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CONTENTS
Volume 32, Number 4, April 2021 (Special Issue)
 

Abstract


Key Words


Address


Abstract
Performance-based seismic design (PBSD) is currently used for retrofitting of older buildings and the design of new buildings. Whereas, application of performance-based design for wind load is still under development. The tendency has been in the codes to increase wind hazard based on recent recorded events. Since tall buildings are highly susceptible to wind load, necessity for developing a framework for performance-based wind design (PBWD) has intensified. Only a few guidelines such as ASCE (2019) provide information on using PBWD as an alternative for code prescriptive wind design. Though wind hazards, performance objectives, analysis techniques, and acceptance criteria are explained, no recommendations are provided for several aspects like how to select a proper level of wind hazard for each target performance criterion. This paper is an attempt to explain current design philosophy for wind and seismic loads and inherent connection between the components of PBSD for development of a framework for PBWD of tall buildings. Recognizing this connection, a framework for PBWD based on limits set for serviceability and strength is also proposed. Also, the potential for carrying out PBWD in line with ASCE 7-16 is investigated and proposed in this paper.

Key Words
wind load; seismic load or force; performance-based design; serviceability; strength; tall building

Address
Hamidreza Alinejad:Department of Architecture and Architectural Engineering & Engineering Research Institute, Seoul National University,1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea

Thomas H.-K. Kang:Department of Architecture and Architectural Engineering & Engineering Research Institute, Seoul National University,1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea

Seung Yong Jeong:Department of Architecture and Architectural Engineering & Engineering Research Institute, Seoul National University,1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea

Abstract
For the past three decades a significant amount of research has been conducted on bridge flutter. Wind tunnel tests for a 2000 m class twin-box suspension bridge have revealed that a twin-box deck carrying 4 m tall 50% open area ratio wind screens at the deck edges achieved higher critical wind speeds for onset of flutter than a similar deck without wind screens. A result at odds with the well-known behavior for the mono-box deck. The wind tunnel tests also revealed that the critical flutter wind speed increased if the bridge deck assumed a nose-up twist relative to horizontal when exposed to high wind speeds – a phenomenon termed the "nose-up" effect. Static wind tunnel tests of this twin-box cross section revealed a positive moment coefficient at 0° angle of attack as well as a positive moment slope, ensuring that the elastically supported deck would always meet the mean wind flow at ever increasing mean angles of attack for increasing wind speeds. The aerodynamic action of the wind screens on the twin-box bridge girder is believed to create the observed nose-up aerodynamic moment at 0° angle of attack. The present paper reviews the findings of the wind tunnel tests with a view to gain physical insight into the nose-up" effect and to establish a theoretical model based on numerical simulations allowing flutter predictions for the twin-box bridge girder.

Key Words
twin-box girder; flutter prediction; aerodynamic derivatives; nose-up effect; suspension bridges; wind tunnel test; numerical simulations; long-span bridges; force and moment coefficient

Address
Maja Ronne:1)Department of Bridges International, COWI, Parallelvej 2, 2800 Kongens Lyngby, Denmark
2)Department of Mechanical Engineering, Technical University of Denmark (DTU),Niels Koppels Allé, Building 403, 2800 Kongens Lyngby, Denmark

Allan Larsen:Department of Bridges International, COWI, Parallelvej 2, 2800 Kongens Lyngby, Denmark

Jens H. Walther:Department of Mechanical Engineering, Technical University of Denmark (DTU),Niels Koppels Alle, Building 403, 2800 Kongens Lyngby, Denmark

Abstract
The vortex-drift pattern over a girder surface, actually demonstrating the complex fluid-structure interactions between the structure and surrounding flow, is strongly correlated with the VIVs but has still not been elucidated and may be useful for modeling VIVs. The complex fluid-structure interactions between the structure and surrounding flow are considerably simplified in constructing a vortex model to describe the vortex-drift pattern characterized by the ratio of the vortex-drift velocity to the oncoming flow velocity, considering the aerodynamic work. A spring-suspended sectional model (SSSM) is used to measure the pressure in wind tunnel tests, and the aerodynamic parameters for a typical streamlined closed-box girder are obtained from the spatial distribution of the phase lags between the distributed aerodynamic forces at each pressure point and the vortex-excited forces (VEFs). The results show that the ratio of the vortex-drift velocity to the oncoming flow velocity is inversely proportional to the vibration amplitude in the lock-in region and therefore attributed to the "lock-in" phenomena of the VIVs. Installing spoilers on handrails can destroy the regular vortex-drift pattern along the girder surface and thus suppress vertical VIVs.

