Hi all,

I'm toward the end of cutting my own frame for a house/cabin I'm building for my family in central vermont. It measures 26' by 32', and consists of four bents (12', 8', and 12' bays). The roof is principal rafter and purlin system with a 12/12 pitch.

I designed the frame myself but consulted an engineer with timber framing experience for the beam sizing. I have done my own joint design... which is getting us closer to the subject of this post.

I decided to use mostly "drop-in" joists and purlins with, only one tieing (pegged tusk tennon) joist per bay. I've been doing some more reading on the subject and I'm getting cold feet about the structural integrity of this system. Unfortunately, the joists, purlins, and tie girts are already cut. So I was thinking of using a metal fasteners (cheating, I know...) to effectively tie two joists (or purlins) across the supporting beam.

My question is, does anybody have experience or advice with this technique?

Thanks

Jon

Apply Simpson straps ( and proper hardened nails ) to at least every other row of common purlins, take care to bridge the principal rafters and tie purlin to purlin at the inboard bents and to bend the strap and fasten to the vertical side of your principal at the gable bents.

Your floor system should be subjected to little in the way of tensile forces and whtever you deck with will serve to augment your tenoned members and tie things together.

Strap your connecting girts as described above, and here the tieing of the corner posts to the girts is particularly important,( again, bend the strap around to the adjacent face ) in that the braces put the connections into extreme tension as wind loads rack the system alternatively one way then the other and all this is ultimately resolved at the end of the line.

Tenons in shear are both preferable to and superior to tenons in tension

The insurance the straps will provide combined with the redundancy of your enclosure system will see your frame serve you well for your lifetime and on into that of whoever is fortunate enough to follow you.

Thanks Will!
I just visited the Simpson web site and it seems like they have just what I'm looking for. Thanks for the installation pointers as well.

Jon

Is all that steel really necessary? Being instructed to sprinkle so many Simpson fasterners onto a seemingly finished and approved design sure seem to blow the wind out of the sails of the builder who has gone thru much effort to design and build a tru timber frame. I don't see why PROPER wood joinery wouldn't be enough. My 1866 frame is built just as you describe your house/cabin, less any metal fasteners and is still solid as a rock in western MA.

Shaun

Hi folks,

I agree somewhat with shaun's questioning the apparent over-use of simpson hardware, especially since Jon indicated that his frame was for a house/cabin. Depending on the enclosure details, mainly the sheathing and/or internal wall bracing, the straps may either be reduced in number or eliminated all together. What are you planning for the roof and walls, Jon? I wouls recommend Simpson tie-downs on ALL frames, however...

That said, I would agree with Will's approach if this was a barn or outbuilding with simple wood cladding for the enclosure (e.g., clapboards, board and batten, or something similar). The metal straps will help the frame distribute the loads more effectively, and allow it to act as the only structural system needed. This is why I would argue for deleting dovetails for joists and purlins... What exactly do you mean by "drop in" in regards to the joinery you are using on your purlins, Jon?

This topic seems to wander over into the other discussion regarding earthquake loading and the need for a well-defined lateral laod resisting system, regardless of the type of lateral load (i.e., wind or earthquake).

Mark Gillis, P.Eng.

Shaun

Jon had expressed concerns over perceived shortcomings in what I took to be an early revival type, post to rafter connection – common purlin / connecting girt frame, A frame completely without members which bridge from bay to bay, and a typology largely without the existent historical record you suggest.

A 19th century connecting girt frame in western Mass would, I believe be quite a rarity. Which is not to say such a thing could not exist, A framer from my area could have been transplanted. Here common purlins were a dominant system from a very early period , though English tying type frames with continuous plates and ties were by far the most common frame type built up through at least the 1830’s. For a brief period in the latter half of the century and as the evolution of historical timberframing was drawing to a close and largly as a response to the almost total deforestation of the time connecting girts became the dominant system in New Hampshire, parts of Maine and the Connecticut Valley region of Vermont. ( Jon I would be greatly interested in hearing if historical examples of either CG’s or CP’s are common to your part of Vermont )

The difference between these historical frames and the typology described above are multiple, common purlins typically bridge two bays and are often staggered to allow for through tenons or relish beyond daps, connecting girt frames tend to be quite large with girts at the upper and lower arcade lines and in the roof system struts, sometimes at the end of overriding ties, a kind of flying plate to form the eaves. Ten to twelve girts or more per bay, not the four and sometimes only two often common to early revival frames.

In the early 90’s the structural inadequacies of timberframes held together by a minimal number of girts with 3-4” tenons and drop in dovetails on common purlins began to be discussed openly in this community. Most framers chose to evolve their thinking and their product in any of several ways, by moving to free-tenons in their girts, some moved towards a myriad of metal based connections, others looked to historical record and introduced continuous wall and purlin plates, ties, joists and purlins into their designs.

I fall squarely into the latter group, often describing myself as a hyper-traditonalist, but am not, however, too proud to apply a strap to a frame if it means it will be better for it.

Moreover, Jon suggested his engineer was involved only in the sizing of the timber and had no input whatever into how they would be connected. More importantly, it is still common for newbies and owner-builders to base frame design on books and articles from the early years of the revival. I was reading between the lines and guessing that Jon fell into this category and that he subsequently found later writings which suggested there was room for structural improvement in his design

In the end, I was simply stepping up and answering his direct question.

All,

Sorry for my delay in responding... I'm glad to see that my original question has generated some good discussion. I’m learning a lot from it. Will has hit it pretty much on the money with his last few points. I am an owner-builder, timber frame newbie, with some prior building experience and became interested in timber frame construction after reading one of Ted Benson’s earlier books, written (as Will has surmised) in the early stages of the timber frame revival.

I’ve been working on this project for about a year now and the learning curve continues to stay steep. I made some early design choices (with admittedly limited knowledge) which, in hindsight, I might have done differently. For one thing, I milled my own wood from trees thinned from my property. The result has been a stack of beams of middling quality, not very square, and relatively short (mostly under 16’). The limited length of my beams led me to choose the common purlin system, thereby reducing the number of longer beams (no large quantities of common rafters).

Another early choice was to try and make as much of the frame as possible able to be raised by hand. To me, this meant a two stage raising, first floor and then roof. This means, however, that there are no posts which are continuous from deck to peak. While my original frame did not include continuous wall plates, I was convinced by a timber framer to add them to the design.

My resulting plan represents a medley of ideas culled from books and conversations with other builders. As far as enclosing the structure goes, my plan has been to use SIPs, although I’m re-thinking this decision as well.

One thing I’m constantly reminded of is that there are very few obviously right answers, but a large number of bad ideas. Given that, I’m glad to get any advice I can.

Thanks again for all your thoughts.

Jon

Thanks for elaborating, Will.

I hope you didn't take offense to my comment. I was simply looking ot generate some more discussion on the topic for Jon's and my benefit.

I'm relatively new to the trade as well. However, after being on this sight and reading your posts for a couple of years now, knew you had it in you to elaborate on your first response to Jon as to why you thought all the metal fasteners were necessary.

Thanks.

Respectfully,

Shaun