CONTENT 5.1 SEISMIC BEHAVIOUR ... - CIB-W18

Discussion
Being the design ground acceleration PGAcode, equal to 0.35g, by applying
the procedure given in 2.2, the q value is:
q 1,20 / 0,35 3,4
Of course the above value is valid only referring to the used Nocera Umbra
ground motion record. A series of different quakes should be used with
the same procedure. This is obviously impossible; therefore the importance
of a good mathematical model that can simulate different quakes
and cases is obvious.
In any case, the above value has its own significance as an indicator.
Moreover, it must be considered that the building has passed without any
important reparation at least 14 "destructive" quakes in a row. It has kept
its shape even with the last quake that has produced the near-collapse
state. That means that this typology seems very promising when the design
philosophy in seismic areas would convert to the NDD – no damage design
– approach.
39-15-3 A Heiduschke, B Kasal, P Haller
Laminated timber frames under dynamic loadings
Summary
This paper discusses the results of time-history analyses of four-story moment-resisting
timber frames subjected to various seismic loadings. The
purpose of the case study was to document the difficulties, which arise due
to large story drifts of the frames caused by the soft character of moment
transmitting connections using dowel-type fasteners. Two frames were designed:
one according strength and the second according stiffness requirements
stipulated by the seismic code – EC8. From the design procedure
it follows that, in general, the serviceability limit state design is the
limiting criteria. Unacceptable lateral drifts made it necessary to increase
the joint stiffness. Since connection stiffness increases significantly with
the depth of the member, the member size inclusive the number of the fasteners
was enlarged. This, in combination with densified and textile reinforced
wood resulted in a frame design with an adequate lateral stiffness
keeping the inter-story drift within the limit stipulated by the seismic code.
The numerical model used to simulate the structural response of the
frames utilized the hysteretic formulation of nonlinear connection behav-
ior. The results of the dynamic pushover analysis were used to evaluate
and verify the behavior factor q. In the seismic code, the elastic spectrum
is modified with this factor to obtain an inelastic design response spectrum
that is used to determine equivalent static forces.
Conclusions
From the design procedure it follows, that in general the SLS design will
be the limiting criteria in the design process of moment-resisting frames.
The inter-story drift of the frame designed in accordance to the strength
and ductility criteria, exceeded the seismic code limitation at the SLS. The
unacceptable deformations made it necessary to increase the joint stiffness.
Since the stiffness increases significantly with the depth of the member
size inclusive the number of the fasteners was enlarged. This, in combination
with densified and textile reinforced wood, resulted in a frame design
with an adequate lateral stiffness keeping the inter-story drift within the
limit stipulated by the code. Over-sized members and a lateral strength
larger then the design story shear characterize this frame-S, designed according
to the stiffness requirements.
From the modal analysis one can conclude that for the evaluation of the
first fundamental period of the system, the connection behavior has to be
taken into account. For the frame-C, long periods (T > 2s), were evaluated
due to the low rotational stiffness of the connections. The structures were
not within the period range of typical earthquakes and this result in small
inertia forces. In order to get reasonable structural deformations, specific
earthquakes, characterized by soft soil conditions were selected for the
nonlinear time-history analysis.
The frame-S had an adequate lateral stiffness to keep the inter-story
drift at a reasonable level for strong ground motions up to 0.6g. In either
simulation the frame performance improved with story drift reductions of
25% to 160% when using densified wood.
The cyclic test on connections have shown that textile reinforced joints
have at least medium ductility, so that these frame types can be classified
as structures having a medium capacity to dissipate energy. As a consequence,
a q-factor of 2.5 can be assumed for the studied statically indetermined
frames. The numerical time-history analysis showed that a q-factor
of 2.5 is acceptable for the investigated frames.
CIB-W18 Timber Structures – A review of meeting 1-43 5 SPECIAL ACTIONS page 5.25