I just know somebody here has experience . . .
A friend wants an arbor over her patio for vines to climb on. We
envision an open structure with four posts supporting two main
crossbeams. These crossbeams would support "rafters" on two foot
centers. One of the rafters will carry a porch swing, but that rafter
is near the posts, not near the center of the span. No other
significant loads other than the weight of the vines that will
eventually cover the rafters. Main crossbeams would be 20 feet long,
with 18 feet of clear span between the posts. Rafters will be 12 feet
long, with 8 feet of clear span and 2 feet of overhang on each end.
My plan is to buy 3/4 plywood intended for sub-floor and cut it into
strips 7 7/8" wide. These strips will glue and screw together with
Titebond II, and the screws will serve as clamps until the glue
dries. I'll stagger the joints by using 2, 4, 6 and 8 foot strips.
I plan to protect the lamintate beam from the weather by enclosing it
in a 3-sided box of pressure-treated lumber.
Here's the question. How thick does the laminated beam need to
be?
DonkeyHody
"Every man is my superior in that I can learn from him" - Thomas
Carlyle
On Jul 1, 8:33 pm, DonkeyHody <[email protected]> wrote:
> I just know somebody here has experience . . .
>
>
> My plan is to buy 3/4 plywood intended for sub-floor and cut it into
> strips 7 7/8" wide. These strips will glue and screw together with
> Titebond II, and the screws will serve as clamps until the glue
> dries. I'll stagger the joints by using 2, 4, 6 and 8 foot strips.
>
I'd be more inclined to use WEST System epoxy and clamps...forget
about screws.
I'm sure Lew can add to my suggestion.
> Yep, got plenty of experience. Whatcha want to know about?
>
> But if you put the swing there, and later find out that's a bad
> place, it's gonna be a bitch moving it. I'd rather have a stand-alone
> swng, just in case.
>
> Why screws? You figuring on taking it apart later? I'd nail it.
>
> How you planning on lifting those beams in place?
>
> JOAT
> If a man does his best, what else is there?
> - General George S. Patton
The screws are to draw the layers of plywood together, each acting as
a small clamp. To lift it, I thought I'd stand one of the rafter
boards on end and screw it alongside the post. Put an eye-bolt at the
top and use a block and tackle.
DonkeyHody
"In theory, theory and practice are the same, but in practice, they
are not."
>
> I would not touch the above with a fork.
>
> Even with epoxy, plywood is a lousy choice for this application.
>
> With an 18 ft unsupported span, I'd sister a couple of 2x12 Doug Fir
> timbers together with epoxy and clamps.
>
> Faster, cheaper, better.
>
> 3 for 3 isn't bad.
>
> Lew
Thanks to all for the advice. I'm re-thinking the plywood idea. The
reason I considered laminates in the first place is that 20 foot
lumber in any size bigger than 2X6 is pretty scarce around here. I'll
price a factory glue-lam.
DonkeyHody
"Every man is my superior in that I can learn from him." - Thomas
Carlyle
On Jul 2, 10:55 pm, DonkeyHody <[email protected]> wrote:
> Thanks to all for the advice. I'm re-thinking the plywood idea. The
> reason I considered laminates in the first place is that 20 foot
> lumber in any size bigger than 2X6 is pretty scarce around here. I'll
> price a factory glue-lam.
"Don",
As an Architect, maybe I can add a little insight...
I would question the use of 'multiple' plywood laminations if for no
other reason, the material strengths are indeterminate (the two main
things needed for a rational structural design is 1) knowing
accurately the loads one is to carry and 2) knowing accurately the
ability of the material to carry those loads). While the wood
generally used for plywood would have known properties, remember that
plywood is generally used in an application (as in decking) that loads
it in the plane of the laminations and in your case, you are loading
it perpendicular to that. Couple that with the gaps that sometimes
occurs in interior laminations (which is more critical in your
application) and much uncertainty enters the equation - exactly what
you don't want if a rational structural design is the goal.
Don't get me wrong, we're not talking some major critical issue here
(it's not holding up a second floor of a house, etc) - but, it seems
reasonable to not get into areas that a trained structural engineer
would avoid, no? There are better ways, anyway.
