Hammerbeam truss construction and need for cable or buttress

[Farmer Jim]

In Steve Chappell's A Timber Framer's Workshop  he covers hammerbeam truss construction and the need for the hammerbeam to be 2/3 the length of the post and that the lower brace should enter the lower third of the post.  Nowhere does he state the need for a tie-cable or buttress type posts.  On the "Barnwood Builders" show they visit some resort by a manmade impoundment and in the one timberframe building the host is pointing out the hammerbeam construction (where the braces appear to enter at least half-way, or higher, up the pots) and states there must be tie-cables or buttressed walls.  I saw the same thing when searching for hammerbeam designs on the interwebs.  My question, is there a need for the tie-cable/buttress if the construction is done in the way described in Steve's book? Steve's explanation and math make sense and seem logical, but I am planning a sugar house where I will have 3 hammerbeam bents in the middle of the building with a king post bent on each end and want to make sure I'm not missing something.  i.e.  I suspect the tv and interweb are wrong because they missed the little detail called proper construction.

[Dave Shepard]

Hammerbeams are complicated. I would trust Chappel over a random TV program. There is a lot of bad timber framing going on out there. Sadly, the latest issue of Sawmill and Woodlot has an article on some of the worst "timber framing" I've seen in a long time.

[Chilterns]

Farmer Jim,

Hammer beam roofs have a good track record of performing over a long period of time without the need for metal tie rod reinforcement between the hammer beams and infact if the hammer beams are set in an inclined fashion this would have a detrimental effect.

Chilterns

[timberwrestler]

It's worth noting, Chilterns, that those hammer beams sit on top of a masonry wall--one often several feet thick.  The Americanized version is a pretty tacky attempt at the same thing. 



Compare that to Chilterns link below.  I don't know where or how the elegant European version got turned into the monstrosity above.  The American bracing is spindly, and nothing close to an arch.  The roof pitch is often low.  There is no bracing to the purlins.  Here is the late great Ed Levin's version, pulled straight out of 'The Open Timber Roofs of the Middle Ages,'



There are a lot of hammer beam roofs in the US that have had steel tension rods added after they started to spread.  The loads are very high on everything.  It's one of the few times that posts want to bend.  It's been a long time since I've looked at Chappell's book, but I would be reluctant to build straight out of the book.  I think that it would be foolish to not have a (timber frame) engineer look at it.

There's this too:  https://forestryforum.com/board/index.php?topic=24074.0

[S.Hyland]

As Dave said, Hammerbeams are complicated! The term hammerbeam truss, is a bit of a misnomer, since there is no true trussing action going on in one of these. If they needed metal tie rods, it may have been because their roof pitch wasn't steep enough to prevent speading. The tie rods actually create a lower chord and make the system a truss.
The steeper the pitch the more force is directed down rather than out. Therefore a 10/12 is usually the minimum pitch for a hammerbeam, steeper =better. As Chappell says, the braces should terminate low in the post so that all the force is directed into the foundation (as long as your post bottoms are tied in well to channel that force!). A lot of old English hammerbeams are not on posts, they bear on massive masonry walls which resolve those forces and resist spreading.
There are a lot of very unpredictable forces at work in a hammerbeam, and it wouldn't be a bad idea to have a timber engineer look at it, but I would say that Chappell's advice is generally good as a rule of thumb.

[Brad_bb]

Yeah spreading is usually the issue, which is why you see tension rods in most done here (US) commercially.  The tension rod often defeats the purpose of wanting the clear open space as it's not truly open.  Buttresses can be costly and would not be typical here. 

First ask yourself, why do you want hammerbeam?  Looks in a house?  Will tension rods be a problem?  Most engineers are going to want the tension rods to make sure it won't spread.

I figure if it's a smaller frame, you might get away with no tension rod if you do a few things to direct the force downwards and reinforce to reduce spreading effects.  With a larger frame, you're taking chances.

When I first got into timber framing, I was really impressed by hammerbeam bents, but learning about the forces you are dealing with, makes you reconsider whether the headaches or potential ones are worth the extra work and risks.  If you want a hammer beam bent to impress people, you can accomplish the same effect with a variation of queen or king post bent with organic timbers (natural and sawn on two sides, or full natural).   With organic timbers, no two will be alike.

