lifting rafters

[Quebecnewf]

Here is a small pic of what I am thinking about


Quebecnewf

[Don P]

Our codes require that tie every 4', what you are proposing is tieing every 6' and using a heavier top plate, I wouldn't call that bad from where I'm sitting, just pointing it out. Furby beat me to string theory  :D. Steel is great in tension. Yes the diaphragm of the back shed will help brace the wall, how much is a judgement call. Snow is alot about exposure, you know the location, so again this is a judgement call. I have had a customer with a 12/12 pitch in metal call with 3' of ice/snow stuck up there. This makes me lean to the conservative side just in case sometime in the life of the structure a freak event happens. Our last freak snow took out the grocery store and a factory roof, which broke the propane lines, which then blew... so go carefully  ;).

I did make a species and grade assumption on the materials in my post above. Continuing that assumption and using the 30psf table that we use here, the rafters could go 13'9", deduct wall thickness and I think its there if you're comfortable with that load.

For wind resistance it will help if the shed's wall that is in the main building wall's midsection area was well braced also.

[routestep]

I think you will have to have external buttresses on both sides of your building, like the cathedrals of old if you don't go with a truss. Not sure if your Lenny shed will do it unless it is strongly braced. The cable would be a good safeguard allowing you to see how well buttressed the walls are. You could loosen the cable and see the behavior of the walls and if all is well remove it later.

[Quebecnewf]

Buttresses are not an option . No room on one side and the other side is over the water. The building sets mostly on a rock and wooden crib. There is water under the shed when the tide is high.

Quebecnewf

[tim1234]

You might want to check to see if you have a high snow load if 2x6 walls are required as well.  I know my buddy needed them in nothern lower MI.  This will also help stiffen the wall between trusses or ties.

Tim

[Don P]

Doing some more math  : :D
I muffed the thrust math yesterday, ignore that method above, it'll give low results 
The axial force in the rafter, at the eaves is given by the formula;
wS{1-cos²(rafter angle from horizontal)/2}/sin(rafter angle from horizontal)
w is the total load in pounds per lineal inch of horizontal span
S is horizontal span of one rafter in inches
an 8/12 pitch is 33.7 degrees

Or doing it the easy way, that formula is part of this calc;
http://www.windyhilllogworks.com/Calcs/Sloped%20Rafter%20Design%20for%20Bending1.htm
(rafter span in that calc would be referring to one rafter or 168")
I ran it for the load and put the spacing at 72" to get the thrust on the plate between the ties. I got 3960 lbs outward thrust. Checking against a table I have here it gave 3955 lbs, think I got it right tonite.

I then checked the top plate as a horizontal beam to see if it could take that;
https://forestryforum.com/members/donp/beamclc06b.htm

Not saying I'm right or wrong but I'm still coming up needing a hefty plate at a 40 lb total load, an 8x6.

Hope we're Hope I'm, not frustrating you too much 

[shinnlinger]

When you figure out what you are going to do structure wise, what do you have to raise the trusses?  Tractor?  FEL?  Truck?  oxen?  150 amish neighbors? 

[Don P]

Its going up a stick at a time and we've eliminated a glulam or I beam, piece of cake  .
Before lightweight trusses were the norm everywhere, we built "trussed rafter" garages to get the clearspan. Basically building a kingpost truss in air. The engineer would draw it out and specify nail counts on the various parts.

[Don P]

I had a rafter to tie connection pic I liked on disc, just another one for the back of people's minds.


shinnlinger, I had to call in my wife to help with this lift. The first bent took a truck hooked to a tractor, not real safe. Using a cable and windlass was stronger and more controlled but slower. Given a lever long enough and a firm place to stand 


I thought more about the top plate today. The size seemed pretty big for no more than a 6' span. Really the load isn't uniform, its 2 point loads of a rafter each (1320lbs per) and the rafters at the ties aren't loading the top plate thrustwise, that's going right to the tie. Using the calc for 2 point bending at third points and applying #1 spf numbers it did pass with a 6x6, #2 didn't though.

[Quebecnewf]

Don P

Are you saying that even with the pipe beams holding the walls together every 8 ft I still should use a 6x8 top plate. ??????????

