Timber Framers Guild Forums General General Forum Questions king post truss

It seems as though the king post truss is always used in conjunction with the ridge beam. Is it necessary to have a ridge beam in the design to use the king post or can it still be used when the design is without a ridge beam?(for instance, using the queen post truss on the other bents, with a principal purlin plate supporting common rafters. I was trying to design a 24 x 48 frame with 5 bents spaced at 12 feet using queen posts on 4 of the bents with intermediate support below, but the 4th bent being a clear span of the full 24 foot width. I thought the king post seemed the best, but it may not be possible? Perhaps the hammerbeam may be the only option?

TB: I usually use a ridge with a king, because then you can put a brace between the two, stiffening the frame longitudinaly. And those braces look really cool! Here\'s an example.

Regarding the 24 foot span, the king post truss is well suited to the task. Hammerbeam trusses are quite inferior structurally to king post trusses.

daiku,
Shame, shame. Denigrating hammerbeams is not your style. Properly designed and executed they last and last and last. And the tendency to bulge along the axes of the timbers is a myth spawned by bad design and execution. Yes, they can deliver the net forces straight down (or vertical if you prefer).
The king post is probably stronger than the calculations for a give species yield. A house circa 1750 in Fredericksburg, VA has what visually seem to be "small in timber cross section" king post trusses. Yet it holds up a very heavy slate roof (the original cover was wooden shakes). The newer pavillion at Ferry Farm, also Fredericksburg, has massive timbers in it's king post trusses. See the article here on the TFG website.
deralte (avid hammerbeam fan)

I am also an avid fan of the hammerbeam but more of a fan of structures that stay standing without help down the line. A few articles, one specifically by Ed Levin, talk of the many hammerbeams seen with the dreaded steel tie bar added later to fix an obvious failure. This sure would be a disaster for a timber framing purist like myself. At the same time you are completely correct that a properly designed hammerbeam truss, well engineered and well executed will last and be beautiful to boot. I am by no means knocking the hammerbeam truss. I just dont feel confident enough to design one and I have a hard enough time finding decent frame plans that are simple, let alone one in which utilizes the more complicated hammerbeam truss.

I think what we miss sometimes is that we are supposed to be designing buildings, not timber frames.

The hammer beam system (Brungraber swears it does not meet the classical definition of a truss) worked quite well for centuries when placed atop and within substantial masonry walls.

Contemporary timberframers and their engineers have been driven to dramatically upsize the lower posts to try to resist the bending that logic and science tell us flow downward from that big lower hammer brace (often) into the middle (most bendy) part of the column. Straw bales and stress skins do not offer the same push back as buttressed stone.

Likewise, hammer beam systems on end walls can display alarmingly little resistance to wind-driven distortion, especially when that wall is mostly glass, with a fabulous ocean view.

What to do? Always take a long hard look at the historical successes (the failures are long gone); they have likely endured a great deal more than the some of the frames we see in the lifestyle magazines.

A willingness to learn from the patterns of our predecessors does not (necessarily) mean we are compelled to use hand tools while wearing animal skins.

EG:

I anticipated some dissent when I made that comment, so I worded my posting carefully and I stand by it. My intent was not to denigrate the hammer beam truss. I simply made a general comparison of the structural properties of the two truss designs. CB.

Hi all once again great topic!

Personally speaking what is wrong with forgetting about the king post , just frame in a regular timber trusses every 12 feet, drop down a support rod from the peak of the truss to the centre of the span to support and keep the bottom chord straight, and then using a plate centre span on top of the trusses lay on your rafters overtop of the trusses, it will be still nice and true many years hence.

NH

I've stepped into the hammerbeam debate previously and would like to add a historical oddity from the sketchbook of Villard de Honnecourt circa 1230 with a hope it will clarify the basic hammerbeam form.

http://classes.bnf.fr/villard/grand/carnet/34.htm

The drawings at the top left represent a possible early stage in the hammerbeam developement. The hammerbeam in this case acts as a cantilever beam supporting the roof system in a balanced fashion. The principle rafter ties to the beam over a corbelled wall, thus counter weighting and bedding the beam. The beam extends modestly into the room and receives the hammer post that extends to the rafter below the collar. This seems to be a conservative design low in drama, well suited to thick walls and small rooms.

The later English hammerbeams maintained this basic principle of the well designed cantilever but added lenght, struts, spandrels and Gothic motif. The current American style has lost the structural system of old but has kept the style and suggests the form, a rendering into a genuine ersatz hammerbeam.

My suggestion is to tie the rafter to hammerbeam and bed (not bracket) the hammerbeam to create a cantilever with a second post in the room and then complete struts. I'm sure a good form could be developed, but not what is expected.

I think its absolutely hilarious that the posting is entitled King post truss and we all end up on a thread about hammerbeams. I love it. Northern hewer: still not too keen on the idea of using " rods" in my "timber" frame. Also I am not quite sure what you mean by " using a plate center span on top of the trusses and lay on your rafters over top of your trusses" ?? Are you not using your rafters as a member in the truss? What is a "plate centre span" ? I am in the dark on this one. Perhaps you can elaborate on this. I am intrigued and always respect your feedback. Thanks tb

daiku:

What's up with the roof boards in the above linked picture? Don't the boards usually go from ridge to eave in roof system with purlins?

Jim Rogers