24x32 in Southern Illinois - New member.

[MountainDon]

I understand what you are saying about advice given, and structural codes... and the only thing I can think of is... those who give advice do so freely and I am reading what is said, just thinking that so much code is referenced, but it doesn't appear anyone here follows most of it.

1.  It's not fair to say that no one follows most of the code; many do, some may not. Codes also evolve; what was prescribed one or more versions ago may now be different and therefore some illustrated works may not be current.

2.  Read the line in the "News" section in the upper area of every CountryPlans Forum page:  
• Postings on this forum may or may not meet local building codes. Confirm structural questions with your inspector.

3.  Code is a prescription; build it as prescribed and it should be safe under the design conditions. There are other ways to achieve the end result in any construction detail; but to know that it is at least as strong as what is prescribed takes engineering knowledge and calculations, not simply casting an appraising glance at it and saying "hmmmm, that looks about right".

4. Read my signature line.

Best of luck.

5. Anyone can build in any manner they choose; future readers/viewers of all projects should reference numbers 2 and 5 above.

[interex]

Don, I understand that very well.

As for the image below, I like how this is, and would like to achieve something similar to it.

[Don_P]

I started this before dinner, the conversation has moved on, hopefully I'm not flogging a dead horse but there might be something of use, ignore the rest. I have an old post up on another forum that might help explain some of this. I'll need to move some pics and will post it in a little bit.

Squirl, that is not a truss, it is a rafter pair that has too high a tie. I know you didn't label it and I don't want to step on toes but calling a table a chair does not change what a thing is. Can it work? Maybe, but as either a truss or a high tied rafter it needs to be checked by a registered design pro. As the span increases the load goes up, also as the pitch lowers the thrust increases. Add it up and this puts the tie and its' connections into more tension.

I'm not sure where the problem was with the two purlins spanning the length of the room, that is a viable option. The loading would be a bit different than what I checked but it is quite do-able. In older timberframing one way to break that 24' long purlin beam span down further was to build one or more heavy timber trusses across the room and span the purlins across them with the common rafters supported by the purlins. This is one way large halls were spanned.

The ridgebeam option is also workable, obviously the load on one beam is going to be larger than the load on two purlin beams so the ridge would need to be larger. There is also a problem with foundation alignment below, that can be solved.

This is a picture of my current job. The room is 20' wide x 36' long. We installed heavy timber trusses that are really supporting a structural ridgebeam that in turn the common rafters hang from. Since my trusses are only about 6' apart a double 2x12 was adequate as a ridgebeam. The spacing was dictated by the architect's design, the trusses shown could have carried the roof at a greater spacing.




Using typical lightweight trusses spanning the width of the room the entire roof load is borne by the exterior walls. Something to consider with respect to the pier loadings. Do bear in mind the load never goes away, you do have control over how it is borne.

I think I'm seeing the thought that "since others here ignore building to code minimums, why shouldn't I". As far as I'm concerned you can build whatever you wish. What I try to point out is what is considered to be good construction, to our minimum standards. I often see owner builders venture outside of standard construction. This is fine but they are sometimes reinventing the square wheel out of a lack of knowledge of why things are done the way they typically are. Some are out there on the bleeding edge, there are some laws one simply cannot break, gravity comes to mind  . Some of the things I see done wrong may not show up at all for most houses most of the time, some may take years, some will only fail in a serious natural event. I do try to help those who are willing to help themselves but am nothing more than a carpenter. I try to steer you all in the direction of least bureacratic entanglement when we discuss design, that is why I reference the codebook's prescriptive design options whenever possible. When we step outside of that basic method of building the plan should really be checked by an engineer.

[Don_P]

I've been working with a potential client and their designer and was writing an email
today. Seemed like alot of work not to bore someone else with it ..

I've attached some quick sketches of various ways to frame the greatroom roof.

These show a couple of approaches. I have a little example that helps
me explain how they work structurally.

Place a Christmas card on a smooth table, tent fashion. This is a
roof. Push down on the fold of the card, the ridge of our roof. The
feet of our roof slide out and it collapses. The vertical load travels
down the rafters and becomes a horizontal outward thrust when it gets
to the walls.

If you put some tape from side to side across the rafter feet and
repeat the experiment the roof will be stable. Notice a string or
piece of tape will work for that bottom chord, it is a tension member.
The 2 sides of the card, our rafters, are compression members. A
triangle is immutable, we have just built the simplest truss. If you
flattened the card you've ventured into strength of materials. In the
case of our greatroom trusses, the horizontal force, the tension in
the bottom chord is roughly 1000 lbs under a very full load. The
joinery at the rafter heeljoint needs to be capable of restraining
that force, the rafter, or top chord of the truss is trying to slide
off the bottom chord with a half ton of force, I have to hook it to
the bottom chord with a connection capable of restraining that. I've
drawn a birdsmouth that hooks the rafter feet down into the bottom
chord. This typically has an open mortise and tenon joint at the ridge



Now look at the kingpost picture. A kingpost, the vertical member, is
sometimes called a kingrod in this orientation, its function is not
what it seems at first. It is not a post sitting on the bottom chord
and holding up the rafter tops.



The kingpost is pinched between the top chords and hangs down to the
bottom chord. It is usually connected to the bottom chord and helps support
it from sagging. Think of it as a rope lashed around the rafter tops and slung
down under the bottom chord to hold it up. It is actually another tension
member. The reason we do not have to have one is that the greatroom span
is not so great that the bottom chords need help to prevent them from
sagging. We can add them as a decorative element but they do get into the
view from the loft rail.



