• Structural Engineering and Mechanics
• /
• 제60권1호
• /
• pp.51-69
• /
• 2016

A large number of welded steel moment-resisting framed (SMRF) structures failed due to brittle fracture induced by ductile fracture at beam-to-column connections during 1994 Northridge earthquake and 1995 Kobe (Hyogoken-Nanbu) earthquake. Extensive research efforts have been devoted to clarifying the mechanism of the observed failures and corresponding countermeasures to ensure more ductile design of welded SMRF structures, while limited research on the failure analysis of the ductile cracking was conducted due to lack of computational capacity and proper theoretical models. As the first step to solve this complicated problem, this paper aims to establish a straightforward procedure to simulate ductile cracking of welded joints under monotonic tension. There are two difficulties in achieving the aim of this study, including measurement of true stress-true strain data and ductile fracture parameters of different subzones in a welded joint, such as weld deposit, heat affected zone and the boundary between the two. Butt joints are employed in this study for their simple configuration. Both experimental and numerical studies on two types of butt joints are conducted. The validity of the proposed procedure is proved by comparison between the experimental and numerical results.

• Steel and Composite Structures
• /
• 제8권1호
• /
• pp.53-83
• /
• 2008

A rational and efficient seismic design methodology for irregular space steel frames using advanced methods of analysis in the framework of Eurocodes 8 and 3 is presented. This design methodology employs an advanced static or dynamic finite element method of analysis that takes into account geometrical and material non-linearities and member and frame imperfections. The inelastic static analysis (pushover) is employed with multimodal load along the height of the building combining the first few modes. The inelastic dynamic method in the time domain is employed with accelerograms taken from real earthquakes scaled so as to be compatible with the elastic design spectrum of Eurocode 8. The design procedure starts with assumed member sections, continues with the checking of the damage and ultimate limit states requirements, the serviceability requirements and ends with the adjustment of member sizes. Thus it can sufficiently capture the limit states of displacements, rotations, strength, stability and damage of the structure and its individual members so that separate member capacity checks through the interaction equations of Eurocode 3 or the usage of the conservative and crude q-factor suggested in Eurocode 8 are not required. Two numerical examples dealing with the seismic design of irregular space steel moment resisting frames are presented to illustrate the proposed method and demonstrate its advantages. The first considers a seven storey geometrically regular frame with in-plan eccentricities, while the second a six storey frame with a setback.

• Earthquakes and Structures
• /
• 제19권5호
• /
• pp.315-329
• /
• 2020

Through bolt connections in Concrete Filled Steel Tubes (CFSTs) has been proved to be good in terms of seismic performance and constructability. Stiffened extended end plate connection with full through type bolt helps to avoid field weld altogether, and hence to improve the quality of joints. An experimental study was conducted on the hysteretic performance of square interior beam-column connections using flat extended end plates with through bolt. The study focuses on the effect of the bond between the tie rod and the core concrete on the cyclic performance of the joint. The study also quantifies how much the interior joint is getting strengthened due to the confinement effect induced by bi-directional bolting, which is widely used in 3D moment resisting frames. For a better understanding of the mechanism and for the prediction of shear capacity of the panel zone, a mathematical model was generated. The various parameters included in the model are the influence of axial load, amount of prestress induced by bolt tightening, anchorage, and the concrete strut action. The study investigates the strength, stiffness, ductility, and energy dissipation characteristics. The results indicate that the seismic resistance is at par with American Institute of Steel Construction (AISC) seismic recommendations. The bidirectional bolting and bond effect have got remarkable influence on the performance of joints.

