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jules4 07-07-2010 06:46 PM

Seeking advice on framing plan
 
Iím building a freestanding screen house in the woods behind my parentsí cottage, and Iím hoping those with some framing experience can comment on whether my framing plan is structurally sound. (I know how this may look being my first post and all, but this isnít just a drive-by posting - Iím actually very interested in getting some constructive criticism.)

My objectives in planning this are to minimize the amount of lumber used (since I have to carry it through the woods to the site) and to limit the number/complexity of cuts since theyíll be done on site using hand tools. My other objective is to avoid building something that is likely to collapse on my parents.

Additional details:
  • Iím using post-framing instead of the usual stick-framing, so none of the wall studs are load bearing.
  • Vertical sheathing joints are centred on rafters.
  • All lumber is 2&Btr SPF (and posts are ground contact).
  • The floor will be 3/4" waterproof G1S Douglas fir. The back wall and sides up to the first stud will be covered with lap siding, rest will be screened (see 3rd image).
  • Iíve dug the post-holes already and the posts will be set 1-3 feet deep, with concrete necklaces, on top of bloody huge rocks.
I've attached two different views of my framing plan as well as an image of what it will look like finished (minus roofing).

Julia

http://i569.photobucket.com/albums/s...ramingplan.jpg
http://i569.photobucket.com/albums/s...amingplan2.jpg
http://i569.photobucket.com/albums/s...creenhouse.jpg

Scuba_Dave 07-07-2010 09:00 PM

Looks good
I can't tell if you have (1) or (2) 2x6's as the cross beams front & back that the ceiling joists will rest on ?

Gary in WA 07-07-2010 10:29 PM

Add solid blocking over the bearing on-edge double beams between posts (to prevent rotation) and at the ends over the screening between over-hang blocks in the roof.

Sec. 2326.12.8. Blocking. Roof rafters and ceiling joists shall be supported laterally to prevent rotations and lateral displacement when required by Section 2306.7. Roof trusses shall be supported laterally at points of bearing by solid blocking to prevent rotation and lateral displacement.

Figure the size differently as you will waste 1 full sheet of roof decking in pieces. Use all p.t. wood with water-proofing applied and field painted on all cuts against the snow and rot. # 11, 12: http://www.ufpi.com/literature/ptfaq-204.pdf

http://www.thompsonswaterseal.com/advice/faq.cfm

Backfill with gravel against frost heave: http://www.waltersforensic.com/artic.../vol1-no11.htm

1/3 below is the ultimate: http://www.brainerdhomeinspection.com/frosth~1.pdf

http://books.google.com/books?id=q37...sult&resnum=10

Clear it with your Providence Building Department for safety.

Be safe, Gary

jules4 07-08-2010 10:22 AM

Quote:

Originally Posted by Scuba_Dave (Post 466672)
Looks good
I can't tell if you have (1) or (2) 2x6's as the cross beams front & back that the ceiling joists will rest on ?

Thanks - at the eves the rafters are sitting on 2x4 double headers notched into the posts (see detail in 1st image below).
Quote:

Originally Posted by GBR in WA (Post 466705)
Add solid blocking over the bearing on-edge double beams between posts (to prevent rotation) and at the ends over the screening between over-hang blocks in the roof.

Great point Ė I used 5/8 ply to get away from needing to using blocking between the rafters, but I totally overlooked the supporting beams/girts on the rake.

Those arenít actually double beams, theyíre single 2x6ís notched into the posts (and will be anchored to the posts with carriage bolts and to the double headers with lag bolts Ė see 1st image below). I thought single 2x6ís would be enough since theyíre only carrying the weight of the rake overhang, do you think I need to use double 2x6ís on each side?

