Overview and Preliminary Validation of a Self-Contained Rapid
Modal Testing System for Highway Bridges*
J. DeVitis, D. Masceri, J. Braley, N. Romano, A. Emin Aktan, and Franklin Moon
Abstract - A novel approach to large scale, quantitative evaluation of highway bridges is introduced in which global
structural properties of a bridge are rapidly extracted via modal analysis and correlated with a Finite-Element model.
Preliminary validation of the system’s experimental components and test methodology is presented through a
comparison of traditional modal testing techniques during field trials of a steel stringer highway bridge.
I. INTRODUCTION
There are over 60,000 bridges in the United States that are posted for loads under the legal limit. Many of these structures
are placed under load restrictions due to the inherent conservatism of the single-line girder rating method, which is widely used
throughout the U.S. This conservatism is recognized by the allowance of more advanced modeling and/or load testing
procedures which often prove valuable in assessing critical or atypical structures. However, bridge owners must balance the
cost and time required by such refined methods, which often prove excessive and thus are seldom employed. Static load tests
directly measure in-situ characteristics of the bridge, however, they require full bridge closure, loaded and weighed trucks, and
the acquisition of local and global responses using numerous sensors and data acquisition equipment. Due to this complexity,
the time requirement for planning, execution, and subsequent data analysis hamper the utility of such evaluations for
emergencies. In contrast, dynamic tests are capable of capturing both direct and indirect measures of global performance, with
significant reduction of execution time compared to conventional static load testing. However, despite the reliability of these
tests, the high costs make such an undertaking prohibitive for the majority of common highway bridges. As a result, bridge
owners have limited options for quantitatively determining the health of a bridge population.
The proposed solution, dubbed the Targeted Hits to Measure Performance Responses (THMPR) system, attempts to
establish a new industry standard for cost-competitive bridge evaluation by pairing leading-edge technology with current
structural engineering best practices. The THMPR system consists of a modal testing device and custom semi-automated
modal analysis software and is paired with the Rapid Automated Modelling for Performance of Structures (RAMPS) software
for semi-automated finite element model development, model/experiment correlation, and live load simulation. The following
will present an overview of the current THMPR system (test device and methodology), as well as present preliminary results
from recent field trials in which modal parameters are extracted from a local steel stringer bridge and compared between the
THMPR system and a traditional instrumented sledge.
II. THMPR SYSTEM
The THMPR system (Fig. 1) combines a self-contained rapid modal testing trailer and streamlined data processing
software to extract modal parameters of a structure, such as frequencies and modes shapes, that may be used to determine
anomalous behavior as well as provide experimental data for finite element model calibration. The system is comprised of a
physical test device utilizing a significantly reconfigured Falling Weight Deflectometer trailer with modifications to (1)
provide a single, large (~30 kip) broadband impact source, and (2) collect the resulting free-decay response of the bridge’s
surface in a spatially distributed manner (and in-turn be able to capture local mode shapes). The trailer is towed along the
bridge deck and local single-input-multiple-output (SIMO) impact testing is performed at targeted locations. The data is
wirelessly acquired at each location and passed to semi-automated modal processing software which performs (1) data quality
checks, (2) frequency response function development and (3) modal parameter estimation. The local mode shapes are then
linearly combined utilizing selected stationary references previously secured along an available sidewalk (out of the way of
traffic) and cabled to an independent, GPS synchronized data acquisition. The resulting global modal parameters may then be
passed to the RAMPS software where an FE model is capable of being developed, correlated to the experimental results, and
ultimately utilized to perform a refined AASHTO Load and Resistance Factor Rating (LRFR) of the structure.
*Research supported by National Institute of Standard and Technology - Technology Innovation Program
J. DeVitis is a PhD Candidate at Drexel University, Philadelphia PA, 19104 (phone: (215)-895-6099; fax: (215)-895-1363; e-mail: jd465@drexel.edu)
D. Masceri is a PhD Candidate at Drexel University, Philadelphia PA, 19104 (email: drm57@drexel.edu)
Figure 1.
