Bracing Members Research Articles

Experimental and numerical research on fire resistance of stainless steel tubular X-joints

The fire test was carried out on six stainless steel square hollow section (SHS) tubular X-joints to investigate the influences of axial loading ratio in brace member, brace to chord width ratio and inclined angle between brace and chord member on the fire resistance performance of joints. The furnace temperature-time curve, surface temperature-time curve, vertical deformation-time curve, lateral deformation of chord web-time curve and failure mode for all specimens were obtained by the fire test. The test results show that, the failure modes of stainless steel SHS tubular X-joints at elevated temperatures can be divided to three types: plastic failure of chord flange, plastic failure of chord flange and web, and buckling failure of chord web. Then, the failure criterion of stainless steel SHS tubular X-joints under fire conditions was proposed: the joint can be considered to be damaged when its concave deformation of chord flange reaches 0.03 times of chord width. The finite element models of stainless steel SHS tubular X-joints were established and validated against the test data. Finally, based on the validated models, a parametric analysis was performed on the influencing factors of the fire resistance of tubular joints, and the results show that the failure temperature of stainless steel SHS tubular X-joints will be enhanced with the increase of brace to chord width ratio and chord width to thickness ratio, or with the decrease of axial loading ratio in brace member and chord preload ratio.

Simplified Lateral Torsional Buckling (LTB) Analysis of Glass Fins with Continuous Lateral Restraints at the Tensioned Edge

Within multiple design challenges, the lateral torsional buckling (LTB) analysis and stability check of structural glass members is a well-known issue for design. Typical examples can be found not only in glass slabs with slender bracing members but also in facades and walls, where glass fins are used to brace the vertical panels against input pressures. Design loads such as wind suction give place to possible LTB of fins with LR at the tensioned edge and thus require dedicated tools. In the present investigation, the LTB analysis of structural glass fins that are intended to act as bracers for facade panels and restrained via continuous, flexible joints acting as lateral restraints (LRs) is addressed. Geometrically simplified but refined numerical models developed in Abaqus are used to perform a wide parametric study and validate the proposed analytical formulations. Special care is spent for the prediction of the elastic critical buckling moment with LRs, given that it represents the first fundamental parameter for buckling design. However, the LR stiffness and resistance on the one side and the geometrical/mechanical features of the LR glass members on the other side are mutually affected in the final LTB prediction. In the case of laminated glass (LG) members composed of two or more glass panels, moreover, further design challenges arise from the bonding level of the constituent layers. A simplified but rational analytical procedure is thus presented in this paper to support the development of a conservative and standardized LTB stability check for glass fins with LR at the tensioned edge.

Energy Dissipation in Frame Structures using Sliding Lever Mechanism Technique

Aim: The aim of this study was to investigate the seismic energy dissipation mechanism of a novel and newly proposed sliding lever damping energy dissipation through dynamic shake table testing. Background: Typical energy dissipation systems consist of brace members and installed dampers, which are directly connected to structural members such as beams, columns and joint regions. This can cause additional load concentrations and may require retrofitting or strengthening of existing structures. In order to avoid the load demand on the main structural system, a new energy dissipation technique based on a sliding lever mechanism is proposed and tested through dynamic testing. Objective: The objective of this study was to test a new sliding lever damping energy dissipation through dynamic shake table testing within the scope of steel frame structures. Methods: In order to investigate the proposed energy dissipation configuration, a 1/3rd reduced scaled, three-story and one bay steel frame model has been fabricated and tested in a uni-directional shaking with increasing excitation and, without and with the new technique. For the sliding lever energy dissipation configuration, a non-structural frame (i.e., carrying no gravity loads) has been constructed and provided with an installed ramp-damper assembly. The shaking responses in the form of acceleration and displacement histories have been obtained during the experimental program and compared in order to check the efficiency of the proposed configuration. Results: The results showed a reduction of 55% to 6% in stories deflections and 36% to 12% in acceleration with the newly proposed sliding lever mechanism energy dissipation technique. The top story peak displacements for the damped frame case decreased by 36.55% in case of 0.1 g, 37.95% in case of 0.2g, 31.89% in case of 0.3g, 38.05% in case of 0.4g, 29.37% in case of 0.5g and 12.06% in case of 0.6g shaking excitation. Conclusion: It has been confirmed from the current experimental studies that the new configuration was quite effective in reducing the overall displacement and acceleration response. The reduction in the structural response parameters was very significant during low excitation shaking, whereas, with the increase in shaking intensities, the responses varied with much less difference.

