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Residential Wood Framed Floors
and Aquarium Weights
by Kevin Bauman (StructureGuy)
Introduction


One of the questions that is inevitably asked in every aquarium chat room, newsgroup and bulletin board is "just how large an aquarium can my floor support." Then the answers follow from people who usually use basically correct structural principles to come to often incorrect conclusions. Unfortunately, I then jump into the fray and try to explain in just a few words what cannot possibly be explained in just a few words. So the result is that no one fully understands my explanation since it seems contrary to his or her experience. So here is the long winded explanation from someone (me) that has been working as a structural engineer since 1976.

This is what the basic residential floor framing layout looks like: click here.

Let's begin with a few definitions:

Dead Load: This is the weight of everything that is permanent such as the floor joists, walls, piping, ductwork, floor tile, etc.

Live Load: This is the weight of everything that you add to the house or apartment when you move in. Furniture, bookshelves, people, appliances, and of course, your computer and your aquarium(s).

Safety Factor: Your floor was designed to support loads without collapsing using a "safety factor". The "safety factor" in most structures is usually somewhere between 1.5 and 2.0. So, if I tell you that your floor can "safely" support 1000 pounds then that also means that your floor might theoretically fail when it receives a load of 1500 to 2000 pounds.

Bearing Wall: This is a wall in your house or apartment that was designed to support the weight of the floor, wall, ceiling or roof. (Most or all of the concrete or masonry block walls in your basement are bearing walls.)


Photo courtesy Philip Greenspun
Partition Wall: This is a wall in your home that acts only to separate rooms. While it might be able to support some load, it was not designed as a part of the structural system that carries the weight of your floor or roof down to the foundation.

Floor Joists: These are typically 2 x 8's or 2 x 10's at 16 inches on center that support your floor. Each end of the joists are supported by bearing walls or beams.

Subfloor: The sheet of wood (usually plywood) that is nailed to the top of the floor joists to form the floor itself before carpeting or tile etc.

Beams: The beams act to support the floor joists. These beams might be constructed of wide-flange steel beams (commonly and incorrectly called an I-beam) or they might be wood triple 2 x 10's, etc.

Column: A vertical post that supports the floor beams. In a home this is usually a round hollow pipe.


Wood Floor Design Loads


In the United States the minimum design floor live loads are usually stipulated in pounds per square foot (psf) by either state or local building codes. An example of typical design live loads might be 200 or 150 psf for a storage warehouse, 100 psf for a public meeting room, 50 psf for an office and 40 psf for a single family residence. So, your home most likely has the capacity to safely support a uniform live load of at least 40 psf. But keep in mind that this design live load is theoretically spread uniformly over the entire floor from wall to wall throughout your entire house. It is not a maximum load on any given area of the floor, it is just a theoretical average load that is used to design the floor for loads that are initially unknown. Some people find this confusing because in reality it is not the floor pressure (in psf) that matters at all, it is the floor load in pounds that really creates the stress in the primary structural framing members.

Myth #1: "According to the building code my house can only support a maximum total load of 40 psf anywhere on the floor." No, the 40 psf is a theoretical uniform design live load over your entire floor. You might have a whole lot more than 40 psf directly under your aquarium but that's okay because you didn't fill your entire room with aquariums either.

Myth #2: "So then, if I fill my entire room with aquariums that weigh more than 40 psf my floor will collapse." No it shouldn't. I said that the 40 psf was a MINIMUM design load and I also said that it is a SAFE load. That means that your floor could be (probably is) stronger than the 40 psf minimum in many places and it also means that the full safety factor is still there to prevent a collapse.

Myth #3: "A structural engineer designed my floor for a live load of 40 psf." No probably not. First of all, your floor was probably never actually custom designed. What builder would ever want to pay a structural engineer to design something this repetitive and simple. All an experienced contractor has to know is that 2 x 10's span ?? ft then he starts using 2 x 12's. (See Allowable Spans for Floor Joists.)

