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  #21   Report Post  
Old February 12th 20, 05:40 AM posted to rec.woodworking
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Default Wide shelving advice needed

On Tue, 11 Feb 2020 22:10:03 -0500, Clare Snyder
wrote:

On Tue, 11 Feb 2020 14:14:19 -0600, Markem
wrote:

On Tue, 11 Feb 2020 14:41:14 -0500, Clare Snyder
wrote:

On Tue, 11 Feb 2020 12:46:38 -0600, Leon [email protected]
wrote:

On 2/11/2020 8:39 AM, Scott Lurndal wrote:
whit3rd writes:
On Monday, February 10, 2020 at 3:21:15 PM UTC-8, Ivan Vegvary wrote:
Tearing out sagging shelves in a 10ft. by 6ft. pantry. Would jike to put 18" deep shelves on the back wall. These would not be "cabinet" quality construction.
So, rip 18" wide plywood? Particle board? Bisquit join solid lumber?

Solid is the strongest, and you have the option to put a pretty wood on the leading edge.
Knotty softwood is the inexpensive way to go (vinyl surface
can be applied so the occasional leaky can won't hurt the wood). Biscuits are good.

Plywood
is weaker than solid wood (half the grain runs the wrong way).

Scratching head...



Which actually makes it stronger.



Stronger and dimensionally stable. Much, much, much less likely to
split along the grain

Or "cup". I like Baltic plywood - higher ply count - and hardwood
(generally birch) all the way through. More expensive too.

I really like my bifold door shelves. They are a box structure - VERY
stiff. Light weight too. Most of mine are Luan/mahogany faced but
hardboard faced works just as well - they have a cardboard honeycomb
in them to help make them even more rigid.


Finding a ten foot long door might be a bit pricy though


Make 1 out of 2 short ones - not easy but doable


They make good shelves as long as you do not hit hard with the edge of
a can.

  #22   Report Post  
Old February 12th 20, 07:56 AM posted to rec.woodworking
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Posts: 3,129
Default Wide shelving advice needed

On Tue, 11 Feb 2020 22:40:30 -0600, Markem
wrote:

On Tue, 11 Feb 2020 22:10:03 -0500, Clare Snyder
wrote:

On Tue, 11 Feb 2020 14:14:19 -0600, Markem
wrote:

On Tue, 11 Feb 2020 14:41:14 -0500, Clare Snyder
wrote:

On Tue, 11 Feb 2020 12:46:38 -0600, Leon [email protected]
wrote:

On 2/11/2020 8:39 AM, Scott Lurndal wrote:
whit3rd writes:
On Monday, February 10, 2020 at 3:21:15 PM UTC-8, Ivan Vegvary wrote:
Tearing out sagging shelves in a 10ft. by 6ft. pantry. Would jike to put 18" deep shelves on the back wall. These would not be "cabinet" quality construction.
So, rip 18" wide plywood? Particle board? Bisquit join solid lumber?

Solid is the strongest, and you have the option to put a pretty wood on the leading edge.
Knotty softwood is the inexpensive way to go (vinyl surface
can be applied so the occasional leaky can won't hurt the wood). Biscuits are good.

Plywood
is weaker than solid wood (half the grain runs the wrong way).

Scratching head...



Which actually makes it stronger.



Stronger and dimensionally stable. Much, much, much less likely to
split along the grain

Or "cup". I like Baltic plywood - higher ply count - and hardwood
(generally birch) all the way through. More expensive too.

I really like my bifold door shelves. They are a box structure - VERY
stiff. Light weight too. Most of mine are Luan/mahogany faced but
hardboard faced works just as well - they have a cardboard honeycomb
in them to help make them even more rigid.

Finding a ten foot long door might be a bit pricy though


Make 1 out of 2 short ones - not easy but doable


They make good shelves as long as you do not hit hard with the edge of
a can.

