CONTENT 5.1 SEISMIC BEHAVIOUR ... - CIB-W18

per describes the simplified calculation model, in the second part the test
results are compared to the simplified calculation model.
Summary and conclusions
The paper presents a simplified design method for the calculation of the
fire resistance of timber slabs made of hollow core elements. The simplified
design method is based on the reduced cross-section method according
to EN 1995-1-2 and takes into account two different charring phases,
before and after the fire-exposed layer is completely charred. For simplicity
linear relationships between charring depth and time are assumed for
each phase. Further it is assumed that the vertical timber members are not
exposed to fire on 3 sides. This can be achieved in two different ways:
– the fire-exposed timber layer is so designed that a fire penetration into
the cavities is prevented
– the cavities are filled with insulation material, so that after failure of the
fire-exposed timber layer charring occurs mainly on the narrow side of
the vertical members, while the wide sides are more or less protected by
the insulation.
As cavity insulation, rock fibre batts which remain intact up to 1000°C and
in place after failure of the fire-exposed timber layer can be used. On the
other hand, cavity insulation made of glass fibre batts is not recommended
because it melts when exposed directly to fire temperatures, being incapable
of protecting the wide sides of the vertical member.
Before the fire-exposed timber layer is completely charred, the timber
assembly is exposed to fire only on one side and a more or less homogenous
regular one-dimensional charring similar to that of a heavy timber
slab can be assumed, as confirmed by fire tests on timber assemblies performed
within the framework of the research project. The charring rate
measured during the fire tests at the fire-exposed lower layer as well at the
vertical members varied between 0.60 and 0.82 mm/min. For the calculation
of the charring depth during the first phase a notional charring depth
β1,n= 0.8 mm/min can be assumed giving safe results. This value corresponds
to the notional charring rate given in EN 1995-1-2 for solid timber.
Because of the small thickness of the vertical members of the hollow
core elements, a superposition of the heat flux from the sides and below
occurs during the second phase and increased charring has to be considered
in comparison to one-dimensional charring. Thus the notional charring
rate β2,n during this phase is mainly influenced by the thickness of the
vertical members. For the hollow core elements tested with a thickness of
the vertical members of 33 mm a notional charring rate β2,n = 1.6 mm/min.
can be assumed. This is confirmed by fire tests conducted within the
framework of this research project and other fire tests. Although the insulation
material is able to protect the wide sides of the vertical members, the
fire tests showed that because of the small size of the vertical members the
temperatures measured in the vertical members are higher than in comparison
to heavy timber cross-sections. For the calculation of the factor d0,
which takes into account the temperature-dependent reduction in strength
and stiffness in the heat affected zones of the vertical members, an advanced
calculation model has been used. The cross-section of the timber
assembly has been divided into n finite elements with different stiffness
and strength properties as a function of the measured temperature Θi
(t).The reduction of the E-modulus and bending strength has been assumed
according to EN 1995-12. Under assumption of a factor d0 = 20 mm a
good agreement between the advanced and the simplified calculation
model was observed.
The global behaviour of the timber slabs made of hollow core elements
was analysed with two fire tests on slabs performed in ENIPA's horizontal
furnace (3.0 x 4.85 m). The fire tests showed a fire resistance of more than
60 minutes and 90 minutes respectively. When verifying the simplified design
method, a good agreement between fire test results and the simplified
design method was observed.
40-16-1 J König, J Schmid
Bonded timber deck plates in fire
Abstract
Laminated deck plates, made of edgewise or flat wise laminations, are increasingly
used as structural elements in housing and commercial buildings,
both in floors and walls. In structural fire design, EN 1995-1-2 gives
a simplified method for the calculation of the mechanical resistance of
structural timber members. Apart from charring, the effect of elevated
temperature is taken into account by assuming a zero-strength layer below
the charring depth of thickness 7 mm, reached after the first 20 minutes of
the fire exposure. For rectangular cross-sections this model gives reasonably
good agreement with advanced calculations. EN 1995-1-2 also permits
the application of this model to timber slabs exposed on one side. This pa-
CIB-W18 Timber Structures – A review of meeting 1-43 5 SPECIAL ACTIONS page 5.41