Constructed beams

[John_M]

I have a question.  I know that when you make a man-made beam, you usually put a piece of 1/2" OSB inbetween the 2X material.

Is this for strength purposes or for making it a specific thickness (the fact that a 2 x 4 is actually 1.5 x 3.5).

The reason that I ask is that I may be using Amish milled lumber and they mill it at actual 2x thicknesses.

The beams will be used to support a loft floor.

[cronenwett]

I was wondering about this myself...I thought maybe I could stick foam insulation board in there to keep out some of the cold.  I'll be interested to hear the answer.
Liz

[Billy Bob]

In the case of a header over a door or window, the purpose is to resize the member so it will be flush with the rest of the framing.  I'm not aware of any other reason, and have seen lots of older homes framed with rough sawn lumber that didn't use any fillers.
I like the idea of the foam as a thermal break, but not sure it wouldn't compress a bit.  Also wonder how it might affect the holding power of fasteners?
BTW, lumber used to be sawn to actual dimension, then when they started planing and sanding for smooth finish, that's where the dimensional loss crept in. I think I noticed it got skimpier in a couple of stages, like they figured on saving a couple million board feet by cutting lean an extra 1/8".  Anyone know the story on that?  This is what I was told a long time ago, but always wondered if it were true.

[JRR]

In most applications, the "web" is an important member in the built-up beam.  It must keep the compression and tension members in their places.  

In the few that I have made, pt plywood was used instead of OSB.

[PEG688]

John I assume you are talking about headers , over doors and windows , not beams meaning spanning longer areas like ten feet or more .
  If that is the case , headers not beams , either the 1/2 plywood / osb / or ridgid insulation will do what your asking , making two 1 1/2 members = 3 " fill out the 3 1/2 stud space.

 In 2x6 wall construction we put a 2x6 under the header which becomes drywall/ window trim  backing and put the dbl 2 x8 / 10 (generally) to the exterior side so the insulator's can fill in the space (2 1/2 ") with insulation .

  Good luck, PEG

  In a true beam your web filler could be  helping , although most laminated beams I have made were all lumber , no filler required , the stager of the joints and position of post / lally columns ( steel post, concrete filled ) proved the required support.

[John Raabe]

When laminating built-up beams out of 2x material, such as might be used in the floor beams of the Little House projects, I suggest using plywood between two 2x's. Stagger ALL the joints 4' o/c and glue and nail the whole thing together.

Structurally I rate it as a double 2x (not giving anything to the plywood). There are two reasons for the plywood:
• It does make the beam stronger, primarily because you now have three joints staggered and more glue holding everything together.
• Now the beam fits the standard brackets sized for a 4x (3.5")

When doing headers I like to use double 2x's with a 2-1/2" block of foam in the middle and a 1x6 on the bottom. This provides nailers for both sides of the wall and up to an R-20 insulation value (about the same as the cavity in a 2x6 wall).  Sometimes you can eliminate the double header, use a single (maybe 2x10 or 12) and have even more room for insulation.

[PEG688]

I have a question.  I know that when you make a man-made beam, you usually put a piece of 1/2" OSB inbetween the 2X material.

Is this for strength purposes or for making it a specific thickness (the fact that a 2 x 4 is actually 1.5 x 3.5).

The reason that I ask is that I may be using Amish milled lumber and they mill it at actual 2x thicknesses.

The beams will be used to support a loft floor.

  John lets start at the beginning , what will the beams span be , what will the loft flooring be 1x6, 2x6 t+g , 3/4 ply / osb ?

 If your span was 16 feet or less why not have the Amish mill you some full 4X12 beam stock so you won't need to laminate it just detail it let the beams show off it natural stuff

  If you use T+G  2x lumber your could space those beams every 30 to 36" apart , and get your floor / ceiling all in one .   And yes there are other ways to do the floor/ ceiling deal this is just one of many.

  Good luck, PEG  

[John_M]

The span will be 18 feet and I would like to use 2x6 T&G flooring for the loft.

What dimensions wood I use for a solid beam.

(I mispelled wood on purpose as a crude attempt at humor)

[John Raabe]

As you can probably imagine, we can not do on-line engineering in the forum .

