Bracing Configurations Research Articles

Design-parameter study on seismic performance of chevron-configured SCBFs with yielding beams

Special concentrically braced frames (SCBFs) designed to meet the 2016 American Institute of Steel Construction Seismic Provisions for Structural Steel Buildings require beams in chevron configurations to develop expected brace forces associated with large lateral deformations. The braces are assumed to fully yield in tension and buckle and deteriorate in compression. These brace forces impose axial forces, shear forces, and bending moments in beams that usually require relatively deep and heavy beam cross sections in comparison to gravity framing and beams in other bracing configurations. A research study was undertaken to develop a new design approach that permits the use of smaller beams engaged as secondary yielding mechanisms. The proposed design approach was developed from previous large-scale experiments and nonlinear response-history analyses. However, the variety of brace sizes and frame geometries used in research to-date is limited. Therefore, two studies were conducted using nonlinear analysis to consider practical design parameters that could impact seismic performance. First, a large parametric study was conducted on three-story chevron SCBFs under static cyclic loading. Second, different methods were used to design typical three-story chevron SCBF buildings for a site in Seattle, WA, which were then subjected to response-history analysis using earthquakes representative of the 2%/50-year hazard level. The studies indicate that low-rise buildings designed by the proposed method have equal or improved seismic performance compared to those designed by conventional design methods, and in conjunction with previous research, they help support future use of the proposed method in practice.

Lateral Stability Analysis for Composite Plate Girder Bridges

The methods that are currently available in the Eurocodes for the design of bridge plate girders to lateral torsional buckling are outlined and discussed. The inapplicability of the classic elastic critical moment theory for deep girders, which are common in bridge practice, is demonstrated. The simplified method of the equivalent flange in compression is applied instead, and its results are compared with those of specialised software which can accurately predict the elastic critical load of bridge girders. These widely accepted methods of design are compared to a previously published truss model which has been proposed for the simplified, yet accurate, modelling of bridge plate girders. The results of both the critical load and the design load according to the Eurocodes are compared. For the truss model, the general method for the calculation of the design load is used. A variety of cross-sectional dimensions, lateral restraints and bridge configurations is examined. Emphasis is given in a two-span, two-girder steel concrete composite bridge with various bracing configurations, where many aspects of lateral torsional buckling design can be demonstrated. The advantages of the use of a 3D truss model from practicing engineers are discussed.

Seismic robustness of self‐centering braced frames suffering tendon failure

AbstractThis paper comprehensively discusses the behavior and failure risk of self‐centering braced frames suffering tendon fracture. The fundamental mechanism of tendon failure in self‐centering braces (SCBs) is first introduced, followed by the design and analysis of a series of prototype buildings with different tendon materials and brace configurations. Assuming a normal distribution of tendon fracture strain, the dynamic behavior of the frames is then assessed by a suite of ground motion records, covering both far‐field and near‐fault ones. The collapse and residual deformation fragilities of the frames are further evaluated, and the study ends with a risk assessment considering a 50‐year service period. Among other findings, the study indicates that tendon fracture tends to increase the peak interstory drift, especially for the structure with smaller tendon fracture strains. Tendon fracture also compromises the self‐centering capability significantly, although there is no obvious statistical correlation between tendon fracture and the peak floor acceleration. The probability of collapse and that of exceedance of certain residual drift both increase evidently when tendon fracture is considered. The failure probabilities are closely related to the available deformability of the SCBs, where dual‐core SCBs show less sensitivity to tendon fracture. The probability of collapse of the considered frames over 50 years of service increases from 1.25‐2.12% to 3.58‐6.52% when tendon fracture is considered. Considering a residual drift threshold of 0.5%, the probability of exceedance of the same structures over the same life span increases from 1.78‐3.54% to 5.46‐9.71%.

