Strong motion arrays monitoring large structures

This new project, comprising 80 Guralp strong-motion accelerometers, gathers real-time data from structures along a newly-built high-speed railway line, and transmits it to operators using a combination of Scream! and DSS protocols.

5T accelerometers

40 stations have been installed at regular intervals over 250 km of the railway line. Each station contains two CMG-5T triaxial accelerometers, connected to the network using DM24 digitizers and DCM data modules.

For example, stations along a viaduct on the route have one CMG-5T unit installed in a recess in one column, and a second within the hollow concrete superstructure. Each instrument is secured to the structure with a single fixing bolt through its centre.

These instruments are connected to a single 6-channel DM24 mk3 digitizer in a nearby building, which is linked to the local TCP/IP network with a DCM. The DCM runs a Scream! server so that real-time data can be viewed from anywhere on the network.

Only one of the two digitizer input ports on the DCM is used, giving the array scope for future expansion as necessary.

The layout of a single station.

DSS

DSS (Data Subscription Service) is a packet format, widely used in strong-motion projects, which enables data and statistics to be requested from a seismic installation. A DSS server is designed to handle many concurrent requests from clients with varying levels of privilege, and may prioritize requests according to their origin and urgency.

Each DCM in this network runs a DSS server providing data on:

• peak acceleration levels
• RMS (root mean square) average acceleration levels
• spectral intensity
• magnitude of horizontal acceleration (combined N/S and E/W components)

These statistics are relayed to a central data centre once every second, where they can be used to trigger automatic warning systems. When the statistics indicate that an event has occurred, or there is a problem with the structure, DSS allows station operators to request the raw seismic data and determine the best course of action.

Results

In between, the sensor in the superstructure (Z2,N2,E2) shows regular peaks in acceleration. There are 21 peaks in all. This is consistent with the passage of a 20-carriage train, where the sets of wheels at the end of each carriage are giving rise to the peaks.

The sensor in the column beneath shows this detail to a much lesser extent, implying that the concrete has flexed and evened out the accelerations in the structure

Conclusion

This network combines the high performance of Guralp CMG-5T sensors with our highly flexible digitizers and data modules. With the new installations, the operators of the railway can

• monitor the safety of structures along the line;
• assess how the structure responds to seismic events and normal vibration arising from activity on the line; and
• contribute to the knowledge of seismicity in the region.

Guralp Systems’ strong motion instruments are ideal for safety monitoring in large structures of all types, from railways to bridges, dams, and towers.

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