Key Words
streamlined closed-box girder; Vortex-Induced Vibrations (VIVs); simplified vortex model; vortex-drift pattern; time-frequency characteristics

Address
Chuanxin Hu:Department of Civil Engineering, Wuhan University of Science and Technology, Wuhan 430065, China

Lin Zhao:1)State Key Lab of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China
2)Key Laboratory of Transport Industry of Wind Resistant Technology for Bridge Structures (Tongji University), Shanghai 200092, China

Yaojun Ge:1)State Key Lab of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China
2)Key Laboratory of Transport Industry of Wind Resistant Technology for Bridge Structures (Tongji University), Shanghai 200092, China


Abstract
In 2019, 5.6% of the total energy produced worldwide came from wind. Offshore wind generation is still a small portion of the total wind generation, yet its growth is exponential. Higher availability of sites, larger producibility and potentially lower environmental impacts make offshore wind generation attractive. On the other hand, as the water depth increases, fixed foundations are no more viable, and the new frontier is that of floating foundations. This paper brings an overview of why and how offshore wind energy should move deep water; it contains material from the Keynote Lecture given by the first author at the ACEM20/Structures20 Conference, held in Seoul in August 2020. The paper is organized into four sections: the first giving general concepts about wind generation especially offshore, the second and the third considering economic and technical aspects, respectively, of offshore deep-water wind generation, in the fourth, some challenges of floating offshore wind generation are presented and some conclusions are drawn.

Key Words
wind generation; offshore wind farms; offshore platforms; economic planning; life-cycle cost assessment; hydrodynamic loads; aerodynamic loads

Address
Francesco Ricciardelli:Department of Engineering,University of Campania "Luigi Vanvitelli", via Roma 29, 81031 Aversa (CE), Italy

Carmela Maienza:Department of Engineering,University of Campania "Luigi Vanvitelli", via Roma 29, 81031 Aversa (CE), Italy

Mustafa Vardaroglu:Department of Engineering,University of Campania "Luigi Vanvitelli", via Roma 29, 81031 Aversa (CE), Italy

Alberto Maria Avossa:Department of Engineering,University of Campania "Luigi Vanvitelli", via Roma 29, 81031 Aversa (CE), Italy


Abstract
Articulated towers are one of the class of compliant offshore structures that freely oscillates with wind and waves, as they are designed to have low natural frequency than ocean waves. The present study deals with the dynamic response of a double-pendulum articulated tower under hydrodynamic and aerodynamic loads. The wind field is simulated by two approaches, namely, single-point and multiple-point. Nonlinearities such as instantaneous tower orientation, variable added mass, fluctuating buoyancy, and geometrical nonlinearities are duly considered in the analysis. Hamilton's principle is used to derive the nonlinear equations of motion (EOM). The EOM is solved in the time domain by using the Wilson-Θ method. The maximum, minimum, mean, and standard deviation and salient power spectral density functions (PSDF) of deck displacement, bending moment, and central hinge shear are drawn for high and moderate sea states. The outcome of the analyses shows that tower response under multiple-point wind-field simulation results in lower responses when compared to that of single-point simulation.

Key Words
multi-point wind field; dynamic analysis; double pendulum articulated tower; wind-induced response; offshore

Address
Mohd Moonis Zaheer:Department of Civil Engineering, Zakir Hussain College of Engineering and Technology, AMU Aligarh-202002, India

Nazrul Islam:Department of Civil Engineering, Jamia Millia Islamia, New Delhi, India

