My new build on my old slab

[dablack]

Some of you might remember that my Rusk build burned down back in July.  Well, some of the guys in the forestry forum are coming to my rescue and cutting the lumber for the rebuild. 

Here is the basic plan.  The slab is 26x52.  I want to build another 1.5 story.  I'm going to balloon frame using 10' long 2x6s.  For the 2nd floor I want to use a let in and mount 26' long Ijoists so I will have a 8' ceiling downstairs.  The I joists are 11.5" tall so that only leaves about 1' of 2x6 wall sticking out above the 2nd floor.  On the tiny 1' "pony wall" I want to stick frame a 8/12 pitch roof using 2x10s.  That puts the rafter length at 17' and spanning 13'.  I will place the collar ties to give me a 7' 6" ceiling upstairs.  The collar ties are in the top third and I don't have rafter ties but the pony wall is only 1' tall and balloon framed. 

Does anyone see a problem with this cross section? 

I will post pictures of the new build and the lumber cutting. 

thanks
Austin

[MountainDon]

.... but the pony wall is only 1' tall .......

Technically, this is not good as it is not prescriptively allowed by the IRC.  (The collar ties do not assist with the horizontal forces where the rafters connect to the walls. They could be as high as you want in the upper third and do the job they are designed for.)  An engineer would have to have a look at the structure and expected loads to give an authoritative "Yes", to the plan.  The rest of us would be guessing. 

Nice to have the forestry folks volunteer to help with the timber.

[dablack]

Well, if it helps any I'm panning on two "walls" down the 52' length of the upstairs that are about 4' tall and 4' from the 1' pony wall.  This will give additional roof support but that load is only carried down to the 26' I joists running perpendicular to the wall. 

Also, I thought on the 1.5 story plans, John has a two or three foot pony wall balloon framed and then sight built trusses?  I know my 2x10 rafters don't come anywhere close to sight built trusses but I do plan to have some plywood up at the peak.  hmmmmm

Also, metal roof and no snow. 

thanks
Austin

[MountainDon]

two "walls" down the 52' length of the upstairs that are about 4' tall and 4' from the 1' pony wall.  This will give additional roof support

Those will do nothing to resolve the issue of outwards horizontal forces from the rafters on the wall tops.  That can make great storage space. If you intend to use it as storage space give thought to insulating the lower slanted part of the rafters and not that short wall. Openings in short walls like that are difficult to seal well; includes electrical, plumbing, doors.


Yes, John's 1.5 story has "pony" or "kneewalls". He will be among the first to advise that in an area with code inspections that will not pass. We may be arguing semantics, but those are more like heavy duty rafters than being a true truss. 

Ajbremer had an engineer draw and spec some heavy duty rafters like that. It's way back in his thread. The engineers alculations showed a very small horizontal vector. However they were Select Structural grade; Douglas Fir maybe. And they were a large cross section. Doubled 2x8's, maybe larger. There were also expensive; but they were also stamped by the PE, or would have been if he bought them. Lots of room for insulation as well.

The other good choice would be a ridge beam; laminated in a factory and designed by the beam company engineer. Then you know what you got.

[dablack]

No go on the beam.  The beam would have to be 52' long and there is no way I can move it!  HA!  Really though, I don't have footers in the foundation for supporting a beam.  I have a perrimeter footer and a 26' one centered on the 52' length. 

As a mechanical engineer I do know that John's site built truss isn't a real truss.  We would need another verticle wall inboard of the exterior knee wall and an angled support from the base of that inboard wall to the point where the rafters meat the top of the knee wall.  Really, that isn't a truss either, but it is much closer.  HA! 

If I was doing a taller knee wall that is what I would do.  With all that being said, I'm pretty sure a 1' knee wall will be ok.  That is a very short hinge. 

[Redoverfarm]

Austin have you considered "attic trusses".  Mine give me a 12' wide 63" high kneewall.  I did have the option of a 15' room and shorter kneewall but chose the narrower.  The storage room behind was more benifical to me than a short kneewall.  If you have a truss company see what they can make you up to get the most bang for your buck.  My garage is 26' X 44' .  I am going to frame a short wall and insulate it as well as the slope of the rafter until I reach the ceiling using rafter mate baffles.  Trusses eliminate the horzontal thrust.  Someone here had some made up with a very short kneewall for maximum floor space.  I can't recall now who it was.   Did we have this conversation on your other house.  Too much water under the bridge for me to remember.





