P. The axial load part gets doubled in there. It's a conservative error in this case.

I've sent an email to API on the matter, although I wouldn't expect any changes until the next edition, whenever that is.

## RE: API 650 Addendum 4 Sec. 3.11.2 Moment Definitions

## RE: API 650 Addendum 4 Sec. 3.11.2 Moment Definitions

## RE: API 650 Addendum 4 Sec. 3.11.2 Moment Definitions

At least one of the questions to clarify ambiguious information in this section has been asked. Here is the response from API.

John,

The answers to all these questions is yes. We will fix these problems in the next addendum.

Best regards,

Gordon

API

Gordon Robertson

1220 L Street NW

Washington, DC 20005

P-202-682-8190

robertsong@api.org

--------------------------------------------------------------------------------

From: John Walters [mailto:Jwalters@ptctanks.com]

Sent: Wednesday, August 16, 2006 11:49 AM

To: Gordon Robertson

Subject: API650, Tenth Edition, Addenda 4, Paragraph 3.11.1

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

Main Section; In Addenda 4, paragraph 3.11.1 includes a definition of Mf and refers to wL as defined in E.2.1.1. We do not find a definition for wL in E.2.1.1.

a. Is wL the same as wL found in paragraph E.4.1 of Addenda 3?

b. Should this be shown as wa as defined in E.2.1.6 of Addenda 4?

c. In the formula for wa, is the factor Fby the same as Fby as found in paragraph E.4.1 of Addenda 3?

d. Should the factor Fby be shown as Fy as defined in E.2.1.6 of Addenda 4?