Unbraced Frames Research Articles

Seismic Behaviour of Steel Building with And Without Bracing System by Performance Point

Steel structures have been playing an important role in the construction industry in recent years. It is necessary to create a model that performs well under seismic loading. Seismic performance of multi-storey steel framed buildings is designed as per the provisions of the Indian Code (IS 800 -2007). The ductility of the structure can be increased by adding steel support to the structure. There are different types of support for retrofitting. X-brace, diagonal brace, V-shaped brace, inverted V-shaped brace etc. There are many ways to prepare steel supports such as: The frame of the building is designed with or without different types of connections such as diagonal, X-brace, V-brace and inverted V-brace, which will be considered in this study. The performance of each frame was examined by nonlinear static analysis (pushover analysis) using the ETABS-2021 software package. The comparison of braced and unbraced frames was based on the examination of base shear forces, hinge consequences and lateral displacements of different frames. Compare the response curves and performance points of different frames with and without support systems to know the relationship between the various frames involved.

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.

Parametric Study on the Buckling of Unbraced Steel Frames under Fire Situation

A parametric investigation of several unbraced steel frames with regular and irregular geometry subjected to elevated temperatures is carried out in this study to determine the most accurate procedure and buckling lengths to be considered during the structural design under a fire situation. In such conditions, the stiffness and strength of steel decrease considerably due to high temperatures, and uncertainty remains in the application of the fire design rules of Eurocode 3 Part 1-2 (EN 1993-1-2) for unbraced frames as no information is given regarding the treatment of the deformed geometry (the so-called second-order effects). More precisely, it is unclear in the norm whether the verification based on the buckling length concept could be used or if a second-order analysis to calculate the internal forces is sufficient to ensure the stability of the frame in case of fire. Based on the linear buckling analysis of the steel frames accounting for the temperature development during a fire, recommendations for the appropriate buckling lengths to be used are given. Finally, it is demonstrated that using the recommended buckling lengths together with the design rules of EN 1993-1-2 leads to results in favor of safety when compared to the results obtained with the finite element method. On the other hand, it is concluded that using second-order internal forces and the real length of the columns as the buckling length, as suggested for room temperature design, yielded results outside safety when compared with the finite element method, and this analysis and verification procedure should not be used for the case of fire.

The effective length factor of columns in steel framed buildings with end restraint effects

The present article presents a practical method to determine the effective length factor of multi-storey frames based on a rigorous analytical approach applied to a basic model judiciously parameterized in terms of geometry and load arrangement. The adopted novel model considers the different interactions and effects of varying the boundary conditions at the ends of the columns and beams that are connected to the column under consideration. Subsequently, parametric studies are presented to evaluate the influence of the boundary conditions at the ends of the columns located above and below the column under consideration on the effective length factor value K. It is worth specifying that these variations are considered for the braced and unbraced frame cases. A comparative study revealed substantial differences between the values of the effective length factors thus obtained and those calculated using the method recommended by Eurocode 3 and CCM97. The proposed charts and relating to the effective length factor value K associated with the double cross model adopted by Eurocode 3 and CCM97, are of practical significance as they are very useful in the conception and design of such frames.

NUMERICAL MODELLING OF REINFORCED CONCRETE FRAMES RETROFITTED WITH BUCKLING RESTRAINED BRACE INCORPORATING STEEL AND SUPERELASTIC SHAPE-MEMORY ALLOY CORES

The numerical response of one-storey reinforced concrete frames designed following design standards before the enactment of modern seismic provisions was investigated in this study using the nonlinear finite element method. An unbraced frame and two frames retrofitted with a buckling restrained brace (BRB) incorporating either a stainless steel or chrome molybdenum core bar were modelled. The nonlinear static reverse cyclic loading performance of the frames was assessed. Modifications to the BRB are proposed to mitigate deficiencies identified with the original BRB and steel core bars. The modified BRB incorporating a superelastic shape-memory alloy (SE-SMA) core bar was further modelled. The results illustrate the benefits of implementing SE-SMA as a retrofit strategy to control permanent displacements. The modified BRB with a SE-SMA core exhibited improved lateral strength and displacement capacities relative to the frames retrofitted with steel cores and the control bare frame.

