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Truss foot ideas?

Started by Don P, January 03, 2007, 10:02:05 PM

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Don P



This is a sketch of the rafter foot of a 24' truss with a 6/12 pitch. The bents are spaced 12' apart, combined roof load is 30 psf. I'm coming up with about 7245 lb compression load coming down the rafter into the notched pocket. The 8x10 rafter and bottom chord are #2 Eastern White Pine. I've drawn the notch bisecting the pitch angle, giving the same grain orientation to rafter and tie. At that angle the wood can handle 390 psi compression without crushing. I drew the notch 3" deep x 8" wide. 24 square" x 390=9360 lbs compression allowable, the notch could be as small as about 2.5" on the bearing face.

The plane in red is trying to be sheared off by the resulting tension in the bottom chord, I'm getting about 6480 lbs. The allowable shear in EWP is 125 psi. I have 133 square inches of beam beyond the notch x 125 psi allowed = 16,625 lbs in shear resistance, that ain't splitting off.

As I see it those crushing and shearing forces are the requirements to design for in the joint. I like the simple notch, I feel comfortable that I can get it to bear uniformly. I need help with the rest of that joint or modifying what's there. I didn't want to get into a double bearing for fear of only really bearing on one face.  The post can wander around some, I wanted to keep it supporting the notch without getting the joints into eccentricity trouble.I'd sure like to hear from those with more mastery, or just an idea though  ;D.


beenthere

I'd add at least one dowel down through that 'red' shear area, and not trust the clear wood numbers for shear, which I'd expect to be free of all defect (albeit with some built-in safety factors).  Just for good times sake.
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

Jim_Rogers

Some standard joint sizing rules say that the tenon should be a 2" thick tenon for an 8" thick timber.
That tells us what size the tenon should be coming off the top of the post into the bottom cord.
If these timbers are rough sawn then there should be some type of a housing into the bottom of the bottom cord. If they are planed four sides you could do without the housing.
This housing would be standard which would be 1/2" for example.
That makes the bottom cord at the point where the tenon enters 9 1/2" tall. Now remove 3" off the top for the rafter foot notch and the bottom cord is now 6 1/2" tall.
Standard joint sizing rules say that for a tenon this is 2" thick the peg should be 1/2 the thickness or 1". The spacing of the peg off the bottom cord's housing should be 1 1/2" minimum. And the hole in the tenon should be according to NDS standards 7 diameters from the end of the tenon. So that makes the tenon 8 1/2" tall. Which in this case won't work as the tenon will be up into the bearing area of the rafter foot.
It was suggested at the engineering workshop that the peg spacing should/could be modified if the engineer could figure the force being placed on the peg. He would then figure what diameter size steel bolt would resist this force. Then find a wooden peg that matched that number of pounds of strength of the steel bolt and use that wooden peg, but use the diameter of the steel bolt for the spacing requirements.

So, for example lets say that this joint will have a 1000 lbs of force on it. And a 3/8" steel bolt will resist 1000 lbs of force. And a 3/4" hardwood peg would have equal strength. So we'd use 3/4" hardwood pegs and space them based on the 3/8" diameter steel bolt. So the hole in the side of the bottom cord needs to be 4-d (d=diameters) off the housing (4 x 3/8"= 1 1/2"). And the tenon hole has to be 7-d from the end of the tenon (3/8" x 7 = 2 5/8 + 1 1/2= 4 1/8" long). This tells us that the tenon should be 4 1/8" long from the shoulder of the post.

The above were just examples of how to figure tenon lengths based on forces, strengths, and NDS spacing. None of the numbers are correct....
Upon further review and research.....
Recently my information about hole spacing in eastern white pine says the holes have to be 4-d off the housing shoulder and 4-d from the end of the tenon. But you may have to review the actual current spacing numbers from your NDS books.

I do remember asking about the hole spacing, in this case in the bottom cord, and I was told that the housing isn't considered a reduction in the overall strength of the bottom cord. And therefore wasn't considered when figuring the hole spacing. So all edge spacing was figured from the surface of the bottom cord. This would mean if you place a hole 1 1/2" off the shoulder of the 1/2" housing it is considered to be 2" off the surface.

If you didn't do all the math and figured the old method of hole spacing to determine the tenon length then a standard 3/4" hardwood peg would need to be 4-d off the bottom cord or 3" off the bottom surface which would place it 2 1/2" up off the housing ( a little more than the old standard 1 1/2"). The tenon would need to be 4-d long plus the 2 1/2" or (4 x 3/4"= 3" + 2 1/2"= 5 1/2"). And this would be under the 6 1/2" of wood we have to work with.

