Quasi-Static Cyclic Testing of Two-Thirds Scale Unbonded Posttensioned Rocking Dissipative Timber Walls

Abstract

Posttensioning low-damage technologies were first developed in the late 1990s as the main outcome of the U.S. Precast Seismic Structural System (PRESSS) program coordinated by the University of California, San Diego, and culminated with the pseudo-dynamic test of a large-scale five-story test building. The extension of posttensioned techniques to timber elements led to the development of new structural systems, referred to as Pres-Lam (prestressed laminated timber). Pres-Lam systems consist of timber structural frames or walls made of laminated veneer lumber, glue laminated timber (Glulam), or cross-laminated timber (CLT). Pres-Lam walls consist of a rocking timber element with unbonded posttensioned tendons running through the length and attached to the foundation, which provides a centering force to the wall, while energy dissipation is supplied by either internal or external mild steel dissipaters. Previous tests carried out on posttensioned timber walls focused on small-scale (one-third) specimens with the main objective of evaluating the general response of the system. The main objective of the experimental program herein presented is the testing and estimating of the response of a series two-thirds-scale posttensioned walls, with alternative arrangements and combination of dissipaters and posttensioning, focusing on the construction details adopted in real practice. The paper first presents a brief discussion on the seismic demand evaluation based on the displacement-based design approach. The construction detailing of the steel dissipater connections, posttensioning anchorage, and shear keys are then presented. The main objectives of this experimental program were the investigation of the experimental behavior of large-scale posttensioned timber walls, with particular focus on the system connection detailing and optimization of posttensioning anchorage, fastening of the dissipation devices, and shear keys. The program consisted of several quasi-static cyclic tests considering different steel dissipater configurations, different levels of posttensioning initial stress, and different dissipater options were considered: both internal and external mild steel tension-compression yield devices were used. The experimental results showed the performance of posttensioned timber wall systems, which provide a high level of dissipation while showing negligible residual displacements and negligible damage to the wall element. The final part of the paper presents the experimental evaluation of the area-based hysteretic damping for the tested specimens, and the results highlight the great influence of the connection detailing of the dissipaters.