Steel deflection

[Solly]

So, I'm designing a steel stand. I know how to calculate deflection, I have the data tables for steel tube, etc, but I'm missing one (very key) part: What level of deflection is considered safe under an aquarium?

Thanks.

-Solly

[BeanAnimal]

Solly, what size tank? What is length and width?

[Solly]

Throwing ideas around with tank size; minimum 6X2X2 ft, maximum 8X3X30" tall. Largely depends on my budget when the project gets started. If 6' length, I don't want a center brace in front. If 8', I want 4' openings.

I know that deflection is calculated as

Deflection = (P x L^3) / (192 x E x I)

Where P is the load in pounds, L is the span in inches, E is 30,000,000 PSI, and I is the steel tube's moment of inertia. I can look up various steel tubes' moment of inertia on a table I have downloaded.

So, for example, if I want a maximum of .1" deflection, and I was doing a 6' (72") tank that weighed 1440 pounds (each of the two lengthwise tubes bear 720 pounds), I would say,

.1 = (720*72^3)/(192 x 30000000 x I)

Solving for I, we get I = .46. We then look this up in the tables, and find that 12-gauge 2" square tubing has a moment of inertia of .493, and so would be good enough.

So, to summarize, if we have a tank that is 6' long, has no center brace, and weighs 1440 pounds, and we want less than .1" of deflection, we need 12-gauge 2" square tubing.

But the .1" of deflection I just used as an example was completely arbitrary. I want to know what deflection is in reality safe. I am a firm believer in shaving any possible penny from large projects, and so I'm not going to splurge on paying more for overkill-sized steel. That's why I'm running these calculations so carefully.

-Solly

PS I'm just putting together a 100 gal right now, and am planning to start this bigger tank come summer. That's why I'm posting a lot of these "random detail" question threads.

[Solly]

Oh, and if anyone wants the table of the moments of inertia for different tubing sizes, feel free to PM me.

-Solly

[BeanAnimal]

Solly, 1x2 rectangular tube will likely be overkill... but would certainly be easy enough to fabricate with and not break the bank compared to 1x1 tube. I am not sure what deflection is acceptable. Are these going to be glass? Plywood and glass? Acrylic?

I imagine a structure engineer will be able to give you more insight. Also look here...http://www.reefcentral.com/forums/showthread.php?threadid=997791&perpage=25&highlight=steel&pagenumber=5

HUGE tank and not really that much steel!

[Solly]

No plywood. Not sure glass or acrylic yet.

16-gauge 1X2" tubing would give me about .25" of deflection over 6' and a 1440 pound tank. And foam/plywood under the tank would further minimize that deflection, of course.

-Solly

[BeanAnimal]

Solly remember that that front rail is only support part of the load. The weight is also resting on 2 short sides (which I did not see you account for) and 1 other long side (that you appear to have accounted for) and distributed evenly. Like I said I do not have the answer... but somebody may. You appear to be moving forward rather well yourself :)

[Solly]

That's right, I did not account for the side rails; I don't know how I would. I'm a mathematician, not an engineer!

Let's hope someone else chimes in. Thanks for your help so far.

-Solly

[BeanAnimal]

MY guess is that you would take the length of the entire perimeter and divide it by the total weight.... That would give you the load per unit length.... I could be VERY wrong though!

[jcm2m9]

I would do it exactly as bean has said above. By doing it this way you are assuming you have the load evenly distributed over the entire bottom of the tank. That is not exactly how it will be, but I believe that assumption is good enough.

Send me that table and I will take a look at it.

jcm2m9 at yahoo dot com

[03LightningSVT]

<a href=showthread.php?s=&postid=8998667#post8998667 target=_blank>Originally posted</a> by Solly
I am a firm believer in shaving any possible penny from large projects, and so I'm not going to splurge on paying more for overkill-sized steel. That's why I'm running these calculations so carefully.

If shaving any possible penny is a firm belief - consider lumber.

IMO, your interest would best be served by doing a side by side cost comparison for all raw materials and labor on both materials.

If you're also planning on a canopy, don't forget to allow for the additional weight of the canopy itself, light fixtures, ballasts, fans, etc. On large tanks this weight adds up quickly.

[Solly]

JCM2M9,

I emailed you the data table.

Lightning,

Lumber is not an option. First, I want the room under the stand, and second, I'm a much better welder than I am woodworker.

-Solly

[Bergovoy]

I would not build difelction into the top beams. With that short of a span and teh load being evenly dispersed along the entire perimeter, you want the tank to sit on the bearing points evenly.

Diflection could be considered requireed if you had a span that you wanted to make sure withstood teh all the loads to be placed on there without leaving some safety leftover. Without camber builtin, it could be more likely to load the roof to the point of negative camber and catastrophy

An example would be to plave air conditioning equipment or other equipment midspan on the roof.

This is not similar to a tank stand where the load is equal at all points alont the perimeter. If there were camber and the tank make contact with that first and with calcs being correct then displacing the camber to become truly level would still create a stress point at where the camber is still trying to recover its shape.

Does this make sense/?

Bill

[Solly]

I agree that deflection creates stress points, and thus should be minimized. But no matter what, there will be some deflection because the beam will be supported at the ends only. The question is what amount of deflection is "safe."

Or did I miss something you're getting at?

-Solly

[03LightningSVT]

Check out the "Large Reef Tanks" forum here on RC. Many posts with steel frame stands.

[pvtschultz]

Actually, if you are using a glass tank, with rigid walls and bottom, you'll have no deflection at all. Place the vertical columns directly under the tank walls (or very close) to reduce the moment acting on the joints, and you'll have now problems. For glass tanks, the side walls bear the vast majority of the weight which is subsequently taken out by the colums, not the horizontal structures.

[Solly]

So, effectively, the top frame has no point? It just adds some minimal stability to help the tank rest on the vertical supports?

-Solly

[spitefull]

Solly, deflection is not a consideration for this project, you need to worry about the welds, the tubing will not crush down, but a good side bump can tear it apart, I made my 250 from 2x2 .065 crew tube, you need surface area not wall thickness, good mitre joins for upper and lower frames and at 1.00$ a foot, is reasonable

[BeanAnimal]

Why not just use some gussets in the corners to add strength? Skinning the top and 3 sides should also do a great deal of stiffening that will help prevent racking or swaying.

[Bergovoy]

There should be zero camber so no diflection should be placed intot he tubing. Longer lengths have a camber, and smaller lengths do not. (depnds on where the xmaller section was tanke from)

The welds need to be good, and gussets are a great idea.

The seight of the tank will be borne by the area /entire perimeter directly under the tank.

Sheeting with ply or the use of cross braces will hold the frame rigid so there is no motion in any lateral direction. (I am interested in seeing a detail for fastening the ply to the steel. is it just tek screwed to the tubing?)