Steel Concentrically Braced Frames Research Articles

Development of pinching‐free tension‐only CBFs based on the application of metal foam dampers

AbstractMetal 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 structures for concentrically braced steel frames (CBFs). The bracing system consists of a steel rod or tube linked to an 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 presented. 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.

Optimal seismic design of multiple-rocking steel concentrically-braced frames in 3D irregular buildings

This paper presents a gradient-based optimization approach for the seismic design of multiple-rocking systems in irregular buildings. The proposed design method tackles general 3D buildings with high torsional and vertical irregularities is developed. The design problem is formulated as an optimization problem that minimizes the construction cost of steel self-centering concentrically braced frames. The constraints are formulated based on current seismic design code requirements and recommendations from the literature. The structural response is computed using nonlinear time history analyses of general 3D models subjected to ensembles of ground accelerations.A dual-phase optimization approach is developed to reduce the computational effort and improve convergence In the first phase, a simplified numerical model is adopted for the analyses while the topology of the framing elements is not optimized. The second phase adopts a more detailed numerical model and starts with fixed location of the rocking sections. This enables topology optimization of the framing elements as well. The design method was implemented for the design of an 8-story building with two setbacks and high torsional irregularities subjected to an ensemble of ground accelerations, each having three components. The results show that this method provides efficient designs of these complex systems with a reasonable computational effort.

A Technique for Optimizing the Sequences Yielding under Load of Concentrically-Braced Steel Frames

Concentrically-braced steel framing is widely used in tall buildings. Designing with adequate but not excessive seismic resistance is a challenge because of the limited experience of seismic failure and the huge variety of components used. A quantitative method for defining an acceptable range of component parameters is proposed and tested using published experimental data and finite element modeling. The method involves the structural yield mechanism control method of the steel concentrically-braced frame. It is proposed by inequality iteration of different structural components’ bearing capacities. It generates acceptable ranges for the parameters defining the properties of the columns, beams, and braces. The test results show that concentrically-braced steel frames designed within the recommended ranges will have the desired sequence of component yielding. The sequence is, however, highly sensitive to components’ parameter values. In practical engineering stochastic variability in the parameters must be considered.

Performance-based rules for the simplified assessment of steel CBFs

In this work, a simplified method for the evaluation of the seismic performance of steel Concentrically Braced Frames (CBFs) in everyday practice is presented. This method, applicable in the immediacy of a seismic event and for quick mapping of the seismic vulnerability of a built area is fully analytical, allowing the representation of the capacity curve of a structure through a trilinear approximation and considering the second order-effects. To set the methodology, extensive parametric analysis is performed on 420 simple CBFs designed according to 3 different approaches namely the Theory of Plastic Mechanism Control ensuring the design of structures characterized by global collapse mechanism (Global Concentrically Braced Frames (GCBFs), the Eurocode 8 provisions (Special Concentrically Braced Frames (SCBFs)) and the design for seismic forces only (Ordinary Concentrically Braced Frames (OCBFs)) without any hierarchy criterion. The capacity-demand checking procedure according to the code-compliant approach and Nassar & Krawinkler is also reported.

Hybrid braced frame with buckling-restrained and strong braces to mitigate soft story

The steel concentric braced frames (CBFs) are the common structural system for buildings. Unlike high strength and stiffness, brace buckling in CBFs results in high nonlinear behavior, which causes damage concentration in one or a few stories during a severe earthquake. This paper proposes a hybrid CBF bracing system to reduce soft-story mechanisms and improve the structural behavior of the CBF system. The proposed system is composed of a buckling restrained brace (BRB) or an energy dissipation device and a strong brace (SB) in one span. The strong braces are designed to have an elastic performance during an earthquake. The direction of BRB and SB are changed, inversely, story by story in a zigzag form. This configuration leads to paradoxical results: stiffness and ductility. This improves both the soft story mechanisms and the buckling capacity of CBFs. In this study, a set of 6-story structural models with different bracing patterns including inverted-V, multi-X, zipper, hexa (hexagonal bracing configuration), and strongback braced frames were evaluated through nonlinear static (pushover) and dynamic analyses (IDA). The collapse performance of structural models was assessed by comparing the seismic behavior of conventional CBFs with similar bracing configuration based on FEMA P695 criteria. From the evaluation of collapse fragility curves, the hybrid bracing system had more safety margin against collapse during severe earthquakes than conventional CBFs.