I'll assume that the post is 8x8 and the brace is a 4x6.



Here I have recreated your joint and placed a 5 1/2" tall tenon into the bottom cord with a 1/2" housing and placed the peg hole at the correct location.

If it were me, I'd move the post out further toward the end of the bottom cord and make the tenon less then a full 8" wide, something like 6 or 6 1/2" so that you'd leave more relish on the end of the bottom cord.

I hope you find this somewhat useful.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Don P

That's what I'm looking for, thanks guys. Please feel free to rip this apart and give me the strongest way if I'm off track.
This is the first drawing I made of that area, The post was all the way out there and I started worrying about the notch, there's a happy medium in there somewhere;


I was leaning toward a through tennon for the reason Beenthere mentioned. The idea of flipping it to the other side to leave some relish is a good idea, as well as not having the post all the way out.
The notch was well locked down in the bottom chord in the pic above but I couldn't guarantee simultaneous bearing of all three notch faces, something was probably going to take all the load till it crushed enough to let another face share.

How about this, the hole in the notch would contain a hidden allthread, the post tennon goes all the way?


Jayson

Hey Don P,
               I'm not sure but I think you are saying that the inside corner of your notch will not be square. Is the bearing side of the of the notch on the rafter perpendicular to the roof line? What I'm getting at is...if I were cutting those I would not want to try and cut an angle on both pieces(the rafter foot and tie).Did I misunderstand?

Don P

No you didn't, and point well taken, I was making the math work elegant , or lazy ::) :D. It was close enough to cut either way at that pitch, or make a tad deeper to cover it either way. Where I'm getting the bisect from is some sketches in the notes from that engineering workshop Jim mentioned. I'm at about 13* to bisect, I don't think squaring one will lose more than about 15 psi in compression when I check it.

Don P

I chopped one bottom chord end today just to try it, a 10" mortise takes awhile  :D. I dropped its tennon down to a 2x6 to give a 1" flat in front and back of the tennon.
The post has about 5000 lbs coming down on about 50 square inches, so about 100 psi in crushing against the bottom of the tie. Its allowed 350 psi in side grain compression so that's ok. I did bisect the notch angle, I've since read tonight to do it both ways, square to the top chord and bisecting the angle, you pays your money...  ;D.


I was looking at the other end of the bottom chord and thinking about its joint at the kingpost. There's about 7500 lbs of tension in the tie when the roof is fully loaded. I think I'll use a hardwood spline, it'll take 5 -1" oak pegs on each side to restrain that. The pegs need to be 5" from any end grain, 4" apart down the row and more than 1-1/2" from an edge and between rows. The spline is a 4'10" long 2x5, I think I have some sycamore, gum or elm that should be about unsplittable?

Again holler if you see something or another way. 4 different bents wouldn't hurt my feelings 

all pitches being equal :D

Thehardway

Don,

If I see your drawing correctly the spline is located at the top of the tiebeam?  I would think the bottom would be a better location, the kingpost actually holding the tie beam up and the spline sharing the load of the tie beams weight with the shoulder on the kingpost rther than the weight on the spline pegs and the shoulder.  What is the span of this truss and dimension of timbers?

As to the truss foot design, the design you show is found in many of the Scissor trusses surveyed in the TF guilds historic truss document complete with through bolts.   The kingposts shown ended not to use a rafter foot notch but rather just a simple tie beam mortise with the rafter tenoned and pegged into it.  I assume this was to avoid additional weakening of the tie beam by the rafter foot notch but also the end of the tenon was formed to be perpendicular to the tie beam so that it would not tend to rise out of the mortise.

Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

Don P

Yes you are seeing the spline at the top, viewed from below. I was trying for what seemed the clearest closeup view that gave some orientation. I'm still a sketchup newbie.

Hmm, I hadn't even thought of the splice or pegs holding weight, simply tension. They certainly would where they are, as the ties shrink and the spline supports the shrinking timbers off the lower housing shoulder.  I had intended the tie beam's weight being on the shoulder. I was also trying not to look at the spline. Maybe make the mortise in the king 1/4"+ lower than the spline's bottom and still keep the spline at the top? Your idea is definitely stronger and better oriented for shrinkage. That trough stands a good chance of initiating the major check.

Aside, I can take the struts up the kingpost, off the tiebeam, and mortise them in there to make sure their load doesn't go into the tie/king joint.

