Timber Framers Guild Forums General General Forum Questions hammer beam truss layout

Hello,

I'm about to start my second season as a timber framing apprentice. The first project of the summer involves building a timber frame consisting of 4 bents, all of which a hammer beam trusses. I have experience with H bents but have never layed out a hammer beam truss.
I would like to have some general understanding of laying out a hammer beam truss before I get to the shop.
Could anyone give me some quick pointers or a good reference.

Thanks,

Ian

Hi Ian.

Steve Chappel's book is the only one I'm aware of that discusses Hammer Beams in any detail. It contains some rules of thumb about their design, including the goal of having the main brace meet the post approximately 2/3 of the way down. My understanding is that this design was originally used in buildings with stone buttresses to resist the rafter thrust. Since this is rarely done today, pay special attention to how well your design will handle that thrust. CB.

Hi Ian,

Daiku gives you good advice to design your bents with good outward thrust resistance, however having just spent a pleasant afternoon with my wife listening to the choir practice in Pilgrims Hall, Winchester (circa 1300 - 1309) my eyes cannot confirm Steve Chappell's recommendations.

This hammer beam hall has stone (flint) walls built with buttresses of dressed ashlar limestone at the hammer post locations. The main cross frame (bent) braces spring from the centre of the arcade tie beam (collar) and join with the hammer post a matter of six inches or so above the canted hammer beam. The design employs massive timbers to help resist the excessive deflections to which hammer beam roofs are prone.

There is a small drawing of this roof frame in a new book - Hampshire Houses by Edward Roberts just published by Hampshire County Council - (ISBN 1 85975 633 6 price £19.95).

Given that you live in California you have the added complication of earthquakes to take into consideration and therefore I would strongly recommend that you engage someone to run some numbers on your design prior to committing to a cutting programme and this will confirm Daiku's recommendation - big posts !

Please feel free to email me a typical cross section of your proposed bent for comment.

Regards

Ken Hume
Visit our website at http://www.clik.to/WorldofWood

Greetings, Ken.

Pilgrims Hall sounds like a sight to behold! Regarding Steve Chappel's advice, he was referring to the situation where there are not stone buttresses. And due to a lack of clarity on my part, you misunderstood which brace I meant when referring a 'main brace'. I was actually referring to the point at which the brace supporting the Hammer Beam (from below)intersect the outside post.

Ken and Daiku thank you both. You certainly have got me thinking. I'm actually not involved in the design processes for the said hammer beam trusses. I simply want to have a feel for the layout. Never the less I do appreciate the wise words and I'm sure at some point in my career I will be faced with design considerations.
I guess the bottom line is to pay attention to what is tried tested and true.
Thanks for the reference.
By the way Hamilton is in Ontario, Canada

Cheers,

Ian