Structural engineers: Please HELP with 7 foot DIY stand build.

[RocketEngineer]

Ok, more complicated maths but it’s what I’ve got:
2X8: 74.75” span with 1335# load (74.75/84*1500) gives .152” deflection. Too much IMO
2X8 w/ narrowed opening: 66.75” span with 1192# load(66.75/84*1500) gives .097” deflection. Borderline IMO.
2X6 w/ extra leg: 51.25” span with 915# load (51.25/84*1500) gives .077” deflection. Best of those 3 options.

In truth, you may as well do a center leg at that point.

 

[mjovic]

Good input here. Use wood resting on wood with fasteners used only to prevent shifting or for marrying beams together. Use Douglas Fir 2x6 beams for extra strength. Double beams should be fine but being an engineer, I would go for triple. Is the plywood underneath to block the light through the tank? I have always had glass tanks and the bottom is designed to support the water weight with underneath support only around the edges.

My tank manufacturer does not support open bottom stands. They require use of a foam support pad to honor their warranty even though it is a rimmed tank.

 

[mjovic]

Agree. Especially if I'm leaning toward 5 or 6' sumps, I negate the openness anyway. Just curious what are your thoughts on doing 2x4 upper rail with 2 center supports equally spaced so spans would be ~24"?

 

[SteveMM62Reef]

For an Aquarium that size I’d go with two Sumps, one Sump and or with a Refugium, or two Sumps and a Refugium. That way you could remove one for maintenance and still have some filtration in operation. In this set up center brace or even two braces wouldn’t be a problem. Multiple return pumps for redundancy. Also I’d have at least two separate breakers power sources, personal I’d put three in and have each one power up other things in the room. IE, The TV is on one of my power sources the other is the WiFi. That way if either goes down, someone in the family will be complaining.

 

[mjovic]

Yes I was planning on running two pumps for redundancy and have already installed an additional 20A GFCI to run this system on a dedicated circuit. There is a 15A outlet nearby I can also use to run smaller wattage equipment if needed but it is connected to the same circuit as our treadmill so can't load it up too much, but it's there if I need it.

As far as running two separate sumps how would I split the flow of the overflow evenly to the sumps? Is it as simple as just t-ing off the main 1.5" drain and because it's in siphon it would just drain exactly half to each? Same with the overflow pipe and emergency drain? Or would I need to have gate valves after the T to dial in each sump independently?Then just have a return pump in each sump? Sorry for the noob question but never built a system this large before.

"Also I’d have at least two separate breakers power sources, personal I’d put three in and have each one power up other things in the room. IE, The TV is on one of my power sources the other is the WiFi. That way if either goes down, someone in the family will be complaining."

I literally LOL'd when I read that but it's smart!

 

[SteveMM62Reef]

How many Holes do have, in your Aquarium? My 120 Gallon has Three in the Overflow and two, on each back side for Returns. In the Overflow, are Two Durso, and a Return Pipe. In this setup, I could easily run two separate sumps.

 

[mjovic]

How many Holes do have, in your Aquarium? My 120 Gallon has Three in the Overflow and two, on each back side for Returns. In the Overflow, are Two Durso, and a Return Pipe. In this setup, I could easily run two separate sumps.

I will be running shadow overflow which has 3 1.5" pipes. It will be set up in the bean animal style with one in full siphon, one as a partial flow to ensure siphon of first pipe, but not enough to gurgle and make noise, and one purely as an emergency drain (unused under normal operation). The tank will have 2 3/4" return lines.

 

[BeanAnimal]

I started to write an entire how-to for Second Moment of Inertia, Bending Moment and Young's Modulus....

I gave up because even simple explanations become complex due to unit conversion. As you see, just explaining WHY this is not easy to explain will take many many paragraphs, let alone explaining the engineering itself.

OP and others. In general there are (3) things that we use to determine how much a beam will bend or deflect.

