Ambient Vibration Method Research Articles

DAMAGE INSPECTION ON PIER OF SG. PERAK RESERVOIR BRIDGE USING ACCELEROMETER AND GNSS

Sg. Perak Reservoir Bridge where it is commonly known to be heavily utilized by heavy vehicles on daily basis. Therefore, the practices of inspection and maintenance of this bridge are essential tasks in prolonging its lifespan especially as it is utilized heavily with heavy vehicles. The act of inspecting damage that occurred at a pier is done through the use of technical equipment as an additional method invalidating visual inspection obtained through vibration reading, and validating in inspection reading analysis as additional information to confirm for any structural damages. The use of using Global Navigation Satellite System (GNSS) can be employed for continuous monitoring and the use of accelerometers, which are components of the ambient vibration method, served to be an integral part of the information obtained on the changes in the dynamic structural features detected. The vibration measurement can display natural frequencies that depended on the weight, material, pressure, and tension as well as the geometry of the object. This data obtained can therefore be used to furnish additional information on the capacity and condition of the structure. The results indicated that maximum vibration on two piers inspection are recorded at 53.7 mm/s2 and 49.6 mm/s2 using an Accelerometer indeed heavy vehicles traffic flow is a factor in influencing bridge vibration by traffic transport diversity between west and east lanes of both sides of the bridge.

Measured dynamic properties of web-core sandwich panel FRP composite footbridges and their relation to pedestrian comfort analysis

This paper reports experimental data for the dynamic properties (i.e. first fundamental flexural frequency, damping ratio, comfort class) of ten web-core sandwich panel FRP composite footbridges in Belgium, which contributes to the assessment of relevant input parameters for design and assessment of this promising bridge type, quickly gaining popularity in recent years. The data is gathered based on smartphone accelerometers, enabling easy, quick, affordable, and abundant measurements, while at the same time yielding reliable experimental values. Given the relatively short spans, the heel and excitation test methods are used, rather than the ambient vibration method. The tests indicate damping ratios of one to three percent, which are strongly dependent on the number of people on the bridge during the measurement. Additionally, comfort analysis tests with up to 58 people (0.5P/m2) were conducted on five out of the ten bridges. The results indicate that the current design guidelines for pedestrian comfort analysis are overconservative and do not reflect the effect of pedestrian-induced damping, which is especially apparent for this bridge type given its very low modal mass and relatively low damping ratio.

A new attempt for early warning of unstable rocks based on vibration parameters

Rock collapse is a type of fast-moving mass wasting process common in mountainous areas worldwide, and it is hard to predict when collapse will occur. In the experiment, we monitored vibration parameters to predict the time of rock collapse. The results show that the vibration amplitude and fundamental vibration frequency had obvious variations before rock failure, which was consistent with the predicted results of the theoretical model. As the rock became unstable, the vibration amplitude increased by more than 5 times, and the fundamental vibration frequency decreased to less than 41%. Therefore, the monitoring method based on the vibration parameters can record obvious failure precursors for unstable rocks. Our results further develop and refine ambient vibration methods used for implementing the early warning of rock collapse.

Modeling the Seismic Response of Unstable Rock Mass With Deep Compliant Fractures

AbstractAn experimental quantification of the strength and volume of real, heterogeneous, fractured rock masses is crucial when assessing rock slope stability. In order to quantitatively characterize the internal structure of fractured rock slopes, we present three‐dimensional numerical simulations of seismic wave propagation and compare with observations. We introduce a simple, effective model for fractured rock mass, which can easily be applied to simulate weak‐motion seismic wave propagation. The macroscopic compliant fractures cutting the rock mass are modeled as finite‐width zones of reduced elastic parameters characterized by shear and normal stiffness. The widths of such zones are not fixed and can be adjusted to fit the grid step in the numerical method. The proposed rock mass model is applied and tested for the Walkerschmatt site in southwest Switzerland. Synthetic ambient vibrations are generated using a finite‐difference method for the fractured rock mass, shaped by the real terrain geometry, and compared with the measurements. The observed seismic response is satisfactorily reproduced in a broad frequency range (0.5–10 Hz). The synthetized response is primarily controlled by the stiffness, depth, number of fractures, and inertial mass of the fractured rock. The simulated amplification and ground‐motion directionality correspond with the observed levels, unless (1) the simplified cracks reach depths of 200–300 m; and (2) the fracture network is larger with respect to the mapped network. This illustrates the potential of ambient vibration methods in combination with numerical simulations to infer depth, volume, and mechanical characteristics of slope instability.

