Shaketable Testing of Rectangular Post-Tensioned Concrete Masonry Walls

The in-plane seismic response of partially grouted post-tensioned concrete masonry (PCM) walls with unbonded tendons is investigated by means of shaketable testing. The principal intent of this study was to validate use of this wall system for residential construction, before the first PCM house is built in New Zealand. An introduction and description of the testing program is followed by the presentation of results from dynamic testing of four rectangular walls, of which one contained a shrinkage control joint. Discussion of the results is concerned with wall structural response in terms of flexural strength, displacement capacity, and tendon stress. The shaketable tests demonstrated the self-centering nature of post-tensioned masonry walls and their ability to achieve large displacements with minimal accumulation of damage. The level of initial tendon prestress was found to have a significant effect on peak wall displacements.

Keywords: concrete masonry; post-tensioned; prestressing; seismic; wall.

(ProQuest-CSA LLC: ... denotes formulae omitted.)

INTRODUCTION

The evolution of performance-based design during the 1990s has resulted in more effort directed towards designing structures to not only withstand seismic loading, but also to perform in a way that minimizes both residual displacements and structural damage. By designing structural elements to rock at their joints, the area of plastic deformation can be restricted, and if a restoring force is provided, the structure will have a tendency to self-center, returning to its original vertical alignment at the conclusion of seismic loading. This behavior assists in mitigating the large financial cost of repairing or rebuilding structures after an earthquake and ensuring continuous building functionality.

Priestley and Tao1 were first to present the concept of incorporating unbonded prestressing in moment frames to provide a self-centering mechanism. This was later tested by Stanton et al.2 and led to the design, construction, and pseudo-dynamic testing of a 60% scale five-story precast concrete building.3 This building had prestressed shearwalls in one direction and a combination of prestressed and non-prestressed moment frames in the other. Although the concept of self-centering was first applied to post-tensioned concrete structures, the method has more recently been applied to other building types. For example, Ricles et al.4 have investigated the performance of post-tensioned steel moment resisting frames.

Historically, researchers have been concerned with the out-of-plane response of post-tensioned masonry walls, as discussed in review papers by Schultz and Scolforo5 and later by Lissel et al.6 Two early studies investigated the in-plane performance of clay masonry walls using a monotonic loading history.7,8 The first in-plane cyclic tests of post-tensioned clay masonry walls were conducted by Rosenboom and Kowalsky9 and demonstrated the advantages of unbonded tendons, mild steel energy dissipaters, and providing confinement to the lower wall corners.

Previous testing by Laursen and Ingham10,11 investigated the in-plane response of post-tensioned concrete masonry walls when subjected to cyclic loading. They found that walls constructed from unconfined masonry could reliably develop displacements (drifts) of 1% before the onset of strength degradation due to masonry crushing in the wall toe regions. By confining these areas, drifts of 1 to 1.5% could readily be developed, depending of the method of confinement used. The use of supplementary energy dissipation devices, in the form of dog-bone-type dampers, were trialed in one wall to provide additional damping to a system that typically has limited hysteretic energy dissipation.11

NZS 4229:1999,12 the New Zealand Standard for concrete masonry buildings not requiring specific engineering design, provides simplified guidelines for the design and construction of reinforced concrete masonry residential structures. The scope of this standard extends to buildings with heights of less than 10 m (33 ft), and a plan area of less than 600 m^sub 2^ (6460 ft^sub 2^) or 350 m^sub 2^ (3770 ft^sub 2^) for single and multi-story dwellings, respectively. Lower-story walls are constructed using structural concrete masonry, with either concrete masonry or timber framing in the upper story. Mid-floors can be constructed of timber or reinforced concrete and the roof consists of timber trusses. As a result, this standard covers the majority of concrete masonry residential construction in New Zealand.

Structures within the scope of NZS 4229:1999 are designed using a bracing approach, where lateral loading from wind and earthquake is resisted by isolated in-plane shear panels, without considering wall boundary conditions. Loading on out-of-plane walls is transferred to the in-plane shearwalls via a 400 mm (16 in.) deep reinforced bond beam, which is constructed in the top two block courses of each story and encompasses the entire structure. A more detailed account of the development and methodology of NZS 4229:1999 is provided by Ingham et al.13