Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 524

energy dissipated before the joint failure in the fractured members was not clearly
apparent. From these circumstances, it has been questioned whether the current
philosophy, based on the dissipation of seismic input energy by means of plastic
rotations, is effective also in case of near-source earthquakes. Among the causes
assumed to explain these brittle fractures, the effect of high velocity of the pulse
seismic loading, producing important strain-rate effects in the structural material, is
considered by the specialists to be the main influencing factor (Gioncu and
Mazzolani, 2002). Therefore, there are two different types of member behavior
influenced by the earthquake type:
- In case of pulse seismic loading (Fig. 10.42a), characteristic of near-source
thrust earthquakes, the great velocity induces very high strain-rate with the
consequence of increasing the yield stress, reducing the ductility and producing the
brittle fracture of members or joints at the first or second cycle. The reduction is
brittle.
- In case of cycle loadings (Fig. 10.42b), characteristic of near-source strike-
slip, intraslab earthquakes and soft soils, an accumulation of plastic deformation
occurs, producing a degradation of the structural behavior. The reduction is ductile .
Figure 10.43 presents a proposal for the values of the reduction factor q , in
function of the earthquake type. This proposal refers to framed structures and
normal soil conditions; for other structure types and bad soil conditions, the values
can be modified accordingly. Only 1-2 pulses and short durations for thrust
sources, 3-4 pulses and moderate durations for strike-slipe sources characterize the
interplate crust near-source earthquakes. As a consequence, the seismic energy
dissipation is very reduced for thrust sources and reduced for strike-slip sources.
Figure 10.42 Member behavior under seismic actions: (a) Pulse seismic loading;
(b) Cyclic seismic loading (Gioncu and Mazzolani, 2002)