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Author Topic: Minimum Pitch for a Hammer Beam Truss?  (Read 21283 times)

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Offline Woodbender

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Minimum Pitch for a Hammer Beam Truss?
« on: February 18, 2007, 10:51:37 AM »
Hi guys

Another quick question.  I'm laying out plans, elevations and sections on the designs I'm considering. I am thinking of using Hammer Beam Trusses in our "great room" space which will span 24'.

I initially wanted a 6/12 pitch but comparing it's 26.565 degree diagram to other photos and diagrams I have seen on the internet and this forum it seems quite shallow.

Do I need to be thinking more like 9/12 pitch on a Hammer Beam Truss??

Also do the braces NEED to be 45 degrees or do they need to be parallel to the primary rafter?
Tim Eastman (Woodbender)
Be an example worth following.

Offline Raphael

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Re: Minimum Pitch for a Hammer Beam Truss?
« Reply #1 on: February 18, 2007, 01:19:52 PM »
  I've got a hammer beam under a 6/12 roof designed and engineer approved for my office.
Never got it cut.  ::)
  The span is 18' and the tension on the main tie was over 6000# (IIRC) with CT snow loads.
So it's possible but should to be engineered sooner rather than later in your planning stages to be sure it's feasible.

My braces were 45.
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Offline Jayson

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Re: Minimum Pitch for a Hammer Beam Truss?
« Reply #2 on: February 18, 2007, 04:10:15 PM »
Typically the braces are the same as the pitch. They are basically a second set of rafters. I have not seen one under a 10 inch pitch. This makes sense to me considering horizontal thrust (the force that wants to push the walls out) is the weak link in the design. Building with a 12 inch pitch would certainly simplify execution but would add some cubic footage to heat and maintain. Another thing to consider in planning your hammerbeam is the fact that the brace will come down to the bottom third of the post (possibly intruding on useable space). How you tie down those post should also be considered carefully because of that. The force acting on them is in effect, kicking them to the outside. Let us know how it works out.

Offline Roger Nair

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Re: Minimum Pitch for a Hammer Beam Truss?
« Reply #3 on: February 18, 2007, 05:57:02 PM »
Disclaimer, I am a carpenter not an engineer, so take this with a grain of salt.

First the hammer beam configuration should not be considered a truss form.  A number of hb elements ie the post, rafter and hammer beam act in bending not in strict axial loading found in a properly designed truss.  So care should be exercised in member sizing, keeping in mind that the forces are perpetual and wood is plastic.

Second, as the slope decreases the spreading forces increase quickly. 

Third, green wood shrinkage effects the overall geometry, so carefully balance choices.

Fouth, things I would try to incorporate.   Foot the rafter into the hammer beam with the best tension joinery I can produce.  Calculate bending to a higher standard, L/480 for example.  Deeply house the braces, consider the braces as inclined posts and land the braces close to the floor.

Remember the pictures in the glossy magazines are selling sizzle so design for steak.
An optimist believes this is the best of all possible worlds, the pessimist fears that the optimist is correct.--James Branch Cabell

Offline Thomas-in-Kentucky

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Re: Minimum Pitch for a Hammer Beam Truss?
« Reply #4 on: February 18, 2007, 09:21:17 PM »
Roger is spot on.  I housed (or shouldered) every joint in my hammer beam bents.  As he indicates, there are some tremendous forces trying to pull the hammer beam itself out of the post and into the center of the room.  I think this is why he suggests (and you will see pictures of elsewhere) to tie the rafter foot directly to the hammer beam.  This way, the forces would resolve themselves between these two elements (bypassing the post).  I was not able to do this (precisely) in my design, so I used a massive through tenon (3"x12" with more than 2 feet of relish) and five pegs to connect the hammer beam to the post.

Like he said, watch out for the pictures in the glossy magazines.  They could be using hidden metal fasteners, and if you can't see the "naked frame," you might not realize that they have an entire house on the other side of the hammer beam post to keep it from going anywhere (which can be a good thing to emulate - I did for one side of my hammer beam bent).  Also, I have noticed some of them lower the "collar tie" to the bottom third of the rafters - and just hang hammer-beam-like-elements beneath the tie beam.  At first (and second) glances, these look like real hammer beam structures, but they are actually functioning as king post trusses.  This could work for you as well, just make sure you know what is actually holding the roof up.