All of my garage/shop shelves are doors - in use fie well over 10
yeaers - holding tools, supplies, car parts, etc and I have yet to
damage one. I'm not the bull in a chinashop type, but I do USE my
stuff. As $2 each from the local ReStore they were the cheapest I
could come up with. I used them to build the base for the building
table for buildingthe plane too. It was EXTREMELY rigid and light
weight at the same time We used 2 layers of 3/4" MDF glued together
for the top surface - 4X16 feet
  #23   Report Post  
Old February 12th 20, 07:18 PM posted to rec.woodworking
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Posts: 891
Default Wide shelving advice needed

On 2/11/2020 8:24 PM, dpb wrote:
On 2/11/2020 8:00 PM, DerbyDad03 wrote:
On Tuesday, February 11, 2020 at 6:46:01 PM UTC-5, dpb wrote:

....

....[snip for brevity]...

Sagulator gave 0.11" for "Plywood, fir" and while Doug fir wasn't one of
the firs it gives specifically, they all were less (altho not by factor
of 2 which does seem somewhat excessive by common experience).* It
doesn't have any other plywood to compare against.

And just about any kind of edging on just about any kind of wood makes
all those specs moot. ;-)


No, it does not.

All it does is change the geometry somewhat but the effect is also
easily calculated.* sagulator has the option to add the edging.

What you can't see in it, unfortunately, is just what data it is using
for the materials properties.* I don't recall whether it has the ability
to input the desired properties manually or not.


Just to confirm what sagulator does; went to the "Engineering Toolbox"
beam calculator for rectangular, uniformly loaded, pinned joint beams
and entered the necessary info for a 3/4" x 36" shelf with no edge
material included in the calculation of the moment of inertia and an
assumed modulus of elasticity of 1.2E6 to guess what it uses for
"Plywood, fir".

And indeed, voila! it returned maximum beam deflection of 0.108" while
sagulator only reports two digits of 0.11". It's a simple beam
deflection calculation.

Also confirms the plywood E appears to match that from the FPL handbook
(which is what I thought I recalled from doing the exercise in the past).

If one doubles E in the beam deflection calculation, the maximum
deflection is then 0.054", precisely half as the maximum deflection for
the assumed geometry and loading is proportional to L^4 but inversely
proportional to both E and I.

Let's see if sagulator is more sophisticated in the edging calculation
than just adding the extra I, not counting for the induced asymmetry...

Adding a 3/4 x 1 edge also of the same material, the beam calculator
returns max deflection of 0.0941" where as sagulator is again limited in
its output precision so returns only 0.09".

Again they match so that's all it's doing -- the total moment of inertia
is simply the sum of the two composite pieces.

https://www.engineeringtoolbox.com/beam-stress-deflection-d_1312.html

The only additional sophistication in the sagulator is that it can
select a different material for the edge so the overall effective
modulus of elasticity is a little different whereas the beam calculator
is primarily intended for structural beams, etc. so is single material.

--



  #24   Report Post  
Old February 13th 20, 04:19 AM posted to rec.woodworking
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Posts: 1,905
Default Wide shelving advice needed

On Wed, 12 Feb 2020 12:18:51 -0600, dpb wrote:

On 2/11/2020 8:24 PM, dpb wrote:
On 2/11/2020 8:00 PM, DerbyDad03 wrote:
On Tuesday, February 11, 2020 at 6:46:01 PM UTC-5, dpb wrote:

...

...[snip for brevity]...

Sagulator gave 0.11" for "Plywood, fir" and while Doug fir wasn't one of
the firs it gives specifically, they all were less (altho not by factor
of 2 which does seem somewhat excessive by common experience).* It
doesn't have any other plywood to compare against.

And just about any kind of edging on just about any kind of wood makes
all those specs moot. ;-)


No, it does not.

All it does is change the geometry somewhat but the effect is also
easily calculated.* sagulator has the option to add the edging.

What you can't see in it, unfortunately, is just what data it is using
for the materials properties.* I don't recall whether it has the ability
to input the desired properties manually or not.


Just to confirm what sagulator does; went to the "Engineering Toolbox"
beam calculator for rectangular, uniformly loaded, pinned joint beams
and entered the necessary info for a 3/4" x 36" shelf with no edge
material included in the calculation of the moment of inertia and an
assumed modulus of elasticity of 1.2E6 to guess what it uses for
"Plywood, fir".

And indeed, voila! it returned maximum beam deflection of 0.108" while
sagulator only reports two digits of 0.11". It's a simple beam
deflection calculation.

Also confirms the plywood E appears to match that from the FPL handbook
(which is what I thought I recalled from doing the exercise in the past).

