Behaviour of Loaded Cross-Laminated Timber Wall Elements in Fire Conditions

Abstract

Cross-laminated timber (CLT) is increasingly being used in medium-rise timber buildings for a number of reasons, such as rapidity of construction, cost effectiveness and robustness. Like for other building materials, verification of the loadbearing performance in fire conditions is an important issue. Experimental fire tests have been performed on loaded CLT wall elements at research institutes in Sweden and Italy. In total, three large-scale and four medium-scale tests have been carried out. The aim was to gain information about initially protected and unprotected elements, to be used for classification and also for validation of calculation models. In the test series, reference tests at normal temperature were included to obtain information (e.g. stiffness, strength) about the material tested in fire conditions. In addition, model-scale fire tests were performed to investigate the loss in stiffness resulting from fire exposure and the effect of different protection types. Loaded fire tests varied in the range of 41.8 min to 120 min, depending on the CLT structure, the level of load, and the type of protection. Data on temperature within specimens and residual crosssections were collected. Charring rates evaluated from experimental results were comparable with values proposed by Eurocode for the design of timber structures. Conservative solutions were obtained by using simplified design methods and comparing their results to test results and results of advanced modelling. It was shown that the load-bearing performance of CLT may show abrupt changes due to its layered structure. It is strongly recommended that a minimum residual depth depending on the CLT structure should be required in order to ensure robust building products.

Structural response of cross-laminated timber compression elements exposed to fire

“…Thus, a few millimetres difference in the assumed ZSL can have a major impact on the predicted deflection, and hence on the predicted failure time. This issue has also been discussed by Schmid et al [11], who showed that CLT wall elements exposed to standard fires failed when the charring had reduced the initial strong parallel layer to only a few millimetres. On this basis, Schmid et al [11] subsequently proposed that a minimum residual depth (of 3 mm) of the strong layers should be imposed in design to account for uncertainties in the charring rates arising from changes in density, moisture content, or fire exposure.…”

“…However, the constant 7 mm ZSL depth currently suggested in design codes [4] is based largely on models calibrated from a relatively small number of flexural standard furnace tests on glulam beams undertaken in the 1980s [8]. The applicability of the current ZSL value to CLT in general [9], and to engineered timber compression elements more specifically [10,11] is doubtful.…”

“…Schmid et al [11] performed tests on compressively loaded CLT wall elements exposed to fire from one side, and postulated that a minimum residual depth of 3 mm should be imposed when considering CLT, in which the cross layers have negligible strength and stiffness in the primary loading direction, particularly considering the propensity for buckling of compression elements and the comparatively fine margins of residual cross section depth that could result in instability failures. Schmid et al also state that the precise depth of the ZSL is potentially irrelevant to the fire resistance time if the ZSL penetrates into a weak layer, since only the loadbearing function of the strong layers is critical.…”

Cross-laminated timber (CLT) – a state of the art report

“…Additional research aimed at defining charring velocity β and thickness of thermally modified layer of material d o was conducted by Schmidt [81] and Fragiacomo [82] for the case of CLT plates, i.e. by Schmidt et al [83] for the case of CLT panels. General conclusion was that, in the foreseeable future, the work should be focused on optimisation of the value d 0 , as related to the required fire resistance time for CLT (30, 60 and 90 minutes), or as related to the CLT composition (thickness, arrangement and number of layers).…”

Special Issue on Timber in Fire

“…The papers compiled within this special issue of Fire Technology address a range of key fire safety issues and applications of timber in modern buildings, including: the structural fire resistance for both solid timber [5] and engineered timber elements in glulam [6] or CLT [7], and light frame engineered timber flooring systems [8] as well as connections in timber [9,10]; the specific fire dynamics in building compartments made from massive or light frame timber elements [11]; the fire protection capabilities of timber materials [12]; and the fire-safe use of timber cladding materials for multi-storey buildings [13].…”