PEER has just published Report No. 2015/14 titled “Seismic Evaluation and Retrofit of Existing Tall Buildings in California: Case Study of a 35-Story Steel Moment-Resisting Frame Building in San Francisco.” It was authored by Jiun-Wei Lai, Shanshan Wang, Matthew J. Schoettler, and Stephen A. Mahin.
The seismic performance of new tall buildings located in regions of high seismic hazard has been recently investigated by Pacific Earthquake Engineering Research Center (PEER) under its Tall Buildings Initiative (TBI) program. This program has now expanded to assess the seismic performance of existing tall buildings. Buildings being considered are 20 stories or more in height and were constructed on the west coast of the U.S. between about 1960 and 1990. During that period, several hundred tall buildings were constructed in California, but earthquake resistant design procedures were not as fully developed as they are today.
A 35-story steel building located in San Francisco and designed in 1968 is selected as the subject of the detailed seismic evaluations presented in this report. Results from several three dimensional (3D) nonlinear dynamic analyses are presented to examine the potential seismic performance of this structure in future earthquakes. Two primary earthquake hazard levels are used for these evaluations. Several different numerical models are developed for the building to represent different behavior characteristics for the as-built structure, as well as for the building following various retrofits. The models were developed in accordance with recommendations of ASCE 41-13 and other relevant guidelines, and simulations were carried out using the analysis framework OpenSees. Analysis results for the as-built and upgraded structure are interpreted considering the methodologies and performance criteria suggested in ASCE 41-13, FEMA 351, and FEMA P-58. With an understanding of the building’s potential seismic vulnerabilities, the sensitivity of its behavior to ground motion and structural characteristics, and the capabilities of different guidelines, several retrofit strategies are identified and explored, focusing on modification of the existing structure and the addition of fluid viscous dampers (FVDs).
In terms of the evaluation of seismic response, performance objectives based on ASCE 41 were used; i.e., damage control under the BSE-1E hazard and limited safety under the BSE-2E hazard. The evaluations conducted indicate that the as-built case-study structure does not satisfy these objectives. Three major seismic deficiencies are identified as part of the ASCE 41 Tier 3 evaluations conducted: (1) the case-study building tends to form weak-story regions in the lower third of the building; (2) pre-Northridge beam-to-column connection details result in a high percentage of connection failures under BES-2E events; and (3) there is a high probability of brittle failures of column splices under BSE-1E and BSE-2E hazard-level excitations.Considering the oriented evaluation approach and performance criteria presented in FEMA 351, the case-study building in its as-built condition is found to be unable to achieve the global collapse prevention performance goal at the 90% confidence level for either a BSE-1E or BSE- 2E hazard-level event.
Several retrofit scenarios are explored wherein modifications were made to the existing structure. These included replacing the heavy exterior cladding with a lightweight curtain wall system, retrofitting the column splices, and retrofitting the beam-to-column connections. Additional analyses are then carried out to assess enhanced retrofit schemes that introduce Fluid Viscous Dampers (FVDs) over the full or partial height of the building. In the studies presented, the FVDs improve the performance at the BSE-1E and BSE-2E hazard levels. They reduce the peak drift ratios, residual drift ratios, and localized connection fractures; in addition, they help suppress floor accelerations and lead to more rapid decay of seismic vibrations. However, the analysis results indicate that large damper force capacities are needed to achieve the desired improvement in performance, especially in the most deformed stories. To obtain equal or better improvements in performance, and to reduce the number and size of the damping devices, a simplified optimization method is used to identify several more retrofit schemes that consider different damper properties, distribution of damper properties, and damper placement configurations. A simplified performance-based evaluation of the incremental costs and likely loss reductions is then made for the final retrofit strategies as well as for the as-built structure. Retrofit of the building is found to be very beneficial, but additional study is recommended to better estimate costs of implementing the retrofits, including costs associated with disruption of function during retrofit and business interruption losses following earthquakes. Recommendations are offered for changes that should be considered in developing future editions of ASCE 41 and for further research to improve the certainty with which response of existing tall buildings—in their as-built and retrofit condition—might be predicted. Because only a single structure is evaluated herein, it is finally recommended that other tall buildings be evaluated, and that a group of researchers and practitioners be gathered to develop improved guidelines for assessing and retrofitting existing tall buildings.