Structural response of fire-exposed cross-laminated timber beams under sustained loads

Published Date
October 2016, Vol.85:23–34, doi:10.1016/j.firesaf.2016.08.002
Open Access, Creative Commons license, Funding information

Author 

Sean A. Lineham

Daniel Thomson

Alastair I. Bartlett

Luke A. Bisby ^,

Rory M. Hadden

School of Engineering, The University of Edinburgh, The King’s Buildings, Mayfield Road, Edinburgh EH93JL, UK

Received 8 November 2015. Revised 20 July 2016. Accepted 9 August 2016. Available online 23 August 2016.

Highlights

• Aspirations for mass timber buildings are limited by knowledge gaps regarding their fire behaviour.
• •
  Novel fire tests are presented on CLT beams under sustained flexural loading.
• •
  The zero-strength layer design method fails to capture the key physics to predict response in fire.
• •
  Detailed thermo-mechanical analyses should be used and the zero-strength layer method discarded.

Abstract 

Cross-laminated timber (CLT) is a popular construction material for low and medium-rise construction. However an architectural aspiration exists for tall mass timber buildings, and this is currently hindered by knowledge gaps and perceptions regarding the fire behaviour of mass timber buildings. To begin to address some of the important questions regarding the structural response of fire-exposed CLT structures in real fires, this paper presents a series of novel fire tests on CLT beams subjected to sustained flexural loading, coincident with non-standard heating using an incident heat flux sufficient to cause continuous flaming combustion. The load bearing capacities and measured time histories of deflection during heating are compared against predicted responses wherein the experimentally measured char depths are used, along with the Eurocode recommended reduced cross section method and zero-strength layer thickness. The results confirm that the current zero-strength layer value (indeed the zero-strength concept) fails to capture the necessary physics for robust prediction of structural response under non-standard heating. It is recommended that more detailed thermo-mechanical cross-sectional analyses, which allow the structural implications of real fire exposures to be properly considered, should be developed and that the zero-strength layer concept should be discarded in these situations. Such a novel approach, once developed and suitably validated, could offer more realistic and robust structural fire safety design.

Keywords

Charring

Cross-laminated timber

Fire resistance

Mass timber

Reduced cross section

Zero-strength layer

---

Nomenclature

b₀

original breadth of structural element [m]

b_T

transformed breadth of structural element [m]

β₀

one-dimensional charring rate under a standard cellulosic fire exposure [mm/min]

Young’s Modulus parallel to the grain direction [N/m²]

Young’s Modulus perpendicular to the grain direction [N/m²]

L

span length [m]

M

applied bending moment [Nm]

M_A,c

ambient temperature bending moment capacity established from control tests [Nm]

P

applied vertical load [N]

θ_n

free rotation at node n [rads]

v_n

free vertical translation at node n [m]

V

applied shear force [N]

Xa-b-c

specimen naming scheme (X=ambient temperature (A) or in fire (F), a=number of layers; b=load level; c=test number)

1 Introduction and background

Cross-laminated timber (CLT) is an engineered mass timber product that is increasingly being used as a primary structural material in multi-storey construction. It is typically made from lamellae of softwood lumber, which are bonded one on top of another in a crosswise fashion using a polymer adhesive. The resulting alternating grain directions give CLT strength and stiffness in two directions, making it suitable for two-way spanning slabs, walls, and diaphragms. Cross-laminated timber falls within the “mass timber” family of engineered wood products, alongside glued-laminated timber, which has been widely used in buildings for decades. Construction using mass timber building systems is, however, becoming ever more popular due to various sustainability advantages, both real and perceived, alongside considerable benefits in terms of the speed and ease with which CLT buildings can be constructed in congested urban centres, the use of advanced offsite and modularised construction methods, and reductions in foundation size due to the reduced overall building mass. However, the use of mass timber as a primary structural material in multi-storey buildings is often limited due to the fact that timber is a combustible material, unlike traditional multi-storey building materials such as masonry, concrete, and steel. Before taller mass timber buildings can be designed with full confidence, particularly in cases where there is a desire to express (i.e. expose) the timber elements in the completed structure, the structural response of CLT elements during real fires must be better understood.

