principal/common rafter frame

[cbecker]

looking for comments on the two different styles
both could be covered with 1.5 inch popular and shingles/slate
http://cbecker940.googlepages.com/TimberFrameDrawing.xls
it wont fit in the gallery. 

Thanks

Chris

[Brad_bb]

I see you made some drawing changes/corrections.  As I said in the other post, the first one- the kingpost is the most appealing in my opinion.  It uses less wood.  I only have two concerns.  1. A kingpost usually is not done as a posted cape from what I've seen (the 3 ft of post above the tie beam).  You'd really need to do the stress analysis on this- bending and shear on the posted section. Usually the rafters tie directly to the tie beam(S), putting the tie beam in tension.  With the posted cape version, you'll also have a bending moment on the posted intersection with the tie.  What will this do? Given that your span is 20 ft and not 30ft, it's a lot less stress.  But you also need to analyze this with all the loads involved.  2. With a kingpost you also need knee braces from the kingpost to the rafters.

[Jim_Rogers]

What is the length of the building?

[cbecker]

The length of the structure is going to be 30 feet long.  There will be 4 posts down each side. 

[Don P]

Is there a floor load on the tie beam?


A kingpost truss does not have to have struts, they can be used to cut the rafter span in half.
In this one, struts from the kingpost to the intersection of the rafter to plate  do some interesting things. Here's a quick model;

The struts are in tension acting like a cocked bowstring pushing up on the kingpost. It just removed the thrust from the post top though.

[moonhill]

Don, could you comment some on what the numbers mean and the arrows at 38 and the red and yellow spots.  Are they direction of loads?  I wonder what the joint would look like at the eve where the strut meets the top plate and rafter?  I don't think I have ever seen this configuration in traditional joinery.  Would you be using this in conjunction with steal plates?  Is the king post the arrow?  And if so did the archer release the arrow? And upon release there is out ward thrust at the plate?  Am I seeing this correctly.  I understand a truss with a tie at eve level but am having a problem following load/force directions with this example.   I will have to go back and look at cbeckers drawings.   I would not use struts on such a short truss-20', unless they were just for looks and then they would have a natural curve.  What happens if the struts are moved up the rafters, with this dropped tie setup?  Thanks, Tim B.

[Don P]

I think I should have kept the observation to myself   . Brad's comments made my mind wander into "What if". I'm not sure that this would be at all pleasing or very functional, I just saw another way to tie it.



the previous pic was a cropped screenshot of the JHU truss calc. I hung a 100 unit load from the peak, that is the black downward pointing arrow. The red and yellow dots are the bearing points of the truss. The 49 and 50 upward pointing arrows are the reactions to the load (for every action, the 100 unit load, there is an equal and opposite reaction, the reactions at the bearings). It's off a little because one bearing pin is moveable. Members in compression are shown in blue, parts in tension are in red.

Following the 100 unit load down the rafters, at that pitch each rafter has a compressive column type load of 44 units each, 38 units of the compressive load is in the kingpost itself. What we are visualizing as struts, are actually the tension tie. The "tie" is the red member marked 36, think of it as a cable maybe. It is taking the thrust which is why the dropped tie's force dropped to 0. This thing is then acting pretty much opposite of the kingpost truss you normally think of. The ties could move up the rafter but the rafters would get into bending with tension and the plate joint would probably get into trouble. Over my head, it was just a train of thought.

See if this helps with where my mind was wandering to. Top pic is a true kingpost, a column under a bridge deck with a tension cable under it, supporting and lifting the center. Middle picture is what I think of as a kingpost truss, sometimes and maybe more correctly it is called a kingrod truss, visualize 2 columns, one leaned in from each brown support. A cable dropped from the top of the connected columns is in tension as it supports the tie from sagging. The tie is in tension. When I say cable it's to help visualize the force, the cable could be wood.

Bottom pic is actually the same as the top pic, it is stable and does not require the lower tie nor does it put thrust on the post tops. It's also not doing anything different than plopping a kingpost truss on top of the posts would and I assume there's a reason for not doing that.


In any truss if you lose the tension ties the thrust that's released will probably go to the walls

Didn't mean to derail us, it would help to know if the tie in the original is also supporting a floor.

