Seismic performance of precast hollow-core floors: Experimental findings and updates to C5

Precast, prestressed hollow-core floors are susceptible to earthquake-induced damage and collapse. While significant progress has been made in New Zealand in understanding and assessing their seismic behaviour, the 2016 Kaikōura earthquake and recent testing demonstrated several unexpected damage patterns. This paper presents experimental evidence and proposes modifications to assessment procedures to account for the detrimental effect of web cracking and the heightened damageability of hollow-core floor units that are seated at or on intermediate columns (so-called ‘beta units’). The experimental investigation involved two full-scale super-assembly experiments on a two-bay by one-bay reinforced concrete moment frame structure with hollow-core floors. Results showed that web cracking can initiate at low inter-storey drifts (~0.5%) and become widespread as drifts increase. Beta units exhibited distinct damage patterns and higher vertical dislocations at lower drifts compared to other units. A comparison between the tested response and predictions from the 2018 version of the New Zealand Assessment Guidelines C5 demonstrated low accuracy in the positive moment failure assessment, particularly for beta units. A revised positive moment failure assessment is proposed to simplify the assessment and account for the damageability of beta units. Additionally, the experimental data showed that beam elongation predictions according to C5 (2018) are overly conservative within the elastic range, and a mechanics-based modification is proposed to enhance the accuracy of the assessment. The proposed assessment changes aim to improve the predictive accuracy and better indicate when seismic retrofitting is necessary.