Deflection behavior and load-bearing period of structural glued laminated timber beams in fire including cooling phase

Abstract

Purpose

This paper aims to discuss the fire performance of glulam timber beams based on their deflection behavior and load-bearing period, which were obtained from load-bearing fire tests under constant load conditions.

Design/methodology/approach

In this report, the fire performance, primarily deflection behavior and load-bearing period of glued laminated (glulam) timber beams will be discussed from the standpoint of load-bearing fire tests conducted during the cooling phase under constant load conditions. Then, based on the charring depth and the per section temperature transformation obtained from loading test results, the load-bearing capacity of the glulam timber beams will be discussed using the effective section method and the strength reduction factor, which will be calculated in accordance with the European standards for the design of timber structures (Eurocode 5).

Findings

In the cooling phase, the charring rate is decreases. However, as the temperature in the cross section rises, the deflection is increases. The failure mode was bending failure because of tensile failure of the lamina at the bottom of the beam. Moreover, a gap caused by shear failure in a growth ring in the beam cross-section in the vicinity of the centroid axis was observed. Shear failure was observed up until 1 to 3 h before end of heating. The calculated shear strength far exceeded the test results. Shear strength for elevated temperature of glued laminated timber is likely to decrease than the shear strength in Eurocode 5.

Originality/value

Unlike other elements, a characteristic problem of timber elements is that their load-bearing capacity decreases as they are consumed in a fire, and their bearing capacities may continue to degrade even after the fuel in the room has been exhausted. Therefore, the structural fire performance of timber elements should be clarified during not only the heating phase but also the subsequent cooling phase. However, there are few reports on the load-bearing capacity of timber elements that take the cooling phase after a fire into consideration.

Citation

Kinjo, H., Katakura, Y., Hirashima, T., Yusa, S. and Saito, K. (2018), "Deflection behavior and load-bearing period of structural glued laminated timber beams in fire including cooling phase", Journal of Structural Fire Engineering, Vol. 9 No. 4, pp. 287-299. https://doi.org/10.1108/JSFE-01-2017-0009