Have any of you built your own parallel chord trusses?

[barbender]

  Just wondering about this, I could save a load of dough if I could build these with my own lumber. To buy them, they are in the range of $3-4 per foot. I would like to use them for floor joists and rafters. Anybody got any info??

[beenthere]

Are you familiar with the makings of the truss? 

Are you in an area where you can make your own, and do not have to meet the local building codes that are most everywhere now...and probably good to have in some cases?

Here is a pdf doc. on some of the requirements that mfg's have to consider when putting their trusses on the market, in order to get the $3-4 per foot.

http://www.icc-es.org/criteria/pdf_files/ac224.pdf

Seems to me that you certainly can build them, and have them work..in most cases.

[Thehardway]

I would consider using "Larson trusses" for the rafters.  They are a form of parallel chord truss and very simple to make even out of scrap lumber.  Floor trusses would be another matter. 

[Don P]

 A larson "truss" is a spacer to provide deeper insulation, it is only sized to support itself and drywall. They work fine for their purpose but are no more structural than the strength of the joist or rafter. They are not a parallell chord truss.

Doodling on the truss calculator to show some of the thinking behind one,
I quickly drew a 16' longx12" deep joist and loaded it with a typical load;


The truss calc is here;
http://www.jhu.edu/~virtlab/bridge/bridge.htm

The blue shows compression members and the forces within them, the red is tension members and their forces.

With the force numbers in hand I would find the highest stress in the members and check them for strength first, adjusting the truss depth or web spacing to meet the materials. Then check to see if I could make a connection to resist those forces. Again modifying the truss to the materials. The truss I drew is going to pass with 2x4's just great at a glance. It's going to be tough to make connections that strong though. Going deeper would help that out pretty fast if it would work in the building.

[Joel Eisner]

We made and used larsen trusses on our timberframe roof.  They were nailed through the roof decking into the purlins.  We then attached skip sheathing, insulated and tin roofed it.  They worked great.

The nice thing about them is that they used up the osb from the window cut outs etc and anyone could staple and glue them together.

[Thehardway]

Don,

The Larsen Truss I am referring to can be seen on page 101 of Tedd Benson's " the Timber Frame Home.  He shows them used as a structural roof system. It is built using 2X2 and OSB as Joel mentioned.  The trusses he shows for the roof are much deeper than those he shows for the walls.

I guess the determining factor here would be what the span and load is.  I see them as parallell chord trusses only because the top and bottom chords are indeed parallell.  In the strictest sense of the word I guess they are not really trusses at all but a form of fabricated beam or Joist similar to a TJI.

I went back and did a search on Larsen trusses  just to make sure I wasn't hallucinating. It seems the roof application is not commonly used.  Non structural sidewalls seem to be far more common and in that application they are not as deep and as you indicated they are only supporting themselves and the sheetrock.

Hopefully I haven't thoroughly confused anyone. Just thought if the application was a short span cathedral style roof it could be a esy and economical substitute for solid sawn rafters.

Bob

[Don P]

No, Thanks, I'll do some more research. I only knew of the non structural one  . If you come across links to designing structural ones I'd like to know more.
Doing it the way I described or by trying to copy in some way what the truss plant offers will cost more in connections than their completed truss I think.

[Don P]

I played around with the same 16' span but changed the depth to 2'. I checked the AWC's connection calculator. It looked like using 1/2" osb gussets with about 7 nails per member end on each side through double side plates would make me happy.
http://www.awc.org/calculators/connections/ccstyle.asp?

The next check would be the members. The most heavily stressed compression member is 4' long with 928 lbs pushing on it. I've set the column calc below to check that member.

The maximum tension in a member is 1050 lbs.
Allowable tension in a member is the net section area times allowable tension per square inch.
1.5*3.5*450=2362.5 lbs allowed in tension in a #2 spf 2x4 so its plenty good.

Here's the basic idea of what I'm thinking, I got tired of drawing nails but you get the idea.  ???

[Gilman]

On page 17 there is a picture, is this a chord truss?

Link to the APA
www.gp.com/build/documentviewer.aspx?repository=bp&elementid=3817

[Don P]

Wow, that is a cool pdf. It is geared more towards an in plant type operation I think.
What they are describing is an I beam, a simple beam in bending, so its not really a truss. The math is pretty similar to sizing a sawn beam. Technically a beam handles load by resisting it with its bending strength where a truss resists load by using the compressive and tensile strength of the wood. This approach is using a center web to handle the lower stresses in the center of the beam and built up flanges where the stresses are greatest to make one that is "efficient", putting wood where it is needed most. I'm not sure that names matter if it gets the job you need done. If you replaced their plywood web with strut webs rather than a solid one the math would change to truss math, analyzing the parts as columns 
The downsides I see;
Its all plywood so everything is bought.
For me I'd probably have to have some kind of third party inspection of the lamination for anything but ag use.
It uses alot of high quality glue, you'll be buying in 5's for a single beam or by drums for a set of beams.
I think for a typical application I could get an LVL cheaper that was made under controlled conditions though.

I bet with the other tech sheet they mentioned some neat shapes would be possible, that's where I think it would shine .

