Ambient Vibration Test Results Research Articles

Dynamic Characteristics of a 1950s Heritage Building: A Comparison of Original Design Methods and Modern Techniques

Research on design rules and methods for architectural heritage is an important aspect of conservation practice. Nevertheless, efforts to recover and divulge design methods for Modern Heritage remain limited. This paper is related to the recent structural assessment of a 15-storey heritage building built in 1950, during which a document describing the original seismic analysis of this structure was identified. The methodology employed is of particular interest, as it involves the application of pioneer concepts of dynamic analysis in the design of the first tall buildings in Mexico. The primary aim of this paper is to review the seismic design criteria for the case under study in order to contribute to the state of the art in Modern Heritage. The review includes a comparison between the dynamic characteristics estimated during the design and the results of recent ambient vibration tests and numerical modeling. Several sources of error among the design criteria were identified. Notably, the fundamental period estimated during the design was 38% larger than the experimental value due to an underestimation in stiffness, which introduces significant uncertainty into the design. Overall, the review shows the evolution of seismic analysis over time and provide valuable insights for the study of similar buildings.

Operational Modal Analysis as a Support for the Development of Digital Twin Models of Bridges

Many transportation infrastructures all around the world are facing new challenges in terms of ageing and loss of performance. The infrastructural asset managers are required to perform scrupulous control of the health condition of the infrastructures over time and to execute the required maintenance works. In this context, digital twin models of the infrastructures should have a key role to simplify and speed up the procedures for proper maintenance. This paper discusses the advantages of developing digital twin models for the management of infrastructures, with a focus on bridges. In particular, the role of dynamic tests performed on bridges for the development of digital twin models is addressed, paying attention to test procedures and requirements. Issues such as the quality of instrumentation, the numerosity, and layout of sensors, and the acquisition and post-processing procedures are addressed through applications to two real bridge case studies. Both infrastructures are multi-span pre-stressed RC bridges that were dynamically tested after the restoration and seismic upgrading works. Results of ambient vibration tests and operational modal analyses are described, providing an idea of dynamic test requirements, as well as their use within the framework of the digital twin model creation.

Dynamic Testing in Support of the Seismic Assessment of a Century Old Masonry Building Complex

The vulnerability assessment of existing masonry buildings is a largely investigated research topic with some aspects still to be faced. In historic towns, masonry buildings are aggregated and together confined, and their final appearance is derived from interventions and additions during their lives in different times and with different masonry textures or different construction materials. Demolitions and reconstructions of some parts were frequent, with the difficulty of now understanding the effectiveness of the mutual constraints. The seismic assessment of a case study of a 175-year-old building complex in Udine (Italy) provides an opportunity to use the results of ambient vibration tests to face the problem of modelling aggregate buildings for their seismic assessment. The “Padiglione Lodi” building complex was built in 1847 and extended and renovated several times afterwards. It was built mostly using URM with limited use of reinforced concrete. It consists of a main building and three wings (western, central and eastern). The inspections, experimental survey and analysis of the available documentation are used to suitably calibrate a Finite Element Model of the whole complex. Moreover, this allows the singling out of the central wing, as the unit needs more careful investigation. Non-destructive dynamic testing is then applied to the central wing in order to further validate the model and improve the knowledge of the interaction of the unit with the rest of the building. General remarks on the effective application of non-destructive dynamic analysis in conjunction with other methods to the seismic assessment of large URM building complexes are drawn.

Vibration Serviceability Assessment of a Historic Suspension Footbridge

Experimental and numerical studies for the structural and vibration serviceability assessment of a historic suspension footbridge adopting non-invasive surveys and low-cost equipment are presented. Field surveys have been carried out to determine geometric properties, ambient vibration tests have been performed to estimate the dynamic properties, and the dynamic response of the footbridge under the action of a single crossing pedestrian has been recorded. Based on field surveys, a 3D Finite Element model was built and was then calibrated against ambient vibration test results. The experimentally-measured maximum acceleration under the action of one crossing pedestrian is compared with the ones obtained numerically and analytically. Furthermore, vibration serviceability assessment under multi-pedestrian loading is carried out, adopting the simplified procedure recommended by a recent guideline. Results show that low-cost non-invasive dynamic testing is suitable to correctly identify the footbridge vertical natural frequencies and mode shapes, including higher-order ones, and to draw considerations about the state of degradation of the structure. Moreover, the level of vibration under the action of a single pedestrian can be estimated with sufficient accuracy using a simplified loading model, provided that the modal damping ratio is properly tuned.

