Wednesday, November 7, 2012

Strong Attic Floors

At 11/19/2012, this post is edited to include the lead-in phrase that follows, a sub-title:

Composite Beam Method In Residential Construction
I would like to claim those simple words for my practice of making wood beams taller as reinforcement, rather than wider (called sistering or doubling). I stitch rips of plywood (called web), to found 2x framing, as bridges to added 2x4 (called nailer). The added 2x4 are longest-possible match of the length of found framing.


In fact, "composite beam" is more commonly descriptive of material mixes including wood/ steel (Flitch Beam), and wood-concrete.

In my work in attics, it is sometimes structurally necessary that I provide substantial decking of the attic floor. 

That attic flooring then provides novel, efficient roof supports.

Here are three examples in a Picasa Web Album. In the first two, I have repaired broken floor joists where they must support needed attic ladders. In the third, I have provided load bearing capability over 2x4 truss bottom elements otherwise flimsy. In all three, the strength of a composite beam is assured only where 2x elements at bottom (ceiling of rooms below) and at top (attic deck surface), are not free to rotate. No deck, less-strong floor, and dangerous navigation with trampling of insulation. 

Here is a graphic illustration of the composite beam application with 2x4 trusses:

I apply ample decking, with needful disregard of restraints. Search at Google "Energy Trust of Oregon". "Fraction of attic floor decked." Find 2012 Weatherization Specifications. Find AT 2.4:

Sadly, there is no forum for public input to rules. I can only try to influence, by this kind of public expression. Here then, let me ask: What is a "cavity to be filled"? I will link this by email to Energy Trust, and see what they have to say, on the broad topic. I invite them to respond by comment here.

This post was imagined, in dealing with structural analysis of composite beams, as presented in another public forum I have joined, DIY Chatroom:

Perhaps readers here will see value in composite beam construction, and might comment. I think floor reinforcement by "sistering" has no validity, where added lumber couples that found, with rotational freedom and unlikely bearing on supports below.

I wish for academic and professional comment. Lacking that, I can attempt my own defense. The ultimate would be deflection testing of sample beams, which is within my capability, but would best happen with support of an interested college or university. For now, note that beam stiffness and strength are in linear proportion to thickness, and cubic to height. Sistering at best doubles strength. A composite 2x10 is likely at least four times stronger than a 2x6, and more than ten times stronger than a 2x4. A composite 2x12 of the same 2x lumber is strengthened by a further factor of two. What wonderful and easy control of strength! Testing and questioning are hardly justified. Does it matter that you might increase 2x4 strength by times thirty? Maybe it is just the added insulation depth that matters. Often it is sufficient to just do all that the situation allows for strength and insulation, as in availability of headroom. Sistering is best effort, only if space for making the floor thicker is absolutely unavailable.

Interest in this has appeared in a re-post to Fine HomeBuilding. I have graphics to share there, which I will copy from expansion of this post. I explain in a further example, that of the repair and strengthening of a 2x8 floor joist in my crawl space. Here is a photo.

And, here is a cross-section detail. This beam bears more load than any other in my crawl space, yet suffers a carve-out passing the bathtub drain. I go way beyond compensating for the divot. Surely the reinforced beam is as strong as a joist of 1 1/2 x 12" cross section. Better strength is by the ratio (12/7.25), cubed. That is, by times 4.5.

I worried some about that beam before my remodel. I won't anymore.

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