iDRY Vacuum Kilns

Sponsors:

lifting rafters

Started by Quebecnewf, January 07, 2008, 07:49:37 PM

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

Quebecnewf

Heres my problem. I have a 26x32 ft building with 10 ft walls. I am planning a 8/12 pitch roof and my rafters are 17 ft long each side 3x6"'s .I want a catherderal ceiling.

How would you lift these rafters into place. No crane and I need to do this pretty much myself with one helper. I do not want to use a ridge beam.

I am thinking some kind of jim pole???????????????

Any suggestions or pics of how you did it would be great .

Quebecnewf

sbishop

Are these pre-engineer trusses? When I built my 28x36 10ft wall garage. We hang the rafters upside down and flip them one by one. went up pretty easy.

hope that helps

Sbishop

logwalker

I have two genie lifts that I used to put up all my trusses. The tmber tresses wee about 1200 lbs each. I also sued thwm as sheetrock lifts with a 4 in 12 pitch. Just made a wooden fixture to create the pitch. I found mine at United Rentals used for $350 each. Most good rental yards have them to rent. Here is what they look like.

http://www.genielift.com/ml-series/ml-1-4.asp

Mine lift 650 lbs to a height of 24 feet. Very stable too. Joe
Let's all be careful out there tomorrow. Lt40hd, 22' Kenworth Flatbed rollback dump, MM45B Mitsubishi trackhoe, Clark5000lb Forklift, Kubota L2850 tractor

Don P

In the absence of scaffold, I've slapped together tall sawhorses or lightweight studwalls (with a diagonal brace) that I can make a small "floor" on to make the ridge man happy. A 3x6x17' isn't very heavy to manhandle.

I am lost though. There needs to be something to resist the outward thrust of the rafters on the walls. That's usually either ceiling joists or a ridge beam, I think you're into needing some form of truss  ???

I've tried to sketch how the vertical load turns into thrust on the walls

Quebecnewf

I cannot rent anything in this region.

Don P

I plan to have "collar beams" across the middle of  most of the rafters down 2 ft from the peak. I will have one beam 6x7 across the building at 8 ft from the end. this beam will have a post in the middle supporting it.

I plan to have a steel or glulam beam across the center of the building . this will have no post supporting it.

I feel these 2 beams are enough to keep the walls in. if not i can add another but it cannot have a post supporting it.

Thoughts?????

Quebecnewf




Don P

The collar ties will do very little to prevent spread, the leverage is just too great against them. The other 2 beams will help, they need to be tied quite well and the wall plate would probably need to be looked at as a beam resisting the thrust between those tieing beams.

In the rafter tables as the ceiling joist comes up off the wall plate, the allowable rafter span decreases and the depth of the rafter increases. The rising tie is putting a greater and greater bending load into the rafter, using up alot of its strength. By the time a tie is raised halfway the rafter has lost 42% of its allowable span.

Just exploring the options, is there a reason not to make each rafter pair into a truss with a low tie?

Quebecnewf

Don P

Good points . I know the farther up the rafter the collar beam is the less support it gives. The shed will have a loft in one end this would be on top of the beam that is 8 ft from the end with the post under it. Collar beams will cut down on the head room in this section. Maybe I could do away with collar beams on the sets of rafters that are over this platform and lower down the the collar beams on the rafters that are over the rest of the shop. I could put a triple top plate on the side walls as extra support for the rafters and to help the walls from bowing.

What size steel I beam would you think would do as a cross beam in the center. I am thinking a 6x6.

What size glulam instead of steel?

So many questions

Quebecnewf

Don P

The section with the floor joists is tied so doesn't need the high collar ties. When they're up high they are really preventing the roof from coming apart if a high wind tries to inflate the building, not a bad idea if the building is open or has a large bay door somewhere.

The other beam I'm not sure about, is it simply a 26' tie holding only its self weight and resisting the spread or is there some other load on it?

I'll need to know the design snow load for your area and rafter spacing if we want to make an educated guess at whether the top plates could resist the thrust. That is getting out of my ken though. This would be getting out of our prescriptive codes (conventional construction) and into engineering required here. A quick plan view (birds eye) sketch of the building with as much information as possible would help make sure were on the same page.

Architect Frank Lloyd Wright used a pretty neat concept that I've seen once. He built a boxed soffit from the top plate into the room. I suspect it was plywood wrapped and designed as a horizontal beam, laying on its side to resist the thrust. I've done something similar on tall greatroom gable walls to help stiffen them and sold it as a "plant shelf/ light bar". I've used 2 layers of ply top and bottom with seams offset.

