Dome roof structure calculation according to DIN 4119

[yuntislol]

Good afternoon everyone,

Currently involved in a project that needs to check suitability for change of service in a tank.
Although the service change has no impact in the roof, i replicated the design in AME Tank.
Major concerns:
Construction code used: DIN 4119 (not supported in my AME TANK License)
Tank ID: 31 090 mm
Largest normal force in the ribs = 3.388 ton
beta assumed to be = 1.0 (unconnected ribbed grid-type);

After checking results, AME Tank warned me that roof structure was not well dimensioned which led me to check calculation note.

Manufacturer calculated minimum moment of inertia for profile and got the result Jx min = 676 cm^4. They then proceed to chose a profile IPN 140 with a moment of inertia lower than this value! (Jx IPN 140 = 573 cm^4).
They justify this choice by calculating maximum moment of 2nd order and then maximum normal stress of 2nd order. Result is a moment 1.334 Tn/cm^2 just below the limit of 1.4 Tn/cm^2 of API 650 (95% soliciation limit).

This is not a confortable result for me. At this moment i am wondering if this approach is correct or if we should advise a roof structure repair.


Best regards,

 

[JStephen]

Is this a steel dome with internal ribs? Or aluminum geodesic dome?
In either case, I'm not aware of any standardized method to do the analysis, so I would expect results to vary considerably depending on the design assumptions made, method of analysis, etc. And with that in mind, I would be very reluctant to pay any money to "fix" an issue shown in one analysis that wasn't shown in the original analysis.
In one of my college classes, the old professor said "Never measure the hardness of the same piece of material twice. If you do, you'll have two different numbers, and won't know which one to use." And similarly, the proverb that "A man with one watch knows what time it is. A man with two watches is never sure."

 

[yuntislol]

Yes steel dome with internal ribs.
We have a signed contract with the client to check for tank integrity for new service. Service change doesn't affect roof so it would be a problem since original construction.
Even though there is no standardized method, if the manufacturer chose DIN 4119, and even after calculating minimum profile moment of inertia they chose one that does not meet requirements. What would be your opinion on this?

Best regards.

EDIT: I should add that AMe tank, through API 650 is telling me allowable stress is 260% above limit state (limit being 100%)

 

[HTURKAK]

ER. yuntislol (Mechanical),

- DIN 4119-2 ( Oberirdische zylindrische Flachboden-Tankbauwerke aus metallischen Werkstoffen; Berechnung) Released at 1980 and withdrawn at 2004 ..

- DIN EN 14015 (released at 2004 ) and AFAIK ,final version released at 2017

Apparently , you are looking for to change the service of an exisitng tank probably 30+ years old. Your approach , to check the tank as per API 650/653 seems not reasonable since , it is not API 650 tank ( probably will not comply the requirements of API 650)

Instead of hybrid approach ( loads as per EC, standard 650 , material of construction St 37-2, St-44..) why you do not check as per EN 14015 ?
Is the only problem , non-conformance rib stress ? which requirements ?

Can you post some details of the roof and rib calcultion and provide info. for the tank?

 

[yuntislol]

Mr. HTURRAK.

Tank construction code is API 650 with the exception of the roof structure! Roof radius is over the API max curvature >1.2 OD (roof radius is 1.5 OD)

TANK ID = 31090 mm
Tank height = 14630 mm
Dome radius = 46 635 mm
Roof thk = 5mm
Top angle = 150x150x14 mm
bottom slope 1:120
bottom thk = 10 mm
1 stifferner IPN 200 @ 10330 mm
all plate materials are A283 Gr. C

Seismic info:
SUG 1
Soil type C
T_L = 2 sec
Ss (g) = 0.23
S1 (g) = 0.09

Wind load info:
wind velocity = 30 m/s
wind pressure profile:
0~5 = 1.07 kPa
5~10 = 1.32 kPa
10~15 = 1.46 kPa
roof and shell pressure coef= 0.7


According to manufacturer calc note, they proceed to calculate what is attached. Not sure if i should raise the question for a future study, since the tank has been constructed for 30+ years. But if there is an error i feel obligated to let the client know.

 

[HTURKAK]

I am not sure if the tank construction code is API 650 with the exception of the roof structure.. Probably a mixture of codes .. ( tank roof radius 1.5 OD is allowed at BS 2654 ).. If the tank bottom and the shell complies with API 650 and the roof structure ( when checked with AME TANK ) does not comply , (AME tank showing allowable stress is 260% above for the roof structure ), IMO, this non-compliance shall be digged more rather than concealing.. In this case, checking the roof structure by an experienced structural eng. with a different software ( structural eng. softwares SAP 2000 etc ) should be way to go..

I looked the excerpt of roof calculation report. Without knowing if adding a center column would hinder the process with new service, i would go with Adding a central column to the dome roof..With this option, the roof no longer would be self supporting and the max.allowed curvature ( API standard ) ≤ 1.2 OD could be circumvented . Moreover, the axial stress will approach to zero and bending stress will also substantially reduce..

I would like to see the roof bracing .

My opinion..

 

[yuntislol]

Mr. HTURRAK,

Thank you for your opinion. I cannot disclose any drawings outside of the organization.
What are your thoughts on calculation the maximum normal stress of second order through maximum moment of 2nd order despite the calculated profile does not meet the requirements of minimum moment of inertia?

Thanks in advance.

 

[HTURKAK]

IMO, the minimum moment of inertia ( Ixxmin = 676 cm^4 ) still not enough and will not meeet the requirements of API 650 and new loading. Just for an example, similar roof with tank diameter 36 m, i remember INP 240 rafters..

 

[yuntislol]

Exactly my thougt. Thank you for your input. I only get valid results after IPN 200.

Even though the tank is already built for 30+ years. I know that steel only gets plastic deformation at about 210 Mpa, and we are assuming 140 Mpa, but still not a safe construction