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Thread: Compression strength of a 2X4.

  1. #1
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    Default Compression strength of a 2X4.

    I'm about to build a base for my two rain barrels, 55 gal each. I plan on them being side by side, approx. 30 inches off the ground and I'd like to use regular 2X4's vertically to build this with a plywood top. These two barrels will weigh over 900 lbs total when full.

    How can I calculate how many vertical supports I require so the 2X4's don't collapse under the weight? The floor surface is a wooden deck, but if necessary I can install 2X4's flat on the deck to help spread out the load.

    I found this website, but I don't know how to use the numbers as for sure the length of the 2X4 makes a difference.

    http://www.southernpine.com/speciescomparisons2x4.shtml
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    Last edited by Guido; February 25th, 2011 at 08:08 AM.

  2. #2
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    I was going to guess a few hundred PSI for wood in compression (as a post). Looking at the table you provided, even the worst wood is over 600 PSI but 'm not sure if the measurement relative to the grain is applicable.

    Since a 2x4 is about 5 square inches, it would only need to support 200 PSI to hold up your water. So one 2x4 post is strong enough. In a perfect world, this is all you care about whether the post is a foot long or 100 feet long. If the posts are long though, you should have some bracing to prevent a post from snapping, especially if 900 pounds of water can fall on someone.

    I think your most important structural area is the deck this is sitting on. It would be best if you had a 2x4 at each end of the water platform that sat on the ground on a concrete block. The much weaker links are the soil or the deck, so you need to spread out this post load to at least a square foot each if not more (assuming 2 or more posts). If there is a beam in the deck under this water platform, you may be OK. If not, I'd consider putting a post or something to the earth under this water platform.

    Also watch the horizontal members that support your water platform. This platform is holding a lot of weight so the building structure is like loading a floor joist. You probably need to design for at least 100 PSF to hold up this water, but can also probably tolerate more deflection than you would for a floor.
    Last edited by suemarkp; March 15th, 2006 at 09:46 PM.
    Mark
    Kent, WA

  3. #3
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    OK so it looks like it'll work. I don't think a 30" post would buckle as I plan on having 6 posts - 2 rows of 3. I'd have the outer posts located directly below the barrel and the 2 middle posts share the load from both barrels. Then have some horizontal members to keep the thing from folding over.

    It will actually be located inside a shed that's on my deck, so I don't have much choice in placement. I plan on using metal straps placed horizontally around the barrels and attached to the shed studs to minimize any side movement, so the only way the barrels can fall is straight down.

    If I use one square foot of "footing" under each post it equals 144 sq in. x 6 = 864 sq in. Just over one PSI on the deck. I'd like to make the footing as small as possible because I will be using the space under the barrels as storage. The posts alone without any footing would be about 30PSI. Is that too much for a deck?
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    Last edited by Guido; February 25th, 2011 at 08:08 AM.

  4. #4
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    You're kind of mixing point loads (typically PSI) with evenly distributed loads (typically PSF). You can use a block of wood or metal to spread the weight of a point load to other structural members. But you can't exceed the overall bearing weight of your deck joists or beams. You also don't want to exceed the footing weight on the soil for the soil bearing strength (this can be a small point load if using posts, or a distributed load if using a long poured footing).

    I'm not sure I understand your use of the word footing. To me, this goes into the dirt. You can use the space under the barrels for storage, but the posts holding it up should really go into their own footing into the dirt. Just putting these posts on your deck is asking for trouble unless your deck was way overbuilt, or you happen to put the water barrel posts right over a main structual support beam that is also close to a pier block or footing. Even if you're right on top of it, that deck was probably designed at 40 PSF overall. If you can distribute the water barrel weight over all of the deck surface, then the PSF hit is small and you'd probably be OK. But I don't know how you can do that easily. I can see how you can spread out these barrels over a perhaps 4x8 foot area, but anything larger would be rather difficult.