[Tom]

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.

[moonhill]

Don, that makes things clearer from here, hope it helps others as well.  Picturing tension members as cables can make a big difference in how things are viewed.  One can't push a load with a cable.  A steal rod can stand a little compression, but nothing in a truss to speak of.  I see a lot of tension rods in the transitional buildings here in my area, churches and barns. 

Tom, these buildings you mention do they have a tie at the eve or just dropped a bit?  You didn't show one and that looks odd.

cbecker, have you selected an example for critique?  The 8"x16" tie is quite large and I wonder if it alone would hold up a floor.  If you do not use a post to support the ridge beam you will eliminate the extra roof loads placed on the tie, and just use the common rafters.  This will leave you with more space in the second floor area, if this is what you are after.  If you are going to go in the truss direction the tie needs not to be so large, 8x8 will do just fine, remember the cable.  Tim

[Tom]

By tie, do you mean a longitudinal beam?  Yes there are beams that tie one post to the other, longitudinally, but not at post height across the building.  The spreading forces apparently are handled by the configuration in red.   The rafters/truss are sitting on birds-mouths on top of the beam.  There is also a 1"x 12, or 1x10,  ridge board.

I guess this isn't timberframing, but I thought y'all might lend some light to the structural integrity.

[Don P]

There are probably ways to look at it and make it work Tom. When I mention tie it is pretty much in the context of the codebook, something that ties the rafters together across the building to keep them from spreading. From this angle it looks like the tie across the rafters is too high to do a whole lot of good. Under a full roof load the forces on the tie and especially its connections to the rafters is going to be pretty substantial. Or, the roof would be much, much stronger if the tie were lower.

A couple of ways to look at it differently; The plane of the roof deck can be treated as a diapragm. If the roof in that plane is viewed a very thin but very deep beam that spans from corner to corner the thrust can then travel across the diaphragm of the roof to the corners where there is usually reinforcement and then down. This is what holds up most hip roofed houses. If its a pole barn with posts buried deeply enough to be considered fixed and if they are of large enough dimension, they can act as flag poles with the roof on top.

I've also seen that setup with braces or ties from the post to rafter.

At the bottom of the span tables in the codebook there is an adjustment chart. It lowers the allowable rafter span according to how much the tie has been lifted. This is because once the tie leaves the plate it ceases to be a rigid triangle pinned at the corners, it becomes something like the letter A with the lower legs trying to bend out against the tie as the downward force increases. Or another way, the raised tie is using up some of the bending strength of the rafter. A tie raised halfway up derates the allowable rafter span by 40% requiring a beefier rafter than normal be used.

Looking in the codebook a #2 SYP 2x10 on 2' centers in the lightest roof load chart, can span as far as 18'11' horizontal measure. If the tie is raised 2/3 the allowable span is cut in half. This chart is for 20 pounds per square foot live load and 10 psf dead load. I doubt you have 10 psf total gravity load. However when I want to step off their charts its time for an engineer. 

I built one house in the hills of SD where the plans called for high ties, they were drawn by a designer in NC, he was not an engineer. I was young(er) and dumb(er) but still a little worried so we connected well, nailed the roof ply well, counted on the log walls acting as beams and hoped somebody knew what they were talking about. Unlike the designer I was shovelling snow out of the building so firsthand the idea of the weight was worrying me. I lost the bid on the next house in the subdivision to a lowballer who was younger, greener and last I heard was crossing state lines with the company backhoe in tow  :. Anyway, he came by to quiz me on ties and we proceeded to impress each other with how much we knew. He eliminated the ties. Both houses still stand. Only difference I know of is we still get Christmas cards.

There are many possible load paths through a building, we are supposed to be able to trace one and show it is adequate for the expected loads. I have been in a house where the jammed door had become the post supporting the second floor. Probably not the designed load path but it was at least at the moment adequate for the load. I hear people all the time say they overbuild. To me someone who overbuilds has identified that load path and built it at least up to snuff, then he goes and makes several others capable of supporting the loads. Generally the folks I see use the term don't really know if they have one adequate support.

