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Any structural engineers out there?

Post in 'DIY and General non-hearth advice' started by mithesaint, Oct 17, 2012.

  1. mithesaint

    mithesaint Feeling the Heat

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    Long story short, I'm in the process of putting a 150 gallon aquarium in my home office. I've beefed up the floor, but am still concerned about the weight. I filled it for the first time today, and took measurements in the basement.

    I'm dealing with a 13' joist span, and in the middle of that span I have a deflection of approximately 1/8". Is that too much?

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  2. Waulie

    Waulie Minister of Fire

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    What size joists? You can use an online calculator to determine what your maximum span is with your joist size and loading. Just google it, there are a bunch of them.

    Building codes typically specify a maximum deflection as a fraction of the joist length (span). I'm not sure what is the most common specification, but I think it is in the 1/400 range. So, 13 feet = 156 inches. 156/400 = 0.39 inches.

    1/8 of an inch = 0.125 inches. So, even if the code states a maximum of 1/600, you're fine.

    You could actually calculate the stress in your joists and compare that to the allowable stress. However, at an 1/8" deflection, I wouldn't bother.
  3. Highbeam

    Highbeam Minister of Fire

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    Is there a room under the floor with the load? If this is an unfinished utility type basement and if it makes you sleep better at night, there is nothing wrong with adding some additional support columns directly under the load. This is cheap and easy to do and can be very temporary. The 1/8" sag is minimal.
  4. stee6043

    stee6043 Minister of Fire

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    Waulie naield it. What would generally be considered "very rigid" would calculate at L/720. Most industrial walkways and mezzanines would come in at either L/240 or sometimes L/360. L/720 is typcially reserved for mezzanines that have rather sensative equipment on them which require minimal movement.

    I can only assume residential requirements would be similar, or less stringent, than those stated above.

    I personally prefer "very rigid" and increased safety factors. I'd add a support column below your tank if it were me. Throw three or four people around your new fish tank admiring your latest batch of flesh-eating fish and you suddenly have a very different equation than a static 150 gallons of water.
  5. Eatonpcat

    Eatonpcat Minister of Fire

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    What is the footprint at the tank base??... I assume it engages more than one joist.
  6. ironpony

    ironpony Minister of Fire

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    residential is usually based on live 40lb/ft2 or dead 20lb/ft2 load
    then joist size by acceptable span, the tank full of water is 1300 lbs
    so personally I would do a little further checking, being the load is in a small footprint
    and as stated above add a few larger friends..................
    is it cross joists or parallel?? along a wall or center span
    it is easier to add bracing now then rebuild later
    could be as simple as doubling every other joist, or adding a column
  7. mithesaint

    mithesaint Feeling the Heat

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    Thanks for the replies. I thought i added some more information, but it must be floating around in cyberspace. I'll try again.

    The tank footprint is 72"x20" or so. 150 (or slightly less) gallons of water, 250 lb tank, and 100 lb stand adding to the weight burden. It sits parallel to the joists, and is fairly evenly situated over two joists. The tank sits along an interior, non load bearing wall.

    Joists are 2x10, spanning approximately 13 feet. I have already beefed up the floor a bit. I added a second 2x10 along each joist, with a piece of 3/4" OSB sandwiched in-between the original and the additional joist. The OSB was held in place with screws and a few tubes of construction adhesive. Long story short, I have 4 2x10 joists supporting that immediate area, plus whatever I can get from the OSB. I figure the OSB should help, provided the adhesive holds up.

    I'd prefer not to add a column, as it would be in the middle of the room in the basement that I'll eventually be finishing.
    Eatonpcat likes this.
  8. pen

    pen There are some who call me...mod. Staff Member

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    If that doesn't hold it, something other than those joists (essentially headers now) gave way.

    pen
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  9. Thomas Anderson

    Thomas Anderson Member

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    If the new joists are also supported on the top of the wall like the originals, I would think that would be sufficient. If they're only sistered together without wall support, you should try to add new joists that go all the way to the top of your walls. Always put the crowns up. If the crowns are up and you're still getting deflection downward, that's worrisome. If you have head room, you could always add a couple of 2x12s instead.
  10. SmokeyTheBear

    SmokeyTheBear Minister of Fire

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    That is quite a dead load 155lbs+ or - per square foot of decking at that point. Not what most flooring is designed to contend with.

