Wireless sensor networks have been shown to be a cost-effective monitoring tool for many applications on civil structures. Strain cycle monitoring for fatigue life assessment of railway bridges, ...however, is still a challenge since it is data intensive and requires a reliable operation for several weeks or months. In addition, sensing with electrical resistance strain gauges is expensive in terms of energy consumption. The induced reduction of battery lifetime of sensor nodes increases the maintenance costs and reduces the competitiveness of wireless sensor networks. To overcome this drawback, a signal conditioning hardware was designed that is able to significantly reduce the energy consumption. Furthermore, the communication overhead is reduced to a sustainable level by using an embedded data processing algorithm that extracts the strain cycles from the raw data. Finally, a simple software triggering mechanism that identifies events enabled the discrimination of useful measurements from idle data, thus increasing the efficiency of data processing. The wireless monitoring system was tested on a railway bridge for two weeks. The monitoring system demonstrated a good reliability and provided high quality data.
Two pedestrian bridges, which were equipped with tuned mass dampers almost two decades ago, have been evaluated for their long-term performance. The response due to pedestrian action is compared to ...the response at installation. Forced vibration tests are used to identify system parameters of the bridges and the tuned mass dampers. The assessment shows that both bridges and tuned mass dampers are still working well. Detuning has been observed in one of the two bridges, which is, however believed to have existed already at installation. Parameters of the tuned mass dampers are shown to depend significantly on seasonal temperatures, which gives some practical limits to exact optimal tuning.
Difficulties may arise in system identification when only the deck response can be measured. Since both inherent bridge damping and absorber effects reduce the overall response, it is difficult to separate the two effects. In some cases even a negative bridge damping together with a high absorber mass has been identified. For reliable results, one of these parameters has to be fixed to a reasonable value. The concept of effective damping is used to show the correlation between the two effects. This concept permits separating the effects of bridge damping, absorber mass, and detuning in an approximate manner.
Lightweight bridges exhibit a high live load to dead load ratio. Thus, their resonance frequencies depend on the current loading state and can vary in quite a broad range. This behavior is a real ...challenge in vibration mitigation of such structures controlled by tuned mass dampers (TMDs), since the performance of single passive TMDs degrades significantly if the resonance frequency of a structure differs even slightly from design frequency. This study proposes an adaptive TMD, which continuously adapts its natural frequency to the current resonance frequency of the structure. The concept is based on pre-stressable leaf-springs that are controlled by piezoceramic stack actuators. Experiments performed on a first prototype show that the TMD is continuously tunable in a broad frequency range.
Monitoring the vibration performance of structures for serviceability assessment purposes usually requires a deployment of several weeks or months to collect a dataset allowing a reliable assessment. ...Since for this type of medium term deployments installation costs are a key factor, wireless monitoring with its fast deployment has an advantage over wired monitoring systems. This paper describes a deployment of a wireless monitoring system on a timber footbridge. The goal of the monitoring was to provide information about vibration amplitudes during operation and to track changes of relevant natural frequencies with the temperature. Accelerations were permanently recorded and the acquired data was processed in the nodes to compute the envelope of the vibration amplitude and the dominant peaks of the Fourier spectrum. The wireless monitoring system that was deployed for one year provided accurate data matching the requirements of vibration performance assessments. The deployment demonstrated that a wireless sensor network is a technically feasible and economically effective mean to monitor the vibration performance of a structure.
Event-driven monitoring policies enable to significantly reduce the power consumption of wireless sensor networks by reducing the recording period to those time intervals that provide valuable data. ...The resulting longer operation lifetime increase discloses fields of application that require long monitoring periods. This paper presents a structural monitoring system that uses specialized sentinel nodes for detecting possibly heavy road vehicles and for alarming monitoring nodes, which are specialized on strain sensing. Heavy vehicles are identified by estimating nearly in real time height and length of vehicles of a traffic flow by processing data recorded from low-cost ultrasonic and magnetic displacement sensors. Field tests demonstrated that while height detection is very reliable, length detection is too imprecise to discriminate with high success rates between trucks and delivery vans.
Summary
Due to the increasing traffic volume on the European railway network, the remaining fatigue life of existing steel bridges is a major concern. Several investigations demonstrated that ...supplementing the assessment with monitoring data enables to achieve more reliable remaining fatigue life estimations. In this paper, an event‐driven monitoring system based on a wireless sensor network that consists of two functionally different components was designed and tested. Sentinel nodes, which were mounted on the track, were used for detecting approaching trains and alerting with alarm messages the monitoring nodes. These nodes, which were mounted on the bridge, started strain sensing and data recording after receiving the alarm message and went back to a power saving mode upon completion. An embedded data processing algorithm transformed the recorded raw data into a much smaller data set representing strain cycles. A test deployment on a railway bridge demonstrated that train detection and alarming was fast and reliable. The combination of event‐driven monitoring and embedded data processing allowed to extend the battery lifetime of monitoring nodes to several months.
