• Steel and Composite Structures
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• v.42 no.4
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• pp.513-538
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• 2022

This paper reports on experiments addressing the buckling and collapse behavior of an innovative built-up cold-formed steel (CFS) columns. The built-up column consists of four individual CFS lipped channels, two of them placed back-to-back at the web using two self-drilling screw fasteners at specified spacing along the column length, while the other two channels were connected flange-to-flange using one self-drilling screw fastener at specified spacing along the column length. In total, 12 experimental tests are reported, covering a wide range of column lengths from stub to slender columns. The initial geometric imperfections and material properties were determined for all test specimens. The effect of screw spacing, load-versus axial shortening behaviour and buckling modes for different lengths and screw spacing were investigated. Nonlinear finite element (FE) models were also developed, which included material nonlinearities and initial geometric imperfections. The FE models were validated against the experimental results, both in terms of axial capacity and failure modes of built-up CFS columns. Furthermore, using the validated FE models, a parametric study was conducted which comprises 324 models to investigate the effect of screw fastener spacing, thicknesses and wide range of lengths on axial capacity of back-to-back and flange-to-flange built-up CFS channel sections. Using both the experimental and FE results, it is shown that design in accordance with the American Iron and Steel Institute (AISI) and Australia/New Zealand (AS/NZS) standards is slightly conservative by 6% on average, while determining the axial capacity of back-to-back and flange-to-flange built-up CFS channel sections.

• Steel and Composite Structures
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• v.42 no.3
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• pp.407-426
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• 2022

Superior to traditional welded studs, high strength friction-grip bolted shear connectors facilitate the assembling and demounting of the composite members, which maximizes the potential for efficiency in the construction and retrofitting of new and old structures respectively. Hence, it is necessary to investigate the structural properties of high strength friction-grip bolts used in steel concrete composite beams. By means of push-out tests, an experimental study was conducted on post-installed high strength friction-grip bolts, considering the effects of different bolt size, concrete strength, bolt tensile strength and bolt pretension. The test results showed that bolt shear fracture was the dominant failure mode of all specimens. Based on the load-slip curves, uplifting curves and bolt tensile force curves between the precast concrete slab and steel beam obtained by push-out tests, the anti-slip performance of steel-concrete interface and shear behavior of bolt shank were studied, including the quantitative analysis of anti-slip load, and anti-slip stiffness, frictional coefficient, shear stiffness of bolt shank and ultimate shear capacity. Meanwhile, the interfacial anti-slip stiffness and shear stiffness of bolt shank were defined reasonably. In addition, a total of 56 push-out finite element models verified by the experimental results were also developed, and used to conduct parametric analyses for investigating the shear behavior of high-strength bolted shear connectors in steel-concrete composite beams. Finally, on ground of the test results and finite element simulation analysis, a new design formula for predicting shear capacity was proposed by nonlinear fitting, considering the bolt diameter, concrete strength and bolt tensile strength. Comparison of the calculated value from proposed formula and test results given in the relevant references indicated that the proposed formulas can give a reasonable prediction.

• Computers and Concrete
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• v.28 no.5
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• pp.521-532
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• 2021

Present paper focuses on the modeling of size effect on the compressive strength of normal concrete with the application of Discrete Element Method (DEM). Test specimens with different size and shape were cast and uniaxial compressive strength test was performed on each sample. Five different concrete mixes were used, all belonging to a different normal strength concrete class (C20/25, C30/37, C35/45, C45/55, and C50/60). The numerical simulations were carried out by using the PFC 5 software, which applies rigid spheres and contacts between them to model the material. DEM modeling of size effect could be advantageous because the development of micro-cracks in the material can be observed and the failure mode can be visualized. The series of experiments were repeated with the model after calibration. The relationship of the parallel bond strength of the contacts and the laboratory compressive strength test was analyzed by aiming to determine a relation between the compressive strength and the bond strength of different sized models. An equation was derived based on Bazant's size effect law to estimate the parallel bond strength of differently sized specimens. The parameters of the equation were optimized based on measurement data using nonlinear least-squares method with SSE (sum of squared errors) objective function. The laboratory test results showed a good agreement with the literature data (compressive strength is decreasing with the increase of the size of the specimen regardless of the shape). The derived estimation models showed strong correlation with the measurement data. The results indicated that the size effect is stronger on concretes with lower strength class due to the higher level of inhomogeneity of the material. It was observed that size effect is more significant on cube specimens than on cylinder samples, which can be caused by the side ratios of the specimens and the size of the purely compressed zone. A limit value for the minimum size of DE model for cubes and cylinder was determined, above which the size effect on compressive strength can be neglected within the investigated size range. The relationship of model size (particle number) and computational time was analyzed and a method to decrease the computational time (number of iterations) of material genesis is proposed.

