CONTENT 5.1 SEISMIC BEHAVIOUR ... - CIB-W18

33-16-1 J König
A design model for load-carrying timber frame members in walls and
floors exposed to fire
Summary
A design model is presented for the calculation of the load-bearing capacity
of timber framed wall and floor assemblies exposed to standard fire
exposure. It is implied that the cavities of the assemblies are filled with
rock or glass fibre insulation, providing partial protection against charring
of the wide sides of the timber members. The design model consists of a
charring, model giving a simplified rectangular residual cross section, and
a mechanical model describing the strength and stiffness properties of the
residual cross section. The charring model takes into account different
charring rates that are dependent on the protection provided by the lining,
and the conditions during the post-protection stage after complete failure
of the lining. Criteria are given for the failure of linings including the
length of fasteners. The reduction of strength and stiffness parameters is
taken into account by multiplying cold values by modification factors. The
model parameters were derived from test data, heat transfer calculations
and mechanical modelling of the cross section using temperature dependent
strain-stress relationships.
33-16-2 J König, T Oksanen , K Towler
A review of component additive methods used for the determination of
fire resistance of separating light timber frame construction
Summary
Verification of fire resistance of separating constructions is performed
with respect to criteria I (insulation) and E (integrity). While fire testing is
still the most common way of verification, design by calculation will be
more common in the future. Several calculation methods called component
additive methods have been in use since several years. All of them stipulate
that in a construction built up of several layers or membranes, the total
fire resistance is the sum of the contributions to fire resistance of each of
the layers. This paper reviews and compares the design method given by
ENV 1995-1-2 with methods being used in North America, the United
Kingdom and Sweden. A proposal for the revised Fire Part of Eurocode 5
is presented.
Conclusions
The determination of the fire resistance of floor and wall assemblies by
calculation is a complex task. In this paper four methods for the determination
of the fire resistance with respect to the separating function of light
timber framed assemblies are presented. Two of them include also the verification
of the load-bearing function. All of them have advantages and
drawbacks, due to conflicting goals: accuracy versus simplicity. The opinion
of the Project Team redrafting the Fire Part of Eurocode 5 is that a design
code should contain rational calculation methods, while tabulated data
for constructions should be found in design manuals.
This criterion would exclude the UK-method.
The North American method is easy to use, however the logic behind is
not obvious to the designer.
Since the Swedish method describes the real performance more adequately
without being too complicated, it is proposed to adopt this method
in the EN 1995-1-2 and to combine it with the existing ENV-method.
Since the method is still incomplete, some corrections must be made to fit
test results. It is also proposed to replace the tabulated data of coefficients
of positions by simple equations. Since the method was developed only for
wall constructions, it must be extended to include floor application, e.g. by
introducing a general reduction of coefficients of position. The effect of
joints should be included as in ENV 1995-1-2. It is proposed to apply the
method for the heat transfer paths given in the ENV with the exception of
heat path b that should include all layers in order to take into account
joints on the unexposed side. For that heat path b the value tins according to
equation (3) should be increased, in order to adapt the time criterion to an
temperature increase of 180 K instead of 140 K.
The main advantage of the proposed method is that it is more general
and open for future extensions both by tests and general calculation methods.
Since integrity failure due to premature failure of linings may occur resulting
from excessive deflections of load-bearing elements, a corresponding
rule should be included. From fire tests can be seen that deflections of
load-bearing constructions become excessive during the last 5 to 10
minutes prior to collapse.
CIB-W18 Timber Structures – A review of meeting 1-43 5 SPECIAL ACTIONS page 5.33