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principal/common rafter frame

Started by cbecker, May 23, 2008, 12:05:05 AM

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Brad_bb

Yeah, I have three 12"X12"X12foot long seasoned white oak timbers that I bought off a guy last year.  Just trying to pick up one end by hand to unload about killed me.  Can the 29 footer be picked up with one forktruck?  I realize that it is a special piece, but what to do with a special piece like that?  It's real hard to handle or turn over.  I hate to suggest cutting it...It could be two one piece tie beams in a kingpost though and slightly easier to handle, though still each half very heavy.  What is the best use for that....hmmm...
Anything someone can design, I can sure figure out how to fix!
If I say it\\\\\\\'s going to take so long, multiply that by at least 3!

cbecker


deeker

Quote from: Tom on May 25, 2008, 01:19:38 AM
Here I am, not knowing what I am talking about again.  What is this called?  It is a configuration that is used around here to get open spans for Hay barns.  Rafters are usually 2x8's or 2x10's.  Spans will be
40+ feet.



At 4500' elevation, we would call that a collapesd roof in the winter.   8) 8)
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moonhill

cbecker, that is about it, some slight differences but the right idea.  Tim B.
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Don P

Well, it took 2 days and multiple tries, locked up the puter twice... Here's the pic for others with connection difficulties. It would sure be over my head to figure the forces in that frame.

moonhill

Don P., I wonder what it is which seems complicated, in that, its over your head to figure?  I know I couldn't put the right numbers in the right spots, but I sure would rather spend a winter in that roof system than the one above, first posted by Tom.  I respect your insight and want to hear more.

Tim B.
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Don P

Tim,
At first look it has 4 sided panels and eccentric joints. I'm not sure whether its a big deal or not. I tend to think not a real big deal here but I cannot quantify the forces, that ain't good. My weakness not yours. This goes into the concepts of where I'm stuck there;
http://en.wikipedia.org/wiki/Statically_determinate

If the struts run from post to ridge then it is a statically determinate truss (again a random 100 unit load to get some sense of the forces);

I think I just drew a cruck frame, could lose the kingpost.

Dragging us backwards a minute, sorry but I think it might be of interest to a few people; raising a tie and the resulting forces, I posted some questions on another forum, I don't think its appropriate to copy and paste the thread or to send you there so I'll plagerize what I wrote. There were 3 engineers contributing to the thread, none ripped it apart, so I think this is good info.
QuoteWhen the tie is attached somewhere other than the foot of the rafter there is additional load and leverage.

Here's what I think I've reasoned out so far, hopefully someone will correct me if I'm wrong. Take a simple truss made up of 2 rafters and a bottom chord on the wall plates. Assume its an 8/12 pitch and has 1000 lbs on it.

arctan(8/12)=33.7 degrees pitch
the reaction to the 1000 lb load is 500 lbs at each rafter foot
500/sin(33.7)= 901 lbs compressive axial load travelling down the rafter
901 * cos(33.7)=750 lbs tension in the tie

This is where I'm drifting out on my own,
Drop the tie halfway from the peak, its a leverage problem, 1/.5=2 The tie force is double or 1500 lbs.

Drop it only one third of the way down from the peak 1/.33= 3 the tie force triples to 2250 lbs of tension ??

That was simple and symetrical, am I off base?

An engineer wrote back that it looked correct but that I left out bending in the lower part of the rafters.

QuoteUp till this point with the tie at the plate I think the rafters are typically sized as simple beams, which is close enough. I believe now they need to be sized as columns with a side bending load. If I did that right I just quantified the axial and the side load.

He responded that he would use combined beam formulas but the result is the same.

moonhill

Don, I went to the wikipedia page and got mired, I am low centered and only front wheel drive but I hope I get good mileage.  In the last quote it mentions side bending, some examples will fail with the wrong size member.  Where if it is of a larger size the example works.  Of course you can go too large as well.  Is the side bending pertaining to the side way bending of a truss, not the downward bending?  This winter, here locally, a builder was putting together a large prefab truss system, the roof was strapped and insulated with 4x8 foam.  They got one side done with sheet metal and all, they had left the other side open.  It snowed a heavy foot of snow! :( :(.  You can guess what happened.  The builder also left out the CLB's, this was a building with a span of over 65'. 