    What is the distance between those sistered joists and the ends of those joists and their sisters have to be fully supported at both ends.
  11. ironpony

    ironpony Minister of Fire

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    being the tank is paralell to the joists, I would do the same thing you have already done to the next joist over each way 4 total and add another osb and joist to the ones directly under the tank. essentially a header for a 2x6 wall.which should carry most of the weight
    as stated above crown up and try to get these on top of the existing supporting walls. If you need to, use a hydraulic bottle jack and a 4x4 to lift the joists up to get the new joists in crown up, keep everything under pressure, attach your osb and joist let set overnite then let the jack down. this will build in an upward crown which will settle down to straight under load (fishtank).
    That is a lot of dead weight in one spot for along time,. the good is the tank is half the span so there is only 3.5 feet on each end in shear load so this should help
    just a qiuck calculation, remember it is 1600 lbs+/-
  12. Highbeam

    Highbeam Minister of Fire

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    1600lbs plus the two or three big guys checking out the fish. That non-load bearing wall, does that add weight to the problem or help resist sag?
  13. stee6043

    stee6043 Minister of Fire

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    From a structural standpoint I don't think you could pick a worse material than OSB for resistance to lateral compression and tension forces (if I'm correctly picturing in my mind what you did). Well, I take that back, American or Cheddar cheese would be worse.

    As stated above those added 2x10's really should be supported by the walls at the ends to be effective. Simply attaching them to the existing joist mid-span might help a little, but probably not a lot. Not to mention the fact that you're adding another X amount of weight to the existing joist when you do that, further stressing the structure with increased dead load.

    Silly question - did you add your OSB/Joists while the tank was filled and on the joists or were the joists in an unloaded state? Too bad you used adhesive...I'd have suggested you ditch the OSB and attach the new joists directly the existing joist. Depending on your choice of pattern for the adhesive application that layer may actually reduce the effectiveness of your newly added joists.
  14. ironpony

    ironpony Minister of Fire

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    if it is truly non load bearing, I think it to be mute. also is it sitting on a joist or not?? if it sitting on the joist and attached properly it is stiffing the joist.

    actually OSB is extremely rigid, just look at I beam (TGI) joists 1/2 inch OSB web, with a 2x4 laying flat to nail too. Almost zero deflection

    add 2or 3 big guys like me 260lbs we all in the basement
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  15. Eatonpcat

    Eatonpcat Minister of Fire

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    ;) Lots of structural engineer's on here apparently...Anybody willing to put their stamp on their design??
  16. ironpony

    ironpony Minister of Fire

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    yes,without hesitation
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  17. Eatonpcat

    Eatonpcat Minister of Fire

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    All the money you have, I know you're not making that as a practicing structural engineer!!:cool: Doesn't mean you're not licensed, just means you were smart enough to do something where you could make real money.

    Anyway, this would be my solution:

    Sister both 2x10 beneath the tank with another 2x10, and add some blocking between joists at 3 locations for a total of 4’-0”.

    I designed it to engage 4 total existing joists and you need a total of 6, the solid blocking helps distribute the load from directly under the tank to the joists adjacent to the tank. If you don’t stick the tank right in the middle of the span I could probably reduce this by a lot.
  18. Thomas Anderson

    Thomas Anderson Member

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    Well, I think it's a foregone conclusion that you won't get an actual professional answer that you can take to the bank on an internet forum. But what you will get is some rules of thumb (backed up by any number of tables from lumber manufacturers -- you can find them online) from experienced amateurs. Much of that info was provided above.

    Getting the precise load bearing capacity would mean knowing exactly what species of wood you have, the grade, moisture content, any imperfections, etc., plus know precisely how much dead and live weight you might place on it as a worst case. Rules of thumb with margin of safety will surely over-build it for most circumstances.

