Quantification of Optimal Target Reliability for Seismic Design: Methodology and Application to Midrise Steel Buildings

Quantification of the optimal target reliability based on the minimum lifecycle cost is the goal standard for calibration of seismic design provisions, which is yet to be fully-materialized even in the leading codes. Deviation from the optimally-calibrated design standards is significantly more pronounced in countries whose regulations are adopted from the few leading codes with no recalibration. A major challenge in the quantification of optimal target reliability for such countries is the lack of risk models that are suited for the local construction industry and design practices. This paper addresses this challenge by presenting an optimal target reliability quantification framework that tailors the available risk models for the countries from which the codes are adopted to the local conditions of the countries adopting the codes. The proposed framework is showcased through the national building code of Iran, which is adopted from the codes of the United States, using a case study of three midrise residential steel building archetypes. The archetypes have various structural systems including intermediate moment-resisting frame (IMF) and special concentrically braced frame (SCBF). Each of these archetypes are designed to different levels of the base shear coefficient, each of which corresponds to a level of reliability. To compute the lifecycle cost, the initial construction cost of buildings is estimated. Next, robust nonlinear models of these structures are generated, using which the probability distribution of structural responses and the collapse fragility are assessed through incremental dynamic analyses. Thereafter, the buildings are subjected to a detailed seismic risk analysis. Subsequently, the lifecycle cost of the buildings is computed as the sum of the initial construction cost and the seismic losses. Finally, the optimal strength and the corresponding target reliability to be prescribed are quantified based on the notion of minimum lifecycle cost. The results reveal a 50-year optimal reliability index of 2.0 and 2.1 for IMF and SCBF buildings, respectively and an optimal collapse probability given the maximum considered earthquake of 16% for both systems. In the context on the case study of the national building code of Iran, the optimal design base shear for IMF buildings is 40% higher than the current prescribed value by the code, whereas that of SCBF buildings is currently at the optimal level.

Investigation of hybrid multi-core buckling-restrained brace components

<p>Contemporary seismic-resistant design of steel braced frames is based on dissipating seismic energy through significant inelastic axial deformation in brace components. Buckling-restrained braced (BRB) frames are a type of concentrically braced frame (CBF) characterised by braces that yield both in tension and in compression. These braces therefore exhibit superior cyclic performance compared with traditional CBFs. However, buckling-restrained braces commonly display a low post- yield stiffness, causing substantial interstory drifts and large residual drifts after seismic events. Moreover, yielding of the core is often only tied to a single performance objective, thus making its performance at other levels of seismicity largely unknown. One promising solution is the use of a hybrid BRB, where multiple cores made from different metals are connected in parallel to work together and complement each other. This research is geared towards first evaluating the potential of different combinations of core materials, followed by the design of a hybrid BRB system that can accommodate multiple core plates. Results show that the post-yield behaviour of hybrid BRBs is improved by employing a combination of 350WT carbon steel and another metal with low-yield and high strain-hardening behaviour, such as stainless steels, aluminium alloys, or other grades of carbon steels. Finally, a detailed overview of one hybrid BRB solution is proposed.</p>

Seismic Performance Analysis of Self-Centering Concentrically Braced Steel Frame Structures

To study the seismic performance of self-centering concentrically braced frame (SC-CBF) structure, the static elastoplastic analysis, low-cycle repeated loading analysis, and elastoplastic time-history analysis were conducted for a four-story SC-CBF structure, compared with the traditionally concentrically braced frame (CBF) structure. The influences of different GAP stiffnesses and cross-sectional areas of prestressed tendon were investigated on the self-centering and seismic performance of the SC-CBF structure. The results show that the SC-CBF structure has a strong lateral resistance, a small base shear under earthquake action, and a slight residual drift after unloading. The SC-CBF structure has a better ductility than the CBF structure. The displacement of the SC-CBF structure under the action of rare and extremely rare earthquakes is large, and the structure can dissipate more energy; the interstory drift is large, but the residual drift is small, exhibiting its ideal seismic and self-centering performance. However, the mechanical behavior of prestressed tendons is significantly affected by the stiffness of the GAP. The mechanical and seismic performances of the overall structure are slightly affected by the stiffness of the GAP, but the cross-sectional area of the prestressed tendons has a remarkable influence on the overall performance of the structure.

Experimental and numerical studies on Seismic Performance of The SCFST Column Eccentrically Braced Frames

The quasi-static experiments and finite element analysis of three groups of special-shaped concrete-filled steel tube (SCFST) column chevron braced frames (two groups of eccentrically braced frames and one group of concentrically braced frame) were carried out. The differences of quasi-static mechanical properties between the three groups frame were compared. The damage mechanism of the concentrically and eccentrically braced frames was significantly different, and the eccentrically braced frame could significantly improve the energy-dissipation ability and ductility. When the single limb of columns was connected by double-steel-plate, the stiffness of eccentrically braced structure could be improved around 10.4% and showed better energy-dissipation capacity. The finite element simulation was built on the basis of experiments, and parametric analysis was examined. The analysis results showed that section forms of the SCFST column and the thickness of brace have significant impacts on the quasi-static properties of such type of structure.