Author Topic: MO 20X40 Cabin  (Read 1800 times)

0 Members and 1 Guest are viewing this topic.

Offline retiredmarine

  • Junior Member
  • **
  • Posts: 38
  • Welcome to the CP-Forum
MO 20X40 Cabin
« on: September 14, 2017, 02:00:41 PM »
Back again, I had to postpone the buld for a year but it's that time.  After listening to Don_P I've changed from a post and beam foundation to a full footer.


Sill plate and center girder placement


and lastly (for now) an elevation view


Comments welcome
« Last Edit: September 28, 2017, 02:41:35 PM by retiredmarine »

Offline MountainDon

  • Administrator
  • Journeyman
  • *****
  • Posts: 16,508
  • Jemez Mountains, NM; that's pronounced HEY-mess
    • My 15.75 x 30 Jemez Cabin
Re: MO 20X40 Cabin
« Reply #1 on: September 14, 2017, 02:39:22 PM »
PB does not let you use their hosting to post forum images anymore, so all we see are placeholders.  There areother services such as imgur, imgbox and many others. We here at the forum don't have a recommendation as we have no control over what they may do if you use them for image hosting.

Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline retiredmarine

  • Junior Member
  • **
  • Posts: 38
  • Welcome to the CP-Forum
Re: MO 20X40 Cabin
« Reply #2 on: September 14, 2017, 03:20:07 PM »
Well that didn't work and the office blocks imgur... guess it will have to wait until I get home.
« Last Edit: September 15, 2017, 05:25:10 AM by retiredmarine »

Offline Don_P

  • Journeyman
  • *****
  • Posts: 4,084
Re: MO 20X40 Cabin
« Reply #3 on: September 15, 2017, 06:28:04 AM »
The pics are coming through here. Looking good  :)
Comments;
Get the floor up out of the ground 18" or more to avoid having to use treated joists. Don't forget to put down a vapor barrier on the ground. I like grade in the crawl to be above outside grade or positively sloped to drain to avoid any possibility of pooling, a pitchers mound. Sand under the plastic or a nice smooth grade is more comfortable and better for the plastic when you do have to go under, remove all organic matter, nothing to attract termites under there. I like a switch near the access (you do need an access somewhere) and a couple of lights near the perimeter (vibration of the floor in midspan kills bulbs faster) and an outlet under there in case you need a dehumidifier, fan or tools.
 
You can reduce the footing depth from 24" down to as little as 8" thick, we usually pour 16" wide to allow more room to center the block and give a better footprint. From there you can go up with block which is cheaper but your way certainly works.

There is no need structurally to install the simpson post base ($) under the center girder. If the lateral gets past your perimeter foundation to this point the house is already gone. Again nothing wrong with your way. I usually tapcon some angle clips from post to footing pad and positively attach to the girder to keep it from being knocked. Typically the rim joist is flush over the mudsill and attached to it, this allows the exterior wall sheathing to be nailed down the wall, over the rim and to the sill keeping a continuous uplift and lateral load path to the foundation anchors. If high wind I'll wrap straps from the underside of the sill out and up over the rim to the studs, coastal or ridgetops.

Not sure how you are doing the porch girders but do not simply hang them on the posts on fasteners, make sure they are bearing on wood or masonry not hanging on nails or bolts, the girder tends to split under the load or shear off if hanging on the sides of the posts.

Offline ChugiakTinkerer

  • Senior Member
  • ****
  • Posts: 437
  • Building a remote cabin in Alaska
Re: MO 20X40 Cabin
« Reply #4 on: September 16, 2017, 10:36:08 AM »
Yeah, I can see the picture as well.  Unfortunately all the image links from your prior posts are showing the Photobucket not-gonna-happen image.

Have you got a link to some of the design details?  I'm curious how you will be supporting the roof.  Will it be trusses or rafters, and if rafters will there be a structural ridge?  If you use trusses or a non-structural ridge, all the load from the roof gets carried by the exterior walls and distributed into the continuous foundation.  In that situation the interior girder supporting the floor only has to meet the requirements in the IRC Table R602.7(2).  Assuming I'm reading that correctly that means a central girder of two 2x12 will span 8'-1".  If on the other hand you're planning on having a ridge beam supported by a load-bearing interior wall or some columns, then you'll want to size the girder and interior footings to match.
My cabin build thread: Alaskan remote 16x28 1.5 story

Offline retiredmarine

  • Junior Member
  • **
  • Posts: 38
  • Welcome to the CP-Forum
Re: MO 20X40 Cabin
« Reply #5 on: September 17, 2017, 04:02:31 PM »
Ok let me catch up on all the great feedback.. 

