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"Concrete construction Designs and plans."
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Fragility-Based Seismic Risk Assessment of Reinforced Concrete Bridge Columns
In earthquake-prone regions, predicting the impact of seismic events on highway bridges is crucial for post-earthquake effective emergency response and recovery planning. This paper presents a methodology for a simplified seismic risk assessment of bridges using fragility curves that integrates updated ductility ratios of reinforced concrete bridge columns from literature based on experimental results on cyclic tests of reinforced concrete circular columns. The methodology considers two damage states (cover spalling and bar buckling) for bridge columns with seismic and non-seismic design considerations and then estimates displacement thresholds for each damage state. The Damage Margin Ratio (DMR) is introduced as an index defined by the ratio of the median Peak Ground Acceleration (PGA) for a specific damage state to the PGA that corresponds to the target seismic hazard probability of exceedance in 50 years that is typically defined in bridge design and evaluation codes and standards. The DMR is then compared to a user-specified Threshold Damage Margin Ratio (TDMR) to evaluate the level of risk at a specific threshold probability of exceedance of the damage state (5% and 10%). Comparative assessment is conducted for the relative seismic risk and performance of non-seismic and seismic bridges corresponding to the seismic hazard values at 10% and 2% probability of exceedance in 50 years for 7 urban centers in the province of Quebec as a case study demonstration of the methodology. The proposed methodology offers a rapid tool for screening and prioritizing bridges for detailed seismic evaluation.
Journal Article
Mass Shotcrete Wall Construction and Thermal Control Plan
Wet-mix shotcrete has been used more and more for structural applications in the past few decades. Recently, wet-mix shotcrete was successfully used to construct a mass structural wall with congested reinforcement and minimum dimensions of 1.0 m in a sewage treatment plant. A low-heat shotcrete mixture that included up to 40% slag was proposed for shotcrete application. A preconstruction mockup was shot to established proper work procedures for shotcrete application and qualify the shotcrete mixture and shotcrete nozzlemen. Extraction of cores and cut windows from the mockup confirmed proper consolidation around the congested reinforcement. A thermal control plan was developed, which included laboratory and field testing requirements, thermal analysis modeling with a three-dimensional (3-D) finite element program, and thermal control requirements, including installation of cooling pipes and thermal blankets. Shotcrete proved to be an efficient means for mass concrete structural construction. Thermal control for mass shotcrete construction was studied, and the proposed thermal control plan was proved to function properly. The general guidance for mass shotcrete construction is provided. Keywords: compaction; consolidation; encapsulation; mass concrete; mass shotcrete; nozzleman; shotcrete; temperature differential; thermal control plan.
Journal Article
Research on the Reverse Order Demolition Scheme of Concrete Suspension Bridges Based on System Transformation Force Analysis
With the continuous development of the social economy and the increasing service life of bridges, in-service bridges generally face multiple challenges such as safety decline, durability deterioration, and insufficient traffic capacity. Demolition and reconstruction have become an important way for some old bridges to achieve functional renewal and ensure traffic safety. This paper takes the first concrete self-anchored suspension bridge in China that has undergone demolition—the Zicai Bridge in Qinzhou—as the specific engineering basis. In response to the safety requirements and smooth progress of its demolition construction, after a comprehensive comparison and optimization of multiple demolition schemes, the core technical solution of reverse sequence removal of the hangers was finally determined. To fully verify the technical applicability, structural safety, and feasibility of this demolition scheme, this study adopts a core research method combining theoretical calculation and numerical simulation, and systematically and deeply analyzes the entire process of bridge system transformation, the evolution law of structural force, and the mechanical responses of key parts during the hanger removal process. The study found that the maximum stress of the hangers in the system during the hanger removal process was much lower than the material breaking stress. The tilt of the bridge tower and the deformation of the main cables were all within the controllable range. Only the local tensile stress at the lower edge of the main beam had a cracking risk exceeding the material’s tensile limit. Based on this, specific construction optimization suggestions and control measures were proposed. This research not only solved the core technical problems of this type of special bridge demolition, but its research ideas and quantitative analysis results can also provide important theoretical references and technical support for the subsequent demolition construction of similar cable-bearing system bridges, and has positive significance for promoting the scientific and standardized development of complex bridge demolition construction.
Journal Article
Computational optimization of shear wall location in a C-shaped reinforced concrete framed building for enhanced seismic performance
Shear wall provide large strength and stiffness to buildings and hence reduces lateral displacement in high rise buildings. With huge inbuilt lateral strength, shear walls are very effective in resisting lateral loads originating from wind or earthquakes. But shear wall can be only useful if it is placed at a suitable location in the building. These locations can be defined by various parameters. In this study, a RCC framework is proposed to obtain the optimum location of shear wall for the ‘C’ shaped structure such that the torsional effect arising in the structure due to its plan irregularity is minimized. The selection of optimum location is done in a way so that it converges to the optimum structural and engineering demand parameters for a ‘C’ shaped structure with a fixed length of shear wall. A G + 15 story structure is considered for an illustrative example of the framework considering fourteen models of shear wall location for a ‘C’ shaped structure with a fixed length of shear wall (0.8% wall to floor area ratio). The analytical results of each model have been compared with that of bare frame model in terms of base shear, peak story displacement, peak story drift, static eccentricity, time period, and modal behavior, in order to determine the optimum model. A novel framework has been proposed to determine the optimal placement of shear walls. The initial selection criterion is based on a model with the first two modes of vibration governed by translation and torsion in the third mode. The structure with the optimal placement of shear walls has a close proximity between its center of mass and center of rigidity. Furthermore, it has been observed that the position of the shear wall has a significant impact on the time period and diaphragm displacement. This proposed framework is anticipated to provide valuable insights for the construction industry, both in India and internationally.
