Final Report of Project CMS 0116005 Hybrid On-Line Experiments and
Monitoring of Structural Systems
By Khalid M. Mosalam, PI
Overview and Summary of Findings
In order to develop new modeling techniques and study the behavior of reinforced
concrete (RC) buildings with unreinforced masonry (URM) infill walls, a two-phase
experimental and analytical study is conducted. The experimental investigation consists
of a shaking table experiment and a pseudo-dynamic (on-line) simulation. The shaking
table experiment serves as a benchmark test for the development of a unified on-line
experimental technique combined with simulations utilizing the pseudo-dynamic
concepts with substructuring. Both experiments also serve the purpose of validation and
calibration of analytical models being developed using Open System for Earthquake
Engineering Simulation (OpenSees). The objectives of the modeling effort are: (1) to
enable accurate representation of the in-plane and out-of-plane behavior of URM infill
walls, and (2) to refine the modeling techniques of hysteretic response and strength and
stiffness degradation in elements and joints of RC moment frames interacting with URM
infill walls. A schematic illustration of the conducted tasks is given in Figure 1.
Prototype structure
C1
C2
Shaketable
tests
Damage sequence
and collapse
mechanism
Development of
element removal
algorithms
B3
A3
Infill-RC
frame
interaction
A1
A2
B1
B2
C4
Pseudodynamic
test
B4
A4
Development
and Validation
of FE model
Improvement of
quasi-brittle
material models
Verification of
pseudo-dynamic with
sub-structuring
Development
of new SAT
models
Development of new “progressively
collapsing” macro-elements for infilled
frames
Development of
hybrid control
algorithm with
mixed-variables
(mode switch
between force &
deformation)
Figure 1. Overview of the project
The combined shaking table and pseudo-dynamic experimental effort is summarized in
Figure 2. The shaking table experiment is divided into three phases. In the phase 1, the
structure is tested as RC frame with URM infill and columns of the RC frames are
prestressed to simulate effect of upper stories. In phase 2, the URM is removed after
complete collapse and the RC columns remained prestressed. In phase 3, the prestressing
of RC columns is removed and the bare structure is tested to significant damage state. On
The pseudo-dynamic test program comprises structural behavior as well as parametric
studies as shown in Figure 2.
Although the shaking table experiment is conducted in the already existing seismic
simulator test facility of the University of California, Berkeley (UCB), the new hybrid
testing system at UCB, developed as part of nees@berkeley, is used to implement the
pseudo-dynamic experimental part of this study. This new system consists of several
components interconnected to allow for an efficient implementation of on-line testing
technique with flexibility in designing the test set up. It is to be noted that the validation
of the nees@berkeley system is sought as part of this project where a structure with
predictable behavior is utilized. Two steel substructures are designed to exhibit linear and
bilinear behaviors, Figure 2. The experiment is carried out and the results are compared
to a purely numerical simulation proving the validity of the system to conduct the hybrid
simulation part of the current project.
Shaketable
test
Phases 2&3
Shaketable
test
phase1
S = Structural evaluation
P = Parametric study
S-1
S-2
Hybrid testing system
validation
Frame
No.1
Infilled
F1I
One story RC frame
with and without URM infill
+ prestressed columns
F2B
F2B
Infill removed
after collapse
+ prestressed columns
Columns
not prestressed
P-1
F1B
P-2
NL
model
Testing rate study
50
Frame
No.2
Bare
F2B
S-3
F1I
F2B
0
-50
-0.1
0
0.1
F1B
Multi-story shear building
with substructures DOF
P-4
P-3
Error compensation
Mixed formulation:
switching between force
and displacement control
Figure 2. Overview of experimental program
The experiments are carried out on a one-story RC moment-resisting frame structure with
URM infill wall. The ¾-scale test structure represents the first story middle frames
substructure of a five-story three-bay by two-bay RC prototype structure designed based
on the requirements of ACI318-02 and NEHRP recommendations in seismic regions.
URM walls are assumed inside the interior frames as shown in Figure 3. As mentioned
before, both the shaking table and the pseudo-dynamic experiments are carried out in
three phases as described in Figure 2. The ground motions for the experiment are
Northridge (Tarzana, CA 1994) denoted by TAR, Duzce (Turkey 1999) denoted by DUZ
and Loma Prieta (Bran, CA 1989) denoted by LomaPr. These ground motions are scaled
based on the NEHRP design spectra to generate different intensity levels. The Scale
factors for these ground motions are summarized in Table 1. The design spectrum and the
spectra for the input ground motions are shown in Figure 5.
