Performance evaluation and cost assessment of weak-story retrofits in RC buildings surveyed after the 2024 Hualien Earthquake
DOI:
https://doi.org/10.5459/bnzsee.1765Abstract
To provide evidence on the performance of retrofitting systems for informing retrofit practice in New Zealand, 18 buildings with retrofits including reinforced concrete (RC) column jacketing, RC wing walls, RC shear walls, and steel frames with braces were surveyed after the 2024 Hualien Earthquake in Taiwan. The primary goal of this study was to quantity amounts of retrofitting installed in vulnerable existing buildings prior to the earthquake observed to be sufficient to prevent severe structural damage and disruption to building functionality. The idea would be to use these quantified amounts as a rough threshold for comparing between competing retrofitting systems in preliminary design. In general, the retrofits observed in Taiwan were low-cost and designed to target mitigation of soft and weak first stories, a well-known structural vulnerability within the Taiwanese building stock. Nevertheless, the scope of retrofits ranged from being installed in first story only to along the full height of the building.
Simple indices estimated as ratios of cross-sectional areas of columns and walls to total floor area used as a proxy for base shear strength showed that on average, retrofitting nearly doubled the base shear capacity compared with the original state. To observe the effect of retrofitting on observed earthquake damage, case studies consisting of sets of building pairs, one building without retrofitting that experienced severe or moderate damage and one building with low-cost retrofitting that had minor or no damage, were investigated. Building pairs were selected to have similar size, structural layout, and seismic demand. Based on three case studies, if the amount of retrofitting increased column and wall indices to a certain threshold, no severe damage was observed.
A cost assessment of typical retrofitting systems was performed based on estimated cost schedules provided by Taiwanese structural consulting firms. For a 5-story mixed-use building consisting of both commercial and residential units, and assuming the cost to build a new structure is approximately USD $1000 per square meter, the cost of installing a retrofitting system comprising RC column jacketing and additional shear walls to double base shear strength was estimated to be between 15-20% of the rebuilding cost of the original building (USD $165/m2) based on current construction practice in Taiwan. The average cost of retrofitting installed in 23 private residential buildings was less expensive at approximately USD $130/m2 indicating that increasing seismic capacity of vulnerable buildings can be achieved at a reasonable cost.
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