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Timber Truss Design with Bolted Connector Plates

Timber Truss Design with Bolted Connector Plates

Timber Truss Design with Bolted Connector Plates

(OP)
Does anyone have any good resources for design of timber trusses with bolted connector plates, both architectural and engineering aspects of the design. This is an area I have little experience in but would really like to gain more, both for personal and professional reasons.

Out of curiosity I attempted to manually create a simple timber truss:



View model here:

https://3dwarehouse.sketchup.com/model.html?id=355...

Specifications:

- King Post truss with 4:12 pitch, 12" overhang and 1/4" buttcut, span 28'
- 6x10 Timbers (TC, BC, King Post), DF No. 2 (or higher depending on the results of the RISA analysis)
- 3/8" Thick Connector Plates
- 5/8" DIA. Heavy Hex Nuts and Bolts
- 2 Rows of Bolts
- 4.5" between rows of bolts
- 4" bolt spacing between bolts in a row
- Plates offset from timber by 3/4"
- Bolts offset from end of plates by 2"
- Bolts offset from edge of plates by 1.5"
- No washers

This particular connector plate configuration uses two additional bolts at the apex of the heel plates, many other variations are possible I'm sure but that is where the questions really begin, what is common practice and why?


A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Timber Truss Design with Bolted Connector Plates

(OP)
As far as the engineering goes, I can throw the timber design into RISA and the bolt checks would be from the AWC and looking at some samples from Breyer's book. I haven't really thought through the checks on the metal plates other than some tension and shear checks taking into account the net area due to the bolt holes. I've got some good steel design textbooks that I can probably find some excellent examples that closely parallel what is going on here.

I am wondering with this sort of manual analysis, how many load cases do you typically consider. Realize that this is a residential design and not some large commercial project with thousands of dollars to budget for the engineering and design.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Timber Truss Design with Bolted Connector Plates

Pretty straight forward... You likely don't need a T plate at the bottom chord at the king post and notching the end plates likely costs more. Just a matter of doing a proper analysis and design.

Dik

RE: Timber Truss Design with Bolted Connector Plates

(OP)
How about group action, eccentricity of the bolted plate connection and splitting. In your RISA analysis do you consider the joints as constrained (moments) or released (pins)?

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Timber Truss Design with Bolted Connector Plates

- AITC Timber Construction Manual may help.

- with software, load casing isn't so much of an issue. Gravity balanced, gravity unbalanced, and worst case uplift at minimum.

- Pinned joints usually. Occasionally, I'll go fixed for deflection only. Sometimes, I'll use TPI truss analogs for heel joint modelling.

- I'd make every effort to keep the plates as compact as possible and as centered on the member work points as possible. Chord and plate bending moments are generally reduced that way. As it stands, I think that you've got a pretty good moment going into your top chord.

- Plate buckling and bending may warrant checks too depending on your final configuration.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Timber Truss Design with Bolted Connector Plates

The bottom member will just act as a simple beam with a point load in the middle.

Little true truss action here.

Mike McCann, PE, SE (WA)


RE: Timber Truss Design with Bolted Connector Plates

Mike... look at replacing the bottom chord with a cable... and you have a 3 pinned arch... might be more appropriate.

Dik

RE: Timber Truss Design with Bolted Connector Plates

True, and that would be a form of a collar tie. But with the bottom chord designed as a beam you do not need the tension tie.

Mike McCann, PE, SE (WA)


RE: Timber Truss Design with Bolted Connector Plates

(OP)
The whole idea is to use a truss both for structural and aestetics, even though I appreciate that a cable could work in this instance.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Timber Truss Design with Bolted Connector Plates

Sorry... wouldn't use a cable either... just illustrating to Mike that it can be done as a truss, not a beam. As a beam, the bottom chord would get pretty big...

Dik

RE: Timber Truss Design with Bolted Connector Plates

Seems like relative stiffness of the bottom chord as a beam will come into play (as dik indicates).
As soon as the "beam" starts to deflect, truss behavior will take over. Axial stiffness of the top chords will be substantially greater than the stiffness in bending of the bottom chord. Unless of course, the connections at the end start to give.

