• Computers and Concrete
• /
• 제12권6호
• /
• pp.803-818
• /
• 2013

In this study, analytical and experimental modal analyses of a scaled bridge model are carried out to extract the dynamic characteristics such as natural frequency, mode shapes and damping ratios. For this purpose, a scaled bridge model is constructed in laboratory conditions. Three dimensional finite element model of the bridge is constituted and dynamic characteristics are determined, analytically. To identify the dynamic characteristics experimentally; Experimental Modal Analyses (ambient and forced vibration tests) are conducted to the bridge model. In the ambient vibration tests, natural excitations are provided and the response of the bridge model is measured. Sensitivity accelerometers are placed to collect signals from the measurements. The signals collected from the tests are processed by Operational Modal Analysis; and the dynamic characteristics of the bridge model are estimated using Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification methods. In the forced vibration tests, excitation of the bridge model is induced by an impact hammer and the frequency response functions are obtained. From the finite element analyses, a total of 8 natural frequencies are attained between 28.33 and 313.5 Hz. Considering the first eight mode shapes, these modes can be classified into longitudinal, transverse and vertical modes. It is seen that the dynamic characteristics obtained from the ambient and forced vibration tests are close to each other. It can be stated that the both of Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification methods are very useful to identify the dynamic characteristics of the bridge model. The first eight natural frequencies are obtained from experimental measurements between 25.00-299.5 Hz. In addition, the dynamic characteristics obtained from the finite element analyses have a good correlation with experimental frequencies and mode shapes. The MAC values obtained between 90-100% and 80-100% using experimental results and experimental-analytical results, respectively.

• Earthquakes and Structures
• /
• 제14권4호
• /
• pp.349-360
• /
• 2018

Reinforced concrete (RC) moment frames supported on two ground levels have been widely constructed in mountainous areas with medium to high seismicity in China. In order to investigate the seismic collapse behavior and risk, a scaled frame model was tested under constant axial load and reversed cyclic lateral load. Test results show that the failure can be induced by the development of story yielding at the first story above the upper ground. The strong column and weak beam mechanism can be well realized at stories below the upper ground. Numerical analysis model was developed and calibrated with the test results. Three pairs of six case study buildings considering various structural configurations were designed and analyzed, showing similar dynamic characteristics between frames on two ground levels and flat ground of each pair. Incremental dynamic analyses (IDA) were then conducted to obtain the seismic collapse fragility curves and collapse margin ratios of nine analysis cases designated based on the case study buildings, considering amplification of earthquake effect and strengthening measures. Analysis results indicate that the seismic collapse safety is mainly determined by the stories above the upper ground. The most probable collapse mechanism may be induced by the story yielding of the bottom story on the upper ground level. The use of tie beam and column strengthening can effectively enhance the seismic collapse safety of frames on two ground levels.

• Computers and Concrete
• /
• 제19권1호
• /
• pp.7-17
• /
• 2017

In Chinese Design Codes, for super high-rise buildings with complex structural distribution, which are regarded as code-exceeding buildings, elasto-plastic time history analysis is needed to validate the requirement of "no collapse under rare earthquake". In this paper, a 117-story super high-rise building is discussed. It has a height of 597 m and a height-width ratio of 9.5, which have both exceeded the limitations stipulated by the Chinese Design Codes. Mega columns adopted in this structure have cross section area of about $45m^2$ at the bottom, which is infrequent in practical projects. NosaCAD and Perform-3D, both widely used in nonlinear analyses, were chosen in this study, with which two model were established and analyzed, respectively. Elasto-plastic time history analysis was conducted to look into its seismic behavior, emphasizing on the stress state and deformation abilities under intensive seismic excitation.From the comparisons on the results under rare earthquake obtained from NosaCAD and Perform-3D, the overall responses such as roof displacement, inter story drift, base shear and damage pattern of the whole structure from each software show agreement to an extent. Besides, the deformation of the structure is below the limitation of the Chinese Codes, the time sequence and distribution of damages on core tubes are reasonable, and can dissipate certain inputted energy, which indicates that the structure can meet the requirement of "no collapse under rare earthquake".