The THMPR system. (The THMPR system during a test at Pennsauken Creek Bridge in Palmyra, NJ)
A. Modal Test Trailer
A schematic of the current modal test trailer is presented in Fig. 2. Utilizing an NI CompactRIO micro-controller and
pneumatic actuators, a local sensor array presses spring loaded accelerometer housings onto the bridge deck. A hydraulic
control system is then utilized to raise an impact carriage with configurable mass and stiffness. The impact carriage is dropped,
impacts the bridge deck and rebounds upwards. The stiffness and mass of the impact carriage are tuned to generate force levels
above 25kips (in order to overcome the presence of light truck traffic and thus not require lane closures) and a usable
frequency band between 0-50Hz to focus the input energy within the bandwidth of the first fundamental modes of typical
highway bridges. A rebound control system is implemented to achieve a unit-impulse and preserve data quality. A
sensing/control system utilizing hall sensors detects the impact and triggers fast acting pneumatic actuators to extend upwards
and catch the mass, preventing subsequent rebounds. The resulting free-decay vibrations are recorded for a duration of 10
seconds to capture the full response record (and maintain a fine frequency resolution) and at a sampling rate of 3200Hz to
ensure adequate characterization of the impulse signal. During testing, an independent data acquisition system utilizing GPS
synchronization records several stationary accelerometers (typically three per available sidewalk) to use as spatial and modal
references for post processing analysis. Once several impact sequences are conducted at a single location, the mobile sensing
array is raised and the trailer is towed to a new location on the bridge deck and the experimental process repeated. During
trailer travel, a series of magnets and hall sensors located along the circumference of the trailer wheels are used to record
wheel rotations to calculate linear distance of the trailer from a reference point - only requiring the engineer to manually record
lane position to determine local positioning of the trailer at each impact location.
Figure 2.
THMPR System Primary Hardware Components.
B. Modal Identification & Test Methodology
Upon successful data acquisition at an impact location, the custom software first performs automated data quality checks to
vet the data records used for further processing before the trailer is moved to another location. This includes checking for
excessive erroneous noise, dropped channels, overloading of the load cells, and proper time synchronization of the
independent data acquisitions. Next, a series of automated filtering and windowing algorithms are applied following the
current best practice approaches. The Frequency Response Function (FRF) is then autonomously developed for each degree of
freedom and coherence and phase angle are computed and displayed for data quality and linearity checks. Semi-automated
modal identification is performed for each impact location via the Complex Mode Indicator Function (CMIF) which extracts
approximate pole locations and corresponding mode shapes for each local test location. A ‘master’ test location is then
selected for each mode shape based upon the impact location’s nearness to an area of high modal amplitude for a specific
mode shape. This enforces high signal to noise ratios and preserves data quality during post processing. The selected master
test location for each mode shape is then passed to an Enhanced Frequency Response Function (eFRF) module which utilizes
each respective mode shape and approximate pole location to perform a single degree of freedom least squares fit on the
experimentally derived FRF data. This provides a refined solution of the structure’s damped natural frequencies and modal
scaling. Finally, the modal properties of each master local impact location are ‘stitched’ together by utilizing the linear
relationship between spatially common reference sensors to form a comprehensive set of global modal parameters (Figure 3).
Figure 3.
Progressive Substructure Integration through Successive Single-Input-Multiple-Output Modal Analysis
III. PENNSAUKEN CREEK BRIDGE CASE STUDY
Consecutive impact tests utilizing two impact testing methods were performed on the Pennsauken Creek Bridge on April
9th, 2014 to validate the THMPR system components and SIMO test strategy. A model 086D50 instrumented sledge with a
force range of 0-5kips and weight of 12.1lbs was selected to represent the state-of-practice and used to performed multipleinput-multiple-output (MIMO) impact tests at five locations on the bridge deck. The THMPR system was then used to perform
multiple local SIMO impact tests at the same locations with seven stationary references located on either sidewalk available to
integrate local modal parameters to global parameters. The modal parameters (frequencies, damping, mode shapes) extracted
from each independent test may then be compared to establish the relative accuracy and viability of the THMPR system in
rapidly and reliably extracting modal parameters of a highway bridge.
