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Old 08-22-2011, 11:49 AM   #16
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I am way old school and can't say much when it comes to the tech end of OSB vs plywood so I will just go with what I know. Plywood would also be my first choice but that is just MHO. If that were my floor I would put a drop girder under the floor joists and forget it. I know a drop girder will work and take bounce out if installed properly.

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Old 08-22-2011, 02:38 PM   #17
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Let me throw something new on the fire here...

What about using OSB rim board inserted in to lets say 2x8 or 2x10 steel track all glued and nailed together?

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Old 08-26-2011, 11:30 PM   #18
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Check with the steel manufacture or a structural engineer for the I-beam.

I was wrong on the OSB for shear strength, it has been discussed here before; Floor bounce joist sistering

OSB Stronger than Plywood?

Let us know your end solution.....

Gary
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Old 05-10-2012, 08:28 AM   #19
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Originally Posted by Daniel Holzman View Post
Assuming the modulus of the OSB is approximately the same as that of the sawn lumber, which it usually is, the deflection will be approximately 1.5/2 = .75, so there will be a 25% decrease in deflection. You get exactly the same result if you nail on a 1/2 inch thick x 9.5 inch thick piece of plywood or sawn lumber, assuming the modulus of elasticity of the nailed on piece is the same as the lumber, and it is usually close.


I came across this old thread and found it very educational. It leaves me scratching my head, though, on how engineered I joists (with an OSB web so much thinner than 2x sawn lumber joists) resist deflection so well, given the statement here that the modulus of elasticity of OSB and sawn lumber is the same. I would expect the 2x4 flanges to contribute hardly anything to resisting deflection.

Last edited by cortell; 05-10-2012 at 08:32 AM.
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Old 05-10-2012, 11:58 AM   #20
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Ancient thread, new question. Deflection is controlled by the modulus of elasticity of the material and the moment of inertia of the section. An I joist works well because the moment of inertia is high, due to the location of the flanges far from the centroid of the section. That is why steel I beams have the large flanges. The web carries vertical shear, and prevents buckling. The web is sized as small as possible to save material, and the web contributes essentially nothing to the moment of inertia, hence the flanges do all the work resisting bending, and the web does virtually nothing. In a sense, the web is only there to hold the flanges apart.
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Old 05-10-2012, 12:13 PM   #21
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Originally Posted by Daniel Holzman View Post
Ancient thread, new question. Deflection is controlled by the modulus of elasticity of the material and the moment of inertia of the section. An I joist works well because the moment of inertia is high, due to the location of the flanges far from the centroid of the section. That is why steel I beams have the large flanges. The web carries vertical shear, and prevents buckling. The web is sized as small as possible to save material, and the web contributes essentially nothing to the moment of inertia, hence the flanges do all the work resisting bending, and the web does virtually nothing. In a sense, the web is only there to hold the flanges apart.
Hm. I'm going to have to study this bugger known as moment of inertia. It's not intuitive to me at all. The following presentation looks promising, but I'm going to need about three reads and two cups of coffee before it sinks in. Thanks, Daniel.

http://paws.wcu.edu/radams/Intro_to_Beam_Theory.pdf
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Old 05-10-2012, 01:07 PM   #22
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Originally Posted by Daniel Holzman View Post
Ancient thread, new question. Deflection is controlled by the modulus of elasticity of the material and the moment of inertia of the section. An I joist works well because the moment of inertia is high, due to the location of the flanges far from the centroid of the section. That is why steel I beams have the large flanges. The web carries vertical shear, and prevents buckling. The web is sized as small as possible to save material, and the web contributes essentially nothing to the moment of inertia, hence the flanges do all the work resisting bending, and the web does virtually nothing. In a sense, the web is only there to hold the flanges apart.
Sorry; follow up question. So would a 4x12 and an I-joist of the same width and height have comparable deflection for a load that does not exceed the design strength of both?
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Old 05-10-2012, 05:12 PM   #23
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Wow, this reads like a post from eng-tips.com Thanks to Daniel for the detailed info, even if this is an old post.

As long as we are on the subject, maybe Daniel can comment on a completed project.

I did the unthinkable, I strengthened the bottom chord of a truss for an attic floor, and to raise the floor to allow for insulation.

The chord is 2x4, and I used a 2x6 stacked vertically on top of the chord. To secure the 2x6 I used 5/8 plywood, 8" tall and 4' long, one one side of the 2x4 and 2x6 stack.