The truss is spanning 24', top and bottom chords are 8x10's, posts are 8x8, the rest is negotiable. I've cut a little over half the chord stock so far and a few 8x8's. White pine takes alot of cleanup, these are mostly around the house and sawmill so I gotta clean up each tree as I go. I can get one popsicle the helicopter trimmers delimbed pretty easily, so at least there's one without the limbery hassles  :D

That's a third option on bearing face angle at the rafter foot. In changing that angle I've been checking the compression values parallell to grain and perpendicular to grain and adjusting the value for the angle, that was why I said I was being lazy by bisecting it, both angles the same to grain. #2 EWPine is 400 psi in end grain crushing, 350 psi in side grain, so really at these low angles and in that wood not much difference there as I square one or the other. Other species and angle combinations might be more critical.

For me to be comfortable with the simple tie beam mortise, if I'm understanding right, I think I would use it in conjunction with that rafter foot notch. Modifying the 3 point bearing notch I posted above is the way I'm seeing it, drop the post's tennon below the rafter tennon's  mortise level in the tie beam and square it out. I think I want more square inches than just the rafter tennon though and I'm too close to the end to bury the toe of the rafter in wood, that's when I started looking to the rear. I may try one that way, 2 are going to be gable ends, so if I'm uncomfortable with my results, there's only about half load there.

Thehardway

Strange how two people can look at a joint and see two different things isn't it.  I can see your logic as well now that you point it out.  The truss you are building is pretty much the same as what I have been cutting in my head for about 2 years now.  Jim Rogers has given me a few great suggestions as well as a few other members.  There has always been some doubt in my mind about using the interrupted bottom chord. Eliminating a 24' timber certainly simplifies things as far as handling timbers by myself goes and I like the interesting visual effect of the joinery but I feared sacrificing structural soundness. Using a spline rather than the wedged dovetail helps more with deflection in the bottom chord. My gut tells me you are right about using some hardwood with an interlocking grain to avoid checks or splits.

Recently I visited the TF Guilds Ferry Farm Pavillion project and looked over the 24' span kingpost truss with struts done there.

They used a continuous bottom chord that is through morticed to receive the kingpost tusk tenon that has two keyhole mortices of which each is double wedged.  I am sure all of the engineering was done and numbers run on this project with plenty overage as the timbers are oak and seem to be oversized for the span but as I looked it over I found myself less comfortable with the wedged key arrangement because they fall exactly where the checks are opening as you expressed fear of in your post.

I see this as a potential point of failure and perhaps less sound structurally than the interrupted bottom chord with spline.  In essence the weight of the bottom chord at the center rests on just a few square inches of wood in the keyhole wedges.




If it would not be imposing could I stop in and see your truss?  It is almost identical to what I am planning.  If memory serves me from past posts you are in NC?  It would be worth the drive for me to see it and I am willing to buck logs, stack boards, shovel saw dust or whatever you have that needs done in exchange.



Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

Don P

Thats a nice looking truss  8)  Did they anticipate the check in the center and placed the wedges where they thought it would be less likely?

This is just a shop, I really don't mind experimenting after I've satisfied myself that any options are safe. Might be kinda neat to watch several options age. Not to mention it'll really mess with someone later  ;D.

You are certainly welcome to come take a look at my truss for ideas... as long as I haven't come to look at yours finished first  :D I'm actually between Wytheville and Independence VA, a little north of the NC line. I'll keep you posted as it gets to looking like something.


Jim_Rogers

I can't see it in the picture but normally the tenon is pegged into the bottom cord/tie beam as well as being wedged.
Maybe they should have used a longer through tenon or made the wedges smaller to make the hole to tenon end spacing longer.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Thehardway

Jim,

I looked for pegs in the tie beam fastening the kinpost tenon but their were none present that I saw.  Resolution and size restrictions don't allow it to be seen plainly but there is actually a large check that has opened up under the right hand set of wedges in the first picture. I will email you a full size/resolution picture so you can see it. I will place additional pictures of the building in my gallery for those interested in seeing.  Maybe I will pose a ? about why it was omitted over on the TF Guild forum. The truss seems to me more Medevial and European in style than typical 1700's era American Timberframing.  I think Mr. Grigg Mullin and Mr. Joel McCarty organized that rendezvous.  I believe Mr. Mullin is a Engineering Prof at VMI so I'm sure he could provide insight as to the why and why not's.