Journal Article
Research on the Deformation Control Measures during the Construction Period of Super High-Rise Buildings with an Asymmetric Plan
Based on the Guangzhou Business Center project, a typical super high-rise building with an asymmetric plan, taking the construction speed, closure time of mega braces and belt trusses as influencing factors, a parametric analysis on its lateral and vertical deformations, as well as the maximum stress of key structural members was conducted. The analysis results indicated that the construction speed had a relatively small impact on the deformation and the maximum stress of key members. However, synchronous closure of belt truss compared with the delayed closure would result in smaller horizontal and vertical deformation differences, as well as the stress of belt truss. Meanwhile, the closure timing of the mega braces had little influence on the vertical deformation difference and the stress of belt truss. And the earlier the closure, the smaller the horizontal drift ratio, the greater the maximum stress of the mega braces. Further, deformation control measurements were brought forward. On the one hand, FEM simulation was carried out according to the above construction suggestions. On the other hand, real-time monitoring was also used. Finally, by comparing both results, proposed construction deformation control measures and simulation methods were verified.
Journal Article
The Effect of Variation of Stories’ Plan Size along Height of Relatively Tall Steel Buildings on Their Seismic Behavior
In urban environments, in many cases, buildings with unequal story plan sizes are desired, resulting in non-uniform mass/stiffness distributions along the buildings’ height. In this study, three groups of 12-story steel buildings with uniform, ascending, and descending story plan size distribution (PSD) along their height, but having the same total architectural space, were investigated to determine how this distribution affects their seismic performance. First, three site conditions—stiff, medium, and soft—were considered, and all buildings were designed by considering moment frames with concentrical bracings as their lateral load-bearing systems. Then, for each site class, seven appropriate sets of accelerograms were selected, and using nonlinear time history analyses, the buildings’ seismic performance levels were compared based on the formation of plastic hinges (PHs). Results show that non-uniform PSD can be quite effective on the seismic performance of buildings’ structures, so that on hard and soft sites, the number of PHs exceeding the collapse prevention performance level in buildings with ascending and descending PSD, respectively, decreases by approximately 30–45% and 20–40% compared with the uniform buildings. Therefore, it is explicitly concluded that employing buildings with uniform plan sizes does not necessarily lead to a higher seismic performance level.
Journal Article
Accounting for Torsional Response in Direct Displacement-based Design of Plan-asymmetric Reinforced Concrete Frame Buildings
The torsional response of Reinforced Concrete (RC) frame buildings within the Direct Displacement-Based Design (DDBD) framework is considered by using semi-empirical equations developed for wall-type asymmetric structures. These equations when applied to RC frame buildings result in the underestimation of engineering demand parameters such as drifts. To alleviate this problem, it is proposed that first-mode effective masses of the frames are connected with rigid bar, constraining the system so that the critical frames’ design displacements are not exceeded and displacement compatibility for all frames is maintained. The proposed method of seismic design is verified by a Nonlinear Time History Analysis (NTHA) of two six-story plan-asymmetric RC frame buildings, the only difference in the buildings being that one is applied with equal mass and the other variable mass along the height. Eleven ground motions are selected and scaled to match a design response spectrum of a given seismic hazard level at the building site. The proposed design procedure produces reasonable response outputs when verified by NTHA of the case studies. Thus, it is a step toward minimizing the use of NTHA for plan-asymmetric frame buildings, thereby saving computational resources and effort.
Journal Article
The Effect of Lateral Load Type on Shear Lag of Concrete Tubular Structures with Different Plan Geometries
Tubular structures are extensively recognized as a high efficiency and economically reasonable structural system for the design and construction of skyscrapers. The periphery of the building plan in a tubular system consists of closely spaced columns connected by circumferential deep spandrels. When a cantilever tube is subjected to a lateral load, it is expected that the axial stress in each column located in the flange frame of the tube is the same, but because of the flexibility of peripheral beams, the axial stress in the corner columns and middle columns is distributed unequally. This anomaly is called “shear lag”, and it is a leading cause of the reduction in efficiency of the structure. In this paper, the possible relation between shear lag and the type of lateral load subjected to these systems is investigated. The above relation is not yet considered in previous literatures. Three various plan shapes including rectangular, triangular and hexagon were modeled, analyzed, designed and subjected to the earthquake and wind load, separately. Further work is carried out to compare the shear lag factor of these structures with distinct plan shapes against different types of lateral load. It is observed that all types of structures with various plan geometry subjected to the wind load had a greater amount of shear lag factor in comparison with structures subjected to the static and dynamic earthquake loads. In addition, shear lag in structures with the hexagon shaped plan was at the minimum.
Journal Article
The Franklin Avenue Bridge
Since the historic Franklin Avenue Bridge was built in 1923, it has served as a vital link between neighborhoods in Minneapolis and St. Paul. The 1050 ft (320 m) long, five-span, open-spandrel concrete arch structure is supported on readily accessible bedrock, and it efficiently spans the Mississippi River between the high bluffs on either side. The bridge's original spandrels, deck, and railings were replaced during a major renovation in 1970. By coupling ABC techniques with prefabricated bridge elements and systems (PBES) that included cap beams, deck panels, and ornamental rail panels, the replacement of the superstructure was completed during a 116-day closure. Planning is paramount to the success of ABC projects, and it requires that all contractual parties are brought to the table. Each operation must be worked out in fine detail; potential issues must be resolved, as much as possible; and contingency plans must be put in place to ensure construction continues when an operation does not proceed as planned.
Journal Article