One basic thing to remember in structural members is the *depth* of a
member -- as this adds rigidity far more than width (i.e. a 2x12 is
far better than a 3x8 or 4x6 (all with the same cross-sectional
area)). It's a 'moment of inertia' (Ix) thing (you can google around
-- or just remember that the 'Ix' goes up with the square of the DEPTH
and you can see the effect). That is - provided the width doesn't get
too slender such that the beam can flex in the horizontal axis (as it
will) - but that is what the rafters above will provide -- 'lateral
support'.
Remember that a beam under load has tension stress in the bottom and
compressive stress at the top and you can see that the wood fibers
need to be oriented horizontally (which every other lamination in
plywood would defeat that purpose - since the fibers would be oriented
incorrectly) and you can see how solid, sawn wood would serve your
purposes better.
I believe Lew mentioned a few 2x12s and I would agree that something
like this would be a smarter solution. Also, the American Plywood
Association (www.apawood.org) has some pretty good tech reports on
built up beams -- and don't get me (us) wrong - one *can* build up
beams with plywood, but it is generally done with a plywood panel
sandwiched between sawn lumber (or, two panels in layers 2 and 4 of a
5 member sandwich).
Then again, we also (now) have LVL (Laminated Veneer Lumber), which is
just a big hunk of plywood (but with the laminations oriented
correctly and with documented strength values) or LSL (Laminated
Strand Lumber (similar). These are (generally) about twice as strong,
size for size, as solid sawn lumber but are, of course, more
expensive. Being an exterior application, you *should* use pressure-
treated or naturally decay resistant wood. I would not use a LVL or
LSL without capping the top surface with a conventional 1x pressure
treated piece - just to keep water from working it's way between the
laminations (or, cap it with some copper sheet - would make a nice
design element -- just paint the covered surfaces with some oil-based
paint and maybe use a few copper washers to space the sheet slightly
away from the wood surface for ventilation (letting it dry between
rains) -- but, then, this begins to complicate things more than it
should be. Simpler is to simply use multiple 2x solid sawn members
(and, maybe, that exterior plywood inner lamination) and cap with a
1x6 pressure-treated 'rain cap'.
Loading? There wouldn't be much - fundamentally the dead load of the
materials and vines, etc. It is possible that wind-loading could be
your greatest load - especially if covered completely with vines. I
would double the rafters where it carries the swing and run your bolts
from the top of the rafters (vs eye-bolts screwed into the bottom of
the rafter) -- there are 'edge distance' issues with bolted
connections and driving a lag-type eye-bolt into a 1.5" member
(nominal 2x width) maybe cutting things a little close. Better to use
a conventional threaded (i.e. machine threads) eye-bolt hanging from
the top (between the two 2x rafters) - which would load the assembly
in compression and not tension (i.e. pullout) - which the conventional
lag-screw type eye-bolt will do. Since running a bolt right thru the
interface between the 2 2x's might could cause delamination over time,
smarter to run the bolt thru a smallish 'cap piece' on top of the
2-2x's (and, with a little overhang, this could be a decorative piece,
as well) and this would not cause delamination.
That's probably enough to get some thinking going...
-- john.
> As an Architect, maybe I can add a little insight...
>
>(Snip of a long and thoughtful explanation . . .)
>
> That's probably enough to get some thinking going...
>
> -- john.
John,
You've given me plenty to think about.
While I can't argue with your structural analysis concerning the
orientation of the wood fibers in plywood, my experience has been that
plywood had less deflection when loaded "edgewise" than solid wood of
equal dimensions. I deduced (perhaps incorrectly) that the advantages
any grain defect being so thin and localized offset the obvious
disadvantage of half the layers being oriented in the wrong
direction. Or maybe I was simply wrong about it being stronger in the
first place. Good food for thought in any case.
I failed to mention one of the reasons I was considering building my
own beams. For esthetic reasons, we wanted the elevation of the
rafters to equal the facia board on the house, even though they won't
be attached to the facia. The house is slab-on-grade, with long eaves
yeilding a fairly low roofline. While I recognize the strength
advantage of a "tall" beam, headroom under it becomes an issue pretty
fast. So . . . I was looking to substitute some width (got plenty of
room for that) for some height. In other words, I wanted to make the
beam wider instead of taller, while realizing the diminishing returns
involved in doing so. I understand the tradeoffs, but I can't
calculate them. All the tables I can find assume loads far higher
than these beams will ever see. Maybe it's time to abandon wood and
go to a steel beam. We could wrap it in pressure treated wood to hide
it.