1 - the Second Moment of Inertia (Moment Area) which is derived from the cross section (shape) of the material. We use this (look up the equation in a table) to determine how a specific shape (in this case, a solid rectangle) resists bending. The issue here is what are the ACTUAL dimensions used for calculation vs the real world beam dimensions. Is the 2x8 1.5 x 7.5 or is it 1.43 x 7.325...

2 - The Bending Moment - the way that the beam is supported (one end or both and fixed or loose supports and where the load is placed (uniform or distributed and at what distance from the support(s).

3 - The Young's Modulus of the material itself. This is the materials properties as they relate to elongation or shortening when under tension or compression.

With these three things (all looked up) we plug in the values and determine the amount of deflection or bending. Where most people run into issues is conversion of the units of each of these. (imperial, metric, SI, etc.)

The biggest variable for our purposes is the Young's Modulus as it relates to wood. -We use a base value based on species and some assumptions for defects (grain, knots, heart or sapwood, etc). Two different pieces of the same size lumber can be vastly different with regard to strength or bending.

While I don't disagree with Rocket's numbers, they can easily be 100% off based on minor changes to the Young's Modulus used and the Moment of Inertia used. For 200 pounds per foot loading over the 19 foot perimeter, I come up with close to half the deflection that he does. But that does not mean that either of us are correct, it means that we made SLIGHTLY different assumptions using the same set of equations. He may have used point supported ends and I used fixed ends... neither is really correct or incorrect, as we don't know how stiff the side supports are to resist inward bending... that is another layer of calculations.

Also the base equation that we would use does not take twisting or torsional vectors into consideration. Tall beams under load want to twist, not just deflect. This is where blocking or other means to prevent twisting are important.

The point here is twofold. One, none of us can give you a 100% correct value. Two, safety factor, especially when there are so many variables in a design and no historical data or testing to consult, is important. That is where the 4x or more (over engineered in layman's terms) design advice comes in.

How do you add safety factor? Use real numbers and multiply the results... or inflate your assumptions and use the standard equations.

Example - Inflate the live load to 300 gallons at 10 pounds per gallon. So 3000 pounds. Make it 3500 pounds to account for your fat uncle and his neighbor leaning on the tank rim at your Halloween party. That increases the pounds per foot of perimeter load. Then run your calculation. Double the deflection value returned. If it is not reasonable then you need bigger beams, even if they are overkill. Is this overkill? Yes more than likely, but you are not an engineer trying to lighten a part in an airplane or reduce the span weight of a bridge on its piers. Likewise, none of us that understand the physics and engineering are certifying an end to end design, we are giving off the cuff advice that should be trusted as that.

 

[SteveMM62Reef]

Has the Aquarium Shipped yet? I’d ask for two more holes to be drilled in the back glass for Returns, to accept 1” Pipe, Bulkheads. My

 

[oreo54]

Wood Beam Span Calculator

Use this wood beam span calculator to check a wood beam of a particular size and length to see if it can support a given uniform linear load by comparing its allowable and required bending and shear stress.

www.omnicalculator.com

All the " maths" are here I believe. Not to mention plenty of unit choices in the drop downs.
See advanced mode as well.

I used 2800#'s (14ish sq ft) or 200# sqft. for tank glass/water/sand weight
Seemed reasonable

Never considered a safety factor though the weight is probably 50% heavier than reality

The joinery will probably matter more

 

[mjovic]

I started to write an entire how-to for Second Moment of Inertia, Bending Moment and Young's Modulus....

I gave up because even simple explanations become complex due to unit conversion. As you see, just explaining WHY this is not easy to explain will take many many paragraphs, let alone explaining the engineering itself.

OP and others. In general there are (3) things that we use to determine how much a beam will bend or deflect.

1 - the Second Moment of Inertia (Moment Area) which is derived from the cross section (shape) of the material. We use this (look up the equation in a table) to determine how a specific shape (in this case, a solid rectangle) resists bending. The issue here is what are the ACTUAL dimensions used for calculation vs the real world beam dimensions. Is the 2x8 1.5 x 7.5 or is it 1.43 x 7.325...