    Having designed and built my own house after having attended an engineering university, though not majoring in structural, and coming from a family of carpenters, I have a pretty good idea of what passes the smell test. And for me, passing the smell test means that it should be built with a big enough margin of safety so as not to even be worried. A structural engineer would tell me that I used way too much wood in my house for that reason. They would have used 2x4s where I used 2x6s, or 2x8s where I used 2x10s, or used 24" spacing instead of 16", or 16" instead of 12". But then, their houses blow over in a 100 year storm, while mine most likely would do fine. For me to feel perfectly comfortable with that fishtank up there, I would put at least 4 2x10s under it fully supported on the walls, and probably if it still felt soft when I hopped up and down on it, I would opt for 6 2x10s not only supported on the walls, but perhaps with an additional 4x4 pillar (or small wall or arch) on each end to reduce the span but still preserve the interior space.

    But obviously if you're taking the advice of people on an internet forum, you must assume the risks of any choices you make based on that.
  19. Eatonpcat

    Eatonpcat Minister of Fire

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    Good points (Mike Holmes)...But try overdesigning like that for a client who is building a 4 story wood hotel...The cost of overdesigning snowballs to a staggering amount. Safety factors are there for a reason. Give ten engineers a problem and you will get ten different answers and they will all most likely work.

    Did not intend to offend anyone, just stating that because you're a carpenter, or you built this or that with your own design, that still does not mean you have any idea what you're talking about. If I had a nickel from every builder that thought they knew more than me or heard "we do it this way all the time, I would be a wealthy man.

    I will stay out of this thread from here on out!
  20. stee6043

    stee6043 Minister of Fire

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    Yikes. Comparing a sheet of OSB to an engineered I joist with an OSB web is like comparing a pile of toothpicks to an engineered ceiling truss. A sheet of OSB will never, ever transfer loads in an even remotely similar fashion to an I joist with dimensional lumber flanges top and bottom and an OSB web. The folks that designed those wooden floor joists with the OSB web did not include the dimensional lumber flanges for your nailing convenience, this I can promise you.
  21. Thomas Anderson

    Thomas Anderson Member

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    For the given design parameters at least. The nice thing about building for yourself is that you get to decide just how safe you'll be for whichever scenarios you choose to entertain. Making a profit doesn't even come into play. Instead it's a matter of sleeping well at night knowing you and your family will be safe and sound. Those are dividends that keep paying for your whole life! I don't ever want to have to live in those minimally-engineered OSB-sheathed-if-you're-lucky cookie cutter egg cartons that pass for houses these days.
  22. mithesaint

    mithesaint Feeling the Heat

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    Thanks again for more feedback.

    I realize that if I really wanted this to be right, I'd have to call someone out to my house and pay them for their information. As always, you get what you pay for, and so far this thread has been free for me:cool: Even still, lots of useful stuff here.

    The two additional 2x10's that I added next to the current joists span the entire distance, and are well supported at each end. They were added before the tank was filled, and were a very tight fit. I'd absolutely have to jack the joists up to have any chance of getting another joist in.

    Eatonpcat - I don't understand what you mean with the blocking. Can you help me out a bit here?
  23. SmokeyTheBear

    SmokeyTheBear Minister of Fire

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    Well I ain't no engineer but I do know there are nice tables and calculators out there that take into consideration dead and live loads, the dimensions and species and grade of the lumber used and will crank out maximum safe spans. There are also calculators that will take a deck (floor) and tell you if there are sufficient supports to bear the weights possible given a fully loaded to design deck.

    Running those can be eye opening.
  24. ironpony

    ironpony Minister of Fire

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    The blocking is perpendicular to the joist, tying the joists together. This will transfer load to the adjacent joist and stiffen the floor.
  25. ironpony

    ironpony Minister of Fire

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    Good addition adding the blocking, I thought about it but passed on it. Sure would help stiffen things

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