X-braced Frames Research Articles

Seismic performance of frame with middle partially encased composite brace and steel-hollow core partially encased composite spliced frame beam

Steel-hollow core partially encased composite spliced frame beam (SHSFB) and middle partially encased composite brace (MPECB) demonstrate the potential to economize on steel consumption while exhibiting superior performance. To investigate the seismic performance of frames with SHSFBs and MPECBs, horizontal low-cyclic loading tests were conducted on four frame specimens, involving two two-story frames with no brace and two multi-story X-braced frames. Numerical simulations were performed using the “Open System for Earthquake Engineering Simulation” (OpenSees). The results indicate that the novel composite frames exhibit commendable seismic performance and energy dissipation capacity. The unbraced frame with SHSFBs has deformation capacity and mechanical properties comparable to bare steel frame. In contrast to the frame specimen equipped with bare steel braces featuring a 6 mm flange thickness, the specimen utilizing MPECBs with 4 mm flange thickness exhibits only a slight reduction of 3.60 % in maximum bearing capacity and 2.68 % in initial lateral stiffness. Significantly, each SHSFB and MPECB component reduces steel consumption by 16.71 % and 24.79 %, respectively, compared to bare steel components. Therefore, while ensuring adequate mechanical properties, frame structures equipped with SHSFBs and MPECBs will require less steel. Based on both experimental and simulation results, the formulas for predicting the ultimate bearing capacity and initial lateral stiffness of the novel composite frame were proposed, and the structural design for the multi-story X-braced frame was analyzed, offering valuable insights for practical engineering applications.

Seismic Behavior Assessment of Special Concentrically X-braced Frame with Through Gusset Plate

Seismic Behavior Assessment of Special Concentrically X-braced Frame with Through Gusset Plate

Shape Optimisation of Grooves in Grooved Gusset Plate Damper used in X-Braced Frame

The X-braced frame represents a specialized variation of concentric-braced frames. It can be used as a resistance towards the lateral load acting on the structural system. X-braced frames generally exhibit lower flexibility when compared to eccentrically braced and moment frames, which is often perceived as a drawback of this structural system. To address this limitation, energy dissipation devices can be integrated into the system with X-bracings to absorb the plastic action and safeguard other structural elements, such as columns, beams and connections from earthquake forces. Specifically, X-concentrically braced frames are tailored through variations in groove shapes. Furthermore, the investigation determines the dampers' load-bearing and energy dissipation capacities. The assessment commences with cyclic load testing conducted using the ANSYS software. The grooves present in GGPD help dissipate seismic energy. The energy dissipation and load-bearing capacity of the X-concentrically braced frame equipped with a grooved gusset plate damper were compared based on different groove shapes. The different shapes used for the analysis were L shape, oval, rectangular and stadium shape. It was observed that oval-shaped grooves have more energy dissipation capacity among four groove shapes, with an increase of 10.74%.

Seismic fragility assessment of the innovative and optimal box-shaped dampers in steel frame structures

Box-shaped dampers (BSDs) are newly introduced, innovative, and low-cost metallic dampers installed along the diagonal members of the frames. Dissipating the input energy by continuously forming plastic hinges, BSDs prove to possess the potential to serve as a fuse during earthquakes. Nonetheless, little literature exists on the seismic response of the BSD-equipped steel frames. Hence, this study aims to evaluate the seismic performance of steel frames equipped with BSDs. First, the failure mechanism and shape effects of these dampers were investigated using non-linear finite element analysis (FEA) in ABAQUS. Using the results obtained from the FEA, the incremental dynamic analysis (IDA) is utilized to determine the vulnerability of BSD-equipped frames during earthquakes. In this regard, IDA analysis was performed on 5- and 10-story steel frames equipped with BSDs, and the fragility curves of the frames were extracted, followed by calculating the structures' collapse margin ratio (CMR). Furthermore, the obtained results were compared to that of traditional X-braced frames. The results revealed that the BSD-equipped frames display better performance compared to that of the X-braced frames, indicating that these dampers can replace the conventional bracing systems for structural strengthening.