A. Pennsauken Creek Bridge
The Pennsauken Creek Bridge (PCB) is a three span, simply supported steel stringer structure carrying two lanes of traffic
in each direction over the Pennsauken Creek in Palmyra, New Jersey (Figure 4). Each span measures 50’ in length, 50’ in
width and consists of a reinforced concrete deck on simply supported rolled steel I-beams with partial-length welded bottom
flange cover plates. Traffic control was established at 9AM and provided a partial closure of the bridge leaving topside access
limited to the southern two lanes and both sidewalks until test conclusion at 3PM.
Figure 4.
Pennsauken Creek Bridge
B. Instrumentation Plan
Twenty-eight model PCB393A-03 accelerometers were fixed to the bridge deck in a dense grid (Fig. 5), cabled to an
independent GPS synchronized data acquisition, and continuously recorded throughout testing. Seven of the twenty-eight
accelerometers were placed on the sidewalk (out of traffic lanes) and chosen to be used as global references for the local SIMO
test integration. A total of five impact locations were selected and shared between test methods. The locations were selected to
impact at areas of high modal amplitude for fundamental modes in order to preserve data quality by exciting modes with high
mass participation. The first set of impact locations were conducted at ¼ span and ½ span along the near lane and the second
set at ¾ span, 5/8 span and 3/8 span along the far lane. MIMO testing with the instrumented sledge began at 10AM and
concluded at 12PM and directly after, the THMPR testing began testing and continued until 2PM. Note the testing time of the
THMPR system is slightly exaggerated as extra care was taken to carefully position the trailer at each impact location (to
achieve a better comparison between test approaches) as well as not to disrupt the sensor grid of MIMO sensors in the
roadway.
Figure 5.
Pennsauken Creek Bridge Test Setup and Execution
C. Data Quality
A total of five impacts were performed at each impact location to use for averaging later in FRF development. Data was
recorded at a sampling rate of 3200Hz in order to define the shape of the impulse signal, and a record length of 10 seconds was
used to capture the full free-decay of the structure post-impact. Typical input force levels of the instrumented sledge were
observed up to 5,000lbs with a usable frequency band of 0-250Hz, and typical input force levels of the THMPR impact device
were observed above 25,000lbs with a usable frequency band of 0-50Hz (Fig. 6). Driving point acceleration levels of the
instrumented sledge test were observed up to +/-.5g, however, the length of the free-decay was relatively short and high
acceleration levels did not typically last long. This is due to the instrumented sledge’s low mass and relatively stiff impact tip.
The frequency content input to the structure suffered from being too broad banded and was not able to fully activate the mass
of the structure and drive the lower frequency, fundamental modes. The THMPR system provided driving point acceleration
levels at +/-2g’s. Due to the large force levels and focused input frequency band, the THMPR system clearly activated the
mass of the bridge better than the instrumented sledge and produced data records with more usable time information. This
resulted in a frequency resolution of .098Hz and, as seen in the figure, aided in the THMPR system’s ability to better
characterize the closely spaced first and second modes. The frequency content of the structural response for both test methods
shows clearly defined, smooth peaks of resonance which indicates linearity as well as good excitation and characterization of
all fundamental modes.
Figure 6.
Pennsauken Creek Bridge Test Typical Force and Response Comparison
D. Partial Modal Parameter Estimation
Modal parameter estimation was performed immediately following each impact test for each test method to provide
immediate feedback of the data quality, structural response of the bridge, and operating condition of the test equipment to the
on-site engineers. The semi-automated modal identification software of the THMPR system was used to perform signal
processing on site during the local SIMO impact tests, and generalized, core signal processing functions within the THMPR
system’s processing toolbox were used to perform custom modal processing during the instrumented sledge MIMO impact
tests. After developing the frequency response functions, the CMIF was calculated for each test method (Fig. 7). Each peak of
the CMIF represents a location of resonance of the structure and the amplitude is directly related to the dominance of the
corresponding shape at that location. Both test methods show well defined areas of resonance and good characterization of
global modes. It is important to note, the THMPR system’s local SIMO modal analyses are independent of each other and may
only leverage one impact location as a reference in the singular value decomposition. This creates an inconvenient necessity to
super impose each independent test’s singular values in order to compare areas of resonance and ultimately select global modal
parameters. In contrast, multi-reference MIMO analysis has the benefit of utilizing all five impact locations to form a set of
global modal vectors and provide solutions to five mode shapes (and corresponding singular values) at each frequency line.
