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:
there is only written pressure on horizontal projected area of roof or umbrella (projected area that mean area toward wind direction for horizontal surface)
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.