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Shear strength of screws



 
 
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  #1  
Old April 6th 12, 05:05 PM posted to rec.woodworking
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Posts: 95
Default Shear strength of screws

I've always taken a serendipity view of screw gauge when doing rough
stuff as in "hey, I'd better use a bigger screw for this." I need to
cobble together some two bys for a frame for a basement storage rack.
I've got a box of number 9 2 1/2 inchers that ought to do the trick.

Question: Is there a formula for determining the minimum gauge for a
screw if you know the load?
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  #2  
Old April 6th 12, 05:18 PM posted to rec.woodworking
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Posts: 9,117
Default Shear strength of screws

On 4/6/2012 11:05 AM, Gramp's shop wrote:
I've always taken a serendipity view of screw gauge when doing rough
stuff as in "hey, I'd better use a bigger screw for this." I need to
cobble together some two bys for a frame for a basement storage rack.
I've got a box of number 9 2 1/2 inchers that ought to do the trick.

Question: Is there a formula for determining the minimum gauge for a
screw if you know the load?


My favorite screw for that application is a "Spax":

http://www.mcfeelys.com/spax-screws

I prefer them for attaching cabinets to walls these days because of the
superior shear strength.

That said, most engineers will tell you that for many
construction/structural projects nails will provide more shear strength
than screws, so it really depends upon the application.

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  #3  
Old April 6th 12, 07:31 PM posted to rec.woodworking
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Posts: 635
Default Shear strength of screws



"Gramp's shop" wrote in message
...
I've always taken a serendipity view of screw gauge when doing rough stuff
as in "hey, I'd better use a bigger screw for this." I need to cobble
together some two bys for a frame for a basement storage rack. I've got a
box of number 9 2 1/2 inchers that ought to do the trick.

Question: Is there a formula for determining the minimum gauge for a screw
if you know the load?


I never thought about it. Just that so many screws today are absolute crap.
If you shear off a screw while driving it into the wood, it is a bad sign.
And it depends where you buy them too. I have had terrible luck with deck
screws and lag screws from the local home depot. But the local ace hardware
store gave me screws that were higher in quality, stronger and a few cents
cheaper too.

If I got any kind of basic repair outside or in the garage these days, I
just use their deck screws. It ain't art or furniture. But it is strong.
They don't shear off and they don't rust outside.

The old standby rule about strength for fasteners is to estimate how many
will do the job. Then put twice as many in there. I have always been
accused of using too many screws anyway.



  #4  
Old April 6th 12, 11:39 PM posted to rec.woodworking
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Posts: 6,091
Default Shear strength of screws

On 4/6/2012 11:05 AM, Gramp's shop wrote:
I've always taken a serendipity view of screw gauge when doing rough
stuff as in "hey, I'd better use a bigger screw for this." I need to
cobble together some two bys for a frame for a basement storage rack.
I've got a box of number 9 2 1/2 inchers that ought to do the trick.

Question: Is there a formula for determining the minimum gauge for a
screw if you know the load?


What you also need to ask is if there is a way to determine the quality
of the screw you are using. I know of #6 screws that are stronger than
#10's.

All things be in equal, McFeeleys.com has the specifically information
that you are asking and IIRC their catalog has a chart of this also.
  #5  
Old April 11th 12, 10:26 PM posted to rec.woodworking
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Posts: 86
Default Shear strength of screws

On Friday, April 6, 2012 12:05:36 PM UTC-4, Gramp's shop wrote:
I've always taken a serendipity view of screw gauge when doing rough
stuff as in "hey, I'd better use a bigger screw for this." I need to
cobble together some two bys for a frame for a basement storage rack.
I've got a box of number 9 2 1/2 inchers that ought to do the trick.

Question: Is there a formula for determining the minimum gauge for a
screw if you know the load?


Those will do. If you are concerned, smear a little construction adhesive in there too. Up here in the Northeast, if we are framing a house, we cannot use screws. Must use nails or you will not pass the framing inspection.

RP
  #6  
Old April 12th 12, 12:33 AM posted to rec.woodworking
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Posts: 2,266
Default Shear strength of screws

On Wed, 11 Apr 2012 14:26:06 -0700, RP wrote:

Up here in the Northeast, if we are framing a house, we cannot use
screws. Must use nails or you will not pass the framing inspection.


That seems a little strange - what's the rational given, if any?

