CONTENT 5.1 SEISMIC BEHAVIOUR ... - CIB-W18

per gives some results from advanced calculations – using the thermal and
thermo-mechanical properties of wood given by EN 1995-1-2 – of the
bending resistance of both homogenous and cross-laminated timber slabs
exposed to fire. It is shown that the depth of the zero-strength layer should
be increased considerably above the value of 7 mm given by EN 1995-1-2
for beams and columns. Also, there is a significant effect of the state of
stress of the fire exposed side of the slab, that is when the fire exposed side
is in compression, the reduction of bending strength is greater than in the
opposite case.
Conclusions and further research needs
For the design of timber beams and columns, EN 1995-1-2 gives two alternative
methods. The effective cross-section method introduces a zero
strength layer of uniform depth of seven millimeters to take into account
the reduction of strength and stiffness properties of the member. The results
of simulations of fire exposed homogeneous and cross-laminated
timber plates show that the application of the effective cross-section method
would require zero-strength layer depths that are dependent on plate
depth, the composition of layers and the state of stress on the fire exposed
side of the plate. A considerable advantage of the method would disappear.
After simplification, the zero-strength layer depths can be given as
linear functions of the plate depth.
Figure – Zero-strength layer depth vs. plate depth
These results are preliminary. More timber plate configurations need to be
investigated and it must be checked if these zero-strength layer depths also
apply to bending stiffness when these elements are used as wall. Also, the
effect of fire protective claddings needs to be investigated. It can be expected
that, in some cases, considerably smaller zero-strength depths
would apply.
Since the accumulation of moisture has a significant effect on strength
and stiffness at elevated temperature, it should also be investigated if there
is any effect of the adhesive on the transport of vapour in the plate.
40-16-2 A Frangi, C Erchinger
Design of timber frame floor assemblies in fire
Introduction
Timber frame floor assemblies are typical structural elements used in timber
engineering. The floor assemblies consist of solid timber beams with
claddings of gypsum plasterboards, wood based panels or combinations of
these layers. The cavities may be filled with insulation made of rock, glass
or wood fibre. The fire behaviour of timber frame floor assemblies is characterised
by different charring phases. After the fire exposed claddings fall
off, the timber beams are exposed directly to high temperatures leading to
an increased charring rate in comparison to timber elements that are initially
unprotected from fire exposure. Thus the fire performance of timber
frame floor assemblies is influenced by the protection provided by the
claddings. Further the size of the timber beams and the protection provided
by the cavity insulation play an important role on the fire performance of
the floor assemblies.
Design models of timber structures in fire usually take into account the
loss in cross-section due to charring of wood and the temperature dependent
reduction of strength and stiffness of the unburned residual crosssection.
For timber frame wall and floor assemblies whose cavities are
completely filled with insulation, Annex C of EN 1995-1-2 provides a
simplified calculation model based on the reduced properties method. The
design model based on fire tests performed on light timber frame assemblies
with studs and joists with small cross-sections. For timber frame wall
and floor assemblies with void cavities only very little information is
available in Annex D of EN 1995-1-2.
CIB-W18 Timber Structures – A review of meeting 1-43 5 SPECIAL ACTIONS page 5.42