José I. Restrepo

INVESTIGATION OF CONCENTRICALLY LOADED REINFORCED CONCRETE COLUMNS CONFINED WITH GLASS FIBER-REINFORCED POLYMER JACKETS

This paper proposes an analytical method to evaluate the axial load-deformation response of short, reinforced concrete rectangular columns (RCs) accounting for the dual confinement effect provided by an external fiber-reinforced polymer (FRP) jacket and by internal steel hoops. A test program comprised of 3 tests on square columns and 3 tests on RCs was carried out to examine the behavior of the effectiveness of the jackets when applied to RCs. Results clearly demonstrate the efficiency of the jackets in enhancing the ultimate strain and strength of the columns. The jackets were also very effective in preventing longitudinal bar buckling from occurring. The analytical model was calibrated using data from the tests. Closed-form equations are proposed for evaluating the short-term load-deformation behavior of columns confined with FRP jackets.

System Identification Study of a 7-Story Full-Scale Building Slice Tested on the UCSD-NEES Shake Table

A full-scale 7-story reinforced concrete building slice was tested on the unidirectional University of California-San Diego Net- work for Earthquake Engineering Simulation (UCSD-NEES) shake table during the period from October 2005 to January 2006. A rectangular wall acted as the main lateral force resisting system of the building slice. The shake table tests were designed to damage the building pro- gressively through four historical earthquake records. The objective of the seismic tests was to validate a new displacement-based design methodology for reinforced concrete shear wall building structures. At several levels of damage, ambient vibration tests and low-amplitude white noise base excitation tests were applied to the building, which responded as a quasi-linear system with dynamic parameters evolving as a function of structural damage. Six different state-of-the-art system identification algorithms, including three output-only and three input- output methods were used to estimate the modal parameters (natural frequencies, damping ratios, and mode shapes) at different damage levels based on the response of the building to ambient as well as white noise base excitations, measured using DC-coupled accelerometers. The modal parameters estimated at various damage levels using different system identification methods are compared to (1) validate/ cross-check the modal identification results and study the performance of each of these system identification methods, and to (2) investigate the sensitivity of the identified modal parameters to actual structural damage. For a given damage level, the modal parameters identified using different methods are found to be in good agreement, indicating that these estimated modal parameters are likely to be close to the actual modal parameters of the building specimen. DOI: 10.1061/(ASCE)ST.1943-541X.0000300.

Experimental and Numerical Seismic Response of a 65 kW Wind Turbine

A full-scale shake table test is conducted to assess the seismic response characteristics of a 23 m high wind turbine. Details of the experimental setup and the recorded dynamic response are presented. Based on the test results, two calibrated beam-column finite element models are developed and their characteristics compared. The first model consists of a vertical column of elements with a lumped mass at the top that accounts for the nacelle and the rotor. Additional beam-column elements are included in the second model to explicitly represent the geometric configuration of the nacelle and the rotor. For the tested turbine, the experimental and numerical results show that the beam-column models provide useful insights. Using this approach, the effect of first-mode viscous damping on seismic response is studied, with observed experimental values in the range of 0.5–1.0% and widely varying literature counterparts of 0.5–5.0%. Depending on the employed base seismic excitation, damping may have a significant influence, reinforcing the importance of more accurate assessments of this parameter in future studies. The experimental and modeling results also support earlier observations related to the significance of higher modes, particularly for the current generation of taller turbines. Finally, based on the outcomes of this study, a number of additional experimental research directions are discussed.

FORCE-DISPLACEMENT CHARACTERIZATION OF WELL-CONFINED BRIDGE PIERS

This paper outlines an approach to estimating the horizontal force-displacement response of well-confined reinforced concrete bridge piers. Special attention is paid to the hollow rectangular piers designed to support 3 new toll bridges in the San Francisco Bay Area. This approach accurately assesses a piers elastic displacement, its spread of plasticity and plastic displacement, and its shear displacement for most ductility levels. The shear transfer mechanism inside a piers plastic hinge region is key to this assessment. This mechanism appears as a fanning crack pattern and results in concentrated compression strains at the base of a pier. These concentrated strains oppose the traditional notion of curvature that assumes plane sections remain plane. The assumption that there is a linear distribution of plastic curvatures inside the plastic hinge region, however, largely overcomes the problem of relating plastic curvatures to plastic rotations, both experimentally and analytically. At most levels of ductility, the mean difference between analytical assessments of the spread of plasticity and results from 12 large-scale structural tests is 16% with a 12% coefficient of variation.

