Forgive me ignorance please!

I have been reading Steve Chappell's " A Timber Framer's Workshop", and I ran upon something that I have a question on. Hopefully someone can give me an answer, or at least point me in the right direction.

In his chapter on tie beam joinery, he states, that tie beams being in tension should be one continuous member, as scarfs eventually spread. This seems to limit me in my home design to what length timber I can obtain. Right now the mill I use can only do 20' so that leaves me with about 19 after squaring up.

What I would like to know is opinions on using scarfs in tension members, and whether this is workable, or should I go back to using only 19' as my widest bent?

Thanks

Mike:

You'll have to give us more information. A tie beam in what context? If it's the lower chord of king post truss, then I'd agree that you should be very cautious about using a scarf or spline in that case. Are you trying to clear span something greater than 20 feet? CB.

Actually The tie beam, as drawn in the book is not actually a free span, but could very well have two or more posts supporting the beam. It was my assumption from reading other books, that as long as the scarf was over a support i.e., post then a scarf would be acceptable. An example would be the beam that ties the two high posts in a high beam cape.

Am I describing this correctly? I hope so.

I guess the question would be? where can you use a scarf? Sills obviously, Plates I assume, Ridge beams, Where else?

One must be careful about the placement of a scarf joint. On top of a post may actually be a poor choice, as this may be the point of the highest bending moment. One can create a scarf joint that is quite strong in tension, but one that is strong in bending is more elusive. This has been discussed at length here on this forum . Also, see Jack Sobon's article: Historic American Timber Joinery part VI Scarf Joints. This is available as a free download from the Guild's web page.

Mike, if my memory serves me right Chappells shop is 32' wide and uses a king post truss with the bottom cord (tie beam)in 2 peices, joined to the king with wedged half dovetail tenons. That might not be the example for your project, and that joint should be in his book as well. I would look it up but I lent the book out a year or so ago and haven't got it back yet. That joint isn't a splice. Tim

Answers to questions posed so far?

(1) I will look at Chappell's book, as I have it here.
(2) I had read the discussion on scarfs over posts, and braces.
(3) The project I have in mind is 24' wide so my timber stretcher doesn't have enough horsepower. :-) actually the basic structure will be 24' wide 32-36 deep. It will be a combination of back 1/2 supported, and front 1/2 freespan.

The mentioned discussion on bending moments on Beams over posts, and braces never discusses the beam being in tension. Isn't a tie beam/girding beam in tension in addition to being subjected to bending. If it is in tension, are scarfs suitable to prevent spreading?

Mike

most mills will overcut their own length, especially bandsaws.
I cut a 40' long set of top plates on my 35' mill. So it should be possible for you to cut 25' tiebeams on a 20' mill.
It takes some time and cussin, but can be done.

There are ways to reduce the outward trust on a post that a tie beam is connected to, in order to reduce the tension to a level that a scarf joint will work.
Each frame design has to be looked at carefully for these reasons. And sometimes a different arrangement of timbers will be needed to accomplish the intended design.

I always like the story about a ladder leaning against a building, with the base of the ladder on a puddle which has frozen and is therefore ice and very slippery. At just about any angle, when you climb this ladder it will slide out and fail/fall down.
Now attach the top of ladder to the building. The ladder can no longer slide on the ice.....

Jim Rogers

good point, Jim.

if you aren't trying to clearspan, then you can support the rafters at the peak with a post (as Jim was illustrating) which if done correctly eliminates thrust on the plate, or you prop them up somewhere along their span with a post and reduce the thrust on the plate. In this instance, you need to tie the posts together to contain the (somewhat) reduced thrust. Either way, you don't need a scarf.

I would not put a scarf in tension without going to a pretty beefy scarf.

Here are some beefy tie scarfs --

http://books.google.com/books?id=PRdDAAAAIAAJ&pg=PA465&dq=timber+framing&as_brr=1#PPA454,M1

It goes without saying that timber frame designs should be reviewed and approved by a structural engineer. (doesn't it?)

gabel

Derek,

Point taken. It is even better to talk to a timber framer first.

By "common sense", I assume you mean an understanding of load paths and the practical application of engineering formulae?


gabel