Capacity Design Methods for Strongback Braced Frames PDF Download

Author: Peter C. Talley
Publisher:
ISBN:
Category :
Languages : en
Pages : 115

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Book Description
When subjected to strong earthquake ground motions, conventional steel braced frames are vulnerable to soft-story mechanisms, whereby the weakest story accumulates more damage relative to the rest of the structure. This reduces the overall strength of the structure, increases the cost of repairs, and can cause issues during the design process due to the reduced redundancy of the system. One method for mitigating this behavior is the use of an elastic spine frame. These frames combine a stiff vertical "spine", such as a truss or shear wall, with a more ductile, energy-dissipating system. The spine typically spans the height of the structure and is designed to remain elastic, distributing earthquake demands across the height of the structure and bridging weak stories. One proposed elastic spine frame is the "strongback" braced frame, which merges a steel buckling-restrained braced frame and an elastic truss, using the buckling-restrained braces for energy dissipation and the truss for force distribution. However, strongback braced frames do not have well-established design criteria. Specifically, there is no generally accepted method for ensuring that the strongback remains elastic, and seismic performance factors have not been developed. Additionally, conventional capacity design underestimates the demands on the spine. It is desirous to have a method for design of these frames that hews closely to existing methods utilizing the equivalent lateral force method. This thesis presents the first phase of a study to address these gaps in the design provisions and to better understand the behavior of this system. A suite of building frames which employ the strongback system were designed with the intent of using them as the basis for parametric analytical studies in the second phase. The suite of frames was selected using the requirements of FEMA P695, the state-of-the-art method for determining seismic performance factors. Three alternative capacity design methods were developed and compared to basic capacity design to identify which is best suited to efficiently achieve the performance objectives. The methods were evaluated for efficiency in the design process, and for feasibility of the resulting designs. However, evaluation of performance objectives is the goal of future study.

Author: Barbara Gwynne Simpson
Publisher:
ISBN:
Category :
Languages : en
Pages : 298

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Book Description
Steel braced frames are characteristically efficient seismic force-resisting systems. However, multi-story steel braced frames tend to concentrate demands in one or a few stories in response to severe ground shaking. Brace buckling and yielding results in a reduction in story strength and/or stiffness. Unless a mechanism exists to re-distribute the inelastic demands to other stories, demands tend to concentrate in the story where the inelastic response was initiated, indicative of story mechanism behavior. Research has identified the advantages of using pivoting seismic force-resisting systems, herein termed strongback-braced frames, to mitigate story mechanism behavior. Strongback-braced frames employ an essentially elastic truss, or “strongback”, that provides an explicit mechanism of re-distributing demands to adjacent stories. Yielding and energy dissipation is provided through inelastic actions, or fuses (e.g., through brace yielding/buckling and/or beam plastic hinging). Forces and moments developed in these fuses are transferred vertically to adjacent stories by the flexural stiffness and strength of the strongback. As such, strongback-braced frames are expected to result in more uniform drift distributions, reduced peak inelastic demands, and improved design flexibility compared to conventional seismic force-resisting systems. Despite being employed successfully in both research and practice, systematic assessment of the strongback’s behavior and practical design methods have not been developed or validated. Since the behavior of strongback systems is not characterized by the formation of story mechanisms, prior studies have found it difficult to proportion the elastic members in the strongback truss and have recognized detailing issues related to large deformation demands induced in the fuses. As such, a series of investigations were aimed at understanding the dynamic behavior and seismic performance of steel strongback-braced frames. Archetype designs were numerically analyzed to characterize the seismic demands in the strongback elements. A four-story strongback-braced frame was used to benchmark the dynamic behavior observed during nonlinear dynamic analysis. Improved numerical models were calibrated to more realistically simulate the buckling-restrained brace response and to characterize the modeling parameters influencing brace buckling and low-cycle fatigue. The FEMA P695 methodology was used to assess potential design methods based on collapse performance. Extensive parametric studies were carried out on strongback geometries with a range of bracing configurations, ground motion characteristics, and design alternatives. Higher mode effects were identified as the cause of substantial force amplification in the elastic strongback truss. Unlike typical yielding systems where force demands are limited by the capacity of the fuses in every mode, force demands in the strongback are characterized by a yielding first-mode “pivoting” and elastic higher-mode “bending” force demands. Since the strongback is designed to remain elastic in all modes, it can exhibit significant strength and stiffness in higher mode bending. Under the second and higher modes, the strongback truss remains elastic and continues to accumulate force demands after the fuses have yielded and as the ground shaking intensifies. These force demands in the strongback members can be significantly larger than those estimated per traditional capacity design assuming first mode-only demands. The addition of a strongback results in improved dynamic response from typical yielding systems, including a more uniform drift profile compared to reference buckling-restrained braced frames. Based on this research, this study proposes recommendations for the design, analysis, and modeling of strongback-braced frames. Simplified static methods to estimate the dynamic demands in the strongback truss are also proposed, including modal pushover and modal enveloping analysis methods.

