Seismic Assessment Methodology for Critical Detailing of Existing Reinforced Concrete Buildings

Abstract

The 2010-11 Canterbury earthquakes enhanced the awareness to identify the potential vulnerabilities and investigate the seismic performance of existing reinforced concrete (RC) structures in New Zealand. The New Zealand Legislation issued a Building Amendment Act 2016 to identify the potentially earthquake-prone buildings, which requires local councils to categorise the buildings using descriptive building profiles. The Act requires engineers to assess the building and to determine the % New Building Standard (%NBS) according to the current NZ seismic assessment guidelines. While the current NZ and overseas assessment guidelines provide detailed guidance for these assessments, they currently lack information on: (a) the typical deficiencies of older RC buildings (pre-1960’s) reinforced with plain round bars in NZ (b) the anchorage behaviour of plain round bars with potentially deficient hooks, and (c) a simplified assessment method to account for the impact of corrosion on RC structures. To address these issues, the current thesis was developed. A database of structural drawings of RC buildings constructed between 1920-1960 in NZ were collected and typical development length and hook geometry of plain round bars were identified. The most common type of end-hook anchorage detailing found in pre-1960’s RC buildings in NZ was 180° hook. Based on these findings, an extensive experimental study of 312 beam-end tests was conducted to understand the anchorage and load-slip behaviour of typical pre-1960 180° hook detailing of plain round bars. It was found that the maximum tensile strength and slip of hooked plain round bars depends on the embedment length, bar diameter, concrete compressive strength, and concrete cover depth. A simplified mechanics-based approach for the seismic assessment of corroded RC structures was developed based on existing analytical models for uncorroded members, but with corrosion dependent modifications implemented. The proposed corrosion dependent models were validated using a large database of experimental results from materials to RC member properties from the available literature. The predicted material (tensile strength, strain, and bond strength) and member (flexure and shear strength, and displacement) properties of corroded RC structures were in rational agreement with the experimental results. A step-bystep procedure to determine the seismic residual capacity of corroded RC structures was developed. In addition, to determine the predominant corrosion mechanism in NZ RC buildings, site investigation of pre-1970’s RC buildings was conducted. As a result, carbonation induced corrosion was found to be a predominant corrosion mechanism in these existing RC buildings. The thesis was concluded by demonstrating the successful application of the developed step-by-step procedure for a case study RC building constructed in 1928 in NZ, which found to be exhibiting pitting corrosion and with plain round bars as primary reinforcing steel. Accordingly, the case study building possesses a 27%NBS, resulting as an earthquake-prone building according to Building Amendment Act 2016. It was further found that the displacement capacity of such case study building will reduce considerably over the years with continued corrosion deterioration without any further retrofit plans. Hence, there is a significant need for timely seismic assessment and maintenance of RC buildings with such critical detailing (corrosion of reinforcement and plain round bars).