I am preparing to build a kingpost truss. I ran across this thread in research and thought it may deserve some more discussion from a slightly different angle.
I like the raised bottom chord design as it gets the kingpost pendant up a little further and makes a kingpost truss more friendly to ones head when used in a sleeping loft with short knee walls.
I have also pondered the use of the discontinuous bottom chord.
I fully recognize the superiority of the continuous bottom chord. In all cases it is beyond doubt the preferred road to take. I also prefer vanilla ice cream to chocolate but on occasion have been known to indulge in eating chocolate. I have found myself no worse for wear unless I get an inferior grade of chocolate ice-cream. (the "lite" versions just won't do)
It is always safer to stay were you are than to cross a highway but someday you may have need to cross and it can be done if you follow rules for safe crossing. In like manner, I see the question at hand being; can a discontinuous bottom chord be done and done well enough to stand the test of time? I have seen pictures of a number of examples that look well executed. And will no doubt stand a considerable time. One of these can be found in the Fox Maple Library.
At some point in time we may all be called upon to execute one of these designs either by owners request, or lack of long suitable timbers as our european predecessors did. Therefore, it is in our best interest not to ignore the issue but to develop some safegaurds.
After considerable thought I propose the following rules or guidelines for use of discontinuous bottom chords or raised bottom chords and offer them up as bowling pins to be knocked around a bit and see what is left standing in the end. I am by no means an expert and would seek to be corrected by any and all means.
1. Never use a discontinuous chord in a low pitch roof. 6:12 min and preferrably 9:12 or greater
2. Never use a discontinuous bottom chord in an application where it will support a floor load as well as resist tension at the eave.
3. Always use a joint designed for extreme tension. ie. a wedged half-dovetail or a spline.
4. Always design a backup plan for repair in event a joint shows signs of failure
5. Never raise bottom chord out of the bottom third of the trusses rise.
6. Always insure deflection is limited at the kingpost by bracing both at raising and during longterm use.
7. Design all extreme tension joinery in a manner that allows regular visual inspection for signs of shear or withdrawl.
8. No extreme tension joint should rely on less than 3 pegs
9. Allow for shrinkage when using green timbers
10. All wedges should be accessible throuout the life of the truss so that they can be tightened should conditions dictate.
So, how far out in left field am I?