Author: Federico M. Mazzolani
Publisher: Springer Nature
ISBN: 3031038118
Category : Technology & Engineering
Languages : en
Pages : 1146

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Book Description
This volume highlights the latest advances, innovations, and applications in the field of seismic design and performance of steel structures, as presented by leading international researchers and engineers at the 10th International Conference on the Behaviour of Steel Structures in Seismic Areas (STESSA), held in Timisoara, Romania, on 25-27 May 2022. It covers a diverse range of topics such as behaviour of structural members and connections, performance of structural systems, mixed and composite structures, energy dissipation systems, self-centring and low-damage systems, assessment and retrofitting, codes and standards, light-gauge systems. The contributions, which were selected by means of a rigorous international peer-review process, present a wealth of exciting ideas that will open novel research directions and foster multidisciplinary collaboration among different specialists.

Author: Jiun-Wei Lai
Publisher:
ISBN:
Category :
Languages : en
Pages : 1023

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Book Description
This dissertation summarizes both experimental and analytical studies on the seismic response of conventional steel concentrically braced frame systems of the type widely used in North America, and preliminary studies of an innovative hybrid braced frame system: the Strong-Back System. The research work is part of NEES small group project entitled "International Hybrid Simulation of Tomorrow's Braced Frames." In the experimental phase, a total of four full-scale, one-bay, two-story conventional braced frame specimens with different bracing member section shapes and gusset plate-to-beam connection details were designed and tested at the NEES@Berkeley Laboratory. Three braced frame specimens were tested quasi-statically using the same predefined loading protocol to investigate the inelastic cyclic behavior of code-compliant braced frames at both the global and local level. The last braced frame specimen was nearly identical to one of those tested quasi-statically. However, it was tested using hybrid simulation techniques to examine the sensitivity of inelastic behavior on loading sequence and to relate the behavior observed to different levels of seismic hazard. Computer models of the test specimens were developed using two different computer software programs. In the software framework OpenSees fiber-based line elements were used to simulate global buckling of members and yielding and low-cycle fatigue failure at sections. The LS-DYNA analysis program was also used to model individual struts and the test specimens using shell elements with adaptive meshing and element erosion features. This program provided enhanced ability to simulate section local buckling, strain concentrations and crack development. The numerical results were compared with test results to assess and refine and the ability of the models to predict braced frame behavior. A series of OpenSees numerical cyclic component simulations were then conducted using the validated modeling approach. Two hundred and forty pin-ended struts with square hollow structural section shape were simulated under cyclic loading to examine the effect of width-to-thickness ratios and member slenderness ratios on the deformation capacity and energy dissipation characteristics of brace members. The concept of a hybrid system, consisting of a vertical elastic truss or strong-back, and a braced frame that responds inelastically, is proposed herein to mitigate the tendency of weak-story mechanisms to form in conventional steel braced frames. A simple design strategy about member sizing of the proposed Strong-Back System is provided in this study. To assess the ability of the new Strong-Back System to perform well under seismic loading, a series of inelastic analyses were performed considering three six-story hybrid braced frames having different bracing elements, and three six-story conventional brace frames having different brace configurations. Monotonic and cyclic quasi-static inelastic analyses and inelastic time history analyses were carried out. The braced frame system behavior, bracing member force-displacement hysteresis loops, and system residual drifts were the primary response quantities examined. These indicated that the new hybrid system was able to achieve its design goals. Experimental results show for the same loading history that the braced frame specimen using round hollow structural sections as brace members has the largest deformation capacity among the three types of bracing elements studied. Beams connected to gusset plates at the column formed plastic hinges adjacent to the gusset plate. The gusset plates tend to amplify the rotation demands at these locations and stress concentrations tended to result in early fractures of the plastic hinges that form. To remedy this problem, pinned connection details used in the last two specimens; these proved to prevent failures at these locations under both quasi-static and pseudo-dynamic tests. Failure modes observed near the column to base plate connections in all of the specimens suggest the need for further study. Both OpenSees and LS-DYNA models developed in this study predict the global braced frame behavior with acceptable accuracy. In both models, low-cycle fatigue damage models were needed to achieve an acceptable level of fidelity. Shell element models were able to predict local behavior and the mode of failures with greater but not perfect confidence. OpenSees analysis results show that the proposed hybrid braced frames would perform better than conventional braced frames and that the story deformations are more uniform. Finally, future research targets are briefly discussed at the end of this dissertation.

