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Old 03-17-2010, 12:06 PM   #1
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Post Load Capacity?


Is there an online resource to lookup post load capacities? Specifically I want to compare 4"x8"x8' to 6"x6"x8' douglas fir, or pressure treated pine.

Background: My unfinished garage, built in 1984 has four posts supporting two 3"x15"x24' (actual measurement) beams. These beams support floor joists below living, dining, and two bedrooms. The existing posts seem insufficient, and I am going to have them replaced before upgrading my 3-tab roof to heavier dimensional shingles.

The existing posts are made from three 2x4 studs nailed together in an I-beam configuration. Two of these posts exhibit visible bowing (one by almost three inches). A contractor I have never used before suggested replacing them with 6x6 posts, but I prefer 4x8, at least on the outside wall so that I will have the option of finishing over the posts in the future. On the outside wall the post could have a header installed to share some of the load the the adjacent 2x4 studs.

Any comments are much appreciated!

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Old 03-17-2010, 01:34 PM   #2
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Post Load Capacity?


in what direction?

1/3b h^3 is the basic formula for resisting bending. (that Base and Height looking at the rectangle cross section)

if you are inline with the 8" dimension that is clearly stronger, but if its bending along the narrow direction, it is weaker.

if you are talking pure compression of an un reinforced post, the 6x6 will be harder to buckle

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Old 03-17-2010, 01:45 PM   #3
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Post Load Capacity?


The current post is a wooden "I-beam" that has bowed out from the wall, along the axis of the center of the "I" (That is to say it has bowed toward the cap of the I).

I guess I want to know how much weight a 4x8 post (not beam) can hold as opposed to a 6x6. I'm sure the 6x6 holds a bit more, but I want to figure out how much of a compromise this would be. (The area is diminished by 4" square)
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Old 03-17-2010, 02:43 PM   #4
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Post Load Capacity?


I’ve put in plenty of glue lamb beams and “never” seen a post wider than the beam. A 3x15x24 isn’t that big. In a 2x6 wall it would be sitting on top of a 4x6 with a 2x6 running up each side from the plate to plate.
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Old 03-17-2010, 03:06 PM   #5
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Post Load Capacity?


The wall is studded with 2x4... is placing a 4X8 under it "the wrong way" going to cause problems with the weight not being distributed properly on the top of the post?
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Old 03-17-2010, 03:27 PM   #6
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Post Load Capacity?


If your have to use a 4x8 you would want so 8 would be fully under the beam and the 4 would flush up with the sides of the beam so you can run a stud from the bottom plate along the post and along side of the beam, creating a pocket. This is suppose to keep the beam from twisting. Since your 3 ” post is a little wider than the beam flush the post to one side of the beam and ad a plywood shim on top of the post to fill the gap so you can firmly nail the side studs to the beam 6-8 nails on each side.

Usually all you would see is a 4x4 in a 2x4 wall. You don’t want the post to stick into the room and anything bigger wouldn’t do any good if it wasn’t sitting on the bottom plate with all of it’s load being transferred to the foundation.

Last edited by kwikfishron; 03-17-2010 at 04:01 PM.
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Old 03-17-2010, 03:43 PM   #7
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Post Load Capacity?


Thank you for your well-informed responses. I'm concerned that a 4x4" would not provide enough load-bearing capacity, but I have no engineering background.

FWIW, the beams are spaced 140" apart, supporting 4x9 joists on 16" centers. I'm thinking they have a lot of weight on them, but again I'm no engineer.

With 1 story+attic above them does this seems like it would be a sound design with 4x4 and studs on the sides? How do you feel about using a lag bolt at the top of the pocket made from the studs, and maybe a few lag screws and/or construction adhesive to marry the post to the studs? Would that effectively add load capacity, or would the screws weaken the post more than they help?

THANKS AGAIN!
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Old 03-17-2010, 03:54 PM   #8
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Post Load Capacity?


If a couple of lags and screws make you feel better, GO FOR IT.

Just don't over do it so you start spliting wood.

I'm no engineer, thats just how we do it.

Last edited by kwikfishron; 03-17-2010 at 03:57 PM.
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Old 03-17-2010, 10:27 PM   #9
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Post Load Capacity?


I wouldn't be concerned about a 4x4. stick a couple of 2x4s beside it to hold the beam in place if you want. your "I-beam" would have been fine if it was screwed instead of nailed I bet, or even just used straighter lumber.

while I have no problem with the concept of a wooden I beam (I plan on building a couple myself for an upcomin project) it sounds like the original builder was cutting corners.

what is your wall these posts are sitting agains? If you can anchor the post to the wall, that will help it to resist buckling a great deal.