Seismic performance of improved multistorey X-braced steel frames

During an earthquake, the brace of a multistorey X-braced steel frame experiences out-of-plane instability. After the brace becomes unstable, the middle span of the brace-intersected beam exhibits a large imbalance force. This force causes a significant vertical displacement in the middle of the brace-intersected beam. To address this issue, an improved multistorey X-braced steel frame is proposed. Adding an arc energy-dissipation structure to the original structure adjusts the bearing capacity of the brace and ensures that the brace does not lose stability. The deformation of the arc energy-dissipation structure reduces the vertical displacement of the brace-intersected beam. A finite element model of the improved multistorey X-braced steel frame is established, and the seismic performance of the structure is analysed. The analysis shows that the brace of the improved multistorey X-braced steel frame does not experience instability, and the lateral bearing capacity is increased by 20%. The vertical displacement in the middle of the brace-intersected beam is small. The improved multistorey X-braced steel frame exhibits good seismic performance.

The comparison of X-braced frame and moment frame on structural building (case study: government office of North Toraja, South Sulawesi, Indonesia)

The seismic force can damage the building and harm the occupants. Therefore, its structure needs to be designed seismic-resistant. One of the essential things in seismic-resistant building is the effect of lateral force to its structural rigidity. It can be improved by using the lateral resistant element such as bracing and shear wall. The lateral element used in this study is concentric X-bracing. This study conducted an optional structural model of the office of North Toraja Regent, Panga, South Sulawesi. The building was originally analyzed as a whole unit structure, but it has already rotated in its first mode shape due its irregularity. The comparison analysis is conducted by using the same existing structure configuration, except with the addition of X-bracing. The analysis results of the dynamic response are the fundamental building period, mode shape, base shear, lateral displacement and P-delta effect. The fundamental building period using X-bracing is smaller than moment frame building. It indicates that the use of X-bracing lead to the stiffer building. The use of the X-bracing results in a larger base shear value than the moment-frame building. The difference in base-shear between X-braced frame with moment frame is about 211.77% and 212.10% for seismic force with X-direction and Y-direction respectively.

Extending substitute frame model as a rapid supersede for moment frames and X-braced structures in seismic response estimations

Substitute Frame (SF) model is a recently introduced equivalent simplified MDOF model by the authors which is applicable to moment-resisting frames, facilitating the modeling efforts and output management besides reducing the analysis time. The use of this model is deliberated only in the code-based OpenSEES software generally used by researchers and is not suitable for the practical works of engineers. This paper aims to develop the SF model to be usable in the practical engineering platforms that are widely used by engineers, and authenticate its accuracy and applicability in such platforms. Moreover, another objective is to develop this model to be applicable to concentric X-braced steel frames with equal or non-equal bay lengths. These objectives will be fulfilled with different examples, and the accuracy of the simplified models will be comprehensively evaluated by means of modal, nonlinear static, and nonlinear dynamic analyzes. The nonlinear time history analyses (NTHA) are performed with 20 real ground motion records. It has been shown that the proposed simplified model can estimate the responses of moment frames and X-braced systems with acceptable accuracy in a considered practical engineering platform. The proposed model could estimate the natural periods of all the original models with less than 1% error. In addition, the proposed SF model could predict the overall behavior of the original models in the pushover curve and estimate the maximum IDRs, PFAs, and story shears of all stories with acceptable accuracy in nonlinear dynamic analysis. Moreover, the use of the SF model can considerably reduce the computational time (by about 85%) for performing the analyses, and also facilitate the modeling efforts and output management. The supplementary example files for the present study would be shared through the link “https://web.nit.ac.ir/∼h.hamidi/software_data.htm”.

