Large High Performance Outdoor Shake Research Articles

Practice-oriented numerical model for seismic response of cold-formed steel-framed mid-rise buildings

Cold-formed steel (CFS) framed buildings are becoming increasingly common, notably in the high seismic regions of the United States (US). However, research to understand the seismic performance of CFS-framed buildings specifically using large-scale (building-level) specimens has begun only recently. Therefore, efforts to validate and improve numerical modeling of whole building systems have been limited due to this lack of experimental data. A recently completed experimental program, conducted at the Large High-Performance Outdoor Shake Table at the University of California, San Diego, involved earthquake testing of a full-scale 6-story CFS-framed building. The test building adopted an assembly of continuous tie-rod and built-up compression stud pack at the ends of individual shear walls to resist seismic overturning moment and uplift forces within the building. The objective of this paper is to present a practice-oriented phenomenological finite element modeling approach intended for design engineers and compare the predictions from this model against the measured seismic response of the 6-story building. The model aims to provide a robust, yet computationally efficient approach, with minimal degrees-of-freedom, while also capturing the salient features of seismic response of mid-rise CFS-framed buildings suitable for nonlinear time history analyses-based design. The building model, developed within the OpenSeesPy finite element platform, includes the shear walls and gravity walls as well as the lateral strength and stiffness contribution from exterior and interior gypsum sheathing. The continuous tie-rod system is also modeled to capture the cumulative floor displacements caused by the axial elongation in the steel rods. The effect of built-up compression stud packs on the lateral behavior of a shear wall was estimated using available experimental data and a detailed numerical model of an isolated shear wall, then subsequently incorporated in the nonlinear hysteretic material model. The numerical model captured the natural frequencies of the translation modes of the building in its undamaged state with <3% error. It also predicted the peak roof drift ratio within an error of 0.03% drift ratio under the design earthquake event. These results provide confidence in the proposed numerical modeling approach as a tool for seismic response prediction of mid-rise CFS-framed buildings with similar structural detailing features.

Modal characterization of a three‐story buckling‐restrained braced frame steel building by non‐destructive field testing

AbstractA full‐scale, reconfigurable, three‐story steel building was constructed and its modal properties identified prior to shake table testing on the UC San Diego Large High Performance Outdoor Shake Table (LHPOST6). The aim of this pre‐shake table test erection was to demonstrate the rapid constructability of the building and identify construction fit issues prior to its assembly on LHPOST6. This posed a unique opportunity to characterize the building's modal properties using a variety of non‐destructive, low‐amplitude techniques. To this end, two novel, non‐destructive methods were used, namely: (1) impact tests conducted by swinging a tire from a crane to impact strategically selected locations, and (2) dynamic base vibration tests, induced by driving heavy machinery in the vicinity of the base of the structure. A system of accelerometers located throughout the structure captured waves propagating from each test to characterize the as‐built dynamic properties of the building. Results from various system identification methods are presented and compared to theoretical and numerical analysis. Comparisons indicate that the theoretical, numerical, and experimentally determined periods are nominally within 15% of each other. The erection of the structure was complete over the course of 3 days and construction fit‐up issues were addressed. The structure was then rapidly deconstructed and stored prior to erection for full‐scale shake table testing. This pre‐shake table test exercise demonstrated the viability of two, simple, low‐amplitude excitation approaches for use in the dynamic characterization of full‐scale buildings, with results consistent with theoretical and numerical models.

