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1. Parameters identification of cable stayed footbridges using Bayesian inference

   https://doi.org/doi.org/10.1007/s11012-019-01019-  

   Pepi, C. ( August 2019 , Meccanica)

   Numerical modeling of actual structural systems is a very complex task mainly due to the lack of complete knowledge on the involved parameters. Simplified assumptions on the uncertain geometry, material properties and boundary conditions make the numerical model response differ from the actual structural response. Improvements of the finite element (FE) models to obtain accurate response predictions can be achieved by vibration based FE model updating which uses experimental measures to minimize the differences between the numerical and experimental modal features (i.e. natural frequencies and mode shapes). Within this context, probabilistic model updating procedures based on the Bayes’ theorem weremore »recently proposed in the literature in order to take into account the uncertainties affecting the structural parameters and their influence on the structural response. In this paper, a novel framework to efficiently estimate the posterior marginal PDF of the selected model parameters is proposed. First, the main dynamic parameters to be used for model updating are identified by ambient vibration tests on an actual structural system. Second, a first numerical FE model is developed to perform initial sensitivity analysis. Third, a surrogate model based on polynomial chaos is calibrated on the initial FE model to significantly reduce computational costs. Finally, the posterior marginal PDFs of the chosen model parameters are estimated. The effectiveness of the proposed method is demonstrated using a FE numerical model describing a curved cable-stayed footbridge located in Terni (Umbria Region, Central Italy).« less
2. A Comparative Study of Frequency-domain Finite Element Updating Approaches Using Different Optimization Procedures

   DONG, Xinjun ; WANG, Yang ( July 2016 , EWSHM)

   In order to achieve a more accurate finite element (FE) model for an as-built structure, experimental data collected from the actual structure can be used to update selected parameters of the FE model. The process is known as FE model updating. This research compares the performance of two frequency-domain model updating approaches. The first approach minimizes the difference between experimental and simulated modal properties, such as natural frequencies and mode shapes. The second approach minimizes modal dynamic residuals from the generalized eigenvalue equation involving stiffness and mass matrices. Both model updating approaches are formulated as an optimization problem with selectedmore »updating parameters as optimization variables. This research also compares the performance of different optimization procedures, including a nonlinear least-square, an interior-point and an iterative linearization procedure. The comparison is conducted using a numerical example of a space frame structure. The modal dynamic residual approach shows better performance than the modal property difference approach in updating model parameters of the space frame structure.« less
3. Beta Probabilistic Databases: A Scalable Approach to Belief Updating and Parameter Learning

   https://doi.org/10.1145/3035918.3064026  

   Meneghetti, Niccolo' ; Kennedy, Oliver ; Gatterbauer, Wolfgang ( January 2017 , Proceedings of the 2017 ACM International Conference on Management of Data)

   Tuple-independent probabilistic databases (TI-PDBs) han- dle uncertainty by annotating each tuple with a probability parameter; when the user submits a query, the database de- rives the marginal probabilities of each output-tuple, assum- ing input-tuples are statistically independent. While query processing in TI-PDBs has been studied extensively, limited research has been dedicated to the problems of updating or deriving the parameters from observations of query results . Addressing this problem is the main focus of this paper. We introduce Beta Probabilistic Databases (B-PDBs), a general- ization of TI-PDBs designed to support both (i) belief updat- ing and (ii) parameter learning inmore »a principled and scalable way. The key idea of B-PDBs is to treat each parameter as a latent, Beta-distributed random variable. We show how this simple expedient enables both belief updating and pa- rameter learning in a principled way, without imposing any burden on regular query processing. We use this model to provide the following key contributions: (i) we show how to scalably compute the posterior densities of the parameters given new evidence; (ii) we study the complexity of perform- ing Bayesian belief updates, devising efficient algorithms for tractable classes of queries; (iii) we propose a soft-EM algo- rithm for computing maximum-likelihood estimates of the parameters; (iv) we show how to embed the proposed algo- rithms into a standard relational engine; (v) we support our conclusions with extensive experimental results.« less
4. Design and Assessment of Innovative Dual-Mode Rolling Isolation Systems

   https://doi.org/11244/321054  

   Hadikhan Tehrani, Mohammad ( August 2019 , The University of Oklahoma Libraries)