Linear and Nonlinear Dynamic Analysis of Reinforced Concrete Building with V Shape Steel Bracing

In this study, the RC-MRCBFs were used with V braced frame. The core objective of this examination is to understand the earthquake behavior of the RC-MRCBFs in steel V braced frames. Response spectrum analysis (RSA) is used to understand the seismic performance of the steel braced and un-braced RC frames. Total 12 steel braced RC frames and 12 un-braced frames for all 4 story, 8story, 12 stories and 16 stories are studied and observed the seismic parameter such as fundamental time period (FTP), top story displacements, inter-story drift, base shear and story stiffness of the structures. After studying the parametric study of the 4 to 16 story buildings with a linear and nonlinear analysis tool it was observed that to get the effective braced frame with expected failure mechanism, ductility, the columns should be designed such that, they resist at least 50% base shear contributions. It is observed that using the steel V bracing in the low rise to mid-rise buildings, improves the seismic behaviors of the structures. The steel bracing reduces the maximum top story displacements, drift and time period of the building and increases the seismic base shear demand, stiffness of the structures. The result shows that as increasing the base shear contribution in the columns, the drift and displacement of the story increases and base shear decreases. Key Words: Response Spectrum Analysis, displacement, Maximum storey drift, Storey shear, Storey stiffness, braced frame, RC buildings.

Part 2: proposed buckling factor for columns in braced and unbraced frames with composite girders

Part 2: proposed buckling factor for columns in braced and unbraced frames with composite girders

Comparative Analysis of Steel Structure with Rigid and Semi Rigid Joint using Analysis Tool ETABS

Generally, in steel structure the connection between beam and column are designed as moment connection and pinned connection, but in actual condition the structure behaves between these two conditions, resulted into semi-rigid condition which is intermediate stage between rigid and pinned joints. Effect of semi-rigid connection on multi-story multi-bay frame is accomplished in this Study. The present study introduces the effect of static and dynamic loading on high rise steel structure of G+12 story with 4m, 6m and 8m three bay span length. Structure is analyzed under two different condition of partial release of semi-rigid connections which is derived by fixity factor of values 0.5 and 0.75 in this study. The analysis is done commercially available software ETABS. From the non-linear analysis, the story displacement and story drift are obtained. The overall performance of the structure from the analysis, semirigid joints display more story displacement and story drift compared to rigid joints. To overcome the results, bracing system is introduced at different location of periphery of structure. These brace frame structure consist of X- bracing and diagonal bracing. Again, comparative analysis is to be performed in ETABS on these three-bay span lengths. It is found that braced semi-rigid frame structure perform quite well as compared to unbraced frame structure.

Second-Order Generalized Plastic Hinge Method for Ultimate Bearing Capacity Estimation of Unbraced Steel Frames

The traditional first-order plastic hinge method (FPHM) utilizes the proportionality property to rapidly estimate the external load increment between any two adjacent plastic hinges, with superior computational efficiency. However, the FPHM cannot consider the influence of second-order effects and residual stresses. Although the refined plastic hinge method (RPHM) overcomes these problems, it must solve the nonlinear structural stiffness equation, resulting in low computational efficiency. In this paper, the second-order generalized plastic hinge method (sGPHM) is proposed to evaluate the ultimate bearing capacity of unbraced steel frames on the basis of using the proportionality property to estimate the external load increment. First, the element bearing ratio (EBR) that considers second-order effects and residual stresses is defined. Since the EBR and the external load are proportional, the plastic yield degree of the cross-section can be easily obtained. Then, the displacement increment and the load increment between any two adjacent plastic hinges can be rapidly estimated through the proportionality property between the EBR and the external load. Accumulating all load increments can determine the ultimate bearing capacity of frames. Finally, several benchmark frameworks in the literature are compared and analyzed with different methods to verify the superior calculation accuracy and efficiency of the proposed sGPHM.

