Truss gusset strength question????
Richard Barnes
I am used to building wooden trusses up to 26 feet span using glued and
nailed plywood gussets on 2x4 chords.
I have a supply of 3/8" ext plywood available free if it can be used. I am
used to using 1/2" for the gussets. If the longest span is going to be 24'
and the roof will be only wooden purlins and sheet metal , with 1/2" T&G
pine ceiling and insulation, would the 3/8" gussets be OK?
--
Richard Barnes
Skwaller Holler Sawmill
Joe Barta
At the risk of being chastised by an engineer for shooting from the
hip, I don't see why 3/8 plywood would do just as well as 1/2. I
say this for two reasons... 1) the strength of the gusset is much
more a factor of gusset size and fastener type/schedule. The
strength difference between 1/2 & 3/8 in such an application is
most likely inconsequential. And 2) I've tipped up an awful lot of
trusses and I'm still amazed at how small and flimsy the standard
metal gussets are. If those are acceptable then I'm sure carefully
applied plywood (whether 1/2 or 3/8) would be just fine.
One might suggest that glue (and or screws) be used as well, and if
you are just doing a few, this certainly wouldn't hurt (regardless
of which plywood thickness you'd use). But... If you're doing any
sort of a production run, the glue/screw step might be messy and add
unneccessary cost.
- Joe Barta
Matthew Whiting
>
> Richard Barnes wrote:
>
> >Hello,
> >I am used to building wooden trusses up to 26 feet span using
> >glued and nailed plywood gussets on 2x4 chords.
> >I have a supply of 3/8" ext plywood available free if it can be
> >used. I am used to using 1/2" for the gussets. If the longest
> >span is going to be 24' and the roof will be only wooden purlins
> >and sheet metal , with 1/2" T&G pine ceiling and insulation, would
> >the 3/8" gussets be OK?
> >
> >--
> >Richard Barnes
> >Skwaller Holler Sawmill
>
> At the risk of being chastised by an engineer for shooting from the
> hip, I don't see why 3/8 plywood would do just as well as 1/2. I
> say this for two reasons... 1) the strength of the gusset is much
> more a factor of gusset size and fastener type/schedule. The
> strength difference between 1/2 & 3/8 in such an application is
> most likely inconsequential. And 2) I've tipped up an awful lot of
> trusses and I'm still amazed at how small and flimsy the standard
> metal gussets are. If those are acceptable then I'm sure carefully
> applied plywood (whether 1/2 or 3/8) would be just fine.
Would we do that? :-)
Seriously, this substitution can't be done by gut feel if you really
care about the integrity of your trusses. Joe may be right, but then he
may be wrong. Are you willing to risk it?
The metal gussets are flimsy, but keep in mind they are loaded primarily
in tension at most joints. Even fairly cheap steel has tensile strength
in excess of 30,000 psi. Wood varies by specie, but is typically in the
neighborhood of 1,000 psi. This makes steel some 30 times stronger than
wood in tension. Divide 1/2" by 30 and you will see that a very thin
sheet of steel can perform as well in tension as can 1/2" plywood.
I'm not saying you need an engineer to check everything you do, but
trusses that support your roof are pretty critical items and probably
not the best place to experiment.
Matt
Ramon M. Riba, P.E.
minimum live [snow, etc.] loads wind loads and other conditions are, since they
will also weight on your roof inaddition to the metal roofing. Given that, it
would be a disservice if I told you whether your proposed design is appropriate
(I might not even be registered in your state). You can call the folks at the
Univ. of Iowa's Midwest Plan Service. They are very helpful in getting the
info to people who call them. They have a pretty good book on designing and
building agricultural structures. One of the sections deals specifically with
glued/nailed plywood gussets just like the ones you want to build and install.
It gives measures, sizes, spans and the like, if I don't remember incorrectly.
Univ. of Iowa
Midwest Plan Service
122 Davidson Hall
Ames, IA 50011-3060
(515) 292-4337
(515) 292-2097
(515) 294-9589 fax
What Joe and others say about the plywood thickness is probably true, i.e. it
will make little difference as far as the strength of the plywood goes.
However, you should be aware that nail strength and stiffness (i.e., how much
your trusses will sag unnder load) will depend on the thickness of the members
joined (your 2x and the plywood). Good luck and email me if there is anything
else I can help you with.
Joe Barta
>Joe Barta wrote:
>>
>> Richard Barnes wrote:
>>
>> >Hello,
>> >I am used to building wooden trusses up to 26 feet span using
>> >glued and nailed plywood gussets on 2x4 chords.