Thanks again for taking the time for such a detailed and thought-
provoking answer.
DonkeyHody
"In theory, theory and practice are the same, but in practice, they
are not."
> Pretty straight forward, you can't get there from here.
>
> The key issue is "Moment of Inertia".
>
> The formula to compute "Moment of Inertia" for a rectangle:
>
> 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.
>
> Perhaps an example:
I guess it's a little late in the conversation to admit that I have an
engineering degree. I understand the concepts well enough, but I
don't have the resources to calculate the amount of deflection I can
expect. Note that my load is lighter than usual and my amount of
allowable deflection is greater than usual. In this case, if the
deflection isn't noticable to a casual observer, it's acceptable,
since there's no drywall to crack or doors to bind etc. etc. I was
hoping somebody would chime in and say, "Oh, I've done that lots of
times. Make your beam 6 inches wide and you'll be fine."
I've about come to the conclusion that, while I CAN get there from
here, the young couple's budget won't support it. So we'll probably
just add a center post, which will simplify life considerably, even
though it's not an elegant solution.
DonkeyHody
"In theory, theory and practice are the same, but in practice, they
are not."
On Jul 3, 4:21 am, DonkeyHody <[email protected]> wrote:
> John,
> You've given me plenty to think about.
"Don" (I'm just just this moniker for convenience)...
Well, your further elaboration of the conditions sheds more light on
possible solutions (as does your engineering degree).
Height limitations always rear their ugly head in everyday design -
and, when that is all you have to work with, well, that is all you
have to work with. Shallower beams can be used - it just takes more
material (as you obviously know) to accomplish the same result - i.e.
less efficient, but then, as noted, that's life. (Lew is correct and
I mistated about the Ix varying as the cube of the depth).
You might consider a smallish (i.e. low slope) truss construction -
even a simple 'king-post' truss (i.e. 1 vertical strut connecting top
& bottom chord at mid-span) would probably work in this case. This
would provide considerable rigidity over a simple beam and even a very
low slope (say, 2 feet of height at mid-span) should work and would
lessen material needs - as well as getting the end "under" the fascia
(as I understand the situation). That may not fit with your aesthetic
desires, tho -- just have to look the situation over.
Just more thoughts.
> Maybe it's time to abandon wood and
> go to a steel beam. We could wrap it in pressure treated wood
> to hide it.
My gut tells me this would be overkill. Just remember with this new
ACQ pressure-treatment the corrosion issues with steel (over the older
(and now banned for residential) CCA treatment). These concerns (with
corrosion) are real.
-- john.
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.
On Jul 5, 2:49 pm, scott <[email protected]> wrote:
> 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.
>
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
> not quite ready to dismiss my original idea as "nutty".
>
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.
> 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."
Morris,
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...
http://www.apawood.org/level_c.cfm?content=pub_tch_libmain
You'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.
This got a little long - sorry,
-- john.
Morris 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?
<snip>
> || What am I missing?
Stephen wrote:
> |
> | 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.
Morris responded:
>
> 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...
>
> --
> Morris Dovey
> DeSoto Solar
> DeSoto, Iowa USAhttp://www.iedu.com/DeSoto/
"DonkeyHody" <[email protected]> wrote in message
news:[email protected]...
>
>
> Here's the question. How thick does the laminated beam need to
> be?
>
> DonkeyHody
> "Every man is my superior in that I can learn from him" - Thomas
> Carlyle
>
One idea no one has mentioned is to laminate the beam out of 2x4's laid
flat. Stack them as high as needed so you end up with a beam 3-1/2" wide by
how ever many 2x4's tall. Probably not cost effective, but possible. When
you get it done you could plan it flat and smooth, as long as you have the
tools and support to do it!
Greg
Sun, Jul 1, 2007, 5:33pm (EDT-3) [email protected] (DonkeyHody)
did query:
I just know somebody here has experience . . .
A friend wants an arbor over her patio <snip> One of the rafters will
carry a porch swing, <snip>
My plan is to buy 3/4 plywood intended for sub-floor and cut it into
strips 7 7/8" wide. These strips will glue and screw together with
Titebond II, and the screws will serve as clamps until the glue
dries.<snip>
I plan to protect the lamintate beam from the weather by enclosing it in
a 3-sided box of pressure-treated lumber.
