Many older and historic structures are built partially or entirely of wood, whose physical and mechanical properties are highly variable and characterized by a complex internal structure, which is porous, inhomogeneous, and anisotropic. The great variety of species, variability of the material, and many environmental factors affecting properties combine to make wood a very difficult material to analyze in structural applications, particularly in existing and historic structures.
Recent advances in computer technology have produced powerful structural analysis programs that allow engineers and architects to model and analyze historic structural systems to predict stresses and deformations in the members for evaluating safety and serviceability. The formulation of a structural model involves making assumptions about the physical behavior of the material. The precision of the model is related directly to the accuracy of the material properties such as modulus of elasticity used in the analysis. Since wood properties are highly variable and anisotropic, it is imperative that these properties be determined as accurately as possible. The model validation procedure requires measurement of forces, strains, and deflections in the members.
This paper discusses a variety of conventional and novel devices for in situ, nondestructive evaluation for determining mechanical properties of wood members in existing buildings. Examples include strain measurement, deflection measurement, visual evaluation, dynamic/vibration evaluation for both modal, and random phenomena. Acoustic emissions, ultrasonics, and stress wave analysis for evaluation of wood material properties are also discussed briefly.