Seismic Performance of X-Braced Frame with Vertical Link Beam (XBF-VLB)

X-braced Frames(XBFs), one of the most prevalent lateral load-resisting structures, has low energy dissipation and ductility. This study investigates the behavior of a single-story, single-span X-braced frame equipped with a Vertical Link Beam (XBF-VLB) under monotonic and cyclic loading using the nonlinear finite element technique in the Abaqus software and validated with experimental data. The VLBs were positioned where the bracing members were connected to the beam in double, single, and wide forms. The impacts of the length, depth, and the number of links, stiffeners, and other VLB section parameters were assessed. The A36 steel ductile damage method (DDM)estimated the damage of the main structural members, such as bracing members. The results revealed that X-braced frames with single, double, and wide form Vertical Link Beams (XBF-VLBs) performed better under cyclic and monotonic loading than X-braced frames without Vertical Link Beam (XBFs). Likewise, because the bracing members did not buckle inthe XBF-VLB specimens, there was no rapid loss in the stiffness and strength of the X-braced frames. Also, theresults of the cyclic loading of the XBF-VLB specimens demonstrated that no rapid drop in stiffness or strength was detected due to no buckling of the bracing members under the cyclic loading, as was the case with the monotonic loading. Additionally, compared to the XBF specimens, the drifts corresponding to the first plastic hinges of the XBF-VLB specimens show that the bracing members experienced nonlinear deformation at greater drifts, with the plastic hinges created in them at bigger displacements. According to the cyclic loading results, the VLB also reduced Von Mises stress and damage in the bracing members of the XBF-VLB specimens. Moreover, theultimate von Mises stress reduction in the bracing members of the models with VLBs equals 33%, which prevents buckling.

Overstrength and ductility factors of XBF structures with pinned and fixed supports

Abstract In today's time, most seismic design codes are based on a linear elastic force-based approach that includes the nonlinear response (ductility and overstrength) of the structure through a reduction factor (named behavior factor q in Eurocode 8 [EC8]). However, the use of a prescribed q-factor that is constant for a given structural system may fail in providing structures with the same risk level. This paper focuses on the estimation of actual values of q-factor for X-braced steel frames (XBFs) designed according to the European codes and comparing these values to those suggested in EC8. For this purpose, a nonlinear pushover analysis has been performed. The effects of specific parameters, such as the stories number, the brace slenderness ratio, the local response of structural members, and the support type, are evaluated. The results show that the most important parameter that affects the q-factor is the brace slenderness ratio, while the support type has less effect on this factor. Furthermore, a local strength criterion has been proposed to implicitly ensure that the suggested value of the q-factor is conservative.

Conceptual design of anti-seismic devices with metal foam core for CBFs

Metal foam is a relatively new and potentially revolutionary material due to its properties such as the low weight-to-stiffness ratio, high damping and energy dissipation capacity. Even if this material is widely used in mechanical, aerospace and automotive industries, it is still not frequently used in civil engineering applications. The large energy dissipation capacity that metal foams possess could be exploited to control the seismic performance of structures, but only few research investigations have been focused on this matter. Within this context, the present paper investigates a possible solution to realize dissipative aluminium foam dampers to be applied in X-braced steel structure resembling a concentrically braced steel frame (CBF). The bracing system consists of a steel rod or tube linked to the aluminium foam damper. The damper device is constituted by aluminium foam that provides the structure with dissipation capacity when activated under compression. The permanent deformations of the aluminium foam are absorbed by a wedge device, avoiding a pinching behaviour in the global response. Analytical equations governing the global behaviour of the bracing system are herein developed. An X-braced steel frame complying with Eurocode 8 provisions is designed and considered as case-study. The structure is subsequently upgraded by introducing the dissipative device into the bracings. The results of preliminary experimental and numerical tests on the components of the device are herein presented, showing the potentiality of the solution.