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.

Direct Displacement-Based Design (DDBD) applied to dual wall-frame buildings and steel concentric braced frames

One of the most efficient seismic design methodologies is Direct Displacement Based Design (DDBD) because it considers the inelastic range of the structure. There are several investigations on the application of this methodology in frames, structural wall and bridges, but those related to steel structures with concentric bracing are scarcer. The objective of this investigation is to determine the damping parameters and the seismic responses of a dual concrete building and one of concentrically braced steel frames, to later compare their base loads and displacements. The procedure first determines the design offset for the 1GDL model; then the desired responses are obtained by this displacement. This methodology uses an effective stiffness, which is composed of an elastic and an inelastic state of the structure to determine seismic responses.

Brace-to-frame connection modelling effects on seismic loss assessment of steel concentrically-braced frames

The main objective of this study is to investigate the implications of the explicit brace-to-frame gusset connection modelling on the seismic performance assessment of steel concentrically-braced frames (CBFs). Two key modelling alternatives are considered, namely through the simulation of CBFs with: i) a benchmark experimentally-validated model, in which the gusset plates are explicitly designed and simulated; and ii) a more simplified option, whereby diagonals are simulated through a centreline-to-centreline approach with fully-pinned boundary conditions. Anticipating the consequences of a less refined modelling approach, a simplified regression-based design process to Eurocode 8 for these gusset connections is proposed, therefore avoiding the need for an explicit connection design for seismic assessment purposes. A third modelling option, an identical approach to the benchmark but with the gusset dimensions being determined through the aforementioned regression-based design process, is also considered. The importance of the explicit modelling of the gusset plate connections to obtain reliable estimates of seismic performance is shown through the comparison of the seismic performance of various building archetypes, considering different metrics (e.g. lateral deformations, earthquake-induced collapse risk and economic losses). Furthermore, the validity of the proposed gusset-design methodology is discussed, by comparing the resulting gusset dimensions against those associated with a rigorous seismic design process, as well as by comparing the seismic performance assessed with numerical models incorporating explicitly-designed or simplified-designed gusset geometries.

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.

Mitigating Inter-Story Drift Concentration of Concentrically Braced Steel Frames Using Energy-Dissipative Columns

ABSTRACT Concentrically braced steel frames (CBSF) always suffer from weak-story failure with a large inter-story drift concentrated on the weakest story. A novel system of Energy-Dissipative Column (EDC) is proposed in this paper to improve the seismic behavior of low and mid-rise CBSFs. The EDC system consists of dual hinge supported steel boundary branches and replaceable steel strip dampers between them. Nonlinear dynamic analyses are conducted on 3-story and 6-story benchmark CBSF buildings with and without EDCs. The effectiveness of EDCs with different stiffness ratios on mitigating the seismic responses of CBSFs under design based and maximum considered earthquakes are investigated. The results show that the presence of EDCs in an existing multi-story CBSF can not only dissipate seismic energy to reduce the maximum inter-story drift, but facilitate more uniform inter-story drift distribution by redistributing the seismic forces along the structural height. It is found that the maximum inter-story drift and drift concentration can be reduced by half when the lateral stiffness ratio of EDC to CBSF reaches 0.2.

Collapse-performance-aided design optimization of steel concentrically braced frames

This study focuses on collapse-performance-aided optimization of steel concentrically braced frame (SCBF) structures optimized in the framework of performance-based design. The proposed methodology in this paper consists of three steps. The first step involves tackling the problem of seismic performance-based size and topology optimization of SCBFs. In the second step, incremental dynamic analysis (IDA) is performed to generate IDA and fragility curves for all the obtained optimal designs. Subsequently, their collapse margin ratios are calculated. In the last step, the SCBFs with fixed and optimal placement of bracings are compared in terms of structural weight and seismic collapse capacity. The efficiency of the proposed methodology is illustrated by presenting three numerical examples of 5-, 10-, and 15-story SCBFs with fixed and optimal topology of braces. The results demonstrate that the topologically optimal SCBFs not only have the least structural weight, but also they are of considerable collapse safety in comparison to optimal SCBFs with fixed topology of braces.