Super elastic system using parallel spine frames with steel braces

AbstractTo reduce the seismic response and residual deformation of structures, this study proposes a super elastic bracing system comprising parallel spine frames with steel rods acting as the braces connecting adjacent spine frames. The brace‐end connections are located on the outer side of the pin supports of spine frames to ensure that the axial deformation of the braces is lower than that of a regular bracing system with identical brace angles. To verify the feasibility and elastic range of the proposed system, cyclic loading tests were conducted using scaled specimens consisting of two spine frames with four different brace configurations. Numerical models created in OpenSees were used to simulate the loading tests appropriately. Four evaluation methods were developed for predicting the force and deformation relation considering the geometrical nonlinearity of the proposed system. Through parametric analyses, the influence of key parameters on structural behaviors is clarified, and the applicability of the evaluation methods is investigated.

COMPARISON STUDY OF BRACING CONFIGURATION WITH SHEAR LINK IN ECCENTRICALLY BRACED FRAME STEEL STRUCTURE

The EBF structural system is identified by the use of bracing and link beams as components that work to receive lateral seismic loads. The position of the link beam makes the EBF system have several choices of bracing configuration; D-Braces, Split K-Braces, V-Braces, Split K & Inverted Split K-Braces, Inverted Y-Braces. Structural analysis was carried out on a 10-story building model for the EBF system with different type of bracing configurations using the ETABS software. All models analyzed according to Indonesian Code (SNI 1729:2015 and SNI 1726:2019) to obtain the structural element. Seismic analysis uses the response spectrum analysis method to obtain the structural response parameters in the EBF system. Result of the analysis for all of bracing configuration are shown that Split K-Braces model has the best response parameters when compared to the MRF system. The lowest value for the parameter is owned by Inverted Y-Braces, although overall it is still larger than the MRF system. The bracing configuration greatly affects the response of EBF system due to the behavior that occurs in the link beam, therefore the selection for type of bracing configuration is also important in the EBF system structure design.

Numerical modelling and design of hybrid cold-formed steel wall panels

Hybrid cold-formed steel (CFS) wall panel, composed of square hollow sections (SHS) and CFS open sections, can offer some great advantages with respect to the lateral performance of light steel frames, in particular for applications in mid-rise construction. This paper presents non-linear finite element modelling of hybrid wall panels, calibrated through experimental tests, and their applications to different designs. The study involves the lateral behaviour of the proposed hybrid walls in terms of load-displacement curve, failure mode, stiffness, ductility ratio, energy absorption, and strength to weight ratio. First, a numerical model, verified and calibrated based on the experimental tests on the hybrid wall panels, is presented. Thereafter, using the validated numerical model, twenty new hybrid wall panel designs are proposed in order to investigate the effect of different SHS brace configurations on the hybrid panels’ performance. The comparison of the experimental and numerical results indicates that superior seismic performance can be achieved from the proposed hybrid CFS wall designs.

Effects of Brace Configuration and Structure Height on Seismic Performance of BRBFs Based on the Collapse Fragility Analysis

The brace configuration and structure height are two factors that have a significant effect on the seismic behavior of braced frame buildings. In the present study, the buckling-restrained braced (BRB) frames were considered to estimate the effect of these two parameters using probabilistic seismic assessment methods. The uncertainty in the different parameters involved in the seismic design of the structural system was also considered. Four, six, and ten-story buildings with the Chevron and inverted Chevron bracing configurations were designed, and their responses due to various ground motions were estimated using incremental nonlinear dynamic analyses. Fragility curves, mean annual frequency of exceeding immediate occupancy (IO), and collapse prevention (CP) states were generated using probabilistic seismic analysis, fragility curves concept, and drift hazard curves. The results demonstrate that the inverted Chevron type BRBFs has better structural performance than Chevron bracing types. Furthermore, an increase of the height of structures, despite lower drift’s hazards, increases the fragility probability.

Experimental testing and analytical modeling of CFS shear walls filled with LPM

Cold-formed steel (CFS) shear walls filled with lightweight polymer material (LPM) is an efficient structural system which can provide high lateral stiffness and strength to resist lateral loads. Despite the significant advantages, the nonlinear behavior of the CFS shear wall filled with LPM is not fully understood. In this study, a total of seven full-scale CFS shear walls filled with LPM were tested under cyclic loads. The test parameters included different LPM types, X-shaped bracing configurations, sheathing types and wall openings. The failure modes hysteresis curves, envelope curves, feature values, ductility ratio, stiffness and energy dissipation of CFS shear wall with and without filling LPM were systematically compared. The experimental data reveled that this type of shear wall showed higher load carrying and energy dissipation capacities than CFS framing wall without LPM. To assist with the design of CFS shear walls filled with LPM, a modified strut-and-tie model was proposed. The modified strut-and-tie model was validated using the experimental data. The result demonstrates that the proposed model can be utilized to accurately estimate the load carrying capacity of CFS shear walls filled with LPM. The presented experimental and analytical results can be used by engineers to design CFS shear walls filled with LPM for earthquake engineering applications.

