Hi,

Am designing an API Storage Tank,

By using Coade Tank program outputs, I selected necessary wind girder section(used as a walkway) from API 650 but as I know, apart from top wind girder, top ring should also be used due to wind shear force. what is the selection criteria of the top angle size for the open top floating roofs? From my understanding, API 650 does not refer it exactly and also Coade Tank software either.

Pls advise

Thx

Hello eugeneee

Section 5.9.1 discusses a top wind girder for an open-top tank. The most critical circumstance for a tank with a floating roof is when the roof is in its lowest position. In that situation, the tank is really an open-top tank, and would thus require a top wind girder. In my opinion, if the top wind girder also serves as a walkay, you need not require any other structure to serve as a top ring girder.

You need only satisfy the equation in section 5.9.6.1.

Hello Ray,

I feel really confused with the sayings "wind girder" and "top angle". I checked some various detail dwgs of the open top floating roof tanks and there, designers had used top angle at the top of shell apart from wind girder used as a walkway at the same time.

Considering wind shear force acting on upper shell course, there may occur crack on the shell edge and this crack may move through the shell, if so, we have to use top angles in order to protect shell edges. Am I wrong?

Thanks for your kind interest

Hi eugeneee

There are two issues here. If you have a tank with gas pressure inside, that gas pressure is trying to separate the roof from the shell. That gas pressure cannot exceed 18 kPa (2,5 psi). As the roof tries to lift, a horizontal inward force is attempting to collapse the top of the tank. A ring (top girder) is placed around the top of the tank to counteract this inward collapsing force. That is found in Appendix F of the code.

The other issue is that of the wind. The wind tries to oval the tan shell. To prevent this, a top wind girder is provided to prevent this. There may be a necessity for an intermediate wind girder further down the tank shell.

Now, if a top ring girder can have sufficient area to act as a wind girder, and at the same time resist the forces from the internal pressure, clearly one ring girder would be sufficient.

It boils down to how the engineer designs the system.

It is extremely unlikely that you would have a crack in the shell unless you bought the steel from a dubious source. American steel would not fall into this class.

shoud I design intermidate wind girdes for fixed roof tanks and self supported roof tanks?
Since it seems that this designs is only for open tanks and for floating roofs!
yours truly Julio Mendez

Hello Julio

You can leave the intermediate wind girder(s) out. Tank will inform you if you need any.

The shell of a storage tank is susceptible to buckling under the influence of wind pressure and internal vacuum, especially when in a near empty or empty condition.

The first point when talking on shell buckling is that the top edge of the shell that must be stiffened.
There is a difference between shell buckling of tanks with roofs and buckling of shell of an open-top tank. Simply, the shell tank with roof is inherent more stiffened in the upper part.
That’s why API 650, Paragraph 5.9.1 concentrates on "open top tank" and says "an open-top tank shall be provided with stiffening rings to maintain roundness when the tank is subjected to wind loads. The stiffening rings shall be located at or near the top of the top course, preferably on the outside of the tank shell. "
In this case "near" means not more than 600 mm below the shell top, because 5.9.3.2 asks "When the stiffening rings are located more than 0.6 m (2 ft) below the top of the shell, the tank shall be provided with a 65 × 65 × 6 mm (21/2 × 21/2 × 3/16 in.) top curb angle for shells 5 mm (3/16 in.) thick, with a 75 × 75 × 6 mm (3 × 3 × 1/4 in.) angle for shells more than 5 mm (3/16 in.) thick, or with other members of equivalent section modulus."

[I guess this answers also to eugeneee old question; it must be a top angle to resist to shear force when the top girder is not "near" the top shell- otherwise the top wind girder is counted also to resist to this load]

The necessary section modulus must be calculated according with 5.9.6.1 where a simple equation is presented. The result is conservative for large tanks, that’s why there is a "Note" saying that for tank diameters over 60 m (200 ft), the section modulus required by the equation may be reduced by agreement between the Purchaser and the Manufacturer.

A tank with roof is inherent more safety against buckling in the upper part. As a general rule, a top angle is required - see for example Roofs-5.10.2.5 "Top Attachment: Roof plates shall be attached to the top angle of the tank with a continuous fillet weld on the top side." Point e of that paragraph says "Except [exceptions listed] tank shells shall be supplied with top angles of not less than the following sizes" and provides the minimum size of top angle for tanks with roof not subject to exceptions.


For the intermediate wind girders there is no difference between constructive types of tanks. For all of them there is a cylindrical shell that must be stiffened against buckling, from top to bottom. As API 650 says, 5.6.1.4 "The tank shell shall be checked for stability against buckling from the design wind speed in accordance with 5.9.7. If required for stability, intermediate girders, increased shell-plate thicknesses, or both shall be used."

Here the calculation problem seems to be more complicated by the fact that the shell is made up of courses of diminishing thickness. This makes analysis difficult, so the method adopted converts the multi-thickness shell into a equivalent shell having a thickness equal to that of the top course, with the height reduced in such a way that the stability of the actual shell is equal to that of the equivalent shell. The results of this analysis must establish number and locations of Intermediate Wind Girders. The required section modulus for girders must be counted according to 5.9.7.6 of API 650.


Best regards.