• 한국지진공학회논문집
• /
• 제20권7_spc호
• /
• pp.553-560
• /
• 2016

After an earthquake occurred in the Gyeongju, 2016, many low-story buildings have been questioned in terms of the seismic performance since mostly they have been exempted from the seismic design requirement since 1988. In this study, a 3-story moment resisting frame (MRF) building was analyzed and evaluated the seismic performance. Due to the insufficient seismic performance required for the seismic performance levels, three different seismic retrofit schemes were proposed and their seismic performances were re-evaluated. While steel brace and open shear wall retrofit systems mainly focused on the strength retrofit, the VES damper retrofit system is mainly to enhance the energy dissipation capacity of the system and resultes in the increased ductility. The original building and 3 retrofitted buildings were evaluated using the nonlinear static and nonlinear dynamic analyses and suggestions were proposed. Through the analysis of nonlinear time history and push-over using MIDAS/Gen program, damages of the building in terms of top story and average story drift and effect of reinforcement were analyzed.

• Structural Engineering and Mechanics
• /
• 제14권1호
• /
• pp.21-38
• /
• 2002

A control algorithm combining viscous and non-linear Reid damping mechanisms has been recently proposed by the authors to command active friction dampers. In this paper, friction dampers and the proposed algorithm are applied to control the seismic responses of a nonlinear 20-story building. Piezoelectric stack actuators are used to implement the control algorithm. The capacity of each damper is determined by the practical size of piezoelectric actuators and the availability of power supply. The saturation effect of the actuators on the building responses is investigated. To minimize the peak story drift ratio or floor acceleration of the building structure, a practical sequential procedure is developed to sub-optimally place the dampers on various floors. The effectiveness of active friction dampers and the efficiency of the proposed sequential procedure are verified by subjecting the building structure to four earthquakes of various intensities. The performance of 80 dampers and 137 dampers installed on the structure is evaluated according to 5 criteria. Numerical simulations indicated that the proposed control algorithm effectively reduces the seismic responses of the uncontrolled 20-story building, such as inelastic deformation. The sub-optimal placement of dampers based on peak acceleration outperforms that based on peak drift ratio for structures subjected to near-fault ground motions. Saturation of piezoelectric actuators has adverse effect on floor acceleration.

• KCI Concrete Journal
• /
• 제11권3호
• /
• pp.153-164
• /
• 1999

The objective of this research is to observe the actual response of a low-rise nonseismic moment-resisting masonry-infilled reinforced concrete frame subjected to varied levels of earthquake ground motions. The reduction scale for the model was determined as 1 : 5 considering the capacity of the shaking table to be used. This model was, then, subjected to the shaking table motions simulating Taft N2IE component earthquake ground motion, whose peak ground acceleration(PGA) was modified to 0.12g, 0.2g, 0.3g, and 0.4g. The g1oba1 behavior and failure mode were observed. The lateral accelerations and displacements at each story and local deformations at the critical portions of the structure were measured. Before and after each earthquake simulation test, free vibration tests and white noise tests were performed to find the changes in the natural period of the model. When the results of the masonry-infilled frame are compared with those of the bare frame, it can be recognized that masonry infills contribute to the large increase in the stiffness and strength of the g1oba1 structure whereas it also accompanies the increase of earthquake inertia forces. However, it is judged that masonry infills may be beneficial to the performance of the structure since the rate of increase in strength appears to be greater than that of the induced earthquake inertia forces.

• KCI Concrete Journal
• /
• 제12권1호
• /
• pp.23-34
• /
• 2000

Experiments were carried out to investigate the seismic behaviors, such as lateral strength, ductility and energy-dissipation capacity. of high-strength concrete (HSC) square short column confined in thin steel shell. The primary objective of the study was to investigate the suitability of using HSC square columns confined in thin steel shell in region of moderate-to-high seismic risk. A total of six columns, consisting of two ordinarily reinforced concrete square short columns and four reinforced concrete square short columns confined in thin steel shell was tested. Column specimens, short columns in a moment resisting frame with girder. were tested under a constant axial and reversed cyclic lateral loads. To design the specimens. transverse reinforcing methods, level of axial load applied, and the steel tube width-thickness ratio (D/t) were chosen as main parameters. Test results were also discussed and compared in the light of improvements in general behaviors, ductility, and energy-absorption capacities. Compared to conventionally reinforced concrete columns, the HSC columns confined in thin steel shell had similar load-displacement hysteretic behavior but exhibited greater energy-dissipation characteristics . It is concluded that, in strong earthquake areas, the transverse reinforcing method by using a thin steel shell (D/t=125) is quite effective to make HSC short columns with very strong and ductile.

