CONTENT 5.1 SEISMIC BEHAVIOUR ... - CIB-W18
CONTENT 5.1 SEISMIC BEHAVIOUR ... - CIB-W18
CONTENT 5.1 SEISMIC BEHAVIOUR ... - CIB-W18
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Concluding remarks<br />
This paper presents the outcomes of experimental tests conducted on small<br />
and larger laminated veneer lumber (LVL) cross-sections subjected to<br />
two-dimensional fire exposure.<br />
A 2D FE conductive model was implemented in Abaqus and validated<br />
on the experimental tests and numerical analyses carried out on timber<br />
sections subjected to one-dimensional fire exposure. Different proposals<br />
for the thermo-physical parameters were investigated. The Eurocode 5<br />
proposal was found to predict the temperature distribution of small cross<br />
sections subjected to 1D fire exposure with excellent accuracy. For small<br />
LVL cross sections subjected to 2D fire exposure, however, the approximation<br />
was lower but still acceptable, whereas the 2D heating process of<br />
the larger cross-sections was predicted with a delayed temperature rise,<br />
particularly in the interior fibres. A new proposal for the conductive model<br />
was therefore made. This proposal assumes the same variations of density<br />
and specific heat as recommended in Eurocode 5 Part 1-2, but considers a<br />
variation of the conductivity according to Frangi up to 550°C, and then<br />
slightly increased values with respect to Eurocode 5 in the range 550°C to<br />
1200°C. The new proposal leads to better predictions for the larger crosssections<br />
and slightly worse, but still acceptable, predictions for the small<br />
LVL cross-section subjected to 2D fire exposure, and for the timber section<br />
subjected to 1D fire exposure, where the heating process of the inner<br />
fibres is slightly anticipated. It must be pointed out, however, that the<br />
small cross-section has less technical relevance than the other crosssections<br />
as it is a narrow member with very low inherent fire resistance,<br />
probably needing additional passive protection if a given fire rating has to<br />
be achieved.<br />
In order to generalize the new proposals, further experimentalnumerical<br />
comparisons should be carried out on large members made from<br />
glulam or different wood-based materials. The numerical model will then<br />
be used for coupled thermal-stress analyses aimed at investigating the fire<br />
resistance of timber beams, timber-concrete composite structures, and<br />
connections.<br />
43-16-1 J Schmid, J König<br />
Light timber frame construction with solid timber members – Application<br />
of the reduced cross-section method<br />
Abstract<br />
In timber members exposed to fire a zone of about 35 to 40 mm depth below<br />
to the char layer, although unburned, is heated above ambient temperature.<br />
Due to the elevated temperature this zone, strength properties and<br />
the modulus of elasticity of the residual cross-section must be reduced.<br />
Two methods, known as reduced properties method and reduced crosssection<br />
method, respectively, are used in practice. In the first one the<br />
strength and stiffness properties of the cross-section, e.g. bending strength<br />
or modulus of elasticity, are multiplied by modification factors for fire,<br />
while in the second one, the residual cross-section is reduced by a socalled<br />
zero-strength layer, whereas the strength and stiffness properties<br />
remain unreduced.<br />
For the calculation of the mechanical resistance of wall and floor assemblies<br />
in fire consisting of light timber frame members with rectangular<br />
cross-sections of solid timber and cavities filled with insulation, EN 1995-<br />
1-2 gives a design model using the reduced properties method. In order to<br />
simplify the calculation the original data were re-evaluated and expressions<br />
for zero-strength layers were derived to allow the use of the reduced<br />
cross-section method. For bending, the zero-strength layers were calculated<br />
to achieve the best fit of bending resistance in the range of load ratios<br />
between 0,2 and 0,4. Only for load ratios smaller than 0,2 the results are<br />
slightly non-conservative. For axially loaded members, the zero-strength<br />
layers were determined to give the same or lower bending stiffness than<br />
according to the reduced properties method. The axial resistance of studs,<br />
however, calculated using the method of EN 1995-1-1 with properties relevant<br />
for the fire situation, is somewhat greater when the reduced crosssection<br />
method is used.<br />
Conclusions<br />
It has been shown that the reduced properties method for the calculation of<br />
the mechan of timber frame members (studs or joists) in wall and floor assemblies<br />
can be replaced 1• cross-section method using zero-strength layers<br />
instead of modification factors for simplify design work. For members<br />
in bending, the bending resistance according to the 1 section agrees well<br />
with results from the reduced properties method since the zero-streng de-<br />
<strong>CIB</strong>-<strong>W18</strong> Timber Structures – A review of meeting 1-43 5 SPECIAL ACTIONS page 5.47