18 inches huh? right now the joists are 17.5" off the ground, not going to lie - that may need to be close enough.. lol  I hear you about the crawlspace and I completely forgot to add an access point.. lol.. next plans update.  I chose to pour 24 inches of concrete due to ease, but tomorrow I'll do the math and see if the cost savings is worth the time it will take to lay block.  The ties in the middle are not simpson, in fact they are just heavier strapping ties I'm going to bed in the concrete.  Porch girders are shown below, no hanging them they will be on the block and then on porch girders fully supported.






ChugiakTinkerer...  More pictures coming.. Here's the roof trusses...

 


 
Full house pictures




« Last Edit: September 28, 2017, 02:42:36 PM by retiredmarine »

Offline ChugiakTinkerer

  • Senior Member
  • ****
  • Posts: 437
  • Building a remote cabin in Alaska
Re: MO 20X40 Cabin
« Reply #6 on: September 18, 2017, 02:00:26 PM »
Nice, I like those scissor trusses.  Definitely want a boom truck to help with installation though!   :D

Bear in mind I'm just a guy on the internet, but it appears to me that since the center girder only has to support the floor and not the roof, then you can get by with a built up beam made of two 2x12.

My cabin build thread: Alaskan remote 16x28 1.5 story

Offline retiredmarine

  • Junior Member
  • **
  • Posts: 38
  • Welcome to the CP-Forum
Re: MO 20X40 Cabin
« Reply #7 on: September 18, 2017, 04:25:27 PM »
145 lbs per....   :o

I'll relook at the tables for the the center girder, thanks!

Running both AC and DC circuits throughout the house.  I'm on the grid, so I'm going to let the solar run the DC circuits, I have several DC items to run, CB radio, HAM radio, a few lights and of course several chargers.  I'll let all of them vampire off of solar and batteries.



Outlets below, already built. (the standard outlets are wired for 12 volt too.  I'll mark each of these covers so everyone knows they're 12 volt..





AC plan also



Offline Don_P

  • Journeyman
  • *****
  • Posts: 4,084
Re: MO 20X40 Cabin
« Reply #8 on: September 18, 2017, 06:43:03 PM »
The porch joists... you're calling out 4x4's, no go, a 2x6 is stronger and stiffer than a 4x4. The 4x4 doesn't make it in deflection at 16" or 24" centers, fails in bending also at 24. The 2x6 does work at 16" but not at 24", at 24" the porch joists would be 2x8. But I do prefer a slight, 1.5" step down from subfloor to top of deck surface... a treated 2x slipped under the door thresholds. I would not notch through the rim, I prefer to use treated for the rim at porches and decks and joist hangers.

In your joist layout sketch you keep stepping forward with the joist laps from top to bottom in the sketch making the 1st lower bay large, lap forward and then back. Block over the girder.

In the truss print notice the 2 runs of bracing on the webs, don't forget them. I'd have them make the gable trusses with the top chord dropped 3.5" for overhang lookouts. Balloon frame up to the underside of the scissors on the gable ends to avoid a hinge in the gable walls. At 145lbs and no taller than they are, they can go either way on install but a truck would sure make for a more fun day.


Offline retiredmarine

  • Junior Member
  • **
  • Posts: 38
  • Welcome to the CP-Forum
Re: MO 20X40 Cabin
« Reply #9 on: September 19, 2017, 04:15:54 PM »
No 4X4's?  Funny I got that idea and the notching from Larry Haun's House building series.  start at the 7:20 min mark.  I liked the 4X4 idea because it gave me a good step down from the door.  I wanna make sure if the snow blows it doesn't build up at the door with only a 1-2 inch lip.  Think it makes a difference that I'm using 2X8 for decking?  So you don't mind joist hangers on the house side as long as the porch side is sitting on the posts and not hung off them huh?  I could use 2X6 and hang it and just make the columns a bit shorter to get the step down I'd like.  I see my lapping error, I meant to show one end falling to one side of the 24" center line and the other end falling to the other side of that same line.  I need to read your truss comments a bit more closely before I tell you I get them...

got it on the bracing...I'm not tracking with you on the gable truss modifications....