Vibration Modeling and Simulation of Braced and Unbraced Steel Structure

Repeated dynamic loads on multiple stories structure may cause damage in the midst of natural disaster, such as earthquakes and landslides, hence it is necessary to observe how the response occurs. Therefore, this paper studies the performance of two-story steel frame without braces and with inverted V concentric braces. Both frames were given a static and a dynamic load in the form of a sinusoidal load for 2 seconds in the form of forced harmonic vibration. After the dynamic load is turned off, harmonic free vibration applies. Vibration is given to the weakest orthogonal axis on the frame with a frequency of 0,5 Hz; 1,03 Hz; 1.7 Hz. To reduce the deviation between stories that occur due to dynamic loads, bracing is provided as a stiffening element. The test variations are called models 1, 2 and 3 with model 1 unbraced frame and model 2 braced frame with the same load between floors, while model 3 unbraced frame model with weight 2nd floor is 3 times larger than 1st floor. It found that the braced frame has a minimum drift and its drift is enhanced with the increase of load capacity.

Progressive collapse analysis of seismically designed braced and unbraced steel frame structures

Progressive collapse analysis of seismically designed braced and unbraced steel frame structures

Teaching-learning-based optimization of unbraced steel frames with semi-rigid connections and column bases

The teaching-learning optimization (TLBO) technique is applied to unbraced steel frames with semi-rigid beam-to-column connections and semi-rigid column-to-base connections. The algorithm for design produces optimized steel frames by choosing appropriate sections from a set of standard steel sections. These standard steel sections, including wide-flange profiles (W), are provided by the American Institute of Steel Construction (AISC). The first example is subjected to the displacement, inter-story drift, and stress constraints of AISC-Load and Resistance Factor Design (LRFD) regulations. In addition, the displacement and stress limitations of the AISC-Allowable Stress Design (ASD) standard and the geometric (size) requirements are implemented in the last two examples. In finite element analysis, optimal designs of three alternative unbraced frames are performed with and without semi-rigid beam-to-column and column-to-base plate connections. All optimization techniques are encoded in the high-level programming language C# version 17.4.4 to integrate with SAP2000 version 22's Open Application Programming Interface (OAPI). The analysis results indicate that the rigidity of connections is determining parameter in the weight optimization of unbraced steel frames.

Performance Assessment of Different Retrofitting Techniques for Steel-Framed Buildings

The requirement for cost-effective critical infrastructure has made employing nonlinear analysis techniques an appealing choice. Estimating seismic loads on structures necessitates explicit consideration of inelastic structural behavior. Hence, nonlinear approaches such as pushover analyses have become more popular. In this study, the performance of a ten-story steel moment-resisting frame retrofitted with various bracings like diagonal-bracing, chevron bracing, x-bracing and k-bracing is evaluated using linear and nonlinear analysis. The responses, such as base shear, lateral load distribution, displacement, drift, and lateral stiffness, are determined using the ETABS-18 building analysis and design program. The results of the study show that the steel-braced frame systems have substantially greater strength and ductility capabilities than their unbraced or alternate steel-braced moment-resistant steel frame. Further, the unbraced steel frame has a low R-factor as compared to the bracing system due to the increased stiffness at the story level.

Performance-based seismic design of laterally braced steel frames

Improving the performance of structural frames is one of the most important focuses of today's researchers. The total base shear capacity of the structural frames will increase by reaching the maximum number of plastic hinges. In this work, the effects of bracing system type on different heights of steel frames were investigated. Static pushover analysis was used to evaluate the performance of 4, 8, and 12-story steel frames with seven structural configuration systems. The results showed that the performance of braced frames increased significantly in terms of number of plastic hinges, total base shear and performance point compared to unbraced frames. The capacity curves were maximum in the one-story X-bracing, multi-story X-bracing, and single diagonal bracing systems.

Part I: Modified Buckling Factor for Columns in Braced and Unbraced Frames with Composite Girders

Part I: Modified Buckling Factor for Columns in Braced and Unbraced Frames with Composite Girders

Elastic stability of unbraced plate-type modular steel frames with semi-rigid corner connections

