Author Topic: Truss Calculators  (Read 369589 times)

0 Members and 1 Guest are viewing this topic.

Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #250 on: January 20, 2016, 10:56:46 AM »
Version 1.2.6 - 01.19.2016
- Added Solid Sawn Floor Joists (metric and imperial)
- Sill plate and Sheathing options (advanced) enabled for TJI and Solid Sawn floor joists.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #251 on: January 21, 2016, 09:28:08 PM »
For the next week or so I'm going to spend some of my free time on wood gusset plate engineering.  I'm going to post engineering questions and some quasi-engineering questions regarding the connections.  Realize that some of these questions are me thinking out loud, contemplating how best to approach this type of truss design.

I'm looking at the nailing of the gussets right now and I'm considering the difference between nails in single shear or double shear. For larger fasteners (ie. 16d nails) it would seem optimal to clinch them on the reverse side and then calculate them in double shear. My question is at what length of fastener exceeding the total thickness can I functionally clinch the nails and consider them in double shear.

For argument sake lets assume a 1.5" truss ply thickness and 1/2" gusset plates each side giving a total thickness of 2.5". An 8D common nail is 2.5", however I would not consider it in double shear in this application. If I were to use a 10D thru 16D common nail in this situation I would have at least 1/2" of nail or more to clinch so in those cases I think I could safely assume clinching was possible and nails are loaded in double shear. Would less than 1/2" of nail protrusion be too small to clinch?

To open up the calculations to as many options as possible I'm considering 8d, 10d, 12d and 16d nails with all the three possible nail types: common, box, sinker.

I also considering 6d and 7d nails but I'm not sure if I will allows those yet.

The plywood or OSB thickness will be: 3/8, 7/16, 15/32, 19/32, 23/32.

Giving this even more thought it would seem that certain gusset thicknesses and nail combinations would not be optimal if the possibility for clinching and double shear is not possible. For instance if I have 23/32" gusset plates on both sides and 1.5" truss ply for a total thickness of 3". If I were to use a common 10D nail or 12D nail I probably could not clinch and therefore double shear is not possible, hence I would have to nail the truss from both front and back. Would this not tend to cause the main member to have more tendency to split since there are double the nails in it.  It would also require roughly double the nails.

I'm also going to assume that the osb/ plywood is Structural I, this would be my recommendation anyways in an effort to eliminate defects and require a stronger material for the gusset plates.  This affects both the shear values of the gusset plates and the lateral loading capacity of the nails.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer


Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #252 on: January 21, 2016, 10:05:46 PM »
Some previous notes and images related to WGC trusses:





Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #253 on: January 24, 2016, 02:16:32 PM »
Assuming only a double shear connection which would prevent excessive nailing of the main truss member and splitting, the following gusset plate thickness and nail combinations would be allowed:

Gusset Plate   Total Thickness Nails
3/8" = 2.25": 10d Commmon, 10d Box, 10d Sinker
7/16" = 2.375": 10d Commmon, 10d Box, 10d Sinker, 12d Sinker
15/32" = 2.4375":  10d Commmon, 10d Box, 10d Sinker, 12d Sinker
19/32" = 2.6875":  12d Common, 12d Box, 12d Sinker, 16d Box, 16d Sinker
23/32" = 2.9375": 16d Box, 20d Sinker

This would allow for at least 3 x diameter for clinching, and also does not violate  NDS 2012 Sec. 11.1.6.5.

Current engineering practice uses the yield limit equations (Sec. 11.3 NDS 2012) found in the AWC National Design Standard to check the capacity of dowel type fasteners like nails. This is the standard I will use to comply with current code and engineering practice. Rather than use the tables I will have to calculate 4 separate equations for each joint in double shear (Im, Is, IIIs, IV). It sounds like a lot of work but once you set it all up in the program the computer does the number crunching. To manually calculate all of the numbers for a typical truss design would probably take me 4-5 hours, with a programmed solution, 10 - 20 seconds to enter the inputs and your done.
« Last Edit: January 24, 2016, 02:32:09 PM by Medeek »
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Don_P

  • Journeyman
  • *****
  • Posts: 4,597
Re: Truss Calculators
« Reply #254 on: January 24, 2016, 03:24:02 PM »
Somewhere... There is a clinching limit, I believe not beyond 12d, I think to prevent tension perp splitting.
Pallet manufacture uses hardened gun nails which will get closer to double shear without clinching but that is not a common gun nail. They also use a technique called "spotting" where they use a nail slightly longer than the assemble thickness and shoot it together over a steel plate. The nail connects then hits the plate and deforms a bit. Again probably not something for the field just info. I'm not sure if it will help in thinking about the yield limit programming but the awc.org connections calc is using the NDS equations. ~3/8" to clinch, that seems doable.


Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #255 on: January 24, 2016, 04:32:51 PM »
Somewhere... There is a clinching limit, I believe not beyond 12d, I think to prevent tension perp splitting.
Pallet manufacture uses hardened gun nails which will get closer to double shear without clinching but that is not a common gun nail. They also use a technique called "spotting" where they use a nail slightly longer than the assemble thickness and shoot it together over a steel plate. The nail connects then hits the plate and deforms a bit. Again probably not something for the field just info. I'm not sure if it will help in thinking about the yield limit programming but the awc.org connections calc is using the NDS equations. ~3/8" to clinch, that seems doable.

The only reference I could find to clinching in the NDS was Sec. 11.1.6.5 which doesn't really say you can't clinch nails with a diameter larger than 0.148" but it does impose at 6 x diameter penetration into the side member for double shear connections which effectively eliminates 16D common nails for 3/4" plywood or less.  This section is also where I am coming up with the min. 3 x diameter for clinching.  In my mind you need about a 1/2" protrusion to get it to clinch, if the nail gets too large (dia.) then this dimension needs to increase.

Also with regard to clinching and  staggering the nails so that they are nailed through both front and back. It probably makes the best sense from an engineering standpoint but from the construction standpoint, you would have to nail the front, then flip the truss over, clinch the ends, nail the back, then flip the truss again and clinch the back nails. Whereas if you just nail the front you only flip once and clinch.  Thoughts?

With truss repairs any adhesives used are typically  not factored into the strength of the truss by engineers, only the fasteners are considered in the calculations.  This adds an additional margin of safety to the assembly.  The primary reasoning behind this is the fact that it is hard to inspect and monitor the proper use of an adhesives in the field.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Don_P

  • Journeyman
  • *****
  • Posts: 4,597
Re: Truss Calculators
« Reply #256 on: January 24, 2016, 06:56:26 PM »
The front and back offset nailing including flipping the truss was how some of the old ag plans show to do it, with glue. IIRC they also called for a concrete nail, a hardened nail, so unclenched but approaching double shear. Fewer manipulations would be nice but I'd probably go back and shoot some from the other side after it is up just to feel better about it. If splitting is an issue, staples? Many small fasteners in single shear but not likely to split, sort of analogous to the metal plate connector.

I agree on field applied glue, it's the gravy but don't design around it. It does stiffen anything that was a pin connection if that matters at typical sizes/loads.

Found what I was remembering while looking up your cite, some wording has changed, this is '05, 11.1.5.5;

Quote
"Exception; Symmetric (!) double shear connections when 12d or smaller nails extend at least 3 diameters beyond the side member and are clinched, and side members are at least 3/8" thick"

Symmetric, I'm reading that as balanced nailing from both sides.

This is the '15NDS, 12.1.6.5 ;
Quote
Exception: The minimum length of penetration need not be 6d for symmetric double shear connections where nails with a diameter of .148" or smaller extend at least 3 diameters beyond the side member and are clinched, and the side members are at least 3/8" thick.

Still symmetric and clinched, unless the nails can get 6d into the far side member, then they don't need the last flop to clinch, you've gotten double shear without clinching but a slew of fasteners I'd bet.

Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #257 on: January 24, 2016, 07:49:40 PM »
Its hard to say what requires more effort, double the nails or clinching.  I've thought about 6d, 7d or even 8d nails in single shear but to avoid splitting the spacing drives the nails apart and the gussets are going to get too large in my opinion.  For now I'm going to start with nails only in double shear but may add in a single shear option later. 

I've got the front end working now so you can look at the input options.  It doesn't do anything yet (actually defaults to metal plates if you choose the wood option), but it gives an idea of the gusset options.

