Conical Roof Tank Design

You can check the roof frame for the load. If necessary you add additional beams and pad plate so that the stresses are within limits.

sir,

thank you for the response.

can you help me out with the references if any for checking the roof frame load. API 650 clause 3.5.3 gives only the allowable stresses and other criteria for various loading, but not the formulas for the design/checking. the same with the IS 803.

regards,

mahesh

If your tank has bracing angles underneath the roof extending to the periphery, then it should take the load. Otherwise you need to provide them. Also it may need gussets if the internal angle of the cone is high.

Alternatively, you can provide external structural members to take away the load.

Assuming the conical roof is 6mm thick, its self weight would be approx. 3 tonnes without the weight of internal truss supports. The stress imposed by the 14" flame arrestor will be [ 250 Kg+Normalised wind load in Kg]/area of existing nozzle flange (mm²). This value should not exceed 3 Kg/mm² (allowable compressive stress of Mild steel).Also, depending on the location of the tank top nozzle, check for rigidity/flexibility of the internal trusses/supports.

Typically, for a central column supported conical roof, the truss beam flexibility can be checked by treating it as a simply supported beam of projected length = Cos (conical base angle) X sloping length of the roof plate. Use (Max deflection) Y(max)= PL³/48 EI, where P=Load, L=Span, E=elasticity modulus and I=Moment of inertia. Accept, if Y(max)≤3.8/2=1.9 mm (i.e. rule of thumb : 1 mm deflection / meter length of beam.

Hope this helps you. If you please, you can send to me the values of the parameters in bold letters above and I can work it out for you, for cross verification only and without any attendent responsibility for consequences.

DRN sir,

thank you for the reply and it is definately helpful to me.

but the tank in question has only rafters (16 nos) as the support for the roof. there is no central column for the support for the rafters as stated by you.

so the radial refters will not be simply supported members (kindly comment).

Kindly tell me about the cross-section of the rafters, (viz. '[' channel, 'L' angle, 'T' T-section, 'I' I-section) and their dimensions or designations as ISMC 50, ISA 75.....

Also, they are likely to be terminated at the center of the conical roof into a central round plate or hoop ring......,what's the arrangement? Then an appropriate formula can be worked out. Please also let me know whether these rafters are "Stitch" welded or continuous welded to the roof plate? If stitch welded, what is the length of the weld and what is the spacing between welds?

We will revert on hearing from you.

the rafters are 16 nos. ISMC100.

the rafters end up connected with the center, with a central round ring of ISMC150 8 mm. thick.

the rafters are not welded to the roof plate. the roof only rests on the rfater with area contact (on falnge of ISMC100).

Regards,

mahesh

Please calculate wind force on Flame arrestor taking total surface area exposed to wind into account

Force Of Wind When Blowing Perpendicularly Upon A Surface Of One Square Foot

- Whittaker's Mechanical Engineer's Pocket Book.

Equation for wind pressure in pounds per square foot:

p=0.00256*v^2

where v is the velocity in mph.

Please take care to convert into Kg/M² .

Now add this wind load (Kgf) to the weight of flame arrestor (250 Kgf.). This total load is to be added to (1/8 X total conical roof weight) and treated as Uniformly Distributed Load on a "Fixed" type Beam ( comprising 2 radial rafters connected by central ring) whose maximum deflection will be given by Y_max = 7 WL³ / 1920 E I at span/2. In addition you can take deflection of the "Fixed" beam caused by the flame arrestor load (Self weight+wind load) acting at span/2 given by : Y _max = PL ³ / 192 EI..

The total deflection calculated as above must not exceed Span (M)/2 in millimeters.Viz.(8M/2)mm≡ 4 mm.

Hope the above has been of use.

dear my friens

i need drawings "self supporting roof with trusses"

can you help me

Please refer BIS 803. You can get back, if you do not find what you want.

Please realise that a drawing can be expected by you only if you provide details of product to be stored, Storage Capacity, ambient conditions (Altitude, temperature variations, rain/snow, wind speeds) etc.

Dear Naidu

Tank Diameter :26800 mm , Tank Height :10000 mm, Roof Type: Conical (Slope:%16.6) Support with trusses. Storage Capacity : Net 5000 m3

1st course :11 mm, 2nd course :9mm, 3rd course :7.5mm, 4th course:6mm, 5th course:6 mm top angle:80x80x8

I am finding trusses detail.(how many trusses, installation, how welding to last shell plates or to top angle

drawing may be *.pdf, *.dxf, *.dwg

thanks

Please reply to my queries:

Please realise that a drawing can be expected by you only if you provide details of product to be stored, Storage Capacity, ambient conditions (Altitude, temperature variations, rain/snow, wind speeds) , Seismic activity (Earthquake Zone number) etc. You must also reveal your identity , so that knowledge of the desirability of owner/beneficiary can be clearly established; The internet has been put to undesirable uses!!

I am confident you'll understand.