Hammerbeam vs. King Post for 24 - 30 foot clear span?

[ArgusBassler]

Background: I am trying to design a timber frame sawmill shed.  I recently bought a woodmizer hd 35 and I would like to continue cutting through the winter.  My end goal is to build my own timberframe home 1600 to 2200 square feet for my wife and I.  We live in PA.  I am lucky enough to have roughly 700 acres of forested land in my family that I can cut and build on.  I was only able to buy the sawmill after a recent selected harvest from this land and I am very great full for what the forest can provide my family and myself.  I work a regular job using solidworks cad, mastercam to run a cnc router and have access to many wood shop machines.  I built a timber frame woodshed for firewood two years ago using a chainsaw to turn the logs into posts and beams, it is about 16 feet by 12 feet and holds all the firewood I need for a season of burning. So I have read 2 timber framing books more than once and am still learning every time I re read them.

My dilemma:  I would like to have the ability to saw logs up to the 21 foot max length my woodmizer can handle.  So I am in need of a clear span that long or longer.  I have drawn up 2 versions with the shed size I believe to be appropriate.  

Questions:  If I am not worried about the shed being pretty would a king post truss be a better way to go over the hammerbeam to get the clear span I would like?
                 King post question, does anyone have a picture or diagram of how 2 tie beams are pegged to the king post?  In my design the span is 26 feet clear the                    tie beams are 7"x12"s and the king post is 7"x10" I know it should be a wedged dovetail but not sure about the angle of the dovetail.  if you could point                  me in the right direction that would be great!
                 Also I would love to learn about the Scarf joint as I would like to use this joint as well on the 2nd bent with a post underneath it.

Please post pictures of king post joints with a description of dimensions if you can.

Here is my King post design 

 
 



  

[ArgusBassler]

Here is a pic of the tie beam to King post joint half dovetail wedge I have now.  The tenon is 2 inches thick.

 

[florida]

How much would a timber-frame barn like you want to build be worth when it's finished?

How long do you think it will take to to fall the trees, move them to your mill, cut the logs, make the joints,  and erect the building? Then you'll still have to cut siding, install that, do whatever interior finish you want and put a roof on.  I'm not a timber framer but I'm guessing on a part time basis you'd be lucky to be done in 2 years.  During that time money is only going to flow in one direction, away from you.

You could have a steel building erected, probably for a fraction of what your timber frame barn would cost,  a month from now and go to sawing and have an income.

[rjwoelk]

Florida you missed the point. The guy wants a timberframed building.
My fil made the same statement when we built the log cabin.  Why mot just frame it with 2x6 like other dwelings.
I tild him then it would not be a log cabin .

[Dave Shepard]

You will need a good timber frame engineer to attempt a hammerbeam that size. Kingpost would be a better approach.

[D L Bahler]

A pure king post truss is the most efficient truss design. It will do the job at this size better and more simply than anything else.

When building things, structural efficiency is a good thing to keep in mind.

Hammer beam trusses are actually terribly inefficient, and technically are not actually trusses at all. They don't tie the walls together in any meaningful way against spreading, and require buttressing of some kind or the inclusion of copious amounts of steel to do this.

Your particular method of king post design is outside of my expertise, so I'll let someone else deal with that. The only I thing I would add though is that there is no reason why your two tenons should not be touching inside the post.

[ArgusBassler]

florida, I am more interested in using this build as another learning experience than trying to make money.  I have over 100 acres of tops from the recent cut to find probably 1/2 of the wood needed to cut for the build.  I Only plan on felling 1 or 2 white oaks for the tie beams and principle rafters.  I do not know how much something like this is "worth" to others to me its an experience.  I am sure I could put up a steel building much quicker but in my opinion that would just look ugly sitting among the trees. I already have all the tools needed.  I would just need to find a good price on metal roofing, and buying the concrete for the floor that is another dilemma I am thinking about.  should I have a dirt floor? That is what I keep asking myself.  