Myth #4: "A building inspector inspected my house or reviews calculations to make sure that homes can safely support a minimum design live load of 40 psf." No probably not. In many locations the building code is only concerned with public buildings. How strong you build your own private residence is of little concern to them. Of course, they still want their cash for the building permit.

So now we know that your floor can safely support no more than a uniform 40 psf live load, right? Wrong! If you go into the basement and look up you will probably see that the exact same floor joist size (and 16 inch spacing) was used throughout your entire house. So 2 x 10's spanning 8 feet in your kitchen are a whole lot stronger than the 2 x 10's spanning 15 feet in your living room.

Myth #5: "If the floor in my kitchen can support the 500 lb refrigerator then the floor in my living room should be able to support my 500 lb aquarium." This isn't necessarily good logic. Do the floor joists span exactly the same distance in both rooms? Do the other items in the rest of both rooms have the same weight? Is your refrigerator and aquarium the same shape so that this same weight is distributed over the same number of floor joists?

The reason that there is rather large safety factor built into the floor design is to take care of as many unknowns and imperfections as possible. Did the plumber cut a notch in the bottom of your floor joists for his piping? Is there any insect damage or maybe a little dry rot? Is there a split or knot in the wood in a zone of high stress? No one wants to have to replace their floor because it is not in perfect condition. So if your aquarium loads your floor over the safe load limit you might not be in danger of collapsing the floor, you might just have less safety factor than recommended. If you're stretching the load limits of your floor structure, then be absolutely sure that your floor structure has no imperfections.

Myth #6: "I put that huge aquarium on the floor and nothing bad happened therefore the floor is safe." If you choose to think of "not collapsing" as safe you are certainly free to do so. But if you have a safety factor of only 1.05 in your floor structure you probably don't know it, and there is not a structural engineer in the land that would tell you that it is "safe."


Where To Put The Aquarium


So I just spent my last dollar on a mega-gallon aquarium and don't know where I can safely put it. I think that everyone knows the best place .... directly on a concrete slab-on-grade floor. The weight is carried directly from the aquarium to the concrete to the ground below and it will take a whole lot of weight to give you any problems. My first house was a split level home where the lower level of the house was a slab-on-grade. I could have had my dream aquarium right there in the family room.

Okay so you can't put the aquarium on a concrete slab because you don't want to put your aquarium in the basement. Well now the best location is directly over top of a bearing wall or a column. Unfortunately, that might not be possible either because there are often bearing walls supporting the second floor over top of the bearing walls and beams supporting the first floor.

Go into your basement and look up at the floor joists. Wherever the joists span the shortest distance will be the room with the strongest floor framing and wherever the joists span the greatest distance will be the weakest room in the house. (Assuming identical size joists) So the next best location is in the room that has the strongest floor.

Well maybe the strongest room is the kitchen and you don't want to put the aquarium there. So, the best practical position is often as close to a bearing wall or column as possible and oriented perpendicular to the floor joists. That way the aquarium weight is distributed to as many floor joists as possible. And the closer to the wall the aquarium can be positioned the more total weight in pounds the floor joists can support. An aquarium stand with a continuous runner at the bottom will distribute the weight a lot better than a stand with just four legs.

Myth #7: "My aquarium is on a metal stand with 4 legs so all I have to do is put a sheet of plywood under the legs to distribute the load to more floor joists." That will help some, but not very much at all. A sheet of plywood laid flat is not very stiff so it will bend and not distribute the load to more floor joists very effectively.


Photo courtesy Philip Greenspun
Just keep in mind that if your aquarium is in the living room then the columns and walls supporting your living room floor are below you in the basement. Some of the walls in your living room might be partition walls and not bearing walls at all. It is important to distinguish which is a bearing wall and which is a partition wall.

And that leaves the worst possible position for an aquarium which is parallel to the joists in the mid span of the joists in the room with the longest joist span. There are probably several partition walls that run parallel to the floor joists in the house so don't assume that just because the aquarium is up against the wall that it is necessarily near a bearing wall or column.

