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#34582 - 10/20/18 09:41 AM Help with design needed for novice
GLO Offline
Member

Registered: 10/19/18
Posts: 2
Loc: MN

Ok, I am a novice at timber framing. I have an add-on project to replace an aging sunroom. I plan build a 18x20 on a slab. I have designed a 3-bay, 6 feet between, 20 foot clear span, 4/10 pitch roof all out of red pine center cut timbers. 1x10 treated sill against concrete, 8x8 sills, 8x8x8' posts, 7x10x20' tie beams, 8x8x10' principal rafters, 8x8x8' collar ties, 3x8 braces on corners and 3x8 arched braces at collar. I would appreciate any feedback or suggestions.

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#34583 - 10/22/18 01:29 PM Re: Help with design needed for novice [Re: GLO]
Roger Nair Offline
Member

Registered: 10/20/99
Posts: 458
Loc: Bakerton, WV, USA
I will keep my comments on the direction I might head toward if I was tasked with your problem. I would tend to use a king post truss with the span and materials indicated. However, the roof slope is very shallow and the resulting forces thrusting at the ends of the ties can be a problem. Also not stated is the roof sheathing and underlaying structure. A little more detail and a drawing would help. Local snow loads should figure in.

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#34584 - 10/22/18 02:54 PM Re: Help with design needed for novice [Re: GLO]
Jay White Cloud Offline
Member

Registered: 11/06/08
Posts: 537
Loc: Vermont
100% second Roger's advice thus far...

Key points...!!!...you have massive snow load potential in your area for that species and span with the size timbers designated...

King Post (or related) truss system...or...interior center post required...

Drawings need to be present for even moderate good advice...PE involvement highly recommended for novice frame projects. Note, I personally have 40 years at timber framing and all "new frame designs" are looked at by a PE...

I don't use oblique bracing in 99.9% of my designs (aka more Asian-Middle Eastern-Eurasian in motif and modality) so will just "hang back" and follow along for the most part...

Good Luck,

j
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#34585 - 10/22/18 11:15 PM Re: Help with design needed for novice [Re: GLO]
GLO Offline
Member

Registered: 10/19/18
Posts: 2
Loc: MN
I have been working on drawings but wanted some feedback before proceeding. I do not have the option of increasing the pitch due to the restrictions of the main house form. Snow load is an issue in that we are in in northern Minnesota.
Certainly king post is an option. I can brace the trusses space allowing. A center post is possible on the freestanding end. The end towards the house can be securely attached to the existing structure. I want to avoid an internal center post. The roof structure would be birch tongue and groove, foam sheets, cdx and asphalt shingles. I do not have PE available locally for timber frame but can certainly send off the plans for review. My biggest concern now is sizing of the timbers. The trees are down and are soon off to the lumber mill. The bolts are big enough for 7x12s. Thanks for any suggestions.

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#34586 - 10/24/18 11:41 PM Re: Help with design needed for novice [Re: GLO]
Jay White Cloud Offline
Member

Registered: 11/06/08
Posts: 537
Loc: Vermont
Hello Glo,

Originally Posted By: GLO
...I have been working on drawings but wanted some feedback before proceeding...


That is always a good thing to do...experienced or not, I still seek advice on every single design I plan on building, and as stated, if it is a new design, it is always looked at by our PE for a final viewing and consideration. Validation comes, when they don't have much to add...LOL...but it is still a wise thing to seek out. I do recommend only using PE with timber framing experience under their belts as many (most?) PE outside the timber framing craft have little to know understanding or experience with traditional joinery and how to properly assess them...

If you choose to forgo a PE's advise, that is your choice, but I would be remiss if I did not suggest such as "good practice" in craft. Especially for a novice framer...

Originally Posted By: GLO
...I do not have the option of increasing the pitch due to the restrictions of the main house form. Snow load is an issue in that we are in in northern Minnesota...


You do indeed have a large challenge with snow load...That is of no doubt for sure!!!

This is not insurmountable though as I have design a very large public frame in that same region with a roof pitch of only (?? if I remember correctly) of 4/10 pitch. That frame has a stone plinth foundation, and no oblique bracing of any kind, but uses much heavier timber. It has a full monitor resting on Queen Post as part of a double chord truss spanning ~30' with ~20' span between bents. With massive eave overhangs making the building close to ~45' wide.

So, as such, low pitches, large spans, and no king post can be achieved even without an oblique bracing system...Yours should just be a matter of "crossing your "T's" and dotting your "i's" if" a King Post truss is the goal...

Timber sizing and proper joinery design and execution will be the challenger for the most part...

Originally Posted By: GLO
...Certainly king post is an option. I can brace the trusses space allowing. A center post is possible on the freestanding end. The end towards the house can be securely attached to the existing structure. ...


Post at the gable end is highly recommended whether a King or a pair of Queen posts...

I do not recommend ever relying on pre-existing architecture as part of any timber frame. This is just asking for issues at some point and time in the structure's future.

Issues of foundation subsidence that is not the equal, frame movement for some reason, and a other reasons abound...not to mention the aesthetics of a good design format. The timber frame additions of this nature I always recommend being self standing alone structures, even though part of another primary structure aesthetically from the exterior...

So, even the frame near the hose should have a center post or some other format...A truss is king of redundant and unnecessary unless it really is part of the aesthetic in the design...

Originally Posted By: GLO
...I want to avoid an internal center post...


This is possible with the correct design...

Originally Posted By: GLO
... The roof structure would be birch tongue and groove, foam sheets, cdx and asphalt shingles...


Birch T&G what...???..slab, plank, plywood, and what thickness...

I would not (nor do I) ever recommend foam insulations. From both a pest control issue, on to other longer term issues in my experience.
I only recommend mineral wools and other traditional/natural mass forms of insulation.

I also would want to see the snow off the roof if possible, so standing seam or metal roofing would be my recommendation...not asphalt but these are more a personal choice than a PE issue for the most part...

Originally Posted By: GLO
...I do not have PE available locally for timber frame but can certainly send off the plans for review.


The project I did near you was in Menomonie Wisconsin, and as stated was a public works Farmers Market Pavilion. As such, there are PE in your region and very good ones that have timber framing experience. Feel free to send me an email at any point...

Originally Posted By: GLO
My biggest concern now is sizing of the timbers. The trees are down and are soon off to the lumber mill. The bolts are big enough for 7x12s. Thanks for any suggestions.


Don't quote me...LOL...but I think 7x12's could work for you and your proposed design if the correct details are worked out...

Good luck, and more questions more than welcome...

j
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