Changes To the 2015 Canadian Building Code for High-Rise Concrete Wall Buildings Following the 2010 Maule Earthquake

Abstract eng:
Compression failures of numerous thin concrete walls in the 2010 Maule Earthquake motivated an experimental investigation that revealed thin concrete walls subjected to uniform compression strain may have much lower compression strain capacity than previously realized, and thin concrete columns and walls may experience very sudden axial compression failures after a small level of damage to the concrete cover because the thin layer of undamaged concrete between the two layers of horizontal reinforcement easily becomes unstable. As thin concrete columns and walls are commonly used structural members in high-rise concrete buildings in Canada, these experimental findings led to extensive changes to the 2015 Canadian Building Code, which are documented in this paper. The new requirements include: (i) The need to check the maximum compression strain demands over the full height of all concrete shear walls used as the seismicforce-resisting system (SFRS) in buildings, whereas this was previously only required at the base of the shear walls where plastic hinging is expected to occur. (ii) New requirements for tied vertical reinforcement at the base of all low ductility SFRS shear walls to protect the compression end of the walls, whereas this was previously only required in shear walls subjected to higher levels of ductility demand. (iii) New significant reductions in the axial load resistance of thin columns and walls (less than 30 cm thick) resisting gravity loads. The reduction is greater for walls without cross-ties (i.e., typical walls) than for columns, which usually contain cross-ties. (iv) A new requirement to account for strong-axis bending of bearing walls, even though such members are defined as members subjected to axial load and weak-axis bending. (v) New requirements that prevent very thin concrete flexural shear walls from being used to resist lateral load due to wind. (vi) Extensive new requirements to prevent failure of thin concrete columns and walls in gravity-load resisting frames due to the seismic deformation demands on the structure. The limit on the induced column drift depends on the type of member (14 different types are defined varying from a column meeting all the requirements for a ductile moment-resisting frame to a thin wall with a single layer of reinforcement). Also, all gravity-load columns and walls must have a curvature capacity greater than the curvature demand in the plastic hinge regions of the SFRS shear walls, and if the interstorey drift ratio exceeds 0.5% at any point in the structure, all walls that are used to support gravity loads must contain a minimum of two layers of reinforcement.

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