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Old 05-21-2011, 04:01 PM   #1
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Beam Support Columns


I am using two 3 1/2 inch columns to support a 15 ft by 11 7/8 inch LVL beam in a one story ranch. The columns will extend from the attic down through the walls to the basement.
Where the columns pass through the floor should the plywood be cut out? In other words does the plywood have the load bearing capacity? The columns will each support 2330 lbs. (Homeowner (me) says remove the plywood ; contractor says its not necessary to remove it).


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Old 05-21-2011, 04:40 PM   #2
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Beam Support Columns


No need to remove it, but do put in solid wood blocking between the joists and between the two columns

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Old 05-21-2011, 05:50 PM   #3
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I am not sure I understand what you are saying. You say you are supporting a 15 foot long, 11 -7/8 inch deep LVL with two 3-1/2 inch columns. I am guessing there is one column on either end, or do you mean that each end of the LVL is supported by a pair of 3-1/2 inch columns?

Assuming it is one 3-1/2 inch column on either end, are the columns 2x4 studs, are they steel, or are they something else? You say that the columns are intended to extend from the attic to the basement, which sounds like approximately 16 feet. I assume there is some sort of footing in the basement to support the column, possibly newly poured.

So what I don't understand is the question about removing the plywood. I am guessing the plywood is the subfloor on the first floor. If so, I expect the designer of the column must have assumed you would cut a hole through the first floor to extend the column to the footing in the basement. If you are asking if the plywood is strong enough to support 2300 lbs over the area of a stud, I suggest you discuss that with your designer. It sounds like a bad plan to me, I assume the concept was to carry the column through to the basement. But then again, a single stud column would not meet code in my town, no idea what the code is like in your area.
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Old 05-21-2011, 06:28 PM   #4
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Beam Support Columns


The only time we cut the subfloor for columns, is when we use steel posts. Just make sure you install solid blocking under the columns.
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Old 05-21-2011, 10:15 PM   #5
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Beam Support Columns


Is this to support a ridge beam?

If so it is not a very long ridge. Do you have a plan of some sort?
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Old 05-22-2011, 08:06 AM   #6
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No I don't have a plan on paper. Yes the two columns are at either end (3,5 X 3.5 each). The columns are wood; I suggested 2x4s; contractor suggested a 4x4.
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Old 05-22-2011, 08:32 AM   #7
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OK, we have a better picture now. You are supporting a nominal 4x4 post on either end of the ridge beam. One option is to support the base of the post on a plywood floor, presumably blocked in some fashion. Before you do it this way, you should carefully review the American Plywood Association (APA) guidance on direct support of posts over plywood, document rr135.pdf, available free from the APA website.

The type of loading you would have is called punching shear, for the reason that the likely mode of failure is that the post would punch through the plywood. Of course, if you install solid blocking underneath the load point of the post, this analysis does not apply. However, it may not be possible to install such blocking, and if you do, you then need to check the blocking for shear. But back to the pure plywood situation.

The actual punching shear area is computed by taking the perimeter of the post (14 inches in this case), multiplying by the effective thickness of the plywood (this comes from a table in APA, and is LESS than the actual thickness of the plywood). For example, 7/8 inch underlayment has an effective nominal thickness of .6 inches. You then take the actual load, which you said was 2,330 lbs, and divide by the punching shear area, and compare this result to the allowable stress in plywood.

Example: In your case, if your floor was 7/8 inch underlayment, your effective shear area would by 7x.6 = 4.2 square inches. The punching shear would be 2,330/14 = 166 psi. This would be unacceptable under APA guidelines for 16 inch on center joists.

Of course, you may have a double layer of floor, and you may be planning to install support blocking, and you probably don't have 7/8 inch thick plywood. I am simply pointing out that supporting a post with a substantial load requires careful detailing of the connections.
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Old 05-22-2011, 05:14 PM   #8
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The 4x4 posts are extending vertically up to the attic from the floor as well as down to the basement. So rather than a 4x4 post to 4x4 post connection (which I prefer) I would have a 4x4 to sheathing to 4x4 post connection. (The sheathing will be sandwiched in between the top and bottom of the 4x4 post. The post will extend from the attic 8 feet down to the kitchen floor (plywood). On the under side of the plywood is another 4x4 post which will carry the load down to the basement floor. A 2x4 plate laying in the basement concrete floor will support the end of the 4x4 post.

My question was whether or not the sheathing needs to be cut out so that the 4x4 post will extend continuously from top to bottom - 16 feet. This way there will be no sheathing in between which my compress under load.

Can sheathing withstand 2330 lbs / 12.25 sq. in. = 190 lbs / sq. in.?

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Old 05-22-2011, 08:49 PM   #9
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This makes more sense now. Unless there is something wrong with the sheathing, i.e. it got wet or is otherwise damaged, the load will not cause significant compression to the sheathing. It is important that you line up the 4x4's carefully so there is no eccentricity introduced between the two posts, else you can get into trouble with column buckling. You also need to adequately fasten the posts at the top and bottom to prevent lateral movement. And you should make sure the support at the base is adequate, usually a column gets a separate footing.

Just curious, you said there was no designer, so how did you come up with such a seemingly accurate column load?
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Old 05-23-2011, 06:52 AM   #10
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I did a hand calculation based on 20 lbs deadload, 10 lbs live load for the attic. Based on the total area I came up with the beam loading and consequently the column loading. I then went to two building supply houses to size the LVL beam. The computer printout also gave the column loading which was close to my estimation. (I am an EE and have taken Statics and Dynamics).
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Old 05-23-2011, 08:30 AM   #11
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I am still doubting the base at the basement floor. Odds say that there is no footer at that point under that basement slab, and that could be a problem, even with a plate thrown down there.
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Old 05-23-2011, 08:46 AM   #12
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You didn't say where you live, so I have no idea what code if any applies, but I believe you said this was a ridge beam to support the roof. If you live in snow country, you would need at least 20 psf live load for the snow. Where I live in Massachusetts snow load is about 35 psf. If you don't live in snow country, you still need to look at wind loading. The 10 psf seems reasonable for dead loading, depending on the thickness of sheathing, type of shingles, framing etc.
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Old 05-23-2011, 11:17 AM   #13
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Beam Support Columns


This is NOT a ridge beam. This beam has very little loading, only unused attic floor space. I live in CT. (The beam is going to take the place of a load bearing wall. The load bearing wall is going to be removed).

When I consulted with the town building inspector he said I did not need a footing under the basement slab.

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