I am building a 6 foot long shelf that is going to be supported at the ends
(no option of support in the middle). The shelf needs to support about 50-
70 pounds of weight (stereo equipment), so the "sag" (or deflection) in the
center is going to be a problem. I plan on building the shelf as a hollow
box, and using steel angle iron on the inside as a stiffener.
Just out of curiosity -- what commonly available wood product is best for
resisting deflection under a continuous load (i.e. shelving)? The easily
available ones at my local lumber store are: laminated pine, mdf, playwood
and particle board, I am assuming that the laminated pine would be best,
since all the grain is running longitudinally. Is my assumption correct?
--
Murray Peterson
"Murray Peterson" <[email protected]> wrote in message
news:[email protected]...
> "todd" <[email protected]> wrote in
> news:[email protected]:
>
>> For the sheet goods, on average, plywood would be the best, then
>> particle board, then MDF for initial sag.
>>
>> I'm a bit unclear on how you're going to build the shelf as a hollow
>> box. If you're going to go to that much trouble, and then include a
>> steel angle, I'd suggest just doubling up on the wood for the shelf.
>> Two of the 3/4" glued-up pine boards would deflect only about 3/32 at
>> the middle under a distributed 70 pound load (assuming a shelf depth
>> of 12").
>>
>> You can get an estimate of sag using the Sagulator at
>> http://www.woodworkersweb.com/sagulator.htm.
>
> Are the sagulator numbers good for long-term loading, or just initial
> deflection? My experience with purchased bookshelves has been poor --
> shelves that progressively sag more as the years go by.
>
> BTW, the loading is not distributed -- almost all of the weight is pretty
> close to centered.
>
> --
> Murray Peterson
>
From the sagulator: "The Sagulator computes initial sag only. As an
engineering rule of thumb, wood beams/shelves will sag an additional 50%
over time beyond the initial deflection induced by the load."
--
Stoutman
http://home.triad.rr.com/brianmelissa/woodworking_frames.htm
(Featuring a NEW look)
You can add an enormous amount of support strength to your shelving by
adding a cleat underneath the front edge of a shelf. This is usually a 3/4
x 1" plywood strip, glued (and preferebly screwed as well) about 1/2" back
from the front edge of the shelf and running the width of the shelf... also
gives a classy look to the shelf.
"Murray Peterson" <[email protected]> wrote in message
news:[email protected]...
>I am building a 6 foot long shelf that is going to be supported at the ends
> (no option of support in the middle). The shelf needs to support about
> 50-
> 70 pounds of weight (stereo equipment), so the "sag" (or deflection) in
> the
> center is going to be a problem. I plan on building the shelf as a hollow
> box, and using steel angle iron on the inside as a stiffener.
>
> Just out of curiosity -- what commonly available wood product is best for
> resisting deflection under a continuous load (i.e. shelving)? The easily
> available ones at my local lumber store are: laminated pine, mdf, playwood
> and particle board, I am assuming that the laminated pine would be best,
> since all the grain is running longitudinally. Is my assumption correct?
>
> --
> Murray Peterson
>
In article <[email protected]>,
Joe Barta <[email protected]> wrote:
>Mike Marlow wrote:
>
>> Serious overkill Murray. The 1/2 inch would weigh a ton and you
>> won't need the strength that it will bring.
>
>I can't imagine 1/2" angle iron bringing much strength at all over 6
>ft... and not that much weight either. Or maybe we're thinking
>different things. I'm thinking about 1/8" thick steel angle with each
>leg being 1/2" wide. Am I missing something??
Yeah. other folks thought you meant 1/2" _thick_ steel -- a continuous
piece running the length of the shelf.
Joe Barta wrote:
> I can't imagine 1/2" angle iron bringing much strength at all over 6
> ft... and not that much weight either.
The angle iron has a modulus of elasticity that is about 15 times that
of wood.
That 1/2" angle iron gives about as much strength as a 1/2"x2" lip of
oak under the shelf.
Of course, if you put the 2" dimention vertically under the shelf, the
lip is about 16x stronger.