[ArgusBassler]

Here is my original idea using the hammerbeam style I do not its not a truss but the design was based off Chappell book a Timber framer's Workshop.

 
D L Bahler and Dave, Thanks for the replies I will work on changing the tie beam tenons to touch one another.

[D L Bahler]

Let's clarify what I mean by truss 

According to the traditional timber framing terminology, yes Hammerbeam is a truss. 
According to engineering principles, it sort of sits on the fence as to whether it really is or not. It doesn't actually perform any trussing action. 

In German, we'd call this "Sprengewerk" and the trussing action would be "Hängewerk" so those terms should clarify a bit what I mean. 
A Hammerbeam "truss" functions by "springing" the loads out toward the walls. This greatly reinforces the rafters and also allows you to span greater distances than a simple beam would allow. But it's not a truss, because nothing is "hanging" that is, nothing is being pulled up by a truss post or cable. In the king post truss, the tie beam is suspended from the upper cords (the rafters). In german the king post would be called a "Hängesäule" or hanging post, because it is hung from the rafters above, and in turn it is holding up the tie beam. Maybe my terms don't lie up with what others are used to, but that's because I'm working from the perspective of another language. 

Since there's nothing hanging, there is really no tying action going on -well there is, but it's pretty negligible. The only thing holding the walls together is the triangulation of the hammer beam struts. These don't want to flatten out. But relying on them to resist thrust means putting stress on the joints in ways they aren't designed for. Some timber framers solve this by adding a steel cable to tie the walls together. Traditional methods frame wings on either side to buttress the structure against spreading, with shed roofs that push back in on the roof structure. 

Other than that, King post trusses are really quite simple. With a steep roof like you have there won't be a tremendous amount of thrust on the tying joints. I really wouldn't fret the specifics of the joint design too much. A modest slope on the tenons into the king post secured with a fairly flat wedge should do the trick quite nicely. 

[ArgusBassler]

D L Bahler, thank you for the lesson I find it very interesting.  I will do just that, a wedge with a 2 or 3 degree slope and I will leave the dovetail the way I have it, a 1 inch taper.  Another newbie question, the grain of the wedge should run the same as the tie beam?  I mean should the grain be vertical or horizontal to the king post? hmm I guess I mean the tangential lines should be vertical or horizontal with the king post?  I am not sure how else to explain it but I cant find anything that says a correct or wrong way. Thanks for the good info.  when I have the design all figured out I will make a cut list and post the stages of the build as I begin and hopefully to the finale before spring gets here.

[D L Bahler]

Grain should run the length of the wedge, so same direction as the tie beam. If it is vertical, the wedge will be very weak and simply snap off. 

[ArgusBassler]

yes but should it lay flat sawn or tangential lines run vertical?  Sorry, maybe I can draw a picture here soon to show you what I mean.

[Don P]

There is a discussion of that in the USFPL's "Wood Handbook" I think it would be in chapter 4, long story short, from memory, negligible difference.
edit; read definition of shear strength parallel to grain 4-3 in my copy, rolling shear in "less common properties" 4-24, fracture toughness 4-25, annual ring orientation 4-30.

The dovetail should be thought through, quantify the horizontal thrust in the tie (looks like a 12/12 roof, gravity load=tie tension, hit the toolbox I have some calcs for that) Then compare the horizontal shear area of the dovetail x the allowable shear for species and grade and compare that to the tension, I'm not sure you are ok there, check it or have an engineer check it. In the same vein the other end of the bottom chord should be analyzed, typically the first check in a truss is at the heeljoint, if you can resolve it you usually have a buildable truss.

I used a spline through the kingpost made of interlocked gum and let into the bottom chord ties on each side then pegged through tie,spline,tie so double shear 1" pegs in 2 staggered rows for my shop kingpost truss. There are some pics in my gallery. The shop isn't worth squat but I had fun, go for it. The English use the term king rod sometimes to help understand the tension role of the kingpost.