Author: Denis M. Regan
Publisher:
ISBN:
Category :
Languages : en
Pages : 180

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Author: Federico M. Mazzolani
Publisher: Springer Nature
ISBN: 3031628888
Category :
Languages : en
Pages : 1034

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Author: Paul William Richards
Publisher:
ISBN:
Category :
Languages : en
Pages : 500

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Author: Shahrzad Dastmalchi
Publisher:
ISBN:
Category :
Languages : en
Pages : 177

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Book Description
Controlled rocking steel braced frames (CRSBFs) have been developed with the goal of minimizing the post-earthquake impact of primary building functions. While there has been significant research to date to demonstrate the viability of the CRSBF as a high-performance system, much less has been accomplished in the development of performance-based design and assessment methods. This research is focused on developing models, tools and techniques for practicing engineers to analyze, design and assess the performance of CRSBFs. To avoid the computational expense of nonlinear response history analyses, an approximate method is formulated to estimate the CRSBF drift demands using the primary design parameters. Additionally, a reliability-based methodology for establishing the load and resistance factors for the force-controlled (braced frame) members is formulated. A key departure from previously developed capacity design approaches is the development of an explicit link between the effect of the failure of the force-controlled components and system level performance limit states (collapse and post-earthquake structural safety). The results from a case study applied to 3-, 6- and 9-story building cases show that the effect of force-controlled components is more significant for the collapse limit state compared to post-earthquake structural safety. Also, even when the resistance to load factor ratio ( / ) is increased to 1.8, the 50-year collapse probability remained below the 1% threshold prescribed by current building codes. The effect of record-to-record and modeling uncertainty on the seismic response and performance assessment of CRSBFs is also studied. The results showed that the impact of modeling uncertainty on seismic performance increases with the building height. To enable practitioners to estimate the service life costs of potential designs, surrogate models are developed to assess earthquake-induced life cycle economic loss and environmental impacts. The effectiveness of the surrogate models is demonstrated by evaluating their accuracy on "unseen" (i.e., not used in the development of the surrogate models) designs.

Author: Wai-Fah Chen
Publisher: World Scientific
ISBN: 9789810241384
Category : Technology & Engineering
Languages : en
Pages : 492

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Book Description
This book summarizes the recent progress in practical analysis for semi-rigid frame design in North America. This encompasses codes, databases, modeling, classification, analysis/design, and design tables and aids. Practical design methods include LRFD procedures, approximate procedures, computer-based procedures and the optimization process. The book can be used as a supplementary steel design textbook for graduate students, as a training book for a short course in steel design for practicing engineers, and as a reference book for consulting firms designing building structures.

Author: Amory Adrien Martin
Publisher:
ISBN:
Category :
Languages : en
Pages :

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Book Description
Rocking spine systems are innovative earthquake-resistant structural systems that dampen seismic shaking through uplift at the base and confine damage to energy-dissipating fuses, thereby significantly reducing the potential of building downtime. Currently, United States building codes and standards provide very limited design guidelines for such systems. This thesis focuses on developing procedures and algorithms for design and optimization of rocking spine systems under nonlinear earthquake response. A new capacity design procedure, the modified modal superposition (MMS) method, is developed for the seismic design of rocking spine systems. The methodology uses an efficient elastic response spectrum analysis to approximate the nonlinear earthquake response through (1) modified boundary conditions to simulate rocking at maximum considered earthquake (MCE) level and (2) a first mode inelastic reduction factor. The methodology is extended to coupled and stacked rocking braced frames, as well as strongback systems, with various hysteretic and viscous dampers. Using nonlinear dynamic analyses on a set of seven archetype frames ranging from 6 to 18 stories, the MMS procedure is shown to accurately capture higher modes effects and estimate axial brace and column forces. A reliability analysis conducted supports applying a load amplification factor of 1.3 for scaling the MMS seismic forces to design the steel braced frame as force-controlled components. A new dynamic topology optimization methodology, called the sum of modal compliances (SMC), is introduced for seismic loading. Recently developed dynamic topology optimization procedures for linear elastic response in the frequency domain are compared and contrasted. The novel procedure is applied to the design of lateral bracing system of high-rise buildings for various earthquake hazards and yields important considerations of the influence of higher modes on the overall dynamic response of the system. The efficiency of the SMC optimization algorithm is demonstrated on a 3D high-rise building with over one million degrees of freedom. Using the modified modal superposition as inspiration, the dynamic topology optimization procedure is extended to design of the elastic spine in rocking braced frames for nonlinear earthquake response. The extruded optimized bracing pattern is compared to a conventional X-bracing system using nonlinear dynamic analyses. An optimization framework is proposed for selecting the number, location and properties of nonlinear dampers in stacked rocking systems, where the total overturning moment in the spine is minimized, subjected to interstory drift and hinge rotation constraints. A ground motion selection routine is developed to facilitate the optimization by estimating the median dynamic response under earthquakes. Algorithmic procedures are developed to solve the structural optimization problem using both modified sequential linear programming (SLP) method and particle swarm optimization (PSO). On a 20-story dual rocking hinge case study, the SLP algorithm is shown to converge to the optimum with less than 40 nonlinear dynamic analyses compared to over 4,000 for an exhaustive search. For a 20-story stacked rocking system with N arbitrary hinges, the SLP optimization yields three rocking joints, whereby the total overturning moment in the spine is reduced by half compared to the initial design, while maintaining drift limits below 2.5% at MCE level. Overall, this thesis introduces design and optimization procedures for both the rocking spine and nonlinear articulated hinges. This research project demonstrates the advantages of rocking spine systems for improved seismic performance and introduces novel optimization algorithms for structural design under earthquake loading.