My King Post Truss Plans?

kantuckid · January 30, 2021, 12:08:24 PM

For my 16 x 20 log cabin I'm planning to build 6 trusses using a simplified (lazy ;D mans design?)plan with 4x6 EWP members and a 4x10 cross beam. I'll have two arms @ 40 degrees for 10:12 pitch roof and one king post. The upper arms would be lap joint to the rafter sections as would the top of the rafters using single oak pegs. The king post to rafters at top would use a three arm top plate and the bottom would use a 5 arm truss fan plate fastening the arms and king post to the cross beam with timber screws
I've seen little mention on this forum of using the steel truss plates in my search.
OZCO makes a fan plate easily seen online using "OZCO # 56620 truss plate" and commonly sold in many box stores. Other mfg.'s I've found are Timberpl;ates.com, Cutting Edge Metals.com along with the guy I'm talking to now in Maine. The actual plates I plan to use from Maine, he operates a CNC Plasma shop and sells very reasonably, easily beats the big guys on price and quick, logical responses. His company name is "Detaileddesignandfab.com" .
I did read here from DonP that one point of connection is suggested by engineers for truss member joints but every plate I see uses multiple holes for fastners and common stick built trusses the pressed plates are quite rigid too?
For those here with experience, I'd appreciate you comments on the use of steel plates, thanks!

Don P · January 30, 2021, 04:36:25 PM

This doesn't sound like a truss in the true sense but more like a 4x10 beam with post and webbing which is one way to start looking at it.

Edit; Looking at it that way it seems to pass if the 4x10's are #1 or better, so there's the short answer.

for a bit of background on my previous comment this is all good but start around pg 75;
2018-02-TX-Heavy-Timber-Trusses.pdf (woodworks.org)

This can be designed as a truss as well, pay attention to the amount of attention payed to the heeljoint connections though.

kantuckid · January 31, 2021, 10:46:06 AM

DonP, thanks, this plan is evolving as the weather continues to (crap out ;D) co-operate. I'm working on the heel joints now having referenced pages 70-76 in the link. My rafter chords will become overhang, so the bottom chord/beam is the resting point. R.e., Figure 82a on page 72 specifically.
BTW, I browsed the entire workshop pdf and was thoroughly entertained. Lots of great truss and frame pictures to feed the brain!
Thanks!

Don P · January 31, 2021, 10:57:19 AM

Uhhh, yeah, I'm thoroughly snowed/iced in and really wishing I'd thrown more temp posts under the punky sills at work

Don P · January 31, 2021, 02:51:16 PM

A thinking exercise.
If the 4x10 and kingpost can instead be cables then you are thinking of this as a truss.
If the 4x10 is holding up the kingpost then it is a beam with a prop.

The first has the bottom chord in tension and requires serious heeljoint thinking, the second is a beam with a center point load in bending so not so much concern over heeljoint restraint.

If it is a truss with the heeljoint configuration in 82a, read the options on pg 76. If there are something like angle iron tabs on the steel plate that support the plate and truss on the top plate of the wall then all is good. if it is bolted up the way you usually see then there is an eccentricity in the joint shown on page 78 and the tendency to a splitting rotation shown on pg 79. Actually, I think I'd use that approach either way.

Then you have the issue Brad has described. It is difficult to maintain air sealing between the truss tails as they penetrate the wall. One way might be to make the plate solid, full height as it crosses the wall and have a vertical angle or channel that the blocking attaches to or drops into. Then as the trusses shrink and move there is a barrier between the blocking and truss. You could also go the concealed rod route and the cut a drop in groove on the sides of the trusses over the wall.

kantuckid · February 01, 2021, 09:52:33 AM

I might end up as a "post on a beam guy"? :D

By angle iron tabs do you mean at the end of the upper chord & the horizontal beam, as an addition to the flat plate shown which would fasten into the top of the wall, thus restricting outward movement of the two parts? or as an end that formed a pocket to hold the end of two parts?
Given that I'm using 4" thick EWP as the wood members, not so much wood movement/shrinkage as with a much larger and typical timber frame dimension involved.
On my own home the rafters are common with ridge beam, all are 3x6 EWP and were dry before I put in the snow blocks so they remain sealed to this day.

Don P · February 01, 2021, 10:24:13 PM

The beam is large enough and it solves the heeljoint tension problem.

Not saying I'm the person who should be designing your connections, that's a far cry from sizing a simple beam but it might be a jumping off point. I turned a tab out and drilled it for connection to the plate. connection to the 4x10 beam down low so that as it shrinks it doesn't lift off the plate and hang by the bolts. Slots for the rafter bolts, install the bolts just snug and high in the slots so the rafter can drop vertically but is restrained horizontally with shrinkage and snug the bolts firm after a couple of winters.

kantuckid · February 20, 2021, 03:56:21 PM

For those who have interest in this topic, check out the link-> Home - DeStefano & Chamberlain, Inc. which has info from the Timber Frame Engineering Council.