I learned the hard way that any carpentry (as opposed to ww) work I do,
someday I, or someone who comes after, will have to take it apart again.
I use screws for all that now.

--
Intelligence is an experiment that failed - G. B. Shaw
  #7  
Old April 12th 12, 12:37 AM posted to rec.woodworking
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Posts: 9,117
Default Shear strength of screws

On 4/11/2012 6:33 PM, Larry Blanchard wrote:
On Wed, 11 Apr 2012 14:26:06 -0700, RP wrote:

Up here in the Northeast, if we are framing a house, we cannot use
screws. Must use nails or you will not pass the framing inspection.


That seems a little strange - what's the rational given, if any?


Nails generally have a greater shear strength than screws, thus the
requirement in some areas are very specific as to the nails used, their
size, makeup, and nailing patterns.


I learned the hard way that any carpentry (as opposed to ww) work I do,
someday I, or someone who comes after, will have to take it apart again.
I use screws for all that now.



--
www.eWoodShop.com
Last update: 4/15/2010
KarlCaillouet@ (the obvious)
http://gplus.to/eWoodShop
  #8  
Old April 12th 12, 01:05 AM posted to rec.woodworking
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Posts: 1,213
Default Shear strength of screws

Used to be that framing had to be toe nailed...
Now I see nails used from top and bottom sil to studs. Not as strong.
The toe nailing really locks it in from both sides.
straight nailing will not withstand storm forces as much.
But then again, most roofs will easily lift before the framing gives.

On 4/11/2012 7:37 PM, Swingman wrote:
On 4/11/2012 6:33 PM, Larry Blanchard wrote:
On Wed, 11 Apr 2012 14:26:06 -0700, RP wrote:

Up here in the Northeast, if we are framing a house, we cannot use
screws. Must use nails or you will not pass the framing inspection.


That seems a little strange - what's the rational given, if any?


Nails generally have a greater shear strength than screws, thus the
requirement in some areas are very specific as to the nails used, their
size, makeup, and nailing patterns.


I learned the hard way that any carpentry (as opposed to ww) work I do,
someday I, or someone who comes after, will have to take it apart again.
I use screws for all that now.



  #9  
Old April 12th 12, 01:15 AM posted to rec.woodworking
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Posts: 6,091
Default Shear strength of screws

On 4/11/2012 7:05 PM, tiredofspam wrote:
Used to be that framing had to be toe nailed...
Now I see nails used from top and bottom sil to studs. Not as strong.
The toe nailing really locks it in from both sides.
straight nailing will not withstand storm forces as much.
But then again, most roofs will easily lift before the framing gives.



Typically hurricane straps add tremendous strength if you are building
for wind storm resistance.
  #10  
Old April 12th 12, 01:30 AM posted to rec.woodworking
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Posts: 10,833
Default Shear strength of screws

On Wed, 11 Apr 2012 18:37:51 -0500, Swingman wrote:

On 4/11/2012 6:33 PM, Larry Blanchard wrote:
On Wed, 11 Apr 2012 14:26:06 -0700, RP wrote:

Up here in the Northeast, if we are framing a house, we cannot use
screws. Must use nails or you will not pass the framing inspection.


That seems a little strange - what's the rational given, if any?


Nails generally have a greater shear strength than screws, thus the
requirement in some areas are very specific as to the nails used, their
size, makeup, and nailing patterns.

Using the right number and pattern of screws will achieve the same
thing. You need a larger screw to give the same strength, generally
speaking, because the root diameter if the screw is significantly less
than the nominal diameter, and the strength of the screw (yield) is
lowered by the stress rizers formed by the malformation of the metal
at the thread root. Screws COULD be made that were almost as strong as
the equivalent sized nail, but they would be way to expensive to be
practical. An "old school" wood screw is stronger than today's
"construction screw" or "deck screw" or, particularly, the "drywall
screw".

The holding ability of a screw excedes that of a nail in most cases -
but with a "deck screw" or "drywall screw" it also often excedes the
yield strength of the screw itself. An "ardox" nail is a compromize -
it's holding ability approaches that of a screw - with the overall
strength of a nail, but without the removeability of a screw. A guy
who used to work in construction with my dad used to drive screws with
a hammer - he said the funny head and other features of the screw were
just to make them easier to remove.
I learned the hard way that any carpentry (as opposed to ww) work I do,
someday I, or someone who comes after, will have to take it apart again.
I use screws for all that now.


 




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