Timber frame help - beginner

[rog_1973]

Morning Guys, 

Thanks for letting me join this forum. I have done lots of general woodwork and have a small shop set up.  I have decided to build a small Gazebo 12 feet square. I have sourced 6X6 green oak beams for the post and ring beam and 8X3's for the knee braces. 

I am struggling to work out which Joint is best to join the post to the horizontal beams.  I want to cut a roof off the ring beam so anticipated it being flat. 

Do I simply mortice the horizontal beams into the top of posts?  or is there a better way?

I have attached a photo of the type of thing I am trying to design and build



 

[Don P]

Hi Rog, welcome to the forum.
I'm hoping some of the more experienced timberframers will weigh in here but till then I'll throw in a couple of thoughts.

I tend to start looking at a building from the top down, accumulating and resisting loads as I go. I don't know your loads, so just some general concepts.

A hip roof is supported by the hips as beams, they are loaded by the jack rafters they are  supporting. Each jack has half its load supported by the hip, half by the ring beam. The jacks get progressively longer from the corners to the center of the ring beam so the ring beam load is non uniform and greatest at midspan.

The hip's load increases from corner to a maximum at the peak.

The feet of the hip pairs in this design are not tied diagonally across the building to resist spreading as the roof tries to toggle, or flatten out, so that thrust must be restrained at the corners by a robust connection tying the beams together into a rigid ring.

The kingpost is not supported from below, it is dangling from the peak. Think of it as a chain dropped from the peak, a tension element.

The struts springing from the kingpost run back up to the hips to support them at midspan, basically breaking the hip span in half.

The ring beam members must all be supported on the posts, in other words a simple half lap will not do, they need to have some amount of full cross section bearing on the post. This is one way of accomplishing this;


 


 
You can see in the notching I've got a bit over an inch of each beam at close to full cross section bearing on the post.
Even at 8x8 that is only a 2x2 tenon, it felt inadequate and wasn't trying to resist any thrust. I'd be tempted to lag a steel L shaped plate to the top of the ring beams at each corner unless someone has better ideas.

[rog_1973]

Hi Don,  Thankyou so much for your help and advice. 

It all Makes sense, I had not considered the side loads,  My roof will have to have a much shallower pitch than the one in the picture I posted.  There will not be much weight on the roof as I am thinking of using Zinc or copper sheets. 

I was thinking about a simple half lap joint on top of the posts with a tennon through just like in your picture. I have 6X6 posts so was thinking of 2" tennons.  

I think I could also cut the joints you have in your pictures - just needs a bit more effort. 

On your pictures I see you have cut a second housing for the knee braces - its that normal and should I consider or does it just look better if you get some shrinkage. 

Thanks again rog

[Don P]

Do remember as the pitch of a roof lowers the horizontal thrust increases. It is in direct proportion to the pitch, so a 3/12 pitch has 4 times the horizontal thrust of a 12/12 pitch. 

Also as the pitch lowers those struts supporting the hips become less and less effective, actually when their pitch is shallower than a 45 degree angle I wouldn't count them in your hip sizing calcs.

The housings on my braces above are indeed hiding gaps and brace angle changes that occur as the timbers dry. Some big timber guys in MT showed me that, must have been 25 years ago. If you are using green timbers and relying on the braces, as you will be here on a stand alone open structure, it would not be a bad idea to make provision for some "packing pieces", wedges driven in the bottom of the mortises to snug the braces tight after the timbers season.

These are some sketches of that modified lap I sent Jeff when he was doing a similar corner joint. Notice how it gives bearing for both beams. If the left hand beam in those pics doesn't have adequate bottom support on the post it will split right down the middle under load.



 


 

 

[Bruce P]

       Hello From near Enderby B.C. Canada. We are new to Timber Framing and rural life as well. We have lots of timber
on the property we are moving to in June and we plan to build a timber frame workshop about 25ft x 35ft on one floor.
We have Douglas fir and pine trees and maybe some cedar to work with. We have a HM130 mill on its way (waiting) and 
are doing research now and gathering tools and a used skid steer and such. So there is my introduction. This form seems to be a great place to share and find information and we are glad we found it. For building a shop we are hoping to not need to hire a engineer to size timbers and such. We do get lots of snow here 50psi is the snow load for the area as best i can tell. It is likely i would over build instead of under. 
  
      So my fist question is this. is there a chart of page where I can put in my snow load and roof size and suggested beam and post sizes and get an idea if i am on the right track?

[Don P]

Hi Bruce,
I wish I had your timber here, lucky guy!
Scroll down to the bottom of the left sidebar and click on the toolbox icon. When it pops up click on Don P's calcs, then beam and column calcs. The most used one is the heavy timber simple beam calc.

Poke around and you'll find a bunch of threads on how to use them or start asking questions.