# CountryPlans Forum

## Internet Finds for Designer/Builders => Referral Links => Topic started by: Don_P on April 10, 2009, 04:13:19 PM

Title: CALC: Simple Truss Design Program
Post by: Don_P on April 10, 2009, 04:13:19 PM
There are several free truss programs on the web. This is the simplest I've seen for checking first to see if you have actually designed a stable truss and then to get an idea of the forces within the truss. http://www.jhu.edu/~virtlab/bridge/truss.htm
Title: Re: Simple Truss Design Program
Post by: glenn kangiser on April 10, 2009, 08:07:51 PM
Thanks Don.  Interesting to play and learn with.
Title: Re: Simple Truss Design Program
Post by: Don_P on April 14, 2009, 05:48:55 AM
I was doodling and figured I'd show a simple 24' wide 6/12 pitch truss. I drew it up as a W or Fink truss. This breaks the rafter spans in half and the ceiling span into thirds. Doing so allows the use of smaller members.

Members in compression, being pushed together, are in blue. Members in tension, being pulled apart, are in red. The vertical black lines are loads hanging from the "nodes", the splices between members. The numbers beside each member are the axial forces in the members, the compression or tension load along the length of each stick. Each stick is assumed to only run from point to point so the stress can vary along its length if the rafter runs full length.

(https://i180.photobucket.com/albums/x109/windyhilll/rooftruss.jpg)

The loading scale I used is X10 so add a zero to all the numbers to get pounds. If this truss were 24' long and set on 2' centers there would be 48 square feet of roof area. I've hung 1600 lbs from the top, rafter, chord of the truss. 1600/48=33.33 lbs per square foot load. That would be pretty close for my 20 lb snow load and 10 lb dead weight. On the bottom chord I distributed about 10 lbs per square foot among the panel points.

Once I drew and loaded the model I had it calculate and give me the forces present. The members and connections need to be sufficient to resist the forces. For small trusses using the codebook tables or the AWC joist and rafter calc works for the rafter and ceiling joist members. The rafter members are spanning 6' and the ceiling joist members are spanning 8'. The AWC connections calculator would work for getting the nail quantity and splice plate thickness. If the side plates are well glued they will actually carry the load, but sufficient nails are also used in case, or when, the glue joint fails.   As a real rough rule of thumb, looking at the heel joint and giving each nail 100 lbs in shear I would use 18 nails into each the bottom chord and the top chord. 9 into each member from each side. This is the type of connection where many small fasteners are better than few large ones, as long as you space them around and don't split anything. Losing one fastener is not as critical if there are many small ones and it makes the connection both fail slower and more noisily. Big bolts can easily deliver large concentrated loads that can split regular 2x dimensions of wood, something to think about. Usually if you can resolve the heeljoint connection the rest get easier.

Notice the tension stress in the bottom chord is 1500 lbs in each outer section but only 1000 lbs in the inner section. If I needed to splice the 24' long bottom chord, I'd do it near a node but in the center panel. Go look at some factory trusses and where they splice the bottom chord, about 1/4 of the way inboard of that panel point  ;).

The yellow and red dots at the ends are my bearing points, the vertical arrows under the are the "reactions" ... for every action there is an equal and opposite reaction, these are the loads heading down the wall framing, about 1000 lbs down the stud under each end of the truss under a snow load here. That load can really vary, and alter the design, there are folks with ten times that snow load.

Title: Re: Simple Truss Design Program
Post by: John Raabe on April 14, 2009, 07:23:02 AM
Thanks for that helpful tutorial on this interesting tool. Helpful stuff! :D :D :D
Title: Re: Simple Truss Design Program
Post by: Don_P on April 14, 2009, 03:23:40 PM
Thanks John,
It is a fun program to play with.
This is a neat comparison;
This picture shows the difference in stress in a steep pitch compared to the shallower pitch in the first picture above. This is the same span and load just a steeper 12/12 pitch roof.

(//i180.photobucket.com/albums/x109/windyhilll/rooftruss2.jpg)