Strain measurement in glulam timber for bridges with embedded strain gauges and detection of moisture-variation effects

ABSTRACT

As an ecological and renewable material with good mechanical and geometrical characteristics, glulam is becoming more and more interesting in the construction of timber bridges. However, due to their exposure to outdoor conditions, these structures are subject to climatic wetting/drying (W/D) cycles, and consequently undergo numerous dimensional variations of shrinkage/swelling, thus impacting the durability of these structures. This study first proposes an accelerated W/D cycle to simulate the moisture content (MC) variations present in timber bridges, i.e. between 10% and 30% MC. For this purpose, a climatic chamber controlled in temperature and relative humidity, and immersion tanks were used to accelerate the wood MC diffusion. Subsequently, the monitoring of strain by gauges embedded in the core of the specimen was carried out to apprehend the shrinkage/swelling mechanisms induced by the W/D cycles. The results show that the bonding pressure applied during the manufacturing of glulam does not alter the measuring capacity of the gauges, and that the strain gauges allow to quantify the behaviour of the different glulam lamellae. The measurements and microstructural observations by X-ray tomography also revealed the damage to the wood under the effect of W/D cycles.

KEYWORDS:

• Glulam
• timber bridges
• durability
• wetting-drying cycles
• strain measurement

Acknowledgements

The authors would like to thank the GEII department (Electrical Engineering and Industrial IT) for its contribution to the development of the measurement system. Thanks also to Frédéric Leroy for his help in setting up the tests. The Research Federation FERMaT (FR 3089) is acknowledged for access to its EasyTom XL X-ray tomograph.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors would like to thank the funders of this work: the Occitanie Region, the Communauté d'Agglomération Tarbes-Lourdes-Pyrénées and the University Institute of Technology of Tarbes.