Informed advice is always valuable, especially when it's freely given. And the only liability incurred is my gratitude. I'll use your rough numbers for now in my drawing. It's relatively easy to change. One question on rafters connected to a structural ridge beam - is it necessary that they sit on top of the beam, with or without a birds mouth, or is it sufficient that the beam is supported on the gable ends and the rafters can be attached in the same way as to a ridge board?

I do need to make some friends with the local lumberyard. They are pretty full service and get a lot of local lumber - there's a sawmill just down the road a mile or so.

As always, much thanks.

Now these numbers are very rough estimates from a novice non-engineer, but here goes.  I tried to use the header sizing guideline from the ICC.  They I took the ratings for a 20 ft wide house at 30 lbs snow load.  For 3 - 2x12's they can span 12 feet 2 inches.  I will round down to 12 ft for calculation purposes.  So the total roof load for that area would be (12 span X 20 width x 30 weight) 7200 lbs.  This is for a header in a bearing wall.  A general assumption I am making is that there would normally be 2 bearing wall in a conventional framed house with rafter ties.  So it would be rated for 3600 lbs for 3-2 x 12's 

Now for your building at 10 ft wide for 16 ft with a 50 lb snow load (Vermont guess) I came up with a total load of 8000 lbs.  My next assumption is that with a ridge beam would displace that load across three areas (2 bearing walls and the beam).  This would give you 2666 lbs per bearing beam.  So with these assumptions 3 - 2x12's would be able to handle the load.  I would be comfortable with this if I was building a ridge beam, remember I am a no engineer.  There may be flaws in my calculations/ assumptions.

You could even size up.  4-2x12's are rated to span 12'-2" for a 50lb snow load in a 20 ft. building and 14'-1" at 30 lbs.  With a building at half the width it doesn't seem too far to stretch it another 3 ft. (estimating 6" each side for studs).

Another thing it says is that it would require 2 jack studs.  I believe a 6x6 would be more than enough. But I am not quite sure about the definition of jack studs.

The best no brainer ultimate safe option would be to buy an LVL board engineered for this.  The reason I gave for these is for calculation purposes and that LVL's are not as easy to lift as one 2-12 at a time.

My only concern is that your center beam is cantilevered and this type of design is usually supposed to go straight over the foundation.

Here's what and how I figured this today. There could be errors.

I used 10 ft x 16 ft for the cabin dimensions. That does not include any overhang. 10 x16 = 160 sq ft.

160 sq ft x 60 psf snow load. VT seems to be mostly 50 or 60 with some 70.
That = 9600 lbs.

Then the roof structure itself, the dead load. Normal seems to be 10 psf, but I used 15 psf.
That = 2400 lbs, for a total of 12000 lbs.

The roof load is carried on three supports; two side walls and one center ridge beam. The ridge carries half the load of each slope, for a total of one half the total roof load. Each side wall carries one quarter of the total. Ridge = 6000 lbs, each sidewall 3000 lbs.

Don_P has graciously offered the use of his Windy Hill Logworks calculators to anyone fpr free. Thay are for use by anyone and used at their own peril. No guarantees or warranties are offered at all. They are great tools for planning. Results should be checked by a qualified engineer. The index page to the calcs are located HERE. In the near future I'm going to list it in an easier to find location.

I used one of the beam calculators, the simple beam, uniform load one.  Data I entered was 6000 lbs load on beam and 192 inches length. If this is measured the same as joists it could be a few inches shorter, but I went with the actual length. Also note if there was a third ridgebeam support someplace along the length that would split the load and permit a smaller beam. That would also chnage the column loads, etc.

For the wood properties I used data from a table for Douglas Fir grade #1.
Maximum Allowable Fiberstress in Bending (PSI)  1322
Modulus of Elasticity (million PSI)  1.7
Maximum Allowable Horizontal Shear (PSI)  207

Then I plugged in different values for the width and depth of the beam. A combination of 6 wide and 11.25 deep passed all three tests.
I re-ran the calculator using wood values for DF #2 and it also passed.

