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Fire Technology

, Volume 54, Issue 5, pp 1113–1148 | Cite as

Simulation of the Fire Resistance of Cross-laminated Timber (CLT)

  • Joachim Schmid
  • Michael Klippel
  • Alar Just
  • Andrea Frangi
  • Mattia Tiso
Article

Abstract

Cross-laminated timber, typical abbreviations CLT or XLAM, is currently one of the most innovative product in building with wood. This solid engineered timber product provides advantages compared to other solid timber slabs as the dimension stability, i.e. swelling and shrinkage, is controlled by the crosswise laminations. As for other components, the fire resistance has to be verified for this type of product. While fire testing is time consuming and costly, simulations provide flexibility to optimize the product or to develop simplified design models for structural engineers. In this paper, a simulation technique is presented which can be used to determine the fire resistance of CLT. The technique was then used to develop simplified design equations to be used by engineers to predict the behavior of CLT in fire resistance tests and verify its fire resistance. Following existing models, the simplified design model aims for a two-step process whereby in a (i) first step the residual cross section and in (ii) a second step the load bearing capacity of the partly heated residual cross section is determined. The presented simulations consider the effective thermal–mechanical characteristics of wood exposed to standard fire and perform an advanced section analysis using a temperature profile corresponding to the actual protection and the location of the centroid together with the possibility of plasticity on the side of compression. It was shown that simulation results agree well with test results and that they can be used to determine layup specific modification factors used by the reduced properties method or zero-strength layers used by the effective cross section method. It was shown that the use of the zero-strength layers is favorable compared to the modification factors to calculate the resistance of the residual cross section. This is due to the large range of modification factors answering the typical layup of CLT comprising layers with their fiber direction cross the span direction. Subsequently, the methodology was used to determine design equations for initially unprotected and protected three-, five- and seven-layer CLT in bending and buckling. While the zero-strength layer for glulam beams in bending is assumed to be 7 mm (0.3 in), for CLT the corresponding value is in most of the cases between 5 mm and 12 mm but is different for other loading modes such as buckling (wall elements) and depending on the applied protection.

Keywords

Cross-laminated timber Modelling Structural fire design Fire design model Fire tests 

List of symbols

b

Width (mm)

d

Depth for charring related measures (mm)

E

Modulus of elasticity (N/mm2)

f

Strength (N/mm2)

H

Half-length of the window (m)

h

Depth for member related measures (mm)

i

Variable

I

Modulus of inertia (mm4)

k

Reduction or modification factor

M

Bending moment resistance (N m)

n

Number of layers

l

Length (m)

q

Heat flux (kW m−2)

T

Temperature (°C)

t

Time (min)

u

Deflection of the bending member

W

Section modulus (mm3)

x

Length coordinate along the axis

Greek

α

Convective heat transfer coefficient (Wm−2K−1)

ε

Emissivity or strain

κ

Curvature

ρ

Density (kg/m3)

Subscripts

0

Zero strength and stiffness

0.12

Reference moisture content

20

Normal temperature

c

Compression

char

Char

E

Elasticity

ef

Effective

fi

Fire

f

Strength

m

Moment (in bending)

mod

Modified

t

Tension

q

Heat flux (kW m−2)

T

Temperature (°C)

t

Time (min)

unexp

Unexposed

pr

Protection

Notes

Acknowledgements

The research described here was conducted at SP Trätek, Stockholm, as a part of the FireInTimber project within the European Wood-Wisdom-Net framework. It is supported by industry through the European Initiative Building With Wood and public funding organizations. The authors would like to acknowledge COST FP1404 where a task group works with this type of product. The test specimens were produced and delivered by Martinsons Trä (Sweden) and Stora Enso Austria. A part of the fire tests were assisted, evaluated and reported by Per Willinder to be included in his Bachelor thesis. The analysis of the reference test results using the Maximum Likelihood Method was conducted by Jochen Köhler.

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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  1. 1.ETH Zurich, IBKZurichSwitzerland
  2. 2.Tallinn University of TechnologyTallinnEstonia
  3. 3.Research Institutes of SwedenStockholmSweden

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