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University of California,
Irvine Bridge column Tests Overview: The University of California, Irvine (UCI) has a research contract with CALTRANS to perform static, cyclic load tests on seismically retrofitted, reinforced-concrete bridge columns. This project is under the direction of Professor Gerry Pardoen at UCI (gpardoen@e4e.oac.uci.edu) and Mr. Tim Leahy (tleahy@trmx2.dot.ca.gov) at CALTRANS. The primary purpose of these tests is to study the relative strength and ductility provided by two retrofit construction procedures. The first procedure extends the diameter of the existing column with cast-in-place concrete. The second procedure extends the diameter of the existing column using shotcrete that is sprayed onto the exterior of the existing column. With funds obtained through Los Alamos National Laboratory’s (LANL) University of California interaction office, staff from the LANL’s Engineering Analysis Group and a faculty member from the Mechanical Engineering Department at Rose-Hulman Institute of Technology were able to perform experimental modal analyses on the columns. These modal tests were performed at various stages during the static load cycle testing. Results obtained from the experimental modal analyses provide further insight into the relative effectiveness of the two retrofit procedures. In addition, data form these modal tests were analyzed
with various parametric and non-parametric damage identification algorithms.
From a damage identification standpoint one of the unique features of
these tests was the ability to examine the effects of column pre-load
on the damage identification process with varying amounts of damage input
to the structure in a controlled and quantified manner. Also, the concrete
column tests provided these investigators’ with their first experience
in applying damage identification algorithms to reinforced concrete structures.
The data from these tests are provided on this web
site for others to analyze. Also, provided on this site are the ABAQUS
input files for the accompanying finite element analyses. A description
of the structures, the testing and results obtained is given below. Test Structure Description
The test structures consisted of two 24-in-dia
(61-cm-dia) concrete bridge columns that were subsequently retrofitted
to 36-in-dia (91-cm-dia) columns. The first column tested, labeled Column
3, was retrofitted by placing forms around existing column and placing
additional concrete within the form. The second column, labeled Column
2, was extended to the 36-in-diameter by spraying concrete in a process
referred to as shotcreting. The shotcreted column was then finished with
a trowel to obtain the circular cross-section. The 36-in-dia. portions of both columns were
136 in. (345 cm) in length. The columns were cast on top of 56-in-sq.
(142-cm-sq.) concrete foundation that was 25-in-high (63.5-cm-high).
A 24-in-sq. concrete block that had been cast integrally with the column
extends 18-in. above the top of the 36-in-dia. portion of the column.
This block was used to attach the hydraulic actuator to the columns for
static cyclic testing and to attach the electro-magnetic shaker used for
the experimental modal analyses. As is typical of actual retrofits in
the field, a 1.5-in-gap (3.8-cm-gap) was left between the top of the foundation
and the bottom of retrofit jacket. Therefore, the longitudinal reinforcement
in the retrofitted portion of the column did not extend into the foundation.