Assuming you will have two pairs of braces, don't skimp on either pair.  Remember there will be wind loading on your roof, and these braces could actually go into tension.  I went with overkill on the braces (8x12's) - using three pegs in each tenon, and lining the braces up like a redundant set of rafters... like Jayson said... and on the same pitch (12:12) as the rafters. 

Here are some pictures...

the foremost bent in this picture is not a hammer beam (but the two behind it are).  I would not use a hammer beam on  the gable end of a house.  Once you've chosen to underdesign :) your house by using a hammer beam, why would you then do it on the gable ends of your house, where neither the freespan nor vaulted aesthetic are  required?


In this second picture, the post on the right is a 12x16, the post on the left is a 10x10, sistered to another 10x10 post which belongs to another bent in the house.  Notice that the hammerbeam in the foreground is shouldered at the top of the beam - not the bottom of the beam.  The brace below the hammer beam is acting as a fulcrum with the result being that one end of the hammerbeam is trying to go up, not down!


This picture might give you a sense of scale.  No, the iron worker on that beam is not a hobbit, but he does weigh less than 140 lbs.  :)  In this picture, you can see the through-tenon on the left.  This is part of the hammer beam that is sticking out the other side of the post.  The rafter foot pegs to this extended tenon which is nice for a llot of reasons.  I ran the rafter past the post primarily to get a lot of roof overhang on this south side of my house.  (passive solar heat design considerations)


This picture shows clearly one aspect of my hammer beam design that a lot of folks might have an issue with...  Specifically, you can see that my lower braces don't hit anywhere near the bottom of the posts.  Roger and Jayson (and Steve Chappell's book) suggest that the braces should hit down low, and I agree with them that it is good design to do so.  The hammer beam bent on the right has a 24' tie beam (to form a loft) and so the braces in this bent effectively hit where most folks suggest.  But what about the hammerbeam on the left - what keeps that post from falling outward?


Here is where I hope to stir the pot a little, or at least explain why I don't think my house will fall down any time soon.  Just as a thought exercise, if you cut the rafters in half on a hammer beam structure and throw away the bottom half of the rafters, the structure would look like the one below.  In fact, you can see that the remaining roof loads do not try to shove the posts outward.  Instead, the loads transmitted to the post are trying to twist the top of the post, and if these were pin joints and the posts were sitting on roller skates, the structure would fall inward, not outward (actually, I think the structure might stand!).  I'm not proposing to cut your rafters - I'm just trying to illustrate a possible load path.  Sagging rafters would transmit similar forces.  Modeling software will illustrate similar loads for the uncut structure.


I'm interested in what people think of this.  Yes, hammer beams are crappy structures compared to legitimate trusses, and my implementation is certainly no better than the one with braces striking the lower third of the posts.  I'm just throwing out the notion that if you look at how a hammer beam is working, and design to take advantage of how it works (and oversize the scrapple out of everything), then you might come to the conclusion that it is not a requirement that the braces hit the lower half of the posts.  I've seen it suggested that old hammer beam roofs worked because they were set atop stone walls - I am skeptical, since tall stone walls (flying buttresses excepted) are really not very good at resisting outward thrust.

I do expect some deformation and creep of my house over time.  In fact, I'm taking care to note the buldges and out-of-squareness that we built into the house (by accident of course), and watching to see if I can detect any movement as time progresses.

I hope this thread gets some good discussion going.  So little useful information is to be found about hammer beams on the internet. I think liability scares most people away from the discussion.  Uh which reminds me:  Disclaimer: my structure should not be emulated unless validated by a licensed engineer.

(and to lob an answer to your question woodbender, I'd also say don't build it with anything less than 10:12... unless maybe you go with a quasi-king-post truss that looks like a hammerbeam truss)

Offline Jayson

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Re: Minimum Pitch for a Hammer Beam Truss?
« Reply #5 on: February 18, 2007, 10:18:21 PM »
Disclaimer, I am a carpenter not an engineer, so take this with a grain of salt.

Ditto...