If one doubles E in the beam deflection calculation, the maximum
deflection is then 0.054", precisely half as the maximum deflection for
the assumed geometry and loading is proportional to L^4 but inversely
proportional to both E and I.

Let's see if sagulator is more sophisticated in the edging calculation
than just adding the extra I, not counting for the induced asymmetry...

Adding a 3/4 x 1 edge also of the same material, the beam calculator
returns max deflection of 0.0941" where as sagulator is again limited in
its output precision so returns only 0.09".

Again they match so that's all it's doing -- the total moment of inertia
is simply the sum of the two composite pieces.

https://www.engineeringtoolbox.com/beam-stress-deflection-d_1312.html

The only additional sophistication in the sagulator is that it can
select a different material for the edge so the overall effective
modulus of elasticity is a little different whereas the beam calculator
is primarily intended for structural beams, etc. so is single material.


Who would have expected differently? It even says it in the text, or
it did at one point. This doesn't change its utility.
  #25   Report Post  
Old February 13th 20, 04:34 AM posted to rec.woodworking
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Posts: 891
Default Wide shelving advice needed

On 2/12/2020 9:19 PM, wrote:
On Wed, 12 Feb 2020 12:18:51 -0600, dpb wrote:

On 2/11/2020 8:24 PM, dpb wrote:
On 2/11/2020 8:00 PM, DerbyDad03 wrote:
On Tuesday, February 11, 2020 at 6:46:01 PM UTC-5, dpb wrote:

...

...[snip for brevity]...

Sagulator gave 0.11" for "Plywood, fir" and while Doug fir wasn't one of
the firs it gives specifically, they all were less (altho not by factor
of 2 which does seem somewhat excessive by common experience).* It
doesn't have any other plywood to compare against.

And just about any kind of edging on just about any kind of wood makes
all those specs moot. ;-)

No, it does not.

All it does is change the geometry somewhat but the effect is also
easily calculated.* sagulator has the option to add the edging.

What you can't see in it, unfortunately, is just what data it is using
for the materials properties.* I don't recall whether it has the ability
to input the desired properties manually or not.


Just to confirm what sagulator does; went to the "Engineering Toolbox"
beam calculator for rectangular, uniformly loaded, pinned joint beams
and entered the necessary info for a 3/4" x 36" shelf with no edge
material included in the calculation of the moment of inertia and an
assumed modulus of elasticity of 1.2E6 to guess what it uses for
"Plywood, fir".

And indeed, voila! it returned maximum beam deflection of 0.108" while
sagulator only reports two digits of 0.11". It's a simple beam
deflection calculation.

Also confirms the plywood E appears to match that from the FPL handbook
(which is what I thought I recalled from doing the exercise in the past).

If one doubles E in the beam deflection calculation, the maximum
deflection is then 0.054", precisely half as the maximum deflection for
the assumed geometry and loading is proportional to L^4 but inversely
proportional to both E and I.

Let's see if sagulator is more sophisticated in the edging calculation
than just adding the extra I, not counting for the induced asymmetry...

Adding a 3/4 x 1 edge also of the same material, the beam calculator
returns max deflection of 0.0941" where as sagulator is again limited in
its output precision so returns only 0.09".

Again they match so that's all it's doing -- the total moment of inertia
is simply the sum of the two composite pieces.

https://www.engineeringtoolbox.com/beam-stress-deflection-d_1312.html

The only additional sophistication in the sagulator is that it can
select a different material for the edge so the overall effective
modulus of elasticity is a little different whereas the beam calculator
is primarily intended for structural beams, etc. so is single material.


Who would have expected differently? It even says it in the text, or
it did at one point. This doesn't change its utility.


No, nor was intended to say anything less...only to confirm precisely
what it does do and how and to illustrate more specifically that the
comment earlier that the edge negates the spec's is false.

--





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Old February 13th 20, 04:10 PM posted to rec.woodworking
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Posts: 891
Default Wide shelving advice needed

On 2/12/2020 9:34 PM, dpb wrote:
On 2/12/2020 9:19 PM, wrote:
On Wed, 12 Feb 2020 12:18:51 -0600, dpb wrote:

On 2/11/2020 8:24 PM, dpb wrote:
On 2/11/2020 8:00 PM, DerbyDad03 wrote:
On Tuesday, February 11, 2020 at 6:46:01 PM UTC-5, dpb wrote:
...

