• Earthquakes and Structures
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• 제20권3호
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• pp.239-260
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• 2021

Torsional response of buildings is attributed to poor structural configurations in plan, which arises due to two factors - torsional eccentricity and torsional flexibility. Usually, building codes address effects due to the former. This study examines both of these effects. Buildings with torsional eccentricity (e.g., those with large eccentricity) and with torsional flexibility (those with torsional mode as a fundamental mode) demand large deformations of vertical elements resisting lateral loads, especially those along the building perimeter in plan. Lateral-torsional responses are studied of unsymmetrical buildings through elastic and inelastic analyses using idealised single-storey building models (with two degrees of freedom). Displacement demands on vertical elements distributed in plan are non-uniform and sensitive to characteristics of both structure and earthquake ground motion. Limits are proposed to mitigate lateral-torsional effects, which guides in proportioning vertical elements and restricts amplification of lateral displacement in them and to avoid torsional mode as the first mode. Nonlinear static and dynamic analyses of multi-storey buildings are used to validate the limits proposed.

• Structural Engineering and Mechanics
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• 제85권1호
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• pp.55-64
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• 2023

Energy-based seismic design and evaluation methods are promising to be involved in the next generation design codes. Accordingly, determining the distribution of earthquake input energy demand among floor levels is quite imperative in order to develop an energy-based seismic design procedure. In this paper, peak floor input energy demands are achieved from relative input energy response histories of several reinforced concrete (RC) frames. A set of 22 horizontal acceleration histories selected from recorded near-fault earthquakes and scaled in time domain to be compatible with the elastic acceleration design spectra of Turkish Seismic Design Code are used in time history analyses. The distribution of the computed input energy per mass values and the arithmetic means through the height of the considered RC frames are presented as a result. It is found that spatial distribution of input energy per mass is highly affected by the number of stories. Very practical yet consistent formulation of distributing the total input energy to story levels is achieved, as a most important contribution of the study.

• 한국항해학회지
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• 제4권2호
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• pp.45-79
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• 1980

The aim of this paper is to analyze the mechanics of price formation in the tramp shipping. For the purpose of this study, the main characteristics of tramp freight rates and the market is examined, and a brief examination of the nature ofthe costs of operation is given which are essential for the understanding of the functioning of shipping firms as well as for the understanding of developments in the tramp freight market. The demand and supply relationships in the market is also analysed in detail. Tramp shipping is an industry that has a market which functions under conditions that are not dissimilar to the theoretical model of perfect competition. However, it does notmean that tramp shipping market is a perfectly competitive market. It is apparent that this realworld competitive system has its imperfections, which means that the market for tramp shipping is near to being a perfectly competitive market on an internaitonal scale and it is freight are therefore subjext to the laws of supply and demand. In theory, the minimum freight rate in the short term is that at which the lowest cost vessels will lay-up in preference to operating, and is equal to the variable costs minus lay-up costs; and this would imply that in all times except those of full employment for ships there is a tendency for newer low-cost, and, probably, faster vessels to be driving the older high-cost vessels in the breaker's yards. In this case, shipowners may be reluctant to lay-up their ships becasue of obligations to crews, or because they would lose credibility with shippers or financiers, or simply because of lost prestige. Mainly, however, the decision is made on strictly economic grounds. When, for example, the total operating costs minus the likely freight earnings are greater than the cost of taking the ship out of service, maintaining it, and recommissioning it, then a ship may be considered for laying-up; shipowners will, in other words, run the ships at freight earnings below operating costs by as much as the cost of laying them up. As described above, the freight rates fixed on the tramp shipping market are subject to the laws of supply and demand. In other words, the basic properties of supply and demand are of significance so far as price or rate fluctuations in the tramp freight market are concerned. In connection with the same of the demand for tramp shipping services, the following points should be brone in mind: (a) That the magnitude of demand for sea transport of dry cargoes in general and for tramp shipping services in particular is increasing in the long run. (b) That owning to external factors, the demand for tramp shipping services is capable of varying sharphy at a given going of time. (c) The demad for the industry's services tends to be price inelastic in the short run. On the other hand the demand for the services offered by the individual shipping firm tends as a rule to be infinitely price elastic. In the meantime, the properties of the supply of the tramp shipping facilities are that it cannot expand or contract in the short run. Also, that in the long run there is a time-lag between entrepreneurs' decision to expand their fleets and the actual time of delivery of the new vessels. Thus, supply is inelastic and not capable of responding to demand and price changes at a given period of time. In conclusion, it can be safely stated that short-run changes in freight rates are a direct result of variations in the magnitude of demand for tramp shipping facilities, whilest the average level of freight rates is brought down to relatively low levels over prolonged periods of time.

