A natural fire test to assess the behavior of modern timber construction techniques: light timber frame vs. glued laminated timber

Wood, used as a building construction material, offers a range of advantages in terms of cost, time for construction, energy efficiency and sustainability. Modern timber construction is able to exhibit high anti-seismic and fire-safety performance, which is at least on a par with more “conventional” construction techniques (e.g. concrete, steel). Fire safety regulations have a major impact on the overall design of buildings with regard to layout, aesthetics, function and cost. In order to achieve the fire resistance requirements proposed in fire safety regulations, a fire protection cladding is commonly applied to load-bearing timber building elements. There are scarce literature reports of full-scale natural fire tests focused on the fire behaviour of load-bearing timber building elements [1, 2]. In this context, the current work aims to investigate the behaviour of two contemporary timber construction systems exposed to realistic fire conditions. A full-scale natural fire test is performed, using a test compartment, measuring
2220mm x 2220mm x 2110mm; an open window, measuring 430mm x 960mm, provides adequate ventilation for the compartment (Figure 1). The Light Timber Frame (LTF) and the Glued Laminated Timber (GLT) construction systems are utilized in the test compartment. A symmetrical layout is utilized; LTF is used in two vertical walls (S, W) and the ceiling, whereas GLT is used in the remaining vertical walls (N, E) and the floor. The test compartment is built following the Eurocode 5 design guidelines. Timber studs and battens are used for the frame of the LTF walls, with plywood panels serving as the “sheathing” material. The GLT walls are formed using pre-fabricated 5-layer CLT panels. In both wall types, a layer of rock wool insulation is used, as well as a final fire protection cladding, comprising two 12.5 mm fireresistant gypsum plasterboard panels, according to relevant energy and fire regulations. The test compartment is externally clad with decorative horizontal timber battens (Figure 1).
Aiming to investigate the fire behaviour of unprotected timber panels, a partially separating wall is also constructed inside the test compartment, using multiple layers of fire-resistant engineered timber panels (i.e. particleboard, plywood, MDF). A fir wood crib is used to provide the required fire load; the crib is located near the NE corner of the room, aiming to provide uniform thermal exposure conditions to the adjacent LTF and GLT walls. Extensive instrumentation is installed inside the test compartment allowing the continuous monitoring of
the temporal evolution of a large variety of physical parameters (Figure 2). Gaseous temperatures and surface temperatures at the interfaces between the wall layers are monitored using 29 K-type thermocouples; an infra-red camera is also aimed at the north GLT wall. Variations in flame shape and position are recorded using 3 video cameras. A gas analyzer is used to monitor the
chemical composition of the gaseous environment. The vertical velocity of the gaseous products emanating from the window is measured using a Pitot tube. The obtained measurements provide a detailed physical description of the main characteristics of the turbulent, reactive and multicomponent flow-field developing inside the test compartment. The temporal evolution of the recorded gaseous temperatures is found to agree reasonably well with the standard time-temperature curves (e.g. ISO 834, EN 1991-1-2). The measured wall surface temperatures, combined with information obtained by optical inspection after the conclusion of the test, suggest that the fire protection cladding does not fail and no charring occurs either in the LTF or the GLT walls. As a result, for both construction techniques, timber building elements are found to retain, at least for the 45 min duration of the test, their separating (compartmentation) and load-bearing functions, thus fulfilling the relevant mechanical resistance (R), integrity (E) and thermal insulation (I) criteria presented in EN 1995-1-2. In contrast, recorded temperatures (evidence of charring) and optical evidence (partial collapse of the wall) suggest that the separating wall, which is constructed using unprotected fire-resistant engineered timber panels, does not meet the aforementioned criteria, thus corroborating the need for adequate fire protection cladding (e.g. gypsum plasterboard layers) in timber building elements.