The concrete foundation was bolted to the 2-ft-thick (0.61-m-thick) testing
floor in the UCI laboratory during both the static cyclic tests and the
experimental modal analyses. The structures were not moved once testing
was initiated. Figure 1 shows the test structure geometry. The columns were constructed by first placing the foundations on July 18th, 1997. Then the 24-in-diameter columns were placed on August 19th and the retrofits were added on September 19th. Corresponding portions of both test structures were constructed from the same batch of concrete. The only measured material property for these columns was the 28-day ultimate strength of the concrete and the test day ultimate strength. The 28-day ultimate strength of foundations was 4600 psi (32 MPa). Test day ultimate strength was Fig. 1 Column Dimensions not measured for the foundations. The 24-in-dia. column 28-day ultimate strength was 4300 psi (30 MPa) and the test day ultimate strength was 4800 psi (33 MPa). The 28-day-ultimate strength of the retrofit portion of the structures was 5200 psi (36 MPa). On test day the strength of the retrofit concrete was found to be 4900 psi (34 MPa). Within the 24-in-dia initial column reinforcement consisted of an inner circle of 10 #6 (3/4-in-dia, 19-mm-dia) longitudinal rebars with a yield strength of 74.9 ksi (516 MPa). These bars were enclosed by a spiral cage of #2 (1/4-in-dia, 13.5-mm-dia) rebar having a yield strength of 30 ksi (207 MPa) and spaced at a 7-in pitch (18 cm). Two-inch-cover (5- cm-cover) was provided for the spiral reinforcement. The retrofit jacket had 16 #8 (1-in-dia, 25-mm-dia) longitudinal rebars with a yield strength of 60 ksi (414 MPa). These bars were enclosed by a spiral cage of #6 rebar spaced at a 6-in pitch (15 cm). The spiral steel also had a yield strength of 60 ksi. Again, 2-in.-cover was provided for this reinforcement. Lap-splices 17-in (43-cm) in length were used to connect the longitudinal reinforcement of the existing 24-in column to the foundation. STATIC LOADINGPrior to applying lateral loads, an axial
load of 90 kips (400 kN) was applied to simulate dead loads that an actual
column would experience. A steel beam was placed on top of the column.
Vertical steel rods, fastened to the laboratory floor, were tensioned
by jacking against the steel beam that, in turn, applied a compressive
load to the column. A photo of the test configuration is shown in Fig.
2. Fig. 2 Test Configuration. An hydraulic actuator was used to apply lateral
load to the top of the column in a cyclic manner. The loads were first
applied in a force-controlled manner to produce lateral deformations at
the top of the column corresponding to 0.25DyT, 0.5DyT,
0.75DyT and DyT. Here DyT
is the lateral deformation at the top of the column corresponding to the
theoretical first yield of the longitudinal reinforcement. The reader
is referred to http:\\www.ics.uci.edu./~athomas/caltrans for a more detailed
summary of the calculation of the deformation corresponding to first yield.
The structure was cycled three times at each of these load levels. Based on the observed response, a lateral deformation corresponding the actual first yield, Dy, was calculated and the structure was cycled three times in a displacement-controlled manner to that deformation level. Next, the loading was applied in a displacement-controlled manner, again in sets of three cycles, at displacements corresponding to 1.5Dy, 2.0Dy, 2.5Dy, etc. until the ultimate capacity of the column was reached. Load deformation curves for columns 3 and 2 are shown in Figs 3 and 4, respectively. This manner of loading put incremental and quantifiable damage into the structures. The axial load was applied during all static tests. Fig. 3 Load –displacement curves for
the cast-in-place column. Fig. 4 Load-displacement curves for the
shotcrete column. Dynamic Excitation
For the experimental modal analyses the excitation
was provided by an APS electro-magnetic shaker mounted off-axis at the
top of the structure. The shaker is shown in Fig 5. The shaker rested
on a steel plate attached to the concrete column. Horizontal load was
transferred from the shaker to the structure through a friction connection
between the supports of the shaker and the steel plate. This force was
measured with an accelerometer mounted to the sliding mass of the shaker
0.18 lb-s2/in (31 Kg). A 0 - 400 Hz uniform random signal
was sent from a source module in the data acquisition system to the shaker,
but feedback from the column and the dynamics of the mounting plate produced
an input signal that was uniform over the specified frequency range.
Fig. 6 shows a typical input power spectrum. The same level of excitation
was used in all tests except for one at twice this nominal level that
was performed as a linearity check. Data Acquisition
Forty accelerometers were mounted on the structure
as shown in Fig. 7. Note that locations 2, 39 and 40 had PCB 302A accelerometers.
