Mehdi Setareh

In Search of Suitable Control Methods for Semi-Active Tuned Vibration Absorbers:

The main purpose of this study is to identify a suitable control method for semi-active tuned vibration absorbers (TVAs) in structural vibration applications. Four control policies are considered. The semi-active control schemes include the following: velocity-based, on-off groundhook control (on-off VBG); velocity-based, continuous groundhook control (continuous VBG); displacement-based, on-off groundhook control (on-off DBG); and displacement-based, continuous groundhook control (continuous DBG). A force-excited model that can be representative of many structural systems is adapted as the baseline model for our analysis. Each of the control policies is applied to the baseline model coupled with a TVA. In order to equally evaluate the control policies, the TVA parameters are optimized according to each policy using numerical optimization techniques. The optimal design parameters are obtained based on minimization of peak transmissibility. The performances of each of the optimized cases are then compared along with the equivalent passive model using the peak transmissibility criteria. The results indicate that all of the semi-active peak transmissibilities are lower than those of the passive, implying that the semi-active TVAs are more effective in reducing vibration levels. The results further indicate that on-off DBG performs the best among the considered control polices.

Virtual-SAP: an immersive tool for visualizing the response of building structures to environmental conditions

Immersive virtual environments (VEs) offer interactive, real-time visualization capabilities for engineers, architects, and scientists. This paper presents Virtual-SAP, an immersive VE application allowing users to assess building structures and their response to various environmental conditions through an interactive design-build-test-redesign cycle. Virtual-SAP has three distinct user interfaces that support its use with a high-end, multi-tracker VE system, with a low-cost portable VE system, or on the desktop. Two of these interfaces have been proven highly usable in user testing, and the third will be tested soon. Virtual-SAP is being used for both research and education.

Experimental Robustness Analysis of Magneto-Rheological Tuned Vibration Absorbers Subject to Mass Off-Tuning

This paper offers an experimental robustness analysis of a semi-active tuned vibration absorber (TVA) subject to structural mass off-tuning, which occurs frequently in practical applications of TVAs. One of the critical problems of a conventional TVA is off-tuning or miss-tuning because off-tuned TVAs may amplify the vibrations of the primary structure. This study intends to evaluate how well semi-active TVAs are able to adapt to structural mass changes (mass off-tuning) as compared with passive TVAs. To this end, a test apparatus was built to represent a two-degree-of-freedom structure model coupled with a TVA. The semi-active TVA considered in this study employed a magneto-rheological (MR) damper as its damping element to enhance overall performance. Using this test apparatus, a series of tests were conducted to identify the optimal tuning parameters of the TVAs. After obtaining each TVA s optimal tuning parameters based on equal peak transmissibility criteria, the mass off-tuning tests were then performed on the optimally tuned TVAs. In order to off-tune the system, the mass of the primary structure varied from -19% to +19% of its nominal mass using a set of steel plates. Overall, the experimental results show that the semi-active MR TVA outper-forms the passive TVA in reducing maximum vibrations. Moreover, the results show that the semi- active TVA is more robust to changes in the primary mass (mass off-tuning) The results further show that the semi-active MR TVA offers larger performance gains as the structure mass increases.

Vibration Serviceability of a Building Floor Structure. I: Dynamic Testing and Computer Modeling

Buildings with large column-free floors or long-cantilevered structures can be susceptible to annoying vibrations due to everyday occupants’ activities such as walking. Computer modeling and analytical representation of building structural properties to predict the floor response subjected to excitations due to human activities are important issues that require further studies. Vibration testing and analysis of built structures can assist in more accurate estimation of structure dynamic properties. This paper presents the results of the modal testing conducted on an office building floor and analysis of the collected vibration measurements. It compares these results with the structural response using computer analyses. It also presents a sensitivity study to assess the importance of various structural parameters on the floor dynamic response. From the results presented, it is concluded that for the structure used in this study the raised flooring and nonstructural elements acted mainly as added mass and d...

Vibrations due to Walking in a Long-Cantilevered Office Building Structure

AbstractInnovative architectural designs create new challenges for structural designers such as annoying vibrations because of the everyday occupant activities such as walking. The designer needs to predict the excitation levels to evaluate the structural response and assess the resulting vibrations for acceptable human exposure. This paper uses a long-cantilevered office building structure to study the floor response subjected to various measured and Fourier series-based forcing functions representing human walks suggested in standards, design guides, and research studies. In addition, the vibration evaluation parameters proposed by several standards are computed and compared by using computer analyses and field measurements. For the majority of the cases studied in this paper, use of the Fourier series-based forcing functions resulted in an overestimation of the structural response. In addition, the in-place marching excitations to represent walking recommended in some design guides can grossly overesti...

Vibration Serviceability of a Building Floor Structure. II: Vibration Evaluation and Assessment

People are generally very sensitive to unexpected vibrations. Very small levels of building floor movements due to activities such as walking can become annoying to occupants. Accurate prediction, evaluation, and assessment of vibrations can greatly assist engineers and architects to design cost-effective building structures without such problems. In an attempt to clarify some of the issues related to this common serviceability problem, this paper presents a study of the various parameters used for the evaluation and assessment of building vibrations. Provisions of several current standards and design guides commonly used in North America and U.K. to evaluate and assess building vibrations as related to human perception and comfort are reviewed. These provisions are then applied using the vibrations measured during a number of walking tests conducted on a large cantilevered office building floor. Based on the results of this study, it is concluded that the vibration dose value (VDV) recommended by some st...

