Small timber bent cabin design critique

[coryallenstaats]

Hello all! Fairly longtime lurker here. I'm building a small starter house, we call it the "cabin" or "barracks," so I can take my time with the design and build of the main house.

I have white/red oak, maples, ash, and hemlock that will be cleared and milled onsite for this build.

Due to topography and logistical constraints it will be a 13' wide by 60' long cabin. 13 bents 5' on center with connecting girts and purlins.

Based on my research and simple calculations the bent I've put together should be more than strong enough load-wise. But, I've never done this before so I'm interested in any feedback or suggestions on the design and raising strategy.

6x6 posts, 6x8 rafter, 6x8 king post, 4x6 scissor chords
12' clear span









Thanks!
Cory

[Ljohnsaw]

Impressive!

But why so tight of spacing? Where are you located, what's your snow load?

[Andries]

I will echo LJohnsaw, impressive for a first post!
 Throwing up some sketchup pictures and asking for advice and critique on your first post shows a lot of quality time spent lurking.
There's a lot of people here that will respond with an incredible amount of detail and care, but here's the prediction: it'll be in direct relation to how much you 'give' to the group.
You're looking for "advice on design and raising strategy."
How in the H (heck) would anyone here give you credible advice without knowing your working environment? Is snowload a factor?  do you have equipment, such as a crane, a forklift or tele-handler, etc. etc. How big is your crew: are they gnarly veterans or well-intentioned spandex specialists? 
I'm not some grouchy old man that's trying to bust your chops. I'm speaking on behalf of too many of us, that have spent wasted time on one-post wonders, trying to guess where they are and what they were trying to do. 
I'm curious to see what you'll do: I'm asking you to lean in.
Better info from you = better advice from this group. 

[Den-Den]

"More than strong enough"  Even without knowing the loads, it looks really strong.
Will you have straight and square timbers?  If not, this is a difficult design.  Do you have the skills to cut and assemble these bents?  If this will be your first timber frame experience, you have created quite a task.

[coryallenstaats]

Wonderful! I didn't want to come out of the gate with too much detail and over-impose. But, there's lots more detail to offer.

I am located in the Monadnock mountains of NH at about 1200' elevation. Ground snow load is 80 pounds per square foot.

The 5' oc bent spacing is because my wife doesn't want huge hulking timbers above her head every day. There is a local architect, First Day Cottage, who designs homes that owners can build themselves using 3 sandwiched 2x6's as "timbers" for bents spaced at 3' oc. She liked the feel of the space, but I don't like the look of all the 2bys. So, we've reached a compromise

Also, smaller bents means more likely we can raise 'em without needing a large crane. I have a Kubota tractor, but I'm thinking a telescoping handler will likely be needed for this job.

The 13'x60' floorplan is to stay out of the way of the subsequent main-house build. We have a relatively small buildable area between 2 brooks, wetlands and some steep slopes. We have been delayed with litigation due to a particularly obstructive neighbor, so our nest egg has dwindled due to cost of living increases, legal fees, and rent we didn't plan to be spending. We don't want to take out any loans. So we are building what we can afford to do now.

My crew is me, mostly. But, I do have some hearty friends I can call in as needed. I've got most of the tools I'll need; i.e. sawmill, chainsaws, most typical carpentry tools. What I don't have yet are the special ones like mortise-chainsaw, 12" portable planer or any square cut drills.

We'll be felling, milling and stacking green timbers onsite. Building bents on the floor and hoping to raise once all pieces are ready.

Pier and beam using at least 10" sono tubes, 10'oc under every other bent post. At least, that's my current plan. Currently working on the sills and main floor structure so nothing is set in stone yet.

My wife is a P.E. working as a building envelope consultant out of Boston. She has some colleagues in line to run the calcs on our structural frame, once it's designed, and give us a stamp so we don't make the Building Inspector's head pop.

We know it's a challenge and a lot of work, especially since I'm inexperienced. But, we are not ones to shy away from hard work worth doing. Even with this starter cabin, I'd rather build something that could potentially stand 100years than put up something cheap and easy.

I've been looking to connect with some local timber framers to trade my labor for learning the skills.

Let me know if I've missed any other details or if you'd like to see something else from the sketchup model.

