Joint Busting! Try Your Skill!
The second installment of the tradition started at the 2009 Eastern Conference in Saratoga.
Bring your best scarf joint to be stressed to the max.
Contest will be held Saturday (outdoors, weather permitting).
You need not be present to win, but need to get your scarf to the venue.
We will load splices in bending, NOT axial tension/compression. We suggest that each specimen have a descriptive name, selected by the designer/builder (for example, “Titanic” or “Big Bertha” or “Finnigan's Folly”). Please do NOT leave it to the testers to do this.
Testers/judges:
Ben Brungraber, Mack Magee, Joe Miller.
Sponsored by Fire Tower Engineering and Fraserwood Industries.
Specifications
- Two @ 8" x 8" sections
- Specimen to be 8' long, when splice is assembled
- The "splice zone" is limited to 24" long; centered on specimen
- There are no particular rules on species and methods. Steel optional.
Other specs
- We will be loading with a single transverse point load in center.
- 24,000 pound capacity. Maximum deflection will be less than 3".
- Testers can limit/truncate tests for safety concerns at deflections past an inch.
Report from Saratoga's Event
Following the lead of the Carpenters Fellowship in England, a group gathered at the Eastern Conference in Saratoga Springs, New York, to watch as scarf joints met their match. The rules were simple: scarf together two 8x8 timbers into one that was 8 feet long, with the length of the scarf not to exceed 2 feet. Four companies stepped up to the plate by bringing a total of six different scarfs, and watched as they were loaded in bending as a hydraulic jack induced midspan point load. Determining which way to bend a scarf joint was no simple task, as some scarfs were capable of resisting a bending moment with the load applied on any of its four faces, whereas others were so specialized that they act as purely one-way connections.
With the aid of a pressure transducer and a string line, we determined the applied load and deflection during testing. Likewise, we recorded several critical points of measurement, including deflection at 1,000lb of applied load and the load applied when 1/4" of deflection was reached. Simply seeing (and hearing) the various failure mechanisms of the joints proved as useful, if not more so, than the load and deflection data.
The first scarf tested was aptly named the Big Dog Bone. This scarf included a dog bone-shaped piece of steel in the tension face, which worked surprisingly well. Despite being the strongest joint in the line up, it was also one of the least stiff at low load levels, as some deflection was required to fully engage the steel. After further investigation, it appeared the steel dog bone had actually yielded.
The SOB joint (“Saratoga or Bust”) was the only scarf joint fabricated from reclaimed timbers (being resawn from old telephone poles.) Due to the number of mating faces, this joint was not overly stiff. However, its tenacity cannot be called into question, as despite being splayed apart with excessive cracking and splintering (which was notably absent from the first scarf test) at high levels of deflection, the joint never ceased to carry load.
The Double Deuce joint appeared deceptively simple, consisting of a tabled scarf, but with a slight twist: the shoulders were slightly inclined such that they bound as the joint flexed. The twist appeared to work, as this was the second strongest joint in the line up. A textbook block shear failure, just as predicted by Jesse Kendall, was the final failure mode.
Jesse also was vindicated with his prediction that the Diamond Dove was going to be a poor performer. While the connection was interesting in appearance, and the use of opposing wedges allowed for an initially stiff joint, things only went downhill from there, with the joint essentially self-destructing under high levels of deflection.
The Pop-Sicle joint was an interesting contender, in that it can only be cut on a CNC machine. The opposing wedges again ensured that the joint was initially stiff, but also proved to be the weak link on this connection as well, as the wedges sheared through at high load levels. By the time the wedges had sheared, Douglas fir's abysmal tension perpendicular to the grain value had reared its head, as the timber was splitting from end to end.
In a moment of last minute flurry, the “LignaTool” scarf was constructed out of a piece of dripping wet white oak. The scarf was cut using the Ligna Tool system, normally used for dovetailing joists into girders where shear is the transfer mechanism, not as an endgrain to endgrain scarf joint. Despite its not being intended for such loading, the joint still behaved admirably, coming in as the stiffest at the 1,000 lb load level.