Another way if I'm understanding things right. If the loft area takes up 8' of the 32' we have 24' left. If the clearspan beam is centered in that at 12' from the loft and 12' from the outside wall. Then the clearspan beam and its rafters could be built as a kingpost truss. A supporting ridge beam could then go from outside wall to truss, another from truss to a post on the loft floor above the post below and another beam from there to the loft exterior wall. Just kicking some options around.

Quebecnewf

Don P

The drawing you did a few replies back was with Sketchup?

If so how do I get mine up in this forum

I have some Sketchup drawings of the building

Quebecnewf

Don P

Yes it was in sketchup.
I clicked "file", "export", "2d graphic" this saves it as a jpeg exactly as viewed when you save it (so rotate to whatever view you want to show)
I saved to my desktop and then opened my jpeg optimizer opened the file in it and sized it down to forum specs, saved it again, then uploaded here.

I think I can do it quicker than I can explain it  :D

krusty

lets see.....28x36 and I am assuming 28' in depth....so 14' supported by a rafter and at 40 PSF snow load approx and 2' spacing on rafters you are incurring 1120 pounds of outward thrust per side. You better be darn sure you take care of that outward thrust!

why not use engineered trusses and still have a vaulted roof?

Quebecnewf

 

Here is a small view of the shed with no roof. You can see the loft floor and beam with post under it. This will hold the walls together . I plan a steel or gluelam beam at the 16' mark or center of the building. This beam would only hold tha walls in and have no weight resting on it.
Krusty

What type of engineered truss are you suggesting. Can you post a pic

Quebecnewf

tim1234

When I built my 24'x38' garage I stick framed the 15/12 roof.  The ridge was 25' above the slab with 8' walls. There is a loft at one end so I framed the loft and put the sub floor down before the roof went on.  For the open part, I extended some temporary floor joists down the center of the building and put a temp sub floor on it.  I made it 8' wide so 4 extra floor joists and about 4 extra sheets of plywood.

I then built some frames out of 2x4's 2 vertical 8' studs with a 4' stud across the top and bottom to make a rectangle frame.  I put the top 4' 2x4 about 3" down from the top of the vertical pieces to make a ledge for a long 2x4 on edge to tie the frames together at the top and to give support for the OSB I used for the top deck.

I added some cross bracing for each frame.  I made 5 of these frames 4' apart, stood them up and cross braced them.  I screwed them directly to the sub floor so I could remove them later and so the frames would not move around.  Then I put 2 16' 2x4's across the top of the 5 frames, used 2 sheets of 1/2" OSB for the top.  Now I had a 8' tall, 16' long, 4' wide scaffold that was directly under the Ridge beam.  I just stood on top, my buddy passed the 24' long 2x8 rafters to me and I nailed them to the Ridge beam.  Worked great. 

I still have the frames in the garage.  I also used them when I put an addition on my house.  It works great and was pretty cheap.  Also transports to a job site in the back of a pickup if necessary.

Trust me if you don't secure the walls somehow, the outward pressure of the rafters on the exterior walls will bow them outward and then you will have a dip in the ridge of the roof.  Don't ask me how I know ::)  I think collar ties need to be in the lower third of the roof height to make any difference.  One of the posts above was exactly correct.  Collar ties are used to transfer wind load from the loaded side of the roof to the other sided and then down to the top plate of the wall.  You really need something else to keep the walls from spreading. 

Tim
You buy a cheap tool twice...and then you're still stuck with a cheap tool!!
Husky 372XP, 455 Rancher, Echo CS300, Alaskan 30" Chainsaw Mill

Quebecnewf

 

Here is another drawing of the shed. There will be a lenny shed on the north side of the building this will help hold that wall straight and transfer the load down.

Quebecnewf

Quebecnewf

 

Here is another pic of the rafter I plan to use they are 8/12 pitch 3x6 and about 17' long. This is the longest I can saw on my mill. Snow load is not a problem. How far down from the peak would be the minimum that you would place collar beams to do any good. I would have no collar beams on the rafters above the loft floor.

I could also easily make the toplate on the side walls out of 2x6x16 and on top of that 6x6x16 ,breaking the butt joints to help resist the outward thrust


Quebecnewf

krusty

http://www.maplevalleytruss.com/common_trusses.htm

has a pic of a cathedral and are designed to absorb the hortizontal thrust that would get transfered to the walls.

ARKANSAWYER


  Just make trusses like these.  You are going to have bottom cords any way and they will look better then any thing else.
  These are 4x8's on 7/12 pitch on 12 ft centers spanning just 20 ft in this carport.  We just put them together upside down and rolled them into place.