...[snip for brevity]...

Sagulator gave 0.11" for "Plywood, fir" and while Doug fir wasn't
one of
the firs it gives specifically, they all were less (altho not by
factor
of 2 which does seem somewhat excessive by common experience).* It
doesn't have any other plywood to compare against.

And just about any kind of edging on just about any kind of wood makes
all those specs moot. ;-)

No, it does not.

All it does is change the geometry somewhat but the effect is also
easily calculated.* sagulator has the option to add the edging.

What you can't see in it, unfortunately, is just what data it is using
for the materials properties.* I don't recall whether it has the
ability
to input the desired properties manually or not.

Just to confirm what sagulator does; went to the "Engineering Toolbox"
beam calculator for rectangular, uniformly loaded, pinned joint beams
and entered the necessary info for a 3/4" x 36" shelf with no edge
material included in the calculation of the moment of inertia and an
assumed modulus of elasticity of 1.2E6 to guess what it uses for
"Plywood, fir".

And indeed, voila!* it returned maximum beam deflection of 0.108" while
sagulator only reports two digits of 0.11".* It's a simple beam
deflection calculation.

Also confirms the plywood E appears to match that from the FPL handbook
(which is what I thought I recalled from doing the exercise in the
past).

If one doubles E in the beam deflection calculation, the maximum
deflection is then 0.054", precisely half as the maximum deflection for
the assumed geometry and loading is proportional to L^4 but inversely
proportional to both E and I.

Let's see if sagulator is more sophisticated in the edging calculation
than just adding the extra I, not counting for the induced asymmetry...

Adding a 3/4 x 1 edge also of the same material, the beam calculator
returns max deflection of 0.0941" where as sagulator is again limited in
its output precision so returns only 0.09".

Again they match so that's all it's doing -- the total moment of inertia
is simply the sum of the two composite pieces.

https://www.engineeringtoolbox.com/beam-stress-deflection-d_1312.html

The only additional sophistication in the sagulator is that it can
select a different material for the edge so the overall effective
modulus of elasticity is a little different whereas the beam calculator
is primarily intended for structural beams, etc. so is single material.


Who would have expected differently?* It even says it in the text, or
it did at one point.* This doesn't change its utility.


No, nor was intended to say anything less...only to confirm precisely
what it does do and how and to illustrate more specifically that the
comment earlier that the edge negates the spec's is false.


Also, I at least was curious to see whether the deflection calculation
in the sagulator reported anything different based on more sophisticated
incorporation of the asymmetry of the geometry. As suspected, it doesn't.

--

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Old February 13th 20, 04:36 PM posted to rec.woodworking
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First recorded activity by DIYBanter: Apr 2011
Posts: 11,470
Default Wide shelving advice needed

On 2/11/2020 5:45 PM, dpb wrote:
On 2/11/2020 3:41 PM, Leon wrote:
On 2/11/2020 1:21 PM, whit3rd wrote:
On Tuesday, February 11, 2020 at 6:39:07 AM UTC-8, Scott Lurndal wrote:
whit3rd writes:

* Plywood
is weaker than solid wood (half the grain runs the wrong way).

Which actually makes it stronger.

Tougher, resistant to splitting, yes.** Stronger in the shelf-sag
sense, no.
Sagulator gets this right.

Shelves need compressive strength in the top surface, and tensile
strength
in the bottom surface (knots on top are less troublesome than on
bottom, for instance).
Plywood has, on bottom surface, a very thin veneer of good
high-tensile strength wood,
* backed by a thicker layer with the grain running the wrong way.


Followed by grain running "in the right direction".* In fact there is
more "in the right direction" plies then the wrong.* If that means
anything.

...

Only that it's somewhat more rigid in the direction of the longitudinal
plies than the other...but it's still less than solid wood longitudinal
of the same species.

