In his series on The Development of the Truss Plate, Joe Kannapell is recounting many interesting details on the evolution of truss plates in the U.S. For another perspective, this article will focus on eight relevant nail plate truss inventions from Finland —two made by Matti Turulin, two by Markku Karhumäki, and four by me. From saunas and ice skates to Fiskars scissors and Nokia phones, Finnish ingenuity continues to evolve with the ages. [For all images, See PDF or View in Full Issue.]
In 2015, Matti Turulin made an automatic truss assembly line invention, patent US9126350B2, where robots make an assembly jig using magnetic grippers on a standing steel plate. The same robots assemble the webs and the chords fully automatically. No manual labor is involved, and only one operator is needed mainly to look after disturbances. This invention has been developed further and successfully commercially implemented in multiple facilities worldwide.
In 2021, Matti Turulin received patent FI128892B for his improved truss assembly invention where the truss is extruded in a short assembly station. Currently, this invention is in the development phase.
Tooth withdrawal from timber is the major reason for failure of the nail plate joint. The withdrawal resistance can be increased by barbing the tooth analogously to barbing a spike, however, such barbing leaves the opening edges serrated which weakens shear and tension resistance. In 1981, I received patent FI59840 for a nail plate where the opening edges are clean. Multiple teeth are stamped from one opening and serrations are made only between the tooth-tooth edges. This nail plate has had almost 100% market share in Finland for recent decades. The plate has good resistance values and can be applied for direct and roll pressing a long tooth (13 mm). Due to turmoil, I enjoyed royalties only for 10 years.
In 1995, I received patent US5440977A where the nail plate is fixed in timber in the timber cutting prior to the truss assembly. It allows simple, accurate, and automatic plate assembly as explained by Joe Kannapell in The Last Word on Pre-Plating in the November 2022 issue. I have sold this patent, and this invention has been recently implemented commercially by House of Design.
In 1993, I received patent US5234279A for a nail plate where teeth are stamped on the plate like in normal plates but also on at least one edge. A normal nail plate has about one-quarter of the steel in the teeth and the rest of the steel is used around the openings to maintain integrity. The edge teeth do not need integrity steel, however, and the efficiency of the steel usage is four times better. According to my studies, up to 20% steel savings is obtained in 1 mm steel and 40% in 2 mm steel. This invention has not been implemented commercially despite several attempts.
My dissertation in 1995 includes a unique truss testing invention, though not patented. Earlier, it was unclear in which way the nail plate joint should be modeled for the structural design. Some half dozen different models were widely used. Researchers argued about various models and used failure tests. I showed in the dissertation that such tests can’t disclose a correct model as the timber resistance in a defect can’t be defined. The comparison unknown-unknown is unfruitful. My invention was to compare the measured and the calculated eccentricities. The eccentricity measurements are made in clear cross-sections, and the problem of defects can be avoided. I had my own test laboratory and financed the whole dissertation by myself. Tests revealed that plate eccentricity, timber eccentricity, and the semi-rigidity of the plate-timber joint must be considered. I made a design program, too. All nail plate trusses made in Finland during the recent decades are designed using the model. Later, the model was implemented in prominent truss design programs worldwide.
In 1985, Markku Karhumäki received patent US4527933A where the tooth is barbed at the flat and stamped using a roller. This technology allows a high pressing force. The tooth can be appropriately formed and especially the root of the tooth can be reinforced. This plate was on the Finnish market for a decade.
In 2000, Markku Karhumäki received patent US6048165A for a nail plate packing method. The plates coming from the stamping machine have all teeth upward. Karhumäki’s device turns every second plate upside down using a unique brush gripping. After the sale of the patent, this invention has been implemented in multiple nail plate stamping facilities worldwide.
These contributions from Finland are helping define the global context of truss plates and component manufacturing.
A professor of Civil Engineering for the Faculty of the Built Environment at Tampere University in Finland, questions about this article may be sent to Tuomo via email: tuomo.poutanen@tuni.fi.