Another quick question.
I don't think that is going to be possible. Although I think this will get over my head real fast, I like Thomas would like to see more info on this topic.
Foot the rafter into the hammerbeam with the best tension joinery I can produce.
What would that be? Maybe a haunched or sloped table so that you produce some friction at the rafter foot. This where I wish I could do illustrations for it would be easier to exhibit but I don't mind going into detail. Haunch and sloped table seem to be interchangeble terms describing when a housing is out of plane. So what I am saying is that at the level line created at the top of the hammerbeam the bottom of the rafter passes farther than the top of the rafter. Therefore the rafter foot would actually have to slide up hill to slide off the hammerbeam. The force pushing that rafter foot out would be the thrust we are talking about right? Now what would you suggest doing to beef this up? I think here the weakness is going to be the lack of relish on the outside end(if you were outside looking at the side of a bent you would be looking at the end grain of the hb) of the hammerbeam. The tenon on the foot of that rafter is going to try and blow that end grain right out. Now I have seen in person in real life with engineer approval and architect oblivion(not a dig on either of those professions just making a point) a tension rod added to in essence hold the hammerbeams together. We went well ( at least 12") into the end grain of hammer beam to bolt the tension rod giving us some relish to rely on in that joint. That stucture was buttressed on both sides by other buildings. I believe that hammer beam bent will rely on those buildings to help it stand up down the road. You guys this tread is going to be so good. I have much more to say but a long week lies ahead. I will try to check in often this week because I know ya'll are about to let it rock. Thomas are you a cook? You really know how to stir a pot!!

Offline Woodbender

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Re: Minimum Pitch for a Hammer Beam Truss?
« Reply #6 on: February 18, 2007, 10:27:53 PM »
Thanks you guys - as always a real good answer has the potential to present new questions and different alternatives.

I'll pick on some structural geeks at work. Yeah they're structural engineers but they are not Timber Framers and they don't work in this field. Which is why I came to you guys FIRST! 8)

Thomas, I never thought about missing the post with the primary rafter and bearing it on the back of the hammer post.  That will be a concept that I will run past the engineer to see how it's model cancels forces etc.  Are you structurally bearing most of the rafter in a joint on top of the post or just some of it - with the remainder acting more as a brace to the end of the hammer beam's extended tenon which passes through the post?

Thanks again - I'd like to see how this develops!
Tim Eastman (Woodbender)
Be an example worth following.

Offline Don P

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Re: Minimum Pitch for a Hammer Beam Truss?
« Reply #7 on: February 18, 2007, 11:25:29 PM »
I can't help much but do have some explanations that I've seen written. The stone churches without flying bu'tresses have massive masonry walls and few windows. The "invention" of the flying buttress allowed the walls to be punctured by large windows. The masons back then were tuning the weight in the buttresses as they watched their mortar joints for stress. Talk about cutting it kinda fine.
The suggestion one engineer gave is to oversize the posts to account for the bending forces in the post as the blue section of Thomas's roof sinks and tries to bow the post out. The post should be designed as an axially loaded column with bending. He mentioned reinforcing distressed hammerbeam homes by bolting another post outside to straighten bowing posts.

I think we're all on the same page, discussion is what moves knowledge forward and so should not be stifled. Any plans falling outside of the prescriptive allowances of the building codes should be checked by a professional, that would be any TF plan. Those are 2 different things. The law of the land in most places, enforced or not, the law says, the plan must be stamped by a professional. No one here is stamping plans, just having a discussion. A person that chooses to not have his plan reviewed by a qualified professional chooses to accept full responsibility and liability for his actions. That was a long winded way of saying, chime in :).

I didn't mean to step in over your question about the overhanging rafter Woodbender, I want to know more there too. That house is one awesome piece of work Thomas.
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Offline Thomas-in-Kentucky

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Re: Minimum Pitch for a Hammer Beam Truss?
« Reply #8 on: February 19, 2007, 08:48:30 PM »
Thanks for bringing up this topic woodbender! No, I'm not a cook, but it sounds like the pot is simmering.  :)  Woodbender started this stew, but I will definitely be following this one.

Are you structurally bearing most of the rafter in a joint on top of the post ....  ..... with the remainder acting more as a brace to the end of the hammer beam's extended tenon which passes through the post?

