The Reduced Cross-Section Method for Evaluation of the Fire Resistance of Timber Members: Discussion and Determination of the Zero-Strength Layer

Abstract

The reduced cross-section method (RCSM) is included in Eurocode 5 (EN-1995-1-2) for the design of timber members in fire conditions. The method considers the strength and stiffness reduction beneath the charred layer by adding an additional depth (known as the 'zero-strength' layer) to the charring depth. The zerostrength layer is one of the key parameters for the fire design of timber members. Recently, some concerns have been raised that the zero-strength layer might be nonconservative in some applications. This paper presents the background to the RCSM, followed by a short discussion on the mechanical assumptions, simplifications and possible limitations of the method itself. Further, it discusses determination of the zero-strength layer thickness for members in bending, tension and compression, and provides guidelines on the use of standard experimental tests to determine this quantity. For demonstration of the determination procedure, the results of fire tests in bending, tension and compression were analysed following the described procedure. Results show that the zero-strength layer exceeds the value used in practice, indicate that the method of Eurocode 5 may be non-conservative and should be revised.

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

“…Figure 7 and Table 4 show that the deflection responses are poorly predicted and the times to failure are over-predicted by more than 100% in some cases; this is partly because the experimentally observed charring rates were somewhat greater than those assumed, but also because the assumed ZSL depth of 7 mm is much too small to properly account for the loss of compressive strength and stiffness in the thermally-affected timber below the char. This hypothesis was confirmed both by repeating the calculations and assuming the experimentally measured charring rates and a ZSL of 7 mm, and furthermore by using the experimental charring rates in combination with an increased ZSL of 18.9 mm [10]. In both cases the computational results were closer to the experimental deflection curves, and the time to failure predictions became slightly conservative for all tests.…”

“…7 show the predicted lateral deflection with time response when constant average charring rates measured during the respective tests are assumed (see Table 3), together with a 7 mm ZSL. The 'ZSL = 18.9 mm' curves (dotted lines) assume the experimentally measured charring rates for each test (Table 3), along with a ZSL depth of 18.9 mm; this is the maximum value within the range suggested by Schmid et al [10] (for situations with timber in flexural compression). Comparisons between the various predictions are made later in the Discussion.…”

“…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.…”

“…For flexural compressive loading (i.e. hogging moment with heating from below) Schmid et al showed that the ZSL should be increased to between 12.5 and 18.9 mm with a mean of 14.8 mm [10].…”

“…Furthermore, the reduction in elastic modulus of timber with elevated temperature is known to be more severe in compression than in tension [37]. Thus, the ZSL method is not, in the opinion of the authors, in its current codified form as presented in the Eurocodes [4] and based on its derivation [10], applicable to structural elements dominated by compression.…”