With wood gussets the web-chord joints are going to be much stiffer than with metal plates so I will probably set up the matrix analysis to consider all joints as rigid instead of pinned.  I'm actually still thinking about that and talking to other engineers with regards to pinned, semi-rigid or rigid joints with this type of construction.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Don_P

  • Journeyman
  • *****
  • Posts: 4,597
Re: Truss Calculators
« Reply #258 on: January 24, 2016, 08:05:34 PM »
Correct me, is a .120x3" nail with 3/4" gussets unclenched in double shear?
Or, is a .120x3.25" same conditions, shot through, unclenched.
Effortwise nailing is pulling a trigger, clenching does take time. That said I think the economical way is to use the thinnest ply and count on clenching these.
« Last Edit: January 24, 2016, 08:19:33 PM by Don_P »


Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #259 on: January 24, 2016, 09:11:22 PM »
Correct me, is a .120x3" nail with 3/4" gussets unclenched in double shear?
Or, is a .120x3.25" same conditions, shot through, unclenched.
Effortwise nailing is pulling a trigger, clenching does take time. That said I think the economical way is to use the thinnest ply and count on clenching these.

For a 23/32" or 3/4" gusset I am only allowing a 16d Box (0.135x3.5) or a 20d Box (0.148x4) that way it is clenched per the NDS.  If I can't clench it I'm not going to consider it for now.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Don_P

  • Journeyman
  • *****
  • Posts: 4,597
Re: Truss Calculators
« Reply #260 on: January 25, 2016, 04:47:46 AM »
I do agree, from my end the sideplates aren't backing out at all if things get torqued in the lift. I worry with metal plates when a lift isn't smooth.
Anecdotally, on the job I've been caught with .131x3.5" nails and 2@ 1/2" sideplates a few times. With ~1" of nail poking through you can hook the claw and make a bent tip then clench the nail driving the hooked tip across grain and back into the sideplate. That is the "dead as a doornail" clench. Nobody is going to intentionally go there though.

Am I reading "symmetrical" correctly, drive half the nails from each face?

There's an advanced wood design short course with a truss repair section coming up at VT, sometimes those are repeated at U Wash. Those rotate thru the Con Ed offerings if you ever have the time. Don Bender there is the nail doctor from his time in Brooks Labs at VT, those kids bend a lot of nails and do some serious pallet smashing  ;D.

Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #261 on: January 25, 2016, 06:05:13 AM »
Yes, I like the idea of half the nails be driven from each side, rather than drive them from one side only and clenched all on one side, probably a bit more work though.

I would really like to see more testing done, comparing a nailed and glued truss to a more conventional MPC truss. 
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #262 on: January 26, 2016, 03:01:30 PM »
Version 1.2.7 - 01.26.2016
- Added advanced roof options for hip rafter roof (sheathing, fascia).



https://3dwarehouse.sketchup.com/model.html?id=u3b08ddf6-b8be-405c-9ae6-912866e7a252
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #263 on: January 27, 2016, 11:17:55 PM »
I've uploaded a test L-Shaped structure with a hip roof:

https://3dwarehouse.sketchup.com/model.html?id=u8d384878-941b-4df9-a8c9-8bba849765bb

I've created both hip roof primitives with the plugin, that was the easy part.

I then placed a valley rafter with its centerline (top) inline with both roof planes.  I think I've got it right.  What I am unsure of however is the best way to terminate the framing at the intersection of the valley, lower ridge and flying hip.  Once I have a handle on how a carpenter would actually construct that junction I think I can proceed to start work on a secondary roof module for both hip and gable rafter roofs.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer


Offline Don_P

  • Journeyman
  • *****
  • Posts: 4,597
Re: Truss Calculators
« Reply #264 on: January 28, 2016, 04:02:59 AM »
I don't have time to sketch at the moment, run the major roof's hip down to the wall like normal, bob its tail at the outboard face of the wall or enough to let the next common rafter by. Set the jack opposite the valley then the ridge, then valley. What I see drawn is correct, on the backside from this view there would be another major hip buried in plane of the roof.

Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #265 on: January 28, 2016, 07:32:28 AM »
One method of framing this roof configuration is to extend the lower ridge past the joint until it meets the next jack rafter, then the flying hip and valley are miter cut to meet the lower ridge. 

I've created a version of the model above with this method of framing at the flying hip/valley/ridge joint:



https://3dwarehouse.sketchup.com/model.html?id=u06386a43-c138-4e90-8b2c-779fd2b63705

What is the thinking on this method of framing this particular configuration?

My other concern is the way I have the valley rafter miter cut where it meets the fascia and the corner of the building wall (top plate).  Is there a more practical way or better way of making those cuts?
« Last Edit: January 28, 2016, 08:31:03 PM by Medeek »
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #266 on: January 28, 2016, 08:43:15 AM »
Compare Rev 2 with Rev 3:



https://3dwarehouse.sketchup.com/model.html?id=u83ae5e44-1b19-4c09-8586-93988684289e

Rev 3 is framed per Don's instructions.