   Loss of operation or devastating damage to buildings and industrial structures, as well as equipment housed in them, has been observed due to earthquake-induced vibrations. A common source of operational downtime is due to the performance reduction of vital equipment, which are sensitive to the total transmitted acceleration. A well-known method of protecting such equipment is seismic isolation of the equipment itself (or a group of equipment), as opposed to the entire structure due to the lower cost of implementation. The first objective of this dissertation is assessing a rolling isolation system (RIS) based on existing design guidelines for telecommunicationsmore »equipment. A discrepancy is observed between the required response spectrum (RRS) and the one and only accelerogram recommended in the guideline. Several filters are developed to generate synthetic accelerograms that are compatible with the RRS. The generated accelerograms are used for probabilistic assessment of a RIS that is acceptable per the guideline. This assessment reveals large failure probability due to displacement demands in excess of the displacement capacity of the RIS. When the displacement demands on an isolation system are in excess of its capacity, impacts result in spikes in transmitted acceleration. Therefore, the second objective of this dissertation is to design impact prevention/mitigation mechanisms. A dual-mode system is proposed where the behavior changes when the displacement exceeds a predefined threshold. A new piecewise optimal control approach is developed and applied to find the best possible mechanism for the region beyond the threshold. By utilizing the designed curves obtained from the proposed optimal control procedure, a Kelvin-Voigt device is tuned for illustrative purposes. On the other hand, the preference for protecting equipment decreases as the earthquake intensity increases. In extreme seismic loading, the response mitigation of the primary structure (i.e., life safety and collapse prevention) is of greater concern than protecting isolated equipment. Therefore, the third objective of this dissertation is to develop an innovative dual-mode system that can behave as equipment isolation under low to moderate seismic loading and passively transition to behave as a vibration absorber for the primary structure under extreme seismic loading. To reduce the computational cost of simulating a large linear elastic structure with nonlinear attachments (i.e., equipment isolation with cubic hardening nonlinearity), a reduced order modeling method is introduced that can capture the behavior of such nonlinear coupled systems. The method is applied to study the feasibility of dual-mode vibration isolation/absorber. To this end, nonlinear transmissibility curves for the roof displacement and isolated mass total acceleration are developed from the steady-state responses of dual-mode systems using the harmonic balanced method. The final objective of this dissertation is to extend the reduced order modeling method developed for linear elastic structure with nonlinear attachment to inelastic structures (without attachments). The new inelastic model condensation (IMC) method uses the modal properties of the full structural model (in the elastic range) to construct a linear reduced order model in conjunction with a hysteresis model to capture the hysteretic inter-story restoring forces. The parameters of these hysteretic forces are easily tuned, in order to fit the inelastic behavior of the condensed structure to that of the full model under a variety of simple loading scenarios. The fidelity of structural models condensed in this way is demonstrated via simulation for different ground motion intensities on three different building structures with various heights. The simplicity, accuracy, and efficiency of this approach could significantly alleviate the computational burden of performance-based earthquake engineering.« less
5. Wind-induced Response Characteristics of a Yawed and Inclined Cable in ABL Wind

   https://doi.org/https://doi.org/10.1016/j.engstruct.2020.110681  

   Jafari, M. ; Sarkar, P. P. ( January 2020 , Engineering structures)

   Inclined cables used in bridges or other infrastructures are vulnerable to unsteady wind-induced loads producing moderate- to large-amplitude vibration that may result in damage or failure of the cables, resulting in catastrophic failure of the structure they secure. In the present study, wind-induced response of an inclined smooth cable was studied through wind tunnel measurements using a flexible cable model for a better understanding of the vibration characteristics of structural cables in atmospheric boundary layer wind. For this purpose, in-plane and out-of-plane responses of a sagged and a non-sagged flexible cable were recorded by four accelerometers. Four cases with differentmore »yaw and inclination angles of a cable with approximate sag ratios of 1/10 were studied to investigate the wind directionality effect on its excitation mode(s) and response amplitude. Cable tension was also measured during all experiments to assess the correlation of wind speed, excitation vibration mode, and natural frequency of the cable with change in cable tension. Additionally, two inclined cables with no sag were tested to determine the influence of sag of a cable on its vibration characteristics. In the second part of this study, a series of finite element analyses were conducted to predict the wind-induced aerodynamic damping of an inclined bridge cable. Experimental results showed that excitation mode(s) of a cable depend on wind speed, inclination angle, and sag ratio and cable tension. First, second, and third vibration modes were observed at a low wind speed for different test cases, whereas higher vibration modes were observed to contribute to the cable response at high wind speeds. Moreover, it was seen that the cable tension significantly increased with wind speed resulting in increased value of the excited natural frequency. Numerical results obtained through finite element analysis of an inclined full-scale cable showed that the criteria that are based on section models can underestimate the critical reduced velocity for dry cable galloping.« less