Since the double header and posts provide lateral support at the ends, could I stabilize the rafter girts by adding 2 additional members as shown in 2nd image?
Quote:

Figure the size differently as you will waste 1 full sheet of roof decking in pieces.
Iíd love to increase the rake overhang, but Iím not sure itís safe to push the bargeboards out any further without adding a lot more reinforcement, and increasing the floor size would require another sheet of waterproof ĺ df flooring (more $$ than CDX).
Quote:

Backfill with gravel against frost heave:
In the soil Iím working with I worry that backfilling with such a different material would wind up creating more problems. The underlying soil on site is sand heavily cemented with iron oxide (ferric podzol) and is extremely hard and non-expansive. My concern is that using gravel would actually allow more water to accumulate than backfilling with the native soil.
Quote:

Clear it with your Providence Building Department for safety.
The local building department (or at least the guy I spoke to) doesnít approve of pole/post framing period. Luckily inspection isnít an issue where Iím building (youíre lucky if homes get inspected) so Iím proceeding with post framing anyway since they couldnít give me any good reasons for their disapproval.


http://i569.photobucket.com/albums/s...girtdetail.jpg
http://i569.photobucket.com/albums/s...rtblocking.jpg

Gary in WA 07-08-2010 04:56 PM

Without the additional roof purlins added, rafters spanning 5-1/2' x 60# (snow load) @16"o.c. = 80# per sq.ft. load A double 2x4, on edge, nailed together spanning 8', will support 47-54# per sq.ft.(Based on 1300-1500 fb) You need to upscale the beams to a single 2x6: 74# (1300 fb) or 85# (1500 fb) or better yet a single 2x8 = 98-113#, ON EDGE, not flat as in the drawing of 2- 2x4's.

Diagonal knee braces at the post tops would help racking of the front, screened only wall.

Be safe, Gary

jules4 07-08-2010 08:33 PM

Quote:

Originally Posted by GBR in WA (Post 467021)
Without the additional roof purlins added, rafters spanning 5-1/2' x 60# (snow load) @16"o.c. = 80# per sq.ft. load A double 2x4, on edge, nailed together spanning 8', will support 47-54# per sq.ft.(Based on 1300-1500 fb) You need to upscale the beams to a single 2x6: 74# (1300 fb) or 85# (1500 fb) or better yet a single 2x8 = 98-113#, ON EDGE, not flat as in the drawing of 2- 2x4's.

Point taken - it’s not just about rotation, the front double header is woefully inadequate.

If a single 2x6 would be enough I’d prefer that to a 2x8 (for aesthetic reasons) - however, an SPF 2x6 (#1 or #2) has 1140 fb, so would probably only support about 65psf (using the ratio of the values you provided). Given 1140 fb, would a double 2x6 (on edge) get me above 80psf?

P.S. I tried to replicate your calculations but I can’t figure out what measurement you’re working with. I understand where you’re getting the 5.5' span and snow load (60psf is dead on btw, assuming 10psf dead load, since the combined ground snow/rain load is 50psf). What I can’t figure out is how the distance between rafters comes into it or what the 80psf is measuring exactly (it obviously isn’t the load bearing on the surface of the header, which would be more like 1200psf). I’d much appreciate it if you could explain (I don’t doubt your math, I’d just like to know for my own interest).

Quote:

Diagonal knee braces at the post tops would help racking of the front, screened only wall.

Be safe, Gary
Will do, that's an easy upgrade.

Thanks,
Julia

Revised plan:http://i569.photobucket.com/albums/s...ramingplan.jpg

Gary in WA 07-09-2010 04:50 PM

The angle braces need to be twice that size and bolted to be effective: Fig. #22: http://www.lancova.com/deckinfo.pdf

A single 2x6 (SPF) at 1100fb = 63# at 8' span. Double would be fine.......

"What I canít figure out is how the distance between rafters comes into it or what the 80psf is measuring exactly (it obviously isnít the load bearing on the surface of the header, which would be more like 1200psf). Iíd much appreciate it if you could explain (I donít doubt your math, Iíd just like to know for my own interest)." ---------- Sorry about that, good catch! I carried the rafter spacing over from previously figuring the rafter loads. (If a 1' over-hang in back, that leaves) 3-1/2' OH in front, total load on the front beam is: 5-1/2' span + 3-1/2' OH = 9' joist span x 8' beam span = 72 sq,ft. x 60# = 4320# total load. This requires a 4x10 beam in front at 1100fb.
The back 1'OH + 5-1/2' = 6-1/2 x 8' x 60# = 3120# requires 2- 2x10's in back. Surprised no one else caught that..... Different if Doug/fir.