Myth #8: "Why all this worry about weight? I am a 200 lb man and I can fill my bath tub with water and get in the tub without crashing through the floor." Well, the builder knew where he was going to put the bath tub so he put extra floor joists there to take care of the higher loads.


Structural Engineering


Warning: Even if you have actually read this far without getting bored it is about to get a whole lot worse. Here is what really happens when you put an aquarium on the floor:

Myth #9: "Kevin is a structural engineer so he knows exactly to the pound how strong his floor framing is." Well we can get close and we can be safe but we really don't know exactly what load will cause the floor will collapse. Man did not manufacture wood from basic elements under controlled conditions as we do steel. Mother nature made wood according to her whim. Oak is stronger than southern pine but not all southern pine trees are identical. Wood has several "grades" based upon its' physical condition but it is usually visually graded. That means a man at the mill looks at the linearity, grain, knots, splits etc and then says: "I think that I'll call this a no 2 grade" Then we structural engineers look in a book which tells us only approximately how strong it is.

More Definitions

Bridging: This is the material that is between your floor joists. It might be a solid 2 x 10 blocking. X-braced wood slats or X-braced metal straps. Bridging does two things. It helps to distribute concentrated floor loads to more joists (If they deflect together then they share the load.) than the ones directly below the load. And it also keeps the floor joists in a verticle position so that they don't tip over on their side. (Lateral-torsional buckling)

Strength and Stiffness: These two items are completely different properties of a material that are indeed related to each other. This single point is definitely the source of a lot of confusion. Strength is obviously how strong something is. Stiffness is how much it bends, deflects, flexes etc.

Shear: If you were to hold a pencil at the edge of a table and then strike it just past the edge of the table you might shear it off. In your floor joists, the shear stresses in the wood joists are the greatest right at the face of the supporting bearing wall. If you have a shear failure in your floor joists it will be a horizontal split, at mid-depth of the joists, parallel to the grain in the wood, near the supporting wall or beam.

Bending: When you hold that pencil in both hands and rotate each hand in oposite directions you will bend the pencil. The pencil should break near the middle. The bending stresses put tension into one side of the pencil and that is where you can see the wooden pencil start to split apart. If you have a bending failure in your floor joists then the wood will pull apart at the bottom of the joist near the mid-span of the joist.

First the weight of your aquarium bears on the subfloor. For the most part this is of little concern unless it permanently puts a dent into your beautiful hardwood floors. It takes an incredible amount of pressure (in psi) for the leg of an aquarium stand to poke a hole through your floor. This type of failure is called punching shear and most plywood or tongue and groove plank subfloors are very strong in punching shear. It could happen, it is just not very likely to happen at all. The subfloor carries the aquarium leg load to the floor joists. Remember that your primary floor structure is the wood joists and that is what was designed to support 40 psf, not the subfloor. The subfloor has to be able to resist the common concentrated loads from refrigerators, water beds, and chairs, etc. and it can do so easily enough since it only has to span the 16 inches between the floor joists. So the subfloor can resist a very large bearing pressure without puncturing the plywood and that is why it is not the bearing pressure that is the greatest concern to us.

Myth #10: "I weigh 250 pounds and can stand on one foot anywhere on the floor without crashing through the floor. My foot is about 36 square inches (0.25 sq ft) so that is 1000 psf. Therefore Kevin's 40 psf makes no sense." Like I said above, the subfloor is very strong in punching shear. And an isolated bearing pressure of 1000 psf on the subfloor has nothing at all to do with a uniform floor design load of 40 psf on the floor joists. The floor joists must be able to support the 250 lb man not the 1000 psf foot pressure. The subfloor easily supports the 1000 psf in punching shear and carries the 250 lb weight to the floor joists.