No guarantees I did all that correctly. Hopefully Don_P will have time enough to critique my procedure.

Thanks in advance.


On to the column. I used the first column calculator from the beam and column calculator list.
(But first I used the ridge height calculator to determine the distance between the bottom of the calculated ridegbeam and the height of the main floor. 156 to 157 inches.)
I used that length for the column. That should likely be longer to reach the foundation, but I'm not sure how that is to be done. So I was happy with this number for general curiosity purposes. I used 5.5 x 5.5 for the column depth and other face values.

The height question brings up a question from me. Should this be calculated for each beam section if there was to be one from ridge to wall top plate level and then a second from there to main floor level? And again another calc for the length to foundation? Another variable will be using headers and two, separated columns to split the load around openings.

Allowable Compression Parallel to Grain (psi) came from a data table for DF.   625
Modulus of Elasticity (million PSI)  1.7  Again from a table.
Load on column entered as 3000 lbs, one half the total beam load.

The 6x6 passed all tests. I tired a 4x6 and it passed as well, but a 4x4 failed 3 of 4 tests.
A re-run with DF #2 also passed in the 6x6 and 4x6 sizes.

It would be interesting to see how others interpret this.

Thanks Don for links to the calculator.  Also thank you for the clarification on the ridge beam carrying half, not one third the load.  For the 6x12, if you only had access to dimensional lumber, could this be 4 - 2x12's?

Quote from: Squirl on March 02, 2011, 03:43:40 PM
..., if you only had access to dimensional lumber, could this be 4 - 2x12's?

I think so...

Sorry to be slow responding, playing catchup after a little lightning fun.

Squirl,
Yes, the 6x11.25 would be a 4 ply built up beam of 2x12's. If anyone gets into heavy timber we need to talk more, there is a different table of design values. Your jack stud comment is a bit right and a bit wrong. It takes care of the crushing into the beam however a jack is a short member, if you use that thinking on too tall a post with a significant load it'll buckle, its part of the check the column calc is doing.

Mt Don,
Your beam methods and calcs look fine. It looks like you used the AWC calc to get the design values... excellent for a 3 ply or greater beam. Either that or you are a pro  

Here's some design values for typical 2.0 E LVL's
Fb-2900
E-2.0
Fv-285
Plugging them in it looks like a 2 ply 11.25" or a single 14" LVL would do the trick. The single 14 would probably be cheaper. A 14" weighs 6.5 lbs/ft so 104 lbs for 16', reasonable for 2 guys on secure ladders.

On to the column calc;
The safe length to use is the longest dimension in an unbraced direction. If there is a floor bracing in one plane and a wall bracing in the other then that floor to ridge dimension is the height... make sense?

One further check is Fc Perpendicular to Grain to make sure the post doesn't crush into the beam. On this example with a 6x6, the post is exerting 99 psi on the beam IF the beam is occupying the entire post top. This would be the case with the built up 4 ply beam and the allowable FcPerp of DougFir is 625 psi, so we're fine there. With a beam narrower than the post top we need to divide the load by the bearing surface and make sure the beam is safe from crushing on the post top. The single 14" LVL is our thinnest beam so checking it; 1-3/4" thick x 5.5" bearing length=9.625 square inches bearing surface on the post top. 3000 lbs /9.625=312 psi perp to beam grain. An LVL has an allowable FcPerp of 750 psi, so that is a safe choice as well. (If you look at the bottom of the calc, Cp... this is the column stability factor. The column at this stifness, load, and length has lost 31% of its capacity due to being a tall slender column.

reb,
The rafters can sit on top or be hung on the side of the ridge with a hanger capable of supporting the load. The rafter load on the connection at the ridge is half the total rafter load. Half of the rafter is bearing on the wall, half is supported by the ridge. Your rafter is spanning 5'. Assuming they are framed on 2' centers the rafter is supporting a tributary area of 10 square feet. 10'x75 lbs per sq ft (60 snow+15 dead)=750 lbs per rafter. Half of this is supported by the ridge and the connection up there, 375 lbs. We can break out the simpson catalog and look at the specs on various connectors, my bet is a framing angle that'll take 5 nails on each leg will do it.

