Cabin Framing Help

[Starvin]

Hello!

I plan to use the Eastern White Pine on my property as framing material for my 24x24 cabin in NH (read: snow loads!).

I have found a few good resources on span tables, calculators, etc. (i like this one the best: http://www.awc.org/calculators/span/calc/timbercalcstyle.asp)

However, the calculator is limited to 2x sizes. I would like to toy around with bigger members for a quasi-post-and-beam construction.

Does anyone know of a sizing chart for eastern pine posts & beams?

[Don_P]

There are some tables in "Wood Structural Design Data" a free pub on the awc.org site
They also have "Supplement to the NDS,  Design Values for Wood Construction" It gives the allowable design strength values of many species, it is where the design values used in the calc you linked to come from.

More often though once you start playing with heavy timber those wussified tables aren't going to help with the problem in front of you, you'll need to understand a little more about what is behind them and solve the problem more manually.

Wood that is 2-4" thick is called dimensional lumber and has one set of design values and adjustment factors... on the calc you linked to, you first selected a species and grade. The calc looked up the bending strength, stiffness and shear strength of that lumber. Each time you clicked a box after that the calc multiplied the base design value by a strength adjustment factor. It then used the adjusted design value in working the problem and determining span.

When wood gets to 5x5" and larger you are into heavy timber, look further to the back of the supplement for design values for posts and timbers, or, beams and stringers. P&T is roughly square in section with the measure of adjoining faces within 2" of each other, generally posts. B&S is more rectangular in section, deeper than wide, generally beams. So species, then basic geometry of the cross section, then grade will give you base design values. Adjustments are fewer for heavy timber, I use just one. For timbers that experience snow loads increase base design value for bending strength, Fb, x 15% (multiply by 1.15), leave all other values at base design strength.

For eastern white pine these are the base design values for beams and stringers 5x5 and larger in #2 grade.
Fb-575 psi
E- .9 million psi
Fv- 125 psi
Fc perp 350 psi

My calcs are here;
timbertoolbox.com
click the "beams and columns" tab
The "simple beam, uniformly supported" is the most commonly encountered situation, a typical rafter or joist.

That was long winded, my finger's tired  have a go and if you need help holler with an example and I'll walk through it with you.

[Starvin]

Thanks Don, this is great information, but I have to admit it is a little over my head at this point. I am more familiar with traditional dimensional lumber span tables (i.e. what span are you trying to achieve @ what load = size of 2x you'll need).

I'd be tickled if you'd be willing to walk me through a few examples. Is it appropriate to ask for structural design help on this forum? I dont know what the policies are... If it is best for me to start a new "help me design" thread, I would be happy to.

In general, I am interested in building a 24x24, 1.5 story cabin on a pair foundation using rough-cut milled eastern white pine.

Given my access to a sawmill and plentiful timber, I was thinking it might be nice to do a quasi-post and beam construction with (4) 12x12 bays and supplemental 2x6 stud framing where needed.

Here is a rough schematic of what i am going for:

[Don_P]

Sure you can ask, just be aware that the answers are unqualified. Continue on this thread, we have the basic picture on the cover of the puzzle box view, now go back and draw in the frame on another layer and post that dimensioned sketch. While working on that find out what your local snow load is in lbs per square foot. If you are up to determining load on a post or beam include that, otherwise we'll talk through that as well.

I'll be blunt. DIY pier foundation, why bother sizing beams.

[Starvin]

I'll be blunt. DIY pier foundation, why bother sizing beams.

Thanks Don! BTW - I am not sure I understand this comment?

Seems the general consensus is 50-60psf for snow live loads. FWIW, I will have a metal roof.

I figure we would start at the beginning. See below. This is based on a 24x24 grid broken into 12x12 bays.

My first problem is how to address the deck... Option 1 or Option 2

Option 1:
6x6 pier posts
6x8x12ish beams
2x10x12ish joists


Challenge with this option is how to frame the corner 1st floor posts (6x6?)



Option 2:
6x6 pier posts
6x8x12ish beams
6x8x12ish joists

My challenge here is how to hang the joists. These will be rough-cut lumber, so I am not sure Simpson makes an applicable hanger...

w/ 6x6 posts


Love to hear your thoughts?

