Wooden structure surveys

The behavior of wooden structures is strongly influenced by the design of their members and by the actual decay level of the material, rather than by their mechanical properties. Therefore, when dealing with timber structures, it is essential to pay particular attention to the evaluation of parameters such as the durability of the wood types and the exposure to atmospheric agents as well.

To evaluate the integrity of wooden structures that usually make the ceiling or the roof of historic buildings, it is essential to carry out a visual inspection of their decay condition, which evaluates, in a preliminary way, the wood defects and its eventual deterioration. For this purpose, the description of the structural members and their detailed analysis with the evaluation of the physical characteristics is typically carried out (in particular density and humidity), the wooden defects (cracks and deformations), the alterations carried from biological (such as mold, brown rot and insects) or non-biological (such as fire and overload) causes. The inspection is also aimed at accurately evaluating the efficiency of the constraints, with special reference to their deterioration and sizing of joints, anchor bolts and brackets. It should be noted that the wood-metal and wood-masonry contact areas are generally the most exposed to deterioration because of the possibility of dew formation.

In many cases, the wooden members are not directly visible because hidden under floors: in these cases, the surveys must be carried out through small holes made in the deck and then by using bore-scope or video-probe.

The mechanical properties of the wood can be evaluated in an expeditious manner with the use of manual tools such as awl and hammer or, with greater precision, through some specific instrumental analysis. It is fairly frequent to assess the integrity of the wooden elements using ultrasonic tests, but this is generally not sufficient to quantify the extent of the damaged area.

Further techniques are based on the measurement of resistance to penetration into the wood of a metallic tip. The penetration resistance can be related to the density of wood, and furthermore to its mechanical characteristics; for this purpose different tools are used to obtain different levels of refinement in the section analysis. The tests conducted with resistograph are based on the same principle: a thin needle is forced to drill into the wood by rotation at a constant velocity. The variation of the twisting force defines the changes in the wood density values that occur along the wooden section.

These penetrometric diagrams are related to the density variation of the cross section of wooden members, and can be compared to identify areas where the wood characteristics are decayed due to cracks, brown rot or other biological attacks. A proper calibration matches the results of such testing techniques with the mechanical characteristics of the type of wood.