Thanks!
Cory

[Ljohnsaw]

Well, you'll be hearing soon from @Don P ! You will get an education on your proposed use of pier foundation. 😉

[coryallenstaats]

Well, you'll be hearing soon from @Don P ! You will get an education on your proposed use of pier foundation. 😉

Looking forward to it. It's certainly not my first choice. We won't be able to afford the earthworks in the first phase that would allow a concrete truck to get up to the site. But, also we live in a hollow that's humid all the time with a high water table and my wife, the building envelope consultant, isn't keen on the success of waterproofing a slab foundation. But, could be something we haven't thought of yet

[coryallenstaats]

One thing I forgot to mention is that we do already have a State Approved septic design for our 6 bedroom waste water management system. So that's a big hurdle passed so far. And it's another reason why we're locked into our house location since it required state and local variances.

[Don P]

Stuff I'm thinking about
I'm always willing to learn, show me how to brace a 13' wide pier foundation when design wind load hits the 60' wall and roof. What is the bearing capacity of the pier on your soil vs the design load per post.
A couple of Ben Brungaber's powerpoints online show a scissor truss they did, the do's and don'ts, which you seem to be fine on at a glance. Anyway worth looking up if you haven't seen it. I've looked at the first day cabins and there is a lot to be said for the concept. I have done built up trusses and as long as everyone is on board I like them. Dimensional design values and repetitive member increases on top of that... and the wood is dry, triple win.

[coryallenstaats]

Bearing capacity of the piers on the soil? Good question! I'll have to look into that.

I am assuming the we'll take the piers down pretty deep unless we hit ledge first, which is very likely. In that case, we'll dowel into the rock and anchor the piers on top.

I'll check out those PowerPoints. Thank you

[coryallenstaats]

I watched a presentation by Ben Brungaber and my IQ increased about 10 pts.

There was one subject that really caught my attention since my truss incorporates a sloped shoulder. Ben says that a sloped shoulder into the king post has potential problems since it may end up relying on friction.

Even though my truss is relatively small and will be utilizing a pegged tenon at that joint, I'm wondering if I should trade the sloped shoulder for the birds mouth notch like Mr. Brungaer advocates for.

Any thoughts?

[Don P]

It looks like you found the TFG's truss standard?

I can sit on either bucket and argue all lunchtime long  . I do a sloped shoulder with a stub tenon vs the potentially compromised re-entrant birdsmouthed corner. KISS. Notice the intentional slop in the upper portion of the birdsmouth. As that timber shrinks, if that plane becomes the bearing surface it will create a splitting potential on the inside corner of that notch. That gap is critical, and hard to make yourself do.

I think I have a pic or sketch of what I've done.



Looking at that white pine top chord and visualizing the amount of the timber compromised by the whorl of knots in that location, is it 1/4(#1), 1/3(#2) or 1/2(#3), just a grading thinking problem.

This was another one, I was carrying a hidden structural ridge on top of the king here but it shows the pockets I put in for that restraint. Shifting to the other bucket, that creates a splitting notch as well.




As far as looking at the critical joint, my bet is that bottom heeljoint. Have you quantified the tension vs shear area beyond the notch.

Is a scissor necessary vs kingpost trusses?

[coryallenstaats]

As far as looking at the critical joint, my bet is that bottom heeljoint. Have you quantified the tension vs shear area beyond the notch.

Is a scissor necessary vs kingpost trusses?

No I have not worked out the forces beyond the notch.

The scissor offers a bit more headroom so that we don't have a tie beam right above our heads every 5ft. The posts are only 7.5ft tall. But also we just like the feel of it better. I figure, no matter what, the first couple bents I put together will be challenging. But after that it's all repetition.

[coryallenstaats]

Looking at this photo, I realized that I will need to do something similar with a ridge beam and purlins. Since I'm planning to use 6x8 rafters, there's not enough width for me to flush mount consecutive purlins. I don't really want to stagger the spacing. I'm planning to use 2" board sheathing on the diagonal.

Can you explain how you fasten the purlins to the rafter in this photo?

Thanks!

[Don P]

I had a healthy bucket of both 3/8" x 10" lags and 10" fastenmaster type screws that I screwed the purlins and blocking down with. Poke out the blocking the other way, I've looked up to as many as 4 screech owls perched up there  .