Assembled modular buildings can improve on-site construction efficiency and reduce environmental pollution and resource waste. As an innovative structural form, a steel building with semi-rigid corner connections composed of plate modular units to form a double-beam and multi-column structure is proposed. This self-load-bearing modular plate-type structure, without an additional internal lateral resistance system forms a low multi-story building with fewer than six stories. To calculate the elastic stability of this structure, based on the assumptions of equal story drifts (case 1) and unequal story drifts (case 2), two different three-column sub-assemblage models were established using effective-length methods to calculate the effective lengths of columns (K-factors). According to the theoretical models, the governing equations and simplified formulas for case 1 and case 2 were obtained, respectively. The governing equations of Li et al.(2018), cases 1 and 2 and the finite element model (FEM) were adopted to analyze the influencing factors of the K-factors. The results showed that the governing equation and simplified formulas in case 2 possess a higher accuracy compared with that reported in existing literature. The variation trends of the curves in case 2 were completely consistent with those evaluated by FEM, and the errors between them were small. Thus, the correctness of the assumption of unequal drifts of a story was verified as well as the applicability and accuracy of the theoretical model for typical engineering in plate-type steel frames with semi-rigid corner connections

Brace stiffness for parallel semi-rigid columns

Proposed in the paper is an analytical method of determining the ideal brace stiffness for a structural system consisting of parallel semi-rigid columns with horizontal tie bracing at mid-height. Based on a newly proposed unbraced frame model, the ideal brace stiffness of the parallel column system can be obtained from buckling of an equivalent unbraced frame with column height being half of the original one. The influence of column stiffness interaction associated with semi-rigid connections, column sizes, and applied loads is theoretically considered in the determination of the ideal brace stiffness, which makes the proposed method applicable to systems with nonidentical columns. For a system with the same columns and end connections, the minimum ideal brace stiffness in elastic analysis can be achieved with the end-fixity factor being 0.392. In addition, a simple expression is presented to quantify the effect of the semi-rigid connection on the ideal brace stiffness of the system. Good agreements between analytical and finite element analysis results of presented examples indicate the proposed method can accurately evaluate the ideal brace stiffness of a semi-rigid parallel column system.

Influence of the imperfection direction on the ultimate response of steel frames in advanced analysis

Initial geometric imperfections are unavoidable in steel members and frames due to erection and manufacturing tolerances. These include frame out-of-plumbness, member out-of-straightness and cross-sectional imperfections, and can have a significant influence on the response and resistance of steel structures. Thus, they need to be accounted for in the analysis and design of steel structures, especially when advanced design procedures are adopted. One of the easiest approaches to introduce geometric imperfections in structural finite element models is through the linear superposition of scaled eigenmodes, which are obtained from a priori elastic buckling analysis. Although the shape and magnitude of frame and member imperfections are specified in international standards, the rules for the combination of different types and directions of imperfections are unclear or impractical, and often require designers to consider many possible combinations to find the critical, or “worst case”, shape of the imperfection including the direction of each eigenmode. This paper investigates the influence of the direction of modes contributing to the imperfection on the ultimate load (i.e., resistance) of steel frames when using advanced analysis. Ultimate loads are estimated from advanced finite element simulations for 20 regular and irregular unbraced frames featuring steel and austenitic stainless steel compact sections, in which initial imperfections are modelled as linear superpositions of six scaled buckling modes considering all possible combinations of direction. The results show that the influence of the imperfection direction on the ultimate frame load is small, and that assuming a combination of all buckling modes with positive amplitudes provides a simple and accurate estimation of the critical imperfection combination.

Effective buckling length of frames with tapered columns and partially tapered beams

This paper presents stability analysis of steel frames composed of linearly tapered columns and partially tapered restraining beams. In the first part of the stability analysis, closed-form equations for the bending stiffness of the symmetrical and unsymmetrical partially tapered restraining beam are derived and presented in form of design charts. Subsequently, slope deflection equations of linearly tapered column subjected to axial compressive load are then assembled along with the bending stiffness equations of partially tapered beams to determine the elastic critical load and the effective buckling length factor for non-sway and sway steel frames. Numerical examples and efficient design charts are presented for several braced and unbraced steel frames. Moreover, the effect of gravity loads acting on leaning columns connected to sway frames is investigated. Based on this investigation it is concluded that correction of effective buckling length factor based on story buckling concept can be extended to cover steel frames with tapered column as well.

On the Columns Buckling Length of Unbraced Steel Frames with Semi-Rigid Joints

On the Columns Buckling Length of Unbraced Steel Frames with Semi-Rigid Joints