For those who care about the math, the deflection of a beam is inversely
proportional to the width of the rectangular cross section. However,
it is inversely proportional to the *cube* of the height of the cross
section.
Chris
todd wrote:
> Unfortunately, it has a moment of inertia (even assuming there are 2 of them
> as the poster suggested) about 350 times less than the shelf.
Oh, certainly. I should have maybe qualified that statement.
>>That 1/2" angle iron gives about as much strength as a 1/2"x2" lip of
>>oak under the shelf.
> i.e. not very much.
Yep.
>>Of course, if you put the 2" dimention vertically under the shelf, the
>>lip is about 16x stronger.
>
> Of course, if the lip is made of tungsten carbide, it would be 30x stronger.
True. But I think my suggestion is more practical. <grin>
> It's also directly proportional to the cube of the length. If the OP could
> reduce the width just 6", the deflection would drop about 25%.
Yep.
It was kind of fun digging up my old statics references--I trained as an
engineer, but I've been doing software for the past 6 years.
Chris
"Murray Peterson" <[email protected]> wrote in message
news:[email protected]...
> Joe Barta <[email protected]> wrote in news:Xns9745D6430F2CAjbartaapknet@
> 207.115.17.102:
>
> > You could make the shelves a torsion box. Think hollow core door.
> > Lightweight and very strong. 1/4" ply top and bottom, hardwood edging
> > and pine innards. No steel needed.
>
> That's pretty much what I had in mind, but with some 1/2 inch steel angle-
> iron screwed to the inside of the front and back edges for increased
> stiffness. Sounds like my design may be overkill for the application.
>
Serious overkill Murray. The 1/2 inch would weigh a ton and you won't need
the strength that it will bring.
--
-Mike-
[email protected]
"Joe Barta" <[email protected]> wrote in message
news:[email protected]...
> Murray Peterson wrote:
>
> > "todd" <[email protected]> wrote in
> > news:[email protected]:
> >
> >> For the sheet goods, on average, plywood would be the best, then
> >> particle board, then MDF for initial sag.
> >>
> >> I'm a bit unclear on how you're going to build the shelf as a
> >> hollow box. If you're going to go to that much trouble, and then
> >> include a steel angle, I'd suggest just doubling up on the wood
> >> for the shelf. Two of the 3/4" glued-up pine boards would deflect
> >> only about 3/32 at the middle under a distributed 70 pound load
> >> (assuming a shelf depth of 12").
> >>
> >> You can get an estimate of sag using the Sagulator at
> >> http://www.woodworkersweb.com/sagulator.htm.
> >
> > Are the sagulator numbers good for long-term loading, or just
> > initial deflection? My experience with purchased bookshelves has
> > been poor -- shelves that progressively sag more as the years go
> > by.
> >
> > BTW, the loading is not distributed -- almost all of the weight is
> > pretty close to centered.
> >
>
> I'm no engineer... more of a shoot from the hip from experience type.
> I think a torsion box with 1/4" glued on ply top and bottom with a
> total thickness of 1-3/4" should be strong enough for even the
> heaviest stereo equipment.
>
> I think the "sagulator" is just a guide and it has no use in
> calculating the strength of engineered assemblies... which is what a
> torsion box would be.
>
> Joe Barta
We're in luck. I'm a mechanical engineer who specialized in materials
engineering. I wasn't attempting to calculate the strength of a torsion
box. I'm sure it would work. But for the original poster, I'm just
imagining that a couple of 3/4" thick boards that he can buy off the shelf
and laminate together would be easier to deal with and would also be very
effective.
Of course the sagulator is just a guide. For one, it doesn't take into
account end conditions, which can have an effect on deflection. It also
makes assumptions regarding the bulk properties of the material that may or
may not be true. And it can't do stuff like figure the deflection if a
thicker piece is applied to the front of the shelf to add rigidity.
todd
"Joe Barta" <[email protected]> wrote in message
news:[email protected]...
> Mike Marlow wrote:
>
> > Serious overkill Murray. The 1/2 inch would weigh a ton and you
> > won't need the strength that it will bring.