Be safe, Gary

jules4 07-09-2010 11:16 PM

Quote:

Originally Posted by GBR in WA (Post 467391)
The angle braces need to be twice that size

I was afraid you were going to say that.

There must be more attractive ways of preventing racking than putting 3 foot 2x4s across the windows. What if I half-lapped the braces into the posts and header so they were technically 3 feet long, but only 1 foot actually showed?

(This goes against my avoid-complicated-joints-due-to-lack-of-power-tools objective, but what the hell, chiselling probably builds character.)

Quote:

Sorry about that, good catch! I carried the rafter spacing over from previously figuring the rafter loads. (If a 1' over-hang in back, that leaves) 3-1/2' OH in front, total load on the front beam is: 5-1/2' span + 3-1/2' OH = 9' joist span x 8' beam span = 72 sq,ft. x 60# = 4320# total load. This requires a 4x10 beam in front at 1100fb.
The back 1'OH + 5-1/2' = 6-1/2 x 8' x 60# = 3120# requires 2- 2x10's in back. Surprised no one else caught that..... Different if Doug/fir.

Be safe, Gary
4x10?! Yikes - I hope not, that will look really awkward.

Okay, let's see, there arenít going to be any free-loaders on my roof, so those two 2x6 purlins I added wonít get away with just loafing around resisting the odd lateral force - I expect them to carry their fair share even if I have to tie them down (or up) to make them do it. So, I think you were right about the front joist span being about 5.5 feet - Iíd put it at 5' 8".

And then thereís the rafter girts themselves, theyíll take part of the load of the rake overhang, so the beamís really only bearing the load from a 7' 3" wide strip down the middle of the roof.
http://i569.photobucket.com/albums/s...ofloadcalc.jpg
So the front beam is carrying about 41 sq ft of roof, or 2460 lbs - is it safe to assume that 2 2x6's, spaning 8' at 1100fb could handle that?

Of course now I have to worry about whether the 2x6 purlins are up to the task . . .

Would the rafters tie the underlying beams and purlins together so that they function as a single unit in bearing the roof load (sort of like blocking under a floor does)? If they would then it would be okay if the purlins were undersized, provided the beams had enough bearing capacity to pick up the slack.

Julia

P.S. Thanks for generously allowing me to waste your time like this, Iím sure you have better things to do than fuss over beams in a piddly little screen house :jester:

Gary in WA 07-10-2010 03:31 PM

"P.S. Thanks for generously allowing me to waste your time like this, I’m sure you have better things to do than fuss over beams in a piddly little screen house" ---- Not a waste of time!!! It is great that someone has the incite to ask important and precise questions. It is our pleasure, that's why we're here!

You figured the loads, here is a helpful site I use so you can figure the rest: http://www.awc.org/pdf/wsdd/C2B.pdf

I can help to check your figures, if you don't mind posting them......

Be safe, Gary

jules4 07-11-2010 02:38 PM

Quote:

Originally Posted by GBR in WA (Post 467735)
"P.S. Thanks for generously allowing me to waste your time like this, I’m sure you have better things to do than fuss over beams in a piddly little screen house" ---- Not a waste of time!!! It is great that someone has the incite to ask important and precise questions. It is our pleasure, that's why we're here!

You figured the loads, here is a helpful site I use so you can figure the rest: http://www.awc.org/pdf/wsdd/C2B.pdf

I can help to check your figures, if you don't mind posting them......

Be safe, Gary

Cool link - thanks.

I’ll definitely post my calculations so you can check them - I think I’ll post them more gory detail than you’d need too, since seeing the details of how I did this might come in handy for someone else who needs to work out what minimum beam size they can get away with.

To use a double 2x6 beam in front I’d need 8' 2x6s capable of carrying 1230 lbs each which, according to your tables, would require an Fb of 2000psi (hahaha - that is so not happening with #2 SPF).

To use a triple 2x6 beam the 8' 2x6s would only have to carry 820 lbs each, which would require an Fb of 1300 (might be doable).