Don P · February 20, 2021, 05:46:55 PM

I'm assuming you've been reading this one?
https://dcstructural.com/wp-content/uploads/2020/09/TFEC-4-2020-Design-Guide-for-Timber-Roof-Trusses.pdf

firefighter ontheside · February 20, 2021, 07:35:29 PM

A king post would be required in this design if the bottom chord is not strong enough to span the building without sagging. As Don said, the bottom chord, in a true truss could be replaced with a cable since the bottom chord should be in tension. The king post should also be in tension, as it essentially hangs from the peak and supports the center of the bottom chord. The one point of connection to me means one gusset as opposed to how many bolts, spikes, etc connect the gusset to the chords. In your picture, I would think you'd be better off with a gusset that spans nearly the whole length of the joint between the top chord and the bottom chord. Bolts would be spaced as far apart as is reasonably possible from one end of the plate to the other. I'm imagining a 1/4" steel plate that is rectangular. The plate in the picture suggests to me a weak point where it could begin to crack under stress at the inside angle. Of course there would either be thru bolts from one side to the other, which may be difficult to line up exactly or there would be lags that are no more than 1/2 the thickness of the members so they do not hit each other.

Don P · February 20, 2021, 09:57:43 PM

It sounds like you have a good understanding of the truss itself. I do agree if the gusset I drew is on a truss, rather than a post on a beam, it has that re-entrant corner stress concentration problem going on. There is one more problem that's always on my mind when thinking about a connection like this and it is not something I'm versed on how to solve ... I know just enough to know most designs I've seen aren't even thinking about it. As a joint like that is loaded the "ideal" connection is indeed one single pin or point of rotation. As you add a line of pins things get wild. Now the pins are probably not acting in an axial line of pure shear but instead there is a tendency of the plate to rotate about one of the connection points. This puts the wood under the rest of the pins in a tension perpendicular to grain splitting type of load. The longer arms reduce the magnitude of that force. On a small lightly loaded truss not a big deal, as it gets larger or more heavily loaded it can be a problem. That's what Brungraber was talking about in the pages I linked to upthread. As one engineer said "If you're asking the question, its time to hire me" :D

kantuckid · February 21, 2021, 07:35:08 AM

My roof support design has evolved!
In the beginning my desire to make a couple of nice trusses for my small cabin wandered around in truss designs. I have the skill set to make most any type and like doing a challenge too but I was not willing to add a bunch of wall logs to gain head clearance in my loft for my truss fetish.
I went from King post designs to really liking scissor trusses. I saw several google search scissor trusses that were beautiful in their simplicity. While they had the angles they lacked what matters. Reality slapped me in the face as I came to learn that any scissor truss that lacks some sort of king post(wood or steel) is a bad design. So, in the end the ones that caught my eye may have better head clearance over my loft area but just not good designs!

I am now fixed on using 4x6 beam common rafters, with typical birdsmouth bases that pass through the wall to act as overhang too, along with a collar beam well above head banging height and a couple of tie beams in the other open area of the cabin. My joints will be Tongue & Fork at the ridge with oak peg and Mortise & Tenon on each end of the collar beam, also pegged. The tie beams will be notched and pinned into the top wall log/plate.
It's now a post and beam roof.

Don P · February 21, 2021, 08:46:58 AM

Notice the comment about "rafter busters" in the TFEC truss paper just above. That raised tie is the problem there.
Your ties at the plate are the ones resisting the wall spreading thrust. That means that the plate logs are acting as horizontal beams between those lower ties and need to be stiff enough and well connected enough to the ties to take care of that force. Quite do-able, just quantify and resist.

kantuckid · February 21, 2021, 10:27:37 AM

See page 38 in my most recent link, where it shows the heel of a Howe truss. My intention is to fully pin the top wall log with more timber screws that the rest of the logs and screws fairly close to each side of the perch of my tie beams. They'll have a perch of at least 6" depth and should be solid enough. My home I used whole logs slightly flattened on top for joists to hold the loft and a round log in what was the original LR, now the DR. They have not changed since 1979 that I can see.

Question:
Is there anything to be gained by creating the angle of the rafters birds mouth as seen on the same page 38 in a truss design, (from a typical flat to the plate common rafter version) with what I'll call a back angle or by notching into the plate log?

firefighter ontheside · February 21, 2021, 10:49:54 AM

Between working construction off and on for years and the training we take on building construction in the fire service and the USAR task force i have learned a lot about truss construction. One of the first things we learned was the dangers of bow string trusses in particular and how truss systems are just that a system. When one fails, the whole system becomes weaker and is more likely to fail catastrophically.

I see what you mean about the one connection, but since a joint is meant to be tight and not have room for hinging, it seems those forces you mention will happen anyway. The end of joint is tight and becomes a fulcrum and great stress is put on the bolt or pin. More than one pin or bolt would at least offer more protection against shear forces.

