Kihak Lee

Evaluating Topological Optimized Layout of Building Structures by Using Nodal Material Density Based Bilinear Interpolation

Abstract This study presents a boundary representation of Eulerian-type in order to achieve conceptual structural layouts of building structures by relieving material discontinuities of structural topology optimization in comparison with classical ones. According to the partition of unity concept, a bilinear interpolation can be employed by nodal densities and shape functions with the use of discretization of four-node square elements and then material properties, i.e. here, densities in finite elements are not constant but variational values. As a result, the improvement of material continuity can be established as optimal topologies, and numerical singularity and zigzag material boundaries which may occur in classical topology optimization design are relieved. Numerical applications verify the efficiency of this method by evaluating this conceptual structural layout approach for designing building structures.

Seismic demand evaluation of steel MRF buildings with simple and detailed connection models

A detailed connection model can take into account effects such as strength and stiffness degradation, pinching and brittle behavior while a simple one cannot. This study investigates the effects of using the detailed and simple connection models on the seismic behavior of steel moment resisting frame (MRF) buildings. The frame models designed by 1994 UBC and 2000 IBC provisions involved the use of clear length dimensions of elements, panel zone models, and connection models subjected to suites of 20 accelerograms with 50/50, 10/50 and 2/50 hazard levels. The interstory drifts, column axial forces, and probability of building failure were investigated. The results show that the discrepancy of the building behaviors between using the detailed and simple connections became larger when the more severe seismic hazard levels and higher percentile values were considered. Therefore, the detailed connection models would be recommended to evaluate and predict seismic performance of steel MRF buildings.

Cyclic Behavior of High-Performance Fiber-Reinforced Cement Composite Coupling Beam Having Diagonal Reinforcement

Coupled shear walls can provide an efficient structural system to resist lateral force. However, the reinforcement detail for diagonally reinforced coupling beams required by ACI-318 often causes the difficulties in construction due to the reinforcement congestion and interference among reinforcement. This paper is to evaluate cyclic behavior of High-Performance Fiber-Reinforced Cement Composite (HPFRCC) coupling beams having reduced transverse reinforcement around the beam perimeter. Experimental test was conducted using three specimens having a beam aspect ratio 2.0. Test results showed that HPFRCC coupling beams with half of transverse reinforcement required by ACI-318 provided similar energy dissipation capacities compared with the coupling beams having reinforcement satisfy the requirement of ACI-318.

Seismic Performance Evaluation of RC Beam-Column Connections in Special and Intermediate Moment Frames

This article presents the results of a research project aimed at investigating the seismic performance criteria of beam-column connections in reinforced concrete (RC) moment frames. A total of 112 RC beam-column connection tests were examined in detail. All the test specimens were qualified as either special moment frame (SMF) or intermediate moment frame (IMF) connections, based on the design and detailing requirements of ACI 318-08 [2008, Ch. 21]. The performance acceptance criteria, originally defined for steel moment frame connections in the 1997 edition of the AISC Seismic Provisions, were employed to evaluate the seismic performance of the RC connection specimens. Most of the specimens that satisfied the ACI 318 design requirements applying for SMFs were found to be ductile up to a plastic drift of 3% without any major degradation in strength, which satisfied the AISC performance acceptance criteria. This is most likely attributed to the stringent ACI 318-08 requirements for joints in SMFs. On the other hand, more than 40% of the IMF connections were not satisfactory based on the AISC criteria applying for steel IMF connections. Joint shear stress level showed more significant effect on their seismic performance than the column-to-beam flexural strength ratio. It is believed that inadequate joint detailing prevented these connections from developing their full beam flexural capacity, which resulted in premature strength deterioration and subsequent failure. Based on the findings, it is recommended that design requirements for beam-column connections in RC IMFs should be revisited and enhanced; otherwise, the R-factor assigned for RC IMFs should be somewhat reduced to accommodate their smaller ductility than intended by the ACI 318 requirements.

Fragility Assessment of Damaged Piloti-Type RC Building With/Without BRB Under Successive Earthquakes

This paper presents the seismic evaluation and prediction of a damaged piloti-type Reinforced Concrete (RC) building before and after post-retrofitting under successive earthquakes. For considering realistic successive earthquakes, the past records measured at the same station were combined. In this study, the damaged RC building due to the first earthquake was retrofitted with a buckling-restrained brace (BRB) before the second earthquake occurred. Nonlinear Time History Analysis (NTHA) was performed under the scaled intensity of the successive ground motions. Based on the extensive structural response data obtained form from the NTHA, the fragility relationships between the ground shaking intensity and the probability of reaching a pre-determined limit state was were derived. In addition, The the fragility curves of the pre-damaged building without and with the BRBs were employed to evaluate the effect of the successive earthquakes and the post-retrofit effect. Through the seismic assessment subjected to the successive records, it was observed that the seismic performance of the pre-damaged building was significantly affected by the severity of the damage from the first earthquake damages and the hysteresis behavior of the retrofit element.

