The jist of this post is a search to design a truss without it’s key member as it is normally configured, and I am at a loss to understand why…

Continuous ties are a proven part of good design.

I know I am blessed to be in a part of country with an almost ready supply of long locally milled timber, in part, this is driven by the demand created by the large number of area framers and this is not true for all regions. There are always national timber brokers or the possibility of convincing your local bandmiller that an investment in a bed extension represents regular orders from you, or you could always set up your own mill, it need not be a fancy one if your intent is only to mill the dozen or so long sticks each frame represents, and maybe some tapers here and there

I and other area framers I work with regularly, routinely include continuous ties, plates, joists, and purlins into our designs / frames. It just makes for a stronger and superior frame and additionally, it is far cheaper labor wise to design with long sticks, than it is to join a bunch of short ones.

Guess I’m sayin… fix whats broke.

And Joe is not only an engineering whiz kid, he’s a highly capable framer, well worthy of a listen.

I'm glad to see there's finally some action on this post. As far as why design a truss with a discontinuous bottom chord: there's all the stuff you mentioned, i.e., not readily available local long timber, expensive to ship from elsewhere, etc., plus, compare moving a 15 footer to a 30. I could go on, but I think the convenience (not to say the
'advantages', these may be dubious) is self-evident.

Anyway, if anyone else would like to weigh or re-weigh in, I'd still like some light on why a discontinuous bottom chord on a kingpost truss is so antithetical if it does the job in a raised bottom chord, as in TF 73. Also, why are compression rings (and what are they in this instance?) necessary in plate w/ bolt tension and not in the traditional joinery?

thanks all,

Matt

By pressure rings, I am presuming he meant shear rings or split rings; common industrial connectors to greatly increase the capacity of bolts subjected to lateral loading.

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?

When using tension joinery for large loads, there is a universal rule of thumb that works in all situations: Calculate the loads and design the members and joints to safely resist them. If you don't know how, hire an engineer. To use any other rule of thumb in such a situation is to tempt fate.

I completely agree with Christopher.

gh

This topic interests me as well. I appreciate the call for an engineer, I also like to understand what is going on.

Let me try an example.

A 24' kingpost truss with 12/12 pitch, 10000 lb combined load and bottom chord at the plate. I come up with ~3750 lbs tension at the spline and ~5300 lbs compression at the heel joint.

If I raise the bottom chord do the forces increase as a proportion of height the chord is raised up the king? In other words, if the tie moves up 1/4 the height of the king does the tension increase by 25%?

It all depends on the locations of the loads on the truss. The truss may be supporting a point load only at the peak, it may be supporting multiple purlins, it may be supporting SIPS spanning between trusses. The bottom chord may or may be not be supporting attic loads. For some configurations, the tension in the bottom chord is increased by a third if the chord is raised 1/4 of the truss depth; the tension is doubled if it is raised half way up; and the tension is quadrupled if it is raised 3/4 of the way up.

Christopher, so I interpret your comments as, a load (at the tie) applied by a lever (the rafter chord) is increasing in factors starting at the bottom going up in quarters as 4/4, 4/3, 4/2 and 4/1. The next problem is bending in the rafter chord resulting in spreading plates and lowering of the ridge. The rafter chord, to resist the bending and plastic deformation, would require substantal increase in section over a conventional axial loaded truss chord.

Don, Also consider, the heel joint (rafter on top of tie) is well suited to handling the compression forces. As soom as you begin to move the tie up the truss, the joinery will have to change to a tenon and peg or wedged dovetail in a mortice that has to deal with the forces in tension. Tom