Timber Framers Guild Forums General General Forum Questions Of Kings and Queens and Princes and Princesses

DanG, I knew there was going to be a test.

4th post, page 1;
I've just reviewed the glossary of term in Cecil A. Hewett's "English Historic Carpentry" and he doesn't list prince post in the list of posts depicted in the book.

I've just reviewed the book "Timber Buildings in Britain" by R.W. Brunskill and he lists a princess post as a post between the queen post and the eave of a queen post truss....

The whole queenpost truss article is available here.
CB.

Thanks Don, somehow I just couldn't see that....
I must have been having a senior moment......
Thanks for that link to the article, Clark...

Has anyone got "engineering" approval for the kind of spans depicted in here
http://www.tfguild.org/publications/queenposttrussTF71.pdf

50 - 60 ft clear span...some over 200 yrs old and still standing.

There are a lot of "approved" engineered structures that are NOT still standing.....not to mention the thousands of old barns around the country that have long survived the stick built house on the same property.

I wonder how long exactly a building structure/system has to stand before it can be considered "SAFE" or "acceptable".....

Went back and read the whole Queenpost truss article. It is definitely incorrect logic to say queenpost= compression. According to article it would be more accurate to say Queenpost = Straining beam present. This, however, means that several prominent members of the TF community have publications which may contain glaring errors regarding kingpost or queenpost trusses.

I have difficulty with the "doesn't really matter what it's called as long as it works idea"

Isn't education the whole goal of the Guild?

Don't feel special Jim, I can't count the times she's reached around me to hand me the tool I'm looking for.

This is from the queenpost article;

"But trusses are not about bending. By definition, a truss is a framework that bears its burden via axial loading, in pure tension and compression. In this capacity, timber (and indeed all framing material) is many times stiffer than it is in bending. As a beam bridge, a 12-ft. 6x6 carries a midspan point load of 1000 lbs. with a half-inch of deflection. Stand the same stick up on end, load its top with half a ton and the newborn column will shorten a few thousands of an inch. Loaded axially the timber is a hundred times—two orders of magnitude stiffer than it is bending.
Conversely, consider the effect of an undersized truss member:
in bending (beam action) it can cause a local distortion of the
frame but, in buckling under axial load, it stands fair to bring
down the whole structure. As specified by the National Design
Specification for Wood Construction (NDS), the controlling design
value used to size a column or other timber loaded in compression
is compression parallel to grain (Fc) as modified by the column stability factor (Cp). Fc specifies maximum allowable axial compression stress by species and grade. To avoid the tendency of long thin members to buckle in compression, Cp lowers the working value of Fc via a complex calculation involving stiffness, slenderness ratio (effective length divided by least cross-section dimension), end conditions and other terms (Sect. 3.7.1, p. 22). Additional equations come into play if the timber in question in loaded in bending as well as compression."

That last sentence gets into section 15, if anyone can help me over a hump there I'd sure appreciate it.

Where my concern came from is the "Queenpost" earlier put the bottom chord in bending. A princepost wandering around on the bottom chord can put a kingpost truss bottom chord in bending. When an axially loaded member is handed too much bending, bad things can happen.

so does this mean not to put a curved piece in a truss? say an bowed down collar tie? because then it would flex and deflect for sure. very complicated equations. like as complicated as guessing.

Well, a wink is as good as a nod to a blind horse.

Do you plan on guessing at the member sizes in a 50 or 60 foot truss, do you really think "they" did? You may feel comfortable guessing, I sure don't.

I've seen a fair number of guesses and rarely seen people guess right, like I can remember one. I was seeing people use logic like, "I've seen a 2x12 span this far, so 2 2x's should span twice as far", guessing falls apart to the square of span. I put a beam calc up after I saw someone guess that a 6x8 would make a span that needed a 8x14.

Complicated enough that an engineer should review your design. I just like to understand what I'm doing. I consider that part of the craftsmanship equation. At the moment when a point load is delivered to the side of an axially loaded member, I'm the blind horse. I'm working on it. The curve, I've seen the math in the glulam guide... that might be next week

Just a doodle flipping the queen.


I think there is some wrong thinking here, engineering simply tries to figure out why the old stuff hasn't collapsed, quantify and understand that and use it when designing new. If your gut is that good, guess, some are, I aren't.

Uncomplicated? I'd try it like a simple beam.

goes from failure, poor, good, very good and excellent.

Some stuff from reading;

While it is recommended that all loads supported by a tension chord be located at a panel point, this may not always be possible,and bending moments may be introduced into the bottom chords. Such additional loads must be accounted for by designing and sizing the bottom chords as combined bending and axial tension members.

The compression chord usually carries most of the external loads, most often applied as uniformly distributed loads along the panel length. When the chord is continuous over several panels, it should first be evaluated as a continuous beam with the panel points assumed to be nonyielding supports. The reactions thus determined become the panel point loads used in the axial force analysis of the truss. The bending moments induced by the uniform loading are included in the combined stress analysis.

The combined axial and bending stresses at the center of the compression chord must also be evaluated. Consider first an initially straight laterally restrained chord, continuous over several panel points, with uniform transverse load and some axial force. Obviously the panels are in reality a series of interconnected beam columns supported at panel points and free to deflect in the plane of the truss.

At first glance the effective column length appears to be the distance between the points of contraflexure (the inflection points as the rafter curves over each strut then deflects down in span under the load). However, it can be shown that the effective length increases with increasing load, and of course, the stresses are nonlinear with load. The designer wishing to persue this complicated analysis can refer to section 15.4 of the NDS.

Normally, in sawn timber the complexities of the preceeding are avoided by assuming a conservative effective length. (it goes on, but the easy way is to multiply the point to point length x 1.5 to get the safe effective length, happy to see that).