Experimental Study on Structural Performance of Prefabricated Composite Box Girder with Corrugated Webs and Steel Tube Slab

We present an innovative prefabricated composite box girder with corrugated webs and concrete-filled steel tube slab to prevent cracking in the web and reduce the self-weight, which is suitable for long-span structures. We carried out systematic experimental and analytical studies to investigate the structural performance, including the loading capacity and dynamic properties of a prefabricated composite box girder bridge before and after erection. Firstly, we tested a prefabricated composite girder with a single box section before erection under monotonic loading, measuring vertical deformation, flexural strain on the slabs, and shear strain on the corrugated steel webs, and evaluating the load-carrying capacity and stiffness reduction. Secondly, we conducted field live-load tests, including a calibration test and a dynamic test, on a composite bridge with twin prefabricated box girders. We hired four-axle heavy trucks for a calibration test to explore the static responses in terms of displacement, bending strain on the slabs, and shear strain on the corrugated steel webs. In the dynamic tests, we carried out a modal test using ambient vibration method and a moving load test in order to determine the dynamic behavior, which involves natural frequencies, the mode shapes, and the dynamic load factor (DLF). Based on the test results, the structural performance was evaluated by the AASHTO bridge rating process. All the findings from the load-carrying capacity test at ultimate state, calibration and dynamic load tests at service state in this study may provide a reference for the design and construction of such type of bridges.

Determination of Period of RC Buildings by the Ambient Vibration Method

Determining the dynamic properties of structures is important for understanding their seismic behaviour. Ambient vibration signal measurement is one of the approaches used to determine the period of structures. Advantages of this method include the possibility of taking real‐time records and presenting nondestructive and rapid solutions. In this study, natural vibration periods are calculated by taking ambient vibration signal records from 40 buildings. The height of the building, infill wall effect, presence of seismic retrofit, and presence of damage are taken into consideration, and their effects on natural vibration periods are investigated. Moreover, the results are compared with the analytical methods to reveal the differences. A significant correlation between the period and height of the building is observed. It is seen that the natural vibration periods of the buildings decrease by 7% to 30% (15% on average) due to infill wall contribution. However, the efficiency of infill walls decreases as the building height increases. Another significant result is that adding shear walls substantially decreases the vibration period values by 23% to 33% with respect to the shear wall ratio. When the analytical estimates and measured building period results are compared, it is seen that analytical models have closer period estimates before infill walls are implemented. The limited data in scope of the study suggest that significant differences may present in the analytical and measured periods of the buildings due to infill wall contributions.

Dynamic Property of a New Type of Postearthquake Temporary Prefabricated Lightweight Steel Structure

For a new type of postearthquake temporary prefabricated lightweight steel structure proposed in this paper, mainly composed of steel frame, prefabricated hanger slabs, prefabricated hanger columns, reinforced concrete superposed slabs, etc., parameters of dynamic property for the structure, including natural frequency, vibration mode, damping ratio, etc., are determined by the test method. For prefabricated hanger columns and prefabricated hanger slabs, they are all produced with construction waste in factory and assembled on‐site, which can form exterior walls. The united method, based on forced vibration method and ambient random vibration method, can quickly obtain accurate natural frequencies of the full‐scale two‐story experimental model. In this paper, damping oscillatory method is used to obtain damping ratio which can be determined only by the test method. In order to analyse the modal of the experimental model, a finite element model for the full‐scale two‐story experimental model is established, where the weight of prefabricated hanger slabs is assumed to be supported by prefabricated hanger columns, and the stiffness of prefabricated hanger columns is also increased. In addition, the connections between lightweight steel frame and prefabricated hanger columns are regarded as flexible connection. Comparing natural frequencies obtained from the finite element method with that obtained from the test method, magnification factor of stiffness for prefabricated hanger column is determined. In the analysis of modal for the full‐scale two‐story experimental model, the results show that the experimental model satisfies the requirement of design for seismic performance.

