I am a novice, and am interested in educating myself on some of the guidelines and recommendatons for hammerbeam trusses (for relatively smaller structures i.e. 14'-22'). I went and spoke with someone who builds and designs timberframes and they were trying to disuade me from their (hammerbeam) use. I have done a front porch addition out of hemlock and am wanting to do some outbuildings/sheds, and then eventually an addition onto the house. Any good references/books etc. that go into some detail regarding hammerbeams would be greatly appreciated. (I have what I think are Benson's and Sobon's first books).
Thanks in advance
Jon Koella

Pick up the Red design book and/or the revised design book published by the Timber Framers Guild. They contain informational and cautionary articles by Ed Levin amoung others.

Jon,
There are those who shy away from hammerbeams because of the characteristic of outward thrust exhibitied by a badly designed/constructed trusses. My personal view says there is no more attractive configuration. Check out the Russell Colbath project in NH here on the TFG website for views of a big hammerbeam barn/educational facility. The design geometry which puts all the thrust downward avoids the outward movement problem. Notice how a flying buttress effect can be achieved which further contains the hammerbeam.

Hi All:
The hammer beam is a tricky truss. The russell colbath truss complete with butresses had to be reinforced with unseen steel to make it work. This was accomplished after the rendezvous that cut and raised it was over.

These trusses need the review of specialty engineers familiar with the possible issues.

Curtis

steve chappell's book "timberframer's workshop" has some info on hammerbeam proportions and speaks about the importance of tying each side of the truss(the post bottoms) to an "immovable object"

To understand the problems associated with hammer beam trusses I reckon its important to consider the origins of the form, and the historic buildings that hammer beam trusses were typically associated with; namely large masonry buildings with massive walls. There's a good reason for this: the trusses are lousy. OK, so they're kinda cute, but they just don't work very well.

Taking these trusses out of context and specifying them in a timber house can cause all sorts of problems...and that's why so many of the contemporary hammer beam roofs out there are either dependant upon their stress-skin panels or metalwork to make them stand up to November storms and February snows.

It can certainly be done, and one solution (rather than building 6 to 8 ft thick stone walls all over the place)is to position the trusses at locations in plan, where adjoining wings can provide some resistance to the thrust. Works a treat with a cruciform plan! BUT...and this is for real: get a decent structural engineer on board early in the design process if hammer beam trusses are your kinda thing.

Enthusiasts might want to check out Timber Framing No 47. More good stuff in Brunskill's 'Timber Building in Britain', Hewitt's 'English Historic Carpentry' and 'English Cathedral and Monastic Carpentry' as well as Brandon's 'Open Timber Roofs of the Middle Ages'.

Properly speaking, the term "hammer beam truss" is an oxymoron. To truss means to tie up, so a truss system will tie opposing plates. Old English hammer beam roofs were in the form of a timber braced arch, not a truss. The reductive design of the current American fashion has tranformed the hammer beam roof system into a queen post truss arrangement with the tie cord severed and removed in the center creating a definitely not a truss structure.

Hi jkoella, Gordon & Roger,

I think that Jkoella's original question pertained to building a garden shed and not building a castle roof - the spans being quite different.

The language symantics are I am sure well meant but if I can make so bold as to suggest that if you adopt the use of the words - "cross frame" then the problem goes away.

Very few hammer beamed roofs in olde England are identical and therefore it would be inadviseable to generalise on what does or does not comprise a hammer beam roof type. There can be a degree of gender identity crises with some roofs where one is not quite sure whether a hammer beam roof is just that or whether it is more akin to an arch braced roof or maybe related to being an upper cruck or from time to time scissor bracing is introduced for good measure. The one thing that they all have in common is an interrupted tie beam and for that reason this device has been employed to span quite large distances using shorter lengths of timber.

Gordon is correct to highlight that where tie beams are interrupted then quite large deflections can develop but in a garden shed this is probably not worth worrying about.

Ken Hume P.Eng.

Thanks Ken for the refocus.