Experimental investigation and low-cycle fatigue behavior of I-shaped steel bracing members with gusset plate connections

Experiments on 12 specimens are conducted to study the low-cycle fatigue behavior of I-shaped steel bracing members with gusset plate connections. The experimental results show that there are three locations vulnerable to damage following three different damage sequences. A small clearance distance (1) makes it more likely to develop damage on the gusset plate, and (2) exacerbates the degradation of the rotational and tensile loading capacity, especially when the specimen is subject to large loading amplitudes. When the loading amplitude is small, properly reducing the clearance distance can increase fatigue life and crack development life, leading to a more ductile performance. Using Bayesian updating, a probabilistic model is proposed to predict the fatigue life.

Evaluation of the slotted-hidden-gap (SHG) connection for square HSS brace members

Hollow structural section (HSS) braces of concentrically braced steel frame structures are often connected using a knife connection, which creates a net area in the brace due to the slot. Tensile rupture of the brace's net section is the controlling failure mode unless reinforcing schemes are utilized. The “Slotted-Hidden-Gap (SHG)” connection, in which a notch is created in the gusset plate to overlap the gross area of the tube to eliminate the net area, represents an attractive alternative to traditional connection reinforcements; however, no design guidelines exist for the SHG connection for square HSS braces. A laboratory testing program and finite element modelling were carried out to determine the minimum overlap length required to develop the yield resistance of a square HSS brace. The test program included monotonic tensile loading tests on short conventional- and SHG- connection specimens, and one reversed cyclic loading test on a 4.86 m long brace specimen with SHG connections. An overlap length of 5% of the weld length was sufficient to develop the yield resistance of the braces with SHG connections. For those specimens, the HSS eventually fractured on their gross area away from the connections, after experiencing extensive yielding along their length. Conversely, braces with the conventional connections suffered localised yielding and fracture at the net section. The long brace with SHG connections attained its yield tensile resistance, including a 15% increase due to strain hardening, while sustaining an axial deformation corresponding to a storey drift of 3.14%.

Parametric Study of the Intermediate External Bracing System of Composite Steel Box Girder Bridges

During the pouring of concrete deck, the installation of external bracing between the inner and outer girders may be necessary when the bridge has sharp curve in order to control the deflection and rotation of the girders. However, it is important to minimize the number of external bracing members, as they have expensive cost and they also have opposite effects for the fatigue features of the steel tub girders. The analysis of curved box girder bridges is carried out numerically by the use of finite element method through (ANSYS 19.2) software. The curved box girder with the intermediate external diaphragms was modeled and the analysis was carried out for many parameters like external bracing sections, girders with or without concrete deck, girders with end diaphragms or without them. The study concluded that ANSYS program has a good ability in evaluating the external bracing force comparing with code equations.

Experimental testing of a self-centring concentrically braced steel frame

Conventional concentrically braced frames (CBFs) undergo many cycles of inelastic deformation during seismic excitation. This inelastic deformation leads to the possibility that a structure will remain in an out-of-plumb position, even if it has performed as required by current design codes. This paper presents an improved steel braced framing system that eliminates such residual deformations in the structure by using a post-tensioning arrangement to ensure the structure self-centres following an earthquake. This is achieved by combining the bilinear elastic rocking response of a post-tensioned frame with the inelastic behaviour of tubular steel bracing members to give a system that both dissipates hysteretic energy and ensures self-centring behaviour, termed the self-centring concentrically braced frame (SC-CBF). This SC-CBF system distinguishes itself from previous self-centring CBFs by employing post-tensioned rocking beam-column connections as opposed to a globally uplifting frame. The mechanics behind the behaviour of the SC-CBF are first described, followed by a discussion of an experimental test setup to validate the concept under quasi-static cyclic testing. Results from a total of nine tests are presented to demonstrate the self-centring behaviour of the SC-CBF. Comparisons with analytical expressions developed for the system demonstrate the SC-CBF performs as anticipated and presents a novel system for the seismic design of steel structures.

Numerical Study to Evaluate the Effect of Ductile Element in the Seismic Performance of Steel Frames with EBF

This paper presents the results of a numerical study, using ANSYS and OpenSees, of Ductile Element in the Seismic Performance of Steel Frames with eccentrically braced frame. This ductile element as dissipater includes a ring with a box around it that increases the ductility and energy dissipation of structures during an earthquake strike. It can be installed in every brace frame. Furthermore, during an earthquake, it contributes a significant portion of dissipated input energy by entry into the non-linear phase. Additionally, flexural plastic hinges are formed. With this, the entrance of other members to the non-linear phase is postponed. As a result, the buckling of the brace members is prevented. The box increases the capacity of the ductile members. The increase in the ring diameter and thickness results in a reduced ductility rate. Using a non-linear finite element analysis, the stresses and deformations under cyclic load are studied. The obtained hysteresis curves indicate that the element can act as an energy dissipating member, as well as a fuse to control the buckling of a brace. Moreover, this member can increase ductility in structures. Additionally, the operation of the ductility element in 2D eccentrically brace frames is considered. The non-linear time history analysis is conducted by OpenSees. The achieved results showed a decrease in the base shear and rotation of the link beam and an increase in ductility.

