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.

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

Christopher,

Point taken, A good TF engineer is worth his weight in golld 10X over, there is however, a disappointing trend taking place in engineering. "CYA" engineering has become the norm rather than the exception. It is due to lawsuits and I understand it is not going away. Today's engineers seldom place any materials in testing these days and do most structural analysis based on tables and computer modeling. These tables are in essence only "rule of thumb" for a given GROUP of species. Seldom is follow up testing done to verify complete working unit. Real world application:

Scenario 1

1. Logger cuts tree without knowing end use.
2. Mill cuts into lumber not knowing end use.
3. Lumber grader grades timber not knowing end use of timber and based on grade tables. Grade tables are based on average safe working loads.
4. Engineer specifies joints and timbers based on average safe working loads per his tables and adds a little CYA factor.
5. Hourly laborer lays out and cuts each timber which is grade stamped based on shop drawings which reflect a enginners stamp.
6. Frame is erected by commercial timberframing company and then they leave.
7.Holes are bored through it by Electricians, plumbers, HVAC techs, security system installer, Cable TV installer, etc. who have no knowledge of timber framing structural elements.
8. Local Building inspector looks at Stamped plans and graded timbers and calls all good as he has little knowledge of timber frames and CO is issued.

Scenario 2

1. A frame is designed with sound historically proven joinery.
2. Local trees are selected for specific purposes by timber framer, based on his cut into logs of appropriate length and labeled for sawyer.
3. Local sawyer cuts logs to specified dimensions. Any questionable timbers are culled and set aside for scantlings.
4. Timbers are layed out and cut by timber framer with knots, crown, twist, shake, check and grain runout taken into consideration for each timber.
5. Frame is erected by neighborhood help without engineer or building inspector involvement.
6. All sub trades work under supervision of timber framer.
7. Timber framer hands keys to the new owner upon completion.

Evaluate:

Which one of these structures would you want to live in?
Which one is most likely standing 200 years form now?
Which one is most environmentally responsible? Which one is most likely to be involved in costly litigation?
Which one costs the least?

Too bad that housing is a function of legality these days rather than craftsmanship isn't it.

I have seen too many high dollar engineering screwups. My faith in engineers is not as great as it is in real world testing and a working knowledge of the materials at hand.

"Leave it to an engineer" is often the kiss of death rather than assurance of positive outcome.

Thought I'd throw these in for comment and fuel. In place 10 years (admittedly not 200..) but look as good as the day. Wisconsin, 35 lb snow, 6" pitch, mixed unseasoned oak, 22' span, KD NRO spline, 1 1/4" pegs. Getting the curve out of one piece, repeating the same radius throughout the series of trusses, was highly impractical without this approach. Comments? C'mon I can take it..

crabtree, I don't have as much experience as you but I recognize a good argument when I read one.

After answering Scenario 2 with all questions except for the litigation one. I wondered, where can you find this sort of deal. In any area or state is this acceptable (to the powers that be)? And if not how do you coup d'etat?

Hi Crabtree,

It might appear to some that you are implying that the two scenarios described above are mutually exclusive i.e. elements of one are not generally found to be present in the other and this is not a reasonable argument.

Regarding litigation being pursued against an "engineered" frame - who is that would be called upon to provide expert advice against the original defficient frame design - a designer, an engineer, a building inspector or a carpenter ?

I understand your frustration but please be assured that there are engineers in this world that do know their arris from their elbow.

Regards

Ken Hume P.Eng.

Two ways to do that...

One: owner pulls ALL the permits and pays 'cash' for everything (no construction loan). House can be lived in and inherited but not sold without a ton of waivers from the buyer.

Two: add one more step, call in an engineer after the fact for an "as built". Might have to go through several engineers (or park a tractor on the roof) to get that done.

Dovetail,

I like it! looks like it exhibits the majority of the "rules" I submitted. Was this an engineered design? Did you mill your own timbers? Can you give us any more details on the joinery selection?

Mo,
Please don't credit me with a lot of experience as I am certainly a novice. The situation I created was a hypothetical of two extremes. I don't neccessarily condone either. Just something to make people think about the laws of averages.

Ken,
I would propose litigation will brought by the owner or a future owner against all involved in the construction from designers and engineers down to local inspector. He will need no expert other than a few pictures of the frame coming apart and estimates for repair or worst case scenario the death certificate of a family member killed in its collapse.

As an engineer, how much of your work is based on computer generated models and tables? What measures do you take to insure that the aforementioned tragedies don't occur in your designs? Please do not take any of this as a personal indictment. As I said a good engineer is worth his weight in gold.

As a professional have you seen builders trending towards a loss of craftsmanship and a turn towards profit margins?

To all:

How is it that codes will permit people to live in mobile homes stacked on concrete blocks and built out of 2X3's with extremely flammable panelling while a guy trying to build with massive timbers in a time proven method has to get in line at the local engineering firm or provide proof it won't fall apart.

Hi Crabtree,

How long have you got ?

It might be more appropriate for TFEC members to provide some answers to your questions as this type of issue falls within their balliewick.

There are significant and well recorded historical precedents as to what types of timber frame design will survive over the longer term.

Timber frame buildings do not usually fail catastrophically but instead tend to slowly yield to the ravages of time.

Fire will affect a timber framed building more slowly than an equivalent size balloon (stud) frame building since it will take longer for the primary components to fail as the fire gradually wastes their section. Design for fire resistance has more to do with providing sufficient escape time for occupants than trying to preserve the structure itself.

Computer modeling is probably the greatest step forward yet made in building structure design. This facility is particularly useful where custom or esoteric designs are being pursued since it helps identify potential problem areas within a frame but also don't forget that it also identifies those components that are not highly stressed and could therefore be made from lower grade materials thus with good design engineering it might be possible to produce a more cost effective design that could be sold more affordably or alternatively provide a higher profit margin to a builder without the need to unduly compromise safety.

Most people do not want to pay for design or engineering and some believe that they are quite capable of handling all technical issues by themselves and up to a point this can be true i.e. when a frame is being built to a building pattern that has passed the test of time then the scope for encountering major problems are probably greatly reduced. It is where the uncertainty or consequence of failure becomes higher that engineering input should be sought.

The issue of legislation must also be considered in that some states / provencies / countries reserve certain building design matters to engineers and it would appear foolhardy to ignore those requirements.

Applying good old fashioned common sense combined with experience is a pre requisite to undertaking any safe building programme but raises the issue of how a person is able to aquire experience and get to know his own limitations without harming himself or others arround him who might have had no say in the decision making process.

Building timber frames is a complex process and I have seen more than my fair share of building solutions with which I have not been particularly happy. Its a terrible feeling when a person is smiling ear to ear proudly displaying their work and inside you are thinking "OMG". How do you tell them ? Should you tell them ? Will they thank you ? Will they hate you ?

The longer term solution lies with bodies like the TFG and CF where free exchange of thoughts, ideas, concerns, and solutions will help to spread knowledge and understanding of timber framing issues and practices in so doing will hopefully improve the general levels of personal safety and building performance.

These are complex issues and cannot really be done justice on a short post on a bulletin board.

Regards

Ken Hume P.Eng
http://www.kfhume.freeserve.co.uk

I've got one of those situations in front of me on another BBS. Nothing to fatal but it could lead to premature failure of a shed/small barn frame. So far I've kept mum, the knee braces look good (in photos) and there is a lot of solid sheathing that will help keep the frame rigid.

The situation is compounded by the fact that it's a wood working project contest entry and the only other TF entered in the contest is my own. To make matters more interesting, the person in question has listed their occupation as Engineer, while I'm listed as a Medical Technologist/Landscape Designer/Wannabe Timber Framer.

As an engineer, what would your reaction be to a "complete neophyte" pointing out flaws in something you designed and built (from tree to finished product)?