[Hilltop366]

I'm thinking that if the bent timbers are just a bit bigger (8" x 8" ?) you could run 6" x 6" purlins long enough to do 4 or 5 bents. The bents will end up being around the right spacing for a natural division of rooms. They don't have to be evenly spaced as long as they are sized right for the longest span the rest will be good.

[Don P]

Are you limited by footprint, or roof area? At only 13' wide I'd be tempted to put quite wide overhangs on it if possible for an exterior "hallway" with dogtrot porch entrances to several pairs of rooms.

[coryallenstaats]

2 things. First, yes we are limited by the footprint and roof area in a sense. We want to leave room to build the main house and we are sandwiched in on all sides as you can see in this image.



The blue is the approximate area of the 13'x60' 1st phase cabin. The brown to the east is steep bouldery slope. The yellow is the actual intended path of the driveway, this is a draft detail that hasn't been updated yet. But, you can see that we have wetland setbacks or steep slope surrounding.

2nd, we will be attaching the eaves to the exterior because my wife has a particular vapor barrier plan in mind to keep the water out.

[coryallenstaats]

Also, I just spoke with the boss and she informed me that she doesn't like purlins. She wants to use 3x8 horizontal boards tongue and groove for the roof sheathing. "Like they do on Acorn Deck Homes."

So, that makes that whole thing much easier

[Don P]

How do those roof lines tie together?

I know Deck House and Acorn Structures pretty well. Go to the AITC website, American Institute of Timber Construction, under their publications I think they will have span tables for various decking spacings and layups. I suspect you can stretch the bent spacing if desired.

[coryallenstaats]

Yeah. I was running the calculations to make sure the 3x8 boards can handle the snow load requirements in a 54" span. And, turns out they are more than strong enough. Looks like even an 8' span still passes the deflection requirement for roof.

So we probably could increase the span between bents. I'll have to ask the boss what she thinks this evening. Spring time in New England means she's on site visits almost every day.

I don't always know if her preferences are based on aesthetics or structural grounds.

[coryallenstaats]

Does this kind of bracing under the structure help with the wind load issue on piers?
(it's a quick mock up. imagine the piers are on footings or anchored to ledge and no more than 2-3ft of concrete pier is above ground.)

[Andries]

Corey, a couple points from up in frost heave country.
The site plan is a challenge in that you don't have much room between steep rock, and wetland.
Your initial build is on piers and the secondary will be on concrete slab. The differential on how water and frost work on the different parts of your build, would be a problem in my mind. The two parts of the house would move differently with seasonal changes in ground frost. Also, when building on piers alone, try to keep the height above ground as low as possible. Use 16" sonotubes with an overkill of rebar. I would expect the ground to be "lively" in that area, so drilling to rock is advised, or to below the frost line at a minimum.
You have mentioned that concrete delivery won't be possible for the first part of your construction plans.
 There may be a 'mud mixer' in your future. 

[Don P]

I would not consider a 16" sonotube overkill at all, take a look at what it takes to keep a light pole up in a parking lot. They have very little wind resistance and are shorter than a house.

Is that one post, on a concrete post, on a footing... or a post, on a little post, on a concrete post, on a footing? Give the 60' wall a design load shove. You've braced what I assume is a stub of a post but nothing else hinged to it in the foundation is braced.

If you put a wall, X, panel of some sort, between the sonotubes then they are braced. If you don't have enough deep pile in the ground to avoid overturning it needs to pick up bracing some other way. Right now it looks like a vertical stack of hinged elements. Brace the stub post and the lateral load simply overturns the unbraced sonotube. The building code calls for engineered design there, and I think for good reason. Basically I'd do it prescriptively on a full perimeter foundation if possible or have it designed locally. Which for me boils down to where to put the money, into the foundation or split between engineer and foundation. Everyone's equation is different.

If the post extended from wall top to footing, all of those hinge points become one solid unbroken post. The walls above provide bracing.

[coryallenstaats]

All points well taken. Yes, we already have a structural PE who will be finalizing the design and providing a stamp. I'm trying to front load as much as I can to minimize the engineering hours. I like learning new things, so even if we go a completely different way it's not wasted time on my part.

I discussed changing the distance between bents with my wife and ultimately it's an aesthetic decision to use 5' oc bents. I'm going to model out more of the cabin and come back with some updates later on.

Thanks!