Offline Don_P

  • Journeyman
  • *****
  • Posts: 4,084
Re: MO 20X40 Cabin
« Reply #10 on: September 19, 2017, 05:30:54 PM »
The dimensions of the 4x4 joists would not meet min code strength or stiffness, the notch invites water into the framing, I don't see it.

The other name for a joist hanger is a "bucket", there is a metal strap under the joist. The nail attachment of the hanger to ledger in shear has been well tested. I wouldn't be keen on toenailing the joists to the rim or house ledger. I do prefer compression to shear, you could build up pilasters when laying the wall to periodically support under the ledger at the house. The ledger attachment tables are the cheaper known way to make that connection but I'll cheer you on if you make the porch self supporting.

There is a heavier hanger for a girder on the side of a post, just another way to think about doing things. Just like with the joist hangers the connection strengths are all supplied, you can figure the load and provide known resistance.

Ex. A joist spans 7.5' they are 2' apart, the total design load is 50 lbs per square foot...
7.5' x 2' = 15 sf x 50 psf= 750 lbs. The hanger at each end carries half of that 375 lbs. The hanger rating for the joist need to be >375 lbs in vertical shear.

The girder span is 8', it is carrying half of the joist 7.5/2= 3.75' x 8' x 50psf= 1500 lbs uniformly loaded on the girder. The definition of a girder is a beam that carries other beams. This girder collects the load from several joists and delivers it to the porch piers. A hanger for a girder would need a higher rating than a joist hanger.


Offline retiredmarine

  • Junior Member
  • **
  • Posts: 38
  • Welcome to the CP-Forum
Re: MO 20X40 Cabin
« Reply #11 on: September 20, 2017, 04:11:42 AM »
Well hangers on the house side make things easier, I won't have to do all that notching.  I'll go with 2X8's 24" OC and hang them on the house side and then adjust my porch columns down.  I can't get to my picture hosting site from work so I'll post a pic tonight.  Thanks for the detailed explanation!

Offline retiredmarine

  • Junior Member
  • **
  • Posts: 38
  • Welcome to the CP-Forum
Re: MO 20X40 Cabin
« Reply #12 on: September 20, 2017, 01:27:45 PM »
Ok new footer/wall picture - with openings





« Last Edit: September 28, 2017, 02:43:27 PM by retiredmarine »

Offline Don_P

  • Journeyman
  • *****
  • Posts: 4,084
Re: MO 20X40 Cabin
« Reply #13 on: September 20, 2017, 07:44:44 PM »
This is the typical detail I usually use;


There is a flashing behind the ledger that extends up the wall, can be a rubber membrane, metal, tarpaper at min.
There is another flashing that runs down the wall over the top of the ledger under the deck boards. The housewrap laps over the flashings, keep directing water out. I would check the joist hanger out carefully while lowering the ledger below where I've drawn it. The lowermost nails in the hanger need to go through the hanger and into wood. Into the sill IMO is fine. Tension perp to grain is disallowed, the hanger can't be somehow nailed to a dangling ledger in a way that makes the hanger want to split or tear the ledger. If you want that 3" drop off a 2x8 floor the 2x6 joists at 16" oc are looking better. Double sill or 4x6 sill if 2x8 might work.
« Last Edit: September 20, 2017, 08:23:30 PM by Don_P »

Offline retiredmarine

  • Junior Member
  • **
  • Posts: 38
  • Welcome to the CP-Forum
Re: MO 20X40 Cabin
« Reply #14 on: September 21, 2017, 03:24:47 AM »
Great graphic.  Using that line of thought couldn't I just use a 2X8 or 2X10 ledger board?  That would extend further down the block, It doesn't have to extend very far for the hangers.  That would give me plenty of nailing surface for both the hangers and to tie the ledger board into the rim joist/sill plate.  Looking at the measurements I only need 1.5" drop - the sill plate adds 1.5# to the house height.  Using your graphic, if your depicting a 2X8 ledger then I'd just use a 2X10 ledger, which would provide more than enough nailing surface for the hanger and still be solidly tied into the rim and sill.

On second thought (although I'm still interested in your thoughts on the idea above) looking through my notes I cant figure out where I decided that the porch joists had to be 2X8 to begin with.  Going with your last thought and using 2X6 joists gives me 1.75" drop and that combined with the door thresholds may be just fine.. just don't want a straight in shot from the porch without a bit of a rise.  Using 2X6 joists is also cheaper especially since I realized at the height of the ground they are I'd need to use PT - still cheaper.  So back to the plans PT2X6X8 joists 16" OC it is!





