UC Berkeley

This dissertation focuses on the use of supplemental energy-dissipation devices to improve the seismic performance of tall steel buildings. It is divided into two parts. Part 1 focuses on exploring cost-effective retrofit strategies to improve the seismic performance of an existing tall building. The selected building is a 35-story steel moment-resisting frame, with representative details from the early 1970s. Detailed seismic evaluations were conducted in the framework of Performance Based Earthquake Engineering (PBEE), using the scenario-based performance assessment methods. A three-dimensional numerical model capturing the mechanical properties of the most critical structural elements was generated using the program: Open System for Earthquake Engineering Simulation (OpenSees). Seismic evaluation of the selected building was done following ASCE 41-13, FEMA 351 and FEMA P-58, and two hazard levels: basic safety earthquake levels 1 and 2 (BSE-1E and BSE-2E) prescribed by ASCE 41 were used for the assessment. Results predicted that this building failed to meet the recommended performance objectives and had a variety of seismic vulnerabilities, and possible retrofits were needed.

Therefore, a two-level retrofit approach was examined that focused on achieving the collapse prevention limit state under the BSE-2E hazard level. In Level-1, the brittle column splices were fixed everywhere in the building, and the massive concrete cladding was replaced with lightweight substitute in the exterior of the building. Level-2 strategies augmented the Level-1 methods by adding different supplemental energy-dissipation devices. Devices investigated include: fluid viscous dampers (FVDs), viscous wall dampers (VWDs) and buckling restrained braces (BRBs). Among these, the scheme that used FVDs was expected to be the most promising to upgrade the seismic performance of the case-study steel moment frame, and thus was examined first. In this approach, feasible damper locations and overall effective damping ratios were evaluated through a series of preliminary studies, and then a two-phase manual design method was used to refine the distribution and mechanical properties of the dampers. Thorough assessments of the refined design were carried out and the results indicated that the proposed retrofit method of using FVDs could achieve the retrofit goal and provide a cost-effective means of improving the structural behavior and reducing economic losses in a major seismic event for this case-study building.

The study was extended to examine alternative measures to upgrade the case-study building by using either VWDs or BRBs, and compared their relative effectiveness and economy with the scheme using FVDs. The locations and effective damping ratios were kept the same for all three schemes to insure a valid comparison. Results indicated that the proposed schemes of VWDs and BRBs both failed to achieve the targeted performance goal for this structure under a BSE-2E event, and special design considerations were required.

Part 2 of the dissertation focuses on developing an automated tool to streamline the design of FVDs in tall buildings. Aided by the high-performance computers and parallel processors, a large amount of complicated nonlinear response history analysis was conducted to facilitate the automate design procedure. The optimization problem was devised in a simplified PBEE framework under one hazard level each time. Basic optimization ingredients were selected to reflect the target performance goal, and several cases using different objective functions were evaluated.

Two tall buildings: the existing steel moment frame examined before and a newly-designed mega-brace steel frame were selected to rely on the automated procedure to optimally design FVDs. In both cases, the automated procedure turned to be very efficient, help identify design parameters of dampers in selected locations and reduce a great amount of engineering efforts. With only limited number of iterations, optimal design patterns of FVDs in a tall building could be found, which were able to improve the structural performance under different hazard events. The suggested optimal design could meet retrofit goal for the existing tall building, as well as achieve enhanced performance goal for both existing and new tall buildings.