Fire tests on glued-laminated timber beams with specific local material properties

Abstract

The fire resistance of glued-laminated timber (glulam) beams is usually determined by calculation models or by tests. The evaluation of the fire resistance tests is accompanied by an uncertainty with regard to the timber material properties, which is, however, important to know when comparing the test results to the effective crosssection method according to EN 1995-1-2. The present paper presents therefore for the first time the results of six fire resistance tests with glulam beams with specific local mechanical material properties, such as exact position and dimension of knots, weak sections and finger joints, densities and strength properties of small cells. The tested glulam beams had an approximate height of 250 mm and reached a fire resistance of up to 68 minutes. The performed fire tests allowed the determination of the zerostrength layer with a higher certainty than investigations performed in the past. Additionally, reference tests at ambient temperature were considered in the evaluation. The determined zero-strength layer was in the range of 7 mm, as given in the current version of EN 1995-1-2 for beams in bending.

Review of the charring rates of different timber species

“…• On average, the charring rates specified in the NDS and Eurocode F I G U R E 7 Char depths of solid softwood and glulam taken from the literature. 19,21,23,25,43,[46][47][48][49] CLT specimen with 10 mm-thick laminations exposed for up to 71 min.…”

“…Thermocouples installed perpendicular to the isotherms may underestimate the temperature of timber as a result of conduction heat losses through the thermocouple wires, leading to underestimation of charring rate. 8,9 Installation of thermocouples perpendicular to isotherms via a drilled hole is convenient for large-scale specimens, and installation of inlaid thermocouples during cross laminated timber (CLT) panel assembly is not always possible. Other studies have found no effect of the thermocouple direction on temperature readings and calculated charring rates of CLT panels exposed to the standard fire curve.…”

“…non-loaded, [19][20][21]23,25,47,49 loaded in bending to 20% or 30% of their ultimate capacity, 46 or were timber-concrete shear specimens loaded axially to 10% of the shear connector capacity. 48 On average, the NDS overestimated the char depth of glulam exposed for up to 60 min by 17%, while Eurocode 5 overestimated char depth by 6%.…”

“…Char depth measurements from the literature for glulam and solid softwood timber are plotted in Figure 7. [19][20][21]23,25,[46][47][48][49] The glulam data include specimens with glue lines oriented both parallel and perpendicular to the direction of heat transfer. No observations of char falloff were reported for the glulam or CLT specimen data presented in Figure 7.…”

Experimental investigation of structural failure during the cooling phase of a fire: Timber columns

“…A layout of the thermocouples inside the timber column is shown in Figure 3. The location of the thermocouples was planned under the viewpoint of the installation parallel to the isotherm 23 . In total 48 thermocouples were installed in the timber column with 16 thermocouples per section.…”