• Steel and Composite Structures
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• v.4 no.2
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• pp.149-167
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• 2004

Offshore platforms have to serve in harsh environments and hence are likely to be damaged due to wave induced fatigue and environmental corrosion. Welded tubular joints in offshore platforms are most vulnerable to fatigue damage. Such damages endanger the integrity of the structure. Therefore it is all the more essential to assess the capacity of damaged structure from the point of view of its safety. Eight internally ring stiffened fatigue damaged tubular joints with nominal chord and brace diameter of 324 mm and 219 mm respectively and thickness 12 mm and 8 mm respectively were tested under axial brace compression loading to evaluate the reserve capacity of the joints. These joints had earlier been tested under fatigue loading under corrosive environments of synthetic sea water and hence they have been cracked. The extent of the damage varied from 35 to 50 per cent. One stiffened joint was also tested under axial brace tension loading. The residual strength of fatigue damaged stiffened joint tested under tension loading was observed to be less than one fourth of that tested under compression loading. It was observed in this experimental investigation that in the damaged condition, the joints possessed an in-built load-transfer mechanism. A bi-linear stress-strain model was developed in this investigation to predict the reserve capacity of the joint. This model considered the strain hardening effect. Close agreement was observed between the experimental and predicted results. The paper presents in detail the experimental investigation and the development of the analytical model to predict the reserve capacity of internally ring stiffened joints.

• Steel and Composite Structures
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• v.44 no.3
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• pp.325-337
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• 2022

In this research, an earthquake-resistant structural system consisting of a pin-connected steel frame and a bracing with metallic fuses is proposed. Contrary to the conventional braced frames, the main structural elements are deemed to remain elastic under earthquakes and the seismic energy is efficiently dissipated by the damper-braces with an amplification mechanism. The superiority of the proposed damping system lies in easy manufacture, high yield capacity and energy dissipation, and an effortless replacement of damaged fuses after earthquake events. Furthermore, the stiffness and the yield capacity are almost decoupled in the proposed damper-brace which makes it highly versatile for performance-based seismic design compared to most other dampers. A special attention is paid to derive the theoretical formulation for nonlinear behavior of the proposed damper-brace, which is verified using analytical results. Next, a direct displacement-based design procedure is provided for the proposed system and an example structure is designed and analyzed thoroughly to check its seismic performance. The results show that the proposed system designed with the provided procedure satisfies the given performance objective and can be used for developing highly efficient low-damage structures.

• Steel and Composite Structures
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• v.44 no.6
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• pp.899-912
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• 2022

To study the seismic performance of the all-steel buckling-restrained brace (BRB) using the novel soft steel LY315 for core member, a total of three identical BRBs were designed and a series of experimental and numerical studies were conducted. First, monotonic and cyclic loading tests were carried out to obtain the mechanical properties of LY315 steel. In addition, the parameters of the Chaboche model were calibrated based on the test results and then verified using ABAQUS. Second, three BRB specimens were tested under cyclic loads to investigate the seismic performance. The failure modes of all the specimens were identified and discussed. The test results indicate that the BRBs exhibit excellent energy dissipation capacity, good ductility, and excellent low-cycle fatigue performance. Then, a finite element (FE) model was established and verified with the test results. Furthermore, a parametric study was performed to further investigate the effects of gap size, restraining ratio, slenderness ratio of the yielding segment, and material properties of the core member on the load capacity and energy dissipation capacity of BRBs.

• Steel and Composite Structures
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• v.27 no.6
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• pp.675-689
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• 2018

Buckling-restrained braces (BRBs) have received considerable attention in seismic design of various types of structures. Conventional BRBs are composed of steel core and surrounding steel tube filled with concrete. Eliminating the steel tube can be advantageous to BRB. In this study the idea of replacing the steel tube by CFRP layers in BRBs is proposed. The advantages of this type of BRB are mentioned, and its design criteria are introduced. The construction procedure of two BRB specimens is described. The specimens are uniaxially tested based on moderate, and severe earthquake levels and the performance of the specimens is investigated. The backbone curves resulted from the hysteresis curve are presented for the design proposes. The results of this study show that CFRP layers can effectively provide the expected performance of the encasing, and the proposed BRB can be considered a viable alternative to the conventional BRBs.