You got it Woodbender.  That's exactly what I intended with my design.  The loads of the rafter mostly terminate at the top of the main post, but the rafter carries on below that point to catch the long tenon of the hammer beam, forming a nice little triangle, giving me one more chance to peg everything together, and catilevering some of the roof loads on the other side of the main post.  I read somewhere that catilevering some of the roof past the post (or stone wall) does nice things for the hammer beam design, and it worked out that I needed about 3 feet of overhang anyway to block summer sun from my windows.

That house is one awesome piece of work Thomas.
Thanks Don.  I am over two (full time) years into it with at least a year to go.  Some parts of the design are risky, but not as risky as the basic notion of spending my entire savings and several years of my life trying to build the house.  And while I'm on the topic of risk, statistically speaking, timber framers who log their own logs are more likely to die while logging than to be crushed by a hammer beam collapse.  :)

Just found this on my hard drive - I watched a frame raising in Cinci Ohio.  I can't be sure who the timber frame company was (Northwoods Joinery?), but the timbers were cut on a CNC machine and the frame was beautiful.  I'm posting it here because it looks like they've gotten by with 8:12(?) pitch.  But look what they've done - each hammer beam is flanked by another bay of the house.  Directly opposing the lower hammer beam brace is a brace that transfers horizontal load to the roof of the adjacent bay.  Very neat - never noticed this before.

Offline Thomas-in-Kentucky

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Re: Minimum Pitch for a Hammer Beam Truss?
« Reply #9 on: February 21, 2007, 08:34:41 AM »
I just took a look at another picture (from a different angle) of that commercial frame.  Assuming the aspect ratio of my camera and screen are correct, it looks like the pitch on that hammer beam is closer to 7.5 : 12 or 32 degrees.  They have employed some nice engineering/design to go to that degree.

Raphael, can you give us a few more details on the 6:12 hammer beam that you have designed.  I certainly believe it can work - just wondering how you did it.  Is the main tie on the lower third of the rafters?  Do the braces hit the post down low?  Do the hammer beams attach directly to the rafter feet?  Would you raise it as one bent or in pieces?  (sorry for all of the questions - just trying to take advantage of an opportunity to learn!)

-Thomas

Offline Roger Nair

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Re: Minimum Pitch for a Hammer Beam Truss?
« Reply #10 on: February 21, 2007, 07:22:09 PM »
Thomas, I have a different view on how the hammer beam system works and why outward thrust will cause the major post top to move outward by force applied by the rafter chord.  I reason that the hb assembly works by a process of dynamic balance rather than the static equilibrium process of a truss.  Let's use your hb design as the model.  What happens, imo, as the load is applied members bend, namely the major post and hammer beam, very slightly no doubt in your robust design.   As the post bends, the upper level and the hammer beam moves downward, the rafter thrusts against the post top, since the post is unyielding in height outward displacement is expected.  Likewise, the effects of differential shrinkage will have a similar effect.  Again post top height to hammer beam seat is fixed but the hammer beam and post will shrink measurably, the rafter will come under increasing load as supporting timber shrinks again forcing the post top outward.  I believe the movement will stop as the competing forces come into balance.  Long term, if the sections are not  large enough the wood will undergo some plastic deformation and the post tops will continue to spread.  Other effects, such as compression set in the pegs and bores, can also contribute to the process. 

A steeply sloped hammer brace, say 3/1, offers great advantage in several respects over a brace sloped 1/1;  horizontal displacement from post shrinkage reults in less vertical displacement under the hammer beam,  greater slope translates to less thrust against the post and reaching to the lower level of the post translates to better capture of thrust into the floor system and foundation.

Btw, super building.
An optimist believes this is the best of all possible worlds, the pessimist fears that the optimist is correct.--James Branch Cabell

Offline Raphael

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Re: Minimum Pitch for a Hammer Beam Truss?
« Reply #11 on: February 21, 2007, 08:23:58 PM »
Hey Thomas,
  I'll see if I can get a scan of my drawings done once I'm fully recovered from last nights "crash".  My PC managed to corrupt the FAT of a 48gig drive so I was up till 6am dragging what I could off the drive and then back onto to the new drives I created after I repartitioned.
  The proportions are more like 2/5 than 1/3.  The rafter feet do land on the hammers.  I was prepared to raise it either way, but if it had come together in time it would have wound up going in as a single unit.  Foard Panel's truck crane raised the office portion of my frame.  I had a number of different designs for that bent all (but the one I wound up with ::) ) geared to having a ceiling fan below the tie rather than above it.
  I spent a lot of my design time on airflow.
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and the truth hit him like a man with no parachute.
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Offline Thomas-in-Kentucky

