Non-destructive and semi-destructive test methods for strength determination of aged wood

摘要

To what extent is there a change in the strength of wood over the course of centuries, when installed under a constant load? Are there non-destructive measurement methods for the determination of strength that are suitable for this investigation? These two questions were the starting point of this research project. Through experience we have learned that the strength of wood is dependent on the growth regions and climatic conditions in which it originates. For example, over centuries, in a northern Italian valley, there were commonly known and clearly defined areas which provided the wood for the construction of the local buildings. Another decisive factor for selecting this research region was that the buildings concerned had already been dated by means of dendrochronology. This created a unique situation in that the strength of the timber in these buildings, which cover about nine centuries (1250 to present), could be compared and analyzed. Local spruce and larch used in the construction typical of this region were selected for the investigations. Starting with 1250AD (the oldest dated wood construction) and in consecutive steps of 100 years, two projects (of the entire 18 projects) are selected and explored. Three non-destructive and respectively semi-destructive measurement methods were used for the strength testing. The measurement methods were: drill resistance measuring, fractometer tests (bending and compression tests) and ultrasonic tests. So the validity of the measurement systems used could be brought into relation with the actual strength. Some small, flawless samples (without knots) from selected projects were harvested and then tested in a laboratory with the universal testing machine.