2 - The Bending Moment - the way that the beam is supported (one end or both and fixed or loose supports and where the load is placed (uniform or distributed and at what distance from the support(s).

3 - The Young's Modulus of the material itself. This is the materials properties as they relate to elongation or shortening when under tension or compression.

With these three things (all looked up) we plug in the values and determine the amount of deflection or bending. Where most people run into issues is conversion of the units of each of these. (imperial, metric, SI, etc.)

The biggest variable for our purposes is the Young's Modulus as it relates to wood. -We use a base value based on species and some assumptions for defects (grain, knots, heart or sapwood, etc). Two different pieces of the same size lumber can be vastly different with regard to strength or bending.

While I don't disagree with Rocket's numbers, they can easily be 100% off based on minor changes to the Young's Modulus used and the Moment of Inertia used. For 200 pounds per foot loading over the 19 foot perimeter, I come up with close to half the deflection that he does. But that does not mean that either of us are correct, it means that we made SLIGHTLY different assumptions using the same set of equations. He may have used point supported ends and I used fixed ends... neither is really correct or incorrect, as we don't know how stiff the side supports are to resist inward bending... that is another layer of calculations.

Also the base equation that we would use does not take twisting or torsional vectors into consideration. Tall beams under load want to twist, not just deflect. This is where blocking or other means to prevent twisting are important.

The point here is twofold. One, none of us can give you a 100% correct value. Two, safety factor, especially when there are so many variables in a design and no historical data or testing to consult, is important. That is where the 4x or more (over engineered in layman's terms) design advice comes in.

How do you add safety factor? Use real numbers and multiply the results... or inflate your assumptions and use the standard equations.

Example - Inflate the live load to 300 gallons at 10 pounds per gallon. So 3000 pounds. Make it 3500 pounds to account for your fat uncle and his neighbor leaning on the tank rim at your Halloween party. That increases the pounds per foot of perimeter load. Then run your calculation. Double the deflection value returned. If it is not reasonable then you need bigger beams, even if they are overkill. Is this overkill? Yes more than likely, but you are not an engineer trying to lighten a part in an airplane or reduce the span weight of a bridge on its piers. Likewise, none of us that understand the physics and engineering are certifying an end to end design, we are giving off the cuff advice that should be trusted as that.

Thank you for taking the time to write this. I have found everyone to be very helpful and I am grateful for that. I have a very expensive project about to unfold and I can't help but feel a little overwhelmed by it (in a good way)!

I seem to recall RE's calcs assuming SFP 2x construction lumber which had a specific modulus number he kept constant (want to say 1.6, but he would need to confirm). I hear you about there being so many variables to account for so I would like to methodically reduce the amount of unknown part by part.

Starting with the wood being used.
Someone in one of the posts turned me on to a very low moisture content lumber called "Framer Series" made by a company called Weyerhaeuser. They claim:

"Each piece of Framer Series lumber with WarpStable technology predicts with 95% confidence which boards will remain stable after being dried below 7% MC. It maintains a stability that is defined by the American Lumber Standards to within the #1 grade limit for bow, twist or crook. Each board is mechanically graded and the crown is marked to eliminate guesswork on the jobsite."

You can read about it here: https://www.weyerhaeuser.com/woodproducts/lumber/southern-lumber/framer-series/

There is an 84 Lumber near my house that carries it. I was going to check them out.

The plan was to follow RE's original plan, using 2x4's for the legs and base and either 2x6 or 2x4 for the top rail. All boards will be screwed using 2-1/2" coated decking screws drilled with pilot holes to prevent splitting. I would also use Titebond III glue in all the joints. Further, I plan to add additional cross braces front to back on the top and bottom boxes as well as skin all 4 sides as well as top and bottom with 3/4" plywood, cutting out the front openings to dramatically reduce wracking. The legs themselves would be 24" or 26" tall depending on if I go 2x6 or 2x4 top rail, so on the shorter side as far as stands go.

I would assume the weight in my tank to be around 3,000 lbs. when filled. 3,500 if we want to be more conservative.