The current paper focuses on a prototype adaptive TMD. Its design concept is based on pre-stressable leaf-springs that are controlled by piezoceramic (PZT) stack actuators. Experiments performed on ...the prototype showed that it is continuously tunable in a broad frequency range. Moreover, they revealed that the device exhibits structural nonlinearities. The current paper focuses on the structural identification of the prototype and attempts for the first time to characterize and classify the observed nonlinearities. Several experiments at different PZT voltage levels are performed. The results indicate PZT voltage dependent nonlinear softening and hardening stiffness. Based upon these observations, static experiments and proper data-pooling techniques, an effective “global” model for the nonlinear stiffness is derived. The estimated nonlinear model is finally validated upon static experiments as well as more realistic operational cases, that are vibrations of the prototype under typical ground excitation.
Large scale tests with progressive damage on a prestressed concrete highway bridge have been performed to investigate the sensitivity of several damage detection, localization, and quantification ...methods based on modal parameters. To investigate the quality of modal parameters, the data set of one damage step was analyzed by several output-only identification techniques. Although the bridge was severely cracked, natural frequencies as well as mode shapes display only minor changes. However, the relative changes of mode shapes are larger than those observed for natural frequencies. A novel damage indicator, called mode shape area index, based on changes of mode shapes, has been developed and found as the most sensitive damage detection approach. Damage detection or localization via changes of the flexibility matrix performed better than natural frequencies or mode shapes alone. The application of the direct stiffness calculation and a sensitivity-based model update technique showed results having a high level of ambiguity about the location and quantification of damage also at the highest damage level. Evaluating the information collected in this study the test results indicate that an early stage damage identification in prestressed concrete bridges is hardly possible because of the nearly complete recovery of stiffness after closing of cracks in prestressed concrete and the effect of environmental parameters on modal data.
A 3-storey residential building with OSB-sheathed light-frame timber walls and timber-concrete composite slabs was subjected to dynamic excitation in two different stages of construction. The ...experiments consisted of measurements of the accelerations resulting from forced horizontal vibrations which were excited by means of a hydraulic actuator shaking a mass of 940 kg. The exciter was positioned on the second floor of the building and rigidly anchored in the slab. It excited the building in its two main directions to vibrations with frequencies within a range of 0.2–14 Hz. Natural frequencies, modal damping ratios and mode shapes of the building were estimated using accelerations of the building measured in the four external corners of each upper floor. For increasing displacement amplitude (1/8000 mm → 0.7 mm) the natural frequencies decreased moderately (−15 %) whereas the modal damping ratios increased markedly (+100 %). Modal damping ratios computed using ambient vibration data were generally smaller than modal damping ratios obtained from forced vibration tests, which suggests that modal damping for the levels of vibration in interest in design might be significantly underestimated if based on results from ambient vibration tests. However, the mode shapes computed using ambient vibration data turned out to be reliable. The differences in dynamic properties assessed in the different stages of construction were much smaller than expected. Compared to the natural frequencies estimated by the designer by applying a simple single degree of freedom model, the measured natural frequencies were considerably higher. This may be attributed to the impact of non-structural internal walls and walls with openings not accounted for in the designer’s model.
Since timber bridges are light-weight structures with a high life-load to death-load ratio, they are susceptible to vibration and thus satisfying vibration serviceability is a governing design ...criterion. A good knowledge of the damping properties of timber bridges is therefore a key aspect for guaranteeing a sufficient serviceability. Since observations suggest that timber bridges with an asphalt pavement increase the damping, this work investigates the influence of asphalt pavement on the damping and the resonance frequencies of two laboratory bridges: (1) a cable-stayed bridge with a main span of 15.6 m, a secondary span of 3.6 m and a width of 1.6 m and (2) a smaller simply supported bridge with a total length of 9.6 m and a width of 1.6 m. The deck plates were made from two different kinds of panels: cross-laminated timber (CLT) and a stress-laminated plate (SLP). Due to the asphalt pavement the first resonance frequency decreases 23%–26% for the cable-stayed bridge and 16%–18% for the simply supported bridge. This result suggests that the additional mass overtops the effect of an additional stiffness. The damping of the first vibration mode is about
ζ
=
0.5
%
…
0.66
%
for both bridges without pavement, and increases to
ζ
=
2.2
%
…
2.6
%
and
ζ
=
6.2
%
…
7.4
%
for the cable-stayed and the simply supported bridge, respectively, after deployment of an asphalt pavement. The comparison of the experimental results for the first resonance frequency and the damping with Finite-Element computations indicate that a noticeable amount of shear forces are transferred during the dynamic measurement and thus the pavement contributes to the stiffness of the bridges. The beneficial effect of the asphalt pavement at ambient temperatures on the damping has to be validated by measurements at different temperatures, since the asphalt is very temperature dependent.
PDF