Here's the question. =A0 How thick does the laminated beam need to
be?
Yep, got plenty of experience. Whatcha want to know about?
But if you put the swing there, and later find out that's a bad
place, it's gonna be a bitch moving it. I'd rather have a stand-alone
swng, just in case.
Why screws? You figuring on taking it apart later? I'd nail it.
How you planning on lifting those beams in place?
JOAT
If a man does his best, what else is there?
- General George S. Patton
My gut agrees. 1/2 the ply is going in the wrong direction to offer any
strength. Of course the middle doesn't do much anyway; it's the top an
bottom of the beam that are exposed to all of the force. 18' is a pretty
big span.
I too think you should check with your local lumber yard about pre-made
beams.
"Swingman" <[email protected]> wrote in message
news:[email protected]...
>
> "DonkeyHody" wrote in message
>> I just know somebody here has experience . . .
>
> You really want to check out the cost of an engineered beam like Anthony's
> GlueLam ... they may be cheaper in the long run than your materials and
> labor.
>
> These are similar to the ones I use:
>
> http://www.anthonyforest.com/stockglulam.shtml
>
> (and I just had an extra 18' one hauled off the other day (Habitat for
> Humanity))
>
> --
> www.e-woodshop.net
> Last update: 6/1/07
> KarlC@ (the obvious)
>
>
--
Posted via a free Usenet account from http://www.teranews.com
> 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.
-Steve
--
Posted via a free Usenet account from http://www.teranews.com
Lew Hodgett wrote:
| 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.
<examples snipped>
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?
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/
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...
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/
Lee Michaels wrote:
| "Morris Dovey" wrote
|| 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. :-)
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/
jcatora wrote:
<with much snippage - I've saved the original>
| 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.
<snip>
| 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.
Interesting! I've looked at SIP panels, but only within the context of
thermally-efficient home wall construction.
| The APA (American Plywood Association) has an extensive library of
| publications - all manner of things - that could lend insight into
| this -- see...
|
| http://www.apawood.org/level_c.cfm?content=pub_tch_libmain
I'll give 'em a visit and look for W605. I think this could turn out
to be a valuable resource. Thanks!
| Every once an awhile, leftover panels from projects show up in your
| 'shopper' mags (and, probably, online) - can occasionally find some
| pretty good deals.
I'll be watching - methinks these might be worth 'playing' with and
have broader application than I'd imagined...
| This got a little long - sorry,
I'm not. Your response is lucid, thought-provoking, and very much
appreciated!
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/
"Lee Michaels" <leemichaels*nadaspam*@comcast.net> wrote in message
news:[email protected]...
>
> "Morris Dovey" wrote
>>
>> 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.
>>
>
> 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.
>
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.
Morris Dovey wrote:
>
> 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?
Lateral stability, fire safety factor, lack of uniformity of
construction lumber, etc, etc.
Time to start thinking about metal studs rather than wood.
Lew
Well I knew a guy that did something like this. He took two 2X8's & routed
out the center of them & glued in a plywood lamination like you speak of.
Then glued up the whole thing so it looked like one beam. I can't remember
how thick he made them, but I think it was three pieces of plywood. Since
your going to do this anyway, why don't you glue up three and see how it
feels? Then you'll know if it feels right. Also I would use Titebond III,
it's a great glue for outdoors.
"DonkeyHody" <[email protected]> wrote in message
news:[email protected]...
>I just know somebody here has experience . . .
>
> A friend wants an arbor over her patio for vines to climb on. We
> envision an open structure with four posts supporting two main
> crossbeams. These crossbeams would support "rafters" on two foot
> centers. One of the rafters will carry a porch swing, but that rafter
> is near the posts, not near the center of the span. No other
> significant loads other than the weight of the vines that will
> eventually cover the rafters. Main crossbeams would be 20 feet long,
> with 18 feet of clear span between the posts. Rafters will be 12 feet
> long, with 8 feet of clear span and 2 feet of overhang on each end.
>
> My plan is to buy 3/4 plywood intended for sub-floor and cut it into
> strips 7 7/8" wide. These strips will glue and screw together with
> Titebond II, and the screws will serve as clamps until the glue
> dries. I'll stagger the joints by using 2, 4, 6 and 8 foot strips.