Automated Model Updating Effect on the Linear and Nonlinear Dynamic Responses of Historical Masonry Structures

Finite element models supply the simulation of the problems to obtain the structural behaviors under different loading conditions. The updating processes based on changing the uncertain parameters such as material properties, boundary conditions, meshing size etc. are made to create actual finite element models in order to represent the real conditions of the problems. This procedure can be made using two methods, namely manual model updating and global/local automatic model updating. The global/local automatic model updating procedures are very popular owing to the applicability to all kind of engineering structures, minimizing the differences nearly to zero, availability of damage localization and structural health monitoring. In this paper a historical masonry armory building selected as an application and its finite element analyses, experimental measurements and automated model updating processes are carried out. The finite element model of the structures was developed with ANSYS software and first three natural frequencies are attained. The experimental dynamic characteristics such as natural frequencies, damping ratios and mode shapes are obtained using ambient vibration method. The differences between experimental and numerical frequencies are minimized up to the range of 0–1%, by using local updating procedure. The linear and nonlinear seismic analyses of the building are conducted on initial and updated models. At the end of the analyses, maximum displacements, principal stresses and strains are given with detail. In addition, crack distributions diagrams are attained to validate the stresses accumulation points. It can be seen that there is an increasing trend in the displacement, stress and strain values after finite element model updating both linear and nonlinear analysis. In addition, linear and nonlinear analysis results are compared with each other to evaluate the nonlinear analysis effect. It is concluded that the displacement and strain values are increased after nonlinear analysis. But, there is not any agreement in stress values.

Identification of Natural Frequency of Low Rise Building on Soft Ground Profile using Ambient Vibration Method

Natural frequency is the rate at which a body to vibrate or oscillate. Application of ambient vibration (AV) excitation is widely used nowadays as the input motion for building predominant frequency, fo, and ground fundamental frequency, Fo, prediction due to simple, fast, non-destructive, simple handling operation and reliable result. However, it must be emphasized and caution to isolate these frequencies (fo and Fo) from spurious frequencies of site-structure effects especially to low rise building on soft ground deposit. In this study, identification of fo and Fo by using AV measurements were performed on ground and 4-storey primary school reinforced concrete (RC) building at Sekolah Kebangsaan (SK) Sg. Tongkang, Rengit, Johor using 1 Hz of tri-axial seismometer sensor. Overlapping spectra between Fourier Amplitude Spectra (FAS) from and Horizontal to Vertical Spectra Ratio (HVSR) were used to distinguish respective frequencies of building and ground natural frequencies. Three dominant frequencies were identified from the FAS curves at 1.91 Hz, 1.98 Hz and 2.79 Hz in longitudinal (East West-EW), transverse (North South-NS) and vertical (UD) directions. It is expected the building has deformed in translational mode based on the first peak frequency by respective NS and EW components of FAS spectrum. Vertical frequency identified from the horizontal spectrums, might induces to the potential of rocking effect experienced by the school building. Meanwhile, single peak HVSR spectrum at low ground fundamental frequency concentrated at 0.93 Hz indicates to the existence deep contrast of soft deposit. Strong interaction between ground and building at similar frequency (0.93 Hz) observed from the FAS curves on the highest floor has shown the building to behave as a dependent unit against ground response as one rigid mass.

Integral Analysis of Seismic Refraction and Ambient Vibration Survey for Subsurface Profile Evaluation

