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Author Topic: Roof Trusses  (Read 1065 times)

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Offline WranglerSS

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Roof Trusses
« on: July 27, 2017, 06:43:12 pm »
Not sure if this is the right forum for this question but I'm planning a 36' x 48' building from lumber cut with my WM LT28. I can buy trusses for around 4k but would rather build them myself.  My question is can I use steel plate to connect chords and webbing instead of the nail plates? I have a CNC  plasma table so designing and cutting the plates are not a problem.  The trusses will be 2x6 for top and bottom chords and 2x4 for webbing. Also what thicknes steel is sufficient?
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Offline Don P

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Re: Roof Trusses
« Reply #1 on: July 27, 2017, 08:10:02 pm »
Yes, but you'll need to do some digging and engineering

The awc.org connections calc gives design values for connections using metal plates once you have the loads figured out

There is a lot of info on this calc;
http://design.medeek.com/calculator/calculator.pl


Offline WranglerSS

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Re: Roof Trusses
« Reply #2 on: July 27, 2017, 09:08:52 pm »
Thank you Don, I'll do some reading.
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Offline fishfighter

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Re: Roof Trusses
« Reply #3 on: July 28, 2017, 08:12:51 am »
I have used 3/8" plywood and glue in the pass with no problems.

Offline Don P

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Re: Roof Trusses
« Reply #4 on: July 28, 2017, 02:10:26 pm »
Glue is a good thing but is not figured in the strength of the connections. That same connections calc does have design values for nails and ply. There again you need to know the load you are resisting in order to know how many nails you need. Driving nails all the way through and clinching them on the backside increases their strength by about 25%.

Offline WranglerSS

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Re: Roof Trusses
« Reply #5 on: July 29, 2017, 09:38:32 am »
I was thinking more along this route.
 

  

 
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Offline Jim_Rogers

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Re: Roof Trusses
« Reply #6 on: July 29, 2017, 05:34:12 pm »
My question is can I use steel plate to connect chords and webbing instead of the nail plates?

You can but you can't just throw some steel at it and place the bolts wherever you want. If you don't do it correctly you can make it so that the bolts split the timbers.
Bolt size and placement is very important. And, I feel, that it has to be done correctly.

I asked the engineers to send me some pictures of the stuff that they see that is done incorrectly.
And they sent me this:
 

  

opposite side:



 

In the above photos the bolt were not spaced out correctly and that caused the timbers to split and shift. The engineers were called in to try and save the building so it wouldn't fall down or fail more.

It is complicated according to one of my engineers but you have to do it right.
I don't know the standard spacing rules as this is what I pay my engineers to do for me when the client wants steel instead of mortise and tenon joints.

Part two:
Quote
Also what thickness steel is sufficient?

The size the engineer says will work based on the loads (and forces) being applied.

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Offline btulloh

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Re: Roof Trusses
« Reply #7 on: July 29, 2017, 07:56:42 pm »
Wow.  It doesn't take an engineer to see that won't work.  They should call that a froe plate because it's a good way to split a timber.
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Offline grouch

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Re: Roof Trusses
« Reply #8 on: July 29, 2017, 10:04:12 pm »
btulloh,
That thar be whatcha call a "slow froe". You use them things when you want to sit back and watch the building do the work instead of you doin' all that pounding.

I bet Jim's engineers have quite a collection of "why would anyone do that" kind of photos.  ;D

When I built my garage, I used some Canadian Plan Service plans for the gambrel roof trusses. (The truss plans have since disappeared from their site). These were very specific in the size, length, number and precise placement of the nails in every gusset. I made a stencil for each type of gusset so I could spray-paint the pattern and not risk collisions when nailing from each side.

Trusses and rafters are not the same animals.

Offline Don P

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Re: Roof Trusses
« Reply #9 on: July 29, 2017, 10:32:45 pm »
I do agree with enlisting an engineer for the design. Go slowly and really study what is going on. You do need to understand them to design or build them right.

Jim's pics are of a poorly designed splice near, but not at, a panel point in a truss. It produced an eccentric force that distorted the truss. You are looking at 2 different members in the bottom chord not a break, but, the rotation is causing the bolts to begin splitting the right hand bottom chord member.