Seismic response of RC frames equipped with buckling-restrained braces having different yielding lengths

<abstract> <p>Buckling-restrained braces (BRBs) have proven to be a valuable earthquake resisting system. They demonstrated substantial ability in providing structures with ductility and energy dissipation. However, they are prone to exhibit large residual deformations after earthquake loading because of their low post-yield stiffnesses. In this study, the seismic response of RC frames equipped with BRBs has been investigated. The focus of this research work is on evaluating the effect of the BRB yielding-core length on both the maximum and the residual lateral deformations of the braced RC frames. This is achieved by performing inelastic static pushover and dynamic time-history analyses on three- and nine-story X-braced RC frames having yielding-core length ratios of 25%, 50%, and 75% of the total brace length. The effects of the yielding-core length on both the maximum and the residual lateral deformations of the braced RC frames have been evaluated. Also, the safety of the short-yielding-core BRBs against fracture failures has been checked. An empirical equation has been derived for estimating the critical length of the BRB yielding cores. The results indicated that the high strain hardening capability of reduced length yielding-cores improves the post-yield stiffness and consequently reduces the maximum and residual drifts of the braced RC frames.</p> </abstract>

Nonlinear Static Seismic Response of a Building Equipped with Hybrid Cross-Laminated Timber Floor Diaphragms and Concentric X-Braced Steel Frames

Simplified seismic design procedures mostly recommend the adoption of rigid floor diaphragms when forming a building’s lateral force-resisting structural system. While rigid behavior is compatible with many reinforced concrete or composite steel-concrete floor systems, the intrinsic stiffness properties of wood and ductile timber connections of timber floor slabs typically make reaching a such comparable in-plane response difficult. Codes or standards in North America widely cover wood-frame construction, with provisions given for both rigid and flexible floor diaphragms designs. Instead, research is ongoing for emerging cross-laminated-timber (CLT) and hybrid CLT-based technologies, with seismic design codification still currently limited. This paper deals with a steel-CLT-based hybrid structure built by assembling braced steel frames with CLT-steel composite floors. Preliminary investigation on the performance of a 3-story building under seismic loads is presented, with particular attention to the influence of in-plane timber diaphragms flexibility on the force distribution and lateral deformation at each story. The building complies with the Italian Building Code damage limit state and ultimate limit state design requirements by considering a moderate seismic hazard scenario. Nonlinear static analyses are performed adopting a finite-element model calibrated based on experimental data. The CLT-steel composite floor in-plane deformability shows mitigated effects on the load distribution into the bracing systems compared to the ideal rigid behavior. On the other hand, the lateral deformation always rises at least 17% and 21% on average, independently of the story and load distribution along the building’s height.

On the behaviour of steel CBF for industrial buildings subjected to seismic sequences

The usual seismic design of buildings, based on current codes of practice, does not account for the occurrence of seismic sequences. Structures are designed to resist one single earthquake and then it is supposed that, before the following event, there is enough time to retrofit the damaged construction. This paper investigates the effects of seismic swarms on single storey Concentric X-Braced steel Frames, a very common construction system for industrial buildings. The selected seismic sequences have been chosen so that the first event is considered as the mainshock, and following events have a similar or greater PGA (Peak Ground Motion). The sequences can represent a swarm or a sequence of earthquakes spaced over time, occurring before the structures are retrofitted. The investigation of the effects on steel frames has been conducted in a simplified way through the analysis of Single Degree Of Freedom (SDOF) systems with a behaviour calibrated on the response of Multi-Degree Of Freedom (MDOF) real Concentrically Braced Frames (CBFs). One of the main focuses of this work is on the additional ductility to be used in the seismic design to account for the effects of seismic sequences. For this purpose, the 95th percentile of the required ductility ratio has been calculated in the typical fundamental frequency range of single storey concentric braced steel frames, as to estimate the increase of ductility or strength to be adopted in design. As a result, a reduction of behaviour factor up to 37% has been found.