• Structural Engineering and Mechanics
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• v.83 no.5
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• pp.627-644
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• 2022

In past major earthquakes (1994 Northridge, 1995 Kobe, Chi-Chi 1999, Kocaeli 1999), significant damages occurred in the liquid storage tanks. The basic failure patterns were observed to be the buckling of the tank wall and uplift of the anchorage system. The damages in the industrial facilities and nuclear power plants have caused the spread of toxic substances to the environment and significant fires. Seismic isolation can be used in liquid storage tanks to decouple the structure and decrease the structural demand in the superstructure in case of ground shaking. Previous studies on the use of seismic isolation systems on liquid storage tanks show that an isolation system reduces the impulsive response but might slightly increase the convective one. There is still a lack of understanding of the seismic response of seismically isolated liquid storage tanks considering the fluid-structure interaction. In this study, one broad tank, one medium tank, and one slender tank are selected and designed. Two- and three-dimensional elastomeric bearings are used as seismic isolation systems. The seismic performance of the tanks is then investigated through nonlinear dynamic time-history analyses. The effectiveness of each seismic isolation system on tanks' performance was investigated. Isolator tension forces, modal analysis results, hydrodynamic stresses, strains, sloshing heights and base shear forces of the tanks are compared. The results show that the total base shear is lower in 3D-isolators compared to 2D-isolators. Even though the tank wall stresses, and strains are slightly higher in 3D-isolators, they are more efficient to prevent the tension problem.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.20 no.1
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• pp.30-36
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• 1984

It is well known that the application of linear elastic fracture mechanics is inadequate to solve the large deformation fracture failures which occurr in ductile manner because of the large scale yielding due to the severe stress concentration in the region adjacent to the crack tip. The authors have been evolved a fracture model, the crack plane equilibrium model, for this kinds of elastic-plastic fracture problems in the previous report. In this report, the crack plane equilibrium model was compared with the Irwin's plastic zone corrected linear elastic fracture mechanics through theoretical comparisons and experimental results to examine the validity of the crack plane equilibrium model as an available tool for nonlinear fracture analysis. Through this study, the main results were reached as follows; Irwin's plastic zone corrected linear elastic fracture mechanics could be applicable only for small scale yielding problems as expected while the crack plane equilibrium model valid as a fracture model for large deformation fracture failure. However, the followings should be considered for the more precise evaluations of CPE model; 1) It is necessary to test more specimens which contain small cracks in the range of 2a/W<0.1. 2) It is important to detect the crack initiation point during the fracture test for determining an accurate fracture load. 3) Effects of specimen thickness in the fracture process zone should be examined.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.1A
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• pp.21-33
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• 2006

The reliability analysis can be conducted more effectively by formulating the stochastic finite element method suitable for the reliability theory about the cable stayed bridge. After conducting the initial equilibrium analysis of the cable stayed bridge, the program which can conduct the linear and nonlinear stochastic finite element analysis using the perturbation method and the reliability analysis considered to the correlation of the random variable is developed. Using the results of this program about the cable stayed bridge, the characteristic of the node displacement, element force and cable tension according to the correlation of the random variable is investigated quantitatively. Also the reliability index and the failure probability are examined by the compounding the correlation of the random variable.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.6A
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• pp.577-585
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• 2009

A 9-node degenerated shell finite element(FE), which has been developed for assessment of ultimate pressure capacity and nonlinear analysis for nuclear containment building is described in this paper. Reissner-Midnlin(RM) assumptions are adopted to develop the shell FE so that transverse shear deformation effects is considered. Material model for concrete prior to cracking is constructed based on the equivalent stress-equivalent strain relationship. Tension stiffening model, shear transfer mechanism and compressive strength reduction model are used to model the material behavior of concrete after cracking. Niwa and Aoyagi-Yamada failure criteria have been adapted to find initial cracking point in compression-tension and tension-tension region, respectively. Finally, the performance of the developed program is tested and demonstrated with several examples. From the numerical tests, the present results show a good agreement with experimental data or other numerical results.

