The Bill Emerson Memorial Bridge in Cape Girardeau (MO) [herein referred as Cape Girardeau Bridge], a new Mississippi River crossing, is a cable-stayed bridge, approximately 80 km from the epicentral region of the 1811–1812 New Madrid earthquakes. The location of the bridge is shown in Figure 1.
This seismically active region requires hazard mitigation programs, including those related to investigation of strong shaking of structures and the potential for ground failures in the vicinity of structures. Design of the bridge (a) accounted for the possibility of a strong earthquake (magnitude 7.5 or greater) during the design life of the bridge and, as a result (b) was based on design response spectrum anchored to a zero-period acceleration (ZPA) of 0.36 g with a 10 % probability of being exceeded in 250 year . Therefore, planning for its seismic instrumentation was initiated before the construction started. Lead institutions that collaborated in the financing and development of the seismic monitoring plan are: the Federal Highway Administration (FHWA), Missouri Department of Transportation (MoDOT), the Multi-disciplinary Center for Earthquake Engineering Research (MCEER), and the United States Geological Survey (USGS).
General view of the 1206 m (3956 ft) total span bridge is depicted in Figure 2. The figure also shows the locations and orientations of sensors. The state-of-the-art, broad-band, real-time seismic monitoring system includes 14 Kinemetrics Q330 digitizers1 and 84 channels of accelerometers. The structural array comprises 66 channels and the two free-field arrays comprise 18 channels including tri-axial downhole accelerometers. Deployed accelerometers facilitate recording motions of and assessing the following responses of the bridge and its vicinity:
- a. Permanent surface and downhole free-field arrays are deployed, one at the Missouri [MO] and another at the Illinois [IL] side of the Mississippi River. The MO free-field array is approximately 100 m (~399 ft) south of Bent 1 and the IL array is approximately 300m (~900 ft) south-east of Pier 15 (Figure 2). Geotechnical characteristics of the boreholes that house the triaxial downhole accelerometers at defined depths from the surface are qualitatively and quantitatively shown in Figure 2. These two free-field arrays are essential in providing the input ground motions that may be used as a surrogate for the various piers of the bridge and also for convolution and deconvolution studies of the free-field ground motion.
- b. Overall motion of the cable-stayed bridge including:
- Motions of extreme ends of the bridge as well as intermediate pier locations to provide data for the translational, torsional, rocking and translational soil-structure interaction (SSI) at the foundation levels. This setup will also provide insight into the horizontal and vertical spatial variation of ground motion.
- Motions of the two towers to assess their translational and torsional behavior – relative to the caissons and deck levels.
- Motions of the deck to assess the fundamental and higher mode translational (longitudinal, transverse and vertical) and torsional components.
The system has the capability to directly transmit to other sites or allow data to be retrieved using an IP address. Antelope software is used for local data acquisition.
An integrated structural monitoring network of this size requires data management infrastructure and strategy. All 84 channels of data are now being transmitted to the Data Management Center (DMC) of the Incorporated Research Institutions for Seismology (IRIS) via the Antelope2 software at the Central Recording System of the Cape Girardeau Bridge Seismic Monitoring System. IRIS is receiving the data in mini-seed format (in general use by seismologists). The latency at IRIS-DMC for all streamed data from the bridge is 4-8 weeks. Identified events (earthquakes) will be windowed and permanently saved at IRIS and at National Strong Motion Program (NSMP) of the United States Geological Survey (USGS) for further processing for use by engineering community. This involves computing processed acceleration, velocity and displacement time-histories for each channel and each event. Data transmitted to and from IRIS is archived according to seed channel naming format (e.g. for this bridge: 7405.NP.HNx.LL. Here 7405 is the station code number assigned by network operator, NP stands for NSMP as the network operator, HN indicates an accelerometer as the sensor for the particular channel, “x” is the sensor orientation (Z, 2, or 3), and “LL” is the two-character L-code (Location Code [see Figure]) developed by the author for this particular bridge in accordance with data format requirements of the Antelope software and IRIS. All horizontal sensors oriented transverse (to the bridge) use the orientation code “2” since they are more closely aligned to NS, and longitudinal sensors have the orientation code “3” since they are more closely aligned to EW. Therefore, directions of components of horizontal motions are hereafter referred to as NS and EW instead of lateral and longitudinal.
The response data will be used by the owner, researchers, and engineers to: (1) assess the performance of the bridge, (2) check design parameters, including the comparison of dynamic characteristics with actual response, and (3) better design future similar bridges. By appropriate configuration of the streamed data, the instrumentation can also be used as a “health monitoring” tool to serve as an early warning system of defects or unexpected behavior, and to assess damage to the structure.
Figure 3 shows an example of recorded quality data recorded that includes an (M=4.1) earthquake that occurred at 12:37:32 (UTC) on May 1, 2005. The epicenter, 175 km from the bridge, was located at 6 km (4 miles) SSE (162°) from Manila, AR (with epicental coordinates 35.830°N, 90.150°W). The figure shows NS time-history plots of motions at center of the deck (C2) , tower tops (T2 and T4) mid-tower locations (M2 and M4) side span deck (L2 and R6) and other pier locations and the free-field arrays. Compared to the ambient motions at deck center and other key structural locations (C2, T2, T4, L2, R6), the free-field motions due to the May 1, 2005 earthquake much smaller. The small amplitude set of data (~10-4 to10-3of g levels) allows insight into the vibrational behavior of the bridge.
Data from the Cape Girardeau Bridge is received by the IRIS DMC in real time and available to the community through all data request mechanisms at the IRIS DMC. The data appear under the Network Code NP for the USGS National Strong Motion Program and represents a very large source of information of interest to structural engineers, among others. Data are in SEED format and fully documented with SEED metadata.
— Tim Ahern, DMS Program Manager
Çelebi, M., Purvis, R., Hartnagel, B., Gupta, S., Clogston, P., Yen, P., O’Connor, J., and Franke, M., 2004, Seismic Instrumentation of the Bill Emerson Memorial Mississippi River Bridge at Cape Girardeau (MO) : A Coperative Effort, 4th Int’l Seismic Highway conference, Memphis, Tenn. , January 2004.
Çelebi,M., 2003, [Final Proposal] Plan for Seismic Instrumentation of the Cable-Stayed Cape Girardeau (MO) Bridge, USGS Administrative Report, August 2002 (revised August 1, 2003).
- Reference to commercial names does not constitute endorsement by U. S. Geological Survey
- The Antelope software is licensed and installed at the site server by Kinemetrics. Citing commercial names does not imply endorsement of the vendor.
by Mehmet Çelebi (United States Geological Survey)