Structural health monitoring of seismically vulnerable RC frames under lateral cyclic loading

The effectiveness and the sensitivity of a Wireless impedance/Admittance Monitoring System (WiAMS) for the prompt damage diagnosis of two single-storey single-span Reinforced Concrete (RC) frames under cyclic loading is experimentally investigated. The geometrical and the reinforcement characteristics of the RC structural members of the frames represent typical old RC frame structure without consideration of seismic design criteria. The columns of the frames are vulnerable to shear failure under lateral load due to their low height-to-depth ratio and insufficient transverse reinforcement. The proposed Structural Health Monitoring (SHM) system comprises of specially manufactured autonomous portable devices that acquire the in-situ voltage frequency responses of a network of twenty piezoelectric transducers mounted to the RC frames. Measurements of external and internal small-sized piezoelectric patches are utilized for damage localization and assessment at various and increased damage levels as the magnitude of the imposed lateral cycle deformations increases. A bare RC frame and a strengthened one using a pair of steel crossed tension-ties (X-bracing) have been tested in order to check the sensitivity of the developed WiAMS in different structural conditions since crack propagation, damage locations and failure mode of the examined frames vary. Indeed, the imposed loading caused brittle shear failure to the column of the bare frame and the formation of plastic hinges at the beam ends of the X-braced frame. Test results highlighted the ability of the proposed SHM to identify incipient damages due to concrete cracking and steel yielding since promising early indication of the forthcoming critical failures before any visible sign has been obtained.

Through gusset connection for two-story X-braced frames, a numerical study

A common and efficient type of concentrically braced frames in practice is the two-story X-braced (split-X) system. Most of the studies conducted on this bracing configuration, investigated the inelastic response of the frame under dynamic loading. While the mid-span gusset connection affects the frame response considerably, a limited number of studies have focused on this type of connection. In this paper, a finite element analysis model is developed for mid-span gusset connections. The model is first validated with experimental results obtained from a two-story X-braced frame tested under quasi-static cyclic loading in the literature. Next, the model is used to investigate the response of a mid-span, through gusset connection, which consists of a double channel beam and a through plate. Case study results show that the brace-intersected beam experiences about 37% lower stresses in the proposed frame compared to the conventional frame. The proposed connection is shown to achieve higher strength and better integrity than the conventional system. Parametric analyses are also conducted to address the shear demands and some geometrical design aspects in both proposed and conventional systems.

Shape optimization of braced frames for tall timber buildings: Influence of semi-rigid connections on design and optimization process

With the recent development of timber as a viable structural material for high-rise structures, glulam braced frames have been recently introduced in lateral load-resisting systems of timber buildings. Based on a simple shape optimization problem of a braced frame, this paper explores one of the specificities of timber structures: the influence of semi-rigid connections on their overall structural behavior and design. Dowel-type connections are first studied to obtain a simplified relation between joint stiffness and axial load-carrying capacity. Then, the established local behavior law is introduced in the shape optimization process and design of a discrete braced frame subject to lateral drift constraint under wind load. The problem is solved by a COBYLA optimization method, combined with Optimality Criteria (OC) member sizing techniques. Solutions are then evaluated and compared with classical steel/concrete design. The semi-rigid behavior of connections finally leads to a significant increase in the volume of timber but also affects the optimal shape and topology of the X-braced frame compared with classical results.

The effects of foundation uplift on seismic response of diagonally braced steel frames

The foundation of buildings commonly assumed to be fixed in practical seismic response analysis. However, in severe earthquake loadings, the uplift phenomenon would be occurred due to the lack of tensile strength in the subsoil. This research aims to investigate this nonlinear phenomenon to obtain the actual behavior of diagonally braced steel frames. For this purpose, four 4-, 8-, 12-, and 15-story X-braced frames are modeled in the software framework of OpenSees. Some key parameters such as the natural period of vibration, inter-story drift ratio, base-shear force, the axial force along with the plastic deformation of braces are examined in two cases consist of fully and partially restrained supports. In the latter case, gap link elements (no tension behavior) are employed to model the soil–structure interaction (SSI). Overall, the results showed that by occurring the uplift the first period and inter-story drifts of the structure are increased; while, the base-shear, axial force and plastic deformation of braces will be reduced.