1.1 Current approach for fire resistance design

Structural fire design guidance for mass timber elements is available in design codes internationally, and takes many forms. The most advanced and rational guidance is likely that set out in Eurocode 5 [1], which can be used to determine the standard fire resistance of timber elements based on a simplified, notional charring rate and a reduced cross section calculation methodology (described later). While CLT is not explicitly treated in the Eurocode, current practice in industry is to design CLT essentially as would be done for solid softwood timber; incorporating suitable modifications to account for CLT's crosswise lay-up. This approach takes advantage of so-called self-protection of the timber by surface charring and loss of an acceptable sacrificial depth of the surface timber, which protects and insulates the underlying (cool) timber.

Two specific, simplified methods are suggested in Eurocode 5 to determine the load bearing capacity of a mass timber (and, by extension, CLT) element during exposure to the ISO 834 [2] standard cellulosic compartment fire; (1) the reduced cross section method, and (2) the reduced properties method. The reduced properties method only applies to elements subject to fire from three or four sides, which is not typically applicable for CLT elements and is therefore not discussed herein (indeed, it is rarely used in practice even when applicable, and is slated for deletion from the upcoming revision to Eurocode 5).

The reduced cross section method assumes that timber will char at a notional charring rate during exposure to a standard fire, and then uses this notional charring rate to predict the depth of charred timber. The char is assumed not to contribute to the element's load bearing capacity and, to account for the presence of a zone of heated timber beneath the char, an additional 7 mm layer of ‘zero-strength’ timber is also assumed to make no contribution to strength or stiffness. The capacity of the timber structural element is then determined based on its ambient temperature mechanical properties, accounting only for the reduced cross section with the charred timber and zero-strength layer ignored.

The reduced cross section approach was originally derived in the 1980s based on numerical simulations of the fire behaviour of glued-laminated timber beams exposed to fire on three-sides by Schaffer [3]. Fig. 1 shows that this approach is fundamentally based on an assumed variation of mechanical properties of the timber below the char, which in turn is based on a small number of tests and on Monte Carlo analysis of the predicted responses. This is also for a specific North American timber species under specific, standard testing conditions (both heating and loading), rather than based on a rigorous assessment of mechanical properties from mechanical tests of the constituent timber materials and adhesives used. Based on his analysis and assumed mechanical inputs for heated timber below the char, Schaffer concludes that timber at depths below 0.3 in. (≈7 mm) from the base of the char layer can be assumed to be at full strength, with all other charred and heated timber ignored (as shown in Fig. 1(b)).

Fig. 1. The origins of the zero-strength layer in the work of Schaffer (after [3]) based on (a) variation of mechanical properties of timber beneath the char layer and (b) an assumed ambient temperature reduced cross section.

It is noteworthy that the 7 mm zero-strength layer approach has not been carefully experimentally assessed for application to CLT elements in bending, and that previous authors have criticised it as being inaccurate and physically unrealistic for solid timber or glued-laminated elements [4]. The current paper presents non-standard fire tests on loaded CLT beams, undertaken to carefully study the applicability of the zero-strength layer concept specifically, and the overall reduced cross section method more generally. It is important to note that the Eurocode 5 [1]reduced cross section method is strictly applicable only to standard fire exposures, however the physical realism of the reduced cross section approach can be interrogated using non-standard heating.

The one-dimensional charring rate (β₀), which is applicable for structural fire design of CLT planar elements subjected to an ISO 834 standard fire, and given in Table 3.1 in Eurocode 5 [1], is 0.65 mm/min. This value is quoted for ‘softwood and beech, and for solid timber with a characteristic density greater than 290 kg/m³’, and strictly speaking would apply for the CLT elements tested in the current study only if they were exposed to a standard fire [2] in a fire testing furnace.