For scarf joints google Cecil Hewett, I've found his treatise on the evolution of scarf joints online. It is also in his book "Historic English Carpentry".
Edit, start on pg 253 here;
http://www.buildingarchaeology.org/wp-content/uploads/2015/03/Structural-Carpentry-Hewett.pdf

A lot of people will advocate scarfing a plate at the inflection point, out near or over a brace. I haven't seen yet reliable engineering for that. Building code clearly prohibits the practice.  In one book advocating it and showing it in historic work, if you look it is splitting right where one would expect. It hasn't failed but its hurt. I like to build on sound engineering principles. My engineer at the time when I asked said he lets other people do that. I've treated it as beams overhanging posts structurally. The AISC steel manual does go into connections at the inflection in their beam equations but their connections are not the same. I sort of view scarfing for the sake of scarfing in the same light as hammerbeams for the sake of hammerbeams, angels tread lightly there. Just to be clear, I am not an engineer, just a carpenter with opinions like anyone else.

[Brad_bb]

I shy away from Hammerbeam bents.  You have to find a way to counteract the outward thrust.  In the old days it was done massive stone buttresses.  Today they tend to use a steel tension rod spanning where a tie beam would be in queen or king post bent.  I see them used up to 24ft typically.  You can go wider, but it just gets more complicated to counteract the thrust.

I prefer queen or king post.  With a king post design, you tend to have tension joinery where the tie beam meets the posts.  I like to avoid tension joinery when possible.  I have a building made of two frames that connect.  One is a Queen post design that spans 36 ft.  The other is a king post design that spans 26 ft. 

King posts can join the tie beam in several ways so don't think you are locked into a wedged dovetail design.  A blind tenon or a thru tenon can be used as well.

No matter what, the design should be reviewed and ultimately stamped by an engineer experienced in timberframing - like firetower engineered timber.  

[ArgusBassler]

Don and Brad, Thank you for the great info I will do more research.  Don, I will be checking out the link and your pictures ASAP.  I will be checking Amazon for a few more books.  I am not sure what a blind tenon is but I know what a through tenon is and I did not even think to use that on the king Post.  If I were to use one of these are they still wedged? The reading I have done the indicates that a half dovetail wedged adds 2 more planes of shear, this is a good thing? Obviously I have much to learn and I greatly appreciate your knowledge and time spent answering questions on this site.  Everyone is very friendly. Glad to have found this site. Thanks again!

[ArgusBassler]

I understand blind tenon    I just didn't put 2 and 2 together in my head.  I thought it was some special thing I never saw before.  Don I took a look at the pics of your king post.  Looks like the way to go, thanks.  How large were your timbers?  Your pitch looks to be on a 4/12 or so.  I will use the formulas I have found to try and figure out load.  I may look into what I can do to find the stresses in Solidworks.  I should be able to plug the formulas into the program and let the computer do the hard work for me, we'll see.  

[D L Bahler]

I mentioned earlier that the sort of wedged tenon design was outside of my comfort zone, so take anything I think about that with  a grain of salt. 
Since Brad mentioned other ways to join to the king post, I'll mention that I am much more familiar with the use of a through tenon on the king post wedged under the tie beam, or the use of a lap joint on the king post through the tie beam. Both of these come with the requirement of a single continuous tie beam. I also like to use a little bit of steel in a truss (Blasphemy!) to reinforce high-stress connections. I am not a fan of the desire of many to totally shun metal,  and believe it to be historically misinformed. That said it is perfectly possible to do what you want without any metal, just a lot harder. 

[wodbutchr]

I enjoyed this thread having been through many points raised here, the main one being the outward thrust.
The hammerbeam/kingpost combo has a 12' span and the total width of the frame is 40'. I didn't like the look of the shop quonset that's sitting behind. I have been working on this project for a while as I taught myself timber framing while lurking around this forum quite a bit! More pics are available on my gallery. The wedged tie beam dovetail in the rafter was a bummer to cut . Thx Jim for describing the square rule.

[ArgusBassler]

Wodbutchr, what you have there is beautiful! nice work!  