• 대한교통학회지
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• 제22권5호
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• pp.139-149
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• 2004

급격히 증가하는 차량수요에 대응하기 위하여 도로의 공급을 증대시키는 양적인 측면의 도로정책 기조가 유지되고 있지만, 이는 근본적인 해결책이 될 수 없다는 인식이 대두되어 왔다. 이에 따라 기존 도로시설의 효율적 활용과 교통수요를 관리하는 방법에 많은 관심이 모아지고 있다. 이중 혼잡통행료는 수요를 관리하고 차량을 도로량에 적절히 배정하는 수단으로 생각될 수 있으며, 교통 혼잡과 환경에 대한 악영향을 줄일 수 있는 가장 효율적인 방법중 하나로 고려되고 있다. 이에 따라 혼잡통행료 산정문제는 그동안 경제학자와 교통전문가에게 중요하고 흥미로운 연구 분야로 여겨져 왔고 다양한 방법으로 연구가 이루어져 왔다. 본 연구에서는 이러한 혼잡통행료 산정문제를 네트워크 설계문제(Network Design Problem, NDP)로 보고 통행배정문제와 혼잡통행료 산정 최적화 문제가 결합된 바이레벨 문제(Bi-level Problem)로 모형을 개발하였다. 여기서, 통행배정문제는 혼잡통행료 징수에 따른 통행자의 행태변화를 고려하기 위하여 Sheffi(1985)가 제시하였던 가변수요모형(Elastic-Demand Network Equilibrium Model)을 적용하였고, 혼잡통행료 산정 최적화 문제로서 사회적 순 편익 최대화문제를 고려하였다. 또한, 이런 형태의 바이레벨 문제는 게임이론측면에서 보면, 두 문제간의 비협력을 전제로 하는 Nash 비협력 게임과 상호 협력을 전제하는 Stackelberg게임으로 구분할 수 있는데, 본 연구에서는 민감도 분석을 통하여 최적해를 찾는 Stackelberg 게임으로 모형을 개발하였다. 통행수요는 단일계층으로 승용차로만 구성되어있다고 가정하였으며, 개발된 모형에 대하여 2개 네트워크에서 실험이 이루어졌다. 실험을 통하여 모형의 알고리즘에 관한 설명과 검증을 하였으며, 혼잡통행료 산정문제와 함께 적합한 혼잡통행료 징수 대상 링크의 선정이 중요한 연구대상임을 제시하였다.

• Structural Engineering and Mechanics
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• 제56권4호
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• pp.569-588
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• 2015

