I am submitting these questions here at the behest of myself and some that I have engaged in conversation with and hope perhaps some insight can be shed by minds greater than mine.
I am very interested in historical buildings and the thought and design that went into them. A recent discussion regarding traditional v modern bent style construction led me to a lot of research and I found myself in the Guilds Historic Truss documents looking at two different frames. First the Rindge Meetinghouse and second the Nevada City Railroad Shed. At first glance they appear to be fairly straightforward in design but as I break them down or rather apart piece by piece I find them violating what I thought were some pretty hard and fast rules of thumb when it comes to design and joinery selection.
First the Rindge Meetinghouse.
The "Flying Plate" really bothers me for the following reasons:
1. This 12"X 24" X 13' behemoth of a beam appears to be turned sideways based on the rule that the greatest cross-section dimension of a timber is its Depth and should be oriented vertically and the Width is the lesser dimension and should be oriented horizontally for maximum strength.
2. This "Plate" rests soley on the 2.5" H X 20" W tenon. (Are there some braces somewhere in the wall but not shown in the sketch?)
3. "Plate" tenons into the trusses tie-beam rather than the post and is interrupted. It actually appears to function like a girt.
4. 4 joints converge in one location of the tiebeam limiting meat and relish.
5. Principle Rafter foot is unsecured.
6. There are no continuous horizontal members to limit vertical movement. (Is the sill plate continuous? It is not shown or described.)
7. Why no continous plate member when it is evidenced that they were available by the 12"X12"+ X 55' timbers present elsewhere in the building.
I assume the Flying Plate was used to form an eave or cornice and that is why it is so wide in relation to its height? Is this assumption correct? This would also explain why it is connected to the tie beam rather than the post. Is this sound design practice or is it better to install a continuous plate and use an English tying joint then put rafter tails on to extend for the eave or cornice?
When I look at this design I see little difference between it and modern bent style construction.
In the Nevada City Rail Shed we see the trusses are tenoned into the post rather than bearing on a wall plate, also in modern bent like style construction. The wall plate is also interrupted though little is stated about where the interruptions fall and the joinery that was used in connection.
An unsupprted scarf joint is used midspan in the bottom chord which would appear to break all the rules but seems to perform admirably due to other design innovations.
Would any of you duplicate this design?
Why resort to an 18' max. timber length? CA was full of trees/timbers that could have easily spanned the distances required and this is the Railroad we are talking about so transportation to the site should not have been and issue.
The last question is regarding the more recently built Ferry Farm Pavillion.
I recently visited to look at the TF Guilds project there. It is quite a nice building. Upon scrutiny it raised two questions in my mind.
1. What factors played in the decision of the Kingpost/tiebeam joint
2. Why are the top plates interrupted rather than contiuous.
The reason I ask about the Kingpost tie-beam joint is it seems to violate the rule that pegs (or in this case keyhole wedges) should be not be weight bearing members but rather just assist the structure as place holders. In this joint it places the center weight of the tie-beam on the keyhole wedges in the tusk tenon. The tusk tenon has a check opening up right under the wedge in one case. The design leaves no wood houseing to prevent this check from spreading to where end grain tear out of the tusk could resulting in joint failure.
This appears on the surface to me to be a design flaw and is perhaps evidence why traditionally tusk tenons were used more in horizontal application (ie. anchor beams) than in vertical. Is this correct or am I way out of bounds? There are no pegs in the tie-beam which are usually seen in this arrangemen in more modern frames. Would the addition of pegs in the tie beam add or subtract from the structural soundness. Why do modern designs use this design so much as opposed to the wedged half dovetail?
What criteria was used in selecting the tusk tenon over say the traditional wedged half dovetail used in many American Kingpost joinery designs. Was it aesthetics, historical accuracy, engineering numbers, etc.? I know many of you were involved in this project and thought perhaps you could clear this up for me.
I know this is a lot of questions and a lot of assumptions and I might be way over my head here but then I've always been told there are no dumb questions.