On page 12 of Sobon's red book, Timber Frame Construction, we can see an example of what I am speaking of.  It's The Old Ship Meeting House in Hingham, Mass, build in 1681.  327 years.  Large boxy timbers, not too big or small, just right.  I feel quit comfortable saying he, the builder, wasn't using wikipedia or a lot of formulas, but ages of experience passed on.  Its when we change into unknown territory we need to prove things in print.  I wish we could do that with politics.  Other wise we are just hoping.  Tim B.
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Don P

Basically its a way of saying its over my head too, using big words  ;D.

QuoteIn the last quote it mentions side bending, some examples will fail with the wrong size member.  Where if it is of a larger size the example works.

In a truss, a structure made up of triangular "panels", loads work mostly axially, along the length of the timber in either compression of the length of the timber or tension along the length.

When the structure is not a triangle (and often when it is one too) there are axial loads and there are also bending loads imposed on one or more faces of the timber. When a timber is under a combined axial and bending load the interrelated effect of both loads must be considered. If a rafter is tied at its feet then the loads can be analyzed as purely axial and a safe structure will result. If the tie is raised to midpoint the axial load is the same, the tie force doubles. It also puts the maximum bending force at midspan in the rafter where the tie is "tied". If I explained that well enough it should be understandeable why the raised tie rafter needs to be a good deal beefier than the bottom tied rafter. I cannot think of a downside to having too large a timber.

The posts sticking above the tie are a similar situation. If there is a vertical load on the post ends from the rafters, that is our axial load. If the lowered tie is in tension, there is a side load. the post will need to be sized larger for the combined loading than if it just had an axial load.

The 5 sided panels over the webs contain joints that are not concentric. You cannot draw a centerline down each timber and have the joinery intersect at common points and in triangles. When the frame deflects there is some degree of rotation in the joints. This needs to be thought about and either dismissed as insignificant or dealt with. If you look in my sketch everything is a triangle and all joints are concentric. That is not to say that either way will not work.

With the truss failure, the 2x dimension of the truss is meant to be braced by the sheathing against buckling in the 2x dimension. This had not happened yet and the axial load was greater than the truss could handle without buckling in the unsupported 1-1/2" wide x 32.5' weak axis. As it begins to buckle it loses capacity which makes it buckle more and a little faster which causes it to lose even more capacity, buckling more..


moonhill

Two reasons off the top of my head for not using a oversized timber,

1-it is a waste of timber in that you could use it in a better location, and trees take a while to grow, so let's use timber that is just right in size. It's similar to when a customer comes to my mill and the wants 3-full 2"x12"x16' cedar planks.  I ask what they are going to do with them?  We are making 4'x4' cold frame boxes.  They are going to cut my hard to find cedar into 4' chunks, when I could of cut 4' butts and procure them very easily.  Similarly, I can squeeze a 6x6 out of a log with a 7" top.  It starts to get harder when customer wants 6x8's when all I have is 7" topped logs.

2-A oversized timber adds unnecessary weight to the truss.   Tim B.
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Thehardway

Seems I saw this general design referred to as a parallell chord truss.  In the parallell chord truss though I beleive they both teerminated at the foot in a common member.  With this setup I can not think of a reason other than economy as mentioned by Tim for not oversizing the tie beam, although the whole strut thing looks awkward and backwards to me.  The only reason i can think of to use a design like this would be severe lateral loading.  In all other categories I would guess a basic kingpost truss with struts would be superior.


Don, when you use the graphic load calculator, how do you allow for the joints not occurring at the end points?  It seems this factor would skew the number?
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Don P

That's why I drew it with joints occuring at triangulated positions and said I could only figure it by statics. That's also the eccentricity I mentioned. I guess I'm being obtuse  :-\

More advanced FEA programs can handle frames as well as trusses.

Found some shots online of the Old Ship Meetinghouse, item 17;
http://www.brynmawr.edu/Acads/Cities/imgb/digcapt1.html

I flipped mistakenly to page 12 in the guilds red book, it was right on topic  :D.


moonhill

Don, is that the new red book?  I have the old one, but can't find it right now.