>
> I can't imagine 1/2" angle iron bringing much strength at all over 6
> ft... and not that much weight either. Or maybe we're thinking
> different things. I'm thinking about 1/8" thick steel angle with each
> leg being 1/2" wide. Am I missing something??
>
> Joe Barta
D'OH!!!! Color me the same color as a brain fart! I'm thinking 1/2" thick
steel - and wondering why anyone would want to do such a thing. Some days
are just really bad...
--
-Mike-
[email protected]
Murray Peterson wrote:
>> You can get an estimate of sag using the Sagulator at
>> http://www.woodworkersweb.com/sagulator.htm.
>
> Are the sagulator numbers good for long-term loading, or just initial
> deflection? My experience with purchased bookshelves has been poor --
> shelves that progressively sag more as the years go by.
Turn 'em over once in a while.
> BTW, the loading is not distributed -- almost all of the weight is
> pretty close to centered.
*That* makes a difference.
--
dadiOH
____________________________
dadiOH's dandies v3.06...
...a help file of info about MP3s, recording from
LP/cassette and tips & tricks on this and that.
Get it at http://mysite.verizon.net/xico
"Murray Peterson" <[email protected]> wrote in message
news:[email protected]...
>I am building a 6 foot long shelf that is going to be supported at the ends
> (no option of support in the middle). The shelf needs to support about
> 50-
> 70 pounds of weight (stereo equipment), so the "sag" (or deflection) in
> the
> center is going to be a problem. I plan on building the shelf as a hollow
> box, and using steel angle iron on the inside as a stiffener.
>
> Just out of curiosity -- what commonly available wood product is best for
> resisting deflection under a continuous load (i.e. shelving)? The easily
> available ones at my local lumber store are: laminated pine, mdf, playwood
> and particle board, I am assuming that the laminated pine would be best,
> since all the grain is running longitudinally. Is my assumption correct?
>
> --
> Murray Peterson
Check out the sagulator shelf deflection calculator. Might be useful.
http://www.woodbin.com/calcs/sagulator.htm
--
Stoutman
http://home.triad.rr.com/brianmelissa/woodworking_frames.htm
(Featuring a NEW look)
Murray Peterson wrote:
> I am building a 6 foot long shelf that is going to be supported at the ends
> (no option of support in the middle). The shelf needs to support about 50-
> 70 pounds of weight (stereo equipment), so the "sag" (or deflection) in the
> center is going to be a problem. I plan on building the shelf as a hollow
> box, and using steel angle iron on the inside as a stiffener.
>
> Just out of curiosity -- what commonly available wood product is best for
> resisting deflection under a continuous load (i.e. shelving)? The easily
> available ones at my local lumber store are: laminated pine, mdf, playwood
> and particle board, I am assuming that the laminated pine would be best,
> since all the grain is running longitudinally. Is my assumption correct?
>
If you are interested, can tell you how to build this shelf using foam,
knitted fiber glass and epoxy along with the basic wooden shelf that is
say 3/4" thick.
It would probably support at leat 200 lbs, weigh maybe 10 lbs max.
Contact me off list if interested.
Lew
"todd" <[email protected]> wrote in
news:[email protected]:
> For the sheet goods, on average, plywood would be the best, then
> particle board, then MDF for initial sag.
>
> I'm a bit unclear on how you're going to build the shelf as a hollow
> box. If you're going to go to that much trouble, and then include a
> steel angle, I'd suggest just doubling up on the wood for the shelf.
> Two of the 3/4" glued-up pine boards would deflect only about 3/32 at
> the middle under a distributed 70 pound load (assuming a shelf depth
> of 12").
>
> You can get an estimate of sag using the Sagulator at
> http://www.woodworkersweb.com/sagulator.htm.
Are the sagulator numbers good for long-term loading, or just initial
deflection? My experience with purchased bookshelves has been poor --
shelves that progressively sag more as the years go by.
BTW, the loading is not distributed -- almost all of the weight is pretty
close to centered.
--
Murray Peterson
Joe Barta <[email protected]> wrote in news:Xns9745D6430F2CAjbartaapknet@
207.115.17.102:
> You could make the shelves a torsion box. Think hollow core door.
