Assessment of minimum vertical reinforcement limits for RC walls

Abstract

During the 2010/2011 Canterbury earthquakes, several reinforced concrete (RC) walls in multi-storey buildings formed a single crack in the plastic hinge region as opposed to distributed cracking. In several cases the crack width that was required to accommodate the inelastic displacement of the building resulted in fracture of the vertical reinforcing steel. This type of failure is characteristic of RC members with low reinforcement contents, where the area of reinforcing steel is insufficient to develop the tension force required to form secondary cracks in the surrounding concrete. The minimum vertical reinforcement in RC walls was increased in NZS 3101:2006 with the equation for the minimum vertical reinforcement in beams also adopted for walls, despite differences in reinforcement arrangement and loading. A series of moment-curvature analyses were conducted for an example RC wall based on the Gallery Apartments building in Christchurch. The analysis results indicated that even when the NZS 3101:2006 minimum vertical reinforcement limit was satisfied for a known concrete strength, the wall was still susceptible to sudden failure unless a significant axial load was applied. Additionally, current equations for minimum reinforcement based on a sectional analysis approach do not adequately address the issues related to crack control and distribution of inelastic deformations in ductile walls.

Understanding Poor Seismic Performance of Concrete Walls and Design Implications

“…The wall response was elastic until the first crack developed at a lateral force of 205 kN and the peak strength was reached at a lateral drift of 0.7% before fracture of the vertical reinforcement occurred. The analysis confirms the findings of other reports that concluded based on section analyses that the vertical reinforcement content in the grid-F wall of the Gallery Apartments building was insufficient to initiate secondary cracking, resulting in a concentration of inelastic actions at the wall base (CERC 2012, Henry 2013).…”

“…As highlighted by the analysis presented in this paper, the observations of buildings such as the Gallery Apartment building should be interpreted with caution as the vulnerability was exacerbated by vertical reinforcing contents being significantly less than that required by current design standards. Nonetheless, CERC, Henry (2013), and the analysis presented in this paper have illustrated that even when the concrete strength is known, the current minimum vertical reinforcement limits in NZS 3101 (i.e., Equation 5) may not be adequate to ensure well distributed cracks form in the plastic hinge region. This concern stems from the use of Equation 5 and the inability of commonly used section analysis methods to include the effects of crack distribution.…”

“…The results of moment-curvature analysis conducted by Henry (2013) have highlighted several deficiencies of Equation 5. This equation was developed for beams with top and bottom layers of reinforcement only and fails to account for the distributed reinforcement in walls, slenderness of wall sections, size effects, aspect ratio and axial loads.…”

Fragility Functions for RC Shear Wall Buildings in Australia

“…One of the limitations of this study is the assumption that the entire RC structural wall building stock has utilized D500N reinforcing bars; because of the paucity of research and experimental testing on other types of reinforcing bars used in Australia (e.g., 230S, 410Y), D500N bars are assumed to be incorporated in the entire RC structural wall building stock. In contrast to the values for some parameters selected on the basis of a normal distribution, the axial load ratio ( ALR ), for example, is randomly chosen between a minimum of 0.01 (1%) and a maximum of 0.1 (10%), based on common values used in previous research (Henry 2013) as well as investigations by Albidah et al (2013) for low-to-moderate seismic regions and, more recently, Menegon et al (2017) for Australia. It should be noted that other seismic assessment methodologies, such as Hazus (FEMA 2010) and EQRM (Robinson et al 2006), also incorporate the variability of the building stock through lognormally distributed capacity functions that are calculated based on a chosen, random number.…”

“…Although Hazus (FEMA 2010) has building parameters for "Pre-Code" buildings, which correspond to structures that have not been seismically designed, it is possible that the findings from Edwards et al (2004) will also hold true for the comparisons made between the fragility curves derived from the generic building parameters provided by Hazus (FEMA 2010) and those derived from an extensive number of capacity curves, which better reflect the RC structural wall building stock in Australia. This is primarily because of the poor performance observed from lightly reinforced and unconfined concrete walls in recent earthquake events (Beca 2011, Canterbury Earthquakes Royal Commission 2012, Henry 2013, Morris et al 2015, Sritharan et al 2014, Wallace et al 2012). Because of the low standard of detailing required in the current materials standards in Australia and the low earthquake return period (RP) typically used in design, it is anticipated that most of the RC walls and cores embedded within structures around Australia are lightly reinforced and unconfined, and this is likely to lead to brittle behavior in an earthquake (Hoult et al 2015).…”