The mechanical properties of timber, as well as their variation with temperature, depend greatly on the grain orientation with respect to loading. The orthogonal crosswise lay-up of timber lamellae in CLT panels means that, when subjected to one-way bending, the crosswise layers contribute comparatively little to the strength and stiffness of the element. To account for this issue during analysis, an effective cross section can be defined by using a simple transformed section analysis, based on an assumed ratio of elastic moduli for the strong (i.e. longitudinal) and weak (i.e. cross-ply) layer orientations. This modular ratio, which is denoted as   is commonly assumed as about 30 for softwood timber [5]. This approach has been experimentally verified at ambient temperatures by Okabe [6], and is shown schematically in Fig. 2. It is assumed herein that this transformed section approach applies equally during fire.

Fig. 2. Schematic showing the transformed section analysis approach commonly used to predict the flexural properties of CLT structural elements.

1.2 Response of Timber to Heating

When timber is heated to temperatures exceeding 200 °C, pyrolysis occurs and a layer of carbonaceous char is formed at the fire-exposed surfaces [7]. This char layer is of low effective thermal conductivity and acts as natural insulation for the underlying timber, reducing the rate of charring and insulating the core of the timber element. Beneath the char layer exists an uncharred but heated drying and pyrolysis zone, which shows visible discolouration and has reduced mechanical properties (see Fig. 1 for example).

A number of controlling processes affect the heat transfer beneath the char layer; these include: species, rate of heating, surface oxidation, crack formation, reaction kinetics, pressure gradients, and moisture content. Evaporation of moisture at temperatures close to 100 °C significantly influences the internal thermal gradients [8]. The available research shows that the resulting temperature distribution through heated timber elements can be predicted with reasonable accuracy [8]. This is recognised by Eurocode 5 [1], and enables thermo-mechanical sectional analysis approaches in lieu of the simplified reduced cross section method, which is currently more commonly used in practice. Sectional analysis approaches, however, require that the mechanical property reductions for heated timber beneath the char layer can be accurately accounted for. While this is not the focus of the current paper, the tests (and novel test method) discussed herein will be used to support the development of such approaches in the future.

1.3 CLT Flexural elements in fire

A number of furnace tests assessing the structural performance of isolated CLT elements under exposure to standard fires [2] and concurrent sustained mechanical loading are available in the literature [9], [10], [11], [12] and [13]. The validity of the reduced cross section and zero-strength layer concepts for CLT elements in bending has also previously seen initial investigation [11]. Based on these tests, proposals have been made for revised zero-strength layer thicknesses, which depend on the specific lay-up of the CLT, the presence of cross layers, section depth, and whether the fire-exposed timber is in the tension or compression zone of an element [9]. All of the proposed revised zero-strength layer depths from available research are greater than the Eurocode's currently suggested 7 mm. It has been concluded from prior research that specific revisions to the reduced cross section method (or entirely new methods of analysis) are needed to properly account for potentially different loading modes, non-standard fire exposures, protected versus unprotected elements (i.e. heating and charring rates), and full-frame assemblies. This large number of revisions raises the question of whether the fundamental basis of the reduced cross section analysis method is adequate to properly account for the necessary physics, and whether an alternative analysis/design method, using available (and future) scientific knowledge on the thermal and mechanical response of heated timber and based on a thermo-mechanical sectional analysis method, rather than one based on a reduced cross section, may be necessary.

The available literature on the fire performance of CLT appears to have been primarily interested in improving the reduced cross section method, typically by proposing revised zero-strength layer depths for different situations, whereas the impetus of the current paper is to better understand the mechanics of heated/charring CLT in bending under sustained loading.

2 Experimental programme

The experimental programme in the current study consisted of tests on 12 one-way spanning CLT beams tested under four point bending. Two different CLT lay-ups with the same overall thickness of 100 mm were studied; one with three lamellae and one with five. The overall dimensions of all specimens were identical. Four control specimens were tested to failure under displacement control at ambient temperature (two of each lay-up) and the remaining eight specimens were subjected concurrently to sustained mechanical loading and severe radiant heating. Heating was from below, within the beams’ constant moment regions until flexural failure. Details of the testing matrix are given in Table 1.

For further details log on website :
http://www.sciencedirect.com/science/article/pii/S0143749615002377