Construction Details and Observed Earthquake Performance of Unreinforced Clay Brick Masonry Cavity-walls

REFERENCE

252 cavity-walls damaged during the 2010-2011 Christchurch eartquakes
Study of typical construction details and common deficiencies in earthquake performance and hence appropriate mitigation interventions.
Data included: wall cross-section, type and spacing of metal cavity-ties, type of connection with diaphragms and roofs, absence or presence of some retrofit, wall orientation and boundary conditions, and type and level of damage.

Continuous cavity across floor levels is the most used detail. Such detail, together with lack of connectivity between masonry leaves is considered the most earthquake-prone.
Less frequent is the presence of an RC beam above the inner wall leaf or across both wall leaves.
Solid or cavity URM parapets were found atop 48% of the cavity-walls, in some cases seating onto a RC beam at roof level.
Existing cavity-ties were often corroded and not suitable for earthquake loading.

28% cavity-walls exhibited in-plane failure, while 72% failed in out-of-plane.
Out-of-plane failure: (i) overturning with collapse of the entire top section of a wall, (ii) one-way bending mostly for long-spanning walls and/or walls without side support; and (iii) two-way bending which exhibited a U-shaped or a V-shaped failure depending on the span length.
The spacing of the original cavity-ties, presence of corrosion, and embedment length largely influenced the performance
The presence of wall-to-floor/roof anchors improved the performance by providing a boundary restraint.
Post-tensioning, external steel strips, and helical ties were found in damaged cavity-wall buildings as implemented retrofit techniques that failed to provide the expected mitigation result.