These accelerometers have a nominal sensitivity of 10mV/g and were not
sensitive enough for the measurements being made. Locations 33, 34, 35,
36, and 37 were Wilcoxon 736t accelerometers that had a nominal sensitivity
of 100mV/g. All other channels were PCB 336C accelerometers with Fig. 5 Shaker used during experimental
modal analyses. Fig. 6 Input Power Spectra. a nominal sensitivity of 1V/g. During the
test on the shotcrete column (column 2) the 336C accelerometer at location
23 had to be replaced with a PCB 308b02 accelerometer that had a sensitivity
of 1V/g. All calibration factors were entered into the data acquisition
system prior to the measurements. A calibration factor of 1.0 was entered
for the accelerometer that monitored the sliding mass on the shaker. Data was sampled and processed with a Hewlett-Packard
(HP) 3566A dynamic data acquisition system. This system includes a model
35650 mainframe, 35653A source module used to drive the shaker, 5 35653A
8-channel input modules which provided power for accelerometers and performed
the analog to digital conversion of accelerometer signals, and a 35651C
signal processing module that performed the needed Fast Fourier Transform
calculations. A Toshiba Tecra 700CT Laptop was used for data storage
and as a platform for the HP software that controls the data acquisition
system. Data acquisition parameters were specified
such that frequency response functions (FRFs), input and response power
spectra, cross-power spectra and coherence functions in the range of 0-400
Hz could be measured. Each spectrum was calculated from 30 averages of
2-s-long time-histories discretized with 2048 points. These sampling
parameters produced a frequency resolution of 0.5 Hz. Hanning windows
were applied to all measured time-histories prior to the calculation of
spectral quantities. A second set of measurements was acquired from 8-s-long time-histories discretized with 8192 points. Only one average was measured. A uniform window was specified for these data, as the intent was to measure a time history only. Fig. 7 Accelerometer locations and coordinate system
for modal testing. Red numbers indicate accelerometers
mounted in the –y direction. Accelerometers 1, 3 – 32 are PCB
336C with 1V/g sensitivity. Accelerometer 23 was replaced during test
on the shotcreted column (Column 2) with a PCB 308b02 that had 1V/g sensitivity.
Accelerometers 2, 39 and 40 are PCB 302A with 10mV/g sensitivity. Accelerometers
33-37 are Wilcoxon 736t with 100mV/g sensitivity. Accelerometers 36 and
37 are located 8 in (20. 3 cm) off axis in the –y direction. Data FilesData that was collected during the experimental
modal analyses are described below. All files are in Universal File (Type
58) Format. In general, FRF and time-history measurements made at each
damage level were done both with and without the pre-load applied. Files are designated TEST#* Where # indicates the sequential test number
and * is a suffix indicating the type of test. The suffixes are: P – indicate pre-load was applied T-indicates time history measurement L- test performed using twice the excitation level Note that no “T” implies a test where FRFs
based on 30 averages were measured. These files include the last time
history used to form the average. No “P” indicates a test where the pre-load
had been removed. Undamaged Tests on Cast-In-Place Column 10/21/97 TEST0 TESTOT TESTOL TESTOP TESTOPT Tests on Cast-In-Place Column cycled to the theoretical first yield of the rebar,
Dy 10/21/97 TEST1 TEST1T TEST1P TEST1PT Tests on Cast-In-Place Column cycled to 1.5 Dy 10/21/97 TEST2T TEST2P TEST2PT Tests on Cast-In-Place Column cycled to 2.5 Dy 10/21/97 TEST3 TEST3T TEST3P TEST3PT Tests on Cast-In-Place Column cycled to 4.0 Dy 10/21/97 TEST4 TEST4T TEST4P TEST4OPT Tests on Cast-In-Place Column cycled to 7.0 Dy 10/21/97 TEST5 TEST5T TEST5P TEST5PT Undamaged Tests on Shotcrete Column 10/23/97 TEST6 TEST6T TEST6P TEST6PT Tests on Shotcrete Column cycled to theoretical first yield of rebar, Dy
10/23/97 TEST7 TEST7T TEST7P TEST7PT Tests on Shotcrete Column cycled to 1.5 Dy 10/23/97 TEST8 TEST8T TEST8P TEST8P Tests on Shotcrete Column cycled to 2.5Dy 10/23/97 TEST9 TEST9T TEST9P TEST9PT Tests on Shotcrete Column cycled to 4.0 Dy 10/23/97 TEST10 TEST10T TEST10P TEST10PT Tests on Shotcrete Column cycled to 7.0 Dy10/23/97 TEST11 TEST11TTEST11PTEST1PT Finite Element AnalysisTwo finite element models were constructed
so that results of analytical modal analyses can be compared to the experimental
modal analysis results. This first model was constructed with 8-node
continuum elements. This model had 20,979 degrees of freedom (DOFs).
