INFLUENCE OF MOISTURE ON COMPRESSIVE BEHAVIOR OF JAPANESE CEDAR AND LARCH STRUCTURAL GLULAM TIMBERS AT ELEVATED TEMPERATURE

Abstract

 Regarding the load-bearing capacity of timber elements subjected to fire heating, knowledge of not only the charring behavior but also the mechanical properties at elevated temperature are required because the temperature of the non-charring area gradually increases during the cooling decay phases in fire. Therefore, in order to predict the fire resistance of timber elements, it is necessary to grasp the changes in strength and elastic modulus of the timber materials with temperature rise.

 In this study, elevated temperature compression tests of structural glulam timbers made of Japanese cedar and larch were carried out for the purpose of understanding the influence of moisture in the timber on the compressive behavior at elevated temperature. The tests parameters were tree species, furnace temperature condition below 200 °C, and heating time. Influences of moisture in the specimens on the compressive behavior were discussed from the test results on the relationships of heating time and weight changing. Another purpose was to obtain a numerical model of stress- strain curves at elevated temperature to analysis the fire resistance of timber elements.

 Main findings of this study were summarized as follows:

 (1) Compressive strength at elevated temperature decreased significantly from the start of heating to 1 hour, and became the minimum during 1 to 2 hours. After that, the strength recovered with drying, and finally, it returned to the strength at ambient temperature.

 (2) The reason why the compressive strength decreased significantly from the start of heating up to 1 hour was that the steam softening and the change of water content due to migration of the moisture had a great influence¹²⁾.

 (3) Compressive strength of both Japanese cedar and larch decreased significantly when the internal temperature of the timber increased from ambient temperature to 70°C, and the lower values of the results up to around 100°C were close to the strength ratio of Eurocode5.

 (4) The modulus of elasticity at elevated temperature decreased significantly from the start of heating to 1 hour, and became the minimum during 1 to 2 hours, similar to change of the compressive strength.

 (5) Results of the load-displacement relationships indicated that the decrease of the rigidity and the maximum load in the initial stage of heating depended on the temperature rising rate.

 (6) The stress-strain curve for compression within 1% strain was approximated by Richard’s equation.