Abstract
Shape optimization of tall buildings is an efficient approach to mitigate wind-induced effects. Several studies have demonstrated the potential of shape modifications to improve the building's aerodynamic properties. On the other hand, it is well-known that the cross-section geometry has a direct impact in the floor area availability and subsequently in the building's profitability. Hence, it is of interest for the designers to find the balance between these two design criteria that may require contradictory design strategies. This study proposes a surrogate-based multi-objective optimization framework to tackle this design problem. Closed-form equations provided by the Eurocode are used to obtain the wind-induced responses for several wind directions, seeking to develop an industry-oriented approach. CFD-based surrogates emulate the aerodynamic response of the building cross-section, using as input parameters the cross-section geometry and the wind angle of attack. The definition of the building's modified plan shapes is done adopting the reduced basis approach, advancing the current strategies currently adopted in aerodynamic optimization of civil engineering structures. The multi-objective optimization problem is solved with both the classical weighted Sum Method and the Weighted Min-Max approach, which enables obtaining the complete Pareto front in both convex and non-convex regions. Two application examples are presented in this study to demonstrate the feasibility of the proposed strategy, which permits the identification of Pareto optima from which the designer can choose the most adequate design balancing profitability and occupant comfort.

Key Words
along-wind accelerations; codes; CFD; surrogate modeling; profitability; tall buildings; reduced basis design; aerodynamic optimization

Address
Miguel Cid Montoya:Structural Mechanics Group, School of Civil Engineering, University of La Coruna, 15071, La Coruna, Spain

Felix Nieto:Structural Mechanics Group, School of Civil Engineering, University of La Coruna, 15071, La Coruna, Spain

Santiago Hernandez:Structural Mechanics Group, School of Civil Engineering, University of La Coruna, 15071, La Coruna, Spain

Abstract
Tornadoes are the most devastating meteorological natural hazards. Many empirical and theoretical numerical models of tornado vortex have been proposed, because it is difficult to carry out direct measurements of tornado velocity components. However, most of existing numerical models fail to explain the physical structure of tornado vortices. The present paper proposes a new empirical numerical model for a tornado vortex, and its load effects on a low-rise and a tall building are calculated and compared with those for existing numerical models. The velocity components of the proposed model show clear variations with radius and height, showing good agreement with the results of field measurements, wind tunnel experiments and computational fluid dynamics. Normal stresses in the columns of a low-rise building obtained from the proposed model show intermediate values when compared with those obtained from existing numerical models. Local forces on a tall building show clear variation with height and the largest local forces show similar values to most existing numerical models. Local forces increase with increasing turbulence intensity and are found to depend mainly on reference velocity Uref and moving velocity Umov. However, they collapse to one curve for the same normalized velocity Uref / Umov. The effects of reference radius and reference height are found to be small. Resultant fluctuating force of generalized forces obtained from the modified Rankine model is considered to be larger than those obtained from the proposed model. Fluctuating force increases as the integral length scale increases for the modified Rankine model, while they remain almost constant regardless of the integral length scale for the proposed model.

Key Words
tornado vortex; velocity components; radial profile; vertical profile; normal stress; aerodynamic force

Address
Yong Chul Kim:1Department of Architecture, Tokyo Polytechnic University, Atsugi, 2430297, Japan

Yukio Tamura:2School of Civil Engineering, Chongqing University, Chongqing, 400045, China

Abstract
A comprehensive knowledge of the wind flow in hilly terrains is of great interest in many engineering applications, be it wind energy distribution for suitable site selection for wind farms, pollution dispersion, forest fire propagation or agro-meterological studies. Several researchers have shown that wind flow over a hilly terrain may be significantly different when compared with the wind flow over a flat terrain. Complex hilly terrains may alter the wind speed to a great extent. Therefore, this effect of terrain must be properly assessed by designers and planners to arrive at a proper wind flow distribution. This paper reviews the work done in this area over the past three decades. Wind flow over two-dimensional hills and two-dimensional escarpments investigated in wind tunnels by various researchers is presented in this paper.

Key Words
wind flow; wind tunnel investigation; Hilly terrain; escarpments; Reynolds number

Address
Abdul Haseeb Wani:Department of Civil Engineering, Indian Institute of Technology Jammu, Jammu-181221, J&K, India

Rajendra K. Varma:Department of Civil Engineering, Indian Institute of Technology Jammu, Jammu-181221, J&K, India

Ashok K. Ahuja:Department of Civil Engineering, Indian Institute of Technology Jammu, Jammu-181221, J&K, India

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