Cognitive scaffolding in Web3D learning systems: a case study for form and structure

In this paper, we describe a case study in usability engineering for Web3D learning systems and introduce a new step to the typical methods of the usability design. Pedagogical applications present a challenge to the usual usability engineering process in that the end-users of the system (students) cannot describe the requirements of the system. For this situation, we engage the latest evidence and principles of cognition to help map requirements to information design for an interactive learning system. Our system seeks to improve the structural understanding of architects and to teach relationships between form and structure in long-span systems. We provide both explanatory multimedia resources and interactive resources including a Web-based modeling and simulation tool that aids architecture students with better understanding of the relationship between structure and form in design. We describe our design process and the system and examine the qualitative impact of the cognitive ergonomic process. This extra step in the usability design process of mapping expert knowledge to human perception and cognition can increase awareness of the requirements of a learning system and improve the effectiveness of the subsequent design.

Experimental evaluation of magnetorheological dampers for semi-active tuned vibration absorbers

The main purpose of this study is to experimentally evaluate the dynamic performance of a semi-active Tuned Vibration Absorber (TVA) with a Magneto-Rheological (MR) damper. To this end, a test apparatus was built to represent a two-degree-of-freedom primary structure model coupled with a MR TVA. The primary structure mass, which is modeled with steel plates, was excited by a hydraulic actuator through four air springs. The air springs represent the stiffness of the primary structure and offer the ability to change the stiffness. The semi-active TVA consists of a steel plate, a MR damper, and four coil springs for physical representation of the mass, the damping element, and the stiffness of the TVA, respectively. Mounted on top of the primary structure, the TVA is connected to the primary structure plates by hardened linear bearing shafts. A series of transducers along with a data acquisition system was used to collect sensory information and implement real time control of the MR TVA. Using this test rig setup, a parametric study was performed to analyze the dynamics of the semi-active TVA and to compare the performance of the semi-active TVA with a passive TVA. Displacement based on-off groundhook (on-off DBG) control was used as the control policy for the semi-active TVA. In the parametric study, the effects of on/off-state damping of the MR damper were investigated and compared with a passive TVA to analyze the relative benefits of a semi-active TVA. When damping increased in the passive TVA, the two resonant peaks merge into one peak, and the peak grows. This indicates that the primary structure and TVA are linked together, disabling the TVA, and it eventually magnifies the vibrations. For a semi-active TVA, however, the two resonant peaks decrease as on-state damping increases (keeping low off-state damping), indicating reduction of vibrations. It is shown that semi-active TVAs outperform passive TVAs in reducing the peak transmissibility, implying that semi-active TVAs are more effective in reducing the vibrations of the primary structure.

Observations from Vibration Testing of In-Situ Structures

Low-frequency floor and footbridge vibration serviceability problems typically arise when the structure is excited in resonance due to a walking excitation and the resulting accelerations exceed human comfort levels. The measures required to resolve an annoying vibration problem after the structure is constructed can be very difficult and expensive to implement. In most cases, the costs of fixing the problem in-situ are much greater than tackling the problem in the design phase, prior to the structures construction, considering the potential cost to building owners from possible legal expenses, loss of rental revenue, and consultation fees. Design guidance for composite steel framed floor systems and footbridges is available in the AISC/CISC Steel Design Guide Series 11: Floor Vibrations Due to Human Acitivity . Although the current design guidance is generally acceptable, there is a need to continue characterizing the often-complicated vibration behavior of these structures in an effort to refine current design and analysis techniques, particularly as researchers gain a better understanding of behavior by collecting high-quality experimental data on in-situ floor and footbridge structures. This paper presents observations from such efforts to further characterize behavior through experimental modal testing of a large in-situ composite steel office floor and a laboratory constructed multi-span footbridge. While not entirely inclusive, some general observations are noted on the dynamic behavior, problems encountered, and the consistency/reliability of the applied testing/analysis techniques employed by the researchers.

Vibration Studies of a Cantilevered Structure Subjected to Human Activities Using a Remote Monitoring System

Long cantilevered balconies used as seating areas in auditoriums, theaters, churches, and stadiums are often susceptible to excessive vibrations because of crowd movements. Measurement and analysis of the responses of such structures when subjected to human movements can provide a reasonable estimate of their dynamic properties. However, it is generally very difficult to artificially excite such massive structures with a measured input force at the same level as that exerted by a crowd. In addition, it is not yet well understood how human occupants’ presence may change the dynamic properties of these structures. This paper presents details of a remote vibration monitoring system (RVMS) installed on a large cantilevered balcony structure to collect the vibration records generated by rhythmic crowd activities. The results of the studies conducted indicate that the presence of human occupants resulted in a consistent reduction in the natural frequencies of the structure and an increase in the damping ratios ...