[Don_P]

A chain doesn't break at the strong link. A pier foundation is specifically called out in the code for engineering for a reason, they have a pretty dismal failure rate when put to nature's tests. I've spoken with an engineer who called that method a collapse mechanism. It doesn't matter how well you build the rest if the foundation fails. If you have more time than money there is plenty of building stone there, a 16" thick rubblestone foundation is not expensive but does take some time. A poured concrete or block wall foundation costs more but is faster.

I'm going to assume the load paths of the building consist of vertical posts stacked over the piers all the way to the top plates. Each floor and the roof is carried on beams that floor by floor carry their loads over to and then down the posts. In other words the main floor beams carry the main floor only, the second floor beams carry the second floor, that load is not carried through the walls down to the main floor beam... etc for the roof.

Let's start with the outer wall upstairs plate, the roof carry beam. I'm going to go ahead and assume there is a structural ridgebeam carrying half the roof load, each outside wall then carries 1/4 of the roof load. The posts are on a 12'x12' grid. A rafter delivers half of its' load to the wall, one side of the 12x12 grid, and half to the ridge, the opposite side of the grid. So the wall plate carries 6' of roof width up and down the roof + overhang, say 1', and it's span length is 12'... so the tributary area is 7.5' x 12', 90 square feet. You have a 60 pound per square foot live load plus a 10 psf dead load, the weight of the roof itself. 90 square feet X 70 pounds per square foot = 6300 lbs on a 12' long beam.

Now we have enough info to go play "what if" on the beam calc
http://www.timbertoolbox.com/Calcs/beamcalc.htm
Enter 6300 for load
Span 144
width 6
depth 8
Fb 661  (this is sized for carrying snow so we take the base design bending strength of 575psi and multiply by 1.15 to account for the short term load duration)
E .9
Fv 125
click "show result"
It should fail spectacularly. I come into range with an 8x12 as about the most efficient size.

Let's jump up and look at the ridge, it is carrying 6'x12' from the rafters on each side of it...144sf x 70psf=10080 lbs, plug that in for load, span is still 144", the design values remain the same, find a beam size 

I'll let you digest this piece of the example, it should give enough to ruminate and possibly modify before going deeper. The material is relatively low in strength, the spans are fairly long and the loads are fairly high. You can adjust material choice or span (post spacing) at this point if the section size is getting too large.

[Starvin]

A chain doesn't break at the strong link. A pier foundation is specifically called out in the code for engineering for a reason, they have a pretty dismal failure rate when put to nature's tests. I've spoken with an engineer who called that method a collapse mechanism. It doesn't matter how well you build the rest if the foundation fails. If you have more time than money there is plenty of building stone there, a 16" thick rubblestone foundation is not expensive but does take some time. A poured concrete or block wall foundation costs more but is faster.

Interesting, something to consider, though I see plenty of cabins on this site and elsewhere built on sonotubes. It's not an a-typical method of building pole barns in New England. I have ledge at my cabin site so I am hopeful that I can pin the piers into the ledge. Either wa, thanks for the advice!

A rafter delivers half of its' load to the wall, one side of the 12x12 grid, and half to the ridge, the opposite side of the grid. So the wall plate carries 6' of roof width up and down the roof + overhang, say 1', and it's span length is 12'... so the tributary area is 7.5' x 12', 90 square feet. You have a 60 pound per square foot live load plus a 10 psf dead load, the weight of the roof itself. 90 square feet X 70 pounds per square foot = 6300 lbs on a 12' long beam.

This is where you lost me. Where does the 7.5' come from? 6' roof + 1' overhang = 7', unless there is a factor you put on it?

Now we have enough info to go play "what if" on the beam calc
http://www.timbertoolbox.com/Calcs/beamcalc.htm
Enter 6300 for load
Span 144
width 6
depth 8
Fb 661  (this is sized for carrying snow so we take the base design bending strength of 575psi and multiply by 1.15 to account for the short term load duration)
E .9
Fv 125
click "show result"
It should fail spectacularly. I come into range with an 8x12 as about the most efficient size.

Can you explain where you get these (2) figures? Thanks!


I think a possible break in communication could be my "post and beam" comment. I have no inclination of having the post/beam structure support all the loads without help from some supplementary stud framing, as well as some bracing.

Maybe the best idea is to get some more of my ideas on paper and available for critiquing.

REALLY appreciate the help!

