Daniel H. Whang

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.

Digitally controlled simple shear apparatus for dynamic soil testing

We describe the characteristics of a simple shear apparatus capable of applying realistic multidirectional earthquake loading to soil specimens. This device, herein termed the Digitally Controlled Simple Shear (DC-SS) apparatus, incorporates features such as servohydraulic actuation and true digital control to overcome control limitations of some previous dynamic soil testing machines. The device is shown to be capable of reproducing sinusoidal and broadband command signals across a wide range of frequencies and amplitudes, although the device has limited control capabilities for very small command displacements (less than approximately 0.005 mm). The small deformation limitation results from noise introduced to the control system from analog-to-digital conversion of feedback signals. We demonstrate that bidirectional command signals can be accurately imparted with minimal cross coupling, which results from an innovative multiple-input, multiple-output digital control system. The capabilities of the device are demonstrated with a series of broadband tests on unsaturated soil specimens subjected to uni- and bidirectional excitation.

Effect of compaction conditions on the seismic compression of compacted fill soils

Seismic compression is defined as the accrual of contractive volumetric strains in unsaturated compacted soil during earthquake shaking. Existing seismic compression analysis procedures are based on laboratory test results for clean uniform sands, and their applicability to compacted soils with fines is unclear. We evaluate seismic compression from cyclic simple shear laboratory testing of four compacted soils having fines contents that are sufficiently large that fines control the soil behavior, but possessing varying levels of fines plasticity. Each soil material is compacted to a range of formation dry densities and degrees-of-saturation. The test results show that seismic compression susceptibility decreases with increasing density and decreasing shear strain amplitude. Saturation is also found to be important for soils with moderately plastic fines (plasticity index, PI » 15), but relatively unimportant for soils with low plasticity fines (PI » 2) across the range of saturations tested (³54 %). The saturation effect appears to be linked to the presence or lack of presence of a clod structure in the soil, the clod structure being most pronounced in plastic soils compacted dry of the line-of-optimums or at low densities. Comparisons of test results for soils with and without low- to moderately-plastic fines suggest that fines can decrease the seismic compression potential relative to clean sands.

Field Testing Capabilities of the nees@UCLA Equipment Site for Soil-Structure Interaction Applications

The nees@UCLA field testing has equipment for field testing and monitoring of structural and geotechnical performance. The equipment includes shakers for exciting structural and/or foundation systems, numerous sensors for monitoring accelerations and deformations within the excited structure (e.g., accelerometers, displacement transducers, strain gauges), and real time data acquisition and dissemination capabilities. A key application area for this equipment is testing of soil-foundation-structure systems. Such testing can, for example, be used to evaluate the stiffness and damping associated with foundation-soil interaction. Existing test data for such phenomena is limited, hence there is a significant need for this type of research. The results would enable the verification and calibration of computational and design models used in practice.