Practical considerations on the design of concentrically-braced steel frames to Eurocode 8

Conventional seismic design of steel buildings is typically conducted, at a preliminary stage, by resorting to idealisations of member joint behaviour (e.g. fully-articulated connections). After the main structural members are defined, one proceeds with the connection design process, whose actual behaviour may or may not deviate from the assumption initially made at the design stage. This research study assesses the implications of connection-related design assumptions for steel concentrically-braced frames (CBFs) designed to Eurocode 8. This goal is achieved through the use of a Python-OpenSees-based automated seismic analysis and design framework to define an extensive set of CBF archetypes. The effect of non-explicit consideration, at the design stage, of the diagonal-to-frame gusset connections is evaluated. Full compatibility of this assumption is shown to be unassured, particularly due to the combined effects of member length reduction and boundary condition flexibility on the member's normalized slenderness. Based on the extensive suite of frame and connection designs considered, a regression-based modified normalized slenderness factor is proposed to realistically estimate, at the design stage, the normalized brace slenderness.

DDBD assessment of steel CBFs using full scale shake table tests with realistic connections

A direct displacement based design (DDBD) methodology for concentrically braced frames (CBFs) is validated using a combination of experimental and numerical data taking into account the behaviour of brace-beam-column connections. Twelve full-scale shake table tests with four different brace cross-section sizes, two different gusset plate design methodologies and two different brace connection configurations are used to evaluate the design procedure at three different performance levels (continued operation, life safety and collapse/near collapse) through successive scaling of test excitations.It is found that the DDBD procedure can predict the behaviour of the shake table CBFs for the three levels of excitation. Accurate results have been found for life safety design where the mean ratio and coefficient of variation of base shear (Fb) calculated using the DDBD method to the values found from shake table tests are 0.96 and 0.17, respectively. Furthermore, the mean ratio and coefficient of variation of brace area (Ab) calculated from the DDBD method to the values used in shake table tests are 1 and 0.08, respectively.The design procedure was also verified using nonlinear time history analysis (NLTHA) using a selection of earthquake records with displacement spectra compatible with those of the exact accelerograms recorded during the shake table tests. At the life safety performance level, the mean peak displacement demand and base shear values obtained from NLTHA were observed to be 82% and 91%, respectively, of the design values determined by the DDBD methodology.

Shake Table Testing of Concentrically Braced Steel Structures With Realistic Connection Details Subjected to Earthquakes

This paper describes an experimental investigation into the seismic response of concentrically braced steel frames (CBFs). Twelve shake table tests were performed on full-scale single storey frames, each containing a pair of identical brace members. The experimental programme examined the behaviour of brace members with four different square and rectangular hollow cross-sections and a range of gusset plate connection details. The aim of the experimental study was to determine the influence of brace and gusset plate properties on CBF response from serviceability to ultimate limit states, including collapse. Consequently, all test frames were subjected to three levels of seismic excitation: (i) low-level excitation to examine elastic frame response, (ii) medium-level excitation to examine brace buckling and yielding effects, and (iii) high-level excitation to induce brace fracture. A detailed set of data on the seismic response of CBFs with realistic brace members and connections were obtained from the tests. The experiments were conducted under representative dynamic response conditions as opposed to the conventional idealised quasi-static loading procedures employed in previous experimental investigations of CBF behaviour. The results faithfully capture the behaviour of brace-gusset plate test specimens with different non-dimensional brace slenderness, brace cross-section slenderness, connection types and gusset plate detailing. The response variables measured in each test included the shaking table and frame accelerations and displacements, brace elongation and axial force, and brace member and gusset plate strains. The experimental observations include elastic frame vibration properties, acceleration and drift demands, ultimate failure modes and ductility capacity. The brace-gusset plate test specimens remained elastic at low-level excitations, brace buckling and yielding occurred in all medium-level excitation tests, while specimens exhibited brace fracture under high-level excitation. Fracture did not occur in the gusset plate connections irrespective of whether these were designed using a conventional design method with a Standard Linear Clearance (SLC), or a balanced design with an Elliptical Clearance (EC). However, the balanced design approach showed more uniform distribution of plastic strains and led to higher brace ductility capacities when compared to the conventional design method. Based on the test results, available methods for predicting the ductility of bracing members are compared and assessed, and a number of considerations for design are highlighted and discussed.

