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Flat Reinforced Tank Roof

Flat Reinforced Tank Roof

Flat Reinforced Tank Roof

Can someone give me a refernece for determining the MAWP of an atmospheric tank with a Flat Roof which is reinforced with angle steel? I know about the ASME flat plate/head calculation method, but, it does not give any credit for the reinforcement (2.5x2.5x1/4 angle). I also know to use the weight of the roof as part of the MAWP based on API.

Thank you,

RE: Flat Reinforced Tank Roof

I assume that this is a non-ASME and non-API code tank. Your flat plate will have near zero stiffness, and the stiffening angles will essentially carry all of the load.  You need look at 3 different scenarios:

1.) Consider the bending stresses in the flat plate between the stiffeners, using appropriate flat plate formula from Roark & Young.

2.) Consider stiffener bending stresses, neglecting any additional stiffness added by the flat plate.

3.) Consider the deflection of the stiffeners and of the flat plate.  Keep it within reasonable limits (span/360).

I believe you are correct in utilizing the roof plate weight in counteracting pressure per API-650 method.  It will likely be where a very large percentage of your total pressure rating comes from.

RE: Flat Reinforced Tank Roof

TAI Mike,

Typically, flat bottomed/flat topped tanks are designed, specified and constructed to API-650 which gives a maximum operating pressure of 2.5 psig (if the tank is constructed to the special provisions of API-650)

Alternately, tanks may be designed to the rules of API-620, which allows higher pressure (up to about 15 psig, as I recall). API-620 tanks are more expensive than API-650 tanks. There are fabricators in the USA that use "no code" and just supply a tank with no guarantee or paperwork. Most flat bottomed tank use either API-650 or a UL Code. Of course, above 15 psig, the "tank" becomes an "unfired pressure vessel" and the rules of ASME Section VIII are commonly required by state law.

For you to try to use the rules of ASME-VIII for a thin walled, reininforced tank would, in my opinion, be a mistake.

Do you work for an MBA or perhaps a graduate of the "Project Management Institute" of Pittsburgh Pennsylvania ?


RE: Flat Reinforced Tank Roof


This tank is not built to API-650....flat roof tanks are not part of API-650 and do not fall under the 2.5 psig allowable pressure. A flat roof atmospheric storage tank would never be able to hold 2.5 psi of pressure.

I was not suggesting the use of ASME Section VIII for an atmospheric storage tank as I am well aware of the proper application of ASME and API standards/codes.....however, the use of the MAWP Equation for a flat plate that is provided in ASME does give conservative results for flat top tanks....especially if limiting the deflection to 1/2 of the plate thickness (even at pressures measured in inches of water column).

I was looking for information on a method to calculate how the stiffening of the roof by the angles would increase my allowable pressure (which I have calculated as 2.5"WC without stiffeners).

Maybe you are a graduate of the "Project Management Institute" of Pitts., Penn.

Mike, M.S., P.E.

RE: Flat Reinforced Tank Roof

In my humble opinion, API 650 does not disallow flat roof tanks.  API 650 only provides four typical roof designs as detailed in Section 3.10.  Because this is a flat roof tank, I will assume that the tank is not very large otherwise, you would have something like a cone roof tank.

Your "MAWP" will be dependent on the longest roof-stiffening member.  You can refer to the Manual of Steel Construction for deflection of a 2.5x2.5x1/4 angle.

I know that you will consider other components of this tank in your MAWP calculations but I will point them out anyway for the benefit of others and perhaps a discussion is generated where I can profit as well.  If this is an existing tank and you are trying to increase the pressure to which your tank was originally designed for, you should also consider the following:

*Analyze the compression area at the roof-to-shell joint.
*Higher internal pressure will change your anchorage requirement.
*Venting design will have to be reconsidered.

In order to prevent a catastrophic failure of the tank, the roof to shell joint should always be the weakest component of the tank.  This roof should be designed to be frangible or non-frangible provided that emergency venting devices conforming to API 2000 are provided.

K. Phan

RE: Flat Reinforced Tank Roof


Your comment "Analyze the compression area at the roof-to-shell joint" does not make sense for a flat roof tank, as the formulas would give INFINITE area required.  For self supporting cone, the slope must be between 9.46° (2:12 slope) and 37° (9:12 slope).

The notes on venting and anchorage requirements are entirely correct.

RE: Flat Reinforced Tank Roof


You're right and I apologize for my error.  I forgot that this is a flat roof tank.

K. Phan

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