I also started looking through Historic American Roof Trusses and found a number of interesting items.  Do you guys have this book? 

On page 37 there is two items of interest, the the 6th century monastery at Mt. Sinai in Egypt with the kingpendant which doesn't support the tie. What is on subject about that drawing is it is only a 20' span.   cbecker query was dealing with a short span as will I believe.  We are not talking entirely about 50' or 60' spans only here, which leads me to believe one can get away with a lot in a shorter span.  cbecker could accomplish what is necessary with no trussing at all.  I am under the impression, aesthetics. 

And on the same page is a "improbable depiction" of trusses in one wall.  What caught my attention was the dropped tie, or what appears to be a dropped tie, if you follow the rest of the rafters it's trickery.(sorry for not being able to post pictures or images from some pdf file, if that is possible)  This is the only one I have found where there is a depiction of a truss with a dropped tie, with the exception of a queenrod truss in the same book with a dropped tie.  See page 25 fig.3-6.  A 32' clear span( with a splice in the tie) railroad freight shed, Virginia City, Nevada, 1875.  The top cord of the truss is not the rafters, the rafters are just a skin on top of the truss.  This being the heart of my point.  Don, if you left the rafters out of the example you would be left with a simple Kingrod truss?  Which should not be miring. 

Thehardway, I have also used a similar design in a building with a true ridge beam.  The rafters set on top of the ridge and did not contact each other, sort of a shed roof on each side of the building.  To keep the roof braced side to side I used these curved struts on the post which support the ridge.  The tie beam was supported with post on the story below. 

The xls drawing cbecker is using is simple lines, very plain.  Once you chuck some organics at it I think it perks up quite well.    I don't see "awkward or backward", I see "Oh Wow, thats different".  Tim B.


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Don P

I was talking about the older red book, TF Joinery and Design Workbook, the new book is green.

Don't have Historic American Trusses in print, I know the Egyptian truss photo you are talking about. The tie is short enough and stout enough that it doesn't need a king holding it up in midspan so the king is there to perform another function. It dangles from the ridge, the webs bear on its bottom and run to the midpoint of the rafters, cutting their unsupported span in half.

That does make my mind wander though;


Here's one way I think it could be figured;
The Pharaoh post terminates above the tie (if only for discussion, the king has essentially no load). Half the roof load goes to the struts that deliver 2 point loads to the tie. The point loads are X distance from the bearings of the posts. The "tie" is a simple beam in bending with 2 point loads. The formula for 2 equal concentrated loads symmetrically placed on a simple beam would be right I think.
The bending moment in the beam is the vertical component of the load travelling down the strut(equals the reaction, 50 units each in the example),  times the distance from the post (P*a in the formulas, output in in lbs or ft lbs). This is then entered into a beam equation to check the tie.

If the struts bear near the posts the bending moment and deflection of the tie are relatively small. As they move in the bending moment increases, till worst case load scenario, the single prop post at midspan. The caution I see is checking to see that deflection under design load is pretty small in the tie. If it bows down the plate rolls outward a corresponding amount.

deflection of the tie would be found by;
(Pa/24EI)(3l2-4a2)



Add braces and a king back in and that is possibly a moot point

edit, the section of Historic American Trusses that Tim described is here (their page 17);
http://www.tfguild.org/publications/kingposttrussTF72.pdf

cbecker

 









Here are some pics of the some of the timbers cut so far and pics of a Dawn redwood tree we cut the other day

The frame that I am attempting to build is going to be an open pavilion 20' wide and 30' long.  It is going to have 4 posts down each side. There are no plans for a second story live load of any significance. 

Thanks again everyone for the great input.  I am learning alot and becoming even more interested in timber framing.

cbecker



This is a pic of 8x8 posts and 4x6 brace stock

The butt end of the redwood was 6'

In the beam pic the 8x12x29' is on the right, the larege ones are 8x16x21-28', the smaller one in the middle is another 8x12, and the smaller ones are 4x8

Brad_bb

I'm guessing the pavillion is to park your saw mill under? 
Just a general question... How do you account for high winds in a pavillion design where the gust can get under the roof?  I would think that the posts would have to be well anchored to footings or a foundation.  Will it just have a sheetmetal roof? Could a storm rip off the ship metal with a gust up under the roof?  Anyone know?  It would definitely be a concern in my area with the high winds we get here in IL in storms and all winter.
Anything someone can design, I can sure figure out how to fix!
If I say it\\\\\\\'s going to take so long, multiply that by at least 3!

