WGC Truss

[medeek]

I've mucked around with wood truss analysis a bit in the last couple of years but until now have never had the opportunity to actually design one for a customer.

Recently I've been contacted by a client who would like to fabricate his own (wood gusset connected) trusses utilizing 2x4's, 8d nails (no glue) and 1/2" (15/32") plywood/osb plates for an out-to-out span of 20 ft with a 4:12 pitch.

They would like to utilize a king post truss (only one web) versus a fink or queen post truss in order to minimize construction time and labor. Hence the need to engineer the design since they feel they may be pushing their design a little with a king post type truss.

My algorithm for the analysis will go something like this:

1.) Determine design criteria of location (snow, wind, seismic) and geometry of the roof (out-to-out span, overhangs, pitch).
2.) Establish all pertinent load cases: Balanced snow, unbalanced snow, wind C&C, wind MWFRS, Dead Only, Construction Live Load etc...
3.) Calculate the tension/compression for each member for each load case.
4.) Calculate the panel point moments and mid panel moments as applicable for the top and bottom chords for each load case.
5.) Calculate the heel joint moment for each load case, (no top chord or bottom chord splices so no splice moments necessary).
6.) Calculate the reactions of the truss for each load case.

7.) Utilize the maximum tensions/compressions and moments of the previous calculations to design the members and the connector plates.
8.) The max. bearing will be used to check the bearing at the heel and the min. bearing will be given to the customer as the max. uplift to design for (ie. install H1 hurricane ties).

I will be using the ANSI TPI 1-2007 as a guide in running the different checks on the members and plates.

I think this should be relatively straight forward, however since I have never actually done a full truss analysis I thought it might be a good idea to throw this out there and see if anyone had any specific hints or suggestions.


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

http://www.medeek.com

 

[manstrom]

The truss design is easy. The connection design is a bear without toothed plates.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[medeek]

Minor Correction:

For plywood/osb plate design I will be referencing some of the sample problems from the NDS 2005 Examples publication as well as Breyer's book.

The key factors will be the loading of the nails in single shear, tensile capacity of the ply/osb, compressive capacity of the ply/osb.

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

http://www.medeek.com

 

[medeek]

What are the main difficulties with wood plate design?

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

http://www.medeek.com

 

[woodman88]

A couple of useful links

http://www.sbcmag.info/article/2011/plywood-vs-osb-truss-repairs

http://www.apmtruss.com/truss-repair.pdf

Garth Dreger PE - AZ Phoenix area
Until we as engineers are willing to read the laws are that the AHJ's and PE's are required to follow. The engineering profession will never be more than second rate.
2012 IBC Section 102.2 "Other laws. The provisions of this code shall not be deemed to nullify any provisions of local, state or federal law."

 

[KootK]

There is a publication by the Canada Mortgage and Housing Corporation called Canadian Wood-Frame House Construction. It contains an appendix that has some recommendations and even some canned designs, which I find a bit shocking from a liability perspective. When I've designed site-built trusses in the past, I've used it as a starting point.

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.

 

[medeek]

I have been finding a number of interesting documents out there, most of them outdated of course but helpful nonetheless.

This particular paper was interesting in that it showed some of the different failure modes of plywood gusset plate:

http://www.csbe-scgab.ca/docs/journal/28/28_2_167_ocr.pdf

The one I had not considered was the rolling shear, I guess there will be quite a few checks applied to the gusset plates (connections).

Site built trusses seem to be more popular in Canada, probably due to the remoteness of some locations and the difficulty in obtaining pre-manufactured trusses.

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

http://www.medeek.com

 

[Brad805]

Home built trusses are only somewhat common in Canada only where engineering is not required. As soon as you add in any engineering cost or the owner (rarely contractor choice) figures out the plywood waste factor they find the correct route is to buy trusses.

 

[medeek]

I would agree, pre-manufactured trusses are the most economical and easy route to go. A large part of this is the automation of the design and engineering of these trusses with the software developed by Mitek, Simpson, Alpine and the like. Without this packaged into the deal the cost of pre-manufactured trusses would not be so competitive. However, in certain situations where a truss plant is not conveniently located nearby there is still a market for site built trusses.

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

http://www.medeek.com

 

[Brad805]

I have worked in many small towns, and trusses are never that far away. For a small garage, or house, sure, home built works, but that's about it. I have been to several small truss plants that use fairly low tech presses to create trusses without all of the automation. The startup cost is not that great in those cases, so you can find plants in towns with a population as low as 10,000. Trucking is also not that expensive. We have shipped 1000's of pounds of wood product from 500miles for hundreds of dollars. The normal problem with this type of client is planning or patience. They decided yesterday they want to build something, and they want to build tomorrow.