Seismic Resistant Steel Structures

Steel structures have been always considered as a suitable solution for constructions in high seismicity areas, due to the very good strength and ductility exhibited by the structural material, the high quality assurance guaranteed by the industrial production of steel shapes and plates and the reliability of connections built up both in workshop and in field (Mazzolani and Piluso, 1996). In spite of these natural advantages, researchers are concerned about the necessity that, in order to ensure ductile structural behaviour, special care must be paid mainly in conceiving dissipative zones, which have to be properly detailed, assuring stable hysteresis loops, able to dissipate the earthquake input energy with high efficiency (Bruneau et al., 1998). As a confirmation, during the recent seismic events of Northridge (Los Angeles, 17 January 1994) and Hyogoken-Nanbu (Kobe, 17 January 1995), even if the cases of collapse of steel buildings have been extremely rare, steel moment frame buildings, considered as highly ductile systems, exhibited an unexpected fragile behaviour. They presented many failures located at the beam-to-column connections, challenging the assumption of high ductility and demonstrating that the knowledge on steel moment frames is not yet complete. Hence, in order to improve constructional details and to propose new design solutions for achieving a correspondence between the design requirements and the actual structural response, the scientific community began to deepen the reasons of this poor behaviour: does it depends on the material quality, on the design concept, on the structural scheme, on the constructional details, on the code provision, or on the seismic input occurred (Mazzolani, 1998)? Most of these questions are still being analysed, but much more has been understood on the seismic behaviour of steel structures. Consequently, during the last years most of the recent knowledge has been already or is going to be introduced into the structural design provisions for seismic resistant design in all the earthquake prone Countries, giving rise to a new generation of seismic codes.

The paper presents the state-of —the art in the design of the steel structures, in the light of the lessons learned from the last great earthquakes: Michoacan (1985), Loma Prieta (1989), Northridge (1994) and Kobe (1995). In the introduction, the very recent progress in the conception and design are presented, showing the challenge for future research works. The design criteria as multi-level design approaches and rigidity, stiffness and ductility demands are detailed, emphasizing the unsolved problems. New aspects for ground motion modeling, as the differences between near-source and far-source earthquakes are examined. The response of the structure to these near-source earthquake is examined, taking into account the influence of superior vibration modes. velocity and strain-rate, vertical components, etc. The final conclusions consider that now is the right moment to introduce some new provisions in the design codes, in order to fill the gap that exists between the accumulated knowledge and design codes.

Earthquake resistant design method in a sense of modern technology started immediately after the Great Kanto Earthquake (1923)(See Table 1.1). Since then, during three fourths century earthquake resistant design has made a great progress. As the scope of the applied structures was extended, the design ideas became versatile. As the electronic digital computers and apparatuses for field observation became available, analytical techniques for response analysis developed drastically and became versatile. On the other hand, successive attacks of big hazardous earthquakes made us continuously recognize the arrived level earthquake resistant design method to be still incomplete. The earthquake resistant design started as a measure by which a structure basically designed for gravity loading is to be supplied additionally with a resistance against earthquakes. Increased experiences of earthquake hazard and the progress of response analysis techniques have raized the importance of earthquake resistant design to such a extent that the design of structures is governed mostly by earthquake resistant design.

Moment-resisting frames (MRF) are structures with a satisfactory behaviour under severe earthquakes. As it has been already noted (Chapter 1, Section 4), they can provide a large number of dissipative zones, where plastic hinges form with potentially high dissipation capacity. In order to maximise the energy dissipation capacity, MRFs have to fail with a mechanism of global type. As a consequence proper design criteria have been conceived to fulfil this condition (Mazzolani and Piluso, 1996a). According to the assumed design approach (see Section 1.2), moment resisting frames can provide different level of strength and ductility.

As well known, structures in seismic regions are designed so that: Seismic building Codes introduce two limit states beyond which the structure no longer satisfies the design performance requirements, an ultimate limit state associated with no collapse and a serviceability limit state associated with damage limitation.

In the beginning of the years 1990’s very little design guidance did exist for the seismic design of composite steel concrete structure. The available test data were rather scarce, like were the numerical analysis.

The use of steel connections is inherent of every structural steel building, whether it is of one story or one hundred stories. Therefore, the beam-to-column connection, because of its importance to all construction, is significant both economically and structurally. Savings in connection costs as well as improved connection quality has an impact on all types of buildings. Because of the repetitive nature of the connections, even minor material or labour savings in one connection is compounded and expanded throughout the entire building. It is important, then, for a design engineer to understand the behaviour of the connection, not only from the point-of-view of the connection as a structural element, but also from the point-of-view of the connection as a part of the complete structural system.