Abstract

Concentrically braced frames (CBFs) are widely used in North America. The CBFs possess high stiffness and moderate ductility, while braces are designed to buckle in compression and yield in tension. However, after a brace experiences buckling, its compression strength diminishes and the system undergoes asymmetrical response, while the distribution of internal forces and deformations is influenced by the frequency content of ground motions. Despite the system’s stiffness, CBFs are prone to concentrate damage within a floor which leads to the formation of storey mechanism. To preserve the stability of the system during the nonlinear seismic response, the National Building Code of Canada (NBCC) imposes limits on a building’s height which depends on the selected ductility-related force modification factor, Rd. Thus, the height limit for buildings with moderately ductile concentrically braced frames, MD-CBFs, is 40 m and for limited ductility concentrically braced frames, LD-CBFs, is 60 m.
To safely increase the height limit of ductile braced frame buildings, a system labelled Outrigger Braced Frame, OBF, is proposed and developed in this study. According to the Council on Tall Buildings and Urban Habitat (CTBUH), a building with more than 14 stories or more than 50 meters in height may be considered a high-rise building. The aim of this research is to develop, design, model, and study the seismic performance of mid-rise (e.g. tweleve-storey) and high-rise (e.g., sixteen-storey) OBF buildings subjected to dynamic loads. It is noted that the outrigger system functions by tying together a core system and a perimeter system. Herein, the core system is made of MD-CBFs and the perimeter system is made of gravity columns. Furthermore, only the core braces are designed to dissipate energy, while the outrigger’s diagonals are designed to respond in the elastic range. The performance of OBF system is controlled by the amount of added stiffness and optimum location of outriggers across the building’s height, the number of levels with outriggers and the intensity of seismic zone. All multi-storey buildings are located in high-risk seismic zone of Victoria, B.C. Canada, on Site Class C. The selection of ground motions was made to capture the seismic characteristics at buildings location. Herein, two sets of crustal and subduction ground motions were considered such as California records and the mega-thrust magnitude 9 Tohoku records, respectively. The nonlinear time-history dynamic analyses were conducted using the OpenSees software.
The main objectives of this thesis are three-fold: i) to identify the effect of subduction versus crustal ground motions on the seismic response of low-rise, mid-rise and high-rise MD-CBF buildings and to study their seismic performance from yielding to failure, ii) to provide design method and optimum location for outriggers of OBF steel buildings, iii) to assess the collapse safety of the proposed mid-rise and high-rise OBF steel buildings using FEMA P695 procedure and to compare their seismic performance against that resulted for MD-CBF buildings.
It is concluded that the OBF buildings are slightly stiffer than the corresponding MD-CBF buildings, and they experienced lower interstorey drift and residual interstorey drift than the MD-CBF buildings. In all case studies considered here, the collapse margin ratio (CMR) is greater for buildings subjected to crustal ground motions than subduction ground motions. Evaluation of seismic performance of sample 12-storey and 16-storey OBF buildings shows that these buildings are able to pass the collapse safety acceptance criteria, ACMR ≥ ACMR10%, when subjected to both sets of ground motions. On the other hand, the corresponding MD-CBF buildings are not able to pass the collapse safety acceptance criteria when subjected to subduction records set. Hence, special attention should be given when designing buildings in seismic regions which are prone to both types of earthquakes.