Niloufar Mashhadiali

Bio: Niloufar Mashhadiali is an academic researcher from Semnan University. The author has contributed to research in topics: Structural system & Braced frame. The author has an hindex of 6, co-authored 10 publications receiving 97 citations. Previous affiliations of Niloufar Mashhadiali include Islamic Azad University.

Progressive collapse assessment of new hexagrid structural system for tall buildings

Abstract: SUMMARY Hexagrid structure is an innovative tube-type system. It is constructed with hexagonal exterior structural grids. The hexagrid works as an effective lateral and gravitational resisting system. This paper presents the progressive collapse-resisting capacity of this new system and the common diagrid system based on the local failure of the structural elements in the story above the ground. The collapse behavior is evaluated by two different nonlinear static and dynamic analysis methods. This study was conducted to design two-type 28-story and 48-story building models to withstand wind load for both structural systems. With the analytical results, the hexagrid has enough potential of force redistribution due to its special configuration. It is observed that the new system had high resistance to progressive collapse than diagrids in similar condition. The complementary studies illustrate that resisting progressive collapse capacity, in both hexagrid and diagrid structures, is increased by using the buckling-restrained elements. The guidelines discussed here can help engineers to understand the mechanism of progressive collapse of the hexagrid structures. Copyright © 2013 John Wiley & Sons, Ltd.

Proposing the hexagrid system as a new structural system for tall buildings

Abstract: SUMMARY In order to improve the efficiency of tube-type structures in tall buildings, a new structural system, called hexagrid, is introduced in this paper. In comparison with diagrid system, it consists of multiple hexagonal grids on the face of the building. In this research, a set of structures using diagrid system having four various diagonal angles and hexagrid system were designed on a strength and stiffness-based approach for buildings with 30, 50, 70 and 90 stories to withstand wind load. The impact of different geometric configurations of structural members on the maximum lateral displacement and architectural performance in both diagrid and hexagrid systems is compared. The stiffness sensitivity using a similar interior bracing system in both systems is also discussed. In this study, the seismic performance of a 30-story diagrid structure and a hexagrid structure was evaluated using nonlinear static and dynamic analyses. According to the results, the hexagrid system has a better architectural view and more ductility and stiffness sensitivity, which are about three times than that of the diagrid system. And finally, in comparison with the diagrid system, the hexagrid system has enough potential to push the height limit. The guidelines discussed here are for architectural and structural engineers to improve freehand design. Copyright © 2012 John Wiley & Sons, Ltd.

Response modification factor of concentrically braced frames with hexagonal pattern of braces

Abstract: This paper presents the response modification factor ‘R’ of the new bracing configuration of concentrically braced frames (CBFs). This innovative bracing system called “hexa-braced frame” is composed of hexagonal pattern of braces in which vertical structural elements connect the V and inverted-V bracings over three stories to form the hexagonal bracing configuration. FEMA P695 provides a global methodology to quantify seismic performance factor for new structural systems. Following this methodology, a set of 4-, 10- and 20-story archetypes representing low-, mid- and high-rise buildings, respectively, were used to evaluate the R factor of the hexa-braced frame. Trial values of R factor were examined through nonlinear static and dynamic analyses to satisfy acceptance criteria of the P695 methodology. The results were compared with the responses of similar X-braced frame models as the benchmark. The iterative process to determine R factor for the hexa-braced frame was performed using values of R factor, 6 and 7. Based on the performance evaluation of hexa- braced frame archetypes by measuring their collapse fragility, the value of R factor, 7 achieved the safety margin against collapse during the earthquakes. As expected, the analysis results confirmed the given value of R factor 6 for X-braced frame system in design codes (ASCE 7).

Vulnerability investigation in 3d framed tall buildings with steel plate shear wall, x-braced and moment frame subjected to progressive collapse

Abstract: In recent decades, vulnerability of tall buildings to unforeseen loads, induced by progressive collapse, has drawn researchers’ attentions. Steel plate shear walls have long been used as a lateral load-resisting system. It is composed of beam and column frame elements, to which infill plates are connected. There is a growing tendency among engineers to design tall buildings using steel plate shear walls. This paper investigates the progressive collapse-resisting capacity of the strip model of steel plate shear wall system compared with X-braced and moment frame system. The 3D models are used to assess the collapse behavior of typical 50-story building models, under sudden loss of elements from the middle and corner of the exterior frame in the story above the ground. The progressive collapse potential of model structures is evaluated by different nonlinear static and dynamic analyses using conventional analysis software. In this study, the vulnerability of candidate structures subjected to progressive collapse is also assessed by a sensitivity index regarding the sensitivity of structures to the dynamic effect induced by progressive collapse. To identify vulnerable members, the resulting actions of two nonlinear and linear static analyses are compared by the factor of redundancy related to the overall strength of the structure. Comparison of the analysis results indicated that in the steel plate shear wall system, the progressive collapse-resisting potential is more than X-braced and moment frame structure. Sensitive index of highly sensitive elements to the dynamic effect stated that in the structural models, beams are significantly more vulnerable in the moment frame than X-braced and SPSW structure. The index depicted that the vulnerability of columns in X-braced and SPSW structure is more than the moment frame structure.

