Seismic Assessment and Rehabilitation of Existing Buildings Using Nonlinear Static Procedures (NSPs) -Pushover Procedures-
Shehadah, Anas Shaher Abdul-Hafeeth
An-Najah National University
Design of buildings for seismic loads is becoming mandatory in Palestine. However, what about the existing buildings? Existing buildings, especially old ones, were mostly designed under the influence of static loads. Such buildings may stand vulnerable to earthquakes and thus need to be strengthened; so that they become safe. To achieve the required level of strengthening, advanced analysis and assessment tools must be used. There is a lack of systematic studies that provide practical "know-how" guidelines for local engineers on the assessment and retrofitting of existing buildings against seismic loads. Generally, the guidelines written in foreign codes (e.g. the ASCE or FEMA) are very broad and general and may pose a challenge to local engineers regarding the consistency of their implementation. This study bridges this gap between local engineers and international codes by putting these guidelines into action through a practical case study. Generally, four procedures are available for seismic analysis of buildings: two linear procedures, and two nonlinear procedures. The nonlinear procedures include the nonlinear static procedure (NSP) and nonlinear dynamic procedure (NDP). NSP's are deemed to be very practical tools to assess the nonlinear seismic performance of structures. On the other hand, NDP's require detailed input data, and it is very time-consuming, which is a relevant drawback in design offices, where the deadlines are restrictive. Also, it doesn’t exist in Palestine neither local earthquake records, nor specialized powerful programs for NDP. This makes the NSP best choice for practical assessment of buildings. The research objective in this thesis is to demonstrate an assessment methodology through studying a local existing building, which was designed under gravity loads only, and then propose retrofitting solutions to remedy the deficiencies in the building. Based on the above, the case study building is assessed using an NSP that is called capacity spectrum method (CSM) as per ATC-40. The behavior of the structure is generated using nonlinear pushover analyses. The seismic assessment were conducted based on FEMA 356 performance criteria. According to FEMA 356, there are two approaches for seismic evaluation: global-level and member-level with three performance levels, which are immediate occupancy (IO), life safety (LS) and collapse prevention (CP). In addition, seismic design requirements that are mentioned in ASCE 7-10 were conducted in order to assess the building for irregularities. Based on the nonlinear pushover analysis and the assessment of the building, it was found that the building suffers from vertical irregularities and concentration of plastic hinges at the ground floor. In order to improve the performance of the building, two possible retrofitting techniques were applied including the addition of RC column jackets, and moment resisting RC frames. The capacity curves for the retrofitted structure were compared to those for the un-retrofitted case. FEMA global drift limits were compared with the drift limits of the performance points of each retrofitting techniques based on the FEMA member-level criteria. In addition to this, the ASCE limits were also rechecked and compared to the ratios of the un-retrofitted building. The retrofitting techniques helped improve the performance of the building. This thesis paves the way to further research on seismic assessment of existing buildings with effective tools for judging the efficiency and suitability of retrofitting techniques.