• Earthquakes and Structures
• /
• 제15권5호
• /
• pp.499-511
• /
• 2018

This paper aims to investigate the seismic performance of the prestressed steel strips retrofitted RC beam-column joints. Two series of joint specimens were conducted under compression load and reversed cyclic loading through quasi-static tests. Based on the test results, the seismic behavior of the strengthened joints specimens in terms of the failure modes, hysteresis response, bearing capacity, ductility, stiffness degradation, energy dissipation performance and damage level were focused. Moreover, the effects of the amount of the prestressed steel strips and the axial compression ratio on seismic performance of retrofitted specimens were analyzed. It was shown that the prestressed steel strips retrofitting method could significantly improve the seismic behavior of the RC joint because of the large confinement provided by prestressed steel strips in beam-column joints. The decrease of the spacing and the increase of the layer number of the prestressed steel strips could result in a better seismic performance of the retrofitted joint specimens. Moreover, increasing the axial compression ration could enhance the peak load, stiffness and the energy performance of the joint specimens. Furthermore, by comparison with the specimens reinforced with CFRP sheets, the specimens reinforced with prestressed steel strips was slightly better in seismic performance and cost-saving in material and labor. Therefore, this prestressed steel strips retrofitting method is quite helpful to enhance the seismic behavior of the RC beam-column joints with reducing the cost and engineering time.

• Earthquakes and Structures
• /
• 제15권5호
• /
• pp.529-540
• /
• 2018

The influences of initial damage paths and aftershock (AS) spectral shape on the assessment of AS collapse fragility are investigated. To do this, a four-story ductile reinforced concrete (RC) frame structure is employed as the study case. The far-field earthquake records recommended by FEMA P695 are used as AS ground motions. The AS incremental dynamic analyses are performed for the damaged structure. To examine the effect of initial damage paths, a total of six kinds of initial damage paths are adopted to simulate different initial damage states of the structure by pushover analysis and dynamic analysis. For the pushover-based initial damage paths, the structure is "pushed" using either uniform or triangle lateral load pattern to a specified damage state quantified by the maximum inter-story drift ratio. Among the dynamic initial damage paths, one single mainshock ground motion or a suite of mainshock ground motions are used in the incremental dynamic analyses to generate a specified initial damage state to the structure. The results show that the structure collapse capacity is reduced as the increase of initial damage, and the initial damage paths show a significant effect on the calculated collapse capacities of the damaged structure (especially at severe damage states). To account for the effect of AS spectral shape, the AS collapse fragility can be adjusted at different target values of ${\varepsilon}$ by using the linear correlation model between the collapse capacity (in term of spectral intensity) and the AS ${\varepsilon}$ values, and coefficients of this linear model is found to be associated with the initial damage states.

• Steel and Composite Structures
• /
• 제36권2호
• /
• pp.163-177
• /
• 2020

Concentrically Braced Frames (CBFs) are commonly used in the construction of steel structures because of their ease of implementation, rigidity, low lateral displacement, and cost-effectiveness. However, the principal disadvantage of this kind of braced frame is the inability to provide deformation capacity (ductility) and buckling of bracing elements before yielding. This paper aims to present a novel Composite Buckling Restrained Fuse (CBRF) to be utilized as a bracing segment in concentrically braced frames that allows higher ductility and removes premature buckling. The proposed CBRF with relatively small dimensions is an enhancement on the Reduced Length Buckling Restrained Braces (RL-BRBs), consists of steel core and additional tensile elements embedded in a concrete encasement. Employing tensile elements in this composite fuse with a new configuration enhances the energy dissipation efficiency and removes the tensile strength limitations that exist in bracing elements that contain RL-BRBs. Here, the optimal length of the CBRF is computed by considering the anticipated strain demand and the low-cyclic fatigue life of the core under standard loading protocol. An experimental program is conducted to explore the seismic behavior of the suggested CBRF compare with an RL-BRB specimen under gradually increased cyclic loading. Moreover, Hysteretic responses of the specimens are evaluated to calculate the design parameters such as energy dissipation potential, strength adjustment factors, and equivalent viscous damping. The findings show that the suggested fuse possess a ductile behavior with high energy absorption and sufficient resistance and a reasonably stable hysteresis response under compression and tension.