Quebecnewf

[Don P]

I was checking a 6' spacing on the ties and coming up with that. It's bigger than I expected but that's what the numbers are saying. Looking at it another way, the codebook at the minimum limit allows 4' tie spacing on a single 2x4 top plate  :-\. Bending stress does square when you double span though.
I didn't take a strength increase for snow, since it is a temporary load you can take a 15% increase. Figuring in the 15% allowable increase on #2 SPF bending strength at an 8' tie spacing a 8x6 just does fail for me. A number 1 works. With three rafters loads on the clearspan of the beam I went back to checking it as a uniformly loaded beam instead of a 2 point load.

What species are you using?

[Quebecnewf]

Don P

Right from the start we are talknig about a 2x6 stud wall. This is what I am planning to build.
Are we on the same page on this.
I don't know codes I have built a few buildings and the 6x8 toplate looks bigbig to me.

That being said I cannot stand to have a hogged roof.

Quebecnewf

I guess the best thing to do is just build it and then take pics and see what she does.

My spruce logs are all #1 and maybe better than that. Its cold here so every tree is SLOW growth and this makes for some good beams no knots and a very dense wood

[Don P]

Yes, I realize you are building a 2x6 wall, with up to a triple top plate  .
I kind of backed into the top plate as a beam and then we began tieing it at closer spacings. When we get down to relatively close spacing of the ties it begins to look like a rather large plate for the span. This may not be the most efficient way to get the desired effect. I suspect that's why the codebook points to a beam at the ridge as the alternative to tieing the rafter feet. That might still be worth exploring in combination with Arky's trusses, a ridgebeam between each truss peak that the rafters hang from, but that's another road, let's stay on this one for the moment.

A couple of thoughts. The prescriptive code that allows a single or double top plate if the rafters are tied on 4' centers is performance based. After many years of experience we know it works whether it works on paper or not. One reason for that is probably the spans of the sheathing up top are short between ties. Getting outside of time tested experience, the basis of many prescriptive codes, being able to show it will work by accepted engineering practices is required. This steps outside of prescriptive and into engineered. It is up to an inspector whether engineering required means "by a licensed engineer" or "according to the principles of accepted engineering practice".

A good bit of what I try to do here is show how to use accepted engineering practices and to point out the prescriptive/ engineering line. Remember I'm just a carpenter so that all gets filtered through my small pea. I go for brutally basic, the ability to point to one thing and say "that can bear the strain".  I'm not going to make any assumptions on the sheathing or anything besides the plate doing the job. The journey is a learning experience for us both. These exercises help make me a better carpenter, I kept checking a beam because I needed a beam at the 16' spacing. I should have dropped back and checked lumber plates as soon as we began dropping the span.

2x stock is rated stronger than 5x5 and larger timbers and has more allowable adjustments to the basic strength values. This is due to the fact that we can see more of the timber so have a better guage of its strength. Let's check a full 2x6 cut triple top plate against the load at 8' tie spacing.

I can take the base bending design value of #1 SPF 2-4" thick and multiply it by all possible adjustments
Base design value is 875 psi
Whenever 3 or more members are closer than 24" to each other we can increase that by 15%, so 1006.25 psi
Design values are based on 12" wide lumber. Testing in the 90's showed that 6" wide lumber can take a 30% increase over that, so now we're up to 1308 psi.
Duration of load, snow is short term, we can take a 15% increase, so finally Fb=1504 psi

plugging that into this calc
http://www.windyhilllogworks.com/Calcs/beamcalc.htm
I used a load of 3960 lbs and a tie to tie span of 96".
E and shear are 1.4 and 135 respectively, those numbers are higher in 2-4" thick lumber than in timbers also.
It passed. Bending passed with about 15% to spare, so we didn't really use up all of the allowed adjustments. So after taking you around the horn  :, triple 2x6 top plates will work, do use good ones.

[Quebecnewf]

Thanks for all the great info. I now have a better grip on how I will build this shop. When I start next spring I will post pics as I go along.

Now another question

I plan to cover the roof with 2 layers of 2" Styrofoam and then 2x4 as firring strips. I need looong screws to lag the the 2x4 down to the roof. I saw something a while back on a post about this type of screw but can't find it now.

Any info would be nice.

Quebecnewf

[Don P]

I've bought olyscrews from these folks.
http://www.loghelp.com/
I've seen other forumites post some other sources of panel screws, hopefully they'll pipe up so you can comparison shop. (and so I can bookmark em this time)

[Thehardway]

http://www.sipbiz.com/

Here's a link to a supplier for panel screws.