NHERI@UC San Diego 6-DOF Large High-Performance Outdoor Shake Table Facility

Since its commissioning in 2004, the UC San Diego Large High-Performance Outdoor Shake Table (LHPOST) has enabled the seismic testing of large structural, geostructural and soil-foundation-structural systems, with its ability to accurately reproduce far- and near-field ground motions. Thirty-four (34) landmark projects were conducted on the LHPOST as a national shared-use equipment facility part of the National Science Foundation (NSF) Network for Earthquake Engineering Simulation (NEES) and currently Natural Hazards Engineering Research Infrastructure (NHERI) programs, and an ISO/IEC Standard 17025:2005 accredited facility. The tallest structures ever tested on a shake table were conducted on the LHPOST, free from height restrictions. Experiments using the LHPOST generate essential knowledge that has greatly advanced seismic design practice and response predictive capabilities for structural, geostructural, and non-structural systems, leading to improved earthquake safety in the community overall. Indeed, the ability to test full-size structures has made it possible to physically validate the seismic performance of various systems that previously could only be studied at reduced scale or with computer models. However, the LHPOST's limitation of 1-DOF (uni-directional) input motion prevented the investigation of important aspects of the seismic response of 3-D structural systems. The LHPOST was originally conceived as a six degrees-of-freedom (6-DOF) shake table but built as a single degree-of-freedom (1-DOF) system due to budget limitations. The LHPOST is currently being upgraded to 6-DOF capabilities. The 6-DOF upgraded LHPOST (LHPOST6) will create a unique, large-scale, high-performance, experimental research facility that will enable research for the advancement of the science, technology, and practice in earthquake engineering. Testing of infrastructure at large scale under realistic multi-DOF seismic excitation is essential to fully understand the seismic response behavior of civil infrastructure systems. The upgraded 6-DOF capabilities will enable the development, calibration, and validation of predictive high-fidelity mathematical/computational models, and verifying effective methods for earthquake disaster mitigation and prevention. Research conducted using the LHPOST6 will improve design codes and construction standards and develop accurate decision-making tools necessary to build and maintain sustainable and disaster-resilient communities. Moreover, it will support the advancement of new and innovative materials, manufacturing methods, detailing, earthquake protective systems, seismic retrofit methods, and construction methods. This paper will provide a brief overview of the 1-DOF LHPOST and the impact of some past landmark projects. It will also describe the upgrade to 6-DOF and the new seismic research and testing that the LHPOST6 facility will enable.

Monitoring physical and dynamic properties of reinforced concrete structures during seismic excitations

This study investigates the feasibility of tracking the structural/dynamic properties of reinforced concrete (RC) structures while subjected to damaging earthquake loads. The methodology proposed implements an unscented Kalman filter scheme that updates the properties of a simplified non-parametric model of the structure. Calibration and validation of the methodology is accomplished using experimental and simulated data from a full-scale RC column test performed at the NEES Large High Performance Outdoor Shake Table. The results obtained show that peak displacement, stiffness, frequency, shear force and damping ratio are well monitored by the proposed approach at all performance levels. However, residual displacements are not captured due to the non-hysteretic nature of the model. Despite its simplicity, the proposed approach seems suitable for identification and damage assessment of highly nonlinear structures exhibiting nonstationary responses, as is the case of RC structures subjected to severe earthquake loads.

Full‐field deformation measurements during seismic loading of masonry buildings

Summary This article demonstrates the potential of the digital image correlation (DIC) method to provide accurate full-field deformation measurements and successfully monitor the development of damage during seismic excitation of a partially grouted reinforced masonry building. The building was subjected to a sequence of earthquake ground motion records using the Large High Performance Outdoor Shake Table at the University of California, San Diego. The DIC setup was capable of measuring surface deformations of the single-story building with high frame rate cameras located at a distance greater than 50 ft away. The accuracy of the measurements was assessed with data obtained using mounted displacement transducers. The full-field deformation data collected by the DIC system was capable to detect strain localization patterns associated with the onset of wall cracking before it could be shown by the displacement sensor data or by post mortem visual inspection. The research findings reported herein demonstrate, for the first time to the authors' best knowledge, the potential of in situ monitoring of actual structures for damage induced by non-stationary loading profiles using optical metrology. Copyright © 2016 John Wiley & Sons, Ltd.

Wavelet-Based Damage Detection in Reinforced Concrete Structures Subjected to Seismic Excitations

The feasibility of using output-only model-free wavelet-based techniques for damage detection in reinforced concrete structures subjected to seismic loads is explored through the analysis of the results of a full scale shake table test of a reinforced concrete bridge column recently performed at the NEES Large High Performance Outdoor Shake Table. The evaluated approaches are based solely in the analysis of the acceleration time histories recorded in the structure. The viability of using numerical models to validate this type of damage detection methodologies is also evaluated. Wavelet analyses were capable of identifying the rebar fracture episodes and partially identified the frequency shifts in the structure as the inelastic demand increased. It was also found that, depending on the methodology employed, the use of numerical models to validate damage detection techniques can oversimplify the actual problem and/or induce spurious irregularities.