[Don_P]

Do a thought experiment. Take a pole barn, the posts running unbroken from a footing up to the top plate. Unsheath the bottom 2 or 3 feet stripping the frame to just the posts in that band between ground and sheathing. Cut the posts off at ground level and nail them back together. Then cut the posts off just below the sheathing and again nail them back together. Apply wind laterally to the building walls. Now look at what you have drawn and what a few misguided DIY'ers on the internet have done. I have no objection to post framing, but that should be done correctly as well. If the examples you are pointing to were a valid method of building, contractor's, always wanting to keep the bid down, would be using it widely. We aren't seeing that. The main problem is that it does not deliver the lateral loads safely to the ground, it instead collapses under design loads. We can design the rest of the building and ignore that problem, but there is the failure. If there is ledge, the footing is essentially there, a rubblestone wall pinned to it would do the job.

I think a possible break in communication could be my "post and beam" comment. I have no inclination of having the post/beam structure support all the loads without help from some supplementary stud framing, as well as some bracing.

This is sort of all wrapped up together, it hinges more on your pier and beam decision. If you deliver the roof load and the second floor load through stud framing to the perimeter girder supporting the main floor, which is spanning 12' and do so using white pine for the girder it will be in the neighborhood of a 12x14. I was describing supporting the loads from the roof and getting them over to the posts, then collecting the loads from the 2nd floor and getting them over to the posts and finally taking the main floor load over to the posts, thus keeping the beam dimensions smaller by supporting each load as it is generated. If the loads come down the building to a full perimeter foundation wall, the loads are uniformly supported along the length of the wall instead of on perimeter girders. Piece of cake, less bouncy because you don't have joists and girders bouncing.

Yes trib width should have been 7' in my example, no adjustment going on there, just a brain fart. Rework the math using actual numbers from your plans. I can tell I poured concrete in the sun before writing that 

E .9
Fv 125
click "show result"
It should fail spectacularly. I come into range with an 8x12 as about the most efficient size.

Can you explain where you get these (2) figures? Thanks!

Reread my first response to this thread on the design strength values of white pine. These are the allowable material "strength" values for this particular species and grade of wood. Those are the design values for stiffness and horizontal shear strength.

Those are the numbers for #2 EWP, look at some other woods and strengths to get a sense of things. It is easier to get a #1 in northern hemlock due to knot structure. As the adelgid wipes them out this species will largely disappear but there is alot coming out of our forests right now.
so in a good relatively clear, small random knotted, non shakey #1 northern hemlock, or tamarack, beam or stringer
Fb 1120
E 1.3
Fv 165

And then for highest strength wood product beams... like the structural ridgebeam, LVL's come in at
Fb 2800psi
E 2.0
Fv 285 psi

Plug those numbers in and you'll see how that affects beam dimensions. Basically your girder spans are exceeding material strength unless you want to hoist big honkin wood, which don't get me wrong, I've got the pics to prove I'm not opposed to medieval dimensions  .

[Don_P]

Keep playing with it, can you keep the joist/rafter span direction at 12' but get the beam spans down to 8' instead of 12? Beam dimensions would probably start to come into the 8x10 range.

[Starvin]

All good info Don. I see where your issues with pier foundations make things complicated. You are right, most of the pole barns or equipment sheds around here that are build entirely with 2xs (sistered to make beams in some instances) sit on haunched Alaska slabs where the weight is more distributed.

I wonder how much sense adding more piers would make. At least more under the (2) exterior walls that bear the rafters. In other words, instead of 12' spans, with 3 piers, I could do 6' spans with 5 piers?
However, I would still have a problem with my ridge beam. Going to 6' spans throw a lot more posts into my open floor plan.

Another option would be to go with (4) bents at 8' instead of (3) at 12'.

Another option would be to buy LVL's for the load bearing beams.

What I really need to do more than anything else is spend some more time up there with the backhoe to explore the subsurface of the site to see how much ledge is there, how deep down, etc. Maybe I will get lucky with some good ledge close to the surface that I can pin a concrete "frost wall" into. I could build the forms myself to keep the cost down.

I made some really good progress on the SketchUp this weekend. Though it sounds like this conversation will change some things, it was good to discover some of the challenges i will be faced with.

[Don_P]

I save each version in skp with a date in the filename, preserving the previous versions just in case you need to backup and head another way. Plus it gives some sort of perverse bragging rights, I got up to 90 some revisions on one  .

[Arky217]

Hello!

I plan to use the Eastern White Pine on my property as framing material for my 24x24 cabin in NH (read: snow loads!).