[Tom]

I'm sure these barns are designed by standing back 100 yards and looking over a fist and extended thumb.  Common  sense tells me that the board holding the rafters together might be too high, though I have no numbers to associate with it.  Florida might be a little different than states where it snows.  Most of the builders here seem to be concerned with "uplift" rather than load.  The building codes want the building tied to a substantial foundation.

These hay barns are nothing but trusses on beams/plates with a covering of tin.  Perhaps the weight is negligable. I was, once, going to get a picture of one these barns but have been remiss.  I'll have to try again.

[cbecker]

There will not be a floor load on the tie beam.  It is going to be a pavillion.

Thanks for all the input

Chris

[Brad_bb]

Tom,
A tie that high up supporting the full spread of the roof is pretty scary looking.  The higher up the tie is, the higher the tension load is on that tie as the legs want to spread under load.  Best/typical practice is usually a tie at the posts.  If you need the clearance, that's usually where hammer beams or other frame types come in. 

It's interesting that this topic has come up - a king post with a dropped tie beam.  I have a workshop I'm planning and want a second floor on half of it.  I'd be spanning 30ft.  I was thinking that truss would be needed possibly.  It would be like having a conventional kingpost truss with a dropped tie beam below it, with braces all between the ties forming an actual truss.  But then how do you connect the bays on the second floor...the truss would be in the way.  (please don't get on my case if you are reading this Carlos.  I haven't progressed design-wise since our last conversation..still finishing up other things).

[Don P]

Brad, do you, or does anyone know how to quantify the tension in a raised tie? I assume its the force if it were connected at the feet times the ratio of where it is in the length of the sides  ???

Chris have you investigated option 2 with the 8x16 tie as a center point loaded beam supporting the ridge beam? I think looking at the tie as a tension member is going to roll the plate off it.

[Dana]

Tom, my Dad's two hay barns use the same construction method you show only they left out the verticle support shown in red. Both barns have been up for 70 plus years and must have seen at least a few snow flakes on the tin. :D

[cbecker]

Option 2 was the original plans were but I was concerned that over a period of time the tie would begin to sag in the middle with the point load on it, thats why i was investigating the kingpost route to hold the tie from sagging.  The live load for my area is 40 psf here in eastern Pa. Whichever option I go with I was only planning on having the posts extend a minimum amount to move the tie joint and the plate tenon apart and not have them right on top of each other.  12" is what I was planning for. 

[Thehardway]

Cbecker,

IMHO, figure 2 is the correct  configuration.   It is time tested, and proven. 

Figure 1 shows more likelyhood of the rafters 8X8 rafters failing than the 8X16 anchor beam.  Also there would no be sufficient kingpost tension in this arrangement to make the friction joint at the top of the kingpost and thrust at the eave is actually being taken by the plate which is only held to the post by a stub tennon. The rafter to plate and plate to post joint would be suspect for failure in my opinion.

A side elevation of this structure might be beneficial.  Is this a post and continuous plate with principal and common rafters or bent style construction with purlins rather than common rafters?

[cbecker]

It is a continuous plate with principal and common rafter configuration.  The common rafters will be 4x8 on 3' centers.

[Brad_bb]

Cbecker,
I don't know why you just don't do a standard kingpost.  I haven't heard any reason why you would want to be
"having the posts extend a minimum amount to move the tie joint and the plate tenon apart and not have them right on top of each other." 
The only feasable reason would be in my mind if you were trying to get a second floor at a certain height.  It doesn't seem like you are doing that here.  It is standard practice to have the tie, rafter, and post come together without separating them.  It sounds like you just don't understand the design of those joints?
There are a few different ways it can be cut, all of which are equally acceptable.  Pick up a copy of Steve Chapell's "Timberframe Workshop" on amazon or wherever, and it illustrates one or two ways to make that joint.  I would suggest, since you don't seem to have a specific design reason for creating something new, that you don't try to reinvent the wheel, but rather use a proven design method with a typical kingpost truss that is not posted cape, or whatever you want to call it.  Also keep in mind that you can have a continious tie beam spanning the distance, and the king post is then inserted in the tiebeam and pegged, or you can have the tie beam in two separate pieces and extend the king post down and insert each half in the king post and peg it.  The "Kingpost Project Begins" post in this forum is a good example of a two piece tie beam in a king post.  If using a one piece tie beam, the king can be half dovetailed and wedged or it can be pegged.  Of course you must understand that a kingpost design uses tension members and therefore must be analyzed/scrutinized more carefully.  All calculations must be done to size the joints, determine number of pegs (or dovetail) to be sure it cannot fail.  Applied loads must be known for this calculation.  Make sure there's enough margin in the design to account for any foreseeable future loads.
So why can't you use a standard kingpost design with rafter, post, tie beam intersecting?

