Research into and Experience of the Use of Finger-Joints in Timber Structures in Czechoslovakia

Summary

Glued timber structures in Czechoslovakia have a rich tradition. Already in the years 1921–1938 before the second World War, many glued laminated structures were produced in Czechoslovakia according to Hetzer’s patent, some of which are still in use, although they were glued with casein adhesive. The basic necessity for reaching the required strength is efficient end-jointing of laminations. This can be produced with scarf or finger-joints. The finger-joint is more effective owing to the possibility of mechanized production and to the considerable saving of timber.

In the State Forest Products Research Institute in Bratislava, the strength efficiency of four types of finger-joints was experimentally verified (see Table 1). The experimental efficiencies were stated from the results of bending tests of jointed and matched control specimens of clear whitewood (Picea excelsa) of 45 mm square cross-section with a length of 750 mm. Both orientations of loading finger profile were tested (Fig. 8).

The Czechoslovak standard for wood structures design (CSN 731701 1971) gives a formula for calculating the finger-joints of whitewood η ≥ η min (1). Evaluation of the finger-joints according to this formula is based on the fact, that the effect of glued end joints on the strength of the jointed member has an influence analogous to the effect of natural defects in timber.

A mechanized line for jointing the elements has started up in the plant ARMABETON, TESKO PRAHA. This line was delivered by the West German producer DOTER GmbH. The full capacity of the line should reach 50 m³ of glued timber daily.

In Czechoslovakia a further two or three plants should be equipped with mechanized lines in the future. The lines should serve for the production of structural glulam members to meet the requirements of our building industry.