Any Engineers in the building? Steel Stand question

I've read this like 5 times now and not sure I understand what you're saying....



...more thoughts....


I'm thinking about taking it to my other shop and putting 2500# of cement on it and seeing if it holds... if I dont get any deflection on that, or no more than 1/64" I think I'm gonna be ok.

Also, the top edge of the stand is going to have some 1x3" lumber on it, between the metal and the tank proper. Should absorb a little bit of play in the tank I think?

...this is frustrating


Oh, and the main reason I didn't do center supports in the first place is because I am going to have a 55g sump/fuge under the tank and the front is about the only way I can get it in once the stand is in place. I can set it over the sump when I get started, but down the road, I'd like to be able to remove/replace the sump without draining and moving the tank off the stand. I won't have enough room on the ends to slide it out from them.

That actually leads me to another thought tho.... what if I placed 4 more legs on it, leaving a central span just wide enough for the 55g tank? It leads me to wonder what's the load and maximum span I can have under the tank....

round robin thought processes leave me going crAaaAAAzY

 

your probubly around 1500 total then divide by 4 so you get about 375 lbs on each leg is no problem. the problem would not be the buckling so no center leg but the stand leaning over onto its side (all the legs bending to one side) so keep the legs strait with an intermidiate bracing such as a lower shelf about a third of the way from the bottom would be great. this is not my thinking my dad told me this ( he works at the Woods hole Oceanographic institution as the boss of the engineering shop so yes he did all the pressure stuff for Alvin and Nerius- the AUV that went to the bottom of the Mary Annas trench so i belive him. we have the same thing with our tank and weve had a similar set up for our 55g and it has worked great hope this helps

 

Let me be more clear.... what you've got is probably fine.... The way I look at these things is through a few "filters"....

First, what are the failure points?? In this case you got two questions.... span length... the 6' between the legs... and leg stability.... how good are your welds?

Second, consequence of failure versus cost of mediation? If a span sags or a leg isn't stable, how bad a day would it be?? In this case, the consequence of failure would be catastrophic.... you could lose your whole tank costing thousands. What is the cost of mediation... trivial to add some braces or center legs.

While what you have is probably fine....I'd just add some braces as was described above or add some center legs. Either solution is fine. It would protect you from any single failure for a trivial effort. So, I'd just do it and stop worrying about "engineering" it. My structural engineers at work waste tons of time trying to get it perfect. Sometimes that is needed. Other times, just brace the darn thing and move on. I think this is the "brace it and move on" case.


-----------------

Second.... integrated solution. To make the math easy.... let's say your system will weight 1800-2000lbs. Spread over 4 legs gives approx 475lbs per leg. Assume you put just a 2x3 block of plywood under each leg... 6 sq in. which would give 80 psi.... What flooring are you putting this on??? Cement? Tile? Plywood decking? My vending machine at my business cracked my tile eventually.... Plywood decking... where are the joists? 6 legs would spread the load.... down to 52 psi. 35% less.

Again, you are probably just fine the way you are, spreading the load is demonstrably better.

So... brace it... add some center legs.... distribute the ground load... skip it all... you are probably fine.... do it all... it'll cost you an hour or two and you'll never worry about it. I wouldn't stress a lot trying to refine it.

Mark

 

Maybe you could do a test to see what the minimum amount of space is that you need to work the 55 into the front of the stand. Once you find the minimum, you could put two supports there instead of just one in the middle. It seems like you want the entire front free of impediments though so this might not be a suitable solution. I don't have experience with metal work but this seems like a practical solution to prevent the possibility of bowing.

 

another thing why center leg would be pointless is most tanks dont touch the stand there on ( the glass part) its the plastic or whatever polymer ring around the tank that touches the stand. the middle is usually hollow so the pressure on the outside of the stand buckles the center of the steel top a little which would just lift the center leg up. all you need is another steel shelf a third of the way up your legs to keep them together that way you can also put a sump underneath

 

the steel legs wont bow it would take thousands of pounds of force to bow a steel leg alone. you just have to worry about keeping them strait which would be fixed with the other steel shelf. i would also do as NASAGeek said and put something underneath each leg to spread the load out better

 

Some of the estimates as to weight seem off. Lets assume this will house a 185 gallon tank. One gallon of freshwater weights about 8.35 pounds.

185 * 8.35 = 1544.75

The density of freshwater is 62.4 pounds per cubic foot. My guess is that LR is closer to 160pcf. That means that any rock in the tank weighs about 2.5 times as much as the water it is displacing. If you have 250lbs of live rock in the display tank, then that live rock is displaying 100 lbs worth of water.

(1544.75 - 100) + 250 = 1694.75
[/B]
The tank itself will weigh between 100 and 220 pounds. Once stocked with livestock and corals you are probably looking at near 2000 pounds of weight for a 185 gallon tank. That is considerably more than some of the estimates of 1500lbs.

To answer the question that was originally posted; I agree with NASAGeek.

 

What if you did arched braces on both sides, and from side to 1/3 down the length. Arches are much stronger(ask the Romans), and would still leave you with a considerable amount of space in the front to put whatever you want inside. See pic...

And the arches should span farther than the pic suggests...MS Paint is very limited. I am not at my PC with my graphics programs.

 

Ok, so after sleeping on it I have some new insight....and more info.

NASAGeek, the whole stand is going on a sheet of plywood cut to fit under the stand with about 1" of over lap. The tank/stand is going on a 6" concrete slab under carpet. I'm thinking about having the carpet removed from where the tank will be to facilitate future carpet installs without needing to move the tank.


Anyone suggesting a shelf... read my original post about why I built a stand 3" talled than the stock wooden stand it came it. I need vertical clearance for the skimmer. If I put a shelf on the stand I will have less space than I had in the wooden stand I built this to replace. It is, frankly, not an option.

I am thinking the best bet might be to brace the legs at the bottom, and make essentially a rectangular box frame. That would keep the legs from splaying out, and I can add an X brace alone the back and sides to keep it from tilting over. Probably just use cables and bolts for that, no need to weld, so no need to re-coat any thing.


Also, thanks for the insight of the forces, I was thinking along the same lines... let me draw you a picture....

#1 shows in elevation the way the forces are actually distributed.... evenly across the span. I am thinking that should be fine - the plastic rim the tank sits on is approx. 100-150 sq inches, the tank is boxed up so I can't measure the exact width, but I estimate it to be between 5/8 and 3/4 wide. That gives me about 13-20 psi across the frame.

#2 shows a single point of force on the middle of the span. Obviously this would be more stress than the stand would handle with no deflection. But it's not the situation. Even then, from charts I have found it is below the failure point of the steel, but I would see some warping/deflection.

#3 is a quick sketch I did as an after thought to show the actual distribution, since the stand is a total of 4 spans, 4 legs.


....I've got an engineer in the family I can bother with this, I just need to get into his schedule. But I suspect the best course of action is going to be brace the legs at the bottom, then skin the frame on the back and ends with plywood (already in the plan) and add a lift off panel in the front that would be cosmetic. ....

Anyone know that wouldn't work?

 

I agree with NasaGeek, Drew3 and the old dude. Most of the weight will be supported on the four corners. Make sure you cross brace it so it doesn't tip over, and don't put the steel legs directly on the floor, distribute the force. The last one is the biggest problem that I have seen with tanks cracking on steel stands. Too much force concentrated in a small area.

All three of my stands have no center brace, and none of the tanks touch in the center of the stand. Two are 125's and one is a 180. They are all made with 2x material and skinned with granite and a plywood front.