>
> I plan to protect the lamintate beam from the weather by enclosing it
> in a 3-sided box of pressure-treated lumber.
>
> Here's the question. How thick does the laminated beam need to
> be?
>
> DonkeyHody
> "Every man is my superior in that I can learn from him" - Thomas
> Carlyle
>
Robatoy wrote:
> On Jul 1, 8:33 pm, DonkeyHody <[email protected]> wrote:
>> I just know somebody here has experience . . .
>>
>
>> My plan is to buy 3/4 plywood intended for sub-floor and cut it into
>> strips 7 7/8" wide. These strips will glue and screw together with
>> Titebond II, and the screws will serve as clamps until the glue
>> dries. I'll stagger the joints by using 2, 4, 6 and 8 foot strips.
>>
>
> I'd be more inclined to use WEST System epoxy and clamps...forget
> about screws.
>
> I'm sure Lew can add to my suggestion.
I would not touch the above with a fork.
Even with epoxy, plywood is a lousy choice for this application.
With an 18 ft unsupported span, I'd sister a couple of 2x12 Doug Fir
timbers together with epoxy and clamps.
Faster, cheaper, better.
3 for 3 isn't bad.
Lew
"Morris Dovey" wrote
>
> 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.
>
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.
DonkeyHody wrote:
> advantage of a "tall" beam, headroom under it becomes an issue pretty
> fast. So . . . I was looking to substitute some width (got plenty of
> room for that) for some height. In other words, I wanted to make the
> beam wider instead of taller, while realizing the diminishing returns
> involved in doing so.
Pretty straight forward, you can't get there from here.
The key issue is "Moment of Inertia".
The formula to compute "Moment of Inertia" for a rectangle:
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.
Perhaps an example:
Assume: b=1, h=1
I=(1)*(1^3)/12=1/12
Next assume: b=2, h=1
I=(2)*(1^3)/12=2/12
Finally assume: b=1, h=2
I=(1)*(2^3)/12=8/12
Tall skinny beams win every time.
Lew
> A friend wants an arbor over her patio for vines to climb on. We
> envision an open structure with four posts supporting two main
> crossbeams. These crossbeams would support "rafters" on two foot
> centers. One of the rafters will carry a porch swing, but that rafter
> is near the posts, not near the center of the span. No other
> significant loads other than the weight of the vines that will
> eventually cover the rafters. Main crossbeams would be 20 feet long,
> with 18 feet of clear span between the posts. Rafters will be 12 feet
> long, with 8 feet of clear span and 2 feet of overhang on each end.
If I'm understanding correctly, your patio roof will be 18 feet long by
16 feet wide (not counting overhang on the ends), with two beams and four
posts?
Typically, you would calculate the loads using 40 lbs per square foot,
but since this is an open structure, I'd probably just use 10 lbs per
foot for the dead weight of the materials (Keep in mind this doesn't
account for any snow loads, weight of the vines, etc.).
So you would be looking at roughly 2880 pounds on the roof structure
(18x16 x 10psf).
Half of that weight (1440 pounds) is carried by each beam.
According to the charts I have, you would need a MINIMUM 4x10 beam to
support that weight over an 18 foot span. If it were me, I'd go with a
4x12 beam. 18 feet is a long distance to free span.
If possible, I would span the beams across the 16 foot distance, rather
than the 18 foot distance. This would let you use 4x8 beams, though I'd
still opt for the next size up, 4x10 beams.
Of course, adding additional beams and posts would make a huge difference
in reducing the spans and the corresponding beam sizes.
> My plan is to buy 3/4 plywood intended for sub-floor and cut it into
> strips 7 7/8" wide. These strips will glue and screw together with
> Titebond II, and the screws will serve as clamps until the glue
> dries. I'll stagger the joints by using 2, 4, 6 and 8 foot strips.
> I plan to protect the lamintate beam from the weather by enclosing it
> in a 3-sided box of pressure-treated lumber.
Sounds expensive and a lot of work. In addition, for outdoor projects you
really want as few joints and seams as possible. I'd go with solid beams,
or a couple of 2x10's nailed together as a bare minimum. Plywood isn't
really a good material for outdoor use.
20 foot long 2x12's would be fairly easy to set in place by yourself,
though it would help if you could get another person to help balance
things while you nail things together.