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.

Behaviour of Steel Transmission Line Towers under Earthquake Loads using Labview

This paper deals with a detailed outline on the design, analysis, response and control measures of steel transmission line towers for the past few years. Due to the technological boom there led a change in our current way of lifestyle which makes necessary for the electricity to be available for 24x7. As the transmission line towers are one of the essential life line structures in the distribution of power from the source to the various places for several purposes and stand forth as lifelines of humans by enlightening our daily lives, it is essential to create a damage free transmission tower since, it is not imaginable to lead the life with absence of such electric serving systems. This review paper shed light on various case studies that deals with the collapse of transmission line towers which are stimulated by the action of wind and earthquake loads along with the creation of three dimensional models using the software. In addition detailed response of towers at different seismic zones under various wind zones are also discussed. The response of the transmission line tower differs depending over several structural bracing configurations which are highlighted in here through the comparisons. This paper also indicates about the response of towers with the foundation at soil surface which is termed as pile-soil-structure dynamic interaction. The present study is the outline of various literature researches over the analysis of steel transmission line towers by using the FEM analysis.

Research on Application of Buckling Restrained Braces in Strengthening of Concrete Frame Structures

This paper examines the application of BRB in strengthening of reinforced concrete frame structures to meet seismic requirements according to Chinese seismic design code. Elastic response spectrum analysis and nonlinear time history analysis are performed by taking a real engineering example that suffers weak first floor irregularity due to added loads and addition of one floor. With the method of equivalent stiffness and displacement-based design method, buckling restrained brace parameters are deduced and are used to model BRB in ETABS using plastic wen model. Three configurations of buckling restrained braces are studied together with ordinary braces. Under elastic state, the relationship between the required cross section area of BRB and ordinary braces is deduced from the formula of calculating elastic bearing capacity where it is shown that the area of the ordinary braces must be 1.25 times that of BRB for ensuring the same performance. The results show that Inverted V brace configuration demonstrated better performance over single brace and V brace configurations and X brace configuration, although not recommended by Chinese code, is simulated and used in this paper and has demonstrated better performance over other configurations, and the further research on the practical use of this brace is recommended. Also, under action of strong earthquakes, by nonlinear time history analysis, buckling restrained braces demonstrated better performance of strengthening the structure and make it meet the requirement of code. Under this same condition, ordinary braces losses their bearing capacity due to excessive buckling.

Investigation of seismic rehabilitation of steel buildings with semi-rigid connections

The Khorjinee connection is a special connection that was widely used in the construction of low-rise steel structures in Iran before the national seismic code was established. However, catastrophic experiences from recent earthquakes have shown that these connections are highly vulnerable to seismic loads. In fact, it is recognised that such connections should be classified as non-seismic, semi-rigid connections with unknown rotational stiffness. As a result of this issue, many structures with these connections may need to be rehabilitated to ensure their safety in future earthquakes. In this study the dynamic properties of a half-scale model of a four-storey steel structure with Khorjinee connections were determined by a forced vibration test. The structure was then separately rehabilitated by means of four different bracing configurations (X bracing, chevron bracing, eccentrically braced frames (EBFs) and knee braces) and the structural responses were investigated. The results, which include the natural frequencies, mode shapes and damping ratios for each case, present a realistic approach to enable engineers to select the best way of strengthening steel structures with Khorjinee connections according to diverse structural limitations. The structural displacement ratios were found to be significantly larger in structures with EBFs and knee bracing systems when the number of braced spans is high compared with the first mode.