« Last Edit: September 21, 2017, 01:46:19 PM by retiredmarine »

Offline Don_P

  • Journeyman
  • *****
  • Posts: 4,084
Re: MO 20X40 Cabin
« Reply #15 on: September 23, 2017, 07:19:28 AM »
I think the 2x6 pt joist is the way to go but to answer the wider dangling girder question. if you go that route make certain that the lowest nail holes on the hanger will allow you to hit the rim or sill through the ledger. 2 reasons, first if you read the details on the hanger they are probably calling out a 10dx3" nail min, not the short hanger nails, so the nail will go into the house framing, yup through that lower flash... which is why vycor/ ice and water isn't a bad thing.. judgment call based on driven rain. simpson hanger screws, approved screws, are another possible alternative, check the specs on everything.

 second and very important always, wood sucks at tension perp. never put it in a situation where the connections pull wood apart across the grain... tension perpendicular to grain. there was a very bad fail on a 2nd floor restaurant deck several years ago. the ledger was attached to the framing by a row of bolts very near the upper edge to allow the ledger to drop down a step below the threshold. the hangers were nailed in below that bolt line. once it had about 50 people bouncing on it the ledger split along the bolt line and the deck dumped intact at the wall side. before you say a residential deck will never have those loads think of weddings, funerals and birthday parties. ours was packed with virgo's having a big birthday party last week.

which gets to that 200lb railing strength callout. one linebacker can push on a railing with 75 lbs before his feet slide out. he does have a 3' lever with that post though, nails would just pry out. 20 people leaning backwards against a railing produces much more and they will tumble backwards if it fails. porch posts are restrained top and bottom, a good thing. all just stuff to keep in mind as you build.
 
http://www.awc.org/codes-standards/publications/dca6
http://www.structuremag.org/?p=11302
http://www.structuremag.org/?p=1691

Offline retiredmarine

  • Junior Member
  • **
  • Posts: 38
  • Welcome to the CP-Forum
Re: MO 20X40 Cabin
« Reply #16 on: September 23, 2017, 09:26:35 AM »
Great resources, thanks!  Yup the 2X6 is the way to go.  On a related topic I'm wondering how much load bearing the outside posts (the ones on the porch) do for the roof? I'm thinking the walls is where all of the load is transferred to.  If that's the case then I can add the porches and columns after I get the place closed up.  Thoughts?  Also how about going back to your comment concerning the gable ends

"I'd have them make the gable trusses with the top chord dropped 3.5" for overhang lookouts. Balloon frame up to the underside of the scissors on the gable ends to avoid a hinge in the gable walls. At 145lbs and no taller than they are, they can go either way on install but a truck would sure make for a more fun day."

I don't understand that..

Offline akwoodchuck

  • Full Member
  • ***
  • Posts: 120
  • Homer, AK
Re: MO 20X40 Cabin
« Reply #17 on: September 23, 2017, 10:53:41 AM »
Also how about going back to your comment concerning the gable ends

"I'd have them make the gable trusses with the top chord dropped 3.5" for overhang lookouts. Balloon frame up to the underside of the scissors on the gable ends to avoid a hinge in the gable walls. At 145lbs and no taller than they are, they can go either way on install but a truck would sure make for a more fun day."

I don't understand that..

This is the dealio he's talking about....2x4 lookouts on edge cantilevered over the gable truss...."balloon frame" in this context means run studs from floor to truss without another plate breaking the continuity.....any such "hinge" point makes the wall flex when the wind blows....not a pretty sight on a dark and stormy night....


Offline Don_P

  • Journeyman
  • *****
  • Posts: 4,084
Re: MO 20X40 Cabin
« Reply #18 on: September 23, 2017, 03:14:07 PM »
look at your truss print under "reactions", the porch gravity load at joints 2 and 8 is 321 lbs AT DESIGN LOAD. the walls support 1238 lbs at each truss at 10 and 14. my gut says that is safe as long as you support it before winter. the truss tech can run the scenario on it easily. worst case you install the porch roof support beam under the trusses and install angled props to grade outboard of the porch framing.

ak got it on the dropped gable end truss. your bottom chord should mimic the cathedral ceiling, then the studs run from floor plane unbroken up to the sloped ceiling where there is again a plane of lateral support.