A methodology for the identification of physical parameters of soil-foundation-bridge pier systems from identified state-space models

A methodology for the identification of the physical parameters of a model describing the transverse dynamics of soil-foundation-pier systems founded on piles is presented in this paper, starting from identified state-space models of the systems obtained from results of dynamic experimental tests and identification techniques available in the literature. By assuming the order of the model to be compliant with that of a numerical one suitably developed to capture the dynamics of the bridge pier, the procedure allows the identification of the stiffness, mass and damping matrices of the analytical model from which the physical parameters of the real system (e.g. masses, pier stiffness matrix and soil-foundation impedance) can be extracted by directly comparing the components of the identified and analytical matrices. The procedure allows the direct definition of the numerical model that best fits the experimental data without the need of any model calibration, and permits the identification of the soil-foundation compliance taking into account its intrinsic frequency-dependent behaviour. Firstly, the dynamics of the analytical model is formulated adopting the continuous-time first-order state-space form. The model includes the frequency-dependent behaviour of the soil-foundation system through the introduction of a lumped parameter model able to reproduce the typical soil-foundation impedances of pile foundations. Then, an identification strategy of the physical parameters of a generic realistic soil-foundation-pier system is proposed starting from the discrete-time state-space model identified from dynamic tests. The procedure is illustrated with some applications, simulating results of forced vibration and ambient vibration tests executed on a known system whose parameters are then identified according to the proposed approach. The procedure revealed to be effective to identify the stiffness, mass and damping matrixes of the known system, and consequently its physical parameters.

On the reliability of the equivalent frame models: the case study of the permanently monitored Pizzoli\u2019s town hall

This paper presents the comparison of the results of modal and nonlinear analyses carried out on a 2-story masonry building with rigid diaphragms, inspired by the Pizzoli’s town hall (AQ, Italy). The case study is one of the Benchmark Structures (labeled BS6) in the “URM nonlinear modelling–Benchmark project” funded by the Italian Department of Civil Protection (DPC) within the framework of the ReLUIS projects. The building has been instrumented since 2009 with a permanent monitoring system by the Osservatorio Sismico delle Strutture (OSS) of the DPC and was hit by the 2016/2017 Central Italy earthquake sequence. In the research first phase, modal and nonlinear static analyses were carried out in a blind prediction, without any preliminary calibration of the models, but referring only to commonly made assumptions on materials and modelling. Five computer programs based on the Equivalent Frame Model (EFM) approach were used. Four different structural configurations were considered: with weak spandrels (A), with tie rods coupled to spandrels (B), with RC ring beams coupled to spandrels (C) and with “shear type” idealization (D). In the research second phase, two of the developed EFMs were calibrated in the elastic range using the results of available Ambient Vibration Tests (AVTs). The models were then validated in the nonlinear range by simulating the dynamic response of the structure recorded during the mainshocks of the 2016/2017 Central Italy earthquake. Recorded and numerical results were compared at both the global and local scale.

Nonlinear Dynamic Response of a RC Frame for Different Structural Conditions Including the Effect of FE Model Updating

After destructive events such as earthquakes, attempts have generally been made to repair and strengthen structures using such methods as filling cracks by injection, wrapping structural elements with fiber-reinforced polymer (FRP). These applications can allow the recovery of lost structural performance. In this paper, the aim was to investigate the structural performance of an RC frame under different conditions—undamaged, damaged, repaired, and strengthened—by comparing results of nonlinear dynamic analyses. In addition, finite-element (FE) model updating effects were investigated in the case of the undamaged condition. To this end, an RC frame model was first constructed in the laboratory, and then, the model was damaged by lateral forces, repaired with epoxy injection, and lastly strengthened by wrapping with FRP. To obtain the dynamic characteristics of the RC frame under the different conditions, ambient vibration tests were performed for each condition. The FE model of the RC frame, obtained through global and local model updating using the ambient vibration test results, was modeled to achieve the nonlinear dynamic analyses. The results of the analyses were presented in the form of maximum displacements, principal stresses, and strains with contours diagrams. The maximum displacement increased significantly (133%) with the degree of damage, sharply decreased with the repaired (32%), and strengthened (36%) applications. The maximum and minimum principal stresses decreased by 24% and 35%, respectively, with the damaged condition. In addition, maximum differences between the undamaged and strengthened conditions were calculated as 17% for the maximum principal stress and 38% for the minimum principal stress. Findings indicated that the epoxy injection with FRP wrapping eliminates effect of the damage and recovers the model’s performance to its initial state. In addition, it is demonstrated in this paper that the updating procedure considerably changes the dynamic analysis results.