Dynamic buckling of braces in concentrically braced frames

AbstractAxially loaded members might experience compressive forces above their static buckling capacity because of dynamic buckling under rapid shortening. Although the subject is studied in the context of engineering mechanics, it has not been thoroughly investigated in the field of earthquake engineering. Such dynamic overshoots in the compressive capacity can also be observed for braces of concentrically braced frames (CBFs) during earthquakes. Consequently, a comprehensive investigation is conducted in this study regarding the effects of dynamic buckling of braces on the seismic behavior of steel CBFs. After providing a theoretical background, recent dynamic experiments on braces and CBFs are simulated and discussed to investigate the occurrence of dynamic overshoot during these tests. Eight archetype CBFs are then designed, modeled, and subjected to a large set of ground motions to provide a quantified insight on the frequency and anticipated level of dynamic overshoot in the compressive capacity of braces during earthquakes. Results of a total of 1600 nonlinear time history analyses revealed that dynamic overshoots occur frequently in braces and affect the behavior of CBFs notably. Considerable increases are recorded in forces transmitted to other members of CBFs as a consequence of such dynamic overshoots. Importance of incorporating these dynamic overshoots in the capacity design procedure of columns, beams, and gusset plates is highlighted. Furthermore, results of a parametric study are presented and summarized in the form of a simple formula that can be used as a guide for estimating the level of dynamic overshoot.

Recommendations for numerical modelling of concentrically braced steel frames with gusset plate connections subjected to earthquake ground motion

Concentrically Braced Steel Frames (CBFs) are commonly used as an economic and effective means of resisting the lateral loading induced during earthquakes and limiting the associated displacements. Here, an integrated experimental and numerical approach is taken to investigate the performance of CBFs subjected to seismic action of varying intensity. As part of the BRACED transnational research project, shake table experiments on full-scale single-storey CBFs recorded the response of test frames employing various combinations of bracing member sizes and gusset plate connection details to simulated ground motions scaled to produce elastic response, brace buckling/yielding and ultimately brace fracture. This recorded experimental data is used to validate a numerical model developed using the OpenSees seismic response analysis software. Key experimental and numerical model responses are compared. The sensitivity of the model to variations in modelling parameters is assessed and recommendations for future numerical modelling are presented. Results indicate model performance is sensitive to the initial camber applied to the brace members, with a value of 0.8% of overall brace length observed to achieve a more accurate representation of global frame stiffness and drift response than the lower values previously recommended in literature, but an underestimate of the compression resistance of the brace.

Seismic behavior of concentrically braced frames designed to AISC341 and EC8 provisions

Steel concentrically braced frames (CBFs) are frequently used as efficient lateral load resisting systems to resist earthquake and wind loads. This paper focuses on high seismic applications where the brace members in CBFs dissipate energy through repeated cycles of buckling and yielding. Widely-used seismic provisions have somewhat different approaches for the seismic design of CBFs. The present study evaluates in detail the similarities and differences between the design philosophies and provisions used in the United States and Europe for these systems. The requirements of both provisions applied during a full design procedure are summarized and compared. Furthermore, X-braced, split X-Braced, and V-braced archetypes are designed accordingly and the differences in the design outcomes are investigated regarding section sizes and the weight of steel used in each design. Finally, inelastic structural models of the designed archetypes are developed and subjected to a large set of ground motions to study their seismic behaviors. The results of a total of 880 nonlinear time history analyses are then synthesized to investigate the way in which the requirements of these provisions affect the seismic behavior of the designed CBF. Notable differences are observed between the performances of the CBFs designed using American and European provisions. The similarities and differences as well as drawbacks of the provisions are thoroughly discussed. Recommendations and future research needs are suggested to enhance the seismic performance of steel CBFs designed according to these provisions.