ARKANSAWYER

Quebecnewf

Thanks for the response but the roof you show in your pic is not the look i am trying for. I want to look and see my 3x6 rafters on 24" centers and then the roughsawn boards that i will cover the roof in with

My plan is /rafters 24" centers/ 1"boards/vapour barrier/4" styrafoam /2x4 strapping horizontal on 16" centers / steel roofing

I could live with an engineered truss I guess.

Quebecnewf



Furby

There is one other option that might be able to be used but it isn't a commonly approved method.
That would be to use cables as your bottom cords.
You would have to spec out the correct cable and hardware and also tension them enough.

ARKANSAWYER


  Not knowing your snow/wind load (yes it matters) my guess is what you want will not work.  The weight of the 3x6's will push the walls out with just 2 or 3 ties.
  Now you can make 5 trusses like in the photo on 8ft centers and then put your 3x6's as perlins and then deck like what you want to do. 
  A ridge beam with bottom chords about every 4th rafter will work for what you want to do.
ARKANSAWYER

Furby

He can't easily cut long enough for a one piece bottom cord Arky, so he'd need a different way to make that truss.

ARKANSAWYER

He can cut 17 ft then he can make the bottom chord.  He can splice it where the "t" is with out any problem.  Just remember it is in tension and through bolt.
ARKANSAWYER

Quebecnewf

Very interesting replies guys.

Arky
Given that i have the walls tied together with the beam under my loft 8 ft out from the end this leaves me with 24 ft of wall to deal with would say 3 of the truss you show in the pic you posted be enough to hold everything together .

I could make 3 of these and then do the rest of my rafters as planned. I have the rafters sawed and drying as we speak/type you see and I really want to look at those roof boards in the horizontal position.

Thoughts????

Quebecnewf

Don P

Out of sequence but I type slow  ::)

With enough bolt(s) to resist the load  ;)

Some stuff I came across trying to find a snow load;
QuoteFinally, the central region, including Ontario and Quebec, is marked by varying winds and snowfalls, and sufficiently low temperatures in many places to allow snow accumulation all winter. In this area high uniform loads as well as high drift loads occur.

Freshly fallen snow is very loose and fluffy, with a specific gravity of about 0.05 to 0.1 (1/20th to 1/10th of water). Immediately after landing, however, the snow crystals start to change: the thin, needle-like projections begin to sublime and the crystals gradually become more like small irregularly shaped grains. This results in settlement of the snow and after a few days the specific gravity will usually have increased to about 0.2. This compaction further increases and specific gravities of about 0.3 will often have been attained after about a month, even at below-freezing temperatures. Longer periods of warm weather as well as rain falling into the snow (a possibility that must be included in proper design loads) may increase this density even further.

As a simple rule for estimating loads from snow depths the specific gravity can be considered to, be about 0.2 to 0.3. In other words, each inch of snow represents a load of about 1 to 1½ pounds per square foot.
Provided I did the conversions right, I didn't find a local official number, I'm coming up a little above 60 psf in the Montreal-Ottawa region. There is a code table for rafters with 70psf loading so lets start there.
#2 SPF 2x6's on 12" ctrs can go as far as a 9'-9" span measured horizontally (you need to go ~14'). You've got the equivalent of a double 2x6 on 24" so basically the same thing. In the 50 PSF table you can go 11'-3". So the 3x6's are too light it looks like, unless I'm missing something.
Doing a little extrapolating from the tables it looks like a 3x8 or 3x9 would be a better fit.

On to the thrust;
You want to tie with some sort of beam across the walls at 16' from the end. A quick check for roughly how much load is on that half of the roof, 16x28x75psf (snow plus the roof weight itself)=33,600lbs gravity load. Half is on each wall plate or 16,800lbs vertical. 8/12=.666, so 16,800 X .666=12,200lbs outward thrust on the 16' wall plate under a full pack of snow.
I'm coming up with a 14x10 top plate using a #1 SPF beam :o

Moving that tie to the 12' point from the wall would make a big difference if you want to keep wandering down this path  ???

Quebecnewf

Thanks for all the great info guys it makes for lots of rethinking and then rethinking again.

On the wall that has the "lenny shed " to it. |Given that this shed is to be 20 to 25 ft long will it not give lots of support to that wall.

Snow load is not a factor here in Harrington. A roof of 8/12 pitch with steel sheeting as a covering will not hold any amount of snow, given the winds we have. We have a 30x40 wharehouse with maybe a 6/12 pitch and rarely if ever do we see any snow on this.

I had another thought last night while turning this problem over in my mind. I have lots of 2" galv pipe in 20' lengths. I could join together and make up 3 spans with plates welded on the end this would set on top of the wall plate and the plates on the end would be down over the outside of the wall holding them together.

Quebecnewf

Thank You Sponsors!