Hammerbeam general Questions

[petehalsted]

First don't panic on this drawing. I'm just starting to play with a few things for my front entry way. There's lots of lines, etc there for reference right now. And although I've got it drawn for with a continous tie beam, What I'm trying to get to is a hammerbeam, but something isn't coming out right. Using Steve Chappell's "A Timber Framers Workshop" as a reference.





He says that the Hammerbrace should start 1/3 of the way up the Post, and the Hammerbeam should be 2/3 the length of the post and that the hammer brace should be at the same angle as the roof pitch. My roof pitch is 6/12. It seems like there no way to make that all work out. You see the line starting at the bottom 1/3 of my post, its at the same angle as the roof, but obviously would not meet with a hammer post if it were only 2/3 the lenght of the post, not to mention that brace would look wierd as heck anyway.

So is a hammerbeam not doable with a 6/12 pitch or did I misinterpet something.

A little background if you want it. We are trying to make a timber frame entryway for our traditional framed/log sided house. The slab is 6' x 24'. I am trying to used rescued 6x6 timbers from a 1923 barn. The longest I have is just over 20' so I don't have one that could be used for a continous tiebeam. So hammerbeam comes to mind both for aestics and use of smaller (6x6) and shorter beams. I know 6x6's sound small for a timberframe but since there's really only going to be 6' of roof (rest of dormer will be traditonal framed and tied into the timber frame portion), my initial calculations seem to say I will be ok (again I'm at the begining of the task). End result is we are trying to make the front entry timber framed giving the impression the house is.

Secondary question if Hammerbeam is out. Can you splice a tiebeam at the king post? Again none of the beams are long enough to make the span continuously, and I really want to stick with 2 post in front (instead of 4) if at all possible.

Here's a pic of the house and a pic similar to what we want to achieve (although that pic isn't a hammerbeam)

[Jim_Rogers]

About using 6x6 timbers for a hammer beam truss, the problem is that all the roof load comes down the braces to the posts and if the post isn't big enough to resist the load then it bends at the point where all this load is applied. This is why the make big buttresses outside of churches that have hammer beam trusses.
If you want to do a truss like the one pictured, which is a king post truss, you can do it with two beams making up the bottom cord and joining the king post.
There are ways to do this with dovetail tenons or through splines.
I have some pictures of drawing in my gallery of king posts with splines, you can look at that to get an idea.
I'm not sure about the 6/12 roof pitch situation with a hammer beam truss, but under the circumstances of the size of timbers you want to use, I wouldn't try a hammer beam with some very strong posts....

Jim Rogers
Jim, I edited your post. Tom.

[petehalsted]

I know the post are undersized for a true hammerbeam, but again this is only for a 6x24' front entry. So total sq ft of roof area over the slab is 168 feet (14' rafters, 6' deep). Ignoring any support recieved from the tie in to the traditionally framed portion of the doomer, I would only be asking each post to carry half that load. So even using a high number like 200 psf of load, I'm only asking each post to carry 16,800 of vertical load.  A 6x6 Eastern White pine can handle 46,800 pounds of vertical load, and if I understand the principles behind the hammerbeam everything is tranfered to a vertical load.

Not trying to argue with you, just giving you my logic, so you can tell me where I went wrong, I've read enough of your post to know you have probably forgot more about Timber Framing than I will ever know.

I will check out your gallery and see if any of the king post designs there help me out. I am defintely not married to the hammerbeam (its a bit agressive of a design for a first timer!). Just would like to continue the discussion on it for general knowlege. Seams like unless I'm missing something it would be hard to use a hammer beam with out a fairly steep roof. As you see the issue with my 6/12 slope and the formulas.

[petehalsted]

If you want to do a truss like the one pictured, which is a king post truss, you can do it with two beams making up the bottom cord and joining the king post.
There are ways to do this with dovetail tenons or through splines.
I have some pictures of drawing in my gallery of king posts with splines, you can look at that to get an idea.

Jim found this drawing in your gallery



Are those pieces on bottom wedges holding the dovetail tennon in place or ?

Is there any requirement for the amount the King Post extends beyound the tie beams? I've got 8' 11" from slab to bottom of the tie beam and since that will be directly in center of door, I don't think I would want it coming below 8'.

I was thinking of a spline joint for the tie beams with the king post having a through tennon with tusk in the spline, but I would imagine the way you show it would be stronger.

Do you have happen to have a "see through" or exploxed drawing of that truss, showing the jointery?

