Assessment of timber element mechanical properties using experimental modal analysis

- A set of eight Douglas-fir beams were tested.
- A Python-MatLab script was created to perform the modal updating.
- A sensitivity analysis was conducted to reduce to number of unknown parameters.
- A validation with existing classical techniques has been achieved.
- Obtained results are insensible to local defects.

Nowadays, timber frameworks become one of the most widespread types of material in building constructions. Their configurations and joints are usually complex and require a high level of craftsmanship to assemble. To properly assess the behaviour of timber elements, many assessment techniques are available but they present some limitations. This paper presents a non-destructive approach to finite element updating using results from experimental modal data. Several timber beams in a free supported condition were analysed in order to extract the frequency response functions and their bending natural characteristics (frequency, damping and mode shape). A finite element model was derived, taking into account the possible effect of local defects. With the intent to estimate the corresponding mechanical properties, a procedure of modal updating was carried out by implementing various modal criteria. The objective function was selected from the conventional comparison tools (absolute or relative frequency difference, and/or modal assurance criterion). This assessment tool was tested to determine the mechanical properties of timber beams (elastic moduli and damping properties). To verify the modulus, a series of static 4-point bending tests and STS04 classifications were conducted. In addition, sensitivity analysis reduced the number of parameters to be updated (Young's modulus along the beam length and the corresponding shear moduli), showing that the present method is an efficient alternative to conventional wood tests.