D L, I would like to try not using steel but I am not totally apposed to that I just don't want to put up a entire steel building I just don't feel like it blends into the woods. The more wood the better just because I can say I cut everything myself 

So I am now at the point of figuring out roof load so I can get the load on rafter per square foot to find the Rafter thrust using the calculator.  One question I am wondering is the purlins, should I add their weight to the roof load? To find the roof load per square (I added the purlin weight) I get 4100 pds and my roof is approximately 440 square feet - so under 10 pounds per square foot? is that what I plug into the calculator or do I divide it by the square feet of the Rafter face? I got 780 for thrust.  My Rafters being 7x10 I divided 780 for thrust by 140 the square inches of the rafter and get 5.57 psi.  Can anyone tell me if the 5.57 psi on the rafter is even close?  This seems low to me but I have not done any real engineering ever.  With the square feet of the rafter face used to carry the load I would get much larger number just not sure if I am calculating things correctly.

A white Oak post sheer strength should be 185 psi but less because I'll be building it green.  If I figured this part correctly then I will work on checking the dovetail joint or the through tenon and I think I'll have a local engineer take a look at what I have done. 

I figured the purlins and ridge beam weigh approx. 1700 pds
I figure the decking to be eastern hemlock or white pine at 1 inch to weigh approx. 1000 pds
I figured metal roof ing to be a max of 1400 pounds although this is just a guess and I think I over estimated to come up with  4100 pound roof load?  

Any pointers here would be great.

Thanks everyone

[D L Bahler]

You need to add snow loads into your equations as well. So you have the dead load, now add the live load. You need to factor in snow and wind. Your county building department should be able to tell you what the number is in your area for these. This could be anywhere from 10 to 40 psf depending on where you are, or even more. 

[Don P]

Chapter 3 here;
https://codes.iccsafe.org/content/VRC2012/chapter-3-building-planning
has a snow load map and wind info, typically snow controls there (that is my VA code based on the model code but your code rules not mine, PA is probably on the 2015 or 2018 version by now and your local jurisdictions weather loads are what matters, especially if you are in a CS area) I'll throw out a number 35psf live load (snow). You can play games with the roof snow load, I don't. Heavy timber, steep roof use 15psf dead load.. total load 50psf, remember this is an example. (correctly you use the rafter length for dead load so for a 12/12 pitch that would be horizontal measure x 1.414 x 15. typically I just use the horizontal projection for both LL&DL)

Assume a 12' principle rafter spacing. The load is collected through the purlins and delivered to the rafters. The purlin is delivering half its load (6') to the rafter on one end of the purlin and half its load(6') to the rafter at its other end, same scenario for the purlins in the adjacent bay. The tributary width of roof bearing on the rafter is then 12', 6' to one side and 6' from the other side). If the building width is 24'the tributary area bearing on a truss is 24'x12'=288sf x 50 psf=14,400lbs uniformly distributed load on the truss.

[ArgusBassler]

First off I have changed the pitch of the roof to 10/12 I will post a picture.

Ok so I have now figured in snow load.  Here is what I have found - total roof load 13,100 lbs. total rafter load 6,550 lbs. and a total of 450 square foot roof.  I found an old thread Jim had talked about checking if the purlins were big enough and mine can support more than 4 times the load of 960 lbs. per purlin that I come up with so that is good.  My Truss Load is 6,550 lbs. My common rafter load is 2,653.92 lbs. and the rafters pass the uniform loaded simple beam test as well. 

Now for Rafter Thrust I might still be missing something.  
Rafter Span = 312
Load on rafter = 30 lbs. per square foot
Pitch       = 10/12
Rafter Spacing = 144"
=
THRUST = 2808 = 2808/2xbxd = 2808/2x7"x10" = 2808/140= 20.05 psi      where b and d = width and depth of rafter

So that would be the Thrust acting on the joint where the rafter lies on the tie beam I believe and this is well within the limits by my understanding as long as the joint is made correctly.  
Now I am not sure how to transfer these calculations to the through mortise joint to the King post which I now have.  Any thoughts on how to figure out what kind of stress is on the tie beam to king post joint mathematically? 