Thanks

[Thehardway]

There have been a number of posts and conversations on the pro's/con's of interrupted tie beams in king posts. You might want to do a search on the topic.  Jim has shared photos of several that are splined as he mentioned.  I have designed one for a 24' span but do not yet have an engineers sign off on it.  6"X6" members sound a little small to me for the span you are trying to acheive.  Using a hammerbeam design would place your braces so low on your posts that it would limit the usable space on your porch or be a headknocker.   Why not do a Queenpost design? I think it would look great if you placed 4 posts where you currently picture the four pots on the slab and used an "anchor beam" with tusk tenons, in the centered between the queenposts.  This would frame your door nicely and also give you some options for securing a railing if you so desire. 6/12 pitch would be well within limits of queenposts. Good luck.

[petehalsted]

I was trying to avoid 4 post in order to keep the front entry very open so the door (btw old photo of house, door is now oak) and two windows  are unobstructed. With the spacing of the windows not the same on each side of the door, I'm afraid the two center post would accentuate that.

I will play with the drawing some and see what we think.

[Thomas-in-Kentucky]

if I understand the principles behind the hammerbeam everything is tranfered to a vertical load.

What you say is true for, say, a kingpost truss, but not true for a hammer beam structure.  The key to a hammer beam is that the roof loads translate into a tremendous moment (or torque) on the tops of the main posts.  With a 6:12 slope, 6x6 posts would not be appropriate.  Neglecting the actual roof loads, the truss has to support itself without sagging.  Also, the tenons on the hammerbeams themselves must be sufficiently long to resist the force that is trying to pull them out of the posts...  6x6 posts would not allow for long enough tenons (unless they are through tenons).  Just an opinion, but I don't think Steve Chappell's book covers the principles of a hammer beam very well.    If you follow his advice, I think you will have a safe structure, but that is because the braces are, for all intents and purposes, carrying the loads to ground, not to the posts.  As you point out, following the advice in his book can result in an impractical looking structure.

you might be interested in this thread...
https://forestryforum.com/board/index.php?topic=24074.0

I'd vote for king post in your particular application, and I think a timber frame entry would look great on your house!  How about a 20 foot span (using one of you available barn timbers for a tie beam), with 2.5 feet of overhang on each side to accommodate your 24 foot slab?  There's more than one way to skin a cat! 

-Thomas

[petehalsted]

Thanks for the link, that confirms that I wasn't off on my hammerbeam attempt, just not pratical in my situation at all.

When we built the house, we planned for the porch to have a roof, so there is a 9' column under the front corners of the slab, the back of the slab is supported by hanches from the basement wall. If I moved just the two post in, I would be off of those columns, and would be asking the 4" slab to handle the load so not sure that would be a good idea either. Thoughts?

[Don P]

I'm not sure there is a situation where a hammerbeam is practical  :D

I've been building some 24' kingpost bents. For the long bottom chord, one was splined and pegged, one was side plated with 2x10s and bolted. Over the weekend I've been nailing a built up bottom chord together of 4 plies and several hundred 4" ring shanked pole barn nails. It has exposed nails, but, no checks, well distributed defects and is dry.

Thus far the bolted side plates win my vote over splines for strength. Aesthetics is a personal call. Tieing the center of a splined bottom chord back to the building would be another way to stiffen that joint up. This is a bolted bottom chord.

[Jim_Rogers]

Are those pieces on bottom wedges holding the dovetail tennon in place or ?

That is one continuous exposed spline.

Is there any requirement for the amount the King Post extends beyond the tie beams?

I'm sure there is, as you have to understand that a king post is hung between two rafters and is holding the bottom cord up whether it is continuous, splined or dovetailed at the point where it meets the king post. Therefore there is a need for the king post pendant to hang down to prevent the mortises from splitting out the wood between the mortises and the end of the king post pendant. This amount of wood may need to be accurately calculated.

Do you have happen to have a "see through" or exploded drawing of that truss, showing the joinery?

I can look through my computer drawings for this king post truss, in order to show you what's inside. But what would be better is to design your bent completely using the timber sizes you actually want to use.
There are some drawings showing splines in my gallery with king post and hanging down pendants but I'm not sure of the roof pitch on that structure. And these should not be exactly copied but used as a reference as to what or how to create a splined king post joint. The reason that they should not be exactly copied is that each situation is different and all the facts of your structure need to be taken into consideration. And your design needs to be done based on these facts.

Some things you have can not be changed, such as the exact point where you want your posts; due to the supporting concrete work. The size of the timbers you have on hand. The roof slope you want to use. And I don't think you've mentioned it yet, but your snow load for your area.

Once all these facts are known then the design can be drawn. Once drawn it has to be evaluated by someone who has experience in calculating the loads and seeing if the design is strong enough. I am not qualified to do such calculations...

