| I=(b)*(h^3)/12 where b = base dimension & h = height dimension. | | As you can see, "Moment of Inertia" is only changed linearly with a | change in width while the "Moment of Inertia" is changed by the cube | of the height dimension. | | "Moment of Inertia" explains why wide flange and I-beams have the | shapes they do. | | Greatest strength for the least amount of material.
Interesting!
A bit of playing with my calculator has me wondering why not use 1x stock for rafters and joists, or at least _some_ rafters and joists? It would appear that if I resawed a 2x rafter and edge glued the two halves, then the resulting rafter could carry a greater load.
You would need aditional blocking to account for the loss in lateral stability. You would also have a tough accurately screwing two edges of a subfloor (plywood) onto a 1-by.
Stephen M wrote: || Interesting! || || A bit of playing with my calculator has me wondering why not use 1x || stock for rafters and joists, or at least _some_ rafters and || joists? It would appear that if I resawed a 2x rafter and edge || glued the two halves, then the resulting rafter could carry a || greater load. || || What am I missing? | | | Not much.... | | You would need aditional blocking to account for the loss in lateral | stability. You would also have a tough accurately screwing two | edges of a subfloor (plywood) onto a 1-by.
Thanks, Steve. I'm obviously not an architect/builder guy :-)
Last week I worked out a gambrel roof design for a friend's garden shed (which started me thinking about offering these things in kit form). The 2x4 rafter segments looked like overkill for the 8' span, and now I'm thinking that 2x4 is only needed where sheets of the ply meet, and that the 'between' rafter segments probably could be 1x4.
CNC drilling pilot holes in sheathing and rafters wouldn't be a difficult - which should make it fairly easy to accurately screw plywood to rafters.
I'd not try using 1x in a larger structure because I suspect structural integrity would vanish fairly rapidly if there were a fire...
That is an interesting idea, to predrill the holes in both the sheathing and rafters. I wonder how well that would work out in the real world. I suspect that there may be problems making things fit.
What a friend of mine did when he built things like this was to predrill the (countersunk) holes in the sheathing. He would put the sheathing in place and drill the pilot hole with a yankee drill driver. Ya know, a drill that works like a yankee screwdriver. It drills holes instead. This worked well for him.
He would then drive the screws with his cordless screw driver.
|| CNC drilling pilot holes in sheathing and rafters wouldn't be a || difficult - which should make it fairly easy to accurately screw || plywood to rafters. | | That is an interesting idea, to predrill the holes in both the | sheathing and rafters. I wonder how well that would work out in | the real world. I suspect that there may be problems making things | fit.
I do this type of thing fairly routinely with solar panel production and it works well for constructs up to 12' (I haven't used the CNC for anything larger) so long as moisture contents don't change much - with "much" being dependent on both materials and required precision of fit. I suspect that as long as the holes line up with about 1/32", there shouldn't be a problem in a framing/sheathing/subflooring context.
| What a friend of mine did when he built things like this was to | predrill the (countersunk) holes in the sheathing. He would put the | sheathing in place and drill the pilot hole with a yankee drill | driver. Ya know, a drill that works like a yankee screwdriver. It | drills holes instead. This worked well for him. | | He would then drive the screws with his cordless screw driver.
I have the large Yankee driver, which came with both drill and screw bits - and I've done what your friend did. It works, but these days I'd be inclined to do the drilling with a tailed drill and use a cordless drill (because of the clutch) to do the driving.
I keep the Yankee and a brace (currently fitted out with a #2 square bit) as backup - been meaning to build an "In case of emergency break glass" case for 'em. :-)
I have three cordless screwdrivers. One is a Klein, another is a Stanley, and the third is a Dewalt 14.4v drill. I have a phillips bit (from Lee Valley) in the Stanley. :-) I also have chrome Yankee drill (like the telephone repairmen used). I was able to buy new old stock bits for it. I do use them all.
I hope the original poster sees his original idea is a bit nutty. I don't think OSB interior flooring would hold up outside, and the orientation of the core does not appear add desired strength.
My buddy and I (who are not engineers) were re-doing his one story home and we got into taking out a supporting wall and remaking it. We made a support beam out of 3/4" plywood sides with 2x4's in the middle, and we used construction adhesive and framing nails from a gun to build it. It was about 18" tall and the length was only about 11' long at the most. The SOB was total overkill just to hold up the roof, it was very very strong with the plywood glued in like that - vertically. I told him if there is every a chance of a tornado or his large trees in his yard falling on the house, go stand under that SOB and you'll be fine.
Outdoors? I think the suggested dimensional lumber make a lot of sense.
