This award is an outcome of the NSF 08-519 program solicitation George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) Research (NEESR) competition and includes Portland State University (lead institution), University of Washington (subaward), and California State University, Los Angeles (subaward). The project will utilize the multiple shake table NEES equipment site at the University of Nevada, Reno. The basic seismic design philosophy for steel frame buildings has been to rely on the gravity load system in order to prevent loss of life. However, the expectations of building owners and society are no longer satisfied with merely providing life safety, so new structural systems are needed for achieving improved performance levels that limit damage. One of the design targets needs to include rapid return to occupancy, especially for earthquakes that are less severe than the maximum expected. The overall objective of this project is to develop a lateral load resisting system, the linked column frame system, for unbraced steel frames capable of achieving specific target performance levels. The proposed structural system includes configurations of novel and conventional structural components that together result in predictable and rapidly recoverable damage. NEES equipment sites offer a unique capability to experimentally evaluate system level response under dynamic loads, which is required to study the interaction of the structural components and evaluate the potential advantages of the proposed building frames. This NEES individual investigator project will transform seismic design approach in regions of moderate and high seismicity by developing a unique seismic load resisting system, thereby contributing to the intellectual merit of the project. The research will use advanced experimental and computational research methods to develop the necessary understanding of system and component behaviors. Data sets from large-scale dynamic experiments will be generated to ensure that analytical models capable of capturing both component and system behavior are developed. The depth of understanding achieved through such an experimental and analytical research program will enable the development of robust design methodologies. The outcomes of this research will not only impact seismic design, but also result in other broader impacts. Utilization of such new structural systems will reduce post-event downtime and building repair costs, and thus will have a significant impact on reducing earthquake losses and life-cycle costs. Further, the research will develop understanding of a novel composite construction that could be adapted to other structural components. This project will also impact engineering education and diversity through active collaboration with faculty and undergraduate students from a minority serving and predominantly undergraduate institution. The proposed activities are combined with an integrated educational component designed to emphasize the importance of capacity and performance design in undergraduate engineering education. These educational aspects will be reinforced through research participation using existing NEES web based telepresence tools. Data from this project will be made available through the NEES data repository (www.nees.org).
While moment frames provide significant architectural advantages in building layout, limited options exist in achieving seismic performance levels that allow rapid return to occupancy and in protecting the gravity load system. Braced systems such as buckling restrained brace frames and, to a limited extent, eccentrically braced frames combine high levels of ductility with component replaceability, but lose out on the architectural utility. The completed NEES research project by Portland State University (PSU), University of Washington (UW) and California State University at Los Angeles (CSULA) concentrated on developing a new lateral load resisting system for unbraced steel frames capable of achieving specific target performance levels including rapid return to occupancy. The developed structural system is referred to as the linked column frame (LCF) and consists of strategically placed dual columns interconnected with replaceable shear links and surrounded by hierarchically flexible moment frames. The system performance level can be targeted, with the most challenging performance level being the rapid return to occupancy following a design level earthquake. The research needed to tackle challenges in development of effective replaceable shear links, understanding system level response of various building LCF arrangements and culminated in system level seismic performance investigation at NEES@Berkeley equipment site. Given the LCF system reliance on the links interconnecting the dual column, the research team proposed, developed, analyzed and experimentally validated replaceable shear link details for cost effective and plastically deformable components to be used for the interconnected column. Detailing of the component was key and the outcome was a cost effective design approach of limiting the inelastic deformation to areas away from the connections. Detailed numerical analyses were validated with component tests on shear links that exhibited ductility levels suitable for use in the LCF system. Broader impact of this aspect of the project was realized when the design approach was ultimately adopted for similarly utilized links in two other NEESR projects. The lessons learned could also be extended for the design of critical components in conventional eccentrically braced frames. Using the experimental data from the shear link component tests, high fidelity numerical models of the LCF system were developed and analyses performed on 3, 6 and 9 story building designs. The design process revealed that similar to conventional moment frames, the LCF system can be drift sensitive. The results of the analyses showed that LCF can significantly outperform conventional moment frame by achieving rapid return to occupancy performance level under 10% in 50 year earthquake probability and collapse prevention under 2% in 50 year earthquake probability. Additional analyses were then conducted to establish seismic design parameters R, Cd, Omega based on FEMA P-695 procedures. These parameters allow for greater and more practical dissemination of the research on this new system to practicing engineers and provide the investigators with means of pursuing codification. The structural system seismic performance investigation required the unique capability of NEES@Berkeley facility, where hybrid simulation allowed for considerations of an entire building frame. The frame was divided into a two-story single bay experimental substructure that incorporated all of the major components of the LCF system and a numerical model of the rest of the frame consisting of the more conventional moment frame components. The outcomes validated the design approach and the incorporation of the shear link components within a LCF system. The LCF system was able to resist numerous earthquake excitations and maintain rapid return to occupancy performance level by limiting the damage to the replaceable links. Ultimate failure mode was also desirably ductile. The research resulted in a strong case for potential incorporation of LCF in the designers toolbox, which would lead to higher level of resiliency and consequently lower economic losses in unbraced frame buildings. In addition to the research related broader impacts, an educational impact was also realized especially touching undergraduates at CSULA and PSU. Junior level course at CSULA integrated research findings and utilized a small scale LCF model collaboratively developed between CSULA and PSU. Students are able to physically interact with inelastic performance and repleacability of links using CSULA table-top shake table. The project results were also disseminated using traditional means of presentations at conferences and workshops as well as papers published in refereed journals and conference proceedings.