Don,

You are only a couple hours from me.  Email me some directions and maybe we can arrange a weekend chiselfest or something.  I'm always eager to see other peoples work and talk timbers.   
Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

Don P

THW,
I'll send directions.
If you want to I can send my sketches and if its anywhere close you can draw your truss within the dimensions we can make one like yours and watch it. Might learn something to help in the house.

Thehardway

Would love to see the drawings.  Email is displayed on my profile.   Seems I have unintentionally managed to highjack your post here is a bump back up.
Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

Don P

If it weren't for twists and turns we'd have mighty short conversations  :)
I'll send pics, do you have sketchup?
I got the triangle of the first truss rough fitted by dark tonite, just the kingpost, rafters, and tie beams. Still need to do the webs, posts and braces. I dropped the spline to the bottom and left 2' of kingpost below it for now. With it there, I liked the pic of the spline hanging down about 2" in Jim's post awhile back, gives it a nice reveal.  I checked 4.5" x 6.5" purlins and they will work @ 2' centers, some leftover kit D logs will dress out to that.

I've been thinking about a steel post base with a knife plate pegged into the post. If I cut a short length of pipe into 3 pieces and weld 2 to a plate cast in the pier and 1 welded on the post base. Insert a piece of rebar and I have a loose pin hinge holding the post feet in place while lifting. Which got me thinking about footing size. Each post under full load could put down about 5000 lbs. The soil here is good for about 2500 pounds per square foot. I normally don't make less than a 2'x2' footing, that'll be plenty.

Don P

I got a pic of the weekends play this morning...boy have I jammed up the mill  :D



In thinking about the webs, I'm not sure whether they should drop below 45*. That is far less than center of the rafter. What have you all seen?

Furby

Maybe you can see the check better.
It's really hard to see, but the check continues a good ways above the bottom chord.




submarinesailor

These kingposts were the hardest piece we had to cut for this job.  Grigg, Joel and Dan Mullen watched over them very carefully to insure they were cut right.  I can tell you this check wasn't there when we cut them and put the truss together.  :-[ :-[ :-[

This is one a good example as to why wood can be the Devil's mistress.  It can be extremely beautiful, very unforgiving, wonderful to work with and as in this case it will bite you in the $ss even if you do it right. :D :D :D  As we all know, a check like this can open at any place and at any time. 

I can say that this was a great project to work on and I gained a great deal of experience while working on it.  And one more thing, green Virginia White Oak in these sizes is very #$%^& heavy – don't ask me how I know. :D :D :D :D

Bruce

Thehardway

Don,

That is one of the very reasons (besides head room) I'm sticking with a 12/12 pitch.  Everything is a simple 45* angle which makes the math easy for an arithmetic dummy like me and creates a nice angle for the struts.  Most of the low pitch kings I have seen are flanked with princeposts or left without struts.  Will the numbers allow you to leave out the struts? The span of your rafters is not that long.  I think I did see a truss once with the struts rising from the tie beam rather than the king post.  Sort of like canted struts in a gambrel roof but closer to the kingpost and perpendicular to the rafter. Not  sure what this does in the engineering. Seems like it would focus all the tension on the splined joint but not sure.


Furby,

Thanks for blowing that up.  If I ever get back up there to see Kenmore I will take a follow up pic only closer up.  From the response I got over on the TF Guild site the tenon is bearing very little weight and should hold up OK. 

Got some interesting pics of some kingpost trusses in a local church yesterday.  Will try and post them soon.  Gotta run right now.  Work is calling...
Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

Don P

Bruce, you worked on those trusses,  8) 8). The response on the TF Guild website was good. The wedged connection to the tiebeam is really just to support the tie from sagging, there is no real load there.

THW, You're right about a 12/12 as far as simplicity, that carries all the way through. I crunched the numbers on mine by dividing the truss into panel point loads spaced at the quarter points of the truss. This put the webs at the same pitch as the rafters and they intersected at midpoint of the rafter. The loads do vary with the pitches and placements. Still need to read more on that, things look different full scale  :D. Built like that, the truss works on paper with 7x8 rafters. That was a fun exercise on paper into an area I am not comfortable yet.  The 8x10's I used will pass as is with no webs, so anything I add is just reinforcing against snow overload or bad grading of a rafter. In theory the present 8x10 rafter will only have deflected about 1/8"under full load .  The splined joint is what I'll be watching for first trouble. We were talking about setting it up temporarily on short legs, nail 2x4 chicken ladders to the top chord and loading it to design. Trouble is it would take 100, 100 lb sacks. I've got that much sand and pea gravel. Not sure about what to use for that many sacks.