[PEG688]

 Hey John like John R. said .   Ask the guy milling your beams he'll more then likely know what size you'll need . A lot has to do with species / quality of the lumber you'll be getting .
  Check with a couple of local / reliable sourses.

  And read the tag line below my post , that will also help, I think. Good luck, PEG

[williet]

 John lets start at the beginning , what will the beams span be , what will the loft flooring be 1x6, 2x6 t+g , 3/4 ply / osb ?

If your span was 16 feet or less why not have the Amish mill you some full 4X12 beam stock so you won't need to laminate it just detail it let the beams show off it natural stuff

 If you use T+G  2x lumber your could space those beams every 30 to 36" apart , and get your floor / ceiling all in one .   And yes there are other ways to do the floor/ ceiling deal this is just one of many.

 Good luck, PEG  

So, on a 20' span with no support posts and a living area upstairs ... what size solid wood beam is needed? or what width 3ply 2x beam is needed?

I'm thinking of using "old" barn beams or re-cut log beams .... +/- 36" apart.

[jraabe]

You need to calculate the "derivative load" that the beam will carry.  This is done through a load trace of the floor plan.

Yours sounds fairly simple. If you have beams 3' o/c and they are carrying only the floor load you would normally use 40 psf (pounds per square foot) for the live load and 10 for the dead - that means 50 psf x 3' or 150 pounds of loading on each foot (plf = pounds per lineal foot).

For a long span like 20' the determining factor will be deflection of the beam under its design load. This is expressed as a ratio of the length (convert to inches) to the max allowed deflection. I would design to a high standard for such a long span floor beam—1/480 deflection. That would mean we will tolerate 1/2" of "bounce" or deflection in a 20' span (20'x12/480).

So you have 150plf of load and 1/480 deflection over a span of 20'. Now you can use this in a structural design computer program or go to span tables to find what types of wood beams would meet your criteria.

I have an old glulam beam table handy published by Willamette industries in 1994. A 24f-V4 Doug fir glulam 3 1/2" x 12" will carry 210 plf over 20' but it is only calculated for a 1/360 deflection. This beam has plenty of strength but may feel too springy. I would probably look further and check some other shapes. (I see a 5 1/8" x 10 1/2" will carry 206 plf.)

This is just an example of how such beams are sized and a built-up beam of the same size will not be as strong as a glu-lam.

PS - I just did a calculation using my structural design program (that allows 1/480 deflection) and a good DF 6x12 will meet the criteria above.

[williet]

You need to calculate the "derivative load" that the beam will carry.  This is done through a load trace of the floor plan.

Yours sounds fairly simple. If you have beams 3' o/c and they are carrying only the floor load you would normally use 40 psf (pounds per square foot) for the live load and 10 for the dead - that means 50 psf x 3' or 150 pounds of loading on each foot (plf = pounds per lineal foot).

For a long span like 20' the determining factor will be deflection of the beam under its design load. This is expressed as a ratio of the length (convert to inches) to the max allowed deflection. I would design to a high standard for such a long span floor beam—1/480 deflection. That would mean we will tolerate 1/2" of "bounce" or deflection in a 20' span (20'x12/480).

So you have 150plf of load and 1/480 deflection over a span of 20'. Now you can use this in a structural design computer program or go to span tables to find what types of wood beams would meet your criteria.

I have an old glulam beam table handy published by Willamette industries in 1994. A 24f-V4 Doug fir glulam 3 1/2" x 12" will carry 210 plf over 20' but it is only calculated for a 1/360 deflection. This beam has plenty of strength but may feel too springy. I would probably look further and check some other shapes. (I see a 5 1/8" x 10 1/2" will carry 206 plf.)

This is just an example of how such beams are sized and a built-up beam of the same size will not be as strong as a glu-lam.

PS - I just did a calculation using my structural design program (that allows 1/480 deflection) and a good DF 6x12 will meet the criteria above.

Thanks,
I've lost my grandfather's old carpenter books. In them there was a table of most often used woods and beam size for standard beam lengths.
I was thinking that a 8"x10" solid wood beam would hold up about anything in normal house construction ....
In building log houses several years ago, we used the "made up" beams, but I couldn't remember the sizes on them either.