Investigation of channel-encased braces incorporating circular HSS

Ductile concentrically braced frames (CBFs) with circular hollow shapes (CHS) have been one of the most common structural systems in seismic regions and are expected to undergo significant inelastic deformations mainly through post-buckling or yielding deformations in their bracing members. However, a comprehensive survey of the experimentally tested CHS specimens carried out in the course of this study indicated that there exists a discrepancy between the expected demand and capacity of conventional CHS bracings. To overcome this quantitatively identified inconsistency, which might result in a poor performance under severe ground shakings, a new and simple performance-enhancing method is proposed and examined. The developed improvement technique, which can be implemented in both new and existing steel buildings, is employed by encasing a CHS bracing in two channels connected with welded stitches. Experimental and numerical investigations into the effectiveness of the developed system showed that the developed channel-encased bracings are promising in terms of improving the cyclic stability and cumulative energy dissipation capability compared with the conventional bracings.

Effect of the Bracing System on the Probability of Collapse of Steel Structures under Maximum Credible Earthquake

Simple bracing frames can be divided into two types in terms of concentric or eccentric. Concentric bracing frames are frames that intersect with other structural members at one point in the structure along the bracing members. Otherwise, the braced frame will be eccentric. It is said empirically that due to this type of shaping, eccentric bracing frames exhibit more ductile behavior and concentric bracing frames exhibit more stiff behavior. This behavioral difference caused this study to be numerically computing for five frames, including unique concentric and eccentric bracing frames of 5 and 10 stories and an ordinary 5‐story concentric bracing frame. Their tensions and drift ratios should be acceptable for the use of residential buildings. Using the primary two steps of the new PEER probabilistic framework, namely, probabilistic seismic hazard analysis and structural analysis, which leads to the drawing of fragility curves, the probability of collapse is obtained to compare the safety capability of these frames according to their different characteristics against earthquakes. The results show that increasing the ductility or increasing the number of floors or the height of these systems can reduce collapse. Also, according to the results of the probability of collapse obtained in frames with 5‐story concentric bracing frames, it can be said that some of the current regulations, which work based on previous approaches of analysis, can lead to unsafe structures with a high probability of collapse.

Earthquake-resistant design of steel frames with intentionally eccentric braces

Braces with Intentional Eccentricity (BIEs) have been proposed to overcome some of the shortcomings of Concentrically Braced Frames (CBFs), namely the implications of their inherently stiff nature, their limited post-yielding stiffness and the susceptibility of Hollow Structural Sections (HSSs) to premature local buckling and fracture, and the excessive overstrength that may result from the design codes limits on the local and global slenderness. However, the application of BIEs for use in buildings has not yet been attempted, nor has their implementation in a global design approach yet been addressed. In this paper, a procedure based on the Direct Displacement Based Design (DDBD) method is employed in the seismic design of Frames with Intentionally Eccentric Braces (FIEBs). Buildings of 4, 8 and 12 storeys are designed as FIEBs with HSS brace members, with target drift ratios of 1.5% and 2.5%, and as Special CBFs for comparison purposes. The performance of the resulting buildings is assessed through Non-Linear Response-History Analysis. The results show that the employed design procedure is well suited to FIEBs, that their seismic performance is satisfactory and complies with the proposed performance objectives, and that they can constitute an economically advantageous alternative to conventional CBFs.

Static resistance of axially loaded multiplanar gap KK-joints of Circular Hollow sections at elevated temperatures

This paper presents the results of a numerical investigation of the behaviour, and proposes a method to calculate the static resistance of axially loaded Circular Hollow Section Double K-joints (DKK) at elevated temperatures. This type of joints is widely used in building construction, but currently, there is no method to calculate their static resistance at elevated temperatures. The numerical investigations of this paper cover a wide range of design parameters, including brace axial load combination, joint temperature, angles between brace members, longitudinal and transverse gap sizes, chord diameter to thickness ratio and brace diameter to chord diameter ratio.The results of this paper indicate that the ratio of joint elevated temperature resistance to ambient temperature resistance may be lower than the reduction factor for the yield strength of steel at elevated temperatures.There are two joint failure modes, one for small and one for large transverse gap. These failure modes and the demarcation between small and large gaps are independent of joint temperature. For joints with large gap, the ratio of joint resistance at elevated temperature to that at ambient temperature is close to the average of steel yield strength and Young’s modulus reduction factors at elevated temperature. For joints with small gap, the joint resistance ratio follows the steel Young’s modulus factor variation at elevated temperature if all four braces are under compression, but the trend changes to the average ratio of steel Young’s modulus and yield strength reduction factors if the axial brace loads are balanced (compression in two brace members equal to tension force in the other two brace members). Linear interpolation can be applied between these two different loading cases.