• Structural Engineering and Mechanics
• /
• 제90권1호
• /
• pp.51-70
• /
• 2024

Poorly designed reinforced concrete (RC) columns of actual moment-resisting frame (MRF) buildings can undergo Axial Compression Ratios (ACR) so high as their demand exceeds their capacity, even for serviceability gravity load combinations, this lack commonly leads to insufficient seismic strength. Nonetheless, many seismic design codes do not specify limits for ACR. The main contribution of this research is to investigate the need to limit the ACR in seismic design. For this purpose, three prototype 6 and 11-story RC MRF buildings are analyzed in this paper, these buildings have columns undergoing excessive ACR, according to the limits prescribed by standards. To better that situation, three types of alterations are performed: retrofitting the abovementioned overloaded columns by steel jacketing, increasing the concrete strength, and reducing the number of stories. Several finite element analyses are conducted using the well-known software SAP2000 and the results are used for further calculations. Code-type and pushover analyses are performed on the original and retrofitted buildings, the suitability of the other modified buildings is checked by code-type analyses only. The obtained results suggest that ACR is a rather reliable indicator of the final building strength, hence, apparently, limiting the ACR in the standards (for early stages of design) might avoid unnecessary verifications.

• Steel and Composite Structures
• /
• 제51권5호
• /
• pp.563-573
• /
• 2024

This research experimentally evaluated the local stress distribution along the cross-section of composite beams under both positive and negative moments. The experiment utilized a large-scale, two-story, two-by-three bay steel gravity frame with a concrete on metal deck floor system. The composite shear connections, which are nominally assumed to be pinned under gravity loading, can develop non-negligible moment-resisting capacity when subjected to lateral loads. This paper discusses the local stress distribution, orshear lag effects, observed near the beam-to-column connections when subjected to combined gravity and lateral loading. Strain gauges were used for measurements along the beam depth at varying distances from the connection. The experimental data showed amplified shear lag effects near the unconnected region of the beam web and bottom flange under the applied loading conditions. These results indicate that strain does not vary linearly across the beam cross-section adjacent to the connection components. This insight has implications for the use of experimental strain gauge data in estimating beam demands near the connections. These findings can be beneficial in informing instrumentation plans for future experimental studies on composite beams.

• Steel and Composite Structures
• /
• 제23권5호
• /
• pp.571-583
• /
• 2017

A kind of accordion-web RBS connection, "Tubular Web RBS (TW-RBS)" connection is proposed in this research. TW-RBS is made by replacing a part of web with a tube at the desirable location of the beam plastic hinge. This paper presents first a numerical study under cyclic load using ABAQUS finite element software. A test specimen is used for calibration and comparison of numerical results. Obtained results indicated that TW-RBS would reduce contribution of the beam web to the whole moment strength and creates a ductile fuse far from components of the beam-to-column connection. Besides, TW-RBS connection can increase story drift capacity up to 9% in the case of shallow beams which is much more than those stipulated by the current seismic codes. Furthermore, the tubular web like corrugated sheet can improve both the out-of-plane stiffness of the beam longitudinal axis and the flange stability condition due to the smaller width to thickness ratio of the beam flange in the plastic hinge region. Thus, the tubular web in the plastic hinge region improves lateral-torsional buckling stability of the beam as just local buckling of the beam flange at the center of the reduced section was observed during the tests. Also change of direction of strain in arc shape of the tubular web section is smaller than the accordion webs with sharp corners therefore the tubular web provides a better condition in terms of low-cycle fatigue than other accordion web with sharp corners.