I have found a few good resources on span tables, calculators, etc. (i like this one the best: http://www.awc.org/calculators/span/calc/timbercalcstyle.asp)

However, the calculator is limited to 2x sizes. I would like to toy around with bigger members for a quasi-post-and-beam construction.

Does anyone know of a sizing chart for eastern pine posts & beams?

Don has some very good points on building with a pier/post foundation.
Even though that's the way that I built,
I readily admit that it certainly is not the optimum way to make a foundation.

If you're going to go with pier/post, here's my 2 cents on the matter.

First of all, make sure that your soil is such that it will support the weight of the house
spread out over the number and sq.ft. area of the piers.
( A finished house weighs a lot ? )

Make sure that you go down below the frost level,
and that the piers are larger at the bottom than at the top.

For stability, either have the posts so short that you don't need bracing,
or have them long enough to get a good stout angle on the bracing.
( I definitely prefer the latter, else how are you going to get under the house. )

Make sure that your posts are large enough for the type of wood that you'll use
so that they will adequately bear the load of the house.

Make sure that everything is tied together well;
piers to posts, posts to beams, beams to joists, joists to walls, walls to roof.

Use more piers, not less; spreads the weight out more and reduces span of the beams.

Below is a picture of my foundation before adding the end beams and the joists.
The house size is 24'x48'; there are 36 piers, 4 rows of nine each.
The piers are 10" at the top, 18" at the bottom and are 20" below grade.
The posts are 6"x6" milled from the red heart of eastern cedar;
they range from 28" to 34" tall.
The beams are 6"x8"x12' milled from yellow pine; they span 6'.

Good luck to you on your cabin,
Arky

[Starvin]

Bumping this to the top. I am looking at two options:

(#1) - "Grade Beam" style frost wall foundation to support the 2 perimeter walls that carry the rafter loads
Plus LVL ridge beam with pier foundations on the interior
I figure this is a cheap alternative to a full perimeter frost wall foundation

(#2) - LVL ridge beam + LVL beams to support the 2 perimeter walls that carry the rafter loads
All set on piers.

Welcome any thoughts/criticisms.

Best value option?

[Don_P]

I've been through #2 enough times. #1 has shear strength along its' length and can resist loads hitting the gable ends but is unbraced if the load comes from the eaves sides... it is able to overturn  if the wind hits it from that direction, or any direction other than directly into the gable end. If you do go that route, this is outside of prescriptive, turn the corners for at least 4' or so to buttress the walls from lateral loads.

[Starvin]

Don, can you clarify what you bean by "turn the corners" in that last sentence. I am not sure I follow. Thanks as always for your help!

[Starvin]

Upon a second and third read, it sounds like you mean to return the grade beam foundation walls along the gable ends at least 4' as a sort of bracing?

[Don_P]

Bingo
When the walls recieve a lateral load (the wind blowing on them out of plane, hitting the eaves side) the return wall on the upwind side is in compression, a buttress wall. The lee return wall is in tension, a counterfort wall. Masonry has no tensile strength, rebar running diagonally from the top of the long wall down to the bottom of the far end of the return wall makes a good tension strap. That rebar effectively doubles the overturning resistance of the foundation.

Disclaimer, there are things I sometimes feel obligated to say educationally because of my background. Sometimes I repeat myself. Some of you probably know that when you do get called an expert they sit you down in a chair and beat you with it. That wasn't particularly aimed at anyone just puttin it out there.
It is probably a misnomer to call this a grade beam foundation, these are two independent walls. When you are driving around in a hilly area notice the retaining walls around properties. The ones that are simple flat walls retaining several feet of soil are often leaning. The soil is providing a lateral load, in this case the wall above with the wind acting on it is providing that lateral load. You'll see some walls that have buttresses and if well done those walls resist the soils' thrust and remain vertical. Usually you cannot see a counterfort, it is underground on the uphill, tension, side of the wall. Our's would be mostly visible and rectangular.

The return walls do help resist the out of plane loads but nothing is going to do that like a complete wall on all 4 sides, under all lateral loading conditions the walls then can resist the loads in shear, along their length rather than out of plane. That creates a strong foundation. In other words, what I suggested isn't right, but it is better than just two flat walls.  A continous footing helps the walls work together, keeps any settlement of the building uniform, doors and windows working properly among other benefits.

[Starvin]

Did a little poking around with the backhoe this weekend. What you see is the proposed site for the cabin with the gable ends facing to and away from the camera. As you can see, there is some ledge that I am going to try and take advantage of for the footings/foundation.