I know that trusses are more money but by shopping around I saved about $50 per truss.  When you consider your floor joist, rafter material and other lumber incidential to building the 2nd floor I think it is a good option.

[dablack]

Hey ROF!  You know I know all about attic trusses.  With the lumber being free, attic trusses just aren't worth it.  Plus, last time, to get the room I needed, I had to go with a 12/12 pitch.  Then because of the pitch, it took me forever to finish the roof.  No, my roof will be stick built. 

thanks
Austin

[Redoverfarm]

Hey ROF!  You know I know all about attic trusses.  With the lumber being free, attic trusses just aren't worth it.  Plus, last time, to get the room I needed, I had to go with a 12/12 pitch.  Then because of the pitch, it took me forever to finish the roof.  No, my roof will be stick built. 

thanks
Austin

I see that you are planning 8/12 pitch.  That to me it is not much difference than 10/12 that I did. Still too steep to work comfortable on w/o walkboards or roof jacks.  I had considered 12/12 with mine but it would have been too steep for what little I would gain plus the main house was 10/12. 

[UK4X4]

regarding ridge beam, do you not have any walls in the whole length of the house ?

that either match up with a wall below or be possible to add a post in to make the ridge beam in 3 pieces for example

If you have the ridge beam you can then do platform frame and add pony walls to whatever height you want.

If the ridge beam is in sections with no snow load one of your friendly timber guys might be able to supply you with the beams too !

[dablack]

I just don't have the interior walls or foundation to support a ridge beam.

I've been looking at maybe a taller balloon wall so my rafter ties end up in the lower 1/3 of the rafters.  I think if I can make that happen then all the problems go away, right?

Thanks
Austin

[Don_P]

Yes, that is the easy solution. You can play with pitch, width, and wall height to affect the "lower third" requirement. Glad to hear the FF folks are helping out  .

[dablack]

Well, I laid it all out to put the rafter ties in the lower 1/3 and it makes the building way too tall.  I'm thinking that I will go with a 2x8 side wall at 10' long.  Balloon frame 12" tall I joists to cover the 26' length.  That will give me a 1' wall above the 2nd floor.  With such a short wall and that wall being 2x8s, I think I'm going to be ok.  I'm also going to put a 4' tall wall down the length of the building between the rafters and I joist. 

The let-in for the I joist in the 2x8 will be a much smaller % loss of material than in a 2x6.  I'm also thinking of putting some strapping over the top of the 10' wall and down to the I joist under the 4' wall.  This will triangulate this area and "truss" it a little bit. 

Thoughts about these improvements?

thanks
Austin

[Don_P]

Well, I wasn't really worried... but for kicks I just spent several hours doing the math;
http://www.timbertoolbox.com/Calcs/ptloadohang.html
Press "show result" , I've set it up with variables installed (I didn't put the pic in the calc, just wrote the code for it, will illustrate later)

I calc'ed the horizontal thrust (the load, http://www.timbertoolbox.com/Calcs/RafterThrust.htm), entered #2 southern pine for the 2x8's, notched them for the ledger (5.75" remaining), centered the hinge point in the 12" deep I joist (18" overhang) and it failed in bending strength and in deflection. I have a rather hard time believing it but would be concerned enough to ask an engineer before building it. We've posted an article by an engineer before where I think he said you shouldn't sit on a kneewall over 6" tall without engineering, it's kind of bearing that out.

Now I know you're going to say, but, but I'm doing this or that, I'm just saying I can't stand behind it myself from what I know, I'd hire someone who can say for certain, that's how I build. I can say that your connections of I joist to stud had better be good, 585 lbs worth or better. I think you're into a 16" I joist, the supplier can spec that. If the 4' kneewalls are doing additional work out in the span he needs to know that. Raising the pitch decreases the thrust, but we are outside of where I can say "sure go ahead". I'm sorry.

[dablack]

Wow Don.   Thanks for the help.  I have been thinking of this all day yesterday and was getting something similar in my head.  I mentioned this below so I will add to it here.  I think the way to resolve this outward thrust is to triangulate between the 4' wall and the 1' kick wall.  My problem is that the top of my 10' wall (1' kick wall) wants to move out.  I think I should put in an angled member from the top of the kick wall to the I joist under the 4' wall.  When you put that member in place, my stick built attic starts looking like an attic truss. 

Again, I really appreciate you weighing in on this one.  Your input has been very valuable. 