A comprehensive methodology is proposed for design of metallic dampers in seismic retrofit of earthquake-damaged frame structures. It is assumed that the metallic dampers remain elastic and only provide stiffness during frequent earthquake (i.e., earthquake with a 63% probability of exceedance in 50-year service period), while in precautionary earthquake (i.e., earthquake with a 10% probability of exceedance in 50-year service period), the metallic dampers yield before the main frame and dissipate most of the seismic energy to either prevent or minimize structural damages. Therefore by converting multi-story frame to an equivalent single-degree-of-freedom system, the added stiffness provided by metallic dampers is designed to control elastic story drifts within code-based demand under frequent earthquake, and the added damping with the combination of added stiffness influences is obtained to control structural stress within performance-based target under precautionary earthquake. With the equivalent added damping ratio, the expected damping forces provided by metallic dampers can be calculated to carry out the configuration and design of metallic dampers along with supporting braces. Based on a detailed example for retrofit of an earthquake-damaged reinforced concrete frame by using metallic dampers, the proposed design procedure is demonstrated to be simple and practical, which can not only meet current China's design codes but also be used in retrofit design of earthquake-damaged frame with metallic damper for reaching desirable performance objective.

• 비파괴검사학회지
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• 제32권1호
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• pp.41-46
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• 2012

MEMS/NEMS 구조체의 개발과 응용기술의 발달로 박판 및 박막의 기계적 물성 평가에 대한 요구가 점차 늘어나고 있다. 기계적 물성은 주로 인장시험이나 초음파의 속도 측정으로 평가되어 왔으나, 박판/박막 구조의 경우 기존의 기술로는 측정에 한계가 있어 나노압입시험법, 유도초음파법 등의 새로운 기술이 개발되고 있다. 본 연구에서는 박판 구조의 금속재료의 탄성계수를 평가하기 위하여 EMAT으로 송수신된 박판내에서의 유도초음파 진행 속도를 측정하였으며, 이론적으로 계산된 유도초음파 군속도와 실험적인 군속도의 최적화 과정을 통해 최종적으로 박판의 탄성계수를 평가하였다. 두께 $50{\mu}m$의 니켈 박판에서 측정된 영률은 201.6 GPa이었으며, 나노압입시험법으로 측정된 207 GPa, 참고문헌의 203.7 GPa과 비교하면 약 3% 내에서 일치하는 결과이다.

• Structural Engineering and Mechanics
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• 제9권6호
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• pp.615-636
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• 2000

Some elevated water tanks have failed due to torsional vibrations in past earthquakes. The overall axisymmetric structural geometry and mass distribution of such structures may leave only a small accidental eccentricity between centre of stiffness and centre of mass. Such a small accidental eccentricity is not expected to cause a torsional failure. This paper studies the possibility of amplified torsional behaviour of elevated water tanks due to such small accidental eccentricity in the elastic as well as inelastic range; using two simple idealized systems with two coupled lateral-torsional degrees of freedom. The systems are capable of retaining the characteristics of two extreme categories of water tanks namely, a) tanks on staging with less number of columns and panels and b) tanks on staging with large number of columns and panels. The study shows that the presence of a small eccentricity may lead to large displacement of the staging edge in the elastic range, if the torsional-to-lateral time period ratio $({\tau})$ of the elevated tanks lies within a critical range of 0.7< ${\tau}$ <1.25. Inelastic behaviour study reveals that such excessive displacement in some of the reinforced concrete staging elements may cause unsymmetric yielding. This may lead to progressive strength deterioration through successive yielding in same elements under cyclic loading during earthquakes. Such localized strength drop progressively develop large strength eccentricity resulting in large localized inelastic displacement and ductility demand, leading to failure. So, elevated water tanks should have ${\tau}$ outside the said critical range to avoid amplified torsional response. The tanks supported on staging with less number of columns and panels are found to have greater torsional vulnerability. Tanks located near faults seem to have torsional vulnerability for large ${\tau}$.

