Structural Question 20 x 30

[Davegmc]

Hello all: Quick question. I'm planning a 20x30 1 1/2 story in a 95 ppf snow area. Planning a ridge board with 2 x 12 rafters with a full loft with 10' 2 x 6 walls and 2 x 12 loft floor joists. I guess I would let in 1 x 4 at 8' height to support loft floor joists. So the rafters and joists would have to be 12" on center, correct? My other question: The floor joists will tie-in to the wall but not the rafters, correct? Will I be able to get this approved without engineering? Do the joists count as rafter ties (thrust) if they are 2' below and not tied in to the rafters? If so, can the rafters be 16" OC while the joists are 12" OC?

Thanks!
Dave

[MountainDon]

Will I be able to get this approved without engineering?

I would be surprised.

The best method to achieve this would be to use an engineered ridge beam seated on proper end wall supports with the load transfer all the way down to the foundation.

You would likely be facing issues with the cathedral ceiling too, depending on what code version is being used. Newer codes call for rafter ties at every rafter. That may be changed if engineered, but then the ridge beam would resolve that issue as well.

[Squirl]

I am a novice, so someone with more expertise can give more insight, but my understanding is that you have the right idea, and it generally would conform to the code.  It seems to conform the all the AWC charts for span and sizing.  Your plan seems like it conforms to what has been said about using joists on the upper part of the wall to prevent outward thrust, if you are not using a rafter tie.  I am not sure there is a code provision for this though. The wall height seems to conform to the maximum wall height.  I have no idea what is required in your area for inspections.  They are all different.  In my area the limit is 1500 ft before you need to get it engineered.  If the inspector in an area my area is familiar with the method, they will approve it.  Don is right. A frequent topic this week has been sizing for a ridge beam.  Many sold come with certifications for span.  Some inspectors require engineering approval for this also.  You never know by jurisdiction.

[Don_P]

actually there is no provision in the code for having the tie, in this case the floor joists, dropped below the plate. A triangle is what is addressed by code. A triangle cannot deform. This would be a 5 sided assembly which can deform under load. It is up to the inspector but this does fall under engineered construction. A ridgebeam does make this go away. At that span and load it would need a center post as well as the end posts. Generally the supplier can provide an engineer stamped letter for a structural ridgebeam.

[Davegmc]

But the 20X30 1 1/2 story says:

"Since all floors and roof bear on the outside walls, the interior of the floor plan can be redesigned to your liking. The framing system is simple and is designed for small crew construction so it can be built by owner-builders. This seems to be one of our most popular plan and many folks are having good luck building it".

is it originally designed for a ridge beam? I thought the 10 ft walls were so you could put the loft floor joists at 8 ft and have a nice loft? How could you do that without rafter ties then? I'm missing something?

Thanks!
Dave

[MountainDon]

Ah!  I didn't realize your were basing this question on John's 20x30 plan set.

They are based on using a site built truss such as can be found HERE. There is also the option of having a truss manufacturer make something similar. The owner built may or may not meet local codes; you would have to check locally on that.    

Then there is also the option of using a ridge beam and I have to admit that I am not certain if that is included in the package.

You can email John and ask him. Refer to the 1  1/2 story plans

[Don_P]

Do note the snow load in that email
Here is what the building code requires of a truss design;
http://publicecodes.citation.com/st/ny/st/b400v10/st_ny_st_b400v10_8_sec002_par029.htm?bu2=undefined

Did anyone mention a ridgebeam?

[MountainDon]

A beam sounds better and better...

[davidj]

My place is a 20x30 built with a 7x18 glulam ridge beam and a 100psf snow load (I should check that - we may have gone with 120psf in the end as there is no good survey for our area and it was something of a guess as the county required 80psf but warned us to uprate it).  Note that, at that load, you're talking some chunky supports under your beam.  Two of our 6x6 beam support posts are over doors, which required 5x12 glulam headers and 4x6 jack studs.  We also needed extra-deep and extra-wide footers where the posts hit dirt.  Even our 8' wide window, with just regular roof load above it, needed double jack studs, a 6x10 header and thicker footings.  In summary, with a 100psf snow load, lots of things you might see in warm regions start to get quite big and chunky.