The third picture is a structural ridge. Go back to our Christmas
card. With no ties and pushing down on the fold we know it has
unrestrained thrust. Now put a pencil under the fold and support each
end on books. Push down on the ridge and there is no thrust. If we
support the ridge on a heavy beam and hang the rafters from that then
we need no ties, there is no thrust. I drew it with a 7x12 glulam
ridge and 4x8 rafters on 4' centers. The ridge beam is supported by a post
at the far end, we would support the near end in a heavy steel hanger
attached to a beam in the loft wall.

[John Raabe]

Those are nice looking timber trusses and could be used to get the open beam effect shown in the earlier photo. They would likely require an engineering workup to meet the local snow and wind loads.

I'm one of many members who appreciate the posts of Don_P and all the building information and experience he has shared on this forum. He doesn't play building inspector, but he does have a good understanding of buildings and how the construction of them has evolved.

Not all the houses posted on this site have followed all the elements of all the code requirements. When you build your own house, especially in a non-code area, you have choices and options. Build responsibly - forum members like Don will try to help you do that.

[PA-Builder]

While I don't post much on this site, I do try to visit every few days and see how everyone's projects are progressing.

It amazes me that there are people here daily (like Don_P, John Raabe, Glenn K, MountainDon, and many others) who freely donate many hours of their time, and offer their suggestions & advice to others who are trying to accomplish a project. 

I sincerely hope everyone appreciates this "free" knowledge, and the ability of these folks to articulate their knowledge in a helpful & professional tone ... never condescending or critical for the sake of being negative.  These folks are true professionals, their knowledge is spot on, and in my humble opinion, this website is without equal.

[Ernest T. Bass]

While I don't post much on this site, I do try to visit every few days and see how everyone's projects are progressing.

It amazes me that there are people here daily (like Don_P, John Raabe, Glenn K, MountainDon, and many others) who freely donate many hours of their time, and offer their suggestions & advice to others who are trying to accomplish a project. 

I sincerely hope everyone appreciates this "free" knowledge, and the ability of these folks to articulate their knowledge in a helpful & professional tone ... never condescending or critical for the sake of being negative.  These folks are true professionals, their knowledge is spot on, and in my humble opinion, this website is without equal.

^^ What he said... Round of applause!

[Squirl]

I can't thank them enough.  They have both taught me to fish and given me a few. (figuratively)

I thought you would have been interested in the second picture.  That is why I left the link below it.



You can see the very large structural ridge beam.  It looks to be supported by the stone fireplace.

So Don, how do you size those timber frame trusses?

Sorry about the nomenclature on the trusses.

[interex]

Don't get me wrong, I appreciate everyone's responses, they are a great deal of help and guidance.

I do like it, very much so, but the problem comes in with the 24 foot span and the beams needed to handle the weight. I can't see myself spending $800 or more per beam or tie, for veneer or lamented wood.

[MountainDon]

I can't see myself spending $800 or more per beam or tie, for veneer or lamented wood.

[Squirl]

Don't get me wrong, I appreciate everyone's responses, they are a great deal of help and guidance.

I do like it, very much so, but the problem comes in with the 24 foot span and the beams needed to handle the weight. I can't see myself spending $800 or more per beam or tie, for veneer or lamented wood.

That is the cost of having the wide open room.  You could alleviate almost all of the concerns by adding posts and cutting down the 24' span.  IIRC you have foundation piers every 8 ft.  Two extra posts would cut the 24 ft down to 16 ft, which by my reading and some of Don's calculations could be spanned by dimensional lumber.  It would  be a pretty cheap fix too.  I have seen many people incorporate these posts into the cabin feel by using trees with the bark peeled off.

You are lucky.  You are in a 20 lb snow load area.  I'm in a 50 lb snow load area.

[interex]

Adding posts in the middle of the room doesn't look too appealing to me.

I like the rafter tie idea most, and if I can do it at every 8 foot, so that our weight is more in line with the piers below, I think this is the route I will take in the open room area.

I am going to contact an engineer to find out how big of a tie I will need, and if I can make it from 2x10s or 2x12s with plywood/glue/nails... and how high up the rafter I can safely go to keep me from having a 24 foot beam resting on the 10 foot walls.

I greatly appreciate all the responses and opinions, just trying to see what I can do and still make it look nice, and keep things sturdy.

[Bob S.]

  I looked into the cost of a 24' parrallel scissors trusses to handle this vary issue.
  I discoverd that the cost difference was not vary great when you priced out the ridge beam and #2 fir 2x12's long enough to span the 24' room.
  I could not set the trusses myself so I would need to hire a framing contractor to set them.
  You can look at rdzone's build (Finally Pictures of our 20'X30' 2 Story) and look at page 8 and see them.
  Your local building supply will do all the engineering for you as part of the deal. I found out this was going to be fairly easy to get approved by the building inspector.
  In the long run I concluded that the cost of the cathedral ceiling was more than it was worth to me.

[Pritch]

Don P,

Thanks for taking the time to explain this.  Your Christmas card illustration and Sketch Up examples really helped explain the forces at work, and why and how various methods are used to accomodate them. 

[duncanshannon]

Don... great post/pics/example! that was very easy to follow and very helpful.

[Don_P]

Y'all are quite welcome, thanks for the kind words.
Now for the humble pie  . I've redrawn the structural ridge pic above to make it more clear. I placed the ridge under the rafters and drew a level seat cut at the bottoms of the rafters.

I would like to see what your engineer comes up with interex. I doubt one would design this site unseen but do make sure they visit the jobsite so they can check the entire load path.