WGC Truss
WGC Truss
(OP)
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.
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
www.medeek.com






RE: WGC Truss
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
RE: WGC Truss
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
www.medeek.com
RE: WGC Truss
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: WGC Truss
http://www.sbcmag.info/article/2011/plywood-vs-osb...
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."
RE: WGC Truss
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: WGC Truss
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_...
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
www.medeek.com
RE: WGC Truss
RE: WGC Truss
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: WGC Truss