Figure 7.
Complex Mode Indicator Function
A partial set of global modal parameters were extracted from each test and presented in Fig. 8. Master SIMO locations
were chosen for each mode of the THMPR analysis based on the relative amplitude of the test location’s singular value. The
SIMO impact location with the largest amplitude for a specific mode contains the highest signal to noise ratio and is then used
as the master set of parameters for integrating the multiple SIMO tests into a global set. A combination of the stationary
references for each ‘master’ local mode shape was chosen on an individual mode basis to avoid using reference sensors in
locations of low modal amplitude for a particular mode. Additionally, impact locations which impacted at a nodal point for a
particular mode were discarded and not used for incorporation within the global modal parameter set. The partial set of global
modal parameters extracted via the THMPR system and instrumented sledge consist of four modes within the frequency band
of 0-15Hz (Table 1) and show very good agreement with a maximum difference in pole location of 1%.
TABLE I. PARTIAL MODAL PARAMETER COMPARISON
Mode
Sledge MIMO [Hz]
1
7.47
THMPR SIMO
[Hz]
7.39
% Difference
-1.07%
2
8.56
8.51
-0.58%
3
10.31
10.23
-0.78%
4
15.07
14.93
-0.93%
Figure 8.
Partial Global Mode Shape Comparison
IV. CONCLUSION
A proposed solution to establish a new industry standard for cost-competitive bridge evaluation is presented. The primary
hardware and software components of the fully functional THMPR system prototype are described and their performance
examined through bench mark field testing at the Pennsauken Creek Bridge. The THMPR system’s experimental testing
components and method were used in a side by side comparison with state-of-the-practice impact device and test method. The
THMPR system was capable of extracting resonant frequencies within a few percent of the traditional testing methods and
mode shapes that showed strong correlation between traditional test methods. The preliminary results indicate the THMPR
system to be an excellent substitute to traditional multi-reference impact testing.
V. REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
Aktan, A.E., F. N. Catbas et al. (1998). “Structural Identification: Analytical Aspects.” Journal of Structural Engineering 124(7): 817-829.
Aktan, A. E., D. Farhey, et al. (1997). “Structural Identification for Condition Assessment: Experimental Arts.” Journal of Structural Engineering
123(12): 1674-1684.
Allemang, R. J. (1999). Vibrations: Experimental Modal Analysis. Cincinnati, OH, University of Cincinnati: SDRL.
Allemang, R. J., Brown, D. L. (2006). “A Complete Review of Complex Mode Indicator Function (CMIF) with Applications”. Proceedings,
International Conference on Noise and Vibration Engineering, Katholieke Universiteit Leuven, Belgium, 38pp.
Brown, D. L., and Halvorsen, W. G. (1977). “Impulse Technique for Structural Frequency Response Testing.” Journal of Sound and Vibration. 8-21.
Brownjohn, J. M. W., P.-Q. Xia, et al (2001). “Civil Structure Condition Assessment by FE Model Updating: Methodology and Case Studies.” Finite
Elements in Analysis and Design 37: 761-775.
Catbas, F.N. (1997). “Investigation of Global Condition Assessment and Structural Damage Identification of Bridges with Dynamic Testing and Modal
Analysis.” Doctoral Dissertation, University of Cincinnati.
Raghavendrachar, M., Aktan A. E. (1992). “Flexibility by Multi-Reference Impact Testing for Bridge Diagnostics.” Journal of Structural Engineering
118(8), p.2186-2203.