• Steel and Composite Structures
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• v.14 no.6
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• pp.621-636
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• 2013

Response modification factor is one of the seismic design parameters to consider nonlinear performance of building structures during strong earthquake, in conformity with the point that many seismic design codes led to reduce the loads. In the present paper it's tried to evaluate the response modification factors of dual moment resistant frame with buckling restrained braced (BRB). Since, the response modification factor depends on ductility and overstrength; the nonlinear static analysis, nonlinear dynamic analysis and linear dynamic analysis have been done on building models including multi-floors and different brace configurations (chevron V, invert V, diagonal and X bracing). The response modification factor for each of the BRBF dual systems has been determined separately, and the tentative value of 10.47 has been suggested for allowable stress design method. It is also included that the ductility, overstrength and response modification factors for all of the models were decreased when the height of the building was increased.

• Steel and Composite Structures
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• v.3 no.5
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• pp.321-331
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• 2003

The paper presents the results of an experimental investigation conducted on welded tubular joints, that are employed in offshore platforms, to study the behaviour and strength of these joints under axial brace compression loading. The geometrical configuration of the joints tested were T and Y. The nominal diameter of the chord and brace members of the joint were 324 and 219 mm respectively. The chord thickness was 12 mm and the brace 8 mm. The tested joints are approximately quarter size when compared to the largest joints in the platforms built in a shallow water depth of 80 m in the Bombay High field. Some of the joints were actually fabricated by a leading offshore agency which firm is directly involved in the fabrication of prototype structures. Strength of the internally ring-stiffened joints was found to be almost twice that of the unstiffened joints of the same configuration and dimensions. Bending of the chord as a whole was observed to be the predominant mode of deformation of the internally ring-stiffened joints in contrast to ovaling and punching shear of the unstiffened joints. It was observed in this investigation that unstiffened joint was stiffer in ovaling mode than in bending and that midspan deflection of unstiffened joint was insignificant when compared to that of the internally ring stiffened joint. The measured midspan deflection of the unstiffened joint in this investigation and its relation with the applied axial load compares very well with that predicted for the brace axial displacement by energy method published in the literature. A comparison of the measured deflection and ovaling of the unstiffened joint was made with that published by the author elsewhere in which numerical prediction of both quantities have been made using ANSYS software package. The agreement was found to be quite good.

• Steel and Composite Structures
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• v.21 no.4
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• pp.863-882
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• 2016

This study aims to introduce a new bracing system by which even super-wide frames with large openings can be braced. The proposed system, hereafter called Cable-Pulley Brace (CPB), is a tension-only bracing system with a rectilinear configuration. In CPB, a wire rope passes through a rectilinear path around the opening(s) and connects the lower corner of the frame to its opposite upper one. CPB is a secondary load resisting system with a nonlinear-elastic hysteretic behavior due to its initial pre-tension load. As a result, the required energy dissipation would be provided by the MRF itself, and the main intention of using CPB is to contribute to the initial and post-yield stiffness of the whole system. Using a stiffness calibration technique, optimum placement of the CPBs is discussed to yield a uniform displacement demand along the height of the structure. A displacement-based design procedure is proposed by which the MRF with CPB can be designed to achieve a uniform distribution of inter-story drifts with predefined values. Obtained results indicated that CPB leads to significant reductions in maximum and residual deformations of the MRF at the expense of minor increase in the maximum base shear and developed axial force demands in the columns. In the case of a typical 5-story residential building, compared to SMRF system, CPB system reduces maximum amounts of inter-story and residual drifts by 35% and 70%, respectively. Moreover, openings of the frame are not interrupted by the CPB. This is the most appealing feature of the proposed bracing system from architectural point of view.