The plywood was secured to the stack with PL Premium (polyurethane glue) and heavy duty 2" screws.

I also glued the 2x4 and 2x6 edges with the PL Premium, and pulled them together using an 8" Timberlok screw (all the way through the top of the 2x6 and into the 2x4) about every 18".

Ignoring the truss issue for the moment (pun intended), have I strengthened the 2x4 to any significant degree? Would I have been better to sister a 2x6 or 2x8 alongside the 2x4?

The floor feels quite solid (for light attic storage), so I know that it did stiffen things up. But if you have a few minutes to give your expert opinion, I'd appreciate it. We can talk about the truss later, but for now let's discuss the strength of my "sandwich."

BTW, I tried to ask this on the truss forum on eng-tips.com before I did it, but the question alone got me thrown off the site, without any explanation from the administrator, who would not return my emails. Seems that they don't look kindly on people trying to DIY solutions with engineering input.

Last edited by M3 Pete; 05-10-2012 at 05:14 PM.
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Old 05-10-2012, 06:11 PM   #24
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Moment of inertia is a bit complicated to wrap your arms around, but it is central to the flexural design of all beams. Without getting into the derivation, the moment of inertia for a rectangular beam is bd^3/12, where b is the width, d is the depth, ^3 means you cube the depth. So for example a beam 2 inches wide, 8 inches deep, has a moment of inertia I = 2*8*8*8/12 = 85.3 in^4.

Note that doubling the depth of the beam increases I by a factor of 8. Sistering a beam of identical size doubles I. So if possible, you get much greater increase in I by increasing the depth of the beam. The limitation is that of course the beam gets deeper, so you may have headroom issues.

The other issue is more complex. When you connect two beams together along the bottom, they need to act as a unit to work. One beam wants to slide past the other when the beams are loaded. The force acting parallel to each beam at the point where they touch is called horizontal shear, and it is critical that the two beams be properly joined so they don't pull apart when the beam is loaded. This can be done with glue, nails, screws, bolts, or pegs, but in EVERY case you have to check the shear force across the connection to make sure the beam does not fail.

By adding a joist to the bottom of the truss chord, you substantially increased the moment of inertia of the chord member, and therefore increased the bending strength of the chord member. However, simple truss members do not act in bending, they are always either in tension or compression, so increasing the bending strength probably has little effect on the overall stiffness of the structure, unless you have a truss with fixed joints (not pinned). In that case, you have a space frame, which is far more complex to analyze than a pinned truss, and way beyond a DIY forum to get into the complex mathematics of space frame trusses.
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Old 05-10-2012, 06:48 PM   #25
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By adding a joist to the bottom of the truss chord, you substantially increased the moment of inertia of the chord member, and therefore increased the bending strength of the chord member.

However, simple truss members do not act in bending, they are always either in tension or compression, so increasing the bending strength probably has little effect on the overall stiffness of the structure, unless you have a truss with fixed joints (not pinned).

In that case, you have a space frame, which is far more complex to analyze than a pinned truss, and way beyond a DIY forum to get into the complex mathematics of space frame trusses.
I need to find a truss primer to remember what pinned means.

I understand that a truss is designed to keep all members in compression or tension. I believe bottom chords are always in tension. Adding tensile strength along the length was not my intent, as I was trying to effectively make the truss chord into a beam for the purposes of avoiding too much floor flex. I also supported the "beam" on two interior walls, and not make the truss try to support itself over the entire span.

But the top chord of a truss does require some bending strength in order to hold up the roof sheathing, right? I know how it's supposed to work, but the bending strength has to be in there somewhere, right? I have a concrete tile roof, so it weighs a fair amount. Bottom chords usually just hold up drywall and insulation, so their bending strength does not need to be as significant.

This is probably too much for this forum. Come on over and I'll buy you a beer and we can talk.

Last edited by M3 Pete; 05-10-2012 at 06:56 PM.
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Old 12-07-2012, 09:00 PM   #26
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Daniel, great info thanks.
I have a very similar issue as jpaz right now and was not sure if should start a new post or not (moderator?). I need to get my floor deflection number up by about 15% to meet L/720. So need to strengthen the joist. Would putting a 2x4 on the bottom of existing 2x8 be sufficient. If so how much glue/screws would be needed to resist the shear?
Thanks again.
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Old 12-08-2012, 04:14 AM   #27
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I suggest you start a new thread--------Moderator---------

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