Edit; the "design methods" section on page 3 of this pdf is a good read
http://www.fpl.fs.fed.us/documnts/pdf1990/leich90a.pdf

This is the opening paragraph of that section;

The design of wood composite I-beams allows the positioning of materials to
take best advantage of their material properties. Combining lumber (or laminated
veneer lumber) and plywood (or oriented strandboard or waferboard) into beams
with an I-section provides a high degree of structural efficiency. In general, the
flanges are designed to provide all moment capacity where cross-sectional sizes
are determined using simple bending theory. The webs are assumed to carry all
shear forces. Shear capacity is most often empirically based. Other necessary
design criteria include bending and shear deflection, bearing capacity, and lateral
stability.

[barbender]

Don, I'm liking all the info you've got. I've just got to study all these numbers, I'd like to get a better handle on figuring all of this stuff.

[Don P]

I've come across a couple more tidbits in reading. The metal gang nail plates in typical lightweight trusses have design shear strength of 80-200 pounds per square inch of plate, depending mostly on edge allowances and species. Typical gun nails have very similar strength per nail again depending on species. So a way to kind of reverse engineer it in your mind is to look at the number of square inches of plate on each side of a joint and figure at least a nail per square inch of plate, don't forget to do this is for both sides. Your "plate" can be larger to get sufficient edge and end distances for nails, but its giving a rough idea of the connection strength required.

The next thing was the proof testing. A metal plate connected wood truss is tested by applying 2x the design load for 10 minutes.

[barbender]

  I was thinking about building one and "testing" it by pressing on it with my skid steer  Just to see how much it could take before it broke. Not very scientific though.  I was wondering, what if the connections are glued as well?  Around here, most of the old carpenters feel the glued/nailed/plywood gusset "W" trusses are stronger than the steel tie plates. I don't know, but they are a lot sturdier when handling them. Those steel gang plates are pretty flimsy when they are on their side. Anyways, the reason I asked about this in the first place, is there are a lot of buildings around here with site built trusses, there's even a lot of sites on the web with plans for building the "w" style trusses. But the parallel chord type I've only seen pre-fab. It's still just a truss I figure, I can saw boards as good or better than the quality that they are using. So then, the magic is in the necessary depth for the span, and the connection strength. That's why I've been trying to swallow all of these numbers and get it sorted through my brain

[Don P]

I'm not qualified to say on the plywood gussets vs metal plate connected trusses,  I think glue can only help. One part of strength is stiffness, which is what I think the old carpenters are feeling. Iron is stiff, steel is ductile, which is stronger, depends. I don't think the glue can be used in your strength calculations for the joint though, that should be all on the nails or bolts or whatever mechanical connection you use. Counting on glue to "help" or mixing connectors to help each other is potentially dangerous if you count on it. For instance glue takes load immediately where nails slip and deform a bit as they take load. If you tried to combine the strengths to come up with a total to resist the load it might not work. An often repeated phrase is "load goes to stiffness". The glue would take the full load until it held or failed and only then would the joint slip a little and load the nails. Assuming we just broke the glueline there would then need to be enough nails, the next stiffest connection, to take the whole load. The same thing happens with mixing bolts and nails, etc. Each connection type takes load differently. I'd glue it, just don't skimp on the nails. Glue joints in the field are notoriously variable... the Boston tunnel comes to mind.

Keeping a single truss in plane without supporting it unintentionally might be tough while testing one. Hanging drums with water would be one way to get the load.

I'm still trying to muddle through this part of my education myself, as long as you all don't mind it too bad I'm learning alot as we go. Do get it checked if the life at risk is more important than a potential early barbeque ;).

[Thehardway]

Don,

Good point about mixed connectors.  I would like to see some data on load calculations for glue only joints and the charachteristics of their failure. This might help us a little in understanding the load assesssment of mixed connections.

I have never seen a glued connection actually fail at the glue line if the surfaces were properly prepared.  Most of the time the wood breaks loose on either side of the glue line. (I know only because I have a knack for breaking things )

I assume this is because the adherance of the synthetic glue is stonger than most natural wood resins.  This applies only in cases were gluing parallel grained wood, not crossgrain or butt joints. 

In joints where glue is applied, the fastners (nails, screws or bolts) most likely function as clamping agents until the glue has cured.  After this, as you mentioned, they serve little purpose until the glue joint fails  after which they would be a pinned connection in either single or double shear depending on whether or not they are plated on both sides and pass all the way throu or only pass thru one plate.   

[JGroebner]

, my father is a master carpenter and has been building stick frame homes for 40 years now.  I asked him about hand-framing rafters a few years back and he said, "it's foolish as a pre-fab truss is backed by an engineer and if you calculate the time lost a truss co. can do it for significantly less".  I literally had a crew just install pre-fab parallel chord trusses on a T.F. in Wi over a 34' space and almost any truss company will do drawings for free.  If you're going to do this yourself you could easily take the FREE truss co. drawings and build them yourself.  As for the gusset connection, if your truss is going to be exposed and it's part of a timber frame It would be VERY disappointing to see steel.  If you want to see some unbelievable 100-150 year-old scissor trusses, let me know as there's some great standing examples.

Jared