Hwasung Roh

Nonlinear Static Analysis of Structures with Rocking Columns

In simple inelastic structures subjected to severe base motions, the maximum acceleration response is bound by the yield strength and is inversely proportional to its mass. It can be demonstrated that a similar effect is approximately produced in structures with multidegrees of freedom. In strong structures, the maximum acceleration response is dependent on the intensity of the earthquake. However, if the strength of the structure is reduced, the maximum acceleration response and associated forces in its structural and nonstructural components can be substantially reduced. If some of the structural columns (carrying gravity loads) are allowed to rock, providing very small resistance to lateral loads, it is possible to reduce the strength of the global structural system and limit the global accelerations in seismic events. However, before overturning such columns may have substantial resistance which may void the effectiveness of weakening. The objective of this study is to (1) develop a simplified analytical model for rocking columns from fixity to overturning; (2) build the computational tools to simulate the behavior of structures including such rocking columns; and (3) examine the global nonlinear static response of a weakened structure. An analytical model is developed using an equivalent flexibility approach, which considers rigid body rotations and flexural deformations. Physical experiments of rocking columns were performed, and the results are used for the calibration and validation of analytical models. An analytical model of a 1:3 scaled structure is developed using IDARC2D modified in this study to evaluate several alternatives of weakening using rocking columns.

Seismic Response of Adjacent Steel Structures Connected by Passive Device

In this study, the seismic response of adjacent steel structures linked to one another at different floor levels by passive devices is evaluated. Two types of connectors are considered: linear viscous dampers and linear springs. Various combinations of type and location of the connectors are analyzed for wide ranges of their characteristics. Influence of the structural properties (ratio of number of stories and ratio of fundamental periods) and the properties of the excitation (frequency) on the story response are also investigated. The results show that reduction of the story drift and the floor acceleration is not possible if the structures are connected to one another solely by the springs. However, it is feasible if the structures are linked to one another by either: (a) viscous dampers or (b) combinations of viscous dampers and springs. Herein, response reductions corresponding to case (b) are generally smaller than those corresponding to case (a). Also, the coupling control approach is more effective in controlling the response than the uncoupled approach.

Seismic Performance of Segmental Rocking Columns Connected with NiTi Martensitic SMA Bars

As well known, the damping or energy dissipation capacity of bridge columns is very important because it is directly related to the design spectrum and affects the seismic performance of whole bridge systems. For this reason, several investigations evaluating the energy dissipation capacity of segmental columns, which has a small energy dissipation capacity itself, have been conducted by adopting various material types of energy dissipating (ED) bars. This paper investigates the damping capacity of post-tensioned (PT) segmental rocking columns connected with large diameter (36.5 mm) martensitic SMA bars at their base as ED bars. Two aspect ratios for the columns are considered: 7.5 for slender and 5.0 for medium size. Moment-curvature relationships and complementary computational tools are adopted to model the behavior of the columns. A bilinear model for the PT tendon and a modified four-spring model for the martensitic SMA bar are used. From the quasi-static cyclic analysis, the martensitic SMA bars leads to an equivalent damping ratio between 10.5% and 12.5% for the different aspect ratios and PT tendon forces. For the sake of comparison, another material type of the SMA bar, which is 25.4 mm diameter superelastic bar, is considered. The damping ratio of the columns with the martensitic SMA bars is much higher than the use of the superelastic SMA bars showing between 5% and 7% damping ratio.

Seismic Fragility Evaluation of RC Frame Structures Retrofitted with Controlled Concrete Rocking Column and Damping Technique

Acceleration response of simple yielding structure is proportional to its own weight, but it is limited by yield strength. Thus, using rocking columns that reduces global yield strength, a limited acceleration is achieved. However, the displacement becomes large due to lower strength and higher inelasticity, but it can be controlled by adding damping. Performing fragility analyses, the seismic response of R/C frame structures with rocking columns and viscous dampers is investigated. Near field MCEER ground motions are considered. The analyses show that the story accelerations are reduced by using rocking columns, while the story displacements are controlled by using viscous dampers.

Seismic Performance of Industrial Sheds and Liquefaction Effects During May 2012 Emilia Earthquakes Sequence in Northern Italy

On May 20, 2012 at 2:03 UTC, a Mw 6.1 earthquake occurred in Emilia Region of Northern Italy. The event was preceded by a Ml 4.1 foreshock on May 19, 2012 at 23:13 UTC, and followed by several aftershocks, twenty of them with a magnitude Mw greater than 4. The epicentral area of the seismic sequence covers alluvial lowland that is occupied by both agricultural and urbanized areas. Liquefaction effects were observed in several villages on the west side of Ferrara which were built upon former river beds such as the Reno River. The Emilia seismic sequence resulted in 27 casualties, several of whom were among the workers in the factories that collapsed during working hours, and there was extensive damage to monuments, public buildings, industrial sites and private homes. Almost no municipalities hit by 2012 earthquake were classified as seismic area before 2003; therefore, most of the existing structures had been designed without taking in account the seismic actions. The main aims of MCEER field mission was to document the emergency response and the most common damage mechanisms of industrial sheds during Emilia earthquake sequence which are shown and discussed in detail.

