• Proceedings of the Korea Concrete Institute Conference
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• 2008.11a
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• pp.221-224
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• 2008

A secant stiffness linear analysis method was developed for moment redistribution of moment-resisting frames. In the proposed method, rotational spring models are used for plastic hinges of the members whose flexural moments are needed to be redistributed. At the plastic hinges, secant stiffness is used to address the effect of the flexural stiffness reduced by inelastic deformation. Linear analysis is repeated with adjusted secant stiffness until the flexural equilibrium is satisfied in the structure and members. By using the secant stiffness analysis, the effect of the inelastic deformation on the moment redistribution can be considered. Further, the safety of plastic hinges can be evaluated by comparing the inelastic rotation resulting from the secant stiffness analysis with the rotational capacity of the plastic hinges. For verification, the proposed method was applied to a continuous beam tested in previous study. A application example for a multiple story moment-resisting frame was presented.

• Steel and Composite Structures
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• v.2 no.1
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• pp.21-34
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• 2002

A model of the process of local buckling in tubular steel structural elements is presented. It is assumed that this degrading phenomenon can be lumped at plastic hinges. The model is therefore based on the concept of plastic hinge combined with the methods of continuum damage mechanics. The state of this new kind of inelastic hinge is characterized by two internal variables: the plastic rotation and the damage. The model is valid if only one local buckling appears in the plastic hinge region; for instance, in the case of framed structures subjected to monotonic loadings. Based on this damage model, a new finite element that can describe the development of local buckling is proposed. The element is the assemblage of an elastic beamcolumn and two inelastic hinges at its ends. The stiffness matrix, that depends on the level of damage, the yielding function and the damage evolution law of the two hinges define the new finite element. In order to verify model and finite element, several small-scale frames were tested in laboratory under monotonic loading. A lateral load at the top of the frame was applied in a stroke-controlled mode until local buckling appears and develops in several locations of the frame and its ultimate capacity was reached. These tests were simulated with the new finite element and comparison between model and test is presented and discussed.

• Computers and Concrete
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• v.6 no.4
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• pp.319-341
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• 2009

A new formulation based on lumped plasticity and inelastic hinges is presented in this paper for nonlinear analysis of Reinforced Concrete (RC) frame structures. Inelastic hinge behaviour is described using the principles of Continuum Damage Mechanics (CDM). Member formulation contains provisions to model stiffness degradation due to cracking of concrete and yielding of reinforcing steel. Depending on its nature, cracking is classified as concentrated or distributed. Concentrated cracking is accounted through a damage variable and its growth is defined based on strain energy principles. Presence of distributed flexural cracks in a member is taken care of by modelling it as non-prismatic. Plasticity theory supported by effective stress concept of CDM is applied to describe the post-yield response. Nonlinear quasi-static analysis is carried out on a RC column and a wide two-storey RC frame to verify the formulation. The column is subjected to constant axial load and monotonic lateral load while the frame is subjected to only lateral load. Computed results are compared with those due to experiments or other numerical methods to validate the performance of the formulation and also to highlight the contribution of distributed cracking on global response.

• Structural Engineering and Mechanics
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• v.26 no.5
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• pp.569-589
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• 2007

This paper presents an analytical model for RC beam-column connections that takes into account bond deterioration between reinforcing steel and concrete. The model is based on the Lumped Damage Mechanics (LDM) theory which allows for the characterization of cracking, degradation and yielding, and is extended in this paper by the inclusion of the slip effect as observed in those connections. Slip is assumed to be lumped at inelastic hinges. Thus, the concept of "slip hinge", based on the Coulomb friction plasticity theory, is formulated. The influence of cracking on the slip behavior is taken into account by using two concepts of LDM: the effective moment on an inelastic hinge and the strain equivalence hypothesis. The model is particularly suitable for wide beam-column connections for which bond deterioration dominates the hysteretic response. The model was evaluated by the numerical simulation of five tests reported in the literature. It is found that the model reproduces closely the observed behavior.

• Earthquakes and Structures
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• v.7 no.4
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• pp.587-607
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• 2014

Progressive collapse, which is referred to as the collapse of the entire building under local damages, is a common failure mode happened by earthquakes. The collapse process highly depends on the whole structural system. Since, asymmetry of the building plan leads to the local damage concentration; it may intensify the progressive collapse mechanism of asymmetric buildings. In this research the progressive collapse of regular and irregular 6-story RC ordinary moment resisting frame buildings are studied in the presence of the earthquake loads. Collapse process and collapse propagation are investigated using nonlinear time history analyses (NLTHA) in buildings with 5%, 15% and 25% mass asymmetry with respect to the number of collapsed hinges and story drifts criteria. Results show that increasing the value of mass eccentricity makes the asymmetric buildings become unstable earlier and in the early stages with lower number of the collapsed hinges. So, with increasing the mass eccentricity in building, instability and collapse of the entire building occurs earlier, with lower potential of the progressive collapse. It is also demonstrated that with increasing the mass asymmetry the decreasing trend of the number of collapsed beam and column hinges is approximately similar to the decreasing trend in the average story drifts of the mass centers and stiff edges. So, as an alternative to a much difficult-to-calculate local response parameter of the number of collapsed hinges, the story drift, as a global response parameter, measures the potential of progressive collapse more easily.

