UC Berkeley

Damage observed near the base of shear walls of reinforced concrete buildings after the Chile (2010) and New Zealand (2011) earthquakes are signs of shortcomings in the design of walls that need to be addressed. This investigation presents results of an experimental test program on ten reinforced concrete rectangular prisms representative of the flexural compression zone of flanged shear walls. The tested elements have transverse reinforcement detailing that matches or exceeds modern code requirements for special boundary elements. The main test variables were the amount and spacing (both vertical and horizontal) of the hoop and crosstie reinforcement. The elements were subjected to monotonically increasing axial compression until failure. Effects of strain gradient (both through the wall length and along the wall height) and effects of wall shear are not represented in the present tests. Nonetheless, the axial compression tests provide insights into the behavioral characteristics of actual wall boundaries. The global force shortening behavior of the specimens was commanded by a thin core which integrity was heavily compromised due to cover spalling, rebar buckling and out-of-plane instability. Measured load-displacement relations did not exhibit an acceptable ductile behavior suggesting that current building code requirements for special boundary elements do not necessarily achieve effective confinement to be protected against brittle axial failure. Enhanced detailing (increasing the volumetric ratio of confinement reinforcement and decreasing its horizontal spacing) improved behavior but did not produce ductile response in all cases. Reported damage extension concentrated over length corresponding to two-and-half times the thickness of the specimens. Compressive strain limits for stable behavior are proposed to be function of the gage length over which they are measured.

Bar buckling reduced the load carrying capacity of the reinforced concrete prisms because of the strength loss suffered by the longitudinal reinforcement, but also because it prevented the effective confinement of the concrete core. An experimental campaign comprising 48 analytical specimens allowed studying the relationship between tie spacing and stiffness, and the diameter of the longitudinal bars, that influenced their response when undergoing lateral instability (inelastic buckling). The behavior of tied bars undergoing lateral instability in the inelastic range is highly influenced by the relative restrictive tie spacing over which bar buckling is forced into, and the relative stiffness of the transverse ties and the longitudinal bar. The experiments assume a rigid contact between the bar and the tie, therefore hook opening is not modeled. For the range of tie stiffness and bar geometries tested, the results indicate that the tie spacing has to be smaller than 4.5 times the bar diameter to prevent bar buckling over a large range of plastic axial strains.

Empirical core stress strain curves, accounting for bar buckling, are reported for point wise strain measurements, as well as for average axial strains recorded within the damaged region. The results show that usable strain limits, to guarantee a stable core response in pure compression, are between 1.1 and 2.0%. Average empirical core stress strain curves are proposed for modeling purposes. Implication of the compressive strain limits observed are evaluated in a hazard-consistent manner by means of the Conditional Scenario Spectra (CSS). The CSS is a set of realistic earthquake spectra with assigned rates of occurrence that reproduce the hazard at a site. Structural responses are obtained by means of numerical analysis of a multistory shear wall under the seismic demand of more than eight-hundred ground motions consistent with the CSS. The case study allows estimating risk curves to evaluate the likelihood of exceeding certain threshold compressive strains in the boundary of the cross section. The single case numerical model showed that the limited strain capacity of these elements is only likely to negatively impact the behavior of the wall system at risk levels beyond the code-based expectations of good behavior.