CONTENT 5.1 SEISMIC BEHAVIOUR ... - CIB-W18

wood-based panels, cavities completely filled with batt-type rock or glass
fibre insulation, and a decking. Heat transfer analyses were performed using
SAFIR. This software permits to study the effect of the lining falling
off at specified times that are known from full-scale testing or using the
criterion of insufficient penetration length of fasteners into unburnt wood.
For the determination of the notional charring depth in the flange and the
modification factors of the whole cross-section, a computer program
CSTFire, written as a Visual Basic macro embedded in Excel, was developed,
using the temperature output from the heat transfer calculations and
relative strength and stiffness values given by EN 1995-1-2, i.e. compressive
strength, tensile strength and moduli of elasticity in compression and
tension. The notional charring depth is calculated such that the notional residual
cross-section of the flange remains rectangular and the section
modulus of the I-section is unchanged. The effect of various parameters on
the notional charring rate is shown, such as charring phases (i.e. a distinction
is made whether the I-section is initially unprotected, protected by a
lining, or unprotected after failure of the lining), flange dimensions and
depth of cross-section. Modification factors for bending and shear strength
are shown as functions of the notional charring depth for different charring
phases. In order to simplify these relationships, simple expressions are
given for increased user-friendliness and code specification.
39-16-3 A Frangi, M Fontana
A design model for timber slabs made of hollow core elements in fire
Introduction
Prefabricated timber assemblies made of hollow core elements are often
used for slabs in residential and commercial buildings. Besides the advantage
of element prefabrication and a high structural performance, the
thermal and acoustic insulation of the timber assemblies can be significantly
improved by insulating batts in the cavities and sound absorbers
placed behind the perforated acoustic layer.
Timber is a combustible material and thus differs from most other
common structural building materials. When sufficient heat is applied to
wood, a process of thermal degradation (pyrolysis) takes place producing
combustible gases, accompanied by a loss in mass. A charred layer is then
formed on the fire-exposed surfaces and the char layer grows in thickness
as the fire progresses, reducing the cross-sectional dimensions of the tim-
ber member. Because of its low thermal conductivity, the char layer protects
the remaining unburned residual cross-section against heat. Because
of the small size of the timber members of the hollow core elements, the
fire action can lead to very irregular residual cross-sections with charring
depths much greater than for heavy timber structures. For fire resistance
calculations it is therefore of primary importance to know the development
of the charring depth during the fire exposure.
A comprehensive research project on the fire behaviour of timber slabs
made of hollow core elements has been recently performed at the ETH
Zurich. The objectives of the research project were to enlarge the experimental
background of timber slabs in fire and to permit the development
of a simplified design model for the fire resistance of timber slabs made of
hollow core elements. In addition to a large number of small-scale fire
tests, the fire behaviour of the timber slabs was experimental analysed
with 2 large-scale fire tests. All fire tests were based on ISO-fire exposure
and performed at the Swiss Federal Laboratories for Materials Testing and
Research in Dübendorf. The test specimens were manufactured by the
Swiss firm Lignatur, Waldstatt. Lignatur elements consist of hollow core
elements made of spruce (picea abies) with a mean density of 450 kg/m3.
The strength properties of the timber elements correspond to the strength
class C24 according to EN 338. Figure 1 shows a typical cross-section of
Lignatur timber assemblies made of hollow core elements. The vertical
members have a thickness of 33 mm.
Figure 1. Typical timber slab made of hollow core elements
The paper describes the simplified design model for the calculation of the
fire resistance of timber slabs made of hollow core elements. Particular attention
is given to the analysis of different strategies used in order to improve
the fire behaviour of the timber slabs in fire. The first part of the pa-
CIB-W18 Timber Structures – A review of meeting 1-43 5 SPECIAL ACTIONS page 5.40