We did actually do this without formal engineering (i.e. it was labeled "conventional construction"), but the county still required calculations for all of this stuff.  However, it was just a house designer, not an engineer or licensed architect, who did them.  (Or, I guess, her computer!).

[Squirl]

Thank you Don_P for the clearer answer.  Check with your local building department, in my jurisdiction trusses must be certified and stamped by a design professional.

[Davegmc]

Thanks All!
Questions for davidj: Does your glulam ridge beam span the whole 30'? Are your non glulam beams and posts number 2 or select grade? Does your ridge glulam have a mid span support? If so is it just supported by a single post (6x6?) resting on a poured single footing? Is your 8' window on an end or side?  Thanks!

[tc-vt]

Dave,

I built a 20x30 in Northern VT.  

http://countryplans.com/smf/index.php?topic=2095.0

Snowload for the area is estimated to be 70psf.  I had tray trusses built which are rated for 40psf, 24 inches on center.  The truss company said it was okay as long as it was a metal roof.  With the metal and the 12/12 pitch, it sheds the snow.  Afterward, I wondered what the rating would have been if the trusses were 16 inches on center.  It never has more than a 18 inches of snow on it, even on the north side.  They cost about $120 each in 2004.

The trusses sit on 32 inch high knee walls, platform framed.  The space upstairs would be more usable if the knee walls were at least 44 inches high.  That would allow a 5'8" tall person be able to stand 2 feet from the wall, as if standing in front of a piece of furniture, without hitting their head.  It also makes more room for taller furniture, like dressers.  Which brings me to my next question...

...Unless the upstairs half-story loft is just bonus space for you, have you considered a full second story?  If I did this again, I would not do the half story upstairs.  Compared to the half story, the full story additional cost is not that much.  The benefits would be more full height vertical wall space for things like furniture, *windows!*, and wall decorations.  Full height walls would also make placement of a bathroom easier.  Small two story houses look nice, too.  Here is the 20x34 two story:
http://www.jshow.com/y2k/listings/57.html

Trusses install quickly and easily and allow you to have an open floor plan with no need for interior weight bearing walls.  I used I-Joists for the upstairs floor which gave me the same freedom from bearing walls downstairs.  My I-Joist floor was bouncy until I put a beam across the bottom.  Going bigger than recommended might lessen the bounce.

Tom

[MountainDon]

Never go for the minimum in sizing a joist. That may have been that design minimum if the bounce was too noticible. Another thing to remember when talking to the supplier/engineer.


FWIW, I grew up in a neighborhood of 1 1/2,  2 story and the odd 2 1/2 story giant. All on 25 foot lots.    My family had a 1  1/2.  I liked my room (full width upstairs with slanted walls) but my friends with the bedrooms in the 2 story houses had more room to hang airplanes from the ceilings without running into them even though their rooms were actually smaller. At least that is what I recall. 

[Davegmc]

Well maybe you folks can put me on a better path. We are purchasing a lot in a cabin community in Leavenworth Washington. It's in the mountains (95ppf snow load area). I was thinking that the 20x30 1 1/2 might be a good way to go. Could be dressed up a bit with a dormer, etc. We want a cabin, not a small house. Like someone else said, a loft is what kind of makes it a cabin. Looks like the only way to make the 20x30 work is with a ridge beam. I guess that's okay but then will need engineering and will add some cost. Any thoughts?

Dave

[Don_P]

First off, alot of people use the reasoning that the snow will slide off their roof to use smaller members. I've had 3' of consolidated snow stick to one of my metal 12/12 pitches so I like to figure that at some point in the life of the house a perfect storm is going to make the roof "sticky". I use the ground snow load. If there are going to be snow guards or a porch roof this might need to be increased for accumulation. The gambrel to shed transition on my barn had over 2' of snow sitting on it a couple of weeks ago, we have not had over an 8" snow this season. It just kept sliding down and piling up on that north side, just stuff to think about.