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Re: Minimum Pitch for a Hammer Beam Truss?
« Reply #12 on: February 22, 2007, 09:00:23 PM »
Roger,

Thanks for the insights.  Anyone reading this thread would do well to read your last post more than once if they plan to design their own timber frame - whether they're building a hammer beam for a house, or a simple barn.  I follow everything you're saying, and it all makes sense.  My point was that the major force on the outside post of a hammer beam bent is a torque, not an outward thrust.   I should have clarified that I was talking about the static case, as you say... neglecting plastic deformations, shrinkage, or idiosyncrasies of pegged connections.  But you are right, all of these things need to be accounted for in a timber frame.

My simple "cut the rafters in half" diagram that I posted previously is no substitute for a full blown finite element model, complete with elastic deformations, but I think it does demonstrate intuitively, if not naively, how a hammer beam bent keeps from spreading by applying a torque to the tops of the posts, not a spreading force.  As for my own case, I think I'm OK as far as elastic deformations are concerned... I modeled the assembly (on a computer) with ridiculous snow loads never seen in these parts, and my 21' long 12x16 post should deflect (horizontally) less than 1/10th of an inch (elastically) under those conditions.

Of course, you are correct that wood can exhibit plastic behaviors (and glass is a fluid isn't it?... depends on the time scale), and I really have no way to account for it in my design - other than the fact that I've over designed w.r.t. elastic deflection and failure.  I can observe, clearly, plastic deformation (maybe "creep" is more appropriate) of wood in some of the ridiculously undersized timbers in an old barn here on my farm.  Hopefully the oak 12x16 that forms the major post of my hammer beam bent will not exhibit significant plastic characteristics in the time scale of interest to me or my heirs, and in a couple hundred years, the owners can tie the two hammer beams together with steel if need be... as is the case with some of the historic hammer beam frames in Europe.

Shrinkage is real, and its effects will become evident within a few years I'm sure.  So this is something I tried to consider back during the design stage.  When choosing (oversizing!) the depth of my timber that forms the actual hammer beam, I thought about the fact that when it shrinks, it will effectively shorten (and therefore unload) the posts that are supporting the midspan of the rafters.  I still went with 12" of depth, but because the brace and small post that intersect it are housed, it is effectively only 9" deep, with maybe 6" of vertical displacement between the pegs of the lower brace and the pegs of the small post... still that leaves 1/2" of shrinkage in the effective height of that small post.  Hmmm.  Fortunately, I remember that particular rafter has nearly an inch of crown.

Which brings me to maybe the only aspect of your description of hammer beam dynamics that I might see differently...  (sorry, just trying to convince myself my house is not going to fall down!)  Rafters do sag - elastically and plastically (is that a word?!).  As I'm sure you know, in the simplest and most common truss imaginable, "two rafters and a collar tie", the collar tie is in compression, not tension, if placed in the upper region of the truss, simply because the rafters tend to sag.   I think the sagging of the rafters must be considered in a hammer beam bent.  It would tend to keep roof loads exerted on the hammer beam (the hammer beam timber), just as all of the forces you mentioned are trying to unload the hammer beam, wouldn't it?  Consider a queen post bent, w.r.t. the same factors you mention in your last post - wouldn't the shrinking height of the tie beam effectively reduce the height of the queen posts until they supported none of the roof load... if rafter sag was not taken into consideration?

What do you think?  I think we're talking about the same thing - and it really depends on the relative sizing of the the timbers.

-Thomas


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Re: Minimum Pitch for a Hammer Beam Truss?
« Reply #13 on: February 23, 2007, 05:39:55 PM »
Thomas, to your point on a queen post truss, if a conventionial approach is used where the rafter chords and the posts land on the atop the bottom chord, the shrinkage in the bottom chord will not change the relative heights of the rafters and the posts, so a consistant geometry is maintained.  Second, the queen posts are in tension rather than a supporting role of a post, so language tends to confuse the actual role.  19th century American engineer and writer John Trautwine wanted to change the way the king post and queen post trusses were named to the  king rod and queen rod truss regardless of material use, ie steel rods or timber.