C2
C1
B1
B2
A2
A1
C1
C2
B3
A3
A1
A2
B1
B2
C4
B4
A4
(a) Prototype
structure
(b) Prototype
(c) Shake-table
substructure
test
Figure 3. Test configuration
(d) Pseudodynamic test
Stage one (Infilled system with
TAR 1
columns prestressed)
AWR-
AWR-
AWR-
AWR-
AWR-
AWR-
AWR-
AWR- LomaPr 2
AWRTAR 1
AWRTAR 2
AWRTAR 3
AWRTAR 4
AWRTAR 6
AWRDUZ 7
AWRDUZ 8
AWR- LomaPr 2
DUZ 9
Stage two (Bare system with
TAR 2
TAR 3
TAR 4
TAR 6
DUZ 7
DUZ 8
DUZ 7-2
columns prestressed)
LomaPr 6 LomaPr 7 LomaPr 8 LomaPr 9 LomaPr LomaPr LomaPr LomaPr
9-2
9-3
9-4
9-5
Stage three (Bare system
LomaPr LomaPr LomaPr LomaPr LomaPr LomaPr LomaPr LomaPr LomaPr LomaPr
2-2
3-2
4-2
6-2
7-2
8-2
9-2-1
9-2-2
9-2-3
9-2-4
without columns prestressed)
LomaPr
9-6
Figure 4. Complete test sequence of the experiments
Table 1. Scale factors for different levels of input ground motions
Level
1
2
3
4
6
7
8
9
Northridge, CA, 1994 (TAR)
0.05 0.17 0.23 0.39 0.59 Duzce, Turkey, 1999 (DUZ)
- 1.50 2.00 2.53
Loma Prieta, CA, 1989 (LomaPr)
- 0.31 0.44 0.67 1.00 1.50 1.95 2.19
2.5
← Begining of the test
← TAR 4
← TAR 6
← After removal
Northridge, Tarzana
of the wall
1
1.57g
0
Acceleration [g]
Spectral acceleration [g]
2
Duzce
Northridge
Design Spectrum
1.5
1
-1
0
5
10
15
Duzce
1
0
0.5
-1
← DUZ 8
0
← DUZ 7
0
0
0.2
0.4
0.6
Period [sec]
0.8
5
10
Time [sec]
15
1
Figure 5. Five percent damping response spectra for the selected ground motions
The results of the shaking table experiment are extensively analyzed and important
aspects of the structural response are studied. The effects of the URM wall on the
dynamic response of the structure in terms of stiffness and damping as well as energy
dissipation and distribution of forces in the structural system are presented in different
publications. For example, the distribution of forces between the URM wall and the RC
frame during the first phase the experiment is shown in Figure 6(a). On the other hand,
Figure 6(b) shows the change in the shear force in the RC slab as the force distribution in
the structure varies during this phase of testing. The peak base-shear versus drift for
different levels of the experiment as well as the change in the damping ratio of the
structure is shown in Figure 7(a) and Figure 7(b), respectively. The change in the natural
period of the test structure during different phases of the shaking table experiment is
shown in Figure 8.
180
800
0.55
URM wall
RC Frame
700
600
120
500
100
400
80
[kN]
Base shear [kips]
140
300
60
200
40
100
20
0
TAR 1 TAR 2 TAR 3 TAR 4 TAR 6 DUZ 7 DUZ 8 DUZ 7-2
Slab shear to base shear ratio
160
0.5
Case: VA=VC=0 → VS=Vb/2
TAR 1 TAR 2
0.45
V
S
B
V
S
TAR 3
V
B
VA
V
b
B
TAR 4
V
C
TAR 6
0.4
DUZ 7
DUZ 8
Case: VA=VB=VC → VS=Vb/3
0
Test level
Test Levels
(a) Distribution of base shear between the (b) Slab-shear demand as a portion of the
URM wall and RC frame
base shear in the test structure
Figure 6. Force distributions in test structure for phase 1 of the shaking table experiment.