(edit)
I've gone back and reviewed my cyphers. Rereading the example I worked it from, it was a 40', 4/12 kingpost truss with 12' bent spacing. The webs did run back at a 4/12, I had strung my math out for so long I was forgetting its roots. I see where to adjust the forces for a different pitch and resulting top chord lengths if desired, that would add some figuring to no good purpose. Long story short the webs can definitely lay down to the same pitch as the rafter, it then bisects the rafter and makes all the panel points equidistant.
The wood in that case is good for 19,200 lbs on the truss. The connections are designed for 10,000 though (the pegs in the spline failing, the heeljoint crushing or shearing off, or the top of the kingpost crushing)

TW

Don P

In engineering school we have test loaded a few wooden trusses. The only result we got was that a truss does not perform nearly similar in theoretical calculations and in a practical test.

Sources of error:
1 At least in Finland we do not use the average strenght of wood for calculations but the 95% fraktile. That means that out of 100 test pieces 95 will be stronger than calculated. Then comes the safety factors oin top of that.
2 Buckling. It was difficult to keep the entire truss from buckling sideways in a way that it does not do when built in in a finished building.


All four trusses could withstand about 1 1/2 times as much load as calculated when trying to find the failiour point.

I edited the post later on to make things more clear:

The reöuired load bearing capacity (you use pounds per square foot)for excample for snow loads is adapted to the safety factors that already are in the strenght rating for the wood.
For excample Eurocode has much higher values for both loads and strenghts compared to Finnish building code.
If the loads are pessimistic, the strenghts may be optimistic or the other way around.
If you use loads following the code when the code assumes 95% fractile, and load wood which may be average, you will end up with a far too weak design.

end of edited chapter

If I had to determine the load bearing ability of a truss by testloading, then I would testload with something like 2 times the teuretical maximum failiour load in order to make up for the differences in strenght properties. The material will most likely be much stronger than in a calculation.

I am not an expert but this is my oppinion. Do not take this too seriously please. I do not guarantee that I am right.
I am not wery good at engineering English either so the explanation may be blurry.

Thehardway

Don,

Sand bagging!! That's my kind of engineering test.  Torture the snot out of it in the real world and forget the calculator.  Load it up to twice what is demanded and then push on it sideways, beat on it and if you really want to know something about it soak it with water, dry it in the sun until crisp, drill a few holes through it and then light a fire under it and see how long it lasts before giving way to the load and collapsing. With this battery of tests I'll put money on your truss outlasting anything but reinforced concrete.

TW is right in my opinion.  Theoreticals require us to way overbuild but still don't account for nature (ie. Ferry Farm check)

It's usually quicker for me to build something and run a real world test that I trust, than figure out the math.  Problem is,  the egg heads want to see things on paper in the form of numbers.  Unfortunately the math won't tell you where a timber will check or an ill placed knot will cause a failure.  Some stuff has to be way overbuilt just to allow for redundancy because the math must be based on averages and statistics.  For instance, enigineers now base calculations on table for "mixed oak".  Any timber man knows there is a vast difference in weight, strength and characteristics between just red and white let alone different species in those classifications and then allowances such as harvest time, growing region, length of drying period for shrinkage, spiral grain the list goes on and on.  Math just takes into account the worst case scenario.  The whole building industry has been dumbed down so that things can be done with computers in an office rather than knowledge in the field.

It would take a heck of a snowstorm for your truss to fail with those numbers and if I am guessing right, the timbers you cut for it probably would spec out better than the grade you chose for the math calculations,  not even mentioning that by choosing the right stick and wood specie for the spline it will be stronger as well.

Was going back over some of my old drawings last night and occurred to me you may want to consider housing the end of the  tie-beam in the kingpost beside the spline rather than using a shouldered joint.  At that joint the tension and weight, is being handled primarily by the spline and its pegs.  Reducing the end dimension of the tiebeam at its intersection with Kingpost and fully housing it beside the spline should go a long ways in preventing a check from opening up the tie beams end, hold it tight to the spline, and help prvent the pegs tearing out.  It will also assist in controlling lateral movement/deflection in the truss at the joint location.

I believe I am going to look at morticing a small timber into my kingpost just above  the tie-beam connection. It will run perpendicular to the trusses and brace the joint from lateral movement at the bottom just as the purlins do at the top.  It would also be a good for chasing electrical wires and mouting lights/ceiling fans etc.  Chamfered edge 4X6s would look pretty sharp I think.  One of my pet peeves in timberframes is that the electrical work always looks out of place.  This would help integrate it into the design and make it look less industrial and more "organic" or at least as if it was planned with the frame.
Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

Don P

Speaking of design load vs real world,
One of our previous clients is putting a hot tub on a porch this weekend  ::). People do things you never intended. I don't think I'll get much done at home this weekend  :).

