Once again here is the land of down under down under (tasmania) I feel as if I am reinventing the wheel or trying to to chart a course to an unknown country without a compass.
Timber framing is an unknown art here and any engineering advice I get generally involves blank looks and large amounts of steel, both of which I would like to avoid.
The current project involves the addition of a hexagonal polygon frame with a 26ft diagonal span to my house. The original plans were submitted to the local building authority before I had caught the timber framing bug and were drawn with a hip rafter pitch of 8:12. I was hoping to create a hexagonal hammerbeam but my research indicates that the low roof pitch may be pushing the boundaries a bit where horizontal thrust is concerned.
My first question.
Is it feasible to create a sound hammerbeam truss at 8:12 hip pitch without resorting to metal fastenings and ridiculous member sizes?
Secondly:
Can anyone reccommend a freeware structural analysis program that will run on a mac and enable me to perform a few peace of mind calculations? I am finding the pen,paper and calculator approach to structural analysis a tad tiresome and difficult.
Unfortunately my lack of technological savvy prevents me from posting images of the initial frame design (all pencil and paper for this luddite) but I will try to describe it below.
6 radially oriented 8x10 posts 10ft long
6x12 crossover plates
8x10 hammerbeam/dragon beam joined directly to post top with significant relish outboard of the post
6x12 hip rafters tenoned directly into hammebeam at bottom and a hexagonal kingpost at top (12" diameter kingpost)
6x7 hammerposts sitting on top of hammerbeam
6x7 collar ties at midpoint of rafters
5x10 arches from bottom third of post to hammerbeam and from hammerpost to collars. Rather than fitting a straight line parallel to the principle rafters, these braces follow as close as possible a catenary curve beginning near the base of the posts and meeting near the kingpost/collar tie intersection.
5x8 jack rafters from plate to hip rafters-4 jacks per roof plane (something like 34" oc) With the jack rafters either screwed or fastened with wedged splines as the hip rafter is too slender to fit full length tenons.
I hope this jarbled description makes some sense.
Two other things to bear in mind here too.
1.It doesn't snow here....no snow loads
2. Australian timber is bloody heavy and strong (some of it sinks when dry) so if some of the timber sizes sound a little scanty they probably are in the north american context but they are what I have in the timber stack to work with.
I hope you can shed a little light on my darkness.
Cheers
Galen