I'd also recommend metal post caps or hurricane straps to hold everything
together. You don't want the whole structure coming down in a strong wind
or earthquake. And, you'll need to sink the posts deeply in the ground
and/or install some kind of diagonal lateral bracing to keep the whole
structure from toppling over.
You might want to pick up a book on outdoor roof structures. I have one
by Sunset books called "Patio Roofs and Gazebo's" that is pretty good
(ISBN 0-376-01440-7).
Have fun!
Anthony
"Greg O" <[email protected]> wrote in
news:[email protected]:
> "DonkeyHody" <[email protected]> wrote in message
> news:[email protected]...
>>
>>
>> Here's the question. How thick does the laminated beam need to
>> be?
>>
>> DonkeyHody
>> "Every man is my superior in that I can learn from him" - Thomas
>> Carlyle
>>
>
> One idea no one has mentioned is to laminate the beam out of 2x4's
> laid flat. Stack them as high as needed so you end up with a beam
> 3-1/2" wide by how ever many 2x4's tall. Probably not cost effective,
> but possible. When you get it done you could plan it flat and smooth,
> as long as you have the tools and support to do it!
> Greg
>
>
Have you noticed that the 2x4s you can buy are generally of lower quality
portions of the tree than the 2x8, 2x10 and 2x12?
Yes, you could save a buck or two, maybe. I'd look elsewhere, though.
Patriarch
"DonkeyHody" wrote in message
> I just know somebody here has experience . . .
You really want to check out the cost of an engineered beam like Anthony's
GlueLam ... they may be cheaper in the long run than your materials and
labor.
These are similar to the ones I use:
http://www.anthonyforest.com/stockglulam.shtml
(and I just had an extra 18' one hauled off the other day (Habitat for
Humanity))
--
www.e-woodshop.net
Last update: 6/1/07
KarlC@ (the obvious)
Why not buy glulams already made up. That's a pretty
wide "clear span (18') and you need to be solid for a
porch swing. You can always "dress" the glulams with
a better looking wood.
http://www.anthonyforest.com/stockglulam.shtml
DonkeyHody wrote:
> I just know somebody here has experience . . .
>
> A friend wants an arbor over her patio for vines to climb on. We
> envision an open structure with four posts supporting two main
> crossbeams. These crossbeams would support "rafters" on two foot
> centers. One of the rafters will carry a porch swing, but that rafter
> is near the posts, not near the center of the span. No other
> significant loads other than the weight of the vines that will
> eventually cover the rafters. Main crossbeams would be 20 feet long,
> with 18 feet of clear span between the posts. Rafters will be 12 feet
> long, with 8 feet of clear span and 2 feet of overhang on each end.
>
> My plan is to buy 3/4 plywood intended for sub-floor and cut it into
> strips 7 7/8" wide. These strips will glue and screw together with
> Titebond II, and the screws will serve as clamps until the glue
> dries. I'll stagger the joints by using 2, 4, 6 and 8 foot strips.
>
> I plan to protect the lamintate beam from the weather by enclosing it
> in a 3-sided box of pressure-treated lumber.
>
> Here's the question. How thick does the laminated beam need to
> be?
>
> DonkeyHody
> "Every man is my superior in that I can learn from him" - Thomas
> Carlyle
>
DonkeyHody wrote:
> I guess it's a little late in the conversation to admit that I have an
> engineering degree. I understand the concepts well enough, but I
> don't have the resources to calculate the amount of deflection I can
> expect.
Beam deflection calculations are a bitch.
Never bothered making them.
Just de-rated the max allowable stress by a factor of 4 and got on
with life when doing bending stress calculations.
Lew
DonkeyHody wrote:
> Thanks to all for the advice. I'm re-thinking the plywood idea. The
> reason I considered laminates in the first place is that 20 foot
> lumber in any size bigger than 2X6 is pretty scarce around here. I'll
> price a factory glue-lam.
SFWIW, right after WWII, materials were still difficult to get and
expensive.
Some how, my dad managed to collect some kind of pallet that used 4x4
runners, about 12 ft long, that were probably "Ironwood" or something
similar that required drilling pilot holes for every nail.
He used those runners for posts, then ran #9 galvanized baling wire
overhead to provide a place for the vines.
That puppy lasted at least 20 years.
Might be an idea or two in there that you might find useful.
Lew