Experimental Study on Load Carrying Capacity Enhancement of System Supports Considering Full Installation of Bracing Members

This study investigated the ultimate load capacity of assembled system supports by comparing cases with and without bracing members. Supports used as temporary structures are essential to resist the concrete casting load. Current trends encourage the use of system supports to prevent the collapse of supports, which occur frequently. Studies regarding the ultimate behavior of system supports in Korea are fewer than those in other countries. Existing studies have only focused on the behavior of unit members, such as vertical members. However, studies that focused on unit vertical members under axial loading reported difficulty in predicting the ultimate behavior of assembled system supports. Furthermore, the ultimate strength of the system support can be decreased significantly in cases without a bracing member, which occur frequently at actual sites. Therefore, the ultimate behavior of the system support considering actual construction site conditions was investigated through experimental tests, focusing on whether bracing members were installed. Experimental results showed that the load-carrying capacity of a three-story system support without a bracing member was 38.4% that of the system support with full bracing members. In addition, the load-carrying capacity of the three-story system support without bracing members decreased by 41.2% compared with that of a two-story system support without bracing members. This indicates that the installation of bracing members is essential for increasing the ultimate load capacity of system supports, and the number of stories of system supports should be considered to determine their ultimate strength.

Deep neural network for detecting earthquake damage to brace members installed in a steel frame

AbstractWe are developing an artificial intelligence system for structural health monitoring that can detect local damage in a building structure by using the E‐Simulator numerical simulation system that is being developed by the Japanese National Research Institute for Earth Science and Disaster Resilience. In this study, we confirmed the applicability of a multiclass classifier using a deep neural network to address the problem of identifying damage patterns in braces installed in a steel frame. Experimental data obtained from shaking table tests were used for training and testing. Cross‐validation tests were conducted for several cases with different numbers of sensors, sensor degrees of freedom, and nodes in the hidden layers of the network. The results demonstrated that the accuracy of the damage pattern detection from the constructed classifier exceeded 77% when the appropriate hidden layers were selected and reached 87.9% for the best case.

The application research of a main machine hall for a thermal power plant with BRB

As a kind of energy dissipation brace members by the yielding of metal, Buckling Restrained Brace (BRB) can overcome shortcomings of the common brace’s compression buckling, which has good energy dissipation capacity and ductility in earthquake. To study the Buckling Restrained Brace effect in the main machine hall, the nonlinear finite element analysis software ABAQUS is adopted to analyze the seismic performance of a main machine hall with BRB under the action of the rare earthquake and compare the elastic-plastic analysis results under the action of different seismic wave in this paper. The results show the structural seismic performance has been improved significantly, “no collapse under strong earthquake” can be achieved.

Quantitative collision risk assessment of a fixed-type offshore platform with an offshore supply vessel

Offshore installations are designed to withstand against potential collisions from offshore supply vessels (OSV). Quantitative risk assessment (QRA) provides an overall picture of expected collision frequencies and consequences for the design life of a platform and subsequently estimates the damage repair cost. However, the main challenge of the QRA study is how well various uncertainties are implemented into the model. This study aimed to introduce and demonstrate an advanced and efficient QRA model for the collision between an OSV and a jacket type offshore platform. A set of fifty collision scenarios were selected using probabilistic sampling techniques, and vessel motion analyses were performed to determine collision load characteristics. Extensive nonlinear structural crashworthiness analyses were conducted using advanced computational modelling techniques, and the repair cost of the damaged brace and column members were calculated. Probability exceedance diagrams were established for different consequence parameters, and asset risks were calculated. A comparison study of the design values of damage parameters and repair costs were carried out in association with the NORSOK and HSE risk acceptance criteria. A sensitivity study was carried out to study the effects of various collision load parameters on the structural consequences. The methods and insights developed in this study could be applied to both new and existing offshore platforms and practically useful for the platform owners to aid in their decision-making process towards the risk-based safety analysis of offshore platforms.

