CONTENT 5.1 SEISMIC BEHAVIOUR ... - CIB-W18

Concluding remarks
This paper presents the outcomes of experimental tests conducted on small
and larger laminated veneer lumber (LVL) cross-sections subjected to
two-dimensional fire exposure.
A 2D FE conductive model was implemented in Abaqus and validated
on the experimental tests and numerical analyses carried out on timber
sections subjected to one-dimensional fire exposure. Different proposals
for the thermo-physical parameters were investigated. The Eurocode 5
proposal was found to predict the temperature distribution of small cross
sections subjected to 1D fire exposure with excellent accuracy. For small
LVL cross sections subjected to 2D fire exposure, however, the approximation
was lower but still acceptable, whereas the 2D heating process of
the larger cross-sections was predicted with a delayed temperature rise,
particularly in the interior fibres. A new proposal for the conductive model
was therefore made. This proposal assumes the same variations of density
and specific heat as recommended in Eurocode 5 Part 1-2, but considers a
variation of the conductivity according to Frangi up to 550°C, and then
slightly increased values with respect to Eurocode 5 in the range 550°C to
1200°C. The new proposal leads to better predictions for the larger crosssections
and slightly worse, but still acceptable, predictions for the small
LVL cross-section subjected to 2D fire exposure, and for the timber section
subjected to 1D fire exposure, where the heating process of the inner
fibres is slightly anticipated. It must be pointed out, however, that the
small cross-section has less technical relevance than the other crosssections
as it is a narrow member with very low inherent fire resistance,
probably needing additional passive protection if a given fire rating has to
be achieved.
In order to generalize the new proposals, further experimentalnumerical
comparisons should be carried out on large members made from
glulam or different wood-based materials. The numerical model will then
be used for coupled thermal-stress analyses aimed at investigating the fire
resistance of timber beams, timber-concrete composite structures, and
connections.
43-16-1 J Schmid, J König
Light timber frame construction with solid timber members – Application
of the reduced cross-section method
Abstract
In timber members exposed to fire a zone of about 35 to 40 mm depth below
to the char layer, although unburned, is heated above ambient temperature.
Due to the elevated temperature this zone, strength properties and
the modulus of elasticity of the residual cross-section must be reduced.
Two methods, known as reduced properties method and reduced crosssection
method, respectively, are used in practice. In the first one the
strength and stiffness properties of the cross-section, e.g. bending strength
or modulus of elasticity, are multiplied by modification factors for fire,
while in the second one, the residual cross-section is reduced by a socalled
zero-strength layer, whereas the strength and stiffness properties
remain unreduced.
For the calculation of the mechanical resistance of wall and floor assemblies
in fire consisting of light timber frame members with rectangular
cross-sections of solid timber and cavities filled with insulation, EN 1995-
1-2 gives a design model using the reduced properties method. In order to
simplify the calculation the original data were re-evaluated and expressions
for zero-strength layers were derived to allow the use of the reduced
cross-section method. For bending, the zero-strength layers were calculated
to achieve the best fit of bending resistance in the range of load ratios
between 0,2 and 0,4. Only for load ratios smaller than 0,2 the results are
slightly non-conservative. For axially loaded members, the zero-strength
layers were determined to give the same or lower bending stiffness than
according to the reduced properties method. The axial resistance of studs,
however, calculated using the method of EN 1995-1-1 with properties relevant
for the fire situation, is somewhat greater when the reduced crosssection
method is used.
Conclusions
It has been shown that the reduced properties method for the calculation of
the mechan of timber frame members (studs or joists) in wall and floor assemblies
can be replaced 1• cross-section method using zero-strength layers
instead of modification factors for simplify design work. For members
in bending, the bending resistance according to the 1 section agrees well
with results from the reduced properties method since the zero-streng de-
CIB-W18 Timber Structures – A review of meeting 1-43 5 SPECIAL ACTIONS page 5.47