[Farmer Jim]

Yeah spreading is usually the issue, which is why you see tension rods in most done here (US) commercially.  The tension rod often defeats the purpose of wanting the clear open space as it's not truly open.  Buttresses can be costly and would not be typical here. 

First ask yourself, why do you want hammerbeam?  Looks in a house?  Will tension rods be a problem?  Most engineers are going to want the tension rods to make sure it won't spread.

I figure if it's a smaller frame, you might get away with no tension rod if you do a few things to direct the force downwards and reinforce to reduce spreading effects.  With a larger frame, you're taking chances.

When I first got into timber framing, I was really impressed by hammerbeam bents, but learning about the forces you are dealing with, makes you reconsider whether the headaches or potential ones are worth the extra work and risks.  If you want a hammer beam bent to impress people, you can accomplish the same effect with a variation of queen or king post bent with organic timbers (natural and sawn on two sides, or full natural).   With organic timbers, no two will be alike.

As stated, it will be a sugar house.  I don't do anything to impress people, it is the design I felt would answer some of the base requirements-width will be 20' and I need clear space overhead as the 3'X12' evaporator being installed to start with will eventually have added hoods etc over it that make me desire the clear space.  If the design isn't sound when constructed with bracing in the right places then I will need to reevaluate whether to live with a narrower than desired frame.  The evaporator will eventually be replaced with a larger one and I am trying to make the building be a non-problem for the future increases of scale.  I have looked at the hammerbeam with the aisle post from floor on up which would give the open area in the center I'm after and remove any side stress on the main posts, but puts extra obstacles in the work area.  I will keep investigating,  you guys keep pointing out the 'what can go wrongs' as I am learning from your input.  Thank you for helping, it is appreciated.

[Hilltop366]

Wondering how long a ridge beam would be practical, or purlins, that would eliminate any risk of spreading.

Not sure what length of building you are looking for but I would think that the bents could be far enough apart to get a clear opening over your evaporator.

[Farmer Jim]

Wondering how long a ridge beam would be practical, or purlins, that would eliminate any risk of spreading.

Not sure what length of building you are looking for but I would think that the bents could be far enough apart to get a clear opening over your evaporator.

Total length 40'.  Length of closed in area is 24' the rest will be open sided wood storage.  With the 12' evaporator and stack thickness  and airspace from the end wall for fire reasons, the 12' evaporator will nearly fill a 16' space (lengthwise) between bents.  The book seems to suggest that 16' is the most you would want and less distance between bents is better, that's where my self inflicted confusion is coming from.  If a longer evaporator is installed in 10 years then I would have the overhead beam, but at this point I'm thinking 10' posts and everything (hoods, steam-away, etc...) should fit under the beams then and the bents could be closer.   I want a trouble free structure that will still be here in 150 years. The open space of the hammerbeam was the easy answer for some of my needs, but as I said the main thing is a solid building we can use next year.   If regular king or queen post bents are the sound trouble-free long-term way to do this then that is the logical approach.  I definitely don't want to go to all the work then have to put band-aids on it because the design won't hold up.

[Jim_Rogers]

There are many a sugar house out there with tie beam over the evaporator. The steam won't effect the timber.
Jim Rogers

[Farmer Jim]

There are many a sugar house out there with tie beam over the evaporator. The steam won't effect the timber.
Jim Rogers

 It was more an issue of being in the way of later equipment that will make height close (when all the hoods and pre-heater stuff that can improve efficiency get stacked up it gets tall), but 10' posts might be the answer.

[Jim_Rogers]

I would think that then you're going to have to design for the future evaporator, and work around your design for the current one.
You can span long distances but you're going to need to have your design reviewed by a qualified timber frame engineer.
Are you going to cut the frame yourself or are you going to hire it out to a timber frame shop/company?
What is the longest timber you can mill? Or what is the longest timber you would care to buy?
A hammer beam can work, you just need to make the posts large enough for the support. Again, proper engineer review and design.
What is the snow load for your area?

Jim Rogers

[Brad_bb]

Just helped with a kingpost frame that had one bay 26' wide and 24' long.  The bents were 8" thick material, and the top plates and purlins were 10"x10".  So if you need only 24' clear, you can do it.

[Farmer Jim]

I would think that then you're going to have to design for the future evaporator, and work around your design for the current one.
You can span long distances but you're going to need to have your design reviewed by a qualified timber frame engineer.
Are you going to cut the frame yourself or are you going to hire it out to a timber frame shop/company?
What is the longest timber you can mill? Or what is the longest timber you would care to buy?
A hammer beam can work, you just need to make the posts large enough for the support. Again, proper engineer review and design.
What is the snow load for your area?