Thank you all for the help I have learned a lot in the last couple days.  

Take a look at what I made for the through tenon I have not seen one like this I just thought it might add an interesting feature while still acting as it should.  I made the through tenon 2.5 inches thick for more beef on the tie beams however I am not sure if this should be done, any suggestions?

[ArgusBassler]

 

 

 

 
Here are some pics of the latest changes to the design.

[Don P]

The terminology is confusing me a bit but I think I see a glitch.
I believe you're saying you have a 30 lb snow load, then add a 15 psf dead load= 45psf total load entered into the calc. At 10/12 that results in 4212 lbs tie tension
There is then a 4212 lb shear stress in the heeljoint acting on the plane in red. How many square inches do you have there in your design. Multiply that by the allowable shear stress... is that number greater than 4212.



Then notice the arrow pointing to the face of the notch. How many square inches is that face. It needs to be capable of withstanding the compression force resulting from the tie tension. It is not quite compression parallel to grain and not quite compression perp to grain, it lies somewhere between the two. The Hankinson formula is used to determine the allowable bearing strength of that intermediate grain direction... or KISS first check that area against compression perp to grain, which is the lower allowable of the two. If it passes that, you're ducky. Else go through Mr Hankinson, google showed a number of hits there.

Then there is the belt and suspenders approach, this is what I built there, the post tenon and buried allthread hopefully will hold the joint down and tight and give me time to run if the heeljoint shears




The same tension stress flows through the entire tie. Your pegs on the left of the king need to be capable of resisting that tension as well as the group of pegs on the right side, also the spline itself. One number I've heard is 1,000 lbs for a 1" hardwood peg in double shear. In a group I swagged it to 500lbs per. Others here might have a better grasp of allowables there.

I'm not really loving your vertical posts on the tie at midspans. You are using them as compression elements. If you look at a typical truss there would be a tension web member dropping from the king's strut down to the tie to hold it up, a rod, then a compression strut running up from the tie to beam at that point. "correctly" the king's struts would divide the top chord in half, effectively halving the rafter spans. Or, in the "corrected" version of your posted truss they would divide the rafter into thirds and the tie into halves.

[florida]

Sorry, I wasn't trying to offend anyone. I'm a pragmatic builder. My point was that you can have a building up quickly, a steel building, or maybe a pole barn would suit your needs better. It's fast and cheap and would give you a covered and/or completely enclosed space to saw your house in and you could be working in it in 30 days.  I love timber frames but they are about the most material and labor intensive buildings you can erect.

No, it wouldn't be a timber frame but would allow you to spend your time cutting your house timbers rather than spending your time cutting timbers to build a shed to cut your house timbers in. It would likely put your schedule ahead by several years.

If you only need a few 21' beams you could use a winch to drag them through a 4' X 4' hole in the end of the building rather than designing  your entire building around those few beams.  

[Don P]

Don't know if it was me but I'm usually just being opinionated rather than offended, it can be hard to tell in typeface rather than face to face . TF doesn't have to be slow. Lots of folks fuss with it enough to make it slow but every farmer back in the day was a timberframer.

[ArgusBassler]

Don P. Thank you for all the info. I am going to rework a few things this week and I will post some updated ideas with the math reworked.  Florida, you did not offend me either.  I am just trying to do as much learning as I can before attempting to conquer my house build.  I know even if I put up 100 timberframes I would still not know enough to build my house perfect.  Thanks for your input I understand where you are coming from.  I'm just not in that type of rush to build (at least not now, I know time goes by quick), my daughter just turned 11 so I have 7 or 8 years before I building my house although I would like to have it well thought out and plans for it before then.

[ArgusBassler]

florida, I do agree with you that cutting over 16' will not happen very often most likely.  I could for sure just use a winch like you said.  This really gets me re-thinking - what are my true needs maybe I should talk to a few more sawyers and see what they have and if they wished they had something different.  Thanks again.