CONTENT 5.1 SEISMIC BEHAVIOUR ... - CIB-W18

Recently, the European Community Commission has issued a draft of
Eurocode 8 "Common Unified Rules for Structures in Seismic Zones". By
these rules, the assessment of seismic loads is based on the frequency and
intensity of the seismic events, the quantity and distribution of masses,
structural geometry; behaviour is assumed to be elastic. The nature of material
and the type of structure are taken into account by introducing a
modifying factor (in order to reduce seismic action, the so-called behavioural
factor "q"), determined by the structure's capacity of dissipating energy
through its hysteretical ductility and resisting severe earthquakes by
reaching domains of non-elastic behaviour.
In the draft of Eurocode 8, values ranging from 1 to 6 were suggested
for the different materials (e.g. 6 for steel), so reducing the seismic design
action considerably, down to 16% of the expected theoretical peak for a
structure with indefinitely elastic behaviour. Since structural timber shows
usually an elasto-linear behaviour before yielding (with a brittle failure), a
factor q = 1 was proposed for timber structures, which apparently makes it
very difficult to obtain cost-effective dimensioning.
That choice does not come from poor performance of timber structures
under earthquakes (experience showed just the opposite); rather it was felt
by the members of the draft panel that what is presently known about sections
and joints behaviour, especially under cyclic loads, is not enough to
allow detailed structural analysis as required by Eurocode 8, and by any
updated design code.
A recent proposal for new Italian seismic design codes, regarding all
building materials, does acknowledge the brittle behaviour of timber sections;
still, it points at structures capacity of dissipating energy at connections,
and the need for further wide and detailed investigations to this effect.
A behavioural factor q = 2 is suggested, to take into account the dissipating
capacity at thousands of nailed connections between plywood panels
and timber frame elements.
However referred to a very simple example, this study is aimed as a
preliminary contribution to the quantitative assessment of the effect of
connections deformability on peak stress conditions that a timber structure
could undergo during an earthquake.
Conclusions and further investigations
The authors are well aware of the following limits in their research: the
geometry and shape of the structure; just two input accelerograms; the cy-
clic behaviour model not fully reflecting actual joint behaviour - cyclic
behaviour laws were in fact derived from a model implemented for steel
structures. Moreover, hysteretic damping could not be considered directly;
rather, the type of analysis involved assuming damping at generic time as
depending on the stiffness matrix: such assumption should be confirmed or
rejected through tests on full-scale models, along with damping value at
time t = 0; in our study, this was prudentially assumed to be 5%, and constant
for all structures.
However, they think that such method, once extended and refined by
means of both improved analysis programs and a greater wealth of experimental
data, could help effectively in studying seismic effects on timber
structures and allow a more exact assessment of the values required by
20-15-1 A Ceccotti, A Vignoli
Behaviour factor of timber structures in seismic zones
Summary
The ideas pertaining to the definition of the structural behaviour factor for
the structures' design in seismic zones are re-examined. A method of evaluating
the behaviour factor of plane timber structures composed of rigid
(glued) joints or seimirigid joints (i.e. dowels) is presented. Behavioural
laws relating to the timber's structural elements and their joints under cyclic
loads were obtained experimentally.
Accelerograms, applied to the base of the structures, were pulled out on
the basis of their response spectrum as defined by the Italian draft proposal
CNR-GNDT. The acceleration values corresponding to the elastic field
limit and the point of collapse (i.e. Ay, Au) were obtained using a direct integration
technique with a DRAIN-2D non-linear analysis computer program.
The behaviour factor was defined as the ratio between Au and Ay.
The values of this coefficient in the case of semi-rigid joints can even be
ten times greater than the values obtained for rigid joints. At the conclusion
of the research a critical analysis of the results was carried out, indicating
suggestions for further research.
Conclusions and suggestions for further research
The conducted analysis clearly allows the assessment of the variation tendency
of some fundamental quantities depending on the most important
constructive factors. More specifically, it can be stated that, in the case of
CIB-W18 Timber Structures – A review of meeting 1-43 5 SPECIAL ACTIONS page 5.7