I would like to have a safety factor of at least 3, meaning the stand could support at least 9,000 lbs. with no issue. Obviously more would be better.

This is all the info I have.

I'd still like to know if I did a 2x4 for the top rail with 2 supports on either long side would there be any issues with only using a 2x4? Has anyone built 8' stands or longer using only 2x4 top rails and uprights? Or do they always go with 2x6? Thanks guys! Appreciate all the help.

 

[The 262 Reef]

I believe the least expensive route would be wood. But i would consider using 2x6 in all aspects if not 2x8 if its that long of a span. Do you really want to not have a center leg brace on back and front? In the 16 years i have in experience of building homes for a living i wouldnt leave out that center brace. To keep it from toppling over on itself it needs to be braced in the back with a panel and screwed to the stand at multiple points. Infact i would screw panels on 3 sides. With your king studs running through and jack studs between the top and bottom interfaces. Honestly it depends how much money and time you want to put into this. Almost anything is possible but what you have drawn looks reasonable.. however i wouldnt remove the center brace ever unless you chose a different material that is stronger then wood and inconveniently im not confident enough to tell you anymore then that as i do not know tensil strength sheer strength of metal options although it would be reletively easy to find this information with a little bit of math as well..one thing i learned is i will never ever set up my next tank with the sump underneath the tank every again as the middle brace is a pain to work around and the fact that i am not sick and tired of ducking underneath and weasiling my skinny but up and underneath for equipment maintenance having to be pulled out and disconnected. At first i said im fine it dont bother me.. 2 years later everytime i even look at it down under neath i get upset lol. Your drawing is pretty much exactly what i have except you have an external overflow box. I highly recommend doing a sump not under the display. Unless you give more then 28 inches from top of sump tank to bottom of 2x6 frame. I only have 20 inches which seemed like plenty and now realise that 8 inches really does go a long way and would be a huge benefit . If you are stuck on not having a center brace then reinforcement would be necessary such as some sort of metal i beam and slotment for fit with studs under the ends for support. I hope you achieve your goals! I am in the same boat and am leaning towards a heavy duty metal stand with a wood panel and custom trim wrapped around it with a sump on the other side of the wall in a dedicated "fish room" where i have easy access to everything and can actually see everything. Good luck to you!

 

[BeanAnimal]

Wood Beam Span Calculator

Use this wood beam span calculator to check a wood beam of a particular size and length to see if it can support a given uniform linear load by comparing its allowable and required bending and shear stress.

www.omnicalculator.com

All the " maths" are here I believe. Not to mention plenty of unit choices in the drop downs.
See advanced mode as well.

I used 2800#'s (14ish sq ft) or 200# sqft. for tank glass/water/sand weight
Seemed reasonable

Never considered a safety factor though the weight is probably 50% heavier than reality

The joinery will probably matter more

I don't trust ANY non verified online calculators, especially for this type of calculation.

Most of them are not built by engineers or qualified people and instead are copies of the work of others. So one bad javascript formula or improper conversion and the results are suspect at best. What is worse, is that the same bad work gets copied into countless calculators and because they "agree" you think the result is confirmed as good.

I did not look at the linked calculator to confirm if the right calculations are being done or if the math and unit conversions are correct, but have found some insanely bad calculators at that site. It is an ad-mill.

 

[mjovic]

