ASCE 7 Table 6-3

[Clamont]

Dear All,

In API 650 5.9.7.1 the Kz = velocity pressure coefficient = 1.04 for exposure C at a height of 40ft.

What about tanks which are not 40ft high? Where can we find the correct pressure coefficient for the different heights? I have searched and found reference to ASCE 7 Table 6-3 however cannot find it online.

Help would be appreciated.

Thanks

 

[fegenbush]

ASCE 7-10 is available from the following vendors. I am sure that there are others that sell it, also.

http://www.amazon.com/Minimum-Desig...UTF8&qid=1390574256&sr=8-1&keywords=asce+7-10

http://www.techstreet.com/products/1677786

http://global.ihs.com/doc_detail.cfm?document_name=ASCE 7-10

For your reference, the values are available in Table 30.3-1 in ASCE 7-10 and in Table 6-3 in ASCE 7-05.

 

[JStephen]

Two things to note-
First, that's the basis of the equation in API-650, but API-650 doesn't specify that you need to use higher loading for higher tanks.
Secondly, the height z is at the centroid of the area, so evaluating at a height of 40' works for a tank up to 80' high or so.
(I would assume the reference is to ASCE 7-05, rather than ASCE 7-10, but haven't compared the Kz factors which may be the same.)

 

[mariog123]

The wind "uniform pressure" of 31 psf was based on ASCE 7-98 reference.
The design wind pressure was taken as
p = qz G = 0.00256(Kz)(Kzt)(Kd)(V^2)(I)(G) = 0.00256(1.04)(1.0)(0.95)(120)^2 (1.0)(0.85) = 31.0 psf
A height of 40 ft is used since the mid-height of most tanks is less than 40 ft.

As an opinion, I think more important than Kz adjustment is to realize that the reference value of 120 mph wind velocity is a 3-second gust wind speed which correspond to 100 mph fastest mile wind speed.
UBC-97 considers basic wind speed as the fastest-mile wind speed associated with an annual probability of 2% (50-year mean recurrence interval) measured at a point 33 feet above the ground of category C.
Especially for non-ASCE sites, it is difficult to understand what is behind the "design wind" written in specifications; it is likely to be something similar to UBC 97.

 

[mariog123]

My mistake, I've written "The wind uniform pressure of 31 psf".
The shell pressure is averaged after applying a force coefficient (drag coeff, shape coeff) of about 0.6, and the result is taken as 18 lbf/ft2

 

[Clamont]

Thanks to all!

JStephen, What about tanks lower than 40ft... Do you keep to 40ft as a minimum?

 

[JStephen]

Normally, the same equation would be used for tanks lower than 40' centroid height as well. IE, no reduction for lower Kz is made.

 

[michek]

Hi
I have to calculate a tank with API650. This tank isn't in USA and ASCE 7 is not applicable in Europe. I can't find the basic wind speed (3-sec gust speed).
The basic wind speed given by Eurocode is averaged on 10 min. So i can't use this value because of the assumptions aren't the same.
How can i handle it?

Best regards

 

[mariog123]

Your case would be considered under provisions of 5.2.1 Loads. point k Wind, third alternative i.e. "-the 3-sec gust design wind speed specified by the Purchaser, which shall be for a 3-sec gust based on a 2% annual probability of being exceeded [50-year mean recurrence interval]."

In case Eurocode has the same mean recurrence interval (I have no idea about!), you may consider the ASCE 7, FIGURE C26.5-1 Maximum Speed Averaged over t s to Hourly Mean Speed.
You know V_600sec, you can determine V_3600sec and after that V_3sec.

Note. The "fastest mile" is the maximum speed of a one mile long column of air passing a reference point.
For the default 100 mph fastest mile wind speed, the wind speed is averaged over a time (1 mile)/(100 miles/hr) = 0.01 hr = 36 sec, so converting a 100 mph fastest mile wind speed to a 3 second gust wind speed (the maximum speed associated with an averaging time of 3 seconds) means to follow the same procedure: V_36sec=100mhr-->V_3600sec-->V_3sec
which leads to the amplification factor of 1.2 as "Wind", Note 4 asks for.
"4) Fastest mile wind speed times 1.2 is approximately equal to 3-sec gust wind speed (V).".

 

[mariog123]

I try to attach FIGURE C26.5-1 Maximum Speed Averaged over t s to Hourly Mean Speed, from ASCE 7.

 

[michek]

Thank you very much Mariog.
One last question, i have to consider wind on conical roof but there are only formulations for domed roofs. Should i consider conical roof as a domed roof?
Regards

 

[mariog123]

API 650 asks to consider "The design wind uplift pressure on roof (PWR) shall be 1.44 kPa (V/190)2, ([30 lbf/ft2][V/120]2) (see item 2) on horizontal
projected areas of conical or doubly curved surfaces.".
In addition they noted that "Windward and leeward horizontal wind loads on the roof are conservatively equal and opposite and therefore they are not included in the above pressures.".
IMO, your case is covered by API 650 assumptions.