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Author Topic: Hammerbeam vs. King Post for 24 - 30 foot clear span?  (Read 1706 times)

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Offline Don P

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Re: Hammerbeam vs. King Post for 24 - 30 foot clear span?
« Reply #20 on: November 14, 2018, 10:58:27 PM »
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.
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Offline ArgusBassler

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Re: Hammerbeam vs. King Post for 24 - 30 foot clear span?
« Reply #21 on: November 15, 2018, 04:37:59 PM »
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?
Argus Bassler

Offline ArgusBassler

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Re: Hammerbeam vs. King Post for 24 - 30 foot clear span?
« Reply #22 on: November 15, 2018, 04:45:54 PM »
 

 

 

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

Offline Don P

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Re: Hammerbeam vs. King Post for 24 - 30 foot clear span?
« Reply #23 on: November 16, 2018, 10:21:25 PM »
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.

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Offline florida

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Re: Hammerbeam vs. King Post for 24 - 30 foot clear span?
« Reply #24 on: November 19, 2018, 09:18:18 AM »
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.  
General contractor and carpenter for 50 years.

Offline Don P

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Re: Hammerbeam vs. King Post for 24 - 30 foot clear span?
« Reply #25 on: November 19, 2018, 07:40:16 PM »
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 :D. 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.
A laborer works with his hands
A craftsman uses his brain and his hands
An artist uses his brain, his hands, and his heart

Offline ArgusBassler

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Re: Hammerbeam vs. King Post for 24 - 30 foot clear span?
« Reply #26 on: November 20, 2018, 09:26:18 AM »
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.
Argus Bassler

Offline ArgusBassler

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Re: Hammerbeam vs. King Post for 24 - 30 foot clear span?
« Reply #27 on: November 20, 2018, 09:33:23 AM »
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.
Argus Bassler


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