I believe the least expensive route would be wood. But i would consider using 2x6 in all aspects if not 2x8 if its that long of a span. Do you really want to not have a center leg brace on back and front? In the 16 years i have in experience of building homes for a living i wouldnt leave out that center brace. To keep it from toppling over on itself it needs to be braced in the back with a panel and screwed to the stand at multiple points. Infact i would screw panels on 3 sides. With your king studs running through and jack studs between the top and bottom interfaces. Honestly it depends how much money and time you want to put into this. Almost anything is possible but what you have drawn looks reasonable.. however i wouldnt remove the center brace ever unless you chose a different material that is stronger then wood and inconveniently im not confident enough to tell you anymore then that as i do not know tensil strength sheer strength of metal options although it would be reletively easy to find this information with a little bit of math as well..one thing i learned is i will never ever set up my next tank with the sump underneath the tank every again as the middle brace is a pain to work around and the fact that i am not sick and tired of ducking underneath and weasiling my skinny but up and underneath for equipment maintenance having to be pulled out and disconnected. At first i said im fine it dont bother me.. 2 years later everytime i even look at it down under neath i get upset lol. Your drawing is pretty much exactly what i have except you have an external overflow box. I highly recommend doing a sump not under the display. Unless you give more then 28 inches from top of sump tank to bottom of 2x6 frame. I only have 20 inches which seemed like plenty and now realise that 8 inches really does go a long way and would be a huge benefit . If you are stuck on not having a center brace then reinforcement would be necessary such as some sort of metal i beam and slotment for fit with studs under the ends for support. I hope you achieve your goals! I am in the same boat and am leaning towards a heavy duty metal stand with a wood panel and custom trim wrapped around it with a sump on the other side of the wall in a dedicated "fish room" where i have easy access to everything and can actually see everything. Good luck to you!

Good info speaking from experience. Thank you.

So to catch you up, I think I've moved away from the idea of having an unsupported span. It would require me to bump up to at the very least 2x8, but most likely 2x10 and I wouldn't have the height necessary to comfortably fit my stand and tank as my ceilings are particularly low in the spot the tank will be placed. This limits me to either 2x4, 2x6 or going with a metal stand.

The way I have it currently configured, if I use a 2x6 I would have a 24" opening for the sump, but I plan to lay my bottom 2x4 cross braces face down, which will gain me an additional 2" giving me 26" of vertical (not to mention the 5.5" additional head space in between each 2x6 cross beam. I may even go with 2x4 cross braces up top to gain me another 2" for 28" of total height under the stand. If I go with an 18" tall sump I would have 10" of clearance above the sump. This seems like it should be enough, although I hear you, bending down already sucks and will only suck more with time (haha). The idea of putting the sump in another area is intriguing, but I honestly don't have the room for it. The tank will be an in-wall installation going into my laundry room and it's already eating up a good chunk of space. Not utlizing the space underneath for the sump would seem like a terrible waste of space.

 

[oreo54]

I don't trust ANY non verified online calculators, especially for this type of calculation.

Most of them are not built by engineers or qualified people and instead are copies of the work of others. So one bad javascript formula or improper conversion and the results are suspect at best. What is worse, is that the same bad work gets copied into countless calculators and because they "agree" you think the result is confirmed as good.

I did not look at the linked calculator to confirm if the right calculations are being done or if the math and unit conversions are correct, but have found some insanely bad calculators at that site. It is an ad-mill.

Fair enough.
As a public service how about your take on just one calculation.

2941 total lbs ( tank glass 1/2 sides, 3/4 bottom, salt water, and 2" sand)
17.5 sqft
169 lbs/sqft

make it simple-ish
200lbs/ sqft

Example: spf grade2 2x8 weight calc of 200 lbf/ft
Beam span of 84( why not)
0.146".deflection.

This will NOT be considered a validation in the event the data is similar.
More out of curiosity.

2nd set of data ..from above.

2X8: 74.75” span with 1335# load (74.75/84*1500) gives .152” deflection. Too much IMO

 

[BeanAnimal]

Sure - we can run through a simple calculation based on that.

The only number that we need for the beam calc is the pound force on the beam and the length of the beam
You have defined that as 200 lbs/ft and 84 inches

2x8 lumber.
1.5" x 7.25" nominal (less when fully dry, sadly)

Moment of inertia for a solid rectangle with a vertical force
Ix = height^3 * width / 12
l = 47.63 in4

Young's Modulus
SPF #2 lumber - properly graded and avg. moisture content using NLGA.ORG values with no modifier for moisture or other factors.
E = 1,400,000 PSI

84 Inch Span
L = 84

200 lbs/ft - we are working in inches (PSI for Young's and in4 for Inetrtial Moment) so we need to convert
w = 16.67 lbs/in


Simply support beam with uniform load
Formula to apply for bending moment, solving for deflection.