Geotechnical site investigation as known as subsurface profile evaluation is the process of subsurface layer characteristics determination which finally used for design and construction phase. Traditionally, site investigation was performed using drilling technique thus suffers from several limitation due to cost, time, data coverage and sustainability. In order to overcome those problems, this study adopted surface techniques using seismic refraction and ambient vibration method for subsurface profile depth evaluation. Seismic refraction data acquisition and processing was performed using ABEM Terraloc and OPTIM software respectively. Meanwhile ambient vibration data acquisition and processing was performed using CityShark II, Lennartz and GEOPSY software respectively. It was found that studied area consist of two layers representing overburden and bedrock geomaterials based on p-wave velocity value (vp = 300 – 2500 m/s and vp > 2500 m/s) and natural frequency value (Fo = 3.37 – 3.90 Hz) analyzed. Further analysis found that both methods show some good similarity in term of depth and thickness with percentage accuracy at 60 – 97%. Consequently, this study has demonstrated that the application of seismic refractin and ambient vibration method was applicable in subsurface profile depth and thickness estimation. Moreover, surface technique which consider as non-destructive method adopted in this study was able to compliment conventional drilling method in term of cost, time, data coverage and environmental sustainaibility.

Structural health monitoring on medium rise reinforced concrete building using ambient vibration method

Monitoring of structural health from initial stage of building construction to its serviceability is an ideal practise to assess for any structural defects or damages. Structural integrity could be intruded by natural destruction or structural deterioration, and worse if without remedy action on monitoring, building re-assessment or maintenance is taken. In this study the application of ambient vibration (AV) testing is utilized to evaluate the health of eighth stories medium rise reinforced concrete building in Universiti Tun Hussein Onn Malaysia (UTHM), based comparison made between the predominant frequency, fo, determined in year 2012 and 2017. For determination of fo, popular method of Fourier Amplitude Spectra (FAS) was used to transform the ambient vibration time series by using 1 Hz tri-axial seismometer sensors and City SharkII data recorder. From the results, it shows the first mode frequencies from FAS curves indicate at 2.04 Hz in 2012 and 1.97 Hz in 2017 with only 3.14% of frequency reduction. However, steady state frequencies shown at the second and third modes frequencies of 2.42 Hz and 3.31 Hz by both years. Two translation mode shapes were found at the first and second mode frequencies in the North-South (NS-parallel to building transverse axis) and East-West (EsW-parallel to building longitudinal axis) components, and the torsional mode shape shows as the third mode frequency in both years. No excessive deformation amplitude was found at any selective floors based on comparison made between three mode shapes produced, that could bring to potential feature of structural deterioration. Low percentages of natural frequency disparity within five years of duration interval shown by the first mode frequencies under ambient vibration technique was considered in good health state, according to previous researchers recommendation at acceptable percentages below 5 to 10% over the years.

Ambient vibration characterization and monitoring of a rock slope close to collapse

We analyse the ambient vibration response of Alpe di Roscioro (AdR), an incipient rock slope failure located above the village Preonzo in southern Switzerland. Following a major failure in May 2012 (volume ∼210 000 m3), the remaining unstable rock mass (∼140 000 m3) remains highly fractured and disrupted, and has been the subject of intensive monitoring. We deployed a small-aperture seismic array at the site shortly after the 2012 failure. The measured seismic response exhibited strong directional amplification (factors up to 35 at 3.5 Hz), higher than previously recorded on rock slopes. The dominant direction of ground motion was found to be parallel to the predominant direction of deformation and perpendicular to open fractures, reflecting subsurface structure of the slope. We then equipped the site with two semi-permanent seismic stations to monitor the seismic response with the goal of identifying changes caused by internal damage that may precede subsequent failure. Although failure has not yet occurred, our data reveal important variations in the seismic response. Amplification factors and resonant frequencies exhibit seasonal trends related (both directly and inversely) to temperature changes and are sensitive to freezing periods (resonant frequencies increase with temperature and during freezing). We attribute these effects to thermal expansion driving microcrack closure, in addition to ice formation, which increase fracture and bulk rock stiffness. We find the site response at AdR is linear over the measured range of weak input motions spanning two orders of magnitude. Our results further develop and refine ambient vibration methods used in rock slope hazard assessment.

Ambient seismic vibration analysis and ground characterization in the vicinity of Algiers seismic zone

The Dar El Beida area, situated in the Mitidja basin, in the north central Tellian Atlas is characterized by its Plio-Quaternary sediments. The site effect analysis in this zone surrounding Algiers city is very important. The soil is constituted from recent soft sediments capable of amplifying seismic motion. Indeed, the analysis thanks to the ratio H/V ambient vibration method confirms these assumptions. An electric resistivity tomography strengthened the results obtained from ambient vibration for site characterization. This demonstrates once again the more or less homogeneous character of formations in this area. However, strongly amplifying its sediments largely explains the seismic acceleration of 0.52 g recorded following the 2003 Boumerdes earthquake.