RE: Timber Truss Design with Bolted Connector Plates

Pay attention to shrinkage and swelling at the bolted connections. As shown, there may be problems with the wood splitting between bolts.

The AITC Construction Manual mentioned by KootK is a free download at http://www.aitc-glulam.org/Shopcart/index.asp

RE: Timber Truss Design with Bolted Connector Plates

(OP)
Looking through some old plans I've managed to dig up I came upon a couple of designs by another engineering firm.

Typically when I've dealt with truss designs I always seen a scarf cut on the bottom chord with the top chord remaining un-notched. In this other example the bottom chord is kept intact and the top chord is scarf cut or notched, while still allowing a continuous section to extend for the overhang with a depth equal to at least 1/2 the top chord depth. I think the picture below explains this far better than I can with words:



For MPC trusses I am used to seeing a 1/4" butt cut, however for timber trusses what is more appropriate?

Which method above would go with (arch. and structural reasons)?

View model here:

https://3dwarehouse.sketchup.com/model.html?id=6d3...


A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Timber Truss Design with Bolted Connector Plates

(OP)
Plates applied to the top truss:



Specs:

- Truss Type: King Post
- 6x10 TC, 6x8 BC, 6x6 kingpost
- Plate thickness = 0.25"
- single row of bolts with two bolts per row.
- connector plate width = 4.0"
- Bolt Dia. 3/4"
- Bolt Edge Distance (timber) = 4.0"
- Bolt Edge Distance (plate) = 2.0"
- Bolt Spacing = 4.0"
- Bolts and Washers not shown.

View model here:

https://3dwarehouse.sketchup.com/model.html?id=ed8...

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Timber Truss Design with Bolted Connector Plates

The heel connection is typically the most heavily loaded and most challenging detail. As a structural guy, ideally I'd like to remove or minimize eccentricities in this connection.

Referring to two of your illustrations:
A. "Typically when I've dealt with truss designs I always seen a scarf cut on the bottom chord with the top chord remaining un-notched." I'd lean towards this system structurally as it seems to me to be easier to resolve the heel connection. I'd tend to push for this system the larger the span and the heavier the load.

B. "In this other example the bottom chord is kept intact and the top chord is scarf cut or notched, while still allowing a continuous section to extend for the overhang with a depth equal to at least 1/2 the top chord depth."
On a smaller truss, such as in a residence, the connection is not too difficult and I'd probably let architectural considerations govern. The plate design and bolt design in your final illustration "Plates applied to the top truss" need to be carefully considered.


RE: Timber Truss Design with Bolted Connector Plates

If you draw the FBD for your heel connection you have a pretty good eccentricity on your connection as drawn. KootK and I had a good discussion a month or so ago on the main Structural Engineering forum a month or so ago.

RE: Timber Truss Design with Bolted Connector Plates

I worked on 6 long, heavy timber, roof truss designs back in 1998. For the design, we used RISA and the NDS Manual which has good info on bolted connections with steel side plates. The heavy timbers were cut by a computerized robot so the dimensions needed to be very accurate. Our CAD plans were then entered into the robot's computer. The fabricator/supplier whined about the design cost and we weren't willing to work for next to nothing. So, that was the end of that relationship.

www.PeirceEngineering.com

RE: Timber Truss Design with Bolted Connector Plates

Another approach is to use your local truss fabricator:
-They have software than can analyze trusses efficiently (should be less than $100 for a specific design)
-Large 4x12 & larger timber trusses can be made with MPC
-Multi-ply trusses can be made easily in a controlled environment

I did over 300 Burger Kings using 2x4 chorded, 2x6 webbed structural trusses using MPC,
then added 2x6 aesthetic scabs to totally cover all the fabricators connector plates.
Added 1" no-thread Bolt/Washer/Nuts with a welded 16d tip with any placement.
Appearance of bolted timber without the steel gusset expense.
Fast and efficient construction with good code approval and building longevity.

RE: Timber Truss Design with Bolted Connector Plates

In the old days, they used multi-ply trusses with split ring connectors.
I've found bolts to be not so effective by the time you deduct for area, and group action.

RE: Timber Truss Design with Bolted Connector Plates

Last time I used split rings was only a few years ago... they work well and have their purpose. Mostly use glulam rivets these days... better than bolts.

Dik

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