Quote from: Don_P on March 02, 2011, 08:50:58 PM

It looks like you used the AWC calc to get the design values... excellent for a 3 ply or greater beam.

Thanks.  Yes, it seemed to be a handy easy access source.     

Thanks Squirl, Mt. Don, and Don P., for all the info. This helps to start filling a gap in my education. I had build a 8x10 shed many years ago, using a Sunset book as a guide (and which had a couple of huge flaws). While the shed came out okay, I learned that I needed to know more about building if I wanted to do it again with a larger structure. Last year I participated in a small house build and learned a bit more, but had taken for granted the designs and methods used there as suitable to my own place. You guys have helped to remedy that assumption. That building was a bit larger than my design, but I don't think it was very well thought out, especially as to roof loads. I doubt that I'll not make any mistakes, but I hope to avoid the major ones. 
Thanks again.

I've altered my loft plan a bit. Not knowing whether I have access to some sizeable beams or, if I do, what they might cost, I moved the joists around so that I have pairs of them set apart by 2.5 inches, the pairs 2 feet apart on center. The idea is to eventually make them look like solid beams. The joists are set in joist hangers (Simpson HUS28, I think).

I've been boning up on the load tables, and see that it looks like I'd be okay using 2 x 8's on 16 inch centers. However, I'm not sure that still applies to my reconfiguration. Mt. Don, Don P or anyone else, how do you go about determining whether this is okay? Thanks.

I like the AWC online calculator for sizing joists and rafters.
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

I'm not sure what variables you used. The variables are, Species (use whatever is readily available in your local area). Size. Grade. Type of use. Deflection (see blow). Spacing. Live load (people, furniture and stuff in the structure). Dead load (the parts of the structure)

If I use Hem-Fir (common where I am), grade 2, 2x8, ceiling, L/360, 24" spacing, live 30#, dead 10#.......... I get a span of up to 11 ft 4 in.
2x6 and everything else the same is 8 ft 9 inch. If the 2x6's were spaced in double sets as you described I believe a well informed inspector might say that is would be fine. I'd be comfortable with that sizing.  Those results will be different with different species, some better, some worse. Use what is locally and readily available.

Examples of common code-prescribed deflection limits and live load values are:
   * Living room floors L/360 & 40 psf
   * Bedrooms and habitable attic floors L/360 & 30 psf
   * Attic floors with limited storage L/240 & 10 psf.
   * Rafters  L/240


That doesn't change anything with the roof.

Thanks, Don, for the confirmation. I'll probably stick with the 2 x 8's. And I've taken your earlier advice regarding the roof, changing the design to have the gable ends support a 14" LVL. I'll be up there in a couple of weeks to talk to the guys at the local lumberyard to get a materials estimate, as well as find a roofer and someone to put a driveway in. Then, all I'll need to do is pay for it all. Oy.

Went up for a day trip to check things out and meet with our driveway conractor. There's still an amazing amount of snow up there, and where there isn't snow there's a good amount of mud. The platform is still as level as it was last fall, though I won't feel confident that there was no heave until everything dries up. Some picts:

Dog flapping might help melt the snow


There's an entire picnic table under me


Yes, it is mud season

That's a lot of snow!  Hopefully, we had our last snow last week    At least our snow melts right away but then we have all the mud, too.