Analysis of Nonlinear Seismic Behavior of Reinforced Concrete Shear Wall Systems Designed with Special and Semi-Special Seismic Details

/ A B S T R A C T /In this paper, analytical models for reinforced concrete shear wall systems designed based on Korean Building Code (KBC2009) are proposed, which have special and semi-special seismic details and are compared with experimental results for a verification of analytical models. In addition, semi-special seismic details aimed to improve constructability and enhance economic efficiency were propos ed and evaluated. The analytical models were performed based on nonlinear static and dynamic analysis. Through the nonlinear analyses, two seismic details showed the similar seismic behavior from the cyclic test and the analytical models for the two different seismic details represented the behavior in terms of the initial stiffness, maximum strength and strength degradation. And newly proposed seismic details(semi-special) provided with similar hysterestic behavior as well as the maximum drift.

Seismic Performance Evaluation and Retrofit of a 2-Story Steel Building Using a Fragility Contour Method

Based on the Korean Building Standard Law, a building less than 3-stories and in area is defined as a small-level building and, as a result, this type of building has been excluded from the requirement to comply with seismic design. In order to prevent the loss of life and property under earthquake loadings, the small-scale building should satisfy the seismic performance specified in the current code through a seismic retrofit. In this study, a seismic retrofit scheme of a Buckling-Restrained Knee Brace (BRKB) was developed for non-seismic 2-story steel buildings, including small-scale buildings, using a fragility contour method. In order to develop an effective retrofit scheme of the BRKB for the building, a total of 75 BRKB analytical models were used to achieve the desired performance levels and analyzed using the fragility contour method. The seismic performance of the retrofitted building was evaluated in terms of the weight of the developed BRKB systems. This study shows that the fragility contour method can be used for rapid evaluation and is an effective tool for structural engineers.

An Seismic Performance Study according to Reinforcement Method of Aramid Rods and SRF of Damaged RC Column

This paper has proposed a reinforcing method for damaged RC columns with SRF sheets and Aramid rods. In order to verify the effectiveness and performance, two original columns and two reinforced columns with SRF sheets and Aramid rods were developed and tested under lateral cyclic displacement and a constant axial load. The test showed that the improvement of energy dissipation capacity was increased in terms of strength and ductility. In addition, an analytical modeling of the standard specimens was proposed using Response-2000 and ZeusNL program. The results of analytical and experimental studies for two standard columns were compared in terms of loading-displacement curve and energy dissipation capacity based on the nonlinear static analysis.

Structural Performance Evaluation of VES Damper System subjected to Cyclic Loadings(CST30)

The performance enhancement of various structural building syst ems from natural hazards has become an inctreasingly important issue in engineering field. In this paper, visco-elas tic(VE) CST30 damping systems were tested under cyclic loadings to evaluate their performance in terms of ductility an d energy dissipation. Main test variables are relative shear stiffness, rate of loading frequency, and thickness of specimens to evaluate the seismic capacity based on the performance criteria. This experiment was performed using a total of 12 spe cimens, subjected to cyclic loadings up to a shear deformation of 500%. All the CST30 dampers provided a ductile a nd stable hysterestic behavior when subjected to the demands of large shear stiffness and different loading frequenc ies. The test results showed that the CST30 dampers are an effective damping systems to enhance the buildings performance for remodeling and retrofit of buildings.

Hysteretic Behavior Evaluation of a RC Coupling Beam using a Steel Fiber and Diagonal Reinforcement

In this paper, a bundled diagonal reinforcement using high performance steel fiber was proposed to enhance the construct ability and seismic performance. Experiments of coupling beam was composed of four specimens and the hysteretic behavior evaluated for reverse cyclic loading to specimens using high performance steel fiber. The main variables of the experiment is a amount of stirrup and bundled reinforcement, depending on whether the mix of steel fiber. Specimen which criteria was applied 100% of stirrup and bundled diagonal reinforcement of ACI318 criteria. With this, by appling same diagonal reinforcement, two specimens were created by adjusting stirrup of 75%, 50%. So, a total of four specimens were produced. When coupling beam was placed concrete, this experiment was mixed in a content of steel fiber 1%. All the specimens were produced by aspect ratio 3.5(l/h=1050/300) to a half-scale. In this result, two specimens as reduced to stirrup of 75%, 50% was no significant difference in the strength, stiffness and energy dissipation capacity, respectively compared to the stirrup of 100%.