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Offline Thomas-in-Kentucky

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Re: Minimum Pitch for a Hammer Beam Truss?
« Reply #14 on: February 23, 2007, 09:28:15 PM »
Roger, you are right again.  And the language is confusing.  I was actually referring to bents like those in Ted Benson's books that use "queen posts" to support the rafters.  These bents are not "conventional queen post trusses," but instead rely on a queen post, acting as a post in compression.  I think bents such as these would have the same shrinkage issues as you referred to in hammer beam bents.

Terminology aside, the bent in the foreground is an example of what I'm talking about.  Not a queen post truss per se, but a common timber frame bent that incorporates queen posts in compression:


Raphael, sorry to hear about your computer crash.  I hate it when that happens!

The reason that I asked about your hammer beam raising plans:  I raised my first hammer beam bent as one unit, and then the second hammer beam bent in three pieces.  Although it took longer to raise the bent in three pieces, that's how I would do it again.  It was far less dramatic to raise it as three pieces, and drama is not what I was seeking!  Without sufficient rigging,  the hammer beam bent wants to fold up like a taco when being lifted from horizontal to vertical... more so than any other bent design I can think of.

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Re: Minimum Pitch for a Hammer Beam Truss?
« Reply #15 on: February 24, 2007, 01:26:46 AM »
Raphael, sorry to hear about your computer crash.  I hate it when that happens!

  I guess it kind of serves me right for ignoring an error I knew was on the drive even though all my tools couldn't find it.  Looks like I lost about 10gig in files, fortunately most can be retrieved or recreated from the original source.
  One of the photo's I lost (temporarily) is one I took at the TTRAG conference in '04 of a large barn with canted queen posts which are definately in compression, they support large purlins and land on the tie beam directly above posts so all that roof load is channelled straight down to the ground.  I'd say the key word is truss...  Anytime I see the word I know forces in the King or Queens are reversed.

Quote
Without sufficient rigging,  the hammer beam bent wants to fold up like a taco when being lifted from horizontal to vertical... more so than any other bent design I can think of.

  I figured I'd need a pair of strongbacks clamping the truss to keep it from hinging, is this how you rigged the one you did raise as a unit?
... he was middle aged,
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 --Godley & Creme

Stihl 066, MS 362 C-M & 24+ feet of Logosol M7 mill

Offline Thomas-in-Kentucky

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Re: Minimum Pitch for a Hammer Beam Truss?
« Reply #16 on: February 24, 2007, 01:12:26 PM »
Yes, I clamped a pair of oak 3x3's (one on each side of the bent), running from one hammer beam to the next.  When I started the lift, the clamps slipped a little and "bowage" ensued.  Scarey to watch - we lowered it, tightened the clamps and added another strongback (or stiffback, or whaler, or whatever they're called!).  We also moved the lifting straps to a slightly different location.  When I watched a commercial hammerbeam being raised, I remembered that they rigged it so that when the thing was vertical, the straps were effectively lifting the bent by pulling up on the bottoms of the hammer beams (not pulling up on the rafters).  This is how I rigged mine, but getting to that vertical position was the tricky part - and I couldn't quite remember how they did that bit.

BTW, the bent we raised this way is the rightmost bent in the picture (picture in previous post that has the two cool dudes standing in front of the SIPs).  That bent had a tie-beam, if you will, that now supports a loft. 

The other bent (to the left in that same picture) had no such tie beam, so I think it would have been even more difficult to stand as one complete bent.  To raise it, we first raised each major post complete with hammer beam.  Then we pointed the hammer beams at each other using a string line.  Then we adjusted the distance between the two post tops, then flew in the upper assembly which contained the rafters and everything else.  Here's a picture of the upper assembly - no stiffbacks, no drama, no sweat.  We flew it in with three tag lines so we could get the four points to land where we wanted them to.  The housings in that bent hide the generous chamfer we put on all of those joints to make things go together more easily.  This is definitely how I'd do it if I had to do another.



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