Shake-Table Tests of Confined-Masonry Rocking Walls with Supplementary Hysteretic Damping

A shake-table investigation is conducted on a 40% scale model frame-wall system to validate the concept of rocking walls as primary seismic systems. The rocking wall concept was implemented on confined masonry walls, but the findings can be extended to any rocking wall system. As the inherent damping of this system is low, a pair of supplemental steel hysteretic energy dissipating dampers is used at the base of the wall. It is concluded that with careful detailing, damage is not only eliminated but the structure re-centers itself following a large earthquake.

Dynamic behavior of rocking and hybrid cantilever walls in a precast concrete building

This paper discusses the dynamic response of precast, post- tensioned, rocking, and hybrid cantilever walls that provided lateral force resistance to a three-story precast concrete building built at half-scale. The building was subjected to extensive shake- table testing on the NEES large high-performance outdoor shake table. The tests provided a landmark opportunity to observe the dynamic response of this type of lateral force-resisting system. Excellent performance was observed overall. Comparison between the assumptions made during the design of the wall and the exper- imental results allowed the validation of the presented design procedure and of the reinforcement detailing in the critical region at the base of the walls.

Nonlinear Cyclic Truss Model for Reinforced Concrete Walls

ACI Structural Journal, V. 109, No. 2, March-April 2012. MS No. S-2010-081.R5 received February 4, 2011, and reviewed under Institute publication policies. Copyright

Displacement-Based Method of Analysis for Regular Reinforced-Concrete Wall Buildings: Application to a Full-Scale 7-Story Building Slice Tested at UC–San Diego

This paper describes a displacement-based method of analysis for the performance-based seismic design of regular buildings with reinforced-concrete bearing walls acting as the lateral-force resisting system. The method considers two performance levels: immediate occupancy (IO) and life safety (LS), each anchored at a specific seismic hazard level. It explicitly accounts for the combined effects of inelastic first mode of response, kinematic system overstrength, and higher modes of response. Quantification of these effects is required to capacity-protect the structure and to ensure the intended performance. As an example, the method is applied to a full-scale 7-story reinforced-concrete building slice, built and tested on the George E. Brown Jr. Network for Earthquake Engineering Simulation Large Outdoor High-Performance Shake Table at the University of California–San Diego. The response of the test building largely verified the method discussed in this paper.

Detection of Initial Yield and Onset of Failure in Bonded Posttensioned Concrete Beams

This paper discusses monitoring of bonded posttensioned (PT) concrete elements using the acoustic emission technique. In particular, a statistical pattern recognition technique based on a multivariate outlier analysis is presented to identify initial yielding and the onset of failure. Experimental tests on large-scale single-tendon bonded PT concrete beams, subjected to multiple load cycles, will be presented to validate the proposed monitoring system.

Study of Loading Protocols in Light-Gauge Stud Partition Walls

This paper examines the influence of two reversed cyclic loading protocols on the response of gypsum light-gauge metal-stud partition walls, which are common in office, hotel, and laboratory buildings. Two identical full-scale three-dimensional specimens were constructed to represent a typical room in an office building. The specimens were tested quasi-statically along two axes using different loading protocols. The loading protocols were applied to observe the sensitivity of loading protocol on damage progression. The loading protocols were developed for the Applied Technology Council ATC-58 project published in FEMA 461, which, among others, addresses the racking protocol of nonstructural building components for use within a performance-based earthquake engineering framework. Details are given about the damage progression of the specimens to the loading protocols and their lateral force-displacement response characteristics.