    I would look at the water barrel structure as its own entity forgetting that the deck exists. If you have 1000 pounds of water and deck to hold up, I would recommend you have your own 5 square feet of earth to hold this up. If you have 6 posts, resting each on a 1 foot square pier block in compacted earth would do it. You can set the posts on the deck if you want, but you'd need the equivalent of 6 more posts under the deck very near (preferably directly under) the above deck posts in order to hold this up. If you reduce the posts from 6 to 4, then you still need about 5 or 6 square feet of concrete to hold this up (so use a larger pierblock or pour a 18" pad and rest the pier block on it.
    Mark
    Kent, WA

  5. #5
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    Thanks for explaining that Mark.
    By "footing" I meant something between the post and deck.

    So you're saying even though the PSI rating may be safe, the actual weight may be too much. Makes sense.

    I have to take a better look and see how the deck was constructed. Maybe it's my fault for not describing it better - attached is a photo of the deck and the shed where I plan to install the two barrels. It's not your typical deck that's 10 or so inches off the ground, if I'm not mistaken the "joists" are actually resting on the ground. I think there is only one center joist going parallel to the house wall.
    What if I try pulling up a few boards and adding support under the area where the barrel posts will sit. I have some extra 8x8 stepping stones I can probably shove in there. If possible I'd rather not make holes in the deck for the posts to pass through.
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  6. #6
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    Try to place your posts directly over the existing joists, and put something solid to distribute the weight under the joist where the post is above it. I don't think a paver is strong enough, but burying a pier block would do it. It would be ideal to have six posts with this much weight.

    If you don't use blocks or something strong to dsitribute the weight on the ground, you'll shove the joist into the ground. There may be a rim joist holding them all in line, but you may bend the nails and pull the joist out of that rim joist. A continual load of 900 pounds is kind of a lot and will make things sag over time.
    Mark
    Kent, WA

  7. #7
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    I'm lost , why wouldn't you just simply use 4 x 4 posts (treated) ??
    Far stronger than 2 x 4's
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    Last edited by pushkins; August 11th, 2011 at 04:20 AM.

  8. #8
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    Because the posts aren't the problem, its what the posts are resting on. I think even a 2x2 would work because wood is very strong in compression. But soil is not, nor can most decks take the total load produced by each post.

    To keep the structure from toppling over, 2x4's should be the minimim and they should probably be braced. Plain old 4x4's may work too without bracing. But whatever is used has to be sitting on something that isn't going to sink when 1000 lbs is placed upon it.
    Mark
    Kent, WA

  9. #9
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    I did more investigating over the weekend.
    The joists are on 16" centers. My barrels are 2' dia, so the platform will be 2' x 4'. I will make 6 post legs. The 4' length will span 4 joists. I estimate 250lbs max on each joist.

    I'll see if I can pull up a couple boards and somehow install a couple pier blocks in the ground. Problem is the deck boards go under the shed wall, so I'll have to cut them to remove them, which is what I wanted to avoid.

    Reason I inquired about 2X4s is because I have a bunch leftover from a recent project. I don't have a truck, so bringing 4X4s home is not a simple task.
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    Last edited by Guido; February 25th, 2011 at 08:09 AM.

  10. #10
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    Quote Originally Posted by suemarkp
    Because the posts aren't the problem, its what the posts are resting on. I think even a 2x2 would work because wood is very strong in compression. But soil is not, nor can most decks take the total load produced by each post.

    To keep the structure from toppling over, 2x4's should be the minimim and they should probably be braced. Plain old 4x4's may work too without bracing. But whatever is used has to be sitting on something that isn't going to sink when 1000 lbs is placed upon it.
    Okay I now see what your talking about. I was just talking about the 4x4's as posts bearing the weight not the fact that they will be actually resting on a deck.
    You could always sister together 2 - 2x4's and lay on their sides on the deck across the joists attached to the floor joists and then use your 2 x 4 legs resting on them, this would share the load more equally across the deck (maybe sistering the 2x4's to make a 4x4).
    Though if it were me, I'd be cutting some boards and digging a small footer, concrete in a 4x4 saddle and sistering the 2x4 to make the 4x4 posts. After all whatever boards you do cut will be 30" under the tanks so it's not like the joins will be easily seen.

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