> Lightweight and very strong. 1/4" ply top and bottom, hardwood edging
> and pine innards. No steel needed.
That's pretty much what I had in mind, but with some 1/2 inch steel angle-
iron screwed to the inside of the front and back edges for increased
stiffness. Sounds like my design may be overkill for the application.
--
Murray Peterson
Murray Peterson wrote:
> I am building a 6 foot long shelf that is going to be supported at
> the ends (no option of support in the middle). The shelf needs to
> support about 50- 70 pounds of weight (stereo equipment), so the
> "sag" (or deflection) in the center is going to be a problem. I
> plan on building the shelf as a hollow box, and using steel angle
> iron on the inside as a stiffener.
>
> Just out of curiosity -- what commonly available wood product is
> best for resisting deflection under a continuous load (i.e.
> shelving)? The easily available ones at my local lumber store
> are: laminated pine, mdf, playwood and particle board, I am
> assuming that the laminated pine would be best, since all the
> grain is running longitudinally. Is my assumption correct?
>
You could make the shelves a torsion box. Think hollow core door.
Lightweight and very strong. 1/4" ply top and bottom, hardwood edging
and pine innards. No steel needed.
Joe Barta
Murray Peterson wrote:
> Joe Barta <[email protected]> wrote in
> news:Xns9745D6430F2CAjbartaapknet@ 207.115.17.102:
>
>> You could make the shelves a torsion box. Think hollow core door.
>> Lightweight and very strong. 1/4" ply top and bottom, hardwood
>> edging and pine innards. No steel needed.
>
> That's pretty much what I had in mind, but with some 1/2 inch
> steel angle- iron screwed to the inside of the front and back
> edges for increased stiffness. Sounds like my design may be
> overkill for the application.
>
Over 6 feet, 1/2" steel angle ain't gonna do squat. You'd get far more
bang for your buck increasing the thickness of the torsion box.
Murray Peterson wrote:
> "todd" <[email protected]> wrote in
> news:[email protected]:
>
>> For the sheet goods, on average, plywood would be the best, then
>> particle board, then MDF for initial sag.
>>
>> I'm a bit unclear on how you're going to build the shelf as a
>> hollow box. If you're going to go to that much trouble, and then
>> include a steel angle, I'd suggest just doubling up on the wood
>> for the shelf. Two of the 3/4" glued-up pine boards would deflect
>> only about 3/32 at the middle under a distributed 70 pound load
>> (assuming a shelf depth of 12").
>>
>> You can get an estimate of sag using the Sagulator at
>> http://www.woodworkersweb.com/sagulator.htm.
>
> Are the sagulator numbers good for long-term loading, or just
> initial deflection? My experience with purchased bookshelves has
> been poor -- shelves that progressively sag more as the years go
> by.
>
> BTW, the loading is not distributed -- almost all of the weight is
> pretty close to centered.
>
I'm no engineer... more of a shoot from the hip from experience type.
I think a torsion box with 1/4" glued on ply top and bottom with a
total thickness of 1-3/4" should be strong enough for even the
heaviest stereo equipment.
I think the "sagulator" is just a guide and it has no use in
calculating the strength of engineered assemblies... which is what a
torsion box would be.
Joe Barta
todd wrote:
> We're in luck. I'm a mechanical engineer who specialized in
> materials engineering.
I have a couple questions for you then. Something I always wondered.
Imagine a torsion box, well made with 1/4" ply top and bottom and 1-
1/2" thick. Let's say it's for a shelf 6ft long and 11-1/4" wide. Now
imagine a solid plank, 2x12 construction lumber (not fir). Would you
think the torsion box would be stiffer or would the solid plank be
stiffer?
I would imagine that if one used oak instead of spf, the oak plank
would be stronger than both.
Also, would I be correct in thinking that 2 pieces of 1x12 laminated
together would be stiffer than a solid 2x12? If so, why? (other than
minimizing and spreading out defects, which is pretty obvious)
I know I could probably do some homework and look this stuff up... but
you're here, right?
Mike Marlow wrote:
> Serious overkill Murray. The 1/2 inch would weigh a ton and you
> won't need the strength that it will bring.