TW,
Our "allowable design values" sound like they are derived the same way. The way it was explained to me was that 95% of the wood within a grade would break at 2.1 times the allowable load or greater. All wood within the grade should break above the allowable design load. It's termed the "5% exclusion limit" in literature here. In grading class I asked the engineer that runs the lab how they check critical timbers. They "proof load" them to 2.1 times design load. Our design loads are also heavier than typically encountered, hot tubs excluded.

Some wood within a grade might be as much as 8 times stronger in bending than calculated. Remember deflection is most often the limiting factor though and those calculations are supposedly pretty accurate.
Only recently have our shear numbers become somewhat accurate. It turns out there was a math error years ago that has had us using numbers that were about twice as conservative as what we've just described.

You have a point about lateral buckling in a lone truss being tested. I would need to brace each panel point on the compression edge, the top chord (each foot, the web point, and the ridge) from lateral, sideways, motion to simulate the purlin's bracing. 

One thing I'm still learning here (of many) is that the members in a truss are considered columns with a force travelling along their axis, or length. Some of those columns have a bending force also, the roof bearing on the top chord. In rafter calculations I've done so far the rafter is treated as a beam. Columns are checked for buckling when a load is placed on one end, a deck post or our webs would be examples. The rafters are checked for this and for bending strength under snow load and roof weight.

When I said the wood was good for 19.2kips I meant that the timbers checked as columns  and columns with bending and were safe to that load. When I was designing the truss foot notch , the spline and the kingpost top I was using our local snow loads and designed for the truss carrying around 10,000lbs.

I wish we were metric, base 10 would be much easier  ::).

THW,
Being the first truss I've drawn, I would like to know if its as strong as I hope.
I figured hanging 2 trucks would be about right, instead of sand, but my friends won't loan me theirs  ;D.

I don't mind being quite conservative with heavy timber, there is very little redundancy in a frame. If a member fails it throws alot of load on the remaining timbers, in a stick frame if you lose a joist or 2 it doesn't really endanger the frame.

Whenever you see a group grade the numbers represent the weakest wood within the group. Mixed oak can be any oak recognized by NeLMA and will carry the design values of the weakest. A white oak in the mix will be stronger than design. If you know you are going to get only our White oak (which is a group) and Northern Red (which is a group), you could use their stronger numbers. The northern red carries the higher numbers and we grow some good northern reds.

I was considering turning an 8x10 deepways for the king and housing the rafter top and tie beams while I was chopping this one. It would help hold things in place while drying.

I've seen the lower bracing you're describing, both as a rat run and as webs going back up to the ridge. The tension chord shouldn't be in danger of buckling... but you never know about some unforseen stress reversal, wood's good  ;D.

I was thinking today, with the spline on the bottom and on a shoulder, plenty of relish on the kingpost bottom, the tie could probably take the weight of princeposts.


Don P

A little reassuring news on the theoretical angle tonight. In order to get the loads at various points, or nodes, I read up on truss design and started crunching numbers (my method of joints question awhile ago). I found a trial version Finite Element Analysis program the other nite and got far enough with it tonight to check my math. Things did check the same, thats a relief   :).

I got a couple of screen shots from the FEA program. The first shows the members. Compression is in red, tension elements are blue. The print in black is the load along the axis of each member.



The next shot is an exagerated view of the deformed truss under load.


Now that I'm more comfortable with the output, it agreed to the pound, here is the calc I've used to make part of the truss. It is just giving axial loads for that style truss, that is what I was using to size connections (the notch's compression and shear check, size of kingpost bearing faces, # pegs in the spline. The actual sizing of the timber members requires more calculation for buckling.
https://forestryforum.com/members/donp/vectormath.htm
For me the total load on the truss works out to 10,000 lbs and it has a 6/12 pitch.

(edit)
I've kept playing with the FEA program. Swinging the webs up to a 45* angle the compression at the top of the rafter climbs by about 1300 lbs, the compression in the webs goes down by about 500 lbs and the kingpost tension climbs by 1200 lbs. Deflection with these dimensions goes up about 1 thousandth.

Also, rereading the last few posts TW don't be shy over the language thing, we understand just fine, I think  ??? ;D. Keep posting, I'll ask you if I don't understand something. Your schooling is something I am envious of  ;).




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