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Residential Wood Framed Floors and Aquarium Weights
By: Kevin Bauman

 

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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 some one (me) that has been working as a structural engineer since 1976.

This is what the basic residential wood floor framing layout looks like: http://www.hometips.com/hyhw/structure/116frame.html

An apartment building might or might not have similar framing.  

First 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.)

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 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 or apartment building.  So, your home should be able 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 the floor structure in my home 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 8's span ?? ft then he starts using 2 x 10's.  See  http://www.mcvicker.com/resguide/page013b.htm  (not my site!)

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."

  • Maybe, but 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 a 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.

Unfortunately, the strongest room is often also the smallest room and you might not want to put your aquarium in the kitchen.  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.

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.  A structural engineer can often figure this out even when the floor structure is covered by the ceiling but maybe you can not.  Most homes are rectangular with bearing walls in the front of the house, the back of the house and a central support system of walls or beams through the middle of the house parallel to the other bearing walls.  In an apartment building it is often the opposite.  The bearing walls are often the walls separating your apartment from your neighbors apartment.

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 to 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 or blocking:  This is the material that is between your floor joists.  It might be a solid 2 x 10 blocking. Or it might be X-braced wood slats or X-braced metal straps.  Bridging and blocking does two things.  It helps to distribute concentrated floor loads to more joists than the ones directly below the load.  (If they deflect together, then they share the load.)  And it also keeps the floor joists in a vertical 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 opposite 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 #10a (male version):  "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.

Myth #10b (female version):  "I am a 100 pound woman who loves her stiletto heels.  My heel is only 1/2 inch diameter so that's a pressure of  over 36,000 psf!"

  • Now instead of a that heavyset male on one foot, we have a woman walking across the floor in high heels.  Same answer:  It is the weight of the person that matters and not the pressure that the heel places on the floor that matters.

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.

 To see how to calculate the weight of your aquarium setup click here=> Weight Calculations

Myth #11:  "I added 210 pounds of rock to my aquarium so now my aquarium setup weighs 210 pounds more."

  • In the above example, when you added the 210 lbs of rock it took up space that was previously occupied by approximately 1.5 cubic feet of water.  So you've really only added 116 lbs (the "buoyant rock weight") to the total 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 or even for a few years.

 

Myth #11:  "My two brothers and I weigh 800 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 800 lb aquarium."

  • Yes, the dropped weight exerted a force greater than 800 pounds, but it was a short term impact load.  If your 800 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 Fiber glass (or graphite?) 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 #13:  "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 over stress However, you should be aware that excessive floor deflection is indeed a sign that the floor might be over stressed 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 #14: "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 these deflections may 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 many, many 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 #15: "My floor is deflecting so I better put shims under it to make it level."

  • Okay, this one might or might not have some truth to it.   If your floor is flat but sloping, then it just doesn't matter structurally if the water level is higher at one end of the aquarium than it is at the other end.  Now if one of the four corners of your aquarium stand is lower or higher than the other three corners then it could be a big problem.  That could put a torsional (twisting) load into the glass tank which could break the glass or the seal at an edge/corner of the glass.

DO NEWER HOMES HAVE STRONGER FLOORS?

Many years ago, hardwood trees such as oak or ash were fairly common.  Lumber companies today generally rely on soft wood for structural lumber.  That is because the softer wood trees such as southern pine are much faster growing and the forest can be replenished more quickly and inexpensively.  Some homes built near the turn of the century were still being constructed with wood that is so hard it is difficult to drive a nail into them.  So an older home will very often be considerably stronger than a brand new home.  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.

Myth #16: "My home is brand new so it is much stronger than those old homes with warped floors."

  • As discussed above, 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".  Even if the new home was constructed with joists that are actually 2" x 10", chances are very good that the wood used to build the older home was a harder stronger wood.  Many older homes were built with the motto"when in doubt, make it stout".  Today's homes are less likely to be custom built and more likely to be built by a developer with an eye to minimize the cost of the structural framing system.  Let's face it, your wife never says "Let's buy this house because the floors are stronger" does she?

 

Conclusion

Aquariums up to 55 gallons can be placed almost anywhere without much worry at all.  Many tanks larger than 55 gallons and no more than 125 gallons will be okay, if they are placed in a good structural location and your floor framing is free from significant defects.  For example, a 125 gallon tank, on a wooden stand, placed perpendicular to the joists up against a bearing wall, will often be okay without any additional structural support.  If your tank is over 125 gallons, then it is likely that you should consider adding supports under your wood framed floor.  Please realize that these are generalities that may or may not apply to your particular situation.

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:http://www.hammerzone.com/archives/framecarp/supplement/floor/joist1/sister.htm ) Unfortunately this doesn't accomplish much since the load was already in the existing 2 x 10 and you haven't removed any stress 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: http://www.hammerzone.com/archives/framecarp/supplement/floor/joist1/raising.htm )

And now for the most commonly perpetuated myth of all.  Someone in the forum asks if they think it is possible to place a 120 gallon tank on the second floor of their apartment.  The answers inevitably go something like this:  "I see no reason you can't.  I've had a 125 gallon aquarium in my bedroom for years."

 

Myth #17:  "If my floor didn't collapse with a ??? gallon aquarium, then your floor should be okay too"  

Since the person posting the question provides no information at all about the composition of the floor construction, the span of the floor framing or the relative position of the tank, there is just no way for anyone to provide a logical answer.  Yet answers flow from people perfectly willing to compare apples to oranges to watermelons to come up with a recommendation. (And in this example the comparison is even a worse because the 125 gallon tank is 6 ft long and the 120 gallon tank is only 4 ft long.)

 

If you know why the answer given in myth #17 is so illogical, then you understand why I decided to sit down and write this all out.  I wanted everyone to have a very basic understanding of the many factors that go into the evaluation of the structural capacity of a floor framing system.  I wanted to give people some guidance on where to best position their tank and when it is best to seek some outside guidance.  And most of all, I wanted people to stop believing in and perpetuating the myths that spread through the Internet like wildfire.  Unfortunately, if you hear the same advise repeated over and over, you can start to believe that it is a commonly recognized fact. Hopefully, now we can get rid of the discussions about the; woman in high heels, the man in the bathtub, the people jumping off the sofa and the cure-all plywood under the tank.

 

 

 

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