tank roof uplift
tank roof uplift
(OP)
Is it typical to dsign for an uplift, due to wind, on the roof of storage tanks? API 650 and AWWA D-100 do not seem to address the topic, they just give a formula for the lateral load. In fact the anchorage calculations in section 3 of API 650 do not include an uplift due to wind in table 3-21b. If you have used an uplift design force, what have you used to calculate the force? Thanks for any ideas.





RE: tank roof uplift
RE: tank roof uplift
RE: tank roof uplift
In this case the roof is a 15 degree cone with external stiffeners. Do you feel that coefficients for a domed roof would apply in this situation? As JStephen mentioned, API, AWWA, and ASCE all give lateral design forces, but do not address uplift on the roof of tanks. The question originated from a request to check the sliding of the tank under wind loads, including an uplift on the roof. With a large tank, an uplift on the roof will quickly reduce the allowable friction force at the base. This will make uplift on the roof a critical load case if there is not enough tank weight to resist the uplift and lateral wind loads.
RE: tank roof uplift
Also, I was told that in the older codes didn't differentate between cone and domed roof uplift coefficient. My doom roof on a 1972 existing tank has a uplift coefficient 3x of a cone roof and it fails the 1.5 in w.c. rating of the tank. It passes with a cone roof. Anybody know the history of uplift coefficients on storage tanks.
RE: tank roof uplift
1) Would the external stiffeners disturb the flow of wind over the roof and cause enough turbulence to at least introduce doubt as to the reliability of the pressure coefficients?
2) If your analysis shows that the tank will slide off the foundation, won't anchors take care of it?
3) How do you determine the friction of the bottom on the foundation? Is the tank bottom cone taken into account?
4) If the uplift exceeds the weight of the cone roof and it's structure then the outer edge of the roof and tank may go into compression, leading to a different failure mode.
5) Can the owner put liquid into the tank if a hurricane approaches? Even water? There is usually plenty of warning.
6) What is the diemater and height of the tank?
RE: tank roof uplift
Keep things in perspective here. Most tanks are operated somewhere between totally full and totally empty. The sliding-in-the-wind problem assumes that the tank is just totally empty, so it's an uncommon condition to begin with. I can't think of why you'd want to approach this particular design aspect in an extra-conservative manner.
As to the wind on the roof with external stiffeners- using the API/AWWA wind loadings, it would probably make sense to consider the stiffened roof as "flat" area at 30 PSF or so instead of coned/double curved at 15 PSF.
Determining the friction is easy- it's stated in one or both codes (I think it's the tangent of 30 degrees, but would have to check). Testing it is easy- take a steel plate, a chunk of concrete, tilt the plate till the concrete slides and measure the angle- that'll give you the coefficienty of friction.
Unless standards say otherwise, I'd include the floor weight in the resisting weight for sliding- I can't imagine that the tank's going to slide off and leave the floor there.
RE: tank roof uplift
RE: tank roof uplift
RE: tank roof uplift
I have already done what you describe and this is not what I am asking. I am asking if there is any requirement to design the roof for a vertical uplift force due to wind. This would act similar to an internal pressure and lift up on the roof. This is typical in building design and the ASCE 7 code addresses it for buildings. I don't think it addresses this for tanks and this is further supported by the lack of coverage of this topic in the API code. It seems that from the responses that are coming, the answer is that this type of uplift would be atypical for a storage tank design.
RE: tank roof uplift
I would have to say that you're right, it's atypical for a tank design. I am not very familiar with building design, but I cannot see how there can be any uplift from wind when there is virtually no projected area on the underside of the roof for the wind to affect. I will look through my ASCE 7 to see if any of the loading cases seem appropriate. Otherwise, I think you have designed the tank correctly.
RE: tank roof uplift
"The tanks are anchored for overturning with bolt chairs, so if the tank moves the bolts will bend and I believe that that is the real concern that they have." The bolts should have some nominal amount of pretension, and that force can be added to the weight in figuring the friction- that's how the bolts resist sliding. If they don't have any nominal amount of pretension, add it- it shouldn't take much. If you needed to, you could add a welded collar at the base of each bolt chair specifically for shear loading, but this shouldn't be necessary.
You might note how they work this at a shear plane in concrete in ACI-318- they figure the rebar will deflect slightly and then develop its full tensile capacity across the joint. It's obviously not a load situation that you'd want to reverse repeatedly.
RE: tank roof uplift
RE: tank roof uplift
Steve Braune
Tank Industry Consultants
www.tankindustry.com
RE: tank roof uplift
When did the wind tunnel study of dome roofs occur. I heard in the mid to late 1990s the increased aerodynamic affects of dome roofs entered into standards.
RE: tank roof uplift
RE: tank roof uplift
RE: tank roof uplift
RE: tank roof uplift
Is that uplift required on all tanks? It sounds high.
RE: tank roof uplift
It also says 30 psf for horizontal projected areas of conical or doubly curved surfaces.
Not being a structual engineer, does that mean you use 18 psf for overturning moment (vertical projection) and uplift I hope and 30 psf for compression ring (horizontal projection)???
RE: tank roof uplift
Steve Braune
Tank Industry Consultants
www.tankindustry.com
RE: tank roof uplift
RE: tank roof uplift
Specifically, the wind uplift is larger than the normal dead weight of a roof. For the overall tank to be unanchored, you need a certain amount of shell weight to anchor the roof. So, for example, my design shows me that a 50' diameter by 16' high tank is not stable for wind uplift/overturn, whereas a 50' diameter by 56' high tank is. (If you want to check, I'm figuring 1/4" shell in both, 3/16" roof plate, 2,400# supported rafter weight, no internal pressure, 120 MPH 3-second gust.)
I'll send a note in to API, but I can't imagine that this is the intended result of the design procedure.
RE: tank roof uplift
then we will have anchor bolts everywhere!!!
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Just for the record I have noticed on an old german standard for storage tank (DIN 4119- part 2)
a nice sketch with the uplift do to wind with a wind uplift pressure equal to 0.6 * the horizontal wind pressure.
The skectch is a a dome roof.