I can't imagine 1/2" angle iron bringing much strength at all over 6
ft... and not that much weight either. Or maybe we're thinking
different things. I'm thinking about 1/8" thick steel angle with each
leg being 1/2" wide. Am I missing something??
Joe Barta
Robert Bonomi wrote:
> In article <[email protected]>,
> Joe Barta <[email protected]> wrote:
>>Mike Marlow wrote:
>>
>>> Serious overkill Murray. The 1/2 inch would weigh a ton and you
>>> won't need the strength that it will bring.
>>
>>I can't imagine 1/2" angle iron bringing much strength at all over
>>6 ft... and not that much weight either. Or maybe we're thinking
>>different things. I'm thinking about 1/8" thick steel angle with
>>each leg being 1/2" wide. Am I missing something??
>
> Yeah. other folks thought you meant 1/2" _thick_ steel -- a
> continuous piece running the length of the shelf.
>
1/2" thick angle iron... in a shelf? I can't visualize that. What are
the length of the legs and how would it be oriented?
Chris Friesen <[email protected]> wrote in news:11s62ile2t3sqb2
@corp.supernews.com:
> The angle iron has a modulus of elasticity that is about 15 times that
> of wood.
>
> That 1/2" angle iron gives about as much strength as a 1/2"x2" lip of
> oak under the shelf.
Thanks -- those numbers help me a lot. I wanted to create a shelf with
quite a thin cross-section (1.2 inches at most), but then ran into problems
with deflection.
Sounds like the interior piece of iron could be enough to get me what I
need.
I'll put the thing together sometime in the next week (if work permits).
I'll post back and let everyone know how it worked (or didn't).
Thanks to all for their helpful responses. Learning new things is always
fun, especially if I can see the results hanging on my wall :-)
--
Murray Peterson
"Chris Friesen" <[email protected]> wrote in message
news:[email protected]...
> Joe Barta wrote:
>
> > I can't imagine 1/2" angle iron bringing much strength at all over 6
> > ft... and not that much weight either.
>
> The angle iron has a modulus of elasticity that is about 15 times that
> of wood.
Unfortunately, it has a moment of inertia (even assuming there are 2 of them
as the poster suggested) about 350 times less than the shelf. In the end,
unless 1/2" angle iron is a lot thicker than I think, it doesn't end up
contributing much to the combined moment of inertia, which means it doesn't
reduce deflection in a significant way.
> That 1/2" angle iron gives about as much strength as a 1/2"x2" lip of
> oak under the shelf.
i.e. not very much.
> Of course, if you put the 2" dimention vertically under the shelf, the
> lip is about 16x stronger.
Of course, if the lip is made of tungsten carbide, it would be 30x stronger.
> For those who care about the math, the deflection of a beam is inversely
> proportional to the width of the rectangular cross section. However,
> it is inversely proportional to the *cube* of the height of the cross
> section.
It's also directly proportional to the cube of the length. If the OP could
reduce the width just 6", the deflection would drop about 25%.
> Chris
todd
"Joe Barta" <[email protected]> wrote in message
news:[email protected]...
> todd wrote:
>
> > We're in luck. I'm a mechanical engineer who specialized in
> > materials engineering.
>
> I have a couple questions for you then. Something I always wondered.
>
> Imagine a torsion box, well made with 1/4" ply top and bottom and 1-
> 1/2" thick. Let's say it's for a shelf 6ft long and 11-1/4" wide. Now
> imagine a solid plank, 2x12 construction lumber (not fir). Would you
> think the torsion box would be stiffer or would the solid plank be
> stiffer?
>
> I would imagine that if one used oak instead of spf, the oak plank
> would be stronger than both.
>
> Also, would I be correct in thinking that 2 pieces of 1x12 laminated
> together would be stiffer than a solid 2x12? If so, why? (other than
> minimizing and spreading out defects, which is pretty obvious)
>
> I know I could probably do some homework and look this stuff up... but
> you're here, right?