Effects of Precast Cladding Systems on Dynamic Characteristics of Steel Frame Buildings by Ambient and Free Vibration Tests

Full-scale tests on a one-story steel frame structure with a typical precast cladding system using ambient and free vibration methods are described in detail. The cladding system is primarily composed of ALC (Autoclaved Lightweight Concrete) external wall cladding panels, gypsum plasterboard interior linings, and window glazing systems. Ten test cases including the bare steel frame and the steel frame with addition of different parts of the precast cladding system are prepared for detailed investigations. The amplitude-dependent dynamic characteristics of the test cases including natural frequencies and damping ratios determined from the tests are presented. The effects of the ALC external wall cladding panels, the gypsum plasterboard interior linings, and the window glazing systems on the stiffness and structural damping of the steel frame are discussed in detail. The effect of the precast cladding systems on the amplitude dependency of the dynamic characteristics and the tendencies of the dynamic parameters with respect to the structural response amplitude are investigated over a wide range. Furthermore, results estimated from the ambient vibration method are compared with those from the free vibration tests to evaluate the feasibility of the ambient vibration method.

Evaluation of Natural Frequency Through Measurement of Ambient Vibration for Wall-type RC Structures

Presently, apartment structure plans tend to depart from the standard pattern based on owners′ requests and frame structures are becoming popular instead of wall-type structures and typical structure systems. Moreover, traditional concepts concerning effective land use in addition to importance of view lead to building high-rise buildings with heights of more than 100 m. According to UBC97, the seismic design of such buildings is gaining more attention, specifically, calculations of exact period are very important for equivalent static analysis. The purpose of this study was to consider the safety factors for the natural period of wall-type and high-rise apartments by comparing the natural period obtained by ambient vibration tests and period simplified formulae recommended by UBC97.Target structures followed by a building′s height, structural system and plan type were selected. The reinforced concrete structure apartment′s natural periods were measured using the ambient vibration method. An approximately 30% error between the eigenvalue analysis and test data obtained by the ambient vibration method has been reported.

Seismic response of Beirut (Lebanon) buildings: instrumental results from ambient vibrations

Resonance period is a key parameter in the seismic design of a structure, thus dynamic parameters of buildings in Beirut (Lebanon) were investigated based on ambient vibration method for risk and vulnerability assessment. Lebanon is facing high seismic hazard due to its major faults, combined to a high seismic risk caused by dense urbanization in addition to the lack of a seismic design code implementation. For this study, ambient vibration recordings have been performed on 330 RC buildings, period parameters extracted and statistically analyzed to identify correlations with physical building parameters (height, horizontal dimensions, age) and site characteristics (rock sites or soft sites). The study shows that (1) the building height or number of floors (N) is the primary statistically robust parameter for the estimation of the fundamental period T; (2) the correlation between T and N is linear and site dependent: T ≈ N/23 for rock sites and N/18 for soft sites; (3) the measured damping is inversely proportional to the period: the taller the building the lower is the damping; (4) a significant overestimation of the period exists in current building codes. However part of the large discrepancy with building code recommendations may be due to the very low level of loading.

