According to the API 650 , 11 th Edition
Section 5.2.1.k - Wind Loads ;

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.

I would like to design Roof steel structure of tank. I need to add wind loads on my model(SAP2000) . Which wind load value should I use ? This value will be act on girders of the tank roof.

1) vertical load : 0.86 kPa (V/190)^2
for example; for V: 150 km/h , 0,86*(150/190)^2 = 0,53 kpa
As per ASCE 7-05 , Page 54 ; shape factor for roof with slope is 0,69 . than ; 0,53*0,69=0,3657 kpa = 36,57 kg/m2
In my opinion this pressure is very low.

2) When calculating wind load as per UBC 97, the result is;
q=95 kg/m2 and shape factor:0,7 than =95*0,7=66,5 kg/m2

When calculating wind load as per ASCE 7-05, the result is;
q=111,7kg/m2 and shape factor:0,69 than = 0,69*111,7=77 kg/m2

I am waiting your comments. Thanks in advance.

Best Regards
Caner

I removed my answer as it refers to your preceded question.
Sorry for the mistake.

First, do API 650 say to apply a shape factor on your calculated pressure as 0,86*(150/190)^2 = 0,54 kPa ?

Second, there is a note "These design wind pressures are in accordance with ASCE 7 for wind exposure Category C. As an alternative, pressures may be determined in accordance with ASCE 7 (exposure category and importance factor provided by Purchaser) or a national standard for the specific conditions for the tank being designed."
If I understand well, you did a calculation under ASCE 7-05. If you can document these figure as applicable for your case, you would use them.

Best regards.

Thanks for your answer mariog.

1) As I understand from API650, 5.2.1.k;
0.86 kPa (V/190)^2 for vertical , 1,44 kPa (V/190)^2 for horizontal wind pressures. And these values are (as you said, just for Category C ) determined from ASCE 7. When we trying to calculate , if there is an anchorage needed or not , we use this values for determining overturning moments. So there is no shape factor is effecting. This formulas are enough.

But I will design the roof steel structure of tank. And I should modify this value in order to acting on girders of roof steel stucture.

I calculate wind pressure directy from ASCE7-05 formula, I calculate as below.
Qz=0,00256*Kz*Kzt*Kd*(V^2)*I (lbf/ft2) (this formula is also available in 5.9.7.1 )
Qz=0,00256*1,105*1*0,95*(93,2^2)*1=23,34 lbf/ft2=111,7 kg/m2
this formula is for wind pressures effect on vertical surfaces.There is a huge difference between 111,7 kg/m2 and 0,86*(150/190)^2 = 0,53 kpa
My first question : Is this calculation correct ? If yes, why the difference is too much ? Both values are determined for vertical wind pressure effect.

This formula is ,as mentioned above, for the vertical cylindirical surfaces. Now According to the ASCE7 , values have to be modified in accordance with acting surface type. For instance if it is a low-rise wall&roofs ( height<60 ft) we will use ASCE 7 page 54 Figure 6-10 , and shape factor for sloped (<5 degree) is -0,69. and we calculate the wind pressure acting on roof as -0,69*111,7=-77 kg/m2 . I am sure this method is true.

My second question: what will I do if I want to use values determined from API 650 formula ( 0,86*(150/190)^2 = 0,53 kpa ). How will this force effect on sloped roof ? is there any shape factor that specialized for cone roofs, tanks . or Do I use the shape factor of ASCE 7 ? (if yes, then the value will be too low. 53 kg/m2 * 0,69 =36,57 kg/m2 )

I am waiting your comments .
Thanks in advance.

Best Regards.

1. In my opinion you make things more complicated than they are.
Following a Code means to apply it "as is". It is a good idea to understand more on a Code subject, but comparing one section of one Code with a similar section of other Code would put you in deep dark.

In ASCE 7 code, qz is the velocity pressure to be considered in ASCE 7 calculation. The design wind pressure is affected by G = gust-effect factor, Cp = external pressure coefficient. The method has a degree of complexity but must be followed "as is" if you want to apply ASCE 7.

Alternatively, API 650 presents only a simplified approach.
For vertical vertical projected areas of cylindrical surfaces, API 650 considers (directly!) the design wind pressure as
[0.86 kPa]*(V/190)^2, i.e. ([18 lbf/ft2]*(V/120)^2
and does not enter into details as windward& leeward and how they are leading to these values of the design pressure (the only indication API 650 gives is "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 if you want to do more in calculation for roof zone, you would consider this fact. What API considers it is limited to a global effect, and this practice has been proved as enough for tank design.

2. API 650 considers only an "uniform" uplift as
[1.44 kPa]*(V/190)^2 i.e. ([31 lbf/ft2]*(V/120)^2
uplift on horizontal projected areas of conical or doubly curved surfaces of tank.
In tank calculation, this uplift affects not only the roof, but also tank's stability and anchorage (if any).
If you want to make a more precise calculation on the roof, you have the freedom to follow ASCE-7 Code.

3. One more comment on the subject. You may consider API 650, 5.10.2.3 Structural Member Attachment: Roof plates of supported cone roofs shall not be attached to the supporting members unless otherwise approved by the Purchaser. In some calculation circumstances this would be important.

Best regards.

Thank you so much for your comments mariog.
I have decided to use [0.86 kPa]*(V/190)^2, i.e. ([18 lbf/ft2]*(V/120)^2 for vertical wind pressure as API declared. And for the roof surfaces, I will use ASCE factors and formulas.
By the way. I consider API 650, 5.10.2.3 in roof calculations. Thanks for you reminding .


Best Regards.
Caner