Dynamic Increase Factor for Investigation of Progressive Collapse Potential in Tall Tube-Type Buildings

Abstract: In recent decades, vulnerability of tall buildings to unforeseen loads induced by progressive collapse has drawn researchers’ attentions. There is a growing tendency among engineers to design tall buildings using complicated structural systems with adventurous load paths that are inconsistent with the existing guideline recommendations. Progressive collapse is a dynamic and nonlinear phenomenon. This paper describes a recommended methodology for calculating the dynamic increase factor (DIF) to consider the dynamic effects for nonlinear static analysis. Based on this method, the specific load factor corresponding to the progressive collapse potential of building structures is evaluated by a proposed collapse index. In the development of the methodology, selected tube-type structural systems such as framed tube, braced tube, diagrid, and hexagrid systems were investigated. This study was conducted to assess the collapse behavior of typical 48-story building models under sudden loss of corner load-be...

Progressive collapse of framed building structures: Current knowledge and future prospects

Abstract: This paper reviews the state-of-art in progressive collapse studies on framed building structures. Such types of failure start with a local damage which extension increases, up to the whole structure. First, emphasis is placed on the current techniques to study collapse propagation, i.e., numerical, experimental and analytical. In particular, the various numerical methods found in the literature are reported and discussed and the experimental studies and technologies involved in the laboratory tests are listed and compared. As reviewed, the method of analysis depends on the collapse mechanism and the triggering event. Thus, an in-depth review of the collapse typologies is proposed. Pure and mixed progressive collapse mechanisms are discussed and debated. The various triggering events, their modeling and their effects on the framed structures are examined. Details on the available literature on multi-hazard scenarios are provided. Finally, robustness techniques against progressive collapse are summarized, compared and contrasted. The paper concludes with an ambitious comprehensive list of open questions and issues covering different aspects of future needs.

Diagrid: An innovative, sustainable, and efficient structural system

Abstract: Summary In recent years, diagrid structures have received increasing attention among both designers and researchers of tall buildings for creating one-of-a-kind signature structures. This paper presents a state-of-the-art review of diagrid structures. First, various diagrid configurations, main factors affecting their behaviors, and related design parameters and approaches are discussed. Then, diagrid applications for free-form steel and concrete structures are introduced showing the diagrid applicability for complex structures followed by recent advances in structural design of diagrid connections, diagrid nonlinear behavior, and structural control of diagrids. Recent studies about a new variation of tubular and diagrid systems, hexagrids, are discussed briefly. Finally, the diagrid potential in design of sustainable buildings is delineated.

A review of the diagrid structural system for tall buildings

Abstract: Summary Unlike vertical columns of traditional structure, diagrid structural systems for tall buildings have special inclined columns. Due to the inclined columns, a diagrid structural system for tall buildings produces axial force along the column direction under horizontal load, which has the advantage of resisting horizontal wind load and seismic load and gives more freedom to architectural design, so a diagrid structural system for tall buildings becomes an effective new structure style for tall and super-tall buildings. Theories and tests regarding the diagrid structural system for tall buildings have been intensely researched since the exterior tube of diagrid structural system for tall buildings was first proposed by Torroja in his seminal book. At present, studies for mechanical characteristics, joint form, theories, and tests have been systematized. This paper systematically summarizes existing research achievements of the diagrid structural system for tall buildings and confirms that the structure has larger lateral stiffness and good seismic performance. Based on the favourable performance of concrete-filled steel tubes, this paper advises the use of concrete-filled steel tube columns as the columns in diagrid structural systems for tall buildings.

Seismic performance upgrade of RC frame buildings using precast bolt-connected steel-plate reinforced concrete frame-braces

Abstract: This paper presents a new strengthening method for existing frames by using outside sub-structures, namely, precast bolt-connected steel-plate reinforced concrete (PBSPC) frame-braces. The working principles, numerical methods, and design procedures are presented, and case studies are carried out. The simulation results were consistent with the results from previous experiments, indicating the effectiveness of the numerical model. The design objective and procedure were put forward, and during the process, a coefficient η that considers the precast influences was considered. A practical engineering retrofitting was performed based on the proposed numerical model and design process, where the structure was subjected to the design basis earthquake (DBE) and maximum considered earthquake (MCE). The analyses demonstrated that the structural demands were clearly reduced within the thresholds, and the inner force was transferred from the existing building to the external sub-structure after strengthening. The incremental dynamic analysis (IDA) and fragility curves were plotted, illustrating the greater reliability in structural capacity and the lesser possibility of structural damage with the new upgrading form.

Strengthening and retrofitting techniques to mitigate progressive collapse: A critical review and future research agenda

Abstract: Abnormal events, that are unforeseeable low-probability and high-impact events, cause local failure(s) to structures that can lead to the collapse of other members and, eventually, to a disproportionate progressive collapse. Ordinary design procedures, which are usually limited to gravity and seismic/wind loads, are inadequate for preventing the progressive collapse. Therefore, a focus on strengthening and retrofitting techniques to mitigate progressive collapse is necessary. Parameters such as topology of the structure, nature of the triggering event, size of the initial failure, typology of the collapse and seismic design requirements affect the strengthening and retrofitting strategy. A discussion on the impact of these parameters on strengthening strategy is first presented. Then, a comprehensive review on strengthening and retrofitting techniques to mitigate progressive collapse is provided. The paper concludes with an ambitious comprehensive list of issues covering different aspects of future research agenda.