Finite-Element Model Updating and Dynamic Responses of Reconstructed Historical Timber Bridges using Ambient Vibration Test Results

AbstractHistorical timber bridges are significant cultural structures and have a long tradition. There are many pre-1950s bridges still in use today. In recent years, great resources have been devo...

Evolutionary-base finite element model updating and damage detection using modal testing results

This research focuses on finite element model updating and damage assessment of structures at element level based on global nondestructive test results. For this purpose, an optimization system is generated to minimize the structural dynamic parameters discrepancies between numerical and experimental models. Objective functions are selected based on the square of Euclidean norm error of vibration frequencies and modal assurance criterion of mode shapes. In order to update the finite element model and detect local damages within the structural members, modern optimization techniques is implemented according to the evolutionary algorithms to meet the global optimized solution. Using a simulated numerical example, application of genetic algorithm (GA), particle swarm (PSO) and artificial bee colony (ABC) algorithms are investigated in FE model updating and damage detection problems to consider their accuracy and convergence characteristics. Then, a hybrid multi stage optimization method is presented merging advantages of PSO and ABC methods in finding damage location and extent. The efficiency of the methods have been examined using two simulated numerical examples, a laboratory dynamic test and a high-rise building field ambient vibration test results. The implemented evolutionary updating methods show successful results in accuracy and speed considering the incomplete and noisy experimental measured data.

Finite element model updating of base-isolated buildings using experimental results of in-situ tests

This paper builds on the results of in-situ release tests carried out on a 3-storey base-isolated reinforced concrete frame building located in the city of Augusta (Sicily). The isolation system consists of a combination of High Damping Rubber Bearings and Low Friction Flat Sliding Bearings. Suitable practice-oriented nonlinear models have been considered to describe the cyclic behavior of the isolation system and the infill panels. The results of ambient vibration and release tests have been used to calibrate the numerical models of superstructure and isolation system, respectively. Nonlinear response-history analyses of the calibrated model are then performed to compare the seismic response of the structure with the expected (design) seismic performance.

Effectiveness of seismic repairing stages with CFRPs on the seismic performance of damaged RC frames

This study aims at evaluating the performance of repairing technique with CFRPs in recovering cyclic performance of damaged columns in flexure in terms of structural response parameters such as strength, dissipated energy, stiffness degradation. A 2/3 scaled substandard reinforced concrete frame was constructed to represent the substandard RC buildings especially in developing countries. These substandard buildings have several structural deficiencies such as strong beam-weak column phenomenon, improper reinforcement detailing and poor material properties. Flexural plastic hinges occurred at the columns ends after testing the substandard specimen under both constant axial load and reversed cyclic lateral loading. Afterwards, the damaged columns were externally wrapped with CFRP sheets both in transverse and longitudinal directions and then retested under the same loading protocol. In addition, ambient vibration measurements were taken from the undamaged, damaged and the repaired specimens at each structural repair steps to identify the effectiveness of each repairing step by monitoring the change in the natural frequencies of the tested specimen. The ambient vibration test results showed that the applied repairing technique with external CFRP wrapping was proved to recover stiffness of the pre-damaged specimen. Moreover, the lateral load capacity of the pre-damaged substandard RC frame was restored with externally bonded CFRP sheets.

Characterisation of structural properties for AlSi10Mg alloys fabricated by selective laser melting

ABSTRACTSelective laser melting (SLM) is a form of additive manufacturing technology that makes it possible to create complex parts directly from metal powders. This technology offers a good balance between investment costs, range of materials and part quality. In the work, in order to investigate the structural properties of SLM AlSi10Mg alloys, the microstructure, structural performance and tensile fracture morphology of the alloys are analysed. The sample study results indicate good prospects, and the application of SLM in terms of aeronautics is studied to verify the structural properties. The ambient vibration test results show that SLM AlSi10Mg alloys could have a better performance in structural analysis and vibration test than the conventional manufacturing part and the reasons are analysed.This paper is part of a thematic issue on Light Alloys.