Don P

That Dawn Redwood is a honker, how did the wood look, never seen it sawn.

Early in the thread we were talking slate on the roof. I had given it 25 psf and the framing dead load another 15 for a DL of 40 psf, snow was also 40 psf. At 200 sf bearing on each bent x 80 psf= 16,000 lbs per bent. I don't think the roof will blow off against that dead weight but we could quantify.

We haven't talked much about wind, this is from the codebook;
Most of PA is in the 90 mph zone
Wind loading in the code book is based on location on the building. Overhangs and ridges carry a different set of wind loads than the main body of the roof. The corners and peak of the overhang carry another set of wind loads. Wind loads are given as positive, acting perpendicular to the plane of the roof pushing down, and as negative, uplift or suction on the roof. Loads are based on effective wind area also, there are sets of values for <10,20,50 & 100+ square feet.

The codebook wind load chart is further broken down by roof pitch, 0-10 degrees, 10-30 degrees 30-45 degrees, and walls. A steep pitch roof in a 90 mph zone, in the main body of the roof has +12.1psf, -12.1 psf. The highest loads generated on such a roof at 90 mph are +13.3,-17 psf on small exposed edges. The highest wind load for large areas at 90 mph is +12.1, -14.6.

So just on dead weight alone it won't pick up a slate roof from my read. -14.6 psf<40 psf DL (actually here I would use 13 psf for light slate and 10 for framing/ sheathing, call us 23 psf to be conservative in this direction)

Wind acts perpendicular to the surface, to convert that to lateral load on the posts we need to know the roof pitch or angle.

Backing up, the struts could also be at the same pitch as the roof.

cbecker

The wood looks nice and smells good.

This is a picture of the lumber from another large Dawn redwood that we cut.  It is the vertical boards, the horizontal boards are pine and the chair rail is black walnut.  All of it has had one coat of tung oil applied to it.   


This is another pic of the same with 2 coats of tung oil applied


The roof pitch is of the pavilion is going to be 8/12.

One of the uses of the pavilion will be to store the woodmizer under.  It will also be used for picnics and possibly storage for other things.  The posts will be set on concrete pads put in the ground 3' and backfilled with crushed stone for drainage.  I am planning on roofing it with 1 1/2" popular boards and then putting either slate or shingles on top of that.

Brad_bb

I was not familiar with the Dawn Redwood so I googled it.  Apparently it is not native to the USA, but was found growing wild in the Szechuan province in China in the 40's.  It was then spread worldwide and used as a landscape tree here in the US.  It's species is one of the oldest - like from dinosaur age- and among the few of it's kind left from that era.  Is the wood soft? has it got tight growth rings?   It looks cool in your pics with the constrasts.  It says it can get to 100', but I bet in an open landscape it would be lucky to get to half that.  In a forest, maybe 100'?
PS.  What's going on in that room?  There's an oak barrel, come kind of compressed gas tank, a clevis hanging in the air... some sort of distillery?
Anything someone can design, I can sure figure out how to fix!
If I say it\\\\\\\'s going to take so long, multiply that by at least 3!

nas

Quote from: Brad_bb on June 11, 2008, 05:31:37 PM
PS.  What's going on in that room?  There's an oak barrel, come kind of compressed gas tank, a clevis hanging in the air... some sort of distillery?
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cbecker

The redwood is very soft and lightweight when dry.  The growth rings are very not very tight at all.  Some of the spaces between groth rings are up to 1" apart. 
That room is going to be my brewhouse.  I homebrew and am refinishing an old chicken pen on the farm to brew beer in.  The basement is 8.5' deep and has hand layed stone walls and a dirt floor so it makes a perfect storage space to age beer.  It was an apple cellar in earlier times. 

Don P

A friend of ours set up a brewhouse in a similar situation with a small basement below ground to age in. Two people got woozy down there before it occured to them that the CO2 was pooling down there. Be careful, think about some ventilation.

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