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niques for monitoring the health of structures have been borrowed from the aerospace industry
to be refined and applied to civil infrastructure. These techniques facilitate the
monitoring of structural health in an automated mode, reducing the uncertainty in the integrity
of the structure and providing valuable information to decision makers after a major
event.
One important issue in SHM is obtaining appropriate structural response data. For best results,
one would typically excite the structure with an actuator or shaker, and measure the
responses. However, for many structures, using forced vibration to induce response for
SHM or system identification purposes may be infeasible or forbidden (due to concerns of
the owners or danger that the structure may already be damaged). However, ambient excitation
(e.g., microtremor, wind, traffic, etc.) can often provide sufficient response for system
identification to be performed. Further, ambient input excitation is often unmeasured
(or actually immeasurable) and methods that require knowledge of the input must be selected.
Additional challenges arise from the nature of large, flexible structures. The characteristics
of these structures include: they are continuous structures that are not wellrepresented
with lumped masses; lateral and torsional motions can be highly coupled; using
a limited number of sensors is feasible; and, they have closely-spaced modes.
The focus of this paper is to examine the capabilities of the Natural Excitation Technique
(NExT) combined with the Eigensystem Realization (ERA) algorithm to extract modal
parameters from a structure with closely spaced modes. A numerical model of the Bill
Emerson Memorial Bridge, developed for the benchmark problem on the control of cablestayed
bridges, is used to perform simulations and obtain response records. For this initial
study, the bridge is excited using a stationary broadband force as a rough simulation of
ambient vibrations. Three sensor configurations are considered to investigate the dependence
of the methodology on the sensor configurations.
DESCRIPTION OF THE BRIDGE AND FINITE ELEMENT MODEL
The cable-stayed bridge used for this benchmark study is the Bill Emerson Memorial
Bridge spanning the Mississippi River (on Missouri 74—Illinois 146) near Cape Girardeau,
Missouri, designed by the HNTB Corporation (Hague, 1997). The bridge is currently under
construction and is to be completed in 2003. Instrumentation is being installed in the
Emerson bridge and surrounding soil during the construction process to evaluate structural
behavior and seismic risk (Qelebi, 1998).
As shown in Fig. 1, the bridge is composed of two towers, 128 cables, and 12 additional
piers in the approach bridge from the Illinois side. It has a total length of 1205.8 m (3956
ft). The main span is 350.6 m (1150 ft) in length, the side spans are 142.7 m (468 ft) in
length, and the approach on the Illinois side is 570 m (1870 ft). A cross section of the
deck. The bridge has four lanes plus two narrower bicycle lanes, for a total width of 29.3
m (96 ft). The deck is composed of steel beams and prestressed concrete slabs. The 128
cables are made of high-strength, low-relaxation steel (ASTM A882 grade 270). The H-
shaped towers have a height of 102.4 m (336 ft) at pier 2 and 108.5 m (356 ft) at pier 3.
Each tower supports a total 64 cables. The cross section of each tower varies five times
over the height of the tower. The approach bridge from the Illinois side is supported by 11
piers and bent 15, The deck consists of a rigid steel diaphragm with a slab of concrete at
the top. Sixteen 6.67 MN (1,500 kip) shock transmission devices are employed in the connection
between the tower and the deck. These devices are installed longitudinally to allow
for expansion of the deck due to temperature changes. Under dynamic loads these
devices are extremely stiff and behave as rigid links. Further details are provided in the paper
describing the benchmark control problem statement Dyke et al, (2002),