Effects of uncertainties on seismic behaviour of optimum designed braced steel frames

Concentrically braced steel frames (CBFs) can be optimised during the seismic design process by using lateral loading distributions derived from the concept of uniform damage distribution. However, it is not known how such structures are affected by uncertainties. This study aims to quantify and manage the effects of structural and ground-motion uncertainty on the seismic performance of optimum and conventionally designed CBFs. Extensive nonlinear dynamic analyses are performed on 5, 10 and 15-storey frames to investigate the effects of storey shearstrength and damping ratio uncertainties by using the Monte Carlo simulation method. For typical uncertainties in conventional steel frames, optimum design frames always exhibit considerably less inter-storey drift and cumulative damage compared to frames designed based on IBC-2012. However, it is noted that optimum structures are in general more sensitive to the random variation of storey shear-strength. It is shown that up to 50% variation in damping ratio does not affect the seismic performance of the optimum design frames compared to their code-based counterparts. Finally, the results indicate that the ground-motion uncertainty can be efficiently managed by optimizing CBFs based on the average of a set of synthetic earthquakes representing a design spectrum. Compared to code-based design structures, CBFs designed with the proposed average patterns exhibit up to 54% less maximum inter-storey drift and 73% less cumulative damage under design earthquakes. It is concluded that the optimisation procedure presented is reliable and should improve the seismic performance of CBFs.

Performance-Based Plastic Design Method for Steel Concentrically Braced Frames Using Target Drift and Yield Mechanism

Under severe earthquakes, steel concentrically braced frames (SCBFs) will experience large inelastic deformations in an uncontrolled manner. According to the energy-work balance concept, a performance-based plastic design (PBPD) methodology for steel concentrically braced frames was presented here. This method uses pre-selected target drift and yield mechanism as key performance limit states. The designed base shear for selected hazard levels was derived based on work-energy balance equations. Plastic design was performed to design bracing members and connection nodes in order to achieve the expected yield mechanism and behavior. The method has been successively applied to design a six-storey steel concentrically braced frame. Results of inelastic dynamic analyses showed that the story drifts were well within the target values, thus to meet the desired performance requirements. The proposed method provided a basis for performance-based plastic design of steel concentrically braced frames.

Development of the Performance Based Plastic Design for Steel Concentric Braced Frames and Evaluation by Energy Methodology

This paper presents the development of performance based plastic design (PBPD) method to design steel concentric braced frames (SCBFs). The design base shear is obtained based on energy-work balance equation using preselected target drift and yield mechanism. Also, the energy-work balance can be applied to estimate seismic demands, herein called the energy spectrum method. PBPD method was originally developed to design SCBFs. The results were too conservative. For further improvement, this paper presents some modifications such as C2-Factor method and P-delta effect in energy-work balance equation to give a better estimate of design base shear. For validity, three baselines frames (3, 6, 9-story) are designed according to AISC 2005 seismic provisions (Baseline frames). Then, the frames are designed based on the Original PBPD method (Original PBPD method). Finally, they are designed based on modifications applied on PBPD method (Modified PBPD frames). These frames are subjected to extensive inelastic pushover and time-history analyses. The Results show that the Modified PBPD frames meet all the intended performance objectives in terms of yield mechanisms and target drifts whereas the Baseline frames show very poor response due to premature brace fractures leading to unacceptably large drifts and instability. Also, Modified PBPD frames show that the structures having 20%–25% weight less than the Original PBPD can meet the performance objectives. Moreover, the drift demands obtained from energy spectrum method significantly correspond with the results of time history analyses.