Nutty? Well maybe. But it just so happens that I have a 24 foot beam laminated from 3/4" plywood holding up the porch of my shop right now. It has 16 feet of clear span with 4 feet of cantilever on each end. The beam itself is only 6 inches tall by 5 inches thick. That headroom thing again. It's painted with waterbase house paint and protected by the roof, but it's only 6 inches from the edge. I built it before I had access to the internet and all the sage advice I can get here. I remember I had read about a new glue called Titebond II that was supposed to be waterproof, but it wasn't available locally, so I used regular Titebond. I didn't even have access to load tables, but it was only supporting one side of a 6 foot roof section, so I did some seat-of- the-pants engineering. It's "almost" strong enough. There's just a bit of sag in the middle, but you have to sight down the beam to see it. It's been there for 13 years so far. From time to time I consider adding a center post to take out the sag, but it's not noticable, and I really like having the clear span, so I haven't done it. I'm pretty sure the beam needed to be taller. But I've seen no sign of deterioration, and it hasn't sagged any more after the first year. There may be better approaches to the problem at hand, but I'm not quite ready to dismiss my original idea as "nutty".
DonkeyHody " We should be careful to get out of an experience only the wisdom that is in it - and stop there; lest we be like the cat that sits down on a hot stove-lid. She will never sit down on a hot stove-lid again---and that is well; but also she will never sit down on a cold one anymore." - Mark Twain
There may be better approaches to the problem at hand, but I'm
Ok, well, I did not mean to insult you, as I've made some decisions I regretted in the past, but I think if you build something out of OSB and put it outside for a while you'd discover while it is not bad outdoors, its certainly not like PT wood for durability. The beam you describe is basically indoors so it would not be a valid comparison in my opinion. I live along the gulf coast where we have year round high humidity and everything deteriorates fairly quickly, even my Trex decking has some mold stains and what not. In a different climate, you might be ok for a while, but I stand by the recommendation for PT wood for outdoors, or steel as one poster recommended for meeting your low profile requirements.
I wasn't insulted. I knew it was "out of the box" thinking, I just didn't know how far out of the box I was. BTW, I'm in central Mississippi, only 150 miles from the coast. Plenty of humidity here too. The PT box I intended to build around the beam should have protected it to about the same degree as the roof over my present beam. But the question is somewhat moot at this point since the young couple's budget just wouldn't cover the materials for a thick beam. They decided to just install a center post, which isn't what I would have done, but does simplify life considerably.
DonkeyHody "Even an old blind hog finds an acorn every now and then."
While the immediate structural needs (bending in this case) can be met (generally) with less material than is normally used, like Stephen noted, there are other considerations (lateral buckling) that need to be considered. When one is talking big construction - many thousands of dollars, it is appropriate to investigate trimming things down to precise materials - because much money can be saved. Smaller projects obviously reach the point of diminishing returns rather quickly.
However, there may be other ways of looking at things here...
What would make the most sense is what is called a 'stressed skin panel' - wherein the rigidity is built up thru the use of 'skins' serving the purposes of 'flanges' (of your standard 'I' beam). By this, *both* surfaces of your framing are 'skinned' with plywood / OSB
- anchored at precisely specified intervals (typically in the 2" to 6" spacing range). This produces a panel product that has some pretty surprising rigidity. Another (similar) approach are the SIP (Structural Insulated Panels), wherein the 'skins' are bonded to a foam core that does the same thing - and adds insulating value as well.
The 'trick' here for SIPs (if there is one) is that - by bonding the surface skins to that foam, then every square inch of the inner surfaces of the skins contributes to carrying the stress - reducing the individual unit stresses down to a very low level (thus, the foam can handle it).
While Stephen appropriately notes 'blocking' for lateral support, placing that 'skin' panel on both sides of the framing, this panel then becomes your lateral support (special circumstances may, indeed, require blocking as well - concentrated loads, etc).
It is not inconceivable for this to be done by the homeowner on smallish projects - but getting an engineer involved probably makes sense for more elaborate undertakings.
As an example of a 'non-analytical' application, one of my hobbies is rocketry (not the small stuff - some of it gets pretty big). I had, in times past, acquired a fair stockpile of some 0.030" G-10 fiberglass panels (like circuit boards are made of). We are always exploring other approaches that can provide strength capable of Mach 3 undertakings while still being lightweight. Stressed skin panels have some real application here. I made up a test panel (about 11"x14") with some 2" foam (that standard, white 'beadboard' product) as a core and the 0.030" G-10 epoxied to both surfaces. Obviously, the bare foam could carry very little - equally, something (anything) 1/32" thick (the G-10) couldn't carry much either. Together (and held rigidly in place with the epoxy), I could set that panel on top of two bricks (13" apart) and it could carry my entire weight (200lbs+) with almost zero deflection - and this for something that weighed less than a pound. Such is the nature of things when the right material is placed in the right place.
The APA (American Plywood Association) has an extensive library of publications - all manner of things - that could lend insight into this -- see...
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'll have to register (and I think the public can do this - no cost). Look for publication W605 "Structural Insulated Panels" as one resource. There are others on that website as well.
Every once an awhile, leftover panels from projects show up in your 'shopper' mags (and, probably, online) - can occasionally find some pretty good deals.
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