Offline retiredmarine

  • Junior Member
  • **
  • Posts: 38
  • Welcome to the CP-Forum
Re: MO 20X40 Cabin
« Reply #19 on: September 23, 2017, 05:00:12 PM »
Amazing how much useful information is on those spec sheets.. lol...  I might as well install the posts form the start before the trusses that way I can get easy access to the column and the post tie.

It took two of you (thanks!)but I finally get what your talking about with the gable trusses.  The order sheet has different trusses for the gable ends but no picture, I'll make sure and check with them on the design.. 


Offline Don_P

  • Journeyman
  • *****
  • Posts: 4,084
Re: MO 20X40 Cabin
« Reply #20 on: September 24, 2017, 07:18:46 AM »
in aks pic, the overhang lookouts are cantilevered from the first inboard common truss out over the top of the dropped gable truss with blocking installed between lookouts on top of the truss top chord. the flying rafter would be next. I normally brace the gable plumb and straight, install the lookouts long, then snap a line and cut them in the air. you can also install the top and bottom lookouts, pull a string and measure for each one.

as you install the lookouts have a 10-12' straight 2x4 that you can lay on top of the lookout and back across several common trusses. you are checking that the lookout is crossing the custom gable truss at the correct elevation to sit in plane with the main body of the roof. the lookout fly end should neither rise above nor drop below the plane of the roof. this can require slight shimming or trimming of the lookout as it crosses the dropped gable truss. small errors between the common and gable trusses either at plant or during installation magnify here... which also happens to be one of those places the eye comes to rest on. its not a fun place to work but is worth taking time on.

you can also omit the gable trusses entirely and just balloon frame the gable wall up to lookout height, tall studs though. in that case you would nail a 2x4 ledger across the inside face of the full height studwall that provides a nailer for the inside ceiling plane. fireblock that at the intersection of planes, that was the downside of balloon framing, it is easy to leave flue channels in the walls, compartmentalize that. I think I remember ak described lofting one out on the floor in a thread recently.
« Last Edit: September 24, 2017, 07:35:29 AM by Don_P »

Offline akwoodchuck

  • Full Member
  • ***
  • Posts: 120
  • Homer, AK
Re: MO 20X40 Cabin
« Reply #21 on: September 24, 2017, 03:55:40 PM »
you can also omit the gable trusses entirely and just balloon frame the gable ceiling plane. fireblock that at the intersection of planes, that was the downside of balloon framing, it is easy to leave flue channels in the walls, compartmentalize that. I think I remember ak described lofting one out on the floor in a thread recently.

Yep love my "life-size blueprints"... [cool]....makes it a breeze to go back and double- or triple-check everything....came to that realization after seeing a whole crew standing around waiting to frame great room gables...while two guys with construction calculators in their hands yelled at each other....lol

Offline retiredmarine

  • Junior Member
  • **
  • Posts: 38
  • Welcome to the CP-Forum
Re: MO 20X40 Cabin
« Reply #22 on: September 25, 2017, 05:10:41 AM »
What a great way to visualize something.  Now you have me wondering if I need/want gable trusses at all.  Another question about the porch support beam.  What do you think is my min size to use there with those posts 8' OC?
« Last Edit: October 06, 2017, 08:44:00 AM by retiredmarine »

Offline retiredmarine

  • Junior Member
  • **
  • Posts: 38
  • Welcome to the CP-Forum
Re: MO 20X40 Cabin
« Reply #23 on: October 06, 2017, 08:51:08 AM »
Truss update.  New design - The 8' segment over the porch is now cantilevered eliminating the need for support.


Also decided not to drop the outside truss.  Can't think of a reason to I'd want or need the overhang.

Offline Don_P

  • Journeyman
  • *****
  • Posts: 4,084
Re: MO 20X40 Cabin
« Reply #24 on: October 06, 2017, 01:15:06 PM »
protects the siding , window and door flashings. you'll understand in the fullness of time.

If it were me I would not use that gable truss... and I'd light up the tech. He is drawing an unbraced hinged wall. If you are going to do it Cape Cod style put a common truss on the ends and balloon frame to the ceiling chord of the truss or balloon frame the full gable wall. On that wall you have points of lateral support at the floor, ceiling and roof sheathing planes. Studs run unbroken between points of lateral support such as floors, ceilings and roofs. He has drawn it broken at 8' with his gable truss, there is no point of lateral support in this vaulted end at 8', it is free to buckle in the wind. In moderate wind it pops the drywall there. In pics after high wind that gable truss is laying on the ground beside the foundation and the roof is gone.