• Computers and Concrete
• /
• 제26권2호
• /
• pp.185-201
• /
• 2020

This research is devoted to investigate the bending and free vibration behaviour of laminated composite/sandwich plates and shells, by applying an analytical model based on a generalized and simple refined higher-order shear deformation theory (RHSDT) with four independent unknown variables. The kinematics of the proposed theoretical model is defined by an undetermined integral component and uses the hyperbolic shape function to include the effects of the transverse shear stresses through the plate/shell thickness; hence a shear correction factor is not required. The governing differential equations and associated boundary conditions are derived by employing the principle of virtual work and solved via Navier-type analytical procedure. To verify the validity and applicability of the present refined theory, some numerical results related to displacements, stresses and fundamental frequencies of simply supported laminated composite/sandwich plates and shells are presented and compared with those obtained by other shear deformation models considered in this paper. From the analysis, it can be concluded that the kinematics based on the undetermined integral component is very efficient, and its use leads to reach higher accuracy than conventional models in the study of laminated plates and shells.

• Computers and Concrete
• /
• 제26권3호
• /
• pp.213-226
• /
• 2020

The present study covenants with the static and free vibration behavior of nanocomposite sandwich plates reinforced by carbon nanotubes resting on Pasternak elastic foundation. Uniformly distributed (UD-CNT) and functionally graded (FG-CNT) distributions of aligned carbon nanotube are considered for two types of sandwich plates such as, the face sheet reinforced and homogeneous core and the homogeneous face sheet and reinforced core. Based on the first shear deformation theory (FSDT), the Hamilton's principle is employed to derive the mathematical models. The obtained solutions are numerically validated by comparison with some available cases in the literature. The elastic foundation model is assumed as one parameter Winkler - Pasternak foundation. A parametric study is conducted to study the effects of aspect ratios, foundation parameters, carbon nanotube volume fraction, types of reinforcement, core-to-face sheet thickness ratio and types of loads acting on the bending and free vibration analyses. It is explicitly shown that the (FG-CNT) face sheet reinforced sandwich plate has a high resistance against deflections compared to other types of reinforcement. It is also revealed that the reduction in the dimensionless natural frequency is most pronounced in core reinforced sandwich plate.

• Advances in concrete construction
• /
• 제10권4호
• /
• pp.345-355
• /
• 2020

In this paper, vibration characteristics of double-walled carbon nanotubes (CNTs) is studied based upon nonlocal elastic shell theory. The significance of small scale is being perceived by developing nonlocal Love shell model. The wave propagation approach has been utilized to frame the governing equations as eigen value system. The influence of nonlocal parameter subjected to diverse end supports has been overtly analyzed. An appropriate selection of material properties and nonlocal parameter has been considered. The influence of changing mechanical parameter Poisson's ratio has been investigated in detail. The dominance of boundary conditions via nonlocal parameter is shown graphically. The results generated furnish the evidence regarding applicability of nonlocal shell model and also verified by earlier published literature.

• Advances in concrete construction
• /
• 제10권4호
• /
• pp.357-367
• /
• 2020

The impulse of this paper is to examine the influence of unsteady flow comprising of Eyring-Powell nanofluid over a stretched surface. This work aims to explore efficient transfer of heat in Eyring-Powell nanofluid with bio-convection. Nanofluids possess significant features that have aroused various investigators because of their utilization in industrial and nanotechnology. The influence of including motile microorganism is to stabilize the nanoparticle suspensions develop by the mixed influence of magnetic field and buoyancy force. This research paper reveals the detailed information about the linearly compressed Magnetohydrodynamics boundary layer flux of two dimensional Eyring-Powell nanofluid through disposed surface area due to the existence of microorganism with inclusion the influence of non- linear thermal radiation, energy activation and bio-convection. The liquid is likely to allow conduction and thickness of the liquid is supposed to show variation exponentially. By using appropriate similarity type transforms, the nonlinear PDE's are converted into dimensionless ODE's. The results of ODE's are finally concluded by employing (HAM) Homotopy Analysis approach. The influence of relevant parameters on concentration, temperature, velocity and motile microorganism density are studied by the use of graphs and tables. We acquire skin friction, local Nusselt and motil microorganism number for various parameters.