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General Forestry => Timber Framing/Log construction => Topic started by: Jixxerbill on January 09, 2019, 08:33:59 AM

Title: Post and beam conversion
Post by: Jixxerbill on January 09, 2019, 08:33:59 AM
Iím new to the forum and thanks for having me. Going to start sawing logs for my wifeís timber frame house soon. (Before Iím to old to do so) 
I have a question on post and beam sizes. I have a set of plans for the house she wants. It calls for 8Ēx8Ē hemlock posts and beams. I donít have enough hemlocks on my property so I was wondering if there is a quick and dirty way of converting to oak or pine? 
So if an 8x8 hemlock post and beam is called for can you use a 8x8 oak instead?  Or a 10x10 pine. Or on the pine have to go up to a 12x12 ? Or is it best to get the whole house plan redone by engineer like they did in the old days ? Lol. I live in southern West Virginia. 
Thanks in advance. 
Title: Re: Post and beam conversion
Post by: Don P on January 09, 2019, 10:32:24 AM
You can go to a stronger wood without resizing, usually plans say for instance #2 spf or better, so you can always go "or better". if you use a wood that is less strong or stiff then you need to resize. Do not assume, the Supplement of design values to the NDS on the publications tab at awc.org is where you will find the strength values, make sure you are looking at the tables marked 5x5 and larger around the middle of that document.
Title: Re: Post and beam conversion
Post by: ljohnsaw on January 09, 2019, 11:21:18 AM
IMO, you need to be careful when you change beam wood.  Posts not so much of an issue (all vertically loaded) but with beams there may be a difference in the bending stress a different wood can tolerate.  And then there is the weight issue.  If you go from hemlock to oak, there is quite a bit more weight.  That weight should have been used to size the original posts and beams - all depends on the style of building you are planning on putting up.  i.e. King/Queen posts sitting mid-span on a beam.  If that beam is now oak and oak posts sitting on it, that's a lot more weight trying to bend the beam.
Title: Re: Post and beam conversion
Post by: Don P on January 09, 2019, 11:03:41 PM
Grade for grade the only clear cut "or better" substitution I see is northern red oak, eastern hemlock has some pretty good strength and stiffness numbers. In #2, mixed oak has lower strength and stiffness in an equal grade substitution with hemlock, shear is identical but rarely controls. Red oak as a group, which for you in WV would most likely be scarlet oak is lower in bending, higher in stiffness and equal in shear to hemlock. White oak has equal bending strength but lower stiffness and superior shear as compared to eastern hemlock in #2.  Red maple also fills the bill. If you can go up a grade in the substitution that opens up more options, what grade hemlock was specified in the plans? The designer may be able to answer the substitution question directly.

Your point is well taken John, I tend to assume standard dead load allowances account for the weight of any typical choice of materials but the designer may have used actual material weights to arrive at the dead load in his design. In a kingpost or queenpost truss the posts are actually in tension, they are holding the bottom chord up rather then bearing down on it so their weight is more critical to the top chord dead load. If you think about them as cables it would be a good way to visualize their function.
Title: Re: Post and beam conversion
Post by: ljohnsaw on January 09, 2019, 11:29:52 PM
Don,
I'm far from your level of knowledge.  What would you call this configuration?

(http://forestryforum.com/gallery/albums/userpics/30640/20170131_c.JPG?easyrotate_cache=1485924127)
 
The short ones mid-span.  Are those queen posts?
Title: Re: Post and beam conversion
Post by: Don P on January 10, 2019, 07:47:03 AM
That doesn't appear to be a truss, they certainly look like compression elements functioning more like a crown post, so again your point is taken, the devil is in the details. I've wandered off into the land of semantics, sorry for that. In your case if the rafters simply rest on the ridge and purlins then their load is point at midspan on the tie beam below them. I suspect that is the case with your substantial dropped tie below the raised plate, that joint would be incapable of taking your loads as horizontal thrust.

In the case of substituting something like northern red oak or red maple for eastern hemlock of the same grades, I wouldn't be worried about materials weight, the northern red is enough stronger to handle that however the design was done. The same thing may be true if we know the hemlock grade specified and can jump up a grade in the substitute material, the strength differences can be fairly good. If one is pushing around the edges then it would be worth looking at the substitution more closely. I think we are both making good points here.
Title: Re: Post and beam conversion
Post by: Brad_bb on January 10, 2019, 03:09:47 PM
Don,
I'm far from your level of knowledge.  What would you call this configuration?

(http://forestryforum.com/gallery/albums/userpics/30640/20170131_c.JPG?easyrotate_cache=1485924127)
 
The short ones mid-span.  Are those queen posts?


Do you really need all those center posts?  That's only what a 20 foot span?  Do you have a lot of extra loading above?
Title: Re: Post and beam conversion
Post by: sawmilllawyer on January 10, 2019, 09:20:39 PM
I think he stated he had a large snow load in that area. :P
Title: Re: Post and beam conversion
Post by: ljohnsaw on January 10, 2019, 09:29:02 PM
Do you have a lot of extra loading above?
 
My Ground Snow Load (GSL) is 351 pounds/sq-ft.  With my roof slope and metal, it dropped to something like 200-230.  I would have to have some massive rafters if I spanned from ridge to eave.  As it is, I will have 5x9s.