COMPARISON OF HYBRID METHODS WITH DIFFERENT META MODEL USED IN BRIDGE MODEL-UPDATING

Two hybrid model updating methods by integration of Gaussian mutation particle swarm optimization method, Latin Hypercube Sampling technique and meta models of Kriging and Back-Propagation Neural Network respectively were proposed, and the methods make the convergence speed of the model updating process faster and the Finite Element Model more adequate. Through the application of the hybrid methods to model updating process of a self-anchored suspension bridge in-service with extra-width, which showed great necessity considering the ambient vibration test results, the comparison of the two proposed methods was made. The results indicate that frequency differences between test and modified model were narrowed compared to results between test and original model after model updating using both methods as all the values are less than 6%, which is 25%−40% initially. Furthermore, the Model Assurance Criteria increase a little illustrating that more agreeable mode shapes are obtained as all of the Model Assurance Criteria are over 0.86. The particular advancements indicate that a relatively more adequate Finite Element Model is yielded with high efficiency without losing accuracy by both methods. However, the comparison among the two hybrid methods shows that the one with Back-Propagation Neural Network meta model is better than the one with Kriging meta model as the frequency differences of the former are mostly under 5%, but the latter ones are not. Furthermore, the former has higher efficiency than the other as the convergence speed of the former is faster. Thus, the hybrid method, within Gaussian mutation particle swarm optimization method and Back-Propagation Neural Network meta model, is more suitable for model updating of engineering applications with large-scale, multi-dimensional parameter structures involving implicit performance functions.

Field Tests on Micropiles Under Dynamic Lateral Loading

Micropiles are increasingly used as foundation support of new buildings in seismic areas as well as for the seismic retrofitting of structures that have experienced seismic damage. Hence, it is essential to enhance the knowledge of the dynamic behavior of micropiles under horizontal loading. In the present paper, first steps of an extensive experimental study carried out on two vertical micropiles in alluvial silty soil are reported. One of the vertical micropiles is injected throughout valves a-manchèttes placed along the steel core of the shaft, while the other one is simply grouted. In particular, experimental results of ambient vibration tests and impact load tests are reported and a comparison between the behavior of the vertical injected and non-injected micropiles is provided. Experimental data of impact load tests are also compared with results obtained from an analytical model.

Finite element model calibration of precast structures using ambient vibrations

In this paper, the finite element model calibration of precast structures was investigated by considering ambient vibration test results. Two precast structures, an overpass and a precast production facility, were selected for investigation. The initial finite element models of these structures were developed by using SAP2000 software and the initial dynamic characteristics were determined. The experimental measurements were carried out by Operational Modal Analysis method under ambient vibrations, such as wind and traffic loads, and the exact dynamic characteristics were identified experimentally. During the ambient vibration tests, structural responses were measured on different points of these precast structures. Measurement time, frequency span and effective mode number were determined by the pretest investigations. The modal parameters were extracted from the collected signals by Enhanced Frequency Domain Decomposition and Stochastic Subspace Identification techniques. At the end of the study, the analytical and experimental dynamic characteristics were compared with each other and the initial finite element models of these structures were updated by changing the stiffness coefficients of the connection joints.

Structural evaluation of Aspendos (Belkis) Masonry Bridge

In this study, the structural performance of a seven span masonry arch bridge was evaluated. Investigations were performed on Aspendos (Belkis) Masonry Arch Bridge which was located on road of Aspendos Acropolis City in Antalya, Turkey. The old bridge was constructed in the early of fourth century AD, but it was exposed to the earthquakes in this region and the overloading by the river water. The old bridge was severely damaged and collapsed by probably an earthquake many years ago and a new bridge was then reconstructed on the remains of this old bridge by Seljuk in the 13th century. The bridge has also been affected from overflowing especially in the spring of each year, so some protective measures should be taken for this monumental bridge. Therefore, the structural performance under these loading has to be known. For this purpose, an initial finite element model was developed for the bridge and it was calibrated according to ambient vibration test results. After that, it was analyzed for different load cases such as dead, live, earthquake and overflow. Three load combinations were taken into account by deriving from these load cases. The displacements and the stresses for these combination cases were attained and compared with each other. The structural performance of Aspendos Masonry Arch Bridge was determined by considering the demand-capacity ratio for the tensile stress of the mortar used in Aspendos Masonry Arch Bridge. After these investigations, some concluding remarks and offers were presented at the end of this study.