Structural framing might be designed for this theoretical uniform 40 psf but it probably doesn't reflect the real world loading conditions in any room of your house. The most likely way for a residential wood floor to fail would be because of excessive shear stresses or excessive bending stresses in the floor joists. So let's say I have a 125 gallon tank and the All-Glass web site says that it weighs 1400 lbs and is 6 ft long. The aquarium is oriented perpendicular to the joists and my floor framing is wood joists spanning 12 ft. So this 6 ft by 12 ft portion of the floor was designed to safely support a total live load of at least 6 ft x 12 ft x 40 psf = 2880 pounds total and may actually be much stronger than that, as discussed above. (And keep in mind that this 2880 pounds includes the weight of any people, furniture, bookshelves etc that are located in that 12 ft x 6 ft area) Does this have much value to you? Yes, but only a little bit. The bending stresses and the shear stresses are distributed much differently due to a large concentrated load like an aquarium than they are when distributed uniformly. Aquariums located close to the wall generate high shear stresses and very low bending stresses. Aquariums located in the middle of the span generate extremely high bending stresses and much lower shear stresses at each supporting wall. But you know that it was easier to break that pencil by bending it, so up against the wall is still the preferred aquarium location.

I think one of the best sources of estimating the weight of your aquarium is to visit the All-Glass web site. Basically you can figure that water weighs 8.34 pounds per gallon or 62.4 pounds per cubic ft. Then there is the weight of the aquarium itself and the weight of the rocks and substrate and stand. You know that the density (weight per unit volume) of rock is greater than the density of water. That's why the rock doesn't float. Depending upon the rock that you use, the density might vary from 120 pounds per cubic ft (pcf) to 180 pcf. The all-glass web site says that a 125 gallon tank weighs 206 lbs (empty) and the total estimated weight of a full aquarium is around 1400 lbs. Let's say that you are adding 210 lbs of limestone rock and gravel to a tank. The density of limestone is around 140 pcf. If this was a solid rock it should be around (210/140 = )1 1/2 cubic ft in size. If it were all gravel it would not all fit into a 1 1/2 cubic ft box but that is because there are lots of pockets of air in that cube of gravel. So 210 pounds of limestone whether solid or gravel should displace about 1 1/2 cubic ft of water. The "bouyant weight" of rock is the weight of the rock minus the weight of the water that it displaces. So this rock has added about (1.5)(140-62.4) = 116 pounds to the aquarium weight. The water in the aquarium weighs (125)(8.34) - (1.5)(62.4) = 950 pounds. So if that aquarium stand weighs 44 pounds, then this is how this works out. Total weight = 206 lb (tank) + 950 lb (water) + 210 lb (rocks) + 44 lb (stand) = 1400 lbs. (Note that since I subtracted out the volume of water that is displaced by the rock, I must use the total weight of the rock and not the bouyant weight.)

Another often unrecognized structural concept is that the duration of the load can be a factor in whether a wood structure collapses or not. Anyone that has ever used a chain saw to cut part way through a tree knows that. You can push on the tree and the tree does not break and fall down immediately unless you use a lot of force. Or you can use less force and sustain it for a longer period of time to make it fall down. A floor failure that I investigated occurred at 3:00 am on Monday morning due to an excessive load that was placed on a wood framed floor on the previous Friday afternoon. So if an entire professional football team crowds around to admire your large beautiful aquarium, it might not be any cause for concern unless they stand there all weekend.


Photo courtesy Philip Greenspun
Myth #11: "My brother and I weigh 400 lbs together and we jumped off the sofa onto my floor and it didn't break, so I know that the floor can safely hold more than a 400 lb aquarium." Yes the dropped weight exerted a force greater than 400 pounds, but it was a short term impact load. If your 400 lb aquarium stays in the same spot for 10 years it might just cause the collapse that the jumping brothers didn't.