If the calculations prove that the design is bad then you may have to add steel to make it stronger....

[petehalsted]

Jim,

I found the thread where you discussed that splined King Post and you had posted some blue prints of the joint but they were to small for me to make out.  I'm just not getting exactly how this joint is put together. The reason I'm asking is without steel, I've only found two joints shown for this. Your and the double half dovetails. The double half dove tails would seem to take alot of wood out of my 6x6 king post, so thats why I'm interested in how your joint is made up.

I may have to resort to steel to use this 6x6's, but if I can avoid it I would like to.

Like I said I am in the preliminary stages with the project. Once I have a pretty good idea of what I want to do I will defintely have someone, run the numbers and stamp it.

BTW, just FYI our roofing requirements are snow load is 20psf, with 90 mph wind gust.

[Thehardway]

Are all of your timbers 6X6?  You said the only 20' timber you had was 6x6 but waht about the shorter ones?

[Jim_Rogers]

Pete:
I sent you an email with the photos from my gallery in larger format....
Jim

[petehalsted]

Got the email and photo Jim, Thanks that helped alot!

Hardway,

All the timbers I have "inventoried" so far a 6x6. I did get a few 6x8 timbers from the barn but I have to check them for quaulity yet, Might be enough for the 2 tie beams, doubt there would be enough for tie beams and king post. The barn was approx 100x80 and I've got 6 trailer loads of wood piled in my driveway awaiting nail pulling and clean up. Some of the timbers are buried in that pile. And its lovely Missouri summer weather here (90+ temperture and hummitity) so not alot of work getting done. But I'm in no hurry on this project, just trying to get some ideals together.

[Jim_Rogers]

Ok, so Pete lets start at the beginning.
You want two free standing bents that are connected, right?
And what are the outside dimensions of these two bents, front to back and left to right?
And what is the desired bottom of tie beam height?
Jim Rogers

[Thomas-in-Kentucky]

throwing another idea out there...

if you want to avoid a two-piece, splined tie beam, and your tie beam must be less than 20 feet, then perhaps you could hold the tie beam up a foot or two, and still keep your posts 24 feet apart.  Essentially, the tie beam would not touch the posts at all.  Here's a picture of what I'm talking about...  This is an 11 foot span (not counting the rafter tails), using a 9 foot tie beam.  The tie beam is "let-in" (or lapped, if you will) to the rafters for simplicity.  The tie beam is also "let-in" to the king post, which let me run the king post on down and have a neat pendant.  I wouldn't recommend an exact duplicate for your application... for instance, struts to support the middle of the rafters would be a good idea for a larger span.  Also, you might come up with a more reliable joint than the lapped half dovetail.



hey - this lets you put a pendant up further too... so it's not a head-knocker.

[petehalsted]

Your close Jim,

I actually plan to only have one bent (don't give up on me there's a method to my madness). Two bents on the 6' slab would be fairly crowded and be a waste of fine timbers. My plan is the frame the a traditional gabled dormer on the existing roof. Tie the two top plates into the existing houses rafters. And basicly make a faux bent (using 2x6's from same barn) on the front the gable. And of course the ridgebeam would tie into the one from the gable on the house.

A little hard to describe and my sketchup skills are a little slow I haven't got enough of the drawning done to make it clear yet. but when its all buttoned up. when you walk you you would see a timberframed king truss/bent in the entry way.

When you walk underneath it and lookup, it will look like there is a second bent, with the post inside the walls of the house.

This second bent is a faux bent attached to the plywood front of the gabled dormer I will build on the roof.

I haven't worked out all the details on how the top plates and ridge beam will be tied in, but my basicly plan is to run the long and then I can use steel to attach them inside the gable where they will be hidden.

Some measurements for you.

Top of Slab to Bottom of Eaves (height of post/bottom of tie beam) = 101"
Slab depth = 6'
Slab width = 24'
Post position = inset 3" from front and side corner (to center them on the hidden columns underneath)
So Measuring from outside of one post to the other would be 23' 6"
Roof Slope = 6/12 this is dictated by the existing house and span. Coming from peak of house and making 24' span, I get a 6/12 slope so thats what I'm stuck with.


Thomas,

Interesting design, but a little more advanced (figuring structual capicity) than I think I want to try with my 24' bent.

[Don P]

This is just an aside post, from when I worked on the math end of a kingpost truss here.
https://forestryforum.com/members/donp/trusswbending.htm
It was stretching my pea so no guarantees. Hopefully it'll help someone.
For inputs on Pete's truss I used some rough numbers
3600 lbs
6/12 pitch
282"
5.5"
5.5"
3.5"
3.5"
species, I tried #2 SYP
Hit "Show Result"

It's figuring the members as axially loaded members, columns, rather than simple beams, the rafter column can have a side bending load, axial+bending.