A rafter couple with a ceiling joist is the simplest form of truss.

The AITC manual is a good resource for this type of construction. It is pretty expensive and get's deep and wide of what you need. You might be able to get it through interlibrary loan. There are selected sections of it available for free download here, click on the typical construction details tab for a good start, detail A15 in the "don't do this" section shows the error in Jim's pic;
http://www.aitc-glulam.org/shopcart/index.asp

I think these are similar to the plans Grouch is describing;
https://www.ag.ndsu.edu/extension-aben/buildingplans/construction

Offline grouch

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Re: Roof Trusses
« Reply #10 on: July 30, 2017, 10:11:15 am »
Don P,
I remember looking through that NDSU collection as well as the one at University of TN (which I can't find any more) and the one at LSU. Those plans seemed a little short on detail and a bit clunky compared to the Canada Plan Service plans (which is also defunct, it appears). The CPS plans gave such things as horizontal, vertical and uplift forces at the sill, snow load and rain load for slippery and non-slippery roofing, and wind load.

Thankfully, much of the information in the CPS archive has been preserved by Canadian Society for Bioengineering.

I can't find the truss plans that I used in that archive, either. It was M-9252P.pdf and the whole 9200 series seems to be missing from the archive. The title was "7.2 m (24'-0") Medium Duty Braced Rafter 600 mm (2'-0") O.C." It's referred to as a "gambrel truss" in other plans, but that fits what you were saying about rafters and ceiling joists.

Offline WranglerSS

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Re: Roof Trusses
« Reply #11 on: August 03, 2017, 08:45:47 am »
I've decided to just buy engineered trusses.
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Offline Al_Smith

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Re: Roof Trusses
« Reply #12 on: August 14, 2017, 05:49:40 am »
Late to the party but just as general info there are many sites on the internet to find truss plans .If nothing else go to the library .
Plywood nails and glue is still an accepted method for tie plates and probably is as strong as metal plates .You can buy metal plates but I never found it necessary .
As far as glue remember this is the stuff that holds plywood together ,just use a good grade of same .

I would imagine the reason commercially built trusses use the metal plates is because it's a faster method .

Offline Don P

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Re: Roof Trusses
« Reply #13 on: August 15, 2017, 06:40:24 am »
Do remember that plywood glue is a structural adhesive applied to clean fresh, tight tolerance machined wood and uniformly pressed under controlled conditions... that isn't readily available or achievable in the field. Glue is a good thing but you cannot give field applied glue any load bearing credit. Most of the glues available to us also creep under continuous load. Back in the day I ran an RF high frequency glue machine and finally sensitized to the urea formaldehyde, rescorcinol was the standard adhesive for exterior grade. These are catalyzed rigid, structural glues, rather than the stickum type titebond pva's and subfloor glues readily available.

You will more than likely get a better truss from the truss plant than you can put together in the back yard. The learning curve on a one off is always steep. I helped with a set of home built trusses last year, in the end I think the savings was negative.

Offline btulloh

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Re: Roof Trusses
« Reply #14 on: August 15, 2017, 08:00:24 am »
Good insight, Don P.  You bring some direct knowledge to the party.

BTW - Resorcinol glue is readily available and is worth keeping around.  It has properties you can't find in standard glues.  When you need a glue joint that will not creep, it is unmatched in glues generally available.  It is especially good on jungle woods and other oily woods.
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Offline Al_Smith

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Re: Roof Trusses
« Reply #15 on: August 15, 2017, 04:17:06 pm »
Some time back,like 1980 I built the trusses for my 30 by 36 foot garage at a place I no longer own .2 by 6's 16" on center 5-12 pitch . Those took a weekend to build with help and were rated much stronger than 2 by 4 commercially made on 24" centers, 2/3 the price too  .Fact the roof load was rated at 240 pounds per square foot .Not my calculations.Those were  done by a civil engineer at work .
As a side note it was 3 tab 240 pound per square roofing which I "quarter" tabbed and believe it or not that roof still looks good after all this  time .