From US FPL Handbook Chap 12 on mechanical properties a summary table
shows Doug fir modulus of elasticity as 1.98x10^6 lb/insq whereas
plywood is 1.01-1.24 or only about half.* Since difference in deflection
of two pieces of same size and species is only the effect of the
different E as the geometrical factor is the same, the computed sag is
directly proportional to the inverse of the E.* IOW, the sag for those
two is almost 2X for ply vis a vis solid of same dimension.

https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_12.pdf

Sagulator gave 0.11" for "Plywood, fir" and while Doug fir wasn't one of
the firs it gives specifically, they all were less (altho not by factor
of 2 which does seem somewhat excessive by common experience).* It
doesn't have any other plywood to compare against.


--



The sagulator determines sag, not strength.
Yes this discussion is mostly about what is best for shelving and it
went in two different directions. What is stronger and what sags less.

The statement I responded to, near the top of this response, was that
plywood was weaker than solid wood, that is not necessarily true
regardless of what the sagulator says.

Considering shelving intended to carry a lot of weight both solid wood
and plywood will bend to some extent and solid wood will likely bow less
along its length, but it will bend.

So how do you get rid of bow along the length when using plywood or
solid wood? You add support. Typically along the front edge and often
along the back edge. Lets say you use equal steel beams to reinforce
the length of the shelves so that there is no bow at all along the front
and back edge

Add weight to the center until one fails. My money is on the plywood
coming out the winner. The hardwood is likely to split/fail along the
length, with the grain.

Have I seen solid wood shelving split along its length? Yes, especially
as it get older. And that was not necessarily with a lot of weight.
And the deeper the shelving the more likely the solid wood will fail
over plywood.

Again, I'm not saying plywood will sag less than solid wood when used
for shelving, I'm saying that plywood is stronger than like sized wood.








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Old February 13th 20, 08:12 PM posted to rec.woodworking
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First recorded activity by DIYBanter: Dec 2017
Posts: 891
Default Wide shelving advice needed

On 2/13/2020 9:36 AM, Leon wrote:
On 2/11/2020 5:45 PM, dpb wrote:
On 2/11/2020 3:41 PM, Leon wrote:
On 2/11/2020 1:21 PM, whit3rd wrote:
On Tuesday, February 11, 2020 at 6:39:07 AM UTC-8, Scott Lurndal wrote:
whit3rd writes:

* Plywood
is weaker than solid wood (half the grain runs the wrong way).

Which actually makes it stronger.

Tougher, resistant to splitting, yes.** Stronger in the shelf-sag
sense, no.
Sagulator gets this right.

Shelves need compressive strength in the top surface, and tensile
strength
in the bottom surface (knots on top are less troublesome than on
bottom, for instance).
Plywood has, on bottom surface, a very thin veneer of good
high-tensile strength wood,
* backed by a thicker layer with the grain running the wrong way.


Followed by grain running "in the right direction".* In fact there is
more "in the right direction" plies then the wrong.* If that means
anything.

...

Only that it's somewhat more rigid in the direction of the
longitudinal plies than the other...but it's still less than solid
wood longitudinal of the same species.

*From US FPL Handbook Chap 12 on mechanical properties a summary table
shows Doug fir modulus of elasticity as 1.98x10^6 lb/insq whereas
plywood is 1.01-1.24 or only about half.* Since difference in
deflection of two pieces of same size and species is only the effect
of the different E as the geometrical factor is the same, the computed
sag is directly proportional to the inverse of the E.* IOW, the sag
for those two is almost 2X for ply vis a vis solid of same dimension.

https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_12.pdf

Sagulator gave 0.11" for "Plywood, fir" and while Doug fir wasn't one
of the firs it gives specifically, they all were less (altho not by
factor of 2 which does seem somewhat excessive by common experience).
It doesn't have any other plywood to compare against.


--



The sagulator determines sag, not strength.
Yes this discussion is mostly about what is best for shelving and it
went in two different directions.* What is stronger and what sags less.

The statement I responded to, near the top of this response, was that
plywood was weaker than solid wood, that is not necessarily true
regardless of what the sagulator says.

Considering shelving intended to carry a lot of weight both solid wood
and plywood will bend to some extent and solid wood will likely bow less
along its length, but it will bend.

So how do you get rid of bow along the length when using plywood or
solid wood?* You add support.* Typically along the front edge and often
along the back edge.* Lets say you use equal steel beams to reinforce
the length of the shelves so that there is no bow at all along the front
and back edge

Add weight to the center until one fails.* My money is on the plywood
coming out the winner.* The hardwood is likely to split/fail along the
length, with the grain.

