iSHM Structural Health Monitoring
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iSHM: Monitoring Structural Health iSHM: Monitoring Structural Health Why monitor? Any large structure is subject to forces that will cause it to deviate from its original design. As a structure ages, components such as bearings will fail, cable stays will weaken and the cumulative effects of cyclic or exceptional loads will reduce the structure’s resistance. Over its lifetime the structure must also cope with geological processes, such as settling of the foundations, soil pressure, slippage and erosion. These processes set up a series of forces on the bridge that will cause structural...

When is iSHM used? An iSHM system can be used in a number of contexts: • Monitoring known faults—if a non-critical fault has been detected during a physical examination, monitoring its effect on the geometry of the bridge allows an informed decision to be made about the necessity and scheduling of repairs. • Bridges nearing the end of their life—a bridge’s design lifetime is a conservative estimate—the majority of bridges are taken out of service prematurely—this has economic and environmental consequences. An SHM system allows engineers to assess the status of a bridge and take corrective...

iSHM: Monitoring Structural Health Figure 1: Overview of iSHM Figure 2: Sensors and their applications Settlement and other geophysical effects may cause small changes in the orientation of parts of the structure; these can be monitored with our inclination sensor. Long term sagging of the span can be measured with our curvature sensor. Issues with bearings and expansion joints can be

iSHM: Monitoring Structural Health monitored with a combination of temperature and displacement sensors. Fatigue can be modelled using data from vibration or strain sensors. iSHMs flexible design and wide range of sensors allows a solution to be tailored to a given task. Smart Logger The smart logger can measure up to 8 channels using a high precision 32 bit ADC and has 2 current loop channels. In addition, there is also a bus system than can measure up to 10 nodes. The sampling pattern is programmable: measurements can be taken at intervals ranging from 10s to 24 hours. Detailed snapshots...

be extracted from measurements made by an iBWIM installation. With appropriate signal processing and spectral analysis these measurements can be used to constantly update the lumped parameters of the dynamic model of the bridge. These oscillations can also be used to record the loading cycles of the bridge and so inform the bridge's fatgue model. The visualisation modules allow the relationships between measurements to be seen more clearly, and unusual behaviour more easily identified. The most obvious example is where we would expect two measurements to follow each other, e.g. the...

iSHM: Monitoring Structural Health Figure 3: Screenshot: Vibration measurement with iBWIM installation Figure 4: Screenshot: Visualisation of bridge components

iSHM: Monitoring Structural Health

Task Our client owns two adjacent bridges, each is a 4 lane road bridge with a total span of more than 400m and piers arranged in pairs; at their highest points, the bridges are 20m above the valley floor. The bridges were constructed in the 1970’s and typically support around 100,000 vehicle journeys daily. During a routine inspection of the bridges in July 2019 a fault was detected in one of the bridge bearings. As temperature varies by hour and season, a bridge expands and contracts. The bearings allow the spans of the bridge to move relative to its fixed points. If a bearing resists...

bridge, our logger is self-powered and communicates using the mobile phone network. Our logger typically runs for six months without maintenance. Analysis of the inclination and curvature of the piers takes place on a remote server. Data is made immediately available to the client via a secure webpage. Reports on the bridge status are prepared every six months. We categorise the bridge status as red, green or amber. Significant changes in the status of the bridge trigger an immediate physical examination of the bridge. We placed instruments on four piers: the pier with the faulty bearing,...

bridge. Temperature drives the expansion and contraction of the bridge, and when that motion is resisted, it deforms the bridge. The bridge's temperature depends on the ambient temperature and on direct sunlight. Our client's bridge runs from east to west; the south side of the bridge is exposed to more direct sunlight and is significantly warmer than the north. Since expansion varies linearly with temperature, if the bearing is functioning, the relative displacement between the bridge and the pier will follow the same curve as temperature. If the bearing should fail, and the bridge and...

Case study Case study Our client commissioned PEC ZT-GmbH to undertake an analysis of the vibrations present on the test bridge. The analysis has two parts: first a numerical model of the bridge is constructed and used to predict the characteristic frequencies of the bridge; second, vibration measurements of the bridge under normal traffic loading were made and compared with the predictions to verify the model. The bridge consists of four spans, one of which crosses the river, our investigation is limited to this span. This span is a steel arch bridge 81m long and 15.3m wide. The...

Case study present on the bridge. The simulation and measurements are used to confirm and support each other. By comparing the measured and predicted resonant frequncies, it is possible to optimise the parameters of the simulation model — improving the quality of the model for other tasks. The four bending modes, and their predicted frequencies are shown below. This report focuses on two topics: the spectral properties of the signals and the symmetry of the sensors. An initial inspection and description of the signals lets us decide on an approach to the subsequent analysis. Spectral...

Case study Figure 4: Vibrational modes, longitudinal.