Deflection = (5 * 16.67 * 84^4) / (384 * 14000000 * 47.63)

Max Deflection at center of span = .16 inches


Hope that helps a bit. I am not sure of that matches the calculator that you listed or not, but I am confident in the formulae that I have applied given your constraints.

Please excuse unit symbolization - don't have easy access to formatters for this type of math and symbol sets right now.

 

[BeanAnimal]

Again - let's put the above in context.

1 - ideal E based on #2 SPF, straight with no defects and proper moisture

2 - simple span - our stand uprights are not simple, but nor are they immovable - so somewhere in between, helping lessen deflection.

3 - I was based on finished dimension for nominal 2x8. Modern lumber is likely to not retain the historical dimensional values due to being milled wetter and/or mills shaving every last sliver off wood off that they can get away with. So the 1.5 x 7.25 is more likely going to end up at 1.375 x 7.125 and increasing deflection

Etc.

Playing with these variables can drastically change the model.

So in the real world if that was the actual load, then I would assume a deflection of .12 to maybe .2 and work from there.

As I mentioned above, being a home engineered project, I would take that .16 number and double it for peace of mind (or double the load, same thing as it is uniform) and run the calcs until I got a deflection that made sense.

I would then do the same with 3x the deflection (or load, same thing).

Choose at least the 2x model for safety (we already fudged the live load up to 200 from 170 or whatever) and if 3x is similar materials, then go with that knowing it is a large safety margin. If you live in a quake zone, than I would go with the 3x number... though joinery becomes more important at this point than vertical load.

 

[areefer01]

Again - let's put the above in context.

1 - ideal E based on #2 SPF, straight with no defects and proper moisture

2 - simple span - our stand uprights are not simple, but nor are they immovable - so somewhere in between, helping lessen deflection.

3 - I was based on finished dimension for nominal 2x8. Modern lumber is likely to not retain the historical dimensional values due to being milled wetter and/or mills shaving every last sliver off wood off that they can get away with. So the 1.5 x 7.25 is more likely going to end up at 1.375 x 7.125 and increasing deflection

Etc.

Playing with these variables can drastically change the model.

So in the real world if that was the actual load, then I would assume a deflection of .12 to maybe .2 and work from there.

As I mentioned above, being a home engineered project, I would take that .16 number and double it for peace of mind (or double the load, same thing as it is uniform) and run the calcs until I got a deflection that made sense.

I would then do the same with 3x the deflection (or load, same thing).

Choose at least the 2x model for safety (we already fudged the live load up to 200 from 170 or whatever) and if 3x is similar materials, then go with that knowing it is a large safety margin. If you live in a quake zone, than I would go with the 3x number... though joinery becomes more important at this point than vertical load.

I like safety. Sort of like taking a Trans Atlantic or Pacific flight in a single engine plane...no thank you.

 

[SteveMM62Reef]

They had a Cabinet Builder, in St. Mary’s MD that built aquarium stands. He used a Vacuum Press Bag System to Laminate pieces of plywood together, to make the support pieces. He used dowels to line up the plywood. I asked him why he didn’t use nails, said they could be ejected in the vacuum process, putting holes in the bags. Said, the dowels were also a lot stronger.

 

[BeanAnimal]

They had a Cabinet Builder, in St. Mary’s MD that built aquarium stands. He used a Vacuum Press Bag System to Laminate pieces of plywood together, to make the support pieces. He used dowels to line up the plywood. I asked him why he didn’t use nails, said they could be ejected in the vacuum process, putting holes in the bags. Said, the dowels were also a lot stronger.

Screws PULL as they are tightened and cause an area of deformation around them. Even pre-drilled they will also slightly deform the wood face due to this pulling, prevent proper face glue contact. Also their small diameter creates a focused stress when under load.

Dowels do not pull or cause wood face deformity during glue-up and in effect become part of the glue-up members as well as spread the force when under load to a larger area.