Ground Motions and Structures - Earthquake Engineering for Structural Design - page 342

342 Earthquake Engineering for Structural Design

(a) (b) (c)
Figure 8. 39 Response control systems: (a) Base isolation; (b) Energy absorption;
(c) Mass effect mechanism (Gioncu and Mazzolani, 2002)

A significant progress has been recently made in development and application of
innovative structural systems for seismic protection. The aim of these systems is the
modification of the dynamic interaction between structure and ground motions, in order
to minimize the structural damage and to control the structural response. So, this
concept is very different from the conventional one. The response control systems can
be roughly classified as shown in Figure 8.39: base isolation, energy absorption and
mass effect mechanism (Gioncu and Mazzolani, 2002).

8.5.2MomentResistingFrames

In case of moment resisting frames, the most significant tendency is to change the
traditional welded connections with bolted joints (Fig. 8.40). This tendency is the result
of the bad behavior of welded connections during the 1994 Northridge and 1995 Kobe
earthquakes, where the site welding type has been incriminated for the cracks occurred
in joints (Gioncu and Mazzolani, 2002). These near-source earthquakes caused to steel
moment resisting frames (MRF) unexpected brittle fractures in the traditional beam-to-
column, welded flange-bolted web connections. In particular the full confidence in
strength, stiffness and ductility of such common moment connections was seriously
undermined: most connections provided small inelastic deformations and their
resistance was smaller than the one of the connected elements. Major causes were
recognized to be the bad quality of groove welds and the composite action with the
concrete slab to the beam strength, producing a large tensile strain in the bottom flange.