Then there is the misunderstood subject of stiffness and strength. The amount of flexing, or bowing or bending that occurs is a function of the elasticity of the material and the shape of the material in the direction of the load. Everyone knows that their fiberglas fishing rod bends alot without breaking. But a wooden stick with the same diameter might bend a lot less and still break under less load. The wood stick is stiffer but weaker. So why does everyone in a second floor apartment constructed with a concrete floor assume that their floor is stronger than the wood floor at their friends house? They assume that their floor is stronger because it is less "bouncy." But this conclusion is not necessarily true since they are comparing two different materials with a much different shape spanning a different distance.

Myth #12: "I know that my floor is strong enough to hold my aquarium because I put it up against the wall and there is no perceptible deflection. And, when I walk across the room it doesn't shake." Could be wrong. If the wood joists below the floor fail in shear then there is little or no perceptible deflection associated with this type of overstress. However, you should be aware that excessive floor deflection is indeed a sign that the floor might be overstressed in bending.

When structural engineers design the floor of a public building with long spans, they have to be aware that floor vibration due to foot traffic could be a problem. It is not a structural problem at all, just a perception problem. People are just not comfortable feeling the floor shake beneath their feet. But this vibration or bounce is really harmless and the floor is probably more than strong enough to support the design load. The same can be said for a high-rise office building that sways too much in the wind. It is not unsafe, it is just uncomfortable.

Myth #13: "My floor was doing fine until I put that 75 gallon aquarium in the room. Now the aquarium is causing the floor to bounce." Actually, your aquarium is doing the exact opposite. The stationary weight of your aquarium is acting to dampen some of the floor vibrations due to foot traffic. It is just that now you see the ripples in the water and so now you perceive that the floor is vibrating more.

Another characteristic of wood is that sustained loads can cause permanent deflections and deflections that increase over time. This "permanent warping" of the wood is called creep. So, if the floor deflects 1/2 inch when you set up your aquarium and you leave the aquarium in the same spot for years that deflection might increase to 3/4". Then when you remove the aquarium you may find that some of that deflection has become permanent. That is why in some older homes the floors are no longer perfectly level.

Myth #14: "My home is brand new so it is much stronger than those old homes with warped floors." Actually older homes are often stronger than new ones. Today a 2 x 10 is really only 1.5" x 9.25" in size. Years ago that 2 x 10 was really 2" x 10". Every year, research adds to our structural knowledge base. As a structural engineer, it is our job to not only make a building safe but also to make it as cost effective as possible. So if the research tells us that we can trim some of the cost of the structural framing out of a building and still maintain the minimum safety factor, then that is what we will do.

You do need to use some common sense in this whole thing. Most 55 gallon tanks on wooden stands with runners can be placed anywhere reasonable without too much worry. Tanks larger than 55 gallons might be okay if they are placed in a very good structural location and your floor framing is in excellent condition. I have a 40 gallon tank under a 55 gallon tank on a metal stand and because I know a little more about my floor structure than most of you, I know that I'm okay. If your tank is over 125 gallons, then it is highly likely that you should consider adding supports under your wood framed floor.

If you do decide to increase the strength of your floor, just keep in mind that this is best done before the aquarium is placed on the floor and the floor has deflected under the load. For example: Let's say that you fill that 180 gallon tank with water and then later get concerned. So you go into the basement and nail another 2 x 10 right alongside the existing 2 x 10 joist.
(see: Joist Illustration 1)

Unfortunately this doesn't accomplish much since the load was already in the existing 2 x 10 and you haven't removed any stess out of the existing 2 x 10 by adding another joist. If you had added the new 2 x 10 first, and then put the aquarium on the floor, then the joists would have deflected together and shared more of the load. Same idea with a post. You should either add the post first (before you fill the aquarium with water) and shim it very tight to the underside of the joists, or you should jack upward so that the post carries more load.
(see: Joist Illustration 2)


Conclusion


I apologize that there are no easy answers or quick fixes. All I've really done is throw some doubt into your mind which maybe isn't so bad. I just don't want anyone to rush out and buy that 180-gallon tank and put it in the middle of the floor based upon some faulty newsgroup logic.

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