I'm trying to design a kingpost truss to span approximately 30 feet. I'm thinking of more or less duplicating the truss from the Lynnsfield Meetinghouse detailed in issue 72 of Timberframing.

My question is this: Since the kingpost is already in tension, what are the potential risks of replacing the 30 foot bottom chord (tie) with two 15 footers splined into the extended (pendant) kingpost rather than wedge-dovetailing the latter into the tie? Should I reduce the dimensions of the ties to reduce weight while somehow still maintaining tension against rafter spread? Should I beef up the upper chords (inner rafters) to help suspend a heavier kingpost (to resist end shear) without buckling?

I'm trying to avoid the difficulty of obtaining and handling such a long tie. Any advice, including suggested dimensions for timbers, would be greatly appreciated.

Matt Champagne
Hammer & Chisel, Inc.

Kingposts are, almost invariably, in a lot less tension than the bottom chord of a king post truss.

Per the article if TF 73, the bottom chord of the Lynnfield truss carries 16,600 lbs of tension, while the king post only 4,200 lbs. This shows the great efficiency in the truss, as it is much easier to let the wood do the work (continuous bottom chord) than any sort of joinery, to carry the tension.

Trying to manage that amount of load (numbers will change based on your application) with traditional joinery, or even a spline, seems quite optimistic. Couple that with the tendency for the bottom chord to want to bend at the king post connection, and you run into other issues as well.

Thanks for the reply Joe. In issue 74 there's detailed a raised collar truss which seems to hold up to analysis. Is there much difference in making a bottom chord discontinuous, especially if you add some metal? My intuition says that a straight bottom chord even if discontinuous would be stronger than the joinery in the issue '74 truss, although the fact that the raised collar is continuous between rafters shakes that intuition a bit. Does the engineering shed any light on this?

Thanks,

Matt Champagne

When you raise the tie, you rather quickly increase the tension in it and increase the bending in the rafter. The problem comes down to heavy tension joinery, is difficult, but certainly not impossible, to achieve with traditional joinery and still have a stiff connection. Steel, unfortunately, seems to be the easiest facilitator in such situations.

All else being equal, a straight tie will have less tension in it than one that is not straight; but, depending on how things frame into it the tie, the increase in tension you get may in fact reduce stresses elsewhere in the truss.

I can't seem to find my copy of #74, so, I cannot comment directly on that. I will search again, however.

I have been working with a company that makes a lot of porches using a 2 piece bottom chord (arched) into the king post. Spans typically 9 feet or less.

The king posts are typically 8x8 and the bottom chord 6x12 or 14 but with an arch cut to make 6x9 or so.

I'm sure they use 2 pieces to save on material but neither they nor myself know if this is inherently a bad practice. I keep encouraging them to get some engineering help but unfortunately they have not moved on this suggestion.

Perhaps with some expert advice I can further encourage them to move in that direction.

Regards,
Steve.

Joe

You mentioned that the bottom chord tends to bend at the kingpost.
I assume this is because as the truss settles the kingpost moves down relative to the bottom chord and so the shoulder of the kingpost at the post bottom tenon presses down causing the chord to bend downward.
If this is the case it would seem that this shoulder serves no useful purpose and is in fact a detriment. Would it make sense then to either move the location of the shoulder upward in anticipation of the movement or alternately eliminate it by moving it well upward and making it a reduction to a tenon that can then move downward whatever distance necessary?
Of course this would mean you would have to either do a thru tenon with wedges at the chord bottom or create slots for your pegs.

J.E.B.

The purpose of the king post is to keep the bottom chord from sagging. The king post is literllay hanging from the rafter peak connection, and holds up the bottom chord in the middle. That's why the king post is in tension, and that's why the bottom chord wants to bend in the middle where the king post is pulling up on it. If a 2 piece chord is used, then the joint would have to withstand not only the large tension force from the rafter thrust, but also the bending force from the king. CB.

Hi all:
great topic, and here is my two cents worth taken from my observations and experience over the years.
trusses that I have noted mainly in spanning the sanctuaries or main areas of churches are in 2 classes, arched or straight across, some of the trusses actually hold up part of the weight of the spires, and have did this remarkably well for hundreds of years. The majority of these trusses bottom chords are 14 inches square, and no matter if they are bridge trusses or king post style, the bottoms of the king posts or otherwise are not pinned but rather dovetailled into the bottom chords and wedged. As far as I can see very little downward movement is evident except when rot happens and lets things sag.
As far as using metal plates and bolts, it just isn't that simple you also need pressure rings for heavy tension joints.
NH

The bending in the bottom chord to which I was referring relates specifically the Lynnsfield truss. The connection between the top chord and the bottom chord does not occur over the supporting post at the eave, which means the bottom chord is carrying that as a point load in span. The bottom chord is then held up at the eave post and at the king post; and since the bottom chord is continuous, there is reveresed bending stress at the king post location. It is because of this that the kingpost is in tension in this instance. It is indeed trying to hold up the bottom chord.

Ahhh ....it is all so simple

Thanks Joe, I retrieved my copy of TF 72 and I see what you were refering to. The location of the upper chord inboard of the eave posts transfers a load to the bottom chord that the kingpost is restraining in addition to the weight of the bottom chord. It would appear that you would want to use only moderatly curved timbers for the top chord as more cuvature would increase the distance from eave post to chord foot and load the kingpost that much more.

CB I did know that but got my head on sideways after reading Joes first post.

NH what are the spans on the trusses you refer to?

And a question to all...why a wedged dovetail?

J.E.B.