내진설계를 위한 공동주택의 고유주기 측정에 관한 연구

최근 공동주택은 여러 가지 원인에 의해 그 형태가 변화되고 있으며, 구조물도 벽식 형태의 구조물에서 보-기둥 형태의 구조물로 선호도가 바뀌고 있다. 따라서, 계획된 공간을 효율적으로 이용하기 위하여 구조물은 점점 더 고층화 되어가는 경향이 있다. 한국건축구조기준(KBC)에서는 이러한 고층 공동주택의 경우 내진설계 시 지진하중을 산정할 때, 구조물의 진동특성은 매우 신중하고 정밀하게 측정해야 하며, 특히 동적주기는 동적 주기는 이에 상응하는 정적 해석 시 매우 중요하므로 정밀하게 측정해야 한다. 본 연구의 주 목적은 상시 미진동 실험으로부터 얻어진 기타골조식과 전단벽식을 고려하는 경우 고층빌딩의 고유주기를 한국건축구조기준의 값과 비교하여 안전계수를 산정하고 내진성능을 평가하여, 보다 경제적인 내진설계방안을 제시하는 것이다. 또한, 연구결과 기타 골조 식은 매우 보수적으로 주기를 산정하는 것으로 나타나므로 내진설계 시 이러한 사항도 고려해야 할 것으로 사료된다. Recently, apartment designs are tend to be changed from the standard pattern by many causes and the beam-column structures are getting popular instead of wall structure system. Therefore, for the effective use of planed space the heights of apartment are tend to be higher and higher. According to Korea Building Code, earthquake resistance designs or seismic design for those high rise apartments must be more attentive and accurate, especially, dynamic periods of structures must be exactly measured, because those are very important for equivalent static analysis. The important subject of this study is to investigate the safety factors and seismic performance for natural period of high rise buildings by comparing the natural periods getting from ambient vibration method with those of Korea Building Code.

Ambient vibration tests of a cross-laminated timber building

Cross-laminated timber has, in the last 6 years, been used for the first time to form shear walls and cores in multi-storey buildings of seven storeys or more. Such buildings can have low mass in comparison to conventional structural forms. This low mass means that, as cross-laminated timber is used for taller buildings still, their dynamic movement under wind load is likely to be a key design parameter. An understanding of dynamic lateral stiffness and damping, which has so far been insufficiently researched, will be vital to the effective design for wind-induced vibration. In this study, an ambient vibration method is used to identify the dynamic properties of a seven-storey cross-laminated timber building in situ. The random decrement method is used, along with the Ibrahim time domain method, to extract the modal properties of the structure from the acceleration measured under ambient conditions. The results show that this output-only modal analysis method can be used to extract modal information from such a building, and that information is compared with a simple structural model. Measurements on two occasions during construction show the effect of non-structural elements on the modal properties of the structure.

Dynamic Modal Assessment of Old Reticular Steel Railway Bridges Undergoing Fatigue Deterioration

To reduce traffic disruption due to a live-cycle and seismic damage it is necessary to study the causes in infrastructures and bridges. When we talk about steel structures and in particular steel bridges, we know that they are very flexible and consequently easy to identify by ambient vibration methods. In a collaborative project between the University of Padua and the Italian Railway Authority, two old reticular steel bridges (1920) were subject to in-situ structural identification, demolition and consequent laboratory fatigue tests. On account to their similarity it was possible to acquire dynamic data for several days by using the same acquisition logger and 12 acceleration sensors After the data of the two analyzed reticular bridges have been filtered, accurate vibrating modes were identified by using both time and frequency domain techniques and the results were outstanding. In order to use best-fitted material characteristics in the elements of the structures, destructive and non-destructive tests were executed. The results permitted to realize very accurate finite element models and calibrate them for successive fatigue scenarios by comparing laboratory and numeric tests.

Determination of stay cable force based on effective vibration length accurately estimated from multiple measurements

Due to its easy operation and wide applicability, the ambient vibration method is commonly adopted to determine the cable force by first identifying the cable frequencies from the vibration signals. With given vibration length and flexural rigidity, an analytical or empirical formula is then used with these cable frequencies to calculate the cable force. It is, however, usually difficult to decide the two required parameters, especially the vibration length due to uncertain boundary constraints. To tackle this problem, a new concept of combining the modal frequencies and mode shape ratios is fully explored in this study for developing an accurate method merely based on ambient vibration measurements. A simply supported beam model with an axial tension is adopted and the effective vibration length of cable is then independently determined based on the mode shape ratios identified from the synchronized measurements. With the effective vibration length obtained and the identified modal frequencies, the cable force and flexural rigidity can then be solved using simple linear regression techniques. The feasibility and accuracy of the proposed method is extensively verified with demonstrative numerical examples and actual applications to different cable-stayed bridges. Furthermore, several important issues in engineering practice such as the number of sensors and selection of modes are also thoroughly investigated.