Rotational behavior of bolted glulam beam-to-column connections with knee brace

This paper investigates the rotational behavior of bolted glulam beam-to-column connections reinforced by knee brace. Due to the low rotational stiffness of conventional glulam beam-to-column connections (i.e., bolted connection with slotted-in steel plate), diagonal braces are usually installed to increase the lateral load resisting capacity of the structure. However, the architectural space will be heavily impacted by such installation. Column and beam members combined with knee braces would be a compromise between the enhancement of connection mechanical performance and the flexibility of architectural layout. Sixteen specimens with different knee brace configurations were designed for a series of monotonic and reserved cyclic loading tests. The failure modes, moment-resisting capacity, stiffness degradation, and hysteretic moment-rotation responses of the specimens were tested and evaluated. Results showed that the installation of knee brace led to a more ductile failure mode. For the investigated connections with knee brace, the initial stiffness was 5.98 times that of the corresponding bolted connection without knee brace, and the moment-resisting capacity was 6.80 times that of the corresponding bolted connection without knee brace. Moreover, the influence of different knee brace configurations (e.g., section modulus of the knee brace, installation angle, installation end distance, etc.) on the rotational behavior of the connections were discussed. An optimized knee brace configuration was proposed to improve the rotational behavior of the connections and to decrease the damage accumulation in the glulam beam and column members. Nonlinear numerical models, including a detailed three-dimensional model and a simplified model, were further developed to comprehend the mechanical performance of such connections. The presented experimental and analytical results can be used as reference for the development of design guidelines for the glulam bolted connection with knee brace in practical projects.

Evaluation of Design and Simulation of Creep Test Rig for Full‐Scale Crossarm Structure

A simulated model was developed in order to design and simulate the mechanical properties of a cantilever beam creep testing rig for a full‐scale size crossarm in transmission towers. Currently, the Malaysian power grid system is implementing several materials, such as Chengal wood, polymeric composite, and galvanised steel, as crossarm structures. However, there is a lack of study regarding the long‐term mechanical behaviour of heavy structures in the literature. Hence, this article explains the design development of creep test rig for a full‐scale crossarm structure using CATIA and mechanical simulation (deformation and safety factors) of the product via ANSYS. The test rig will be used to predict the creep life of the cantilever beam structure. In this study, a tall and large base area structure was designed and replicated from an actual tower to elevate the crossarm above the ground level. In order to select the best performance model, a baseline conceptual test rig was generated in CAD modelling, and the finite element analysis was carried out by using a static structural analysis in ANSYS. Four different bracing configurations were incorporated in the baseline model, and the modified structures were then analysed. The results show that the hybrid bracing configuration has enhanced the mechanical properties and safety factors in the baseline model.

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.

Coupled element and structural level optimisation framework for cold-formed steel frames

Optimisation of cold-formed steel (CFS) structures can be challenging due to the complex behaviour of thin-walled CFS sections affected by different buckling modes. In this paper, a coupled framework is presented for element and structural level optimisation of CFS portal frames, under serviceability limit state (SLS) and ultimate limit state (ULS) conditions, using Genetic Algorithm. First, CFS lipped-channel beam sections are optimised with respect to their flexural capacity determined in accordance with the effective width method specified in Eurocode 3 (EC3). The relative dimensions of the cross-section are considered as the main design variables, while the EC3 plate dimensions and slenderness limits and a number of manufacturing and end-use constraints are taken into account in the optimisation process. The results show that the optimum CFS sections exhibit significantly higher (up to 84%) ultimate capacity compared to the standard lipped channel sections with the same plate width and thickness. The structural level optimisation is then carried out to obtain the optimal design solution for a long-span CFS portal frame with knee braces under SLS and ULS conditions. Compared to conventional optimisation using standard cross-sections, it is shown that the proposed coupled framework leads to more cost-effective solutions (up to 20% less structural material) by using the more efficient CFS cross-sectional shapes optimised for generic applications. The results also indicate that optimising the frame geometry and knee brace configuration can noticeably improve the structural performance and reduce the required structural weight, especially when both ULS and SLS conditions are considered.