Exciting news! Tomorrow we'll have a driveway and we'll be framing and sheathing in 2 weeks. The lumber is ordered and will be delivered a week from Thursday. I just hope we're ready. I went up yesterday to add more supports and play with the new generator. In the process, I discovered I made a pretty stupid error last fall. We should not have bothered to insulate or add a vapor barrier to the platform then. I had been blissfully optimistic that, with a deck and a tarp covering it all, no water would get in. Wrong. A lot of the insulation was merely damp, and air-drying was all I needed to do. But a portion was sitting in puddles, soaked. What was really surprising was the amount of water puddled up on top of the vapor barrier below the deck. How it got there is a bit of a mystery. Unless the plastic tarp is semi-permeable, or snow & rain managed to squeeze up under it and in between the deck and joists.

I'll have to pull the deck off next weekend again and add back some insulation. Annoying, but not too bad. I'm very glad we used screws to put it all down. Here are a few picts:

I think we are framing the same weekend, I was going to ask you and the family up for a BBQ house raising !!  , driveways can be very exciting, wish I had one. Trudy

Hey Trudy - we'd love to come by some time! Good luck with your build, and let's hope for good weather that weekend. And if you get down our way, stop by. And bring your carpentry teacher & students (we're all amateurs down here). 

I have found that the more I think I know the more I do not ! I am so happy to have someone helping me that knows how to build a house !! Did you get that water cleaned up? I think all the Vermonters should get together for a big BBQ at the end of the season  

The Windham county contingent is working this weekend, too. We'd support an end of year b-b-q!

Hurray! We have a driveway! Now we're all set to have the lumber delivered this Thursday, with the build next weekend. It's going to be a busy week. Pictures and possibly a time-lapse video after.

The driveway:

Looking good 

Well, we're part way there. The weather wasn't very cooperative, but my friends and familiy were. Here are some picts:

Lumber delivery:


Pre-building headers with rigid insulation, and creating a temporary addition to the deck (12' walls, 10' deck, needed some room):


One wall up, with sheathing:


Two walls up:


Three walls up:


Fourth going up:


All up:


Cap rail on:


Most loft beams in:


Some of the crew:


More sheathing on:


Green cube on the landscape:


Top of the future stairs loft beams:


As I left it yesterday:


I'm hoping to get a couple of strong bodies up there this weekend to get the ridge beam in place, though it doesn't look good right now. It'd be nice to keep most of the weather out. More next week.

Rich

Hey Rich you got alot more done than I did !!! Looking great and inviting !

Nice! 

This weekend's work, always less than hoped for, but we're got the big ridge beam up at least. We would have gotten the rafters up except someone (your's truly ) forgot to get joist hanger nails. Still, given the unseasonably hot, humid weather, we did okay. Some more picts:

More sheathing on the sides:



Two gable ends up:



Getting the ridge beam in:



Nothing sounded better than the plop when the beam went in:



This is about the time I discovered the lack of nails. D'oh!



Inside the loft view, with the highly expensive temporary roofing  :



More or less as we left it yesterday:



We'll be back up it 2 weeks, as I really need to heal my cuts, scraps, and bruises and rest my poor muscles.

-Rich

A little more progress this weekend, despite some awful weather. Went up on Friday, and I got started adding the remaining lookouts while the wife started building the porch deck. Between us, we got most of the rafters up and the deck all done. Saturday varied between a drizzle to outright downpour. Not real great for working on a roof. I did manage to finish the rafters, and added blocking between them. Also built the stairs, though I'm not entirely happy with how they came out. We had a really nice surprise in the afternoon -- some neighbors we had not met stopped by and invited us up to their place. Beautiful post and beam, with the beams made from their own trees. They used structured panels and had the wood stove going. It was so nice to warm up and dry out while sipping wine.

Today I finished the gable end framing, figured out how to get the fly rafters on by myself and actually managed to wrestle up some of the roof sheathing, before the rain started up again. No pictures of that as it's all covered by a tarp now.

First rafter up:



A bunch more:


Rafters and porch:


With rafters up, we could reposition the ladder:


The dogs seem to gravitate to the new deck:


The weather for much of the weekend:



Hopefully the weather will be better next weekend. I think I can get most of the roof complete if I get just one day without rain. Lets hope,

- Rich