Shake table testing and numerical simulation of a utility‐scale wind turbine including operational effects

ABSTRACTShake table tests were undertaken on an actual wind turbine (65 kW rated power, 22.6 m hub height and a 16 m rotor diameter) using the Network for Earthquake Engineering Simulation Large High Performance Outdoor Shake Table at the University of California, San Diego. Each base shaking event was imparted in two states, whereas the turbine rotor was still (parked), and while it was spinning (operational). Each state was tested in two orientations of shaking direction, one parallel (fore‐aft) and another perpendicular (side‐to‐side) to the axis of rotation of the rotor. Structural response characteristics are presented for motions imparted in both configurations and both operational states. Modal parameters (natural frequencies, damping ratios and mode shapes) were estimated throughout the testing program. It is found that shaking imparted in the fore‐aft direction while spinning is the only observed situation where operational effects appear significant, with reductions up to 33% in seismic bending moment demand near the tower base. Using modifications developed by the research team to the FAST code, experimental results are compared with corresponding simulations to show that dynamic characteristics, acceleration time histories and trends in tower bending seismic demand can be numerically approximated. This experimental evidence and associated numerical simulations suggest that modeling of combined wind and earthquake loading with existing turbine specific codes produce meaningful results. Discrepancies between experimental and numerical results support that further refinement of simulation codes can improve accuracy beyond the current state. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Shake Table Testing of a Utility-Scale Wind Turbine

AbstractShake table tests were undertaken on a full-scale wind turbine (65-kW rated power, 22.6-m hub height, and 16-m rotor diameter) using the Network for Earthquake Engineering Simulation Large High Performance Outdoor Shake Table at the University of California, San Diego. Structural response characteristics and modal parameters are presented for base shaking imparted in two configurations, both parallel (configuration 1) and perpendicular (configuration 2) to the axis of rotation of the rotor. Results are summarized for a series of progressively stronger motions imparted in configuration 1, with analysis identifying damage sources leading to an overall loss in stiffness. Two sources of observed softening are identified and quantified: (1) degradation of grout at the tower base, and (2) loss of bolt torque at the connections between tower segments. Results showed that the two configurations had little difference in structural response and demand parameters. For the tested turbine, with appropriate con...

Real-Time Strong-Motion Broadband Displacements from Collocated GPS and Accelerometers

We present a new solution to the classical problem of deriving displacements from seismic data that is suitable for real-time monitoring. It relies on an optimal combination of collocated high-rate GPS displacements and very high-rate strong-motion accelerometer data using a Kalman filter that takes advantage of the individual strengths of seismic and geodetic networks while minimizing their weaknesses. The result is a more accurate broadband estimate of velocities and displacements at the higher rate provided by accelerometers. In contrast to displacements inferred through integration of seismic data alone, which are degraded by the presence of unphysical drifts and limited dynamic ranges, broadband displacements do not drift and will not clip. Furthermore, they are more accurate and robust than those obtained solely from GPS receivers. We establish the accuracy and precision of the new solution using earthquake engineering data from the University of California, San Diego’s Large High-Performance Outdoor Shake Table. Then we estimate broadband displacements for the 2010 M w 7.2 El Mayor–Cucapah earthquake using 1 Hz GPS displacements and 100 Hz accelerometer data from collocated sensors in southern California. We also reconstruct velocity measurements equivalent to those of broadband seismometers, which clipped at stations hundreds of kilometers from the epicenter during the earthquake. The ability to obtain broadband displacement waveforms in three dimensions with millimeter precision is a breakthrough for a number of areas ranging from earthquake source physics to the response of long-period engineered structures. To do so in real time from a dense monitoring network provides distinct advantages for earthquake early warning and rapid earthquake response.

Experimental study of the dynamic interaction between the foundation of the NEES/UCSD Shake Table and the surrounding soil: Reaction block response

An extensive experimental study of the dynamic interaction between the foundation block for the NEES/UCSD Large High Performance Outdoor Shake Table and the surrounding soil was conducted in 2003. The vibrations induced by the two NEES@UCLA large eccentric mass shakers were recorded at multiple stations within the reinforced concrete foundation block and on the surface of the surrounding soil up to distances of 270 m from the block. The present paper focuses on analysis of the data recorded within the reaction block including the average rigid body motion of the foundation and its dependence on frequency, and the deformation of the block for longitudinal (EW), transverse (NS), and torsional excitation. Comparison of the reaction block response during shaker induced vibrations with that for the much stronger actuator forces shows that linearity holds for the range of forces involved. Comparisons with analytical results for a simplified model of the foundation show good agreement between experimental and theoretical results.

Performance of Suspended Pipes and Their Anchorages During Shake Table Testing of a Seven-Story Building

This paper presents results of shake table tests on pipe systems anchored in a full-scale, seven-story building performed on the Large High-Performance Outdoor Shake Table at the University of California at San Diego on 1 May 2006. The purpose of the tests was to investigate the forces that act on post-installed anchors in buildings during a diverse range of earthquake ground motions. A sound understanding of the force levels and number of cycles is important for developing reliable anchor qualification approaches and seismic design guidelines. The tests also provide data on floor accelerations and acceleration amplification for nonstructural components in buildings during seismic events.