Pile deflection analysis due to asymmetrically distribution loads

Jacket platform is one of the offshore structure where the pile is fixed at the bottom sea level. Pile and brace configuration has a significant influence on local and global structural strength under axial and lateral loads. Pile is one of the most important elements for the fixed offshore structure. On the other hand, the pile is also very critical to resists not only axial compression but also lateral pressure. The objective of the present study is to analyzed pile deflection due to asymmetrically distribution loads. The numerical calculation is used to analyze pile deflection due to asymmetrically distribution loads. In this case, large deflection analysis is separated into two asymmetrically loading conditions those are trapezoidal and triangular with different end support. The result obtained by numerical analysis is therefore compared with the analytical solution using direct integration. It found that the pile deflection obtained by numerical analysis is almost identic with direct integration. It is also observed that the end support condition gives significant influence to support reaction and deflection.

Shake table tests of special concentric braced frames under short and long duration earthquakes

Several long duration and subduction earthquakes recently took place in different locations such as Chile (2015, 2014, and 2010), Japan (2011), China (2008), and Indonesia (2004). These earthquakes motivate the need to prepare for a possible long-duration, large-magnitude earthquake in the Cascadia subduction zone along the Pacific Northwest Coast of the United States. Although earthquake duration is expected to affect the response of structures and damage accumulation, current seismic design specifications do not consider duration effects. One reason might be the lack of sufficient experimental research to better understand the effect of the duration on structural performance. In this paper, an experimental program was conducted to investigate the influence of earthquake duration on structural response of steel special concentric braced frames (SCBFs). Three identical 1/2-scale one-story one-bay SCBFs with chevron brace configuration were tested on one of the University of Nevada, Reno shake tables under unidirectional short and long duration earthquakes. The overall objective was to investigate whether the duration of earthquake have an effect on the collapse capacity of SCBFs dictated by brace rupture. Test results showed that the earthquake duration affects the structural performance of SCBFs and can cause a premature failure, i.e. lower displacement capacity, because it is directly related to the low cycle fatigue life of the braces. The experimental data provided here can be used to verify fatigue damage and degradation numerical models for capturing duration effects, and support initiatives to incorporate the duration effect in future seismic design provisions and performance-based assessment frameworks.

Stable Length in Inelastic Design Considering Ductility Requirement

The region close to plastic hinge is vulnerable to lateral instability due to the formation of the mechanism. A stable length between lateral restraint at the plastic hinge and adjacent lateral restraint plays a significant role in preventing lateral instability and until after the required hinge rotation has taken place. This phenomenon is studied for beams under uniform moment loading by carrying out an extensive parametric study on a mixed set of I-shaped hybrid and homogeneous plate girders using nonlinear finite element analysis. Geometric dimensions and steel grade of plate girder elements are varied to account for their effect on member slenderness. Effect of overall slenderness is included by varying lateral bracing configuration. Attention is given to the interaction between local and lateral buckling and their influence on inelastic rotation. Regression analysis of the database is carried out to arrive at a prediction equation for stable length to achieve the required rotation capacity. The equation is validated by applying it to thirty-nine selected experiments conducted by others and comparing results with the results of available prediction equations. Statistical analysis of the validation study shows that the proposed equation provides more refined results as compared to available equations. It is observed that use of the new prediction equation when used along with rotation capacity prediction equation suggested by authors in the previous study proves to be a rational solution as available stable length equations that are based only on geometric proportions of sections. At the end of the paper, a flowchart and demonstration examples for use of these equations are presented.

Influence of the Brace Configurations on the Seismic Performance of Steel Concentrically Braced Frames

Concentrically braced frame is an effective and prevalent seismic force resisting system which is commonly used in low-rise buildings. This type of structural system utilizes steel braces to provide the stiffness and strength needed to dissipate earthquake energy. Several bracing configurations have been proposed in different building codes worldwide. These codes provide detailed design requirements for the structural members and connections, but no guidance is provided in selecting the best bracing configuration for the design. In this study, the impact of the bracing configuration on the seismic response of a five-story prototype office building located in Vancouver, Canada, is systematically examined. Five different bracing configurations were designed according to the National Building Code of Canada and CSA S16 standard. Detailed structural responses, initial costs, and life cycle costs of the prototype building with five different bracing configurations were systematically compared. The results show that the different bracing configurations play an important role in sizing the structural members, which impacts the initial material usage and the overall life cycle cost of the building.