Don P · February 21, 2021, 05:51:29 PM

Actually for both of you, start reading on pg 36, much of what we're talking about is explained in those next few pages.

Quote from: kantuckid on February 21, 2021, 10:27:37 AM
See page 38 in my most recent link, where it shows the heel of a Howe truss. My intention is to fully pin the top wall log with more timber screws that the rest of the logs and screws fairly close to each side of the perch of my tie beams. They'll have a perch of at least 6" depth and should be solid enough. My home I used whole logs slightly flattened on top for joists to hold the loft and a round log in what was the original LR, now the DR. They have not changed since 1979 that I can see.

Question:
Is there anything to be gained by creating the angle of the rafters birds mouth as seen on the same page 38 in a truss design, (from a typical flat to the plate common rafter version) with what I'll call a back angle or by notching into the plate log?

For a truss, yes the heeljoints as drawn are doing major work and probably will be incapable of resisting sliding out if a typical level seated stick frame style birdsmouth is used.

Take a hard cover book, open it and set it on a slick table tent fashion, then push down on the "ridge". The rafter feet will slide outward. Now look at those heeljoints and think about how the angled bearing or the buried toe work to resist that sliding thrust.

Here is a pic from my gallery of one way to resist that force. I drew in the the shear plane to be checked when you do that. If that blows out, whee!
Also notice the re-entrant cornered notch in the top plate and think about what happens as the top chord shrinks. The lelel cut becomes the bearing surface, the bottom edge of the top chord can lift off its seat and if there is a heavy roof load a split can form from that inside corner. Running a handsaw through the level cut after the bottom of the top chord is down in notch and bearing is good practice, relieving the level cut a bit.

I like to hold the joint together with buried allthread like this, it adds another fail safe as well as uplift protection. If the post tenon extends up like that as another fail safe recalculate the available shear area above.

Fail safe... I'm not saying we can prevent failure, if that happens you want to fail safely. If the bottom chord relish fails the all thread goes into load, the truss distorts visibly. The post tenon starts taking load when the all thread fails, visibly distorting more. Hopefully the occupants have taken the hint by then, a "safe" failure.

What you are describing wanting to do is not going to be a truss, but it can work. But I smell food :D

kantuckid · February 21, 2021, 06:08:28 PM

The food you smell is leftover Navy beans and Kale greens-the cook is over near the WV border on alzheimers duty. I may get ambitious and fry some taters... ;D

I am not placing my tie beams under the rafters as shown in your diagram, so in my case the notching would be in the top plate log on the side walls.
So yes, I realize it's no longer a truss but using this thread to carry the thoughts along as I've changed to rafters.

I'm currently in the midst of a huge industrial surplus website. Talk about wheee! I thought it was a scam site until it showed me almost 800,000 items listed. I doubt there's many crooks that industrious?
Changing up the subject a bit, therein I'm seeing 12" long hex-head log screws. My logs are going to be 6" thick so that seems like overkill? I also see some torx- panhead and washer head screws listed @ 10" which I'm leaning towards most.

Old Greenhorn · February 21, 2021, 06:21:41 PM

Quote from: kantuckid on February 21, 2021, 06:08:28 PM
.....
I'm currently in the midst of a huge industrial surplus website. Talk about wheee! ...

Would you care to share? I am in that mood tonite as I can feel some kind of project developing in the back of my brain that I have neither the time, materials, or funds to pull off, and would like to work on the bill of materials for it. ;D

kantuckid · February 21, 2021, 06:38:09 PM

PM sent

Don P · February 21, 2021, 07:14:41 PM

Quote from: Old Greenhorn on February 21, 2021, 06:21:41 PMWould you care to share? I am in that mood tonite as I can feel some kind of project developing in the back of my brain that I have neither the time, materials, or funds to pull off, and would like to work on the bill of materials for it.

:D :D

I'd like to play but I gotta call a man about a sticky racing slick in the morning. My sawmill drive tire experiment died of dry rot :'(. I hope he has a line on a used one, those things are gold.

My sweetwife cooked up pork chops a friend grew, the corn, taters and punkin pie came from our garden.

The plate logs can act as beams between the ties. The ties need to be securely, uh, tied, to the plates. Steel rod is the best thing for restraining that tension but if you can get the wood well connected that works fine. The force on the tie connections and on the plates in horizontal bending is the accumulated thrust from the rafters in that section of roof. We can figure all that up if you need when you have a sketch.

The 10" screws will be plenty long. If you see some 3/8 or 1/2" lags it would be a good thing.

Percy · March 28, 2021, 10:38:32 AM

Quote from: Don P on February 21, 2021, 05:51:29 PM
But I smell food :D

:D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D ;D ;D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D

kantuckid · March 30, 2021, 06:19:54 PM

My wife's off on alzheimers duty and I smell beer, but zero food. BTW, have you tried the Dos xx's Montezuma Ambar Especial? it's seriously good stuff.
:D Done one a day like the vitamins that I don't take. 8)

I need to get off here and smell some food?