Seismic Performance Evaluation of High Voltage Transformer Bushings

The main objective of this paper is to identify the dynamic characteristics of a realistic model of a commercial transformer (230 kV) and identify the critical components that influence the dynamic response of three high voltage porcelain bushings installed on its flexible roof. For this purpose, a finite element model of the transformer is appropriately modified and enhanced. The improved model is subjected to a wide band random motion excitation and the responses at various locations are processed in the frequency domain. The dynamic response of the high voltage porcelain bushings (196/230kV) and the amplification that occurs between the base and the top of the transformer’s oil tank, is assessed through detailed sensitivity analysis for a variety of potential structural modifications of the model. Each structurally modified model is investigated using a random ground motion, compatible to IEEE-693 spectrum. The accelerations are monitored at the bushings, at the corners of the cover plate and at the top of the turrets for each model and comparisons are made for the effect of each modification to be assessed. The ground motion amplifications at the cover are obtained from the spectra at the corners of the cover and at the top of the turrets. The sensitivity shows that the cover plate has the predominant influence on the response of the bushings.

Performance evaluation of SFRC for tunnel segments based on large beam test

In order to develop SFRC TBM tunnel segment, evaluating the SFRC mixture was conducted through flexural tests of SFRC beams without ordinary steel reinforcement in this study. Considered variables were compressive strengths of SFRC, aspect and mix ratio of steel fibers and total 16 specimens were fabricated and tested until failure. The load-vertical displacement results demonstrates that the effect of aspect ratio is minor when compared to results form small beam test(Moon et al, 2013). A SFRC beam resists the vertical load until the width of crack reaches to 7 mm due to steel fibers across cracked surfaces. Moreover, it is found that flexural moment estimated by equation of TR No. 63(Concrete Society, 2011) is useful for prediction of nominal strength for SFRC structure. From the investigation of fiber distribution in cracked section, it is found that dispersion improved in actual size beam compared to in standard small beam for evaluation of flexural strength.

Experimental Fatigue Evaluation of Prestressed Concrete-Filled Steel Tube I-Shaped Bridge Girders

This paper investigates experimentally the fatigue capacity of prestressed concrete-filled steel tube (CFT) I-shaped girders. The girder consists of steel plates, PT tendons, concrete, and steel shear keys. Three 10 m long girder specimens were prepared for different fatigue stress ranges and subjected to a three-point bending test producing a positive moment along the entire girder span. Displacement control was used to prevent sudden failure of the specimens. The test girder specimens exhibited fatigue capacities classified as Category B′ or C′ of the AASHTO criteria for positive and negative moment, respectively. The special testing arrangement used to produce the negative moment is presented, as well as the fatigue testing procedures and results. At the end of the paper, a modified equation for the S–N curve is derived for predicting the remaining life of the girder.

Experimental Investigation of the Effects of Concrete Alkalinity on Tensile Properties of Preheated Structural GFRP Rebar

The combined effects of preexposure to high temperature and alkalinity on the tensile performance of structural GFRP reinforcing bars are experimentally investigated. A total of 105 GFRP bar specimens are preexposed to high temperature between 120°C and 200°C and then immersed into pH of 12.6 alkaline solution for 100, 300, and 660 days. From the test results, the elastic modulus obtained at 300 immersion days is almost the same as those of 660 immersion days. For all alkali immersion days considered in the test, the preheated specimens provide slightly lower elastic modulus than the unpreheated specimens, showing only 8% maximum difference. The tensile strength decreases for all testing cases as the increase of the alkaline immersing time, regardless of the prehearing levels. The tensile strength of the preheated specimens is about 90% of the unpreheated specimen for 300 alkali immersion days. However, after 300 alkali immersion days the tensile strengths are almost identical to each other. Such results indicate that the tensile strength and elastic modulus of the structural GFRP reinforcing bars are closely related to alkali immersion days, not much related to the preheating levels. The specimens show a typical tensile failure around the preheated location.

Load Carrying Capacity Evaluation of Single Span Bridge using Impact Factor Response Spectrum

In a previous study, the impact factor response spectrum and corresponding method for evaluating the load carrying capacity of bridges was suggested to improve the existing evaluation method. To verify the applicability of the suggested method, which is based on the frequency of bridges, the dynamic characteristic test for an actual single span simply-supported bridge was conducted. Through a field test under ambient traffic conditions, the dynamic response of the bridge was obtained using wireless accelometers and its fundamental frequency was identified. The peak impact factor was determined from the identified frequency and the impact factor response spectrum. The load carrying performance variation of the bridge was estimated considering the performance reduction factor, which was calculated using the current and previous natural frequency and impact factor. From the result, the load carrying capacity of the bridge was decreased, but the capacity was still enough because its value is greater than the design live load. Through the overall procedures and technical details presented in this paper, the suggested evaluation method can be applied to actual bridges with the acceleration data measured under ambient traffic conditions and the impact factor response spectrum.