Derek Skolnik

Identification, Model Updating, and Response Prediction of an Instrumented 15-Story Steel-Frame Building

Identification of the modal properties of the UCLA Factor Building, a 15-story steel moment-resisting frame, is performed using low-amplitude earthquake and ambient vibration data. The numerical algorithm for subspace state-space system identification is employed to identify the structural frequencies, damping ratios, and mode shapes corresponding to the first nine modes. The frequencies and mode shapes identified based on the data recorded during the 2004 Parkfield earthquake ( Mw =6.0) are used to update a three-dimensional finite element model of the building to improve correlation between analytical and identified modal properties and responses. A linear dynamic analysis of the updated model excited by the 1994 Northridge earthquake is performed to assess the likelihood of structural damage.

Critical Assessment of Interstory Drift Measurements

Interstory drift, the relative translational displacement between two consecutive floors, is an important engineering demand parameter and indicator of structural performance. The structural engineering community would benefit well from accurate measurements of interstory drift, especially where structures undergo inelastic deformation. Unfortunately, the most common method for obtaining interstory drift, double integration of measured acceleration, is problematic. Several issues associated with this method (e.g., signal processing steps and sparse instrumentation) are illustrated using data from shake table studies and two extensively instrumented buildings. Some alternative contact and noncontact methods for obtaining interstory drift are then presented.

Forced Vibration Testing of a Four-Story Reinforced Concrete Building Utilizing the nees@UCLA Mobile Field Laboratory

The nees@UCLA mobile field laboratory was utilized to collect forced and ambient vibration data from a four-story reinforced concrete (RC) building damaged in the 1994 Northridge earthquake. Both low amplitude broadband and moderate amplitude harmonic excitation were applied using a linear shaker and two eccentric mass shakers, respectively. Floor accelerations, interstory displacements, and column and slab curvature distributions were monitored during the tests using accelerometers, linear variable differential transformers (LVDTs) and concrete strain gauges. The use of dense instrumentation enabled verification of common modeling assumptions related to rigid diaphragms and soil-structure-interaction. The first six or seven natural frequencies, mode shapes, and damping ratios were identified. Significant decreases in frequency corresponded to increases in shaking amplitude, most notably in the N-S direction of the building, most likely due to preexisting diagonal joint cracks that formed during the Northridge earthquake.

Ambient and Forced Vibration Testing of a Reinforced Concrete Building before and after Its Seismic Retrofitting

AbstractThis paper investigates the effects of seismic retrofitting on the modal characteristics of a 6-story RC building located in Istanbul, Turkey. Ambient vibration surveys were carried out before, during, and after the retrofitting work, which took place between June and December 2010. The building was retrofitted via jacketing of columns, addition of structural walls, and construction of a mat foundation. These studies were complemented with data from forced vibration tests performed with an eccentric-mass shaker after the retrofitting work was completed. During retrofitting, partitions were demolished; as a result, the first modal frequency of the building decreased by 11%, based on the results of the ambient vibration survey. The ambient vibration survey also showed that the modal frequencies after the seismic retrofitting increased by almost 96%. During the forced vibration tests, the building was excited around its modal frequencies using an eccentric-mass shaker. It was found that the modal dam...

A Quantitative Basis for Strong-Motion Instrumentation Specifications

Abstract A quantitative basis for key strong-motion instrumentation specifications is established by analyzing signal distortions and errors associated with data acquisition processes and then performing sensitivity analyses with respect to important parameters. The digital acquisition of 30 digitally enhanced earthquake histories is simulated to gain insight into how specific sources of error, namely clock jitter, randomness in initial sampling instant, and differential nonlinearity, accumulate and influence overall data quality. Sensitivity analyses with respect to several measures of shaking-strength (an engineer’s intensity measure) such as instrumental intensity, peak ground acceleration, peak ground velocity, and peak response spectral pseudoaccelerations are performed. Results from these studies are used to assess potential updates to current strong-motion instrumentation specifications of major strong-motion instrumentation programs. The simulations and analyses detailed here are also groundwork for future sensitivity studies of structural response parameters of peak floor acceleration and peak interstory drift.

A Quantitative Basis for Building Instrumentation Specifications

A quantitative basis for key building instrumentation specifications—namely, sample rate, system resolution, and time synchronization—is established by quantifying the sensitivities of engineering demand parameters of peak floor acceleration and peak interstory drift to the errors associated with data acquisition. Details of the realistic simulation and digitization of structural responses are provided and automation of the seemingly signal-dependent procedure of high-pass digital filtering of relative displacements obtained by double numerical integration of accelerations is presented. Results from these studies, along with prior results from similar sensitivity analyses with respect to intensity measures of peak ground acceleration, peak ground velocity, and peak spectral acceleration, are used to recommend potential updates to structural instrumentation specifications of major strong-motion instrumentation programs.