• Steel and Composite Structures
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• 제19권5호
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• pp.1167-1184
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• 2015

Despite considerable life casualty and financial loss resulting from past earthquakes, many existing steel buildings are still seismically vulnerable as they have no lateral resistance or at least need some sort of retrofitting. Passive control methods with decreasing seismic demand and increasing ductility reduce rate of vulnerability of structures against earthquakes. One of the most effective and practical passive control methods is to use a shear panel system working as a ductile fuse in the structure. The shear Panel System, SPS, is located vertically between apex of two chevron braces and the flange of the floor beam. Seismic energy is highly dissipated through shear yielding of shear panel web while other elements of the structure remain almost elastic. In this paper, lateral behavior and related benefits of this system with narrow-flange link beams is experimentally investigated in chevron braced simple steel frames. For this purpose, five specimens with IPE (narrow-flange I section) shear panels were examined. All of the specimens showed high ductility and dissipated almost all input energy imposed to the structure. For example, maximum SPS shear distortion of 0.128-0.156 rad, overall ductility of 5.3-7.2, response modification factor of 7.1-11.2, and finally maximum equivalent viscous damping ratio of 35.5-40.2% in the last loading cycle corresponding to an average damping ratio of 26.7-30.6% were obtained. It was also shown that the beam, columns and braces remained elastic as expected. Considering this fact, by just changing the probably damaged shear panel pieces after earthquake, the structure can still be continuously used as another benefit of this proposed retrofitting system without the need to change the floor beam.

• 대한교통학회지
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• 제31권2호
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• pp.33-44
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• 2013

토지이용-교통 통합모형은 실증연구와 정책연구에 적합한 풍부하고 유연한 모형요소를 가지고 있다. 그러나 모형의 구성이 복잡한 만큼 후생함수의 정책변수에 대한 변화율 또한 통상 복잡하다. 따라서 최적 정책수단이 충족시켜야 할 1계 도함수 조건을 푸는 최적 정책수단의 수식을 명시적으로 유도하는 것이 매우 어렵고, 그 결과 수치해석적 모형은 이론연구 도구로서 활용하는데 근본적 한계를 가지고 있었다. 이 문제를 해결하고자 Yu and Rhee(2011) 및 Rhee (2012)는 이들 모형에서 목적함수인 후생함수의 정책변수에 대한 변화율을 간단한 수식으로 변환하는 방법론을 제시한 바 있다. 그러나 이들이 사용한 모형에서 교통수요는 고정된 것으로 가정하고 있어, 이들 모형 또한 교통 계획적 관점에서 보았을 때 상당한 한계를 지니고 있다. 이에 본 연구는 이들의 방법론을 교통수요가 탄력적인 토지이용-교통 모형으로 확장한다. 이 방법론을 이용하면 보다 현실적인 모습의 토지시장과 교통망이 구현된 모형에서 정책수단의 다양한 영향을 분석할 수 있게 된다. 이러한 분석은 종래에 존재하지 않았거나 있었다 하더라도 매우 제한된 범위에서 수행되었던 분석들이다.

• Smart Structures and Systems
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• 제18권1호
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• pp.75-92
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• 2016

The design of a semi-active (SA) control system addressed to mitigate wind induced structural demand to high wind turbine towers is discussed herein. Actually, the remarkable growth in height of wind turbines in the last decades, for a higher production of electricity, makes this issue pressing than ever. The main objective is limiting bending moment demand by relaxing the base restraint, without increasing the top displacement, so reducing the incidence of harmful "p-delta" effects. A variable restraint at the base, able to modify in real time its mechanical properties according to the instantaneous response of the tower, is proposed. It is made of a smooth hinge with additional elastic stiffness and variable damping respectively given by springs and SA magnetorheological (MR) dampers installed in parallel. The idea has been physically realized at the Denmark Technical University where a 1/20 scale model of a real, one hundred meters tall wind turbine has been assumed as case study for shaking table tests. A special control algorithm has been purposely designed to drive MR dampers. Starting from the results of preliminary laboratory tests, a finite element model of such structure has been calibrated so as to develop several numerical simulations addressed to calibrate the controller, i.e., to achieve as much as possible different, even conflicting, structural goals. The results are definitely encouraging, since the best configuration of the controller leaded to about 80% of reduction of base stress, as well as to about 30% of reduction of top displacement in respect to the fixed base case.