Austin

[Don_P]

Wood and its' connections are pretty tough to develop resistance in tension and we're loading up the I joist in span the way you're talking. In compression wood shines, in tension steel shines, but I can also see a ply gusset in there doing what you're talking about.

Another thought. If the I joists pass thru the wall to form the underside of the overhang, the rafter birdsmouth sits on the 1' wall and the rafter extends down alongside the extended I joist and is well attached to it. The top of the extended I joist is cut flush to the top of the rafter along that attachment. We've just triangulated out of the floor span and the rafter tail, which is the wall brace, is in compression against the thrust. The downside is I think the I joists just got to be ~30' long. The I joist engineering dept might do the check.

[dablack]

I hadn't thought of extending the I joists.  It is a great idea......until I drew it up.  Crazy lookin'  That over hang is tough to get used to. 

I get what you are saying about the tension and compression.  I'm not sure what I'm going to do right now. 

I had a friend of mine tell me that 2x4 walls and 2x6 rafters would be fine and have been used for years.  That might be true but I just can't do it. 

Just looked up the I joists and it looks like at 16" OC, a 11  7/8" joist will be "ok".  There are some things I can do to stiffen them up.  hmmmmmm

Austin

[Don_P]

I don't think Dad ever built 2x6 walls, what your friend is saying is correct for prescriptive, typical, construction. As soon as you step outside of those norms the forces can get away from you pretty fast. We tend to try to force those rules of thumb onto situations where they no longer fit. In this case you're looking for a healthy clear span, no ridge support and a high tie, all of those are outside of the old rules of thumb. I think that's where his comment is missing the mark. Some days you just can't win, keep thinking  .

[dablack]

Thanks Don.  Oh yeah.  I know his "rules" don't fit with what I'm working with today.  I'm just pointing out the extremes of advice.

I'm having a tough time with the cost of the ijoist so I'm also looking at the possibility of a beam to support 2x8 floor joists.  I'm over on the forestry forum working on that. 
http://www.forestryforum.com/board/index.php/topic,69167.new.html#new

My head is hurting from trying to stop the outward thrust.  I thought it wise to work on something else. 

thanks
Austin

[Don_P]

OK, Looks like we're getting somewhere. I didn't check Jim R's results on the girder dimensions but he's sharp and I trust the calcs he's using   .  (He actually helped when I was developing them). This plan opens up the possibility of a ridgebeam. Stack ridge support posts over the girder support posts below, install a ridgebeam, and the thrust goes away. The rafters then hang from the ridgebeam.

Add the upper post load to the girder load to check the allowable compression perpendicular to grain in the girder if you go this route. I've gotten into trouble there before, one way out is to mount a ~3' x 10x10 bolster on top of the lower 10x10 post forming a T. The bolster is made out of something with a high perp to grain strength, the oaks come in at about 800 psi for instance vs the 375psi compression perp of SYP. We've just applied the post punching load into a dense bolster and then spread that load out over a large surface area. It also gives lots of attachment room for the beam splices. I've been working on 14 bolstered locust posts for a barn repair lately. Can you handle 2 posts upstairs?

[dablack]

Don,

I'm right there with you and was moving the same direction.  Now that I know it will work downstairs, I'm trying to make it work upstairs. 

I read your post about the bolster and I wsa thinking "pad" but now I see the "T".  It is a post laying on its "side" on top of the original post to spread the load.  I get it.  The load in the bolster is perpendicular to the grain.  Ok.  This is all before coffee so I'm doing well without visuals. 

What I'm concerned with right now is the concrete pad.  Going back and looking at my pictures, the 26' footers that support the two posts downstairs are only 12" x 12" under a 4" slab.  I originally designed the slab to take all the roof load on the perimeter.  Is there a way to spread that load out on the downstairs post?  Maybe pour a 6" thick pad that is 14" x 14" or something like that.  I dunno.  It has been a couple of years since I designed that slab. 

thanks
Austin

[Don_P]

Worst case scenario is to saw out a large enough square to pour a new footing, remove the old work and finish the new works' top with the existing slab. Take a look at the floor under the column bases in a store that has concrete floors, like a Lowes.

[dablack]

Right.  I might be resurfacing anyway.  I haven't cleaned off the slab since the fire.  I know some parts of it popped the cream layer up.  Most of those areas only looked about 1/4 to 1/2" deep.  Other areas were still stained, sealed, and looked like they just needed wax.  HA!  Anyway, I'm going to have to do a full inspection of the slab.   And, like you said, worse case is cut it out and pour a big column footer. 

thanks
Austin