CONTENT 5.1 SEISMIC BEHAVIOUR ... - CIB-W18

dowels are respected, for example according to EN 1995-1-1. The loadcarrying
capacity of the connections depends primarily on the embedment
strength of the timber members and the yield moment of the dowels. Due
to the protection of the slotted-in steel plates against fire provided by the
timber side members (see Fig. 1), a high fire resistance may be achieved.
In order to accurately predict the structural performance in fire, knowledge
on the temperature distribution in the cross-section as well as the influence
of steel elements (slotted-in steel plates and steel dowels) on the charring
of the timber members is required.
This is challenging and complex due to the influence of several parameters
like dowel diameter, geometry of the connection, different failure
modes, different thermal properties of timber and steel as well as the thermal
interaction between timber members and steel elements. Thus, only
limited work has been carried out on the fire behaviour of steel-to-timber
connections and current standards do not contain consistent calculation
models for the fire design of steel-to-timber connections taking into account
the influences of the different parameters. The reduced load method
given in EN 1995-1-2 is only valid for laterally loaded symmetrical threemember
connections (two shear planes, one steel plate in the middle) under
ISO-fire exposure. The relative load-carrying capacity versus time given
as a one-parameter exponential model is based on a still limited number
of fire tests carried out on timber connections with bolts and nails. For
multiple shear steel-to-timber connections with two or three slotted-in
steel plates no design models in the fire situation exist so far.
Figure 1 Typical multiple shear steel-to-timber connection with three steel
plates (left) and example of the geometry of tested connection D1.1 (right)
A comprehensive research project on the fire behaviour of multiple shear
steel-to-timber connections with two or three slotted-in steel plates and
steel dowels has recently been performed at ETH Zurich. The objective of
the research project was the development of a fire design model for multiple
shear steel-to-timber connections with slotted-in steel plates and steel
dowels. In addition to 25 tensile tests at ambient temperature to determine
the load-carrying capacity, the fire behaviour of the connections was experimentally
analysed with 18 fire tests under constant tensile load. All
fire tests were performed under ISO-fire exposure on the horizontal furnace
at the Swiss Federal Laboratories for Materials Testing and Research
(EMPA) in Dubendorf. The paper first describes the main results of extensive
experimental and numerical analyses on the fire behaviour of multiple
shear steel-to-timber connections with slotted-in steel plates and steel
dowels. Particular attention is given to the analysis of the influence of the
steel elements on the charring of the timber members. Then, the design
model for multiple shear steel-to-timber dowelled connections with slotted-in
steel plates in fire is presented and compared to fire tests.
Conclusions
The load-carrying capacity of timber structures is often limited by the resistance
of the connections. Thus, highly efficient connections as multiple
shear steel-to-timber connections with slotted-in steel plates and steel
dowels are needed for an efficient design. Connections with slotted-in
steel plates achieve a high fire resistance because the steel plates are protected
by the timber side members. The results of an extensive experimental
analysis showed that shear steel-to-timber dowelled connections
with two or three slotted in steel plates designed for ambient temperature
with a width of the timber members of 200 mm reached a fire resistance of
at least 30 minutes. A reduction of the load level from 30% to 15 or 7.5%
of the average load-carrying capacity measured at ambient temperature did
not lead to a significant increase of the fire resistance. By increasing the
thickness of the side members as well as the end distance of the dowels by
40 mm the connections reached a fire resistance of more than 60 minutes.
Connections protected by timber boards or gypsum plasterboards can
reach a fire resistance of 60 minutes or more depending on the thickness
and type of protection. Thus, from a fire design point of view the increase
of the side member thickness or the protection by boards are efficient in
order to increase the fire resistance significantly.
CIB-W18 Timber Structures – A review of meeting 1-43 5 SPECIAL ACTIONS page 5.45