This version of Hip Roof 4 has the flying hip extending as a full hip to the exterior wall and then the lower ridge framing into it with the valley rafter framed in last.
« Last Edit: January 28, 2016, 08:45:47 PM by Medeek »
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #267 on: January 28, 2016, 01:43:29 PM »
Rev. 5 is a slight variation of the previous roofs.  I have shifted the secondary roof over by 24" to create a T-shaped building.  The question is how to best frame the long and short valleys:

https://3dwarehouse.sketchup.com/model.html?id=uf6d4e102-46d7-44b2-bfa2-b8ac12b6da21
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Don_P

  • Journeyman
  • *****
  • Posts: 4,597
Re: Truss Calculators
« Reply #268 on: January 28, 2016, 03:40:17 PM »
Rev2, the ridge to jack... I'm having trouble following the load.
Rev5 I would probably make the main roof hip the support and run the other valley up to the hip, shorter beam length.

I typically double hips and valleys, the inside miter is then half on each piece, simple compound bevels. I've also run a single bevel on the valley and run that subfascia first, carrying it past the valley to the wall and then butt the adjacent sub into the first one.

Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #269 on: January 28, 2016, 07:35:32 PM »
Rev2, the ridge to jack... I'm having trouble following the load.
Rev5 I would probably make the main roof hip the support and run the other valley up to the hip, shorter beam length.

I typically double hips and valleys, the inside miter is then half on each piece, simple compound bevels. I've also run a single bevel on the valley and run that subfascia first, carrying it past the valley to the wall and then butt the adjacent sub into the first one.

I've seen the double valley used in a number of roofs.  Your second sentence though I'm not quite following.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer


Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #270 on: January 29, 2016, 09:06:13 AM »
The intersection of the supporting valley rafter, valley rafter and lower ridge I had framed incorrectly.  The corrected method is shown below:



Also note that the segment of the supporting valley rafter between the upper ridge and lower ridge would need to be beveled or "backed" otherwise it clashes with the sheathing.  I noticed this when I originally added the supporting valley rafter but confirmed my suspicion when perusing DeWalt's carpentry and framing handbook this morning.  I probably should have pulled this book out before beginning this study but it only confirmed everything I had managed to discover myself once I started examining the model.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #271 on: January 29, 2016, 08:27:18 PM »
Now that I've got a gable and hip rafter roof I was thinking about adding in a dutch gable roof, but I'm a little unclear on the best method to support the end rafters.  Has anyone ever seen something like this used? 
 


I've checked all of my carpentry and construction books and there is absolutely no reference material on traditional framing of this type of roof, online is also quite scant, any resources, framing diagrams etc... would be quite helpful.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #272 on: January 29, 2016, 10:06:49 PM »
This is my first crack at a dutch gable roof framed with rafters:



Without the sheathing:



Please examine the model here:

https://3dwarehouse.sketchup.com/model.html?id=u060d4827-fbb5-400e-8270-0b244d958750
« Last Edit: January 29, 2016, 10:29:53 PM by Medeek »
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Offline Don_P

  • Journeyman
  • *****
  • Posts: 4,597
Re: Truss Calculators
« Reply #273 on: January 30, 2016, 04:02:17 AM »
Going back to yesterday afternoon's pic, one reason I like doubling hips and valleys is the ability to easily rip a backing angle down the longth. That piece's doubler has its appropriate backing angle ripped down its length. Then when you bring in the jacks or other framing it is easier to figure out where the sheathing plane is.

On the dutch gable I suspect the commons on the main roof need to be at least doubled. Lacking that hardware I think I'd run a jack from the underside of the minor ridge/header to the plate. (I've only built this style once, it's more coastal, and it's been too long). Historically we have more jerkinheads up here, kind of the inverse.

Offline Medeek

  • Journeyman
  • *****
  • Posts: 2,063
  • Structural Engineering
    • The Engineering and Design Firm of Medeek
Re: Truss Calculators
« Reply #274 on: January 30, 2016, 12:42:34 PM »
Doubling of the last common rafter is pretty much a given.  I can't seem to find many framing details on a dutch gable roof online.  For a large roof with a low pitch the span of the ledger holding up the end jack rafters would potentially become an issue, this member would need to be sized accordingly if not somehow supported mid span or by some other means.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer