Design of arched bottom chord of timber truss 1

[davidfi]

I am working on the design of a heavy timber truss with an arched bottom chord (30' radius). The architect would like to cut the bottom chord from a 8x18 or 8x20 to be an 8x14. The chords will be spliced at the center king post. Is this even possible? Or does it need to be glulam?

The loads in the bottom chord are as follows:
Tension = 9.0 kips
Moment = 9.0 ft-k

I calculated my radial tension per NDS 5.4.1.3: fr = 3M/2Rbd = 3*12000*9.0/(2*30*12*7.5*13.5) = 4.44 psi <= 15 psi.

Is this correct?

Any ideas on how I can check a solid sawn member?

Thanks!

 

[mike20793]

It should be a glulam. It's very hard to get the curve cut right without introducing stress concentrations, especially with a beam that depth since it will require a specialty bandsaw. The glulams can be manufactured with the proper curve. Usually, it's best to leave this kind of decorative truss design to specialists since they can design it more efficiently, especially at the connections.

Whatever you do, don't forget to account for the thrust caused by the arch.

 

[SlideRuleEra]

I agree with mike20793 to use glulam.

Just out of curiosity, I did a rough check to see if the geometry of cutting a curved (rough) 8 x 14 out of a (rough) 8 x 20 would yield a significant length curved 8 x 14 beam. Assuming the ends of the curved 8 x 14 are perpendicular to the beam's center line, I get a length of less than 12'. You did not mention the required length, but sounds somewhat short.

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[jdgengineer]

Keep in mind that ripping sawn lumber violates the grade of the lumber.

 

[mike20793]

jdgengineer, great point. Thanks for bringing it up.

 

[davidfi]

Sounds like glulam is what it need to be. Thanks for the advice.

 

[KootK]

Usually, it's best to leave this kind of decorative truss design to specialists since they can design it more efficiently, especially at the connections.

I quite agree. One thing that I've always struggled with, however, is how far to take my own investigation of such a delegated item before passing it on. Obviously, not just anything will work, particularly when it comes to connections. Got any advice in that regard? All that I've heard so far is that keeping your member utilization under 70% might facilitate doable connections. Rough science.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mike20793]

It's a hard problem because most of us are used to designing out of steel, concrete, and masonry primarily and wood is just now catching on. The lack of heavy timber construction in my project area pretty much makes anything with heavy timber a specialty item. The non-Simpson connected wood projects I've worked on have all been truss modifications with plywood gussets minus one that used some split ring connectors for a rehab. That being said, usually if Simpson doesn't have a connector that will work, I'll reach out to Woodworks and see what they think. I did a house for a MLB pitcher that had arched decorative trusses throughout and they pointed me towards a specialty contractor that came up with an incredible design out of glulams. I did some rudimentary calcs to figure out a spacing, but the connections for the actual truss were out of my league.

 

[Triangled]

getting similar number to slideruleera, and for glulam you're probably getting into the region of specialty lamination thicknesses, ie thinner than standard to achieve the radius, not that it can't be done, nor is uncommon, nor is even hard to do, just that not every laminator is set up to do it. Glulam like this, I think, can be very attractive and I def agree with others in that I'd look at it and try to 'sell' it to the architect and owner. I'm sure you'll call a local laminator which can tell you how they do it and how to specify it quite easily. They may aid you in design as, I think, residual stresses, depending on the process of manufacturing, may be existent in the laminations that lead to some adjustments in standard design values...but your section size for that span suggests you'll have minimal issues. APA-EWS is often a great resource for glulam trivia, also AITC.