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

Hi,

Do you have a sketch and a camera? From there, we can get you instructions on how to get that sketch on this screen.

Maybe iron tie rods, or timber buttresses around the post perimeter. Almost an additional colonnade if you will.

I don't know design loads in Tasmania and I don't know design values for your timber, but around here (nominal/no snow load) in any wood you would need either tie rods or a serious tension ring design around the eave (like a hoop around a barrel, keeps it from spreading).

Tie rods are pretty straightforward -- they could all go to a decorative/functional hub and radiate out from there.

The tension ring can be done with timber plates at the eave and steel strapping or plating to tie adjacent plates together across the posts. Steel can be expressed or concealed as desired. At 8 in 12, you would have significant loads on those connections at which point strapping won't be enough and you would need a serious weldment.

Or as mo said, you could buttress with another radial colonnade and serious bracing between radial columns. But that's more work and more timber.


Or you could add a column in the center to support the top of the hips, size the rafters to span by themselves and hang whatever hammer beam ornamentation you want to from them.



Short version -- around here, that would be really hard or impossible to do without steel (and meet building regs).

Tension cable and clamps. Used in yurt construction.

Thanks Guys. Sorry about the delay in replying. Have been away for a wedding for a few days.

I will try and figure out how to post a sketch or two tonight but until then...

A compression band around the top plate was the original plan with maybe a couple of secondary bands over the top of the lining boards and anchored to the hip and jack rafters to prevent spreading.

I am not a huge fan of tension cables and definitely not of the timber framing option. I use timbers for their structural capabilities, never just for a purely decorative purpose. What is the point of all that extra framing when a king pin, rafters and a couple tension ties would suffice.
Inspiration for the frame came from Steve Chappels new compound framing book which shows an enormous 56ft clear span hammer-beam octagon of a similarly low pitch. I don't personal know Steve but I get the strong impression that he is not the sort to lace his frames with steel. The design and timber sizing for my frame is simply the result of necessity. What can I put together with what is in the timber stack without having to resort to cutting or buy more timbers. I should point out here that the timbers I plan to use are significantly more sturdy than those specified by the engineer for the initial conventionally framed structure.
On the engineering side of things. Design loads for the structure are as follows
Dead load for roof 90kg/m2 (about 18lb/sqft)
Wind load 1.3Kpa for the roofs cross section which works out at something like 14lb/sqft live load.
This works out at something a little over 30lbs/sqft maximum total design load.
I don't know the exact numbers for timber properties. We grade timber very differently here to the US and it would take me a while to figure out how to convert it all. However the timber I am using for the majority of the frame is said to have very similar properties and characteristics of dense western larch.

I hope all this helps and look forward to your replies.
I will do my best to get some images of sketches online in the next 24hrs.
Cheers

Have finally managed a couple of shots of plan sketches but can't figure out how to post them on here.
Any tips?

You need to find somewhere online to host the files--like photobucket, or picassa or something. Then grab the IMG link, and copy paste it.