Cyclic performance and fracture of wide flanged concentrically steel braced frames

ABSTRACT Concentrically braced steel frames (CBFs) are often used in buildings to resist lateral loads induced by wind and seismic excitations. Large-size wide-flange (WF) steel brace members could be selected and used in these frames. In the current study, a finite element model (FEM) with a strain-based fracture rule is presented to investigate the performance of a large-size WF brace member in single-storey CBF with different sizes of beams and columns when subjected to reversed axial displacements. The effects of restraining the far ends of the beam bottom-flange, increasing the thickness of tab-plates, increasing the number of bolts used to connect the tab-plates to the web of the beam, and the location of the work-point along the brace with respect the beam-column assembly, on the hysteresis response of the frame are investigated. Results suggest that it may be advantageous to reduce the number of bolts and to increase the bolt size to allow for an increase in the thickness of the tab-plates to resist the extreme rotational demand in high seismic regions, thereby preventing fracture of the weld between the tab-plates and the column flange.

Effects of far-field and near-fault cyclic loadings on seismic performance of naturally buckling braces in pairs

Naturally buckling braces (NBBs) have been previously developed through combining low-yield and high-strength steel channel segments with an intended eccentricity along the brace length. The developed NBB formed a novel mechanism and was experimentally verified to provide a ductile and stable seismic performance with the features of early yielding, large post-yield stiffness in tension and well-postponed local buckling in compression compared to conventional buckling braces. In this study, a series of paired NBB specimens have been tested to clarify the seismic performance of NBBs in pairs in frame systems, such as the effects of brace angles, member and local slenderness, and adopted steel grades of the low-yield segment. Two types of cyclic loading protocols, including far-field and near-fault cyclic loadings, were applied to investigate near-fault effects on the seismic performance of the paired NBBs compared to that under far-field cyclic loading. The test results verified that the paired NBBs provided a comparable, ductile, and stable hysteretic performance in tension and compression under both far-field and near-fault loading protocols. A procedure to transfer the nonlinear measured strength backbone curves to bi-linear strength curves was proposed to present the major hysteretic properties of the paired NBBs, and several general equations were established to accurately estimate the major hysteretic properties. Strength degradation was observed among the specimens and was more significant under near-fault loading. An equation was developed to accurately quantify the strength degradation according to the member and local slenderness ratios of the paired NBBs, and the relationship of the strength degradation between far-field and near-fault loadings was established. Cumulative plastic deformation and total energy dissipation of specimens in the study were evaluated to quantify the fatigue life of the paired NBBs. Finally, an improved modelling approach of the paired NBBs was suggested and experimentally validated to more accurately represent the seismic performance of the paired NBBs in the frames.

Prediction of ductile fracture for circular hollow section bracing members under extremely low cycle fatigue

The fracture behaviour of concentrically loaded circular hollow section (CHS) bracing members under extremely low cycle fatigue (ELCF) is examined in this paper. Finite element (FE) models capable of predicting fracture initiation and propagation on cyclically loaded braces were developed. A structural steel ductile fracture criterion, together with a damage accumulation rule that can account for the effects of both stress triaxiality and Lode angle, was adopted in the FE models. The FE models were validated against the available test results from different experimental programmes and shown to provide an accurate prediction of both the hysteretic response and the ELCF fracture cracking process. The coupling effects of buckling and fracture on the ELCF performance of braces were assessed through a parametric study. This parametric study examined the influences of the geometry, material and manufacturing process on the local and global deformation, and the ductility of the braces. Predictive equations for the localized strains and member ductility were proposed based on a plastic hinge model. The seismic performance of a chevron braced frame was also evaluated in terms of storey drift angle according to the requirements of the current design code.

Optimal design of a hanging truss with shape memory alloy wires

In this study, we discuss the dynamic problem of a column-type hanging truss structural system with pseudo-elastic shape memory alloy (SMA) bracing wires. In the case where the sectional area values of the bracing SMA wire members are small enough to be negligible, it is close to the situation where there are no braces. In the case where the sectional area values of the bracing SMA wire members are large enough not to be negligible, the vibration amplitude of the peripheral end apparatus is suppressed from the deformation point of view. In addition, energy attenuation efficiency is improved with larger sectional area values because of the hysteretic characteristic of SMA. Small sectional area values of the bracing SMA wire members near the support ceiling or peripheral end are beneficial to vibration transmission reduction. These findings indicate that the placement and sectional area values of SMA wire members are both significant from the viewpoint of suppression of influence of the support ceiling vibration of the hanging truss. In this study, we obtain the optimal sectional area values of the SMA wire bracing members for the objectives of vibration isolation and attenuation. We discuss influences of different vibration conditions on the optimal solutions.