Dears,
for 41m diameter, 8m height, design liquid level 7m, annular plate thickness 10mm and CA 2mm

As I understand
1- From API650 (5.11.2)The liquid weight (wL)shall be the lesser of 14.8HD for SI Units and wL = 59tb sqrt(FbyH)

by calculations 14.8*41*7= 4247.6
and 59*8*aqrt(250*7)= 19745 N/m

from software report
wa = 154.92 N/cm = 15492 N/m

why is this big difference between 14.8HD (4247.6)(lesser)and 15492 by the software ?

2- overturning moment from horizontal wind pressure= 0.86(160/190)^2*d*h*h/2= 803 kN.m

overturning moment from vertical wind pressure=
1.44(160/190)^2*roof area(1320)*d/2 = 27736 kN.m

from software report
Mw - moment due to wind (hor+ver) press = 779.1 kN.m

can anyone help me what's wrong in my calculations

thank you

Open a Support Request on eCustomer and send in your job file so we can look at it. Also, be aware that the software uses the "Imperial Units Equations", with conversions to user units on the front and back. Unfortunately some editions of the Code have mistakes with some of the SI equations.

mainly u r calculating roof area by wrong method.
projected area of horizontal along the wing direction
formula is:
1.44(160/190)^2*roof area(1320)*d/2 = 27736 kN.m

1.44(160/190)^2*roof area(d/2)*h)*(H+h/3)=

h=Roof height (calculating by roof angle)
H=Shell Height

and also u have to recheck wind parameters may be some area required to proper input.

i think your problem will be solve.

Please see API 650 Figure 5-27: there are two overturning moments: one is due to wind load on shell and one is due to wind uplift load.

First, please note that a pressure of 1.44[kPa]*(V/190)^2 is specific to uplift calculation (according to 5.2.1), not to horizontal component.

Second, your "corrected" formula is not dimensional consistent with "kN*m" units.

Also, according to 5.2.1, windward and leeward horizontal wind loads on the roof are conservatively equal and opposite and therefore you don't need to include a roof correction for horizontal component.

1. API650 dos't define the formula how to calculate the roof area effected by wind force. there is only written pressure on horizontal projected area of roof or umbrella (projected area that mean area toward wind direction for horizontal surface). if we go one example exist in (3.9.4.10 page-97 "Guide to Storage Tanks and Equipment" bob long and bob garner)to calculate Mwr (moment due to wind on roof) which is logical.

2. wind force is creating overturning moment on base of bottom plate, and overturning generate the uplift witch is from both component shell and roof (uplift not only due to roof see.Table 5-21a wind loadcase).

3. Formula Reference (3.9.4.10 page-97 "Guide to Storage Tanks and Equipment" bob long and bob garner)

if we calculate roof uplift as understanding by 1.44(160/190)^2*(pi*d^2/4) that's mean wind force actually hit from in side tank on roof. that is only possible when tank is without shell and roof supported by columns.

Jamil,

Guide to Storage Tanks and Equipment by Bob Long, Bob Gardner, Editor John Wiley & Sons, 2004, ISBN 1860584314, 9781860584312 is an excellent book; this doesn't mean you can substitute API 650 requirements by what is written in that book.

API650 defines both PWR = wind uplift pressure on roof, PWS = wind pressure on shell- see the explanations inside the Table 5-21a.

Please note that always pressure acts on normal to surfaces and by math (or fluid mechanics) you can count the pressure resultant along a direction by multiplying the pressure itself with the area projection on a plane perpendicular to that direction (e.g. to get the pressure resultant along vertical direction you have to multiply the pressure to the area projection on horizontal plane).

About your comment:

I propose you to revise an extract from API 650 and after that to apply the math rule already explained.

5.2.1 Loads
paragraph k
Wind (W): The design wind speed (V) shall be 190 km/hr (120 mph), the 3-sec gust design wind speed determined from ASCE 7, Figure 6-1, or the 3-sec gust design wind speed specified by the Purchaser (this specified wind speed shall be for a 3-sec gust based on a 2% annual probability of being exceeded [50-year mean recurrence interval]). The design wind pressure shall be 0.86 kPa (V/190)^2, ([18 lbf/ft2][V/120]2) on vertical projected areas of cylindrical surfaces and 1.44 kPa*(V/190)^2, ([30 lbf/ft2][V/120]2) uplift (see item 2 below) on horizontal projected areas of conical or doubly curved surfaces, where V is the 3-sec gust wind speed. The 3-sec gust wind speed used shall be reported to the Purchaser.


If you try to apply these requirements:
- shell surface has a rectangle as vertical projection and multiplying PWS by that vertical area you will calculate the wind on shell- horizontal component
- roof surface has a triangle or other shape as vertical projection, but here the wind effect is more complicated (for details you can see ASCE 7) and API concludes "Windward and leeward horizontal wind loads on the roof are conservatively equal and opposite and therefore they are not included in the above pressures"; however, if you don't agree, you can perform your calculation as in "Guide to Storage Tanks and Equipment";
- roof surface has a disk as horizontal projection and multiplying PWR by that horizontal area will get you the vertical wind on roof, i.e. the wind uplift tank.
It is uplift because the wind effect of roof is counted as suction (which is not clear explained in API 650, instead they preferred to say directly "uplift", as you can see).

Best regards.

thanks mariog,

after discussion and reading below link i have build positive approach about the wind suction on roof.

http://www.nrc-cnrc.gc.ca/eng/ibp/irc/cbd/building-digest-68.html

one thing i want to clear
if windward and leeward wind loads on the roof are conservatively equal and opposite (means that windward pressure on half of the roof is +ve and leeward pressure will be -ve "suction" on the other half portion of roof) and therefore they are not include in the above pressure which is as per 5.2.1.k.2
*that mean this pressure can be ignore.
then why we are calculating roof pressure against overturning and uplift loadcase.

API 650 says "Windward and leeward horizontal wind loads on the roof are conservatively equal and opposite and therefore they are not included in the above pressures", so they refer only to horizontal resultant as negligible (i.e. the windward horizontal component is equal and opposite to leeward horizontal component), whereas the vertical resultant (i.e. the resultant of both windward vertical component and leeward vertical component) cannot be neglected.

But here API just tried to simplify the result of math.

The normal approach would be to start with the pressure distribution on roof (which is not uniform) and to integrate it over the roof surface.

By math, this non-uniform pressure integration gives an uplift resultant (which does not act in the "roof center"!) and a "small" horizontal component.

If you try to count both things, you'll see that it's conservative to replace everything as an uniform uplift pressure, to consider the uplift is acting along a vertical axis passing through roof center and to say the horizontal component is zero; normally this approach would be accepted by an engineer obliged to obey to Code's rules...
The only problem is that sometimes is quite hard to understand what they are asking for!

Best regards.

Thanks Mariog for the excellent treatise.

The original poster's query no.1 has remained unanswered. I have also faced the same anamoly. On much research I have come to the conclusion that the software (Ver 3.3) considers bottom course corroded thicknes, though this is only the upper limiting condition as per API, instead of bottom plate corroded thickness.

Regards,

LINUS