The column stability factor is indicating how much strength is lost due to slenderness. To fail a web...Take the web width down to 1.5" and check its stability and buckling. It is down to 23% strength due to its slender dimension, the actual stress exceeds the allowable stress.

[Jim_Rogers]

Pete:
The first problem that jumps out at me when you say you're going to have half stick frame gable entry and half timber framed is shrinkage. The stick framed part most likely will not shrink to the extent that the timber framed part will. Although you are using recycled timbers that are most likely very dry and won't shrink much at all.
But you'll need to be aware of it.....

[Jim_Rogers]

What is the height of the top of the ridge of the existing house?

The reason I ask is you don't have to make the entry the same roof pitch, if you don't want to....

[petehalsted]

Jim,

Shrinkage is a good point, but on 1923 6x6's I think Its less of an issue, but will probably still need to be accounted for with how I attached the 2 halfs. But I just can't see trying to timberframe the half that is setting on the roof and the framing can't be seen at all once its covered.

I'm not matching the pitch of the existing roof, I'm getting max pitch that I can get with a 24' span so that the ridge board of the gable meets the ridge board of the house gable. In otherwords to have any higher of a pitch on the entry I would have to less than 24' or the peak would be above the peak of the main house. Which I guess could be done if you sloped the ridge board back to the house.

And some more measurements for you for the house,
house is 32' deep
Total Roof Rise 86 1/2"
So house roof is about 5.4/12

Don P,

You throw out a couple of terms that I'm not familar with, so I'm not sure exactly what your saying. I'm not sure what the web is?

And another piece of the puzzle, I believe the beams are Eastern White Pine, hard to tell on 84 year old timbers but most are from a creamy yellow to a pinkish read in color and defintiely pine, couple with the fact I'm sure they were harvested locally in Ellington, MO (northern Ozarks) that points to Eastern White to me.

[Jim_Rogers]

Ok, now I understand why you want a 6/12 entry roof pitch....

[Jim_Rogers]

I came up with this:



and:



All the timbers in this entry bent are 6x6 except the braces and struts which are 4x6. Usually you try to have the struts go from the mid point of the rafter to the king post with one end of the strut 90° to the rafter, but in this case that point was no where near the mid point of the rafter.

These are two views of the same bent, but not engineered and/or not all the joinery has been put in....

Just ideas.....

Jim Rogers

[petehalsted]

You got it Jim, I will get my sketchup drawing there at some point, but man I'm slow <G>.

Your drawing and comments do point out the issue struts. Not being a structural guieus and just looking at it, it seems that the strut would not be as effective at preventing rafter sag. Doesn't seem like it would be in compression as much as it would if it were 90 degrees.

I wonder if I wouldn't be better off, moving the struts up to the top 1/3 and adding queen post at the halfway point? Thoughts everyone?


As to 4x6 braces and Struts. Would there be an issue with using 6x6 throughout. Keeping with the plan to use timbers from the barn I don't have 4x6, I could cut a 6x6 down, but that would expose a clean face and I would lose that 84 years of patina that they have now.

Missing from the drawing is the top plates and ridge poll, and those are two areas that have me concerned when it comes to jointery and my 6x6 restrictions.

Top plates: I believe in the drawing they would attach and be at the same level as the tie beams, this means we have the tie beam, wall plate and post all tieing together in a 6x6 area I don't think an english tieing joint would work in that amount of area would it?

Top of King Post, Ridge Poll: Same issue we got the two rafters coming into the top of the king post, plus a ridge pole, again all in a 6x6 area.

Since this is more or less a standalone bent, I don't need the top plates, etc for the next bent, but do need them (or something) to tie into the gable dormer/current house framing to keep the bent standing. If I go with purlians that could be doing some of that.

BTW Jim in the first drawing it appears the post are leaning in, I assume thats an optical illusion caused by the view ange?

Thanks again everyone for the input so far! Once this becomes a reality and really going to boost the front view of my house.

[Don P]

Pete, I meant strut when I used web.

As I swung the struts up on one program, the loads in the rafters got better. This would need to be checked against the lengthening lower part of the rafter's stability.

At a quick check white pine was cutting it pretty fine, use good stuff.

If I spline another one I'll totally house the bottom chord into the kingpost, the spline groove wants to open.

I would consider just purlins to the main roof, with them bolted atop the truss without notching the top chord of the truss.


The hard part with the 6x6's is having enough wood left after doing some of the joinery I think.