200
14
DUZ 8
AWR-TAR 6
DUZ 7
LomaPr 9
DUZ 7-2
12
150
TAR 6
LomaPr 9-6
Damping ratio [%]
Maximum Vb / Wtotal [%]
DUZ 7
LomaPr 8-2
100
LomaPr 9-1
TAR 4
LomaPr 9-4
LomaPr 6
50
0
0
TAR 1
LomaPr 2
2
4
6
Corresponding drift [%]
Stage 1
Stage 2
Stage 3
LomaPr 9-2-1
LomaPr 9-6
10
TAR 3
8
LomaPr 9
AWR-TAR 6
6
LomaPr 9-2-4
LomaPr 2-2
4
2
8
Regression
Energy equivalent
TAR 4
T AR 1
AWR-TAR 1
0
Test levels
(a) Peak force-deformation plots
(b) Change in the damping ratio
Figure 7. Change of dynamic properties of the test structure during the phases (stages) of
the shaking table experiment
1
Results using ground motion signals
Results using white-noise signals
Natural period [sec]
0.8
Removal of the
Removal of the
Prestressing
wall
0.6
Wall
cracking
0.4
0.2
Stage three
Stage two
Stage one
0
Test levels
Figure 8. Change in the natural period of the test structure during the three phases
(stages) of the shaking table experiment.
Efforts are underway to reproduce the experimental results using the finite element
method and extend the findings of the study to more general configurations with URM
infill panels. The calibrated model can be used for reliability analysis of such structures,
providing a probabilistic fragility curves and laying the foundation for performance based
design of structures with URM infill walls.
In the pseudo-dynamic experiment, the structure tested on the shaking table is idealized
as a 3 degree of freedom model, with one spring representing the stiffness of each frame
and a numerical model of a spring representing the RC slab connecting the frames, Figure
9. One RC frame with URM infill is physically modeled while the other bare frame is
numerically simulated using symmetry. These frames correspond to frames B1-B2, A1A2 and C1-C2, respectively, as indicated in Figure 3(b). In the structural phases, S-1, S-2
and S-3, Figure 2, the structure is tested using a series of strong motions until the collapse
of the wall. The same sequence of ground motions used in the shaking table experiment is
adopted in the hybrid simulation to allow the comparison between the two experiments.
m : inertia mass
k : stiffness
BF : Bare Frame
IF : Infilled Frame
S : Slab
mBF
kBF
kS
kIF
loading
direction
mIF
kS
mBF
kBF
Figure 9. Idealized test structure for pseudo-dynamic experiment
In general, reasonable agreement between the results of the shaking table and pseudodynamic experiments is observed. Peak force and peak displacement results for the two
experiments match reasonable well, Figure 10. Moreover, similar trends are observed in
the effect and contribution of the URM wall, Figure 11. However, the shape of the forcedeformation plots and the extent of damage are significantly different. The discrepancies
are attributed to the differences in the nature of two experiments, in terms of the speed of
the test and the inherit assumptions in the pseudo-dynamic test method, e.g. damping.
Extensive analysis of the results on the local level is conducted.
b
Maximum V / W
total
[%]
200
DUZ 7
DUZ 8
150
TAR 6
100
50
TAR 4
TAR 3
TAR 2
TAR 1
0
0
0.5
Shake-table
Pseudo-dynamic
1
1.5
2
Corresponding drift [%]
2.5
Figure 10. Global response of the test structures from the shaking table and pseudodynamic experiments
(a) Shaking table experiment
(b) Pseudo-dynamic experiment
Figure 11. Comparison between crack patterns of the URM infill wall for the shaking
table and pseudo-dynamic experiments
In phase P-1 of the pseudo-dynamic experiment, Figure 2, a hypothetical structure is
tested using a calibrated hysteretic Bouc-Wen model to numerically simulate the URM
infilled frames, while Frame F1B represents the physical substructure. This allows a
better understanding of the effects of the higher modes on the dynamic behavior of the
test structure.
In phases P-2 and P-4 of the pseudo-dynamic experiment, Figure 2, an evaluation of the
error in applying the command displacement and its variation with the different testing
rates is examined. A direct strong correlation between the actuator velocity and the
displacement error is observed, which allows the accurate prediction of the expected
error. An error compensation scheme is thus implemented, and the results show a more
accurate execution of the hybrid simulation, Figure 12.