Seismic Response Assessment of Stiffened RCC Frames Located in Gujarat

Past earthquakes in India, China, Nepal and other nations convey that abundant medium to high rise structures suffered major damages, comprising of life and safety of the people. This research compares the seismic response of RCC frames incorporated with different steel bracing systems that are located in Gujarat. The structural performances of the different bracing systems are analyzed for storey heights of 8, 12 and 16 considering the earthquake data of Bhuj. Diverse Structural Configurations like X-Braced Frames (XBFs), V-Braced Frames (VBFs), Inverted V-Braced Frames (IVBFs) and Moment Resisting Frames (MRFs) were taken under study. Three bayed frames with steel braces incorporated in the exterior frames are taken under consideration. The structural responses of these frames in terms of base shear vs. displacement, hinge formation, storey displacements, global damage index, overstrength factor and design factor were studied through pushover analysis. The roof displacement vs. time parameter was evaluated using time-history analysis.

The Effect of Different Braced Configurations on the Nonlinear Seismic Behavior of Steel Structure

Improvements in construction technologies have allowed steel structural elements to become more frequently used today in order to enable different architectural designs and to meet structural performance more effectively and efficiently. Structural steel has been used for more than a hundred years and has been tested under real earthquakes, which provide the basis of many earthquake-resistant steel construction standards. The major advantage of steel construction material is that it allows for large plastic deformations. Structural deformations vary depending on the deformation capacity of the structural components in addition to the configuration of the structural components. In this study, moment resisting frames (MRF), X braced frame (XBF), Inverse V braced frame (IVBF), K braced frame (KBF), and eccentric inverse V braced frames (EIVBF) were used to examine the effect of different steel braced systems on the plastic deformation capacity of steel structure with the help of nonlinear static pushover analysis. Bilinear material model was utilized to represent nonlinear steel material behavior and inelastic displacement-based frame element were used to represent column and beam element. The analyses' results demonstrated that the braced frame configuration had a significant effect on the lateral response of steel frame structures.

Behavior of ductile steel X-braced RC frames in seismic zones

A satisfactory ductile performance of moment-resisting reinforced concrete concentric braced frame structures (RC-MRCBFs) is not warranted by only following the provisions proposed in Mexico’s Federal District Code (MFDC-04). The nonlinear behavior of low to medium rise ductile RC-MRCBFs using steel X-bracing susceptible to buckling is evaluated in this study. The height of the studied structures ranges from 4 to 20 stories and they were located for design in the lake-bed zone of Mexico City. The design of RC-MRCBFs was carried out considering variable contribution of the two main lines of defense of the dual system (RC columns and steel braces). In order to observe the principal elements responsible for dissipating the earthquake input energy, yielding mappings for different load-steps were obtained using both nonlinear static and dynamic analyses. Some design parameters currently proposed in MFDC-04 as global ductility capacities, overstrength reduction factors and story drifts corresponding to different limit states were assessed as a function of both the considered shear strength and slenderness ratios for the studied RC-MRCBFs using pushover analyses. Additionally, envelopes of response maxima of dynamic parameters were obtained from the story and global hysteresis curves. Finally, a brief discussion regarding residual drifts, residual drift ratios, mappings of residual deformations in steel braces and residual rotations in RC beams and columns is presented. From the analysis of the obtained results, it is concluded that when a suitable design criterion is considered, good structural behavior of RC-MRCBFs with steel-X bracing can be obtained. It is also observed that the shear strength balance has an impact in the height-wise distribution of residual drifts, and an important “shake-down” effect is obtained for all cases. There is a need to improve design parameters currently proposed in MFDC to promote an adequate seismic performance of RC-MRCBFs.