• Steel and Composite Structures
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• v.31 no.6
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• pp.541-558
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• 2019

In Special Concentrically Braced Frames (SCBFs), vertical and horizontal components of the brace force must be resisted by column and beam, respectively but normal force component existing at the gusset plate-to-column and beam interfaces, creates out-of-plane action making distortion in column and beam faces adjacent to the gusset plate. It is a main concern in Hollow Structural Section (HSS) columns and beams where their webs and gusset plate are not in the same plane. In this paper, a new gusset plate passing through the HSS columns and beams, named as through gusset plate, is proposed to study the force transfer mechanism in such gusset plates of SCBFs compared to the case with conventional gusset plates. For this purpose, twelve SCBFs with diagonal brace and HSS columns and twelve SCBFs with chevron brace and HSS columns and beams are considered. For each frame, two cases are considered, one with through gusset plates and the other with conventional ones. Based on numerical results, using through gusset plates prevents distortion and out-of-plane deformation at HSS column and beam faces adjacent to the gusset plate helping the entire column and beam cross-sections to resist respectively vertical and horizontal components of the brace force. Moreover, its application increases energy dissipation, lateral stiffness and strength around 28%, 40% and 32%, respectively, improving connection behavior and raising the resistance of the normal force components at the gusset plate-to-HSS column and beam interfaces to approximately 4 and 3.5 times, respectively. Finally, using such through gusset plates leads to better structural performance particularly for HSS columns and beams with larger width-to-thickness ratio elements.

• Steel and Composite Structures
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• v.30 no.3
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• pp.201-215
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• 2019

In this research, the behavior of tube-in-tube BRBs (TiTBRBs) has been investigated. In a typical TiTBRB, the yielding core tube is located inside the outer restraining one to dissipate energy through extensive plastic deformation, while the outer restraining tube remains essentially elastic. With the aid of FE analyses, the monotonic and cyclic behavior of the proposed TiTBRBs have been studied as individual brace elements. Subsequently, a detailed finite element model of a representative single span-single story frame equipped with such a TiTBRB has been constructed and both monotonic and cyclic behavior of the proposed TiTBRBs have been explored under the application of the AISC loading protocol at the braced frame level. With the aid of backbone curves derived from the FE analyses, a simplified frame model has been developed and verified through comparison with the results of the detailed FE model. It has been shown that, the simplified model is capable of predicting closely the cyclic behavior of the TiTBRB frame and hence can be used for design purposes. Considering type of connection detail used in a frame, the TiTBRB member which behave satisfactorily at the brace element level under cyclic loading conditions, may suffer global buckling due to the flexural demand exerted from the frame to the brace member at its ends. The proposed TiTBRB suit tubular members of offshore structures and the application of such TiTBRB in a typical offshore platform has been introduced and studied in a single frame level using detailed FE model.

• Steel and Composite Structures
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• v.30 no.2
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• pp.81-96
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• 2019

The behavior of a new Three-Tube Buckling-Restrained Brace (TTBRB) with circumference pre-stress (${\sigma}_{{\theta},pre}$) in core tube are investigated through a verified finite element model. The TTBRB is composed of one core tube and two restraining tubes. The core tube is in the middle to provide the axial stiffness, to carry the axial load and to dissipate the earthquake energy. The two restraining tubes are at inside and outside of the core tube, respectively, to restrain the global and local buckling of the core tube. Based on the yield criteria of fringe fiber, a design method for restraining tubes is proposed. The applicability of the proposed design equations are verified by TTBRBs with different radius-thickness ratios, with different gap widths between core tube and restraining tubs, and with different levels of ${\sigma}_{{\theta},pre}$. The outer and inner tubes will restrain the deformation of the core tube in radius direction, which causes circumference stress (${\sigma}_{\theta}$) in the core tube. Together with the ${\sigma}_{{\theta},pre}$ in the core tube that is applied through interference fit of the three tubes, the yield strength of the core tube in the axial direction is improved from 160 MPa to 235 MPa. Effects of gap width between the core tube and restraining tubes, and ${\sigma}_{{\theta},pre}$ on hysteretic behavior of TTBRBs are presented. Analysis results showed that the gap width and the ${\sigma}_{{\theta},pre}$ can significantly affect the hysteretic behavior of a TTBRB.