Advice on Timber Framed structure

Hey all, Just wanted to run a design for a timber framed structure that I am plan ning on building and wanted to appeal to those with more experience than th is weekend warrior for feedback. I plan on building an outdoor kitchen. I will excavate and pour a 20' x 20' 4" slab with the appropriate turn down on which I want to place timber framed structure. The structure will use 4 12" x 12" Douglas Fir posts se t at 16' spacing to support the roof. These will be affixed to the slab us ing a 18" x 18" x 1/4" steel plate with a schedule 80 1 1/2" x 12" pipe wel ded to it that will be tapped into the bottom of the posts. These plates w ill be affixed to the posts with 4 - 18" timber screws. They will be affix ed to the slab using 4 -3/4" x 4" concrete anchors embedded in the concrete slab when it is poured. This will handle the uplift forces on the structu re, or at least that is the intent, as well as any shear at the base of the posts.
The roofs gable ends will use 6" x 8" beams for the king post truss's pr inciple rafters and tie beams as well as for two struts. The tie beams f or the gable end rafters will be framed to the posts using a 2" thick x 4" high x 6" wide tenon on the post and the appropriate mortise on the tie bea ms. They will also be supported by a 2' x 2' x 3' knee braces between the tie beam and post using the same 6" x 8 " Douglas Fir.
The tie beams between the two posts in the other direction will be 6" x 8" Douglas Fir as well. They will be framed to the posts using 2" thick x 4" high x 12" wide tenons on the posts and the appropriate mortise on the t ie beams. These tie beams will be 6" x 8" x 20' and will support 6" x 8" pr incipal rafters that are 4' on center. They will also have 2' x 2' x 3' kn ee braces between the post and tie beam. These principal rafters on the non -gable ends will also have a 6" x 8" collar beam 2' down from the peak. I will also tie all the rafters together using 4" x 4" beam running the lengt h of the roof halfway up the principal rafters and at the peak.
On top of the rafters I will put 1 1/2" x 6" T & G Western Red Cedar ca r decking on which a standard felt / asphalt shingle roof will be placed.
So with that context I want to ask a couple of questions. 1.) I want t o make sure that the tie beams on the non-gable ends are sufficient to supp ort principal rafters that are on 4' on center as well as the rest of the r oof. I had someone say that the span for a 6" x 8" beam shouldn't be any m ore than 12'. These will span 14' but will have additional knee braces tha t will reduce the span to 10'. Is this sufficient? 2.) Is the 4' spacing between the principal rafters too large?
Thanks for any advice,
John
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On Saturday, October 25, 2014 4:27:56 PM UTC-7, John Sheppard wrote:

anning on building and wanted to appeal to those with more experience than this weekend warrior for feedback.

x 20' 4" slab with the appropriate turn down on which I want to place timb er framed structure. The structure will use 4 12" x 12" Douglas Fir posts set at 16' spacing to support the roof. These will be affixed to the slab using a 18" x 18" x 1/4" steel plate with a schedule 80 1 1/2" x 12" pipe w elded to it that will be tapped into the bottom of the posts. These plates will be affixed to the posts with 4 - 18" timber screws. They will be aff ixed to the slab using 4 -3/4" x 4" concrete anchors embedded in the concre te slab when it is poured. This will handle the uplift forces on the struc ture, or at least that is the intent, as well as any shear at the base of t he posts.

principle rafters and tie beams as well as for two struts. The tie beams for the gable end rafters will be framed to the posts using a 2" thick x 4 " high x 6" wide tenon on the post and the appropriate mortise on the tie b eams. They will also be supported by a 2' x 2' x 3' knee braces between th e tie beam and post using the same 6" x 8 " Douglas Fir.

x 8" Douglas Fir as well. They will be framed to the posts using 2" thick x 4" high x 12" wide tenons on the posts and the appropriate mortise on the tie beams. These tie beams will be 6" x 8" x 20' and will support 6" x 8" principal rafters that are 4' on center. They will also have 2' x 2' x 3' knee braces between the post and tie beam. These principal rafters on the n on-gable ends will also have a 6" x 8" collar beam 2' down from the peak. I will also tie all the rafters together using 4" x 4" beam running the len gth of the roof halfway up the principal rafters and at the peak.

car decking on which a standard felt / asphalt shingle roof will be placed.

to make sure that the tie beams on the non-gable ends are sufficient to su pport principal rafters that are on 4' on center as well as the rest of the roof. I had someone say that the span for a 6" x 8" beam shouldn't be any more than 12'. These will span 14' but will have additional knee braces t hat will reduce the span to 10'. Is this sufficient? 2.) Is the 4' spacin g between the principal rafters too large?

Point of clarity, the knee braces will be 3' long. Not 2' x 2' x 3'. That is the triangle they will describe...
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"John Sheppard" wrote:

Point of clarity, the knee braces will be 3' long. Not 2' x 2' x 3'. That is the triangle they will describe... ----------------------------------------------------------------------------------- Some general observations and comments:
1) What is the frost line in your part of the world?
2) 4" of concrete is little more than a dust cover. You need at least 6" and wire mesh of the correct size.
3) What size footer do you plan having?
4) What do you mean by "appropriate turn down"?
5) Is this free standing or do you plan to secure to an adjacent structure?
Questions, questions.
Lew
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On Saturday, October 25, 2014 8:05:36 PM UTC-7, Lew Hodgett wrote:

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Lew, Thanks for the reply. As far as the concrete slab. I will have 4" of 5 /8 minus gravel compacted underneath the slab. The slab will have wire mes h. The turn down I was referring to is making the edges of the slab thicke r. In this case 8".
Frost line here is 18". The footings for the posts will be 2' w x 2' l x 1 8" h. I will also have footings for the counter area. This is because I b uilding the counter from bezer blocks and slate with a granite counter top so I need additional load carrying capacity.
The structure will be free standing.
Thanks,
John
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