Tell me about the construction of the insides of the torsion box. The
makeup of the longitudinal pieces will have the greatest effect on
stiffness. I imagine it will be 3/4x1 SPF of some kind.
todd
Murray Peterson wrote:
> Joe Barta <[email protected]> wrote in news:Xns9745D6430F2CAjbartaapknet@
> 207.115.17.102:
>
>> You could make the shelves a torsion box. Think hollow core door.
>> Lightweight and very strong. 1/4" ply top and bottom, hardwood edging
>> and pine innards. No steel needed.
>
> That's pretty much what I had in mind, but with some 1/2 inch steel
> angle- iron screwed to the inside of the front and back edges for
> increased stiffness. Sounds like my design may be overkill for the
> application.
Yep :)
--
dadiOH
____________________________
dadiOH's dandies v3.06...
...a help file of info about MP3s, recording from
LP/cassette and tips & tricks on this and that.
Get it at http://mysite.verizon.net/xico
On Mon, 09 Jan 2006 01:38:05 GMT, Murray Peterson
<[email protected]> wrote:
>I am building a 6 foot long shelf that is going to be supported at the ends
>(no option of support in the middle). The shelf needs to support about 50-
>70 pounds of weight (stereo equipment), so the "sag" (or deflection) in the
>center is going to be a problem. I plan on building the shelf as a hollow
>box, and using steel angle iron on the inside as a stiffener.
>
>Just out of curiosity -- what commonly available wood product is best for
>resisting deflection under a continuous load (i.e. shelving)? The easily
>available ones at my local lumber store are: laminated pine, mdf, playwood
>and particle board, I am assuming that the laminated pine would be best,
>since all the grain is running longitudinally. Is my assumption correct?
Ply or solid wood deflects the least. You might consider a 2" lip on
front and in the back or a torsion box to help. Six feet is a lot.
"Murray Peterson" <[email protected]> wrote in message
news:[email protected]...
> I am building a 6 foot long shelf that is going to be supported at the
ends
> (no option of support in the middle). The shelf needs to support about
50-
> 70 pounds of weight (stereo equipment), so the "sag" (or deflection) in
the
> center is going to be a problem. I plan on building the shelf as a hollow
> box, and using steel angle iron on the inside as a stiffener.
>
> Just out of curiosity -- what commonly available wood product is best for
> resisting deflection under a continuous load (i.e. shelving)? The easily
> available ones at my local lumber store are: laminated pine, mdf, playwood
> and particle board, I am assuming that the laminated pine would be best,
> since all the grain is running longitudinally. Is my assumption correct?
>
> --
> Murray Peterson
For the sheet goods, on average, plywood would be the best, then particle
board, then MDF for initial sag.
I'm a bit unclear on how you're going to build the shelf as a hollow box.
If you're going to go to that much trouble, and then include a steel angle,
I'd suggest just doubling up on the wood for the shelf. Two of the 3/4"
glued-up pine boards would deflect only about 3/32 at the middle under a
distributed 70 pound load (assuming a shelf depth of 12").
You can get an estimate of sag using the Sagulator at
http://www.woodworkersweb.com/sagulator.htm.
todd
"Joe Barta" <[email protected]> wrote in message
news:[email protected]...
> todd wrote:
>
>> We're in luck. I'm a mechanical engineer who specialized in
>> materials engineering.
>
> I have a couple questions for you then. Something I always wondered.
>
> Imagine a torsion box, well made with 1/4" ply top and bottom and 1-
> 1/2" thick. Let's say it's for a shelf 6ft long and 11-1/4" wide. Now
> imagine a solid plank, 2x12 construction lumber (not fir). Would you
> think the torsion box would be stiffer or would the solid plank be
> stiffer?
Depends on what sort of plywood it is.
Likely to be close. But make the box 2" thick and it will win. 3" and the
plank has no chance.
> I would imagine that if one used oak instead of spf, the oak plank
> would be stronger than both.
>
> Also, would I be correct in thinking that 2 pieces of 1x12 laminated
> together would be stiffer than a solid 2x12? If so, why? (other than
> minimizing and spreading out defects, which is pretty obvious)
No, it would be the same unless you had trouble finding 2x without splits
and things.
j