>> >I have a supply of 3/8" ext plywood available free if it can be
>> >used. I am used to using 1/2" for the gussets. If the longest
>> >span is going to be 24' and the roof will be only wooden
>> >purlins and sheet metal , with 1/2" T&G pine ceiling and
>> >insulation, would the 3/8" gussets be OK?
>> >
>> >--
>> >Richard Barnes
>> >Skwaller Holler Sawmill
>>
>> At the risk of being chastised by an engineer for shooting from
>> the hip, I don't see why 3/8 plywood would do just as well as
>> 1/2. I say this for two reasons... 1) the strength of the gusset
>> is much more a factor of gusset size and fastener type/schedule.
>> The strength difference between 1/2 & 3/8 in such an application
>> is most likely inconsequential. And 2) I've tipped up an awful
>> lot of trusses and I'm still amazed at how small and flimsy the
>> standard metal gussets are. If those are acceptable then I'm
>> sure carefully applied plywood (whether 1/2 or 3/8) would be
>> just fine.
>
>Would we do that? :-)
>
>Seriously, this substitution can't be done by gut feel if you
>really care about the integrity of your trusses. Joe may be
>right, but then he may be wrong. Are you willing to risk it?
>
>The metal gussets are flimsy, but keep in mind they are loaded
>primarily in tension at most joints. Even fairly cheap steel has
>tensile strength in excess of 30,000 psi.
>
>Wood varies by specie,
>but is typically in the neighborhood of 1,000 psi. This makes
>steel some 30 times stronger than wood in tension. Divide 1/2" by
>30 and you will see that a very thin sheet of steel can perform as
>well in tension as can 1/2" plywood.
This is a real life example of experience over education. Sometimes
experience makes for a better education.
You should take a good look at those steel gussets and how they
attach to the members. The tensile strength of the steel is fairly
irrelevant. When I speal of flimsy I speak of their relative size
and how they are attached with prongs. Those prongs will rip out of
the wood WAY before the steel rips. I know because I've had to
repair enough of them. We repair them with plywood gussets which
actually makes the assembly much stronger. In addition, we have
occasionally gotten custom trusses from a truss manufacturer and one
or more of the existing metal gussets can barely hold the thing
together to tip up on a roof (designed I'm sure by an engineer). In
order to build the roof we have to beef up those metal gussets with
plywood. In no case is the tensile strength of steel, or species of
wood an issue.
>I'm not saying you need an engineer to check everything you do,
>but trusses that support your roof are pretty critical items and
>probably not the best place to experiment.
EVERYTHING is potentially critical and "experiment" has nothing to
do with it. It is a simple matter of taking past experience, current
knowledge and good judgement to solve a problem. There are several
ways to skin a cat, most of which will get the job done just fine.
- Joe Barta
eric ryder
eric
"Ramon M. Riba, P.E." wrote:
>
> Richard, I don't know what your particular situation is (i.e., local codes,
> minimum live [snow, etc.] loads wind loads and other conditions are, since they
> will also weight on your roof inaddition to the metal roofing. Given that, it
> would be a disservice if I told you whether your proposed design is appropriate
> (I might not even be registered in your state). You can call the folks at the
> Univ. of Iowa's Midwest Plan Service. They are very helpful in getting the
> info to people who call them. They have a pretty good book on designing and
> building agricultural structures. One of the sections deals specifically with
> glued/nailed plywood gussets just like the ones you want to build and install.
> It gives measures, sizes, spans and the like, if I don't remember incorrectly.
>
> Univ. of Iowa
> Midwest Plan Service
> 122 Davidson Hall
> Ames, IA 50011-3060
> (515) 292-4337
> (515) 292-2097
> (515) 294-9589 fax
>
> What Joe and others say about the plywood thickness is probably true, i.e. it
> will make little difference as far as the strength of the plywood goes.
> However, you should be aware that nail strength and stiffness (i.e., how much
> your trusses will sag unnder load) will depend on the thickness of the members
> joined (your 2x and the plywood). Good luck and email me if there is anything
> else I can help you with.
>
> Richard Barnes wrote:
>
> > Hello,
> > I am used to building wooden trusses up to 26 feet span using glued and
> > nailed plywood gussets on 2x4 chords.
> > I have a supply of 3/8" ext plywood available free if it can be used. I am
> > used to using 1/2" for the gussets. If the longest span is going to be 24'
> > and the roof will be only wooden purlins and sheet metal , with 1/2" T&G
> > pine ceiling and insulation, would the 3/8" gussets be OK?
> >
> > --
> > Richard Barnes
> > Skwaller Holler Sawmill
--
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