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IMO, it is a simple math language Impulsive spectral acceleration parameter, Ai: [Ai=....] (E.4.6.1-1) However, Ai ≥ 0.007 (E.4.6.1-2) and, for S1 ≥ 0.6: [Ai≥....] (E.4.6.1-3) I understand that, for all cases, Ai must be the greater of the value calculated by E.4.6.1-1 and 0.007. For cases...

It is true that, under normal circumstances, the safety coefficients must be uniform thru a Code. From this point of view your last remarks make sense. However, it is hard to believe that during a seismic event, a tank with the actual sigma-h at a value equal to "the basic allowable membrane in...

Yes, see the definition under E.2.2 Notations SP= Design level peak ground acceleration parameter for sites not addressed by ASCE methods. [See EC Example Problem 2 when using “Z” factor from earlier editions of API 650 and UBC. Since 475 years recurrence interval is basis of this peak ground...

Some guidance in API 650, H.5.2.2.3 If circulation vents (per H.5.2.2.1 and H.5.2.2.2) are not installed, gas blanketing or another acceptable method to prevent the development of a combustible gas mixture within the tank is required. Additionally, the tank shall be protected by pressure-vacuum...

API 650 model considers two seismic moments. Mrw is the "ringwall moment", a result of the seismic moving content exerted to the tank shell. Ms is the "slab moment", which is the result of seismic action of the content to the tank (shell and bottom). Ms-Mrw appears to be the seismic action of...

You don't need to mix the requirements applicable to self supported cone roof with requirements of 5.1.5.9 e. 5.1.5.9 e is clear enough saying "Except as specified for open-top tanks in 5.9, for tanks with frangible joints per 5.10.2.6, for self-supporting roofs in 5.10.5, and 5.10.6, and for...

In my understanding, td is the required minimum thickness of shell plates, calculated according to the paragraph 5.6.3.2; let's say you calculate it based on the formula 5.6.3.2 and the result is 19.13 mm, for CA=2 mm. We can say that td-CA corresponds mathematically to Sd stress. t_ordered is...

In the previous post, when I written "uplift force" I referred to wind uplift, due to suction effect on roof.

The new rule allows the designer to ignore the uplift force on structurally supported conical roofs. I cannot say it is something to be supported by engineering. Why ignore the vertical uplift on a supported cone roof, but not on a supported domed roof or self-supported dome roof? In addition...

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...

I try to attach FIGURE C26.5-1 Maximum Speed Averaged over t s to Hourly Mean Speed, from ASCE 7.http://files.engineering.com/getfile.aspx?folder=398d9d65-51a0-466c-95a0-444c60fb1acd&file=MAXIMUM_SPEED_AVERAGED_OVER_T[sec]_TO_HOURLY_MEANS_SPEED.PDF

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...

It is evident that API 650 does not cover all cases; I think the best engineering approach is to perform a "free-body analyses" made at the level of shell-roof joint. We can separate the tank model in two parts and introduce a pair of vertical internal forces keeping them together. For the...

I think I don't understand. For me, "any dead load including roof plate acting on the shell" means I already have there the weight of plates roof i.e. the plates weight is included in W2. As you says, when in Table 5.21b they considered terms with pressure, they substract "8th" in pressure...

JStephen, I do not second the idea API would implement a difference in approach. Of course you are right saying a small internal pressure will appear as diminishing the weight of the roof, without producing uplift. However in this case it will change also the force transmitted to the shell (by...

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

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...

The failure pressure Pf is related to roof failure only and may be expressed in SI units as: Pf=8*Fy*A*tanθ/D^2+4/PI*DLR/D^2 - units in [Pa] (where A is the area subject to stress Fy) or Pf=Fy*A*tanθ/(125*D^2)+0.0012732*DLR/D^2 in [kPa] F.4.1 defines "the maximum design pressure" considering a...

fegenbush has already answered, however I attach a paper with derivation of that formula, hoping you can find useful the sketch (which, of course, it is based on API 650...

"If the fluid pressure is greater than the wind pressure, the tank cannot blow in." Yes, the theory behind API 650 is "the modified U.S. Model Basin formula for the critical uniform external pressure on thin-wall tubes free from end loadings", subject to the total pressure of 1.72 kPa (36...