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ideas for spreading roof load and maintaining continuous purlin plate

Started by mrbrb, September 02, 2014, 07:23:09 AM

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mrbrb

Hello,
Im looking for some feedback/ideas on alternatives to this continuous purlin plate design with vertical posts:


  

 

Wondering about the possibility of placing the posts at a 60 degree diagonal, thus spreading the load out towards the walls and creating a wider alley. Seems like roughly the same amount of joinery, but this is not something I have seen done- and there is probably a reason why:




Brian_Weekley

Although your design might give you a "wider alley" at the base, those slanting posts take up a lot of room.  You lose a lot of working space on either side of the alley.  I think overall, you'll end up with much less usable space.  I think my head would be full of bruises from manuvering around all of those slanting posts rather than ones that were straight up and down.  Just my two cents...
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Jim_Rogers

If you have a heavy snow load that's one thing. But you haven't said where you intend to build.
Larger common rafters closely spaced may eliminate the need for a principal purlin plate.

It all depends on the load, the span, the type of wood.
Lots of things to consider.

Jim Rogers
Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

D L Bahler

If you would push the canted posts clear to the ends of the tie beams, then have them support the collar beam just under the purlin (the purlin is lodged on the collar with a shallow cog and a heavy wooden peg) then you have the Liegender Stuhl which has been used thousands upon thousands of times, in Europe and North America. It exists for precisely the reason you are looking for, to create more room in the middle of the structure. It's considerably less complicated than what you have shown here, and more efficient.

You'll need to add some braces in your roof structure too.

timberwrestler

Yes, I'm with DL on this one (believe it or not).  You need to move the struts out.  And I'd say you need a principle rafter at each bent, that in turn supports the purlin plate.  This is also akin to French jamb de force.
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mrbrb

Thank you for the feedback and ideas. Mr Bahler, is this along the lines of what you were suggesting? Pretty sure I know what you mean by 'cog'- basically a lap joint.

This building is sited in Northern Connecticut- snow load is 30psf for my town, so I would be designing the roof for a total of 40 psf.

I am using mostly no.2 white pine, with oak braces when I can. All of the calculations are for no.2 ewp.

The main idea here is to get the roof load out toward the walls, as the main tie beams just squeaked by with my original design (vertical queen posts), with a good deal of the roof load going straight to the mid point of the tie beams.  Even though the original design passes, I would feel a lot more comfortable directing the roof load to the periphery of the tie beams.

-  total floor load of 35psf (clear span first floor), with calculations taking into account the self weight of the oversized tie beams.

How would you work braces into this type of roof system?

Thanks and all the best!

  

    

D L Bahler

Like this:


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The photograph above has an upper truss used to support a ridge beam. I doubt the size of your structure merits doing this. The design you have is pretty good, I would just recommend to add some bracing like shown.

routestep

Somewhere on this site I have a gallery of pictures.

I made a barn that uses 6 by 8 inch struts very similar to your second set of pictures. They were about 110 inches long. Total of four pair. Your drawing shows many more!

The struts were mortised into the cross ties about thirteen inches from the tie/post connection and angles up to a collar tie in principle rafters. The angle was 52 degrees.

The principle purlins were in the roof, mortised into the principle rafters and supported the common rafters.

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