Modeling of concrete dwellings based on results from ambient vibration tests

Based on both ambient vibration measurements and numerical models using the wide-column analogy and the finite element method, vibrations periods of low-rise concrete dwellings are estimated in this study. A brief review of parametric and nonparametric techniques is presented and discussed, and in-situ measurements techniques are critically compared. The results of ambient vibration tests are used to calibrate numerical models. Advantages, drawbacks and accuracy of parametric techniques, as well as particular recommendations to build numerical models for assessing the elastic behavior of low-rise concrete wall dwellings, are proposed.

Modal Identification of Different RC Frames Using Experimental Measurements

Abstract The main purpose of this study is to determine dynamic characteristics, such as natural frequencies, mode shapes and damping ratios of reinforced concrete (RC) frames with low and normal strength concrete and compare them with each other using the operational modal analyses method under ambient vibration. For this purpose, full-scaled, one bay, and one-story RC frames with low and normal strength concrete are produced. The RC frames are vibrated by natural excitations with small impact effects, and the response signals are recorded using accelerometers during the tests. Measurement time-frequency span and effective mode number are determined by considering similar studies and literature. To obtain experimental dynamic characteristics, enhanced frequency domain decomposition and stochastic subspace identification methods are evaluated together. Results of ambient vibration tests show that dynamic characteristics change significantly depending on the material properties. It is shown that the ambient vibration measurements are enough to identify the most significant modes of RC frames as well. The first five natural frequencies are obtained experimentally for the RC frame with low and normal strength concrete between 14.99–155.20 Hz and 16.64–179.20 Hz, respectively. Dynamic characteristics should be verified using finite element analysis. Finally, inconsistency between experimental and analytical dynamic characteristics should be minimized by finite element model updating using some uncertain parameters such as material properties, boundary condition, and section properties to reflect the current behavior of the RC frames.

Modal testing and finite element model calibration of in-filled reinforce concrete frames

In this study, modal testing and finite element model calibration of in-filled reinforced concrete (RC) frames are studied. For this purpose a full-scaled, one bay and one-story RC frame is produced and tested for plane and brick in-filled conditions. Dynamic characteristics, such as natural frequencies, mode shapes and damping ratios, of plane and in-filled RC frames are determined using the Operational Modal Analyses method under ambient vibration. The RC frame is vibrated by natural excitations with small-impact effects and the response signals are measured using sensitive accelerometers during ambient vibration tests. Measurements of time-frequency range and effective mode number are determined by considering similar studies and literature. To obtain experimental dynamic characteristics, enhanced frequency domain decomposition and stochastic subspace identification methods are employed. Analytical modal analysis is performed on a two-dimensional finite element model of the frames using SAP2000 software to provide analytical frequencies and mode shapes. The results of ambient vibration tests show that dynamic characteristics change significantly depending on the existence of an in-fill wall. The first five natural frequencies are obtained experimentally between 16.64 and 179.20 Hz, and 63.56 and 226.12 Hz for plane and brick in-filled, respectively. Dynamic characteristics obtained by analytical and experimental methods are compared with each other and the finite element model of the frames is updated by changing some uncertain modeling parameters, such as material properties and boundary conditions, to reduce the differences between the results. At the end of the study, maximum differences in the natural frequencies are reduced on average from 39% to 8% and a good agreement is found between analytical and experimental dynamic characteristics after finite element model updating. This result shows the importance of finite element model updating to reflect the current behavior of the structures. In addition, it is seen that material properties are more effective parameters in the finite element model updating of the plane frame. However, for the brick in-filled frame, changes in boundary conditions determine the model updating process.

Seismic Response Analysis of Masonry Minaret and Possible Strengthening by Fiber Reinforced Cementitious Matrix (FRCM) Materials

This paper presents the analytical study on the dynamic response of a natural stone block masonry minaret of a historical mosque located in Istanbul, Turkey. A three-dimensional finite element model of the whole minaret, adjusted with former in situ ambient vibration test results, was used in order to get probable lateral displacements, stresses, and understanding of failure modes under seismic loading. Particularly, FEM has been used to evaluate the seismic safety of the minaret in present-day condition and after the application of a proposed strengthening method. In this method, by utilizing the fiber reinforced cementitious matrix (FRCM) material, the critical cross-sections were jacketed. The appearing damage for the bare minaret obtained from FEM analyses was located at the boot and transition part of the minaret. After FRCM material was wrapped around these critical cross-sections, the analytical results indicated that such method appears effective in terms of the seismic response. It also appears less invasive in terms of the conservation of historical heritage like minarets without causing aesthetic conflict on the existing structure when compared to other available techniques.