Jim Rogers

Timbers will be falled and milled here.  I had planned on black birch and red oak, but I'm concerned with checking so I'm leaning toward cutting some of our EWP.  21 1/2' is my current sawing length limit.  I intend to cut the frame joinery, my sons will be the muscle for rolling and fitting.  Regarding snow load- I can't answer that.  The roof will rise 1 foot for each foot of run. 

[Heartwood]

What SHyland said. Hammerbeam bents are not trusses; their basically very deep rafters with the horizontal tie missing. Bring the braces in low; some designers even run the posts down into the basement and let the surrounding earth act as a buttress. Make the posts wide to resist bending. Have a very heavy bracket or other way to secure the post foot.
There are two great articles analyzing hammer beam roofs by Ed Levin in Timber Framing #s 48 &49.
I think Chappell mentions that a 24 foot span is about the limit before problems start escalating, and an engineer is a must.
Although flying buttresses like the great cathedrals may be impractical, one way it can be done is with well-braced shed roofs with horizontal ties outside the building bearing up against the main building right where the hammer braces hit the post (if they're high enough). Here's an example from the Guild's Russell Colbath (NH) project, engineered.

[S.Hyland]

That's nice having the sheds to either side. I did one once like that with wings to either side, it makes a big difference.

Farmer Jim, I don't think that what you are proposing is at all unreasonable @20' span with a 12/12 pitch. If executed properly you should have no issue. You could use the hardwood for your posts, hammers, and braces and use the EWP for rafters, purlins etc. That will help keep your dead load down. You are also smart to keep your outer bents non-hammerbeam. That is going to help with spreading, wind load, and overall rigidity. I'm assuming there is no basement to this so you will want to make sure your post to concrete connection is well thought out. You'll be putting  a lot of strain there.
What is your finished roofing going to be? If it's steel, snow accumulation is going to be nearly nil.
Good luck with it. I'll bet it would make the coolest sugar shack around!

[Farmer Jim]

That's nice having the sheds to either side. I did one once like that with wings to either side, it makes a big difference.

Farmer Jim, I don't think that what you are proposing is at all unreasonable @20' span with a 12/12 pitch. If executed properly you should have no issue. You could use the hardwood for your posts, hammers, and braces and use the EWP for rafters, purlins etc. That will help keep your dead load down. You are also smart to keep your outer bents non-hammerbeam. That is going to help with spreading, wind load, and overall rigidity. I'm assuming there is no basement to this so you will want to make sure your post to concrete connection is well thought out. You'll be putting  a lot of strain there.
What is your finished roofing going to be? If it's steel, snow accumulation is going to be nearly nil.
Good luck with it. I'll bet it would make the coolest sugar shack around!

The roof will be steel and we just put up a 16'X32' workshop with a steel roof that we roofed Autumn before last. It has a roof with approx. the same pitch and it will gather about 1" of snow then shed it.  There has been no snow load on it.

[D L Bahler]

I don't mean to dissuade you if you want to use a hammerbeam,

but a raised kings post might also be a solution for your situation

The concept is simple. The upper chords of the truss land on the plate, and the lower chord ties into them a few feet higher, and from there it is executed the same as a standard kings post. The assembly can still cause some spreading, much like a hammerbeam, because the upper chords might flex out.

Keep in mind that the cross member (lower chord) has to be joined to the upper chords with tension joinery, because it will be under a great deal of tension. The most common solution to this (used in Germany a lot today) is to make this with two member clapped over the upper chords and king post, then secured with bolts. But you can do this with heavy beams and joinery if designed properly.

Note that the lower chord should not be raised above the lower third of the truss, or it will not be able to efficiently resist spreading forces and bending in the timbers.

There are a number of more modern truss designs used by the Germans and French today that can effectively create an open roof design, but if you prefer timber framing this may not be an option.

Another thing to consider may be a scissor truss.

[thechknhwk]

I don't know if mine is technically a hammer beam, but that's what I call it.  There are a few pics of it here - https://forestryforum.com/board/index.php/topic,73938.80.html.  It's 21' to the tie beam, I think the posts are 16'.  I don't know if that helps you at all, and I'm no expert but it was engineered by my architect.  the pitch is 8/12.