Time to get a bit more exact about the Fb values for my beam members. The allowable design value Fʹb = base value (Fb) x applicable design factors (Great link explaining this: Allowable Design Values).
Base Fb for #2 SPF=875

Duration factor (CD): CD is determined by the critical load combination (See section 2.3.2.2 of Design Values)
dead load: 10/0.9=11
dead plus snow: 60/1.15=52 (critical load)
CD = 1.15

Size factor (CF): For a 2x6 CF = 1.3

Repetitive member factor (Cr): For 3 or more members closer than 24" Cr = 1.15
Taking these factors into consideration, each member of my triple 2x6 beam would have:
Fʹb = Fb x CD x CF x Cr = 875x1.15x1.3x1.15 = 1500 psi
1500 exceeds my requirements, so a triple #2 SPF 2x6 beam will work. (Whew)

jules4 07-11-2010 06:48 PM

The purlins are bearing 1631 lbs each, so they are also undersized - plus, I just realized that I can’t load up the purlins without massively beefing up the side rafter girts, so forget that.

Time to re-design with the central roof load falling entirely on the front and rear beams.

Total load on the front beam will be 3878 lbs: 1293 each for 3 2x6s.

Total load on the rear beam is 2900 lbs: 967 each for 3 2x6s.

At F'b of 1500, a 2x6 can bear 945 lbs. So the rear beam is probably okay, but the front beam is once again not strong enough (sigh).

murray59 07-12-2010 10:12 AM

I'll chime in with this. Why have you run your floor joists the 11ft direction not the 8' direction?

Willie T 07-12-2010 12:16 PM

I've also just sat back expecting someone to address the severe notches in the posts. Maybe they already have, and I missed it. Are you sure you want to weaken those 6x6 posts by removing such a large amount of them? It seems you are going to exaggerated lengths to design the top layer for big loads, but kind of neglecting the continuous post dimensions required to carry all that weight and to resist lateral wind forces.

Chop those posts up like that, and you no longer really have true 6x6's anymore. Might be real susceptible to snapping at those skinny sections.

jules4 07-12-2010 04:38 PM

1 Attachment(s)
I think I may have run across a solution to my undersized front beam that doesnít require using bigger lumber: sandwiching two sheets of 1/2" CDX between the three 2x6's using construction adhesive and 4' fasteners from both sides (see attached image).

Adding the ply layers "significantly" increases the Fb of the beam (although I can find any info of exactly how much).

Quote:

Originally Posted by murray59 (Post 468422)
I'll chime in with this. Why have you run your floor joists the 11ft direction not the 8' direction?

So that the total floor load is carried by 2x8 beams/girts with only an 8' span instead of an 11' span. I know that a 2x8 joist can safely span 11', but I donít think a 2x8 beam can.


Quote:

Originally Posted by Willie T (Post 468464)
I've also just sat back expecting someone to address the severe notches in the posts. Maybe they already have, and I missed it. Are you sure you want to weaken those 6x6 posts by removing such a large amount of them? It seems you are going to exaggerated lengths to design the top layer for big loads, but kind of neglecting the continuous post dimensions required to carry all that weight and to resist lateral wind forces.

Chop those posts up like that, and you no longer really have true 6x6's anymore. Might be real susceptible to snapping at those skinny sections.

I thought that as long I cut the notches carefully (i.e., close fit and flat bearing surfaces) and fastened the connecting members to posts securely, there would be a better vertical transfer of load (so less inherent stress on the framing) and the frame as a whole would be more unitized and so stronger (even though certain members, considered by themselves, would be weaker).

But my experience (restoring, not building) with the extreme notching involved in some timber framing may have made me overly blasť about chopping up posts - my notches are a pretty far cry from mortise and tenon joints, so perhaps I should reconsider all the notching and use some deadmen instead.

(But it's just so damn elegant looking to notch it all into place rather than having deadmen clinging awkwardly to every joint :( )

Willie T 07-12-2010 05:21 PM

1 Attachment(s)
Yes, a lot of the vertical support is regained with good, snug-fitting, cross-members. But when the inertial moments are stressed, as they would be from side loading wind factors, at least one direction proves to be weaker than the un-notched piece would have been.

I happen to love Timber Framing, and I think (although extra work) mortising in the heads of angle braces and cross-members would be sharp.


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