Error [%]
1
Uncompensated Error
Compensated Error
0.5
0
-0.5
-1
0
2
4
6
8
10
12
14
16
Time [sec]
Figure 12. Error compensation in the hybrid simulation (pseudo-dynamic) experiment
A new mixed force/displacement formulation algorithm is derived departing from the
implicit α -method, Figure 13. The limits and validity of the algorithm are examined
through a numerical parametric study. The algorithm is integrated as part of the hybrid
testing system, and a simulation is applied on the test structure in phase P-3, Figure 2.
The mode switch between displacement and force control is implemented smoothly
during the test and the algorithm shows great potential for various possible applications
in hybrid testing, Figure 14.
Define thresholds Kd, Kf , Vd, Vf
Kd < Kf and Vd < Vf
Start
Switch to force
control mode
End
Yes
Yes
No
Stay at current Mode
Switch to displacement
Control Mode
Vi > Vf
Calculate Ki, Vi
Displacement
Ki > Kf
No
Current Mode?
No
No
Vi < Vd
Yes
Ki < Kd
Force
Yes
50
0
-50
-6
-4
-2
Displ. Control
Mixed Formulation
0
2
4
Measured displacement [in]
Energy disspated [kips.in]
Measured force [kips]
Figure 13. Newly developed mixed formulation for hybrid simulations
250
200
150
100
50
0
0
5
Displ. Control
Mixed Formulation
10
15
Time [sec]
(a) Force-deformation relationship
(b) Energy time history
Figure 14. Comparison between displacement control and the newly developed mixed
formulation
Publications
1. Hashemi, A. and Mosalam, K.M. “Transient analysis of reinforced concrete frame
with and without masonry infill wall under blast,” Emirates Journal for Engineering
Research, 9(2), 97-103, 2004.
2. Hashemi, A. and Mosalam, K.M. “Shake-table experiment on one-story RC structure
with and without masonry infill”, NATO workshop: Seismic Assessment and
Rehabilitation of Existing Buildings, May30-June 1, 2005, Istanbul, Turkey.
3. Elkhoraibi T. and Mosalam K.M. “Pseudo-dynamic experiment on one-story RC
structure with and without masonry infill”, Proceedings of the 100th Anniversary
Earthquake Conference Commemorating the 1906 San Francisco Earthquake, April
18-22, 2006.
4. Hashemi, A., Elkhoraibi, T. and Mosalam, K.M. “Dynamic and pseudo-dynamic
evaluation of RC structure containing masonry infill”, Proceedings of the Modern
Problems and Directions of Mechanics Conference, May 17-18, 2006, Tashkent,
Uzbekistan.
5. Hashemi, A. and Mosalam, K.M. “Shake-table experiment on reinforced concrete
structure containing masonry infill wall”, Earthquake Engineering and Structural
Dynamics, Vol. 35, No. 14, 1827-1852, 2006.
6. Hashemi, A. "Seismic evaluation of reinforced concrete buildings including effects of
masonry infill walls", PhD dissertation, UC-Berkeley, Spring 2007.
7. Elkhoraibi T. “Generalized hybrid simulation framework for structural systems
subjected to seismic loading,” PhD dissertation, UC-Berkeley, Spring 2007.
8. Elkhoraibi T. and Mosalam K.M. “Towards error-free hybrid simulation using mixed
variables,” Earthquake Engineering and Structural Dynamics, Vol. 36, No. 11, 14971522, 2007.
9. Hashemi, A. and Mosalam, K.M., “Towards generalized fragility functions of RC
frame structures containing URM infill walls,” Proc. Tenth International Conference
on Applications of Statistics and Probability in Civil Engineering, ICASP10, Kashiwa
Campus, The University of Tokyo, Tokyo, Japan, 2007.
10. Hashemi, A. and Mosalam, K.M., “Seismic evaluation of reinforced concrete
buildings including effects of masonry infill walls,” PEER Technical Report
2007/100, 2007.
11. Elkhoraibi, T. and Mosalam, K.M., “Generalized hybrid simulation framework for
structural systems subjected to seismic loading,” PEER Technical Report 2007/101,
2007.