Elsevier

Materials & Design

Volume 30, Issue 3, March 2009, Pages 518-525
Materials & Design

Temperature distribution within glued laminated timber during a standard fire exposure test

https://doi.org/10.1016/j.matdes.2008.05.063Get rights and content

Abstract

The purpose of this study was to investigate the temperature distribution within glued laminated timbers (glulams) made from five softwood species as determined by a standard fire exposure test (CNS12514). The results reveal that the temperature within the middle section of tested glulam increased with increasing fire exposure time. After a fire exposure of 30 min and 45 min, the average temperature in the center of middle section for tested glulams (with a cross-section of 266 mm in depth × 190 mm in width) ranged from 27.5–28.3 °C to 29.5–33.2 °C; whereas, the average temperature for tested glulams (with a cross-section of 266 mm in depth × 140 mm in width) increased rapidly (from 32.3–51.1 °C to 39.1–89.3 °C) with an increasing exposure time and a decreasing cross-section. It was also found that the temperature in the center of the middle section increased slower than that in the right side or left side of a glulam middle section. The temperature within glulams (78 mm from the surface) remained constant without significant influence of fire exposure time according to CNS 12514. It can also be observed that equations developed by the Swiss Federal Institute of Technology give a closer calculation for the temperature development in tested glulams with heat flux from three sides.

Introduction

A glued laminated timber (glulam) consists of wood laminae, which are bonded together with adhesive. Its members are composed of individual pieces of dimension lumber, each end-jointed; that is, the grain of all lamination runs parallel the length of the member. Glulam is used widely in the European and North American countries because it is engineered and manufactured to meet a range of design stresses and can be used for large spans. In recent years, glulam has brought architectural concerns to the forefront in Taiwan.

Among the properties of glulam as an engineered wood product, fire resistance is important for fire safety. Fire reduces the dimensions of wood cross-sections as well as the strength and stiffness of heated zone close to the char-line. For the calculation of the resistance of load bearing wood sections it is therefore important to know the thermal degradation of wood (charring rate) and the temperature distribution in wood members exposed to fire [1]. In our previous study [2], we reported that the charring rate decreases with an increasing glulam density. We found that the charring rates of all domestic glulam specimens (including Taiwania, Japanese cedar, and China fir) were higher than those of Douglas fir and Southern pine glulam specimens. Furthermore, the average heat release rate and total heat release of Taiwania glulam were higher than those of the other glulam specimens because the Taiwania glulam was seriously damaged after a fire exposure of 60 min. In order to understand the fire performance of glulams, the relationships among temperature distribution within glulam, charring rate, and heat release rate should be studied.

White [3], White and Nordheim [4] conducted ASTM E 119 tests on wood specimens using a small-scale vertical furnace. Adopting the common criterion that the char front corresponds to 288 °C, they found that to char to a 13 mm depth of spruce or pine required 14.6–15.0 min if the specimens were conditioned at 50% RH. For specimens conditioned at 30% RH, the times dropped to 12.1–14.6 min. The corresponding times to char through 25 mm were 31–34 min for 50% RH and 29–33 min for 30% RH. Hardwoods required about 10–20% more time to char. Hakkarainen [5] measured charring of laminated heavy-timber members in a full-scale room fire fueled by wood cribs. On the ceiling, a char depth of 30 mm was obtained in 40 min, while on the walls it required 50 min. For the first 3 min there was no charring and after that the rate was somewhat faster in the early part of the test than later. Lau et al. [6] reported that the outer char layer was observed when the temperature reached 260–300 °C. In other words, the initial charring rate is faster during fire exposure test. Frangi and Fontana [1] reported that the temperature profiles in wood members exposed to fire are mainly influenced by heat flux, but the profiles do not appear to be sensitive to moisture content and density. A pyrolysis zone between 200 °C and 300 °C was indicated by Frangi and Fontana [1]. Lache [7], Schaffer [8], White and Schaffer [9] measured a temperature of the char front of 253, 288, and 360 °C, respectively. However, an accurate prediction of the temperature of the char front is quite difficult.

Because of the good insulation behavior of the charred and uncharred wood, typical temperature profiles through wood members exposed to fire exhibit a steep temperature gradient [1]. The thermal penetration depth (defined as the distance from the char-line to the part of wood) is an important parameter to describe the temperature profile in wood members exposed to fire. Lache [7] measured a thermal penetration depth of about 25 mm. On the other hand, Mikkola [10] gives a thermal penetration depth in the range of 40–50 mm.

The objective of this study was to investigate the temperature distribution within glulams made from five softwood species as determined by a standard fire exposure test (CNS12514). The temperature distribution within the glulam was measured with a standard fire exposure test. These results can provide information for estimating applicability of glulams in architectural design.

Section snippets

Tested glulams

Five softwoods, commonly used as building materials in Taiwan, including: Japanese cedar (Cryptomeria japonica), Taiwania (Taiwania cryptomerioides), China fir (Cunninghamia lanceolata), Douglas fir (Pseudotsuga menziesii), and Southern pine (Pinus spp.), were selected for this study. Two different lumber (lamina) dimensions were tested as well – 38 mm (thickness) × 190 mm (width) × 1800 mm (length) and 38 mm (thickness) × 140 mm (width) × 1800 mm (length). Each glulam was made of seven laminae.

The K

Temperature distribution within glulam

Table 1 shows the temperature distribution within five softwood glulams after a standard fire test CNS12514. During a fire exposure of 60 min, the temperature within the middle section of all glulam specimens (8F group) increased with an increase of fire exposure time. It was also found that the temperature in the center of glulam middle section increased slower than that in the right side or left side of middle section. The temperature in the right side or left side of middle section increased

Discussion

During fire exposure, the height and width of lumber decreases gradually, dependent on the charring rate of the species. In our previous study [2], it was found that the charring rate and total heat release in all glulam specimens showed a decreasing order as follows: Taiwania > Japanese cedar > China fir > Douglas fir > Southern pine glulam. Yang [15] also found that the temperature within Taiwania, Japanese cedar, and China fir glulams increased rapidly (especially within small cross-section glulams)

Conclusions

The temperature distribution within glulams made from five softwood species as determined by a standard fire exposure test (CNS12514) was investigated in this study. During a fire exposure of 60 min, the temperature within the middle section of all glulam specimens (8F group) increased with an increase of fire exposure time. It was also found that the temperature in the center of the middle section increased slower than that in the right side or left side of the middle section. The temperature

Acknowledgments

The authors wish to thank the National Science Council (NSC-94-2211-E-002-058-), and Architecture and Building Research Institute, Ministry of the Interior, Executive Yuan, Republic of China, for financial support.

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