Influence of variability of material mechanical properties on seismic performance of steel and steel-concrete composite structures

Modern standards for constructions in seismic zones allow the construction of buildings able to dissipate the energy of the seismic input through an appropriate location of cyclic plastic deformations involving the largest possible number of structural elements, forming thus a global collapse mechanisms without failure and instability phenomena both at local and global level. The key instrument for this purpose is the capacity design approach, which requires an appropriate selection of the design forces and an accurate definition of structural details within the plastic hinges zones, prescribing at the same time the oversizing of non-dissipative elements that shall remain in the elastic field during the earthquake. However, the localization of plastic hinges and the development of the global collapse mechanism is strongly influenced by the mechanical properties of materials, which are characterized by an inherent randomness. This variability can alter the final structural behaviour not matching the expected performance. In the present paper, the influence of the variability of material mechanical properties on the structural behaviour of steel and steel/concrete composite buildings is analyzed, evaluating the efficiency of the capacity design approach as proposed by Eurocode 8 and the possibility of introducing an upper limitation to the nominal yielding strength adopted in the design

INFLUENCE OF STEEL MECHANICAL PROPERTIES ON EBF SEISMIC BEHAVIOUR

Among the resisting steel types suitable for the design of high ductility structures, Eurocode 8 proposes MRFs and EBFs. Also if the formers are generally considered a more efficient structural solution for high-ductility design, they suffers a strong weakness in the lateral stiffness creating, during the design process, cumbersome procedures to avoid excessive lateral displacements maintaining quite high ductile behaviour under design seismic actions. In many cases, the design process produces not optimized structural members, oversized respect to the minimum seismic requirements due to lateral deformation limitations. On the contrary, EBF combines high lateral stiffness furnished by bracing elements and high dissipative capacities furnished by plastic hinges developed in links. Eurocode 8 proposes a design procedure for realizing high ductility EBF in which iterative checks are required to properly design the links assigning to every link a defined level resistance dependant on all other links resistance. The present paper investigates the seismic behaviour of EBFs using the Incremental Dynamic Analysis technique in order to explore their mechanical response under increasing seismic action levels. A set of steel structures is designed according to Eurocode 8. The numerical simulations are executed considering the variability of both steel mechanical properties and seismic input, aiming to a complete probabilistic characterization of mechanical response of the system and deeply analyzing the effective level of structural safety and the ability to internally redistribute plasticizations during the earthquake. Structural safety conditions will be defined according to a multi-level performance approach. The paper presents also some final suggestions for possible improvements/simplifications in EBF design

Influence of low-cycle fatigue and corrosion phenomena on the ductile behaviour of steel reinforcing bars

Actual standards for constructions prescribe the realization of r.c. structures able to dissipate the energy stored during the earthquake through the development of global collapse mechanisms; the capacity design procedure is generally adopted, individuating specific "plastic hinges" in which the plasticization shall be located and using specific shear design actions opportunely over-dimensioned in order to avoid local brittle failures. The dissipative capacity of the structure is directly related to the rotational capacity of the elements in which plastic hinges are located and strictly depends on the geometrical and mechanical characteristics of the section itself and on the ductile capacity of steel reinforcing bars. The evaluation of the low-cycle fatigue behaviour of the rebars is obviously of relevant importance for the analysis of the structural behaviour of seismic r.c. buildings; two aspects shall be widely discussed, mainly related to the definition of the effective seismic demand on the rebars due to earthquakes and to the effective cyclic capacity of reinforcements. In the present work, developed inside the framework of a European research project called Rusteel (2009) the results coming from an accurate analysis of the seismic behaviour of r.c. structures is presented, evaluating, in particular, the effective level of strain and dissipated energy due to earthquake events on rebars. The mechanical characterization of the low-cycle fatigue behaviour of bars commonly used in r.c. buildings was also executed, leading to the comparison with the data coming from numerical analyses

Seismic Performance of a 3D Full-scale High-ductile Steel-concrete Composite Moment-resisting Frame-Part II: Test Results and Analytical Validation

This paper presents the results of a multi-level pseudo-dynamic seismic test program that was performed to assess the performance of a full-scale three-bay, two-storey steel¿concrete composite moment-resisting frame built with partially encased composite columns and partial-strength beam-to-column joints. The system was designed to develop a ductile response in the joint components of beam-to-column joints including flexural yielding of beam end plates and shear yielding of the column web panel zone. The ground motion producing the damageability limit state interstorey drift caused minor damage while the ultimate limit state ground motion level entailed column web panel yielding, connection yielding and plastic hinging at the column base connections. The earthquake level chosen to approach the collapse limit state induced more damage and was accompanied by further column web panel yielding, connection yielding and inelastic phenomena at column base connections without local buckling. During the final quasi-static cyclic test with stepwise increasing displacement¿amplitudes up to an interstorey drift angle of 4.6%, the behaviour was ductile although cracking of beam-to-end-plate welds was observed. Correlations with numerical simulations taking into account the inelastic cyclic response of beam-to-column and column base joints are also presented in the paper together. Inelastic static pushover and time history analysis.JRC.G.5-European laboratory for structural assessmen