Let's do some quick math;
If you use a ridgebeam the ridge will support a swath down the middle of the building 10'wide, the walls will support halfway to the ridge on each side. Looking at a cross section the wall supports halfway to the ridge, or 5', the ridge supports 10' and the other wall supports 5' of width. The ridge you would like to use is spanning 30'. so 10' wide x 30' long x (95 psf snow+10psf dead load)= 31,500 lbs on a 30' span. Looking that up in the tables in the AITC manual that is a 6 3/4x 24" deep glulam and weighs just under 1200 lbs. the beam reaction at each end is half the load or 15,750 lbs on each end post... we can look at that but I doubt its a 6x6.

If there is a midpost the load on each span is cut in half and the span is cut in half, so the bending forces on the beam are reduced by a factor of 4. A 6 3/4 x 12 or a 5-1/8x 13 1/2 would then work. The supplier can do the stamped engineering for final sizes and this is a free service. The support conditions are normally up to your engineer but that is usually not too expensive.

An attic truss is worth looking into as well, it can have kneewalls, it has the floor syatem built in and the rafters can spring from the overhang rather than the wall giving a bit more headroom on a shorter main wall.

Edit;
Here's some glulam tables, good site to look around in as well.
http://www.aitc-glulam.org/capacity.asp

[davidj]

Thanks All!
Questions for davidj: Does your glulam ridge beam span the whole 30'? Are your non glulam beams and posts number 2 or select grade? Does your ridge glulam have a mid span support? If so is it just supported by a single post (6x6?) resting on a poured single footing? Is your 8' window on an end or side?  Thanks!

Sorry to take so long to get back to you - been off carrying slate tiles in my backpack through 4' of snow...

We've got a single ridge beam but it has a post about 10' from one end, so the free span is about 19'.  The center post touching the beam is 6x6 #1.  This sits on the 6x12 glulam header (above the door), 4x6 jack studs (side of the door) and 4x6 posts (under the floor).  The footings under the 4x6 posts were pretty chunky too - I guess they've got something like 7500lb on each one.  Actually, given the door is slightly off center of the post, maybe a little more.  I think they ended up at 24x24x12 (don't have the plans to hand right now, but I've vague memories of dumping 4 cubic ft of concrete into a pad at some point!).

For concrete pads, your soil type gives the soil bearing capacity (something like 3000 psf for soils with lots of gravel, 2000psf for sandy soils according to google).  From that, and that load, you get your pad area.   And for a given area, that fixes the thickness and amount of rebar (so the pad doesn't crack).  For small loads, I think I needed 12"x12"x8" and 1 piece of rebar each way.  For the big pads, 24"x24"x12" and 3 pieces each way. 

The 8' span window is on the side.  This has the 6x10 header.  I can't remember with this is specified as #1 or #2, but I think I used #1.

[Davegmc]

Thanks DavidJ! MountainDon pointed out your project pictures to me. At that mid span support, looking at the picture it looks like there is a steel (channel) for the 6x6 sitting on top of the double top plates. That correct? I see the glulam and the 4x6s. It looks like there is a single poured footing roughly 2' x 6' that supports the 4x6s. So only that portion of that wall is a bearing wall? Then on top of that footing is 2 round concrete supports that support 4x6 posts? How do the upper ends of the 4x6 posts under the floor tie in at their upper end (below floor)? The other 2 footings (2'x2') must be for a beam that supports your floor joists mid span, correct? It also looks like you had to locally widen the footings by 3" each side at the front of the building to support the ridge beam. Did you have to do that at the back of the building as well? Seems like that could cause problems with running drain tile next to the footer? I wonder if your designer would be willing to sell me a copy of the plans?

Thanks
Dave