Regarding Bulging limit for rectangular steel tank- rest on gound

[Arbu]

Dear All,

I am trying to analyse one Rectangular steel tank (water) having size approx. L X W X H = 8 x 1.5 x 2.2 m. I modelled it in STAAD Pro and applied loads as below:
1. Side walls hydrostatic pressure triangular
2. Base uniform pressure with intensity H x water density.

The tank will be fabricated by MS Plate 3 mm thick and reinforced by MS channels out side. Also inside wall is having a FRP sheet covering of 5 mm.
The tank will be resting on concrete. When I am analysing for the above loads I am getting a lateral deflection of top channel (Bulging) around 70 mm.

My doubt are
1. There is any standard for limiting the bulging (lateral deflection) of the rectangular top open Tank.
2. I modelled the MS plate with channels and I ignored FRP sheet, is this approach is correct?. Do I need to model FRP sheet also?

Please guide.

regards,
Arbu

 

[r13]

I don't recall there is limit on the amount of wall bulging, as long as the strength is adequate, and stresses are within allowable per the design code. But, the 70mm over 2.2m high turns out a deflection-span ratio of 30, it seems low, I believe the ratio preferably should be in the hundreds. The inclusion of FRP in your analysis model shall improve the result.

 

[JStephen]

In some cases, large deflections can invalidate the analysis, if the analysis isn't set up for it. You can start getting tension in plate in addition to bending (catenary effects), etc.
The number of stiffeners, stiffener spacing, and plate thickness would generally be adjusted for the most economical construction, and it might be worthwhile to reduce the number of stiffeners and increase plate thickness, etc.

 

[r13]

Can you add a baffle in the long side (2-4x1.5x2.2 compartments)?

 

[Arbu]

Thank you for quick response.

Dear retired13,
I cannot add baffle as required length of tank is minimum 7.5m . If baffle it will obstruct purpose of tank.

Dear JStephen,
there will be FRP and MS bonded together. So you are asking me model a laminated plate or something like that.

Please suggest how to control bulging without adding baffle, without adding top cover.

Thank you.

 

[r13]

Try double channels for the middle 3 verticals stiffeners on the long side. If still not enough, use double channels for the lowest horizontal stiffeners, then the middle stiffeners, if necessary. Or, increase thickness of the panels.

 

[HTURKAK]

1. The deflection (bulging of top channel ) 70 mm is not acceptable. The deflection should be around plate thickness. The plate thickness 3 mm should be increased and add some corrosion allowance... The inertia of top edge stiffener should be increased or more economic to provide tie rods inside the tank ..

2. The approach is correct. The modulus of elasticity of FRP negligible compared to steel.

 

[human909]

Your general approach is fine though, personally I'd be bumping up your ring stiffeners significantly. Your vertical stiffeners should be smaller. Also the corners connections need significant attention. There is plenty of force that needs to be transferred there.

 

[Geoff13]

I was so surprised by your 3 mm wall thickness I did some quick numbers using Roark's.

Roark's is generally based on small deflection theory for plates, and I wouldn't be surprised if STAAD is the same. A small deflection is normally defined as one-half the plate thickness, however Roark's formulas for your geometry gave me deflections one to two time the plate thickness. I'm estimating values so I could be wrong.

As already noted by JStephen you may need to add more thickness or stiffeners to meet small-deflection theory. This would be my solution. Stiffeners are expensive due to manhours for fabrication and welding, so plate thickness would be cheapest where I am.

You could switch to large deflection theory, but the resulting diaphragm tension in your plates would tear this light structure apart. Roark's has some limited information for plates, but I don't think your cases are covered.

Not sure about the FRP, but my reaction is not to include it. First you would need to suitably ratio down its thickness to account for it being much softer than the steel, and then you would need to have a 100% bond with zero slip so they act as one.

 

[JLNJ]

Have you considered the connections at the corners (and bottom) fixed or pinned? True fixity will be a little difficult to achieve and any flexibility in the fixity will increase your sidewall bulging.

Given the proportions, your best bet might be to use a heavier member at the top to keep the bulging in check.

A tension tie across the top would work nicely as well.

Check out Blodgett's Design of Welded Structures. He has some examples of tank design with easy-to-follow hand calcs, and it's better to hand calc something you can understand as a first pass anyway.

 

[saplanti]

AS 4100 recommends several ratios of span/deflection. Perhaps you may use this ratio as 250 in your case. In case the material is not more ductile than steel you may increase the ratio up to 500.

You may additionally check Connections of frame members (for each horizontal levels) for adequacy as well.

You can use this ratio for all the horizontal members. Probably the vertical stiffeners and the distance between them is going to determine the wall thicknesses of the tank.

I suggest you do a hand calculation first and refine it by the FEA if necessary for your application

 

[Arbu]

Thanks you for your suggestions.

Actually this details I got from the supplier and I need to verify it. Supplier of tank is not sharing the design report that's why I am verifying.

All the joints are rigid and I cannot propose tie rod as it will obstruct the working of tank. I checked the stress distribution in the plate element of STAAD model and found that there is very less membrane stresses in plates. Seems like plate action only there due to which the deflection is high. if there will be some membrane action it will reduce the bulging. Please suggest.

as per my understanding the plate element in STAAD is Hybrid element having Plane stress and bending both action.
As the plates are modelled and supported by steel channels and meshed, the size of Plate element is very small compared to one side of tank and the deflection of plate corner node is relatively small. Might be due to this membrane action is not taking place in STAAD.

Could someone will suggest me the modelling approach such that membrane behaviour will be in action.

Thank you.

 

[dik]

Can you have horizontal tension tie rods on the interior, going from side to side?

Dik

 

[JLNJ]

Barring a cross tie, the only way to reduce the horizontal deflection of the top on the long side is to increase the member size on that top edge. It’s pretty simple.

I would pin the ends and choose a member which reduces the horizontal deflection to about 30mm. Yes, this means the member the fabricator guessed at isn’t large enough and you will have to tell him so.

 

[dik]

I'd be looking at a deflection of about L/100, myself.

Dik

 

[r13]

From Bentley:

"In STAAD, this (plate) element has both attributes - membrane (in-plane effect) and bending (out-of-plane effect). The bending effect can be shut off by declaring it as ELEMENT PLANE STRESS. The in-plane effect can't be shut off.

 

[Arbu]

Now I am planning to increase the thickness of the plate and if need I will strength the members also. I cannot provide tie or baffle as it will obstruct the working.

Dear Dik,

you mean to L is height of length of the tank.
If its height then the allowable bulging = 2250/100 = 22.5 mm

So as you said H/100
and
I will be keeping the bulging in between 22 to 30 mm.

regards,
Arbu

 

[dik]

I'm talking the membrane deflection of the sheet metal (oil canning, or whatever) between superstructure support. I'd design the sheet as a membrane and not a flexural item... I often use L/100 for checkered floor plates or whatever. The actual superstructure can be L/100 or even more stiff...

Dik

 

[human909]

Now I am planning to increase the thickness of the plate

That is unlikely to achieve what you are after. Plate size seems appropriate. Member size not so much.

 

[dik]

Your top member should be a horizontal beam and should have cross members to minimise the size, maybe at each 2nd or 3rd vertical. If not, you have a huge cantilever that would have to be fixed at the bottom by an equally large cross beam at the bottom.

Dik