Alberto Cavalli

How to improve the on-site MOE assessment of old timber beams combining NDT and visual strength grading

For the conservation and restoration of old timber structures, the knowledge of the mechanical properties of each element is fundamental. For this reason, various nondestructive techniques were developed and investigated since the 1990s. Some of them provide very good results, but the solutions and the proposed models were applied only in few circumstances as a consequence of the on-site restrictive working conditions: no possibility to remove the elements, limitation to ends and faces accessibility and unknown density. The on-site inspection, including the visual strength grading, has been identified as the first step for the timber assessment. In this research, 13 old timber members of Silver Fir (Abies alba Mill.) were visually graded and tested with different nondestructive techniques for the density and modulus of elasticity (MOE) estimation: flexural and longitudinal vibrational tests, stress wave transmission time and Pilodyn penetration depth. The timber elements were also tested in a four-point bending test to determinate the local and global MOE. Finally, a reliable method, applicable to the limiting on-site conditions, was proposed and the results were showed.

The Influence of Routed Grooves on the Bending Behavior of Old Timber Beams

Old timber structures represent an important portion of the World cultural heritage: wooden buildings materials and building techniques are part of our history and their conservation is an essential contribution to cultural diversity and global cultural wealth. In recent times, the methods and approaches used to assess and to maintain timber in historic buildings have evolved considerably and various techniques for timber repair and strengthening were developed and deeply investigated. A very common and widely used technique for the timber strengthening-repair is realized by inserting reinforcement materials in grooves cut in the original sound wood. The proposed reinforcement materials are various (fiber reinforced plastic materials, steel bar/s or plate/s, wood and wood products) and the grooves number and depth are variable. Several researches were conducted to determine the Modulus of Elasticity (MOE) of old timber members strengthened with the described technics. A common approach is to consider the strengthened timber beam as a composite one and the MOE of the wooden part correspondent to the original. In practice, the timber members work together with strengthened materials, and the strengthening intervention is planned taking into account both original timber and strengthening material MOEs. The basic assumption of this approach is that the slots executed along the timber length have no effect on the timber MOE. The aim of this research is to investigate the effects of the routed grooves cut along the wooden beams, on the bending MOE of old timber members, to provide important information for the strengthening interventions plan.

Machinability of Minor Wooden Species before and after Modification with Thermo-Vacuum Technology

The influence of the thermal modification process on wood machinability was investigated with four minor species of low economic importance. A set of representative experimental samples was machined to the form of disks with sharp and dull tools. The resulting surface quality was visually evaluated by a team of experts according to the American standard procedure ASTM D-1666-87. The objective quantification of the surface quality was also done by means of a three dimensions (3D) surface scanner for the whole range of grain orientations. Visual assessment and 3D surface analysis showed a good agreement in terms of conclusions. The best quality of the wood surface was obtained when machining thermally modified samples. The positive effect of the material modification was apparent when cutting deodar cedar, black pine and black poplar in unfavorable conditions (i.e., against the grain). The difference was much smaller for an easy-machinability specie such as Italian alder. The use of dull tools resulted in the worst surface quality. Thermal modification has shown a very positive effect when machining with dull tools, leading to a relevant increment of the final surface smoothness.

Beech sawn timber for structural use: A case study for mechanical characterization and optimization of the Italian visual strength grading rule

The potential use of beech (Fagus sylvatica Linnaeus) sawn timber for structures has been investigated. Beech stands in transition from coppice to high forest after thinning interventions have been sampled from different Italian sources. A sample of 160 beams of two different cross sections was extracted and tested, according to the normalized procedures for the characterization of wood for structures. The specimens were visually graded considering the strength relevant defects, according to the Italian standard. To determine the mechanical properties of the timber (namely strength and stiffness), four-point bending tests were performed and the main characteristic values were derived. The results of the study reveal that knot ratio was the principal defect that influenced both resistance and yields. The potential strength class D30 was achieved with the resulting characteristic values, although, due to the small number of samples tested, the reached class could not be formally assigned. To improve the effectiveness of the grading rule in the Italian standard, some modifications and a new grade were proposed for the beech sawn timber. Finally the D40 strength class could be theoretically reached, but with reduced yields.