• Journal of the Earthquake Engineering Society of Korea
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• v.28 no.2
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• pp.77-83
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• 2024

The design shear strength equations of RC shear walls have been developed based on their performance under in-plane (IP) loads, thereby failing to account for the potential performance degradation of shear strength when subjected to simultaneous out-of-plane (OOP) loading. Most of the previous experimental studies on RC walls have been conducted in one direction under quasi-static conditions, and due to the difficulty in experimental planning, there is a lack of research on cyclic loading and results under multi-axial loading conditions. During an earthquake, shear walls may yield earlier than their design strength or fail unexpectedly when subjected to multi-directional forces, deviating from their intended failure mode. In this paper, nonlinear analysis in finite element models was performed based on the results of cyclic loading experiments on reinforced concrete shear walls of auxiliary buildings. To investigate the reduction trend in IP shear capacity concerning the OOP load ratio, parametric analysis was conducted using the shear wall FEM. The analysis results showed that as the magnitude of the OOP load increased, the IP strength decreased, with a more significant effect observed as the size of the opening increased. Thus, the necessity to incorporate this strength reduction as a factor for the OOP load effect in the wall design strength equation should be discussed by performing various parametric studies.

• Steel and Composite Structures
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• v.51 no.6
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• pp.661-678
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• 2024

The utilization of geopolymer recycled aggregate concrete (GRAC) as the infilled core of the concrete-filled steel tubular (CFST) columns provides superior economic and environmental benefits. However, limited research exists within the field of geopolymer recycled aggregate concrete considered a green and sustainable material, in addition to the limitation of the design guidelines to predict the behavior of such an innovative new material combination. Moreover, the behavior of high-strength concrete is different from the normal-strength one, especially when there is another material of high-strength properties, such as the steel tube. This paper aims to investigate the behavior of the axially loaded square high-strength GRACFST columns through the nonlinear finite element analysis (NLFEA). A total of thirty-two specimens were simulated using ABAQUS/Standard software with three main variables: recycled aggregate replacement ratio (0, 30, and 50) %, width-to-thickness ratios (52.0, 32.0, 23.4, and 18.7), and length-to-width ratio (3, 5, 9, and 12). During the analysis, the response in terms of the axial load versus the longitudinal strain was recorded and plotted. In addition, various mechanical properties were calculated and analyzed. In view of the results, it has been demonstrated that the mechanical properties of high-strength GRACFST columns such as ultimate load-bearing capacity, compressive stiffness, energy absorption capacity, and ductility increase with the increase of the steel tube thickness owing to the improvement of the confinement effect of the steel tube. In contrast, the incorporation of the recycled aggregate adversely affected the mentioned properties except the ductility, while the increase of the recycled aggregate replacement ratio improved the column's ductility. Moreover, it has been found that the increase in the length-to-width ratio significantly reduced both the failure strain and the energy absorption capacity. Finally, the obtained NLFEA results of the ultimate load-bearing capacity were compared with the corresponding predicted capacities by numerous codes. It has been concluded that AISC, ACI, and EC give conservative predictions for the ultimate load-bearing capacity since the confinement effect was not considered by these codes.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.32 no.3
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• pp.147-160
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• 2020

A stochastic probabilistic model for harbor structures such as rubble-mound breakwater has been formulated by using the generalized Wiener process considering the nonlinearity of damage drift and its nonlinear uncertainty, by which the damage path with real-time can be tracked, the residual useful lifetime at some age can also be analyzed properly. The formulated stochastic model can easily calculate the probability of failure with the passage of time through the probability density function of cumulative damage. In particular, the probability density functions of residual useful lifetime of the existing harbor structures can be derived, which can take into account the current age, its present damage state and the future damage process to be occurred. By using the maximum likelihood method and the least square method together, the involved parameters in the stochastic model can be estimated. In the calibration of the stochastic model presented in this paper, the present results are very well similar with the results of MCS about tracking of the damage paths as well as evaluating of the density functions of the cumulative damage and the residual useful lifetime. MTTF and MRL are also evaluated exactly. Meanwhile, the stochastic probabilistic model has been applied to the rubble-mound breakwater. The related parameters can be estimated by using the experimental data of the cumulative damages of armor units measured as a function of time. The theoretical results about the probability density function of cumulative damage and the probability of failure are very well agreed with MCS results such that the density functions of the cumulative damage tend to move to rightward and the amounts of its uncertainty are increased as the elapsed time goes on. Thus, the probabilities of failure with the elapsed time are also increased sharply. Finally, the behaviors of residual useful lifetime have been investigated with the elapsed age. It is concluded for rubble-mound breakwaters that the probability density functions of residual useful lifetime tends to have a longer tail in the right side rather than the left side because of the gradual increases of cumulative damage of armor units. Therefore, its MRLs are sharply decreased after some age. In this paper, the special attentions are paid to the relationship of MTTF and MRL and the elapsed age of the existing structure. In spite of that the sum of the elapsed age and MRL must be equal to MTTF deterministically, the large difference has been shown as the elapsed age is increased which is due to the uncertainty of cumulative damage to be occurred in the future.