Have I seen solid wood shelving split along its length?* Yes, especially
as it get older.* And that was not necessarily with a lot of weight. And
the deeper the shelving the more likely the solid wood will fail over
plywood.

Again, I'm not saying plywood will sag less than solid wood when used
for shelving, I'm saying that plywood is stronger than like sized wood.


Well, the poster of the above comment was used the word strength in an
imprecise manner and really was referring to the ability to resist
bending moment which is, for a fixed geometry, directly proportional to
the modulus of elasticity.

What you're describing also is not really "strength" per se, but the
effect of the very high anisotropic nature of wood with and across
grain. Certainly if you don't load it or constrain/support it across
grain, solid wood will failure much sooner in that direction. Plywood
definitely has the advantage in that regards and in dimensional
stability, yes.

--

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Old February 13th 20, 10:33 PM posted to rec.woodworking
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First recorded activity by DIYBanter: Apr 2011
Posts: 11,470
Default Wide shelving advice needed

On 2/13/2020 1:12 PM, dpb wrote:
On 2/13/2020 9:36 AM, Leon wrote:
On 2/11/2020 5:45 PM, dpb wrote:
On 2/11/2020 3:41 PM, Leon wrote:
On 2/11/2020 1:21 PM, whit3rd wrote:
On Tuesday, February 11, 2020 at 6:39:07 AM UTC-8, Scott Lurndal
wrote:
whit3rd writes:

* Plywood
is weaker than solid wood (half the grain runs the wrong way).

Which actually makes it stronger.

Tougher, resistant to splitting, yes.** Stronger in the shelf-sag
sense, no.
Sagulator gets this right.

Shelves need compressive strength in the top surface, and tensile
strength
in the bottom surface (knots on top are less troublesome than on
bottom, for instance).
Plywood has, on bottom surface, a very thin veneer of good
high-tensile strength wood,
* backed by a thicker layer with the grain running the wrong way.


Followed by grain running "in the right direction".* In fact there
is more "in the right direction" plies then the wrong.* If that
means anything.
...

Only that it's somewhat more rigid in the direction of the
longitudinal plies than the other...but it's still less than solid
wood longitudinal of the same species.

*From US FPL Handbook Chap 12 on mechanical properties a summary
table shows Doug fir modulus of elasticity as 1.98x10^6 lb/insq
whereas plywood is 1.01-1.24 or only about half.* Since difference in
deflection of two pieces of same size and species is only the effect
of the different E as the geometrical factor is the same, the
computed sag is directly proportional to the inverse of the E.* IOW,
the sag for those two is almost 2X for ply vis a vis solid of same
dimension.

https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_12.pdf

Sagulator gave 0.11" for "Plywood, fir" and while Doug fir wasn't one
of the firs it gives specifically, they all were less (altho not by
factor of 2 which does seem somewhat excessive by common experience).
It doesn't have any other plywood to compare against.


--



The sagulator determines sag, not strength.
Yes this discussion is mostly about what is best for shelving and it
went in two different directions.* What is stronger and what sags less.

The statement I responded to, near the top of this response, was that
plywood was weaker than solid wood, that is not necessarily true
regardless of what the sagulator says.

Considering shelving intended to carry a lot of weight both solid wood
and plywood will bend to some extent and solid wood will likely bow
less along its length, but it will bend.

So how do you get rid of bow along the length when using plywood or
solid wood?* You add support.* Typically along the front edge and
often along the back edge.* Lets say you use equal steel beams to
reinforce the length of the shelves so that there is no bow at all
along the front and back edge

Add weight to the center until one fails.* My money is on the plywood
coming out the winner.* The hardwood is likely to split/fail along the
length, with the grain.

Have I seen solid wood shelving split along its length?* Yes,
especially as it get older.* And that was not necessarily with a lot
of weight. And the deeper the shelving the more likely the solid wood
will fail over plywood.

Again, I'm not saying plywood will sag less than solid wood when used
for shelving, I'm saying that plywood is stronger than like sized wood.