• Structural Engineering and Mechanics
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• v.39 no.1
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• pp.125-145
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• 2011

Pushover analysis has gained significant popularity as an analytical tool for realistic determination of the inelastic behaviour of RC structures. Though significant work has been done to evaluate the demands realistically, the evaluation of capacity and realistic failure modes has taken a back seat. In order to throw light on the inelastic behaviour and capacity evaluation for the RC framed structures, a 3D Reinforced concrete frame structure was tested under monotonically increasing lateral pushover loads, in a parabolic pattern, till failure. The structure consisted of three storeys and had 2 bays along the two orthogonal directions. The structure was gradually pushed in small increments of load and the corresponding displacements were monitored continuously, leading to a pushover curve for the structure as a result of the test along with other relevant information such as strains on reinforcement bars at critical locations, failure modes etc. The major failure modes were observed as flexural failure of beams and columns, torsional failure of transverse beams and joint shear failure. The analysis of the structure was by considering all these failure modes. In order to have a comparison, the analysis was performed as three different cases. In one case, only the flexural hinges were modelled for critical locations in beams and columns; in second the torsional hinges for transverse beams were included in the analysis and in the third case, joint shear hinges were also included in the analysis. It is shown that modelling and capturing all the failure modes is practically possible and such an analysis can provide the realistic insight into the behaviour of the structure.

• Structural Engineering and Mechanics
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• v.63 no.4
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• pp.521-536
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• 2017

Reinforced concrete core-wall structures with buckling-restrained brace outriggers are interesting systems which have the ability to absorb and dissipate energy during strong earthquakes. Outriggers can change the energy demand in a tall building. In this paper, the energy demand was studied by using the nonlinear time history analysis for the mentioned systems. First, the structures were designed according to the prescriptive codes. In the dynamic analysis, three approaches for the core-wall were investigated: single plastic hinge (SPH), three plastic hinge (TPH) and extended plastic hinge (EPH). For SPH approach, only one plastic hinge is allowed at the core-wall base. For TPH approach, three plastic hinges are allowed, one at the base and two others at the upper levels. For EPH approach, the plasticity can extend anywhere in the wall. The kinetic, elastic strain, inelastic and damping energy demand subjected to forward directivity near-fault and ordinary far-fault earthquakes were studied. In SPH approach for all near-fault and far-fault events, on average, more than 65 percent of inelastic energy is absorbed by buckling-restrained braces in outrigger. While in TPH and EPH approaches, outrigger contribution to inelastic energy demand is reduced. The contribution of outrigger to inelastic energy absorption for the TPH and EPH approaches does not differ significantly. The values are approximately 25 and 30 percent, respectively.

• Proceedings of the Korea Concrete Institute Conference
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• 1996.10a
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• pp.466-472
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• 1996

The nonlinear analysis was perforned for a 2-bay 2-story moment-resisting reinforced concrete plane frame with seismic detail using KDARC 2D program. The analytical models consist of the material model, the member model, the hysteretic model, and the damage model etc. The conclusion based on the results of analysis is as following. : (1) Story shear-displacement relationship is similar to the experiment result but from the energy point of view, the analysis relationship is similar to the experiment result but from the energy point of view, the analysis result was different from the experiment result. (2) Plastic hinges were found to occur mainly in beams at first story while all the columns had plastic hinges throughout the structure. (3) Failure mode is a little different from experiment result in the yielding mechanism. (4) Damage index isabout 0.25. This means that the degree of damage is moderate and can be repairable.

• Journal of Korean Society of Steel Construction
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• v.17 no.1 s.74
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• pp.103-113
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• 2005

In the present study, the Direct Inelastic Design (DID) for steel structures developed in the previous study was improved to expand it applicability. The proposed design method can perform inelastic designs that address the design characteristics of steel structures: Group member design, discrete member sizes, variation of moment-carrying capacity according to axial force, connection types, and multiple design criteria and load conditions. The design procedure for the proposed method was established, and a computer program incorporating the design procedure was developed. The design results from the conventional elastic method and the DID were compared and verified by the existing computer program for nonlinear analysis. Compared with the conventional elastic design, the DID addressing the inelastic behavior reduced the total weight of steel members and enhanced the deformability of the structure. The proposed design method is convenient because it can directly perform inelastic design by using linear analysis for secant stiffness. Also, it can achieve structural safety and economical design by controlling deformations of the plastic hinges.

• Steel and Composite Structures
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• v.29 no.2
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• pp.175-186
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• 2018

The plastic hinge method and the plastic zone method are extensively adopted in displacement-based elements and force-based elements respectively for second-order inelastic analysis. The former enhances the computational efficiency with relatively less accurate results while the latter precisely predicts the structural behavior but generally requires more computer time. The displacement-based elements receive criticism mainly on plasticity dominated problems not only in accuracy but also in longer computer time to redistribute the forces due to formation of plastic hinges. The multi-element-per-member model relieves this problem to some extent but will induce a new problem in modeling of member initial imperfections required in design codes for direct analysis. On the contrary, a force-based element with several integration points is sufficient for material yielding. However, use of more integration points or elements associated with fiber section reduces computational efficiency. In this paper, a new force-based element equipped with stress-resultant plasticity model with minimal computational cost is proposed for second-order inelastic analysis. This element is able to take the member initial bowing into account such that one-element-per-member model is adequate and complied with the codified requirements of direct analysis. This innovative solution is new and practical for routine design. Finally, several examples demonstrate the validity and accuracy of the proposed method.