An elastic modulus of 3.6 x 106 psi (24.8 GPa), a mass
density of 2.17 x 10-4 lb-s2/in4 (2.28
x 103 kg/m3), and a Poisson’s ratio of 0.2
were specified in this model. The nodes corresponding to the base of
the foundation were constrained in the three translation DOFs to simulated
the connection to the structural testing floor. The mesh for this model
is shown in Fig. 8. As shown in Fig. 8, the pre-load hardware
was not modeled in any manner. A similar model was analyzed with the pre-load
simulated. To simulate the pre-load, a uniform pressure was applied vertically
downward on the top surface of the 24-sq.-in. portion of the column.
The deformed shape of the column was then used in a subsequent analytical
modal analysis. This modeling does not account for the kinematic constraint
provided by the pre-load hardware at the top of the column. A second model was constructed from three-node
beam elements. This model had 114 DOFs. The same material properties
specified in the continuum model were also used in this model. One beam
element was used to model the foundation and one beam element was used
to model the 1.5-in-deep section of the 24-in-dia. column. Fixed boundary
conditions were applied to the base of the element that simulated the
concrete foundation. The results from the analytical modal analyses
using both these models are summarized in Table I. Figures 9-11 show
the first three bending modes, first two torsion modes and first axial
mode calculated with the continuum element model and without pre-load.
These are all modes that were calculated below 400 Hz. Note that because
of symmetry, the bending modes occur in orthogonal pairs at the same natural
frequency. The first three bending modes calculated with the beam models
are shown in Fig. 12. Reinforcement was not incorporated in either model.
Both ABAQUS input decks are made available in ASCII format. The continuum
model is named UCI-Brick and the beam model is named UCI-Beam. Both models
have nodes located in positions corresponding to the locations of the
accelerometers.
Fig. 8 Undeformed Mesh of the continuum model. Fig. 9. (A) First bending mode and (B) first torsion
mode calculated with the continuum model. Fig. 10. (A) Second bending mode and (B) first axial mode calculated with the
continuum model. Fig. 11. (A) Third bending mode and (B) second torsion mode calculated with
the continuum model. Fig. 12. (A) First bending mode, (B) second bending mode, and (C) third bending
mode calculated with the beam model. EXPERIMENTAL MODAL ANALYSIS RESULTS
Table II summarizes the modal properties (resonant
frequencies and modal damping values) that were obtained from applying
a rational-fraction polynomial curve-fit algorithm in the DIAMOND experimental
modal analysis/damage ID code to the measured FRFs. Compare undamaged,
no pre-load results for both columns to FEM results. Compare pre-load
to no pre-load results for undamaged columns. Compare results at various
damage levels for both columns. Make conclusion on how similar the columns
are based on these measures.
FRFs: Overlay magnitude from Channels 19,
20, 21 for all damage levels, respectively. Compare pre-load to no pre-load.
Overlay same channels from different columns for each damage level. Overlay
linearity check results Mode shapes: Show 3 bending, 3 torsion, axial,
shows torsion change for each damage level. Other modes? Compare two
columns, pre-load to no pre-load. Coherence: Compare coherence for different
sensitivity accels. (Ch 19 and 40) (Ch23 and 4) (Ch 20 and 35) Compare
coherences to input power spectrum. Compare pre-load to no pre-load. Damage Detection (to come) Flexibility analysis (to come) Model updating analysis (to some) Non-parametric analysis (to come) |