Well, the poster of the above comment was used the word strength in an
imprecise manner and really was referring to the ability to resist
bending moment which is, for a fixed geometry, directly proportional to
the modulus of elasticity.

What you're describing also is not really "strength" per se, but the
effect of the very high anisotropic nature of wood with and across
grain.


Actually the addition of adhesives between ply's, a necessary component,
makes plywood stronger.

  #30   Report Post  
Old February 13th 20, 10:54 PM posted to rec.woodworking
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First recorded activity by DIYBanter: Nov 2017
Posts: 3,129
Default Wide shelving advice needed

On Thu, 13 Feb 2020 15:33:36 -0600, Leon [email protected]
wrote:

On 2/13/2020 1:12 PM, dpb wrote:
On 2/13/2020 9:36 AM, Leon wrote:
On 2/11/2020 5:45 PM, dpb wrote:
On 2/11/2020 3:41 PM, Leon wrote:
On 2/11/2020 1:21 PM, whit3rd wrote:
On Tuesday, February 11, 2020 at 6:39:07 AM UTC-8, Scott Lurndal
wrote:
whit3rd writes:

* Plywood
is weaker than solid wood (half the grain runs the wrong way).

Which actually makes it stronger.

Tougher, resistant to splitting, yes.** Stronger in the shelf-sag
sense, no.
Sagulator gets this right.

Shelves need compressive strength in the top surface, and tensile
strength
in the bottom surface (knots on top are less troublesome than on
bottom, for instance).
Plywood has, on bottom surface, a very thin veneer of good
high-tensile strength wood,
* backed by a thicker layer with the grain running the wrong way.


Followed by grain running "in the right direction".* In fact there
is more "in the right direction" plies then the wrong.* If that
means anything.
...

Only that it's somewhat more rigid in the direction of the
longitudinal plies than the other...but it's still less than solid
wood longitudinal of the same species.

*From US FPL Handbook Chap 12 on mechanical properties a summary
table shows Doug fir modulus of elasticity as 1.98x10^6 lb/insq
whereas plywood is 1.01-1.24 or only about half.* Since difference in
deflection of two pieces of same size and species is only the effect
of the different E as the geometrical factor is the same, the
computed sag is directly proportional to the inverse of the E.* IOW,
the sag for those two is almost 2X for ply vis a vis solid of same
dimension.

https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_12.pdf

Sagulator gave 0.11" for "Plywood, fir" and while Doug fir wasn't one
of the firs it gives specifically, they all were less (altho not by
factor of 2 which does seem somewhat excessive by common experience).
It doesn't have any other plywood to compare against.


--


The sagulator determines sag, not strength.
Yes this discussion is mostly about what is best for shelving and it
went in two different directions.* What is stronger and what sags less.

The statement I responded to, near the top of this response, was that
plywood was weaker than solid wood, that is not necessarily true
regardless of what the sagulator says.

Considering shelving intended to carry a lot of weight both solid wood
and plywood will bend to some extent and solid wood will likely bow
less along its length, but it will bend.

So how do you get rid of bow along the length when using plywood or
solid wood?* You add support.* Typically along the front edge and
often along the back edge.* Lets say you use equal steel beams to
reinforce the length of the shelves so that there is no bow at all
along the front and back edge

Add weight to the center until one fails.* My money is on the plywood
coming out the winner.* The hardwood is likely to split/fail along the
length, with the grain.

Have I seen solid wood shelving split along its length?* Yes,
especially as it get older.* And that was not necessarily with a lot
of weight. And the deeper the shelving the more likely the solid wood
will fail over plywood.

Again, I'm not saying plywood will sag less than solid wood when used
for shelving, I'm saying that plywood is stronger than like sized wood.


Well, the poster of the above comment was used the word strength in an
imprecise manner and really was referring to the ability to resist
bending moment which is, for a fixed geometry, directly proportional to
the modulus of elasticity.

What you're describing also is not really "strength" per se, but the
effect of the very high anisotropic nature of wood with and across
grain.


Actually the addition of adhesives between ply's, a necessary component,
makes plywood stronger.

Even a unidirectional laminated wood is stronger than "solid" wood if
properly designed and implemented (with the layers alternated "cup up"
to "cup down") and is also more dimensionaly stable than "solid wood"
as it resists warpage pretty effectively.


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