Just be sure you get very strainght clean 4x4s for this project, and you'll be fine. defects will start the buckle, then you have a problem. It really doesn't take much strength to hold a great deal of weight in pure compression if its all in- line. I remeber my cousin as would ballance on an empty popcan. then the slightest tap on the side of the can and it would colapse.
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Old 03-17-2010, 10:49 PM   #10
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Post Load Capacity?


I am not sure I understand the arrangement, a photo would help immensely, or a sketch. If I understand you correctly, you have four posts in the garage. You indicate that the four posts support two 24 foot long beams. This suggests two posts per beam, which would place the posts 24 feet apart. Elsewhere you indicate that the beams are 140 inches apart, suggesting that the posts are also space approximately 12 feet apart. I could not tell how wide the garage is, perhaps 14 feet or so?

At any rate, the load bearing capacity of an 8 foot long post is most probably controlled by buckling, not by axial compression. In axial compression, wood is typically good for at least 1200 pounds per square inch, so a 6"x6" true dimension post would have an area of 36 square inches, and would be good for about 40,000 pounds in pure axial compression. However, the post will typically buckle under much lower load than the axial compression.

Computation of allowable load where buckling controls is not simple, and there are typically not tables in the code books as there are for beams under horizontal bending. The difficulty in computing allowable buckling load is determining the end conditions. If the post is fixed on both ends, it buckles at a much higher load than if one end is free or pinned. Similarly, if the post is restrained in the middle, it buckles at a much higher load than an unrestrained post.

For an unoccupied garage, total vertical load is typically relatively small, however you indicate that you have occupied space above the posts and beams, so it would be prudent to have an engineer check the capacity of the posts, especially since it sounds like the ones you have in there have either buckled or warped severely. Living space adds considerably to the load on the posts, especially if the posts are carrying all the load, and the exterior walls are not load bearing.
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Old 03-17-2010, 11:26 PM   #11
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Post Load Capacity?


Thank you for the important information regarding buckling, and the importance of securing the posts in the centers.

I will make an effort to post pictures, or a sketch tomorrow.

The posts are located at the ends of the 24' beams. The two beams run parallel to one another on 12' centers. They are located approximately 12' from the garage door, and 12' from the rear wall of the garage.

The home is a ranch with a garage below on one side, and a basement on the other.

The garage is approximately 36' x 25'. The left posts (when you are facing in from the door) are positioned in the unfinished exterior (load bearing) wall. The right posts are actually positioned about 1' from a (poorly) finished interior wall, which I believe is also load-bearing.

With the information garnered here it seems that a 4x4 placed on the left where the beam goes into the unfinished wall would probably have sufficient axial load bearing capacity, and I could provide some protection against buckling along the axis of the wall with a diagonal strap of some sort.

I'm at a loss as to how to provide reinforcement against buckling in the axis perpendicular to the wall, but I would think that a couple of two-by fours glued and screwed to either side (within the wall space) would help with this, at low cost.

Then there is the matter of the other posts, which are almost free-standing. I'm thinking this is a good place for a 6x6, as it would clearly have greater resistance to buckling than a 4x8.
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Old 03-18-2010, 02:15 AM   #12
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Post Load Capacity?


so you have 1 of the posts sort of free floating presuably because the extra 1' of span was too much?

you might as well incorperate the posts into some tool storage closet. It could be much stonger, look better and give you some garage storage.

another option yould be to get a 4x4 square steel tube (this was holding up a big I beam used for a chainfall in my last garage) or even one of the round adjustable basement posts.

I think keeping the posts hidden within the wall might be a good idea just for safety reasons in a garage. hate to make it home way too tired (or drunk) and hit the post and have your house land on top of you.
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Old 03-18-2010, 06:46 AM   #13
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Post Load Capacity?


Yes, the posts on one side of the beam are sort of "floating" ... actually they are attached to the wall by a 2x4 "plate" at the bottom, and tied in with a sort of cross brace at the top. Still, frankly I don't know what the builder was thinking (or drinking, as it seems the case may have been).

Great idea on the closet, but the positioning doesn't lend itself well to this, besides they are not in a spot that a car would hit them... the garage door is off-center, placed about two feet from the left wall, 8 feet from the right.
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Old 01-09-2011, 01:33 PM   #14
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Post Load Capacity?


I am building a supplies loft that is 24' x 24'. I am using 9 - 3" x 6" posts, 3 beams of 2" x 8" (double) beams, 2" x 6" floor joices (16 " inches apart), and the floor is 3/4" plywood. I am going to place wooden shelves and supplies on top. Am I within load bearing limits?

Thanks, Rick

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