Offline Don P

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Re: Roof Trusses
« Reply #16 on: August 20, 2017, 08:54:35 pm »
Where I think site built or homemade trusses shine is sort of like where I think small sawmills shine. I can saw commodity sizes but am not really competitive with the guys that bang out tens of thousands of board feet per day. The same is true with trusses. You can make your own lightweight trusses but IMO if you're going to go to the trouble, make fewer, nicer trusses and show them off, like in Wrangler's pic, outside the commodity stream. They can be simple and still look better than the lightweight stuff.



That said, I've sawn framing too, what works at the time is always a moving target. This is a cheat sheet for a simple Fink truss. It give member forces and so joint forces as well;


An example; Take a 30' truss with a 35 pound per square foot total load set on 2' centers.
30' x 2' x 35 psf=2100 lbs total load
divide by the number of w's in the diagram 2100/4=525 lbs per panel point
The bottom chord tension is the first thing I check, usually if you can get that joint to work the truss is buildable. So check the tension in the bottom chord DC, it's 4.5 times the panel point load 525 x 4.5=2362.5 lbs tension. You need to resist that amount of pulling force on each side of the joint at the heel and along the bottom chord.

The AWC.org connections calc is here;
http://awc.org/codes-standards/calculators-software/connectioncalc
Divide the load by the allowable load per fastener.

"Nails are not ideal fasteners for truss joints because of the potential for movement in the joints and possible loosening of the joints due to initial shrinkage of the wood or of repeated swelling and shrinkage over time due to fluctuations of moisture and temperature. In light trusses with plywood gusset plates it is common to glue the plates in place, using the nails to clamp the glued joint, although the nails will also actually be designed for the joint loads"

Offline grouch

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Re: Roof Trusses
« Reply #17 on: August 20, 2017, 09:51:06 pm »
Don P:
If those are ordinary Edison base incandescent bulbs in that picture, those are some hefty pieces of wood. Are those called king post trusses, same as in a king post bridge?

Thanks for posting that chart. That is interesting information! I've always heard those referred to as "W" trusses; didn't know "Fink".

Nails may not be ideal fasteners for trusses, but all of the commercial ones I've installed are held together with nail plates. :) (Yes, I'm being a bit disingenuous there, just for a lame joke. You're talking about nailing gussets on site-built trusses vs the nail plates pressed on and in factory-built trusses).

The gambrel gussets I referenced earlier were to be nailed on with concrete nails and there was note of making sure they completely pierced the gusset on the other side. I was guessing at the time that it had something to do with resistance to pull out and shear. Concrete nails typically have ribs, ridges or are even square like blacksmith hammered "plancher" nails.




Sometimes they don't put enough oomph in the press. Might've been a new guy in a hurry, there.

Offline Don P

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Re: Roof Trusses
« Reply #18 on: August 20, 2017, 10:44:55 pm »
Those are ordinary Walmart bulbs  ;D If memory serves it was 6x8 and 6x10's. Yes that is a kingpost truss, or as the brits sometimes call it, a king rod truss, which is probably more descriptive. The kingpost is pinched between the top chords and dangles down to support the bottom chord, supporting it at midspan to keep it from sagging. Same concept in a kingpost bridge. Think of it as a rope dangling from the peak and lashed around the bottom chord. From that kingpost you can angle web members back up to support the top chord breaking its' span in half.

This is how the top of the kingpost joint works, there is a tennon on the bottom that is pegged in the bottom chord.



I recall that cement nail callout on the Canadian plans, a hardened nail is going to perform better and I suspect they had some numbers allowing for double shear but this book and the NDS only give single shear numbers to nails. The book "Design of Building Trusses" (James Ambrose), the NDS and the connections calc all line up within a couple of pounds/fastener.

I've had trusses look like that more times than I like. It does give credence to his concerns with nail type connections. I do prefer hand framing a roof whenever possible but there's a time and place for everything. I've kind of reverse engineered a few trusses, basically each square inch of the gang nail plate equates to an 8d common nail.

Remembered this pic in the gallery of a kingpost and queenpost bridge reinforcement idea for a simple span beam bridge. One cuts the beam span in half, the other into thirds.