I just noticed that you said you were planning continious top plates.  Any special reason?  What I mean is, that will require either two long pieces of wood, or scarf joints.  That's either more handling of the longer pieces, or more careful work to scarf them.  If you use a typical bent system you can use shorter pieces of wood.  Something to think about.  I'm trying to think how not to complicate it any more than need be unless there is a reason.  It doesn't make it less skilled or anything else, just another method to achieve the same thing.  Interested to hear your reasons if I haven't missed them.

[moonhill]

I would go along with brab bb in that maybe you should just use a english tying joint and a king rod truss.  Also, I have done trusses with a dropped tie, long plates and common rafters.  I used two cruck blades as the top cords, connecting the top of the king to the outer ends of the tie beam about 10" in from the post creating a typical clear span truss. Rafters set on purlins setting on the crucks.  The peak used a tongue and fork.  Wedged half dovetails on the ends of the tie in the posts.  I am using this in my next frame, it has a second floor and clear span first.  The client brought the White Oak crucks , in log form, up from Pennsylvania.  It can be done both ways, the tie at the plate is the most direct, in my opinion.  Tim

[Thehardway]

Cbecker,

Details which approximate your desired assembly can be viewed in Part One of the "Historic American Timber Frame Joinery" which can be seen online in pdf. form or bought as a book from the TFGuild website.  Jack Sobon illustrates the details of joinery and assembly well.  It is actually a hybrid of bent style construction and post and continuous plate.  It was primarily used by the Dutch in story and a half houses. They also added lean-to's and expanded the design for 3 aisle barns. They used an "H- Bent" which was a post and "anchor beam" using a through tenons, and then a rafter pair with raised tie. 

In your drawing you show a ridge plate supported by a post to the anchor beam which is really unnecessary and complicates the design. It changes the weight transfer to the anchor beam rather than directly to the plate, post and into the ground.  It would tend to cause the anchor beam to sag over time and do little for the roof.  Take a look at the h-bent with continuous plate and principal and common rafter setup and then let us know if it clarify's anything.

A kingpost would certainly handle the span but if used I would change setup to beef up the rafters to at least 8X10 and cut down the size of the tie to 8X10.  In the kingpost config, you want the tie as small and light as possible while still allowing necessary tension joinery and able to control outward thrust at the plate.  I personally would also change the way the plate is attached to the post if the kingpost arrangement is chosen.

[cbecker]

I have that book and have read through it.  That makes sense about the anchor beam sagging over time and that is why I originally was looking at a different way to frame it.  My other concern was thrust at the plate from the rafters.  Thats why I was originally exploring the kingpost with a ridge supported by the kingposts, intern supporting the rafters.  The reason I was trying to stick to the continuos plate design is that I already have a 29' 8X12  and a 24' 8x12 white oak cut so I would only have to make one scarf joint. 

Thanks everyone for all your input.  I greatly appreciate it.
Chris

[Brad_bb]

So where does that leave you?  Do you know the direction you are going to go?  Will you have the frame design  analyzed by an engineer to verify beam sizing and joint design (shear area for pegs and bearing area)?
So this 29' white oak stick of that size is good as far as defects and grain runout?  If so, that sounds like some stick! And one heavy sonofagun!

[Thehardway]

You ain't kiddin'  not many places you can find a white oak that tall without getting into a lot of branches and thus knots, and/or grain runout. Those must have been some kinda trees and some kind of equipment to handle the logs.   My 7"X8"X12' timbers took a 18' dia. log and the 7"X10"X 18' were 22"+ dia. logs.

Can you post some pics of these timbers?  Would like to see them as I am sure others would.

At 64 lbs per cubic ft. green that puts the timber itself close to 1400 lb.