Comparison of damage identification algorithms on experimental modal data from a bridge Page: 2 of 8
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COMPARISON OF DAMAGE IDENTIFICATION ALGORITHMS ON
EXPERIMENTAL MODAL DATA FROM A BRIDGE
David V. Jauregui and Charles R. Farrar
Engineering Analysis Group (ESA-EA)
Los Alamos National Laboratory
Los Alamos, NM 87545
ABSTRACT. Over the past 25 years detecting damage
in a structure from changes in dynamic parameters has
received a considerable amount of attention from the
aerospace, civil, and mechanical engineering communities.
The general idea is that changes in the structure's physical
properties (i.e., stiffness, mass, and/or damping) will, in
turn, alter the dynamic characteristics (i.e., resonant
frequencies, modal damping, and mode shapes) of the
structure. Properties such as the flexibility matrix, stiffness
matrix, and mode shape curvature, which are obtained from
modal parameters, have shown promise for localizing
structural damage. In this paper, several different
techniques for damage assessment are demonstrated and
compared using experimental modal data from an
undamaged and damaged bridge.
Because the Interstate 40 (I-40) bridges over the Rio Grande
in Albuquerque, New Mexico were to be razed during the
summer of 1993, investigators from New Mexico State
University (NMSU) were able to introduce simulated cracks
into the structure in order to test various damage
identification methods. To support this research effort, Los
Alamos National Laboratory (LANL) performed experimental
modal analyses [1, 2], and developed experimentally verified
numerical models of the bridge .
In this paper five damage identification methods that have
been reported in the technical literature were applied to the
experimental modal data measured on the I-40 Bridge.
Subsequently, the same methods were applied to
numerically generated modal data obtained from the finite
element models previously mentioned as discussed in an
accompanying paper .
Length limitations of this paper preclude a detailed
discussion of the experimental modal analyses. The
pertinent data obtained from both the experimental studies,
required by all of the damage identification algorithms, were
resonant frequencies and mode shapes for the undamaged
and damaged bridge. For a more detailed summary of the
tests performed on the I-40 Bridge and the results that were
obtained, the reader is referred to .
2. I-40 BRIDGE GEOMETRY AND DAMAGE
The I-40 Bridge over the Rio Grande formerly consisted of
twin spans (there are separate bridges for each traffic
direction) made up of a concrete deck supported by two
welded-steel plate girders and three steel stringers. Shear
studs were not found when the concrete deck was removed.
Loads from the stringers are transferred to the plate girders
by floor beams located at 20-ft ( 6.1 m) intervals. Cross-
bracing is provided between the floor beams. Figure 1 shows
an elevation view of the portion of the bridge that was tested.
The cross-section geometry of each bridge is shown in Fig.
2. It should be noted that the actual bridges have concrete
crash barriers on either side of the concrete slab. Each
bridge is made up of three identical sections. Except for the
common pier located at the end of each section, the
sections are independent. A section has three spans; the
end spans are of equal length, approximately 131 ft (39.9 m),
and the center span is approximately 163 ft (49.4 m) long.
Five plate girders are connected with four bolted splices to
form a continuous beam over the three spans. The portions
of the plate girders over the piers have increased flange
dimensions, compared with the midspan portions, to resist
the higher bending stresses at these locations.
Connections that allow for thermal expansion as well as
connections that prevent longitudinal translation are located
at the base of each plate girder, where the girder is
supported by a concrete pier or abutment. These
connections are labeled "exp" and "pinned" in Fig. 1.
The damage that was introduced was intended to simulate
fatigue cracking that has been observed in plate-girder
bridges. This cracking results from out-of-plane bending of
the web at locations where the seats of the floor beams are
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Jauregui, D.V. & Farrar, C.R. Comparison of damage identification algorithms on experimental modal data from a bridge, article, December 31, 1995; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc668451/m1/2/: accessed April 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.