Author: Khalid Mosalam

1. Efficient Analytical and Hybrid
Simulations using OpenSees
Khalid M. Mosalam, PhD, PE
Taisei Professor of Civil Engineering
Director, Pacific Earthquake Engineering Research (PEER) Center
University of California, Berkeley
OPENSEES DAYS EUROPE: FIRST EUROPEAN CONFERENCE ON OPENSEES
June 19-20, 2017, Porto, Portugal, http://eosd2017.weebly.com/
Keynote session #8: June 20, 2017 [5:45-6:15 pm “GMT+1” via skype]
Thanks to: Dr. Selim Günay & Dr. Xiao Liang

2. Outline
2
Analytical Simulation
 Overcoming convergence challenges:
▪ Evaluating time integrators, parameters & solution algorithms
▪ Development of a Lyapunov-based nonlinear solution algorithm
▪ Progressive collapse with direct element removal [Briefly]
Hybrid Simulation Applications
 Background
 Wind turbine blades
 Curtain wall systems

3. 3
Analytical Simulation

4. 4
Motivation
Convergence problems occur at high level of nonlinearity
Approaches:
 Alternative Integration Methods
➢ Explicit Newmark (EN)
➢ Operator-Splitting Methods (OS)
➢ TR-BDF2*
 Improvement of Implicit Newmark (IN)
 Lyapunov-based nonlinear solution algorithm
 Progressive collapse simulation w/ direct element removal
*Trapezoidal Rule with the second order Backward Difference Formula
(also referred to as Trapezoidal Rule with three-point Euler method)

5. 5
Implicit
Newmark (IN)
Alternative Methods:
1. Accurate
2. Iterative
Explicit Newmark (EN)
Operator Splitting (OS)
TR-BDF2
1. No convergence issues
2. Numerical damping
Motivation

6. 6
Analytical Bridge Models
Superstructure, column bent & abutment of the
Bridge B used in NLTA [Kaviani 2011]
40 pulse-like
three-component ground
motions [Baker et al. 2011]
Three standard bridge structures
in California:
 Jack Tone Road Overcrossing
(Bridge A), a bridge with 2
spans supported on a single-
column bent
 La Veta Avenue Overcrossing
(Bridge B), a bridge with 2
spans & a 2-column bent
 Jack Tone Road Overhead
(Bridge C), a bridge with 3
spans & two 3-column bents

7. 7
Abutment Modeling
Springs and gap elements used to model components of the abutment [Kaviani, 2011]
Backfill
Expansion joint
Shear key
Bearing pad
and stemwall
Type I abutment modeling (detailed)
Backfill
Shear key
Expansion joint
Backfill
Bearing pad
and stemwall
Type II abutment modeling (simplified)

8. 8
Accuracy of Alternative Integrators
Type I abutment modeling Type II abutment modeling
Max error of the OS algorithm for the three selected EDPs of Bridge B
IN
OSIN
error
Max
MaxMax
Max



9. 9
Accuracy of Alternative Integrators
0 10 20 30 40
-3
-2
-1
0
1
2
3
4
5
Time (s)
LongitudinalDisplacement(in)
IN
OS
-1.2 -0.8 -0.4 0 0.4 0.8 1.2
x 10
-3
-0.8
-0.4
0
0.4
0.8
1.2
x 10
5
M
Abutment unseating displacement Moment-curvature response
OS & TR-BDF2 provide very close results to those of IN 
They are suitable alternatives to IN for NLTA of RC highway bridges.

10. 10
Adaptive Switching of Integration
Algorithms
Convergence failure time [sec] of simulations for different integration methods
NGA Sequence
Number
Bridge
Scale
factor
Implicit
Newmark
Switching Integration
algorithms
TR-BDF2
182 A 2.80 6.160 Completed Completed
1271 A 1.08 13.720 Completed Completed
964 A 1.47 2.690 Completed Completed
1263 B 3.00 17.420 Completed Completed
1011 B 2.10 1.310 Completed Completed
1541 C 1.00 31.120 Completed Completed
755 C 1.70 0.495 Completed Completed
1542 C 1.37 25.040 Completed Completed
TR-BDF2 & adaptive switching algorithms
show improved convergence compared to IN.

11. 11
Improving Convergence of IN:
Nonlinear Solver
Scale Factor Newton KrylovNewton Broyden NRLS BFGS
1.0 Completed 21.820 Completed Completed Completed
1.1 35.645 21.820 35.660 Completed 35.820
1.2 35.650 21.820 Completed Completed 41.010
1.3 35.655 6.115 35.655 Completed Completed
1.4 Completed 6.115 35.260 Completed 28.985
1.5 Completed 6.115 78.505 Completed Completed
1.6 Completed 6.115 Completed Completed 42.600
1.7 Completed 6.115 Completed Completed 36.155
1.8 35.710 6.115 Completed Completed 37.270
1.9 Completed 6.115 35.265 Completed 35.915
2.0 35.730 6.115 52.540 Completed 24.675
Convergence failure time [sec] of simulation for different initial nonlinear solvers
under GM31 for Bridge A
As long as a suitable initial solver is selected, order of subsequent solvers
do not significantly impact convergence.
Most suitable initial solver: NRLS (Newton-Raphson with Line Search)

12. 12
Scale
Factor
EnergyIncr NormDisIncr
RelativeNorm
DisIncr
RelativeTotal
NormDisIncr
Relative
EnergyIncr
1.0 31227 48225 55821 55817 40949
1.1 31510 48514 56481 56472 41726
1.2 31671 48983 56656 56664 42324
1.3 32021 49340 56734 56733 42505
1.4 31979 49567 57093 57075 42605
1.5 32272 49844 57187 57187 42715
1.6 32580 50035 57023 57023 42522
1.7 32573 50074 57015 57011 42162
1.8 33077 50683 57291 57219 42077
1.9 33397 51537 57381 57372 42439
2.0 33780 Failed 61986 57876 42664
Total # of iterations for simulations with different convergence tests under GM31 for Bridge A
Most suitable convergence test: Energy Increment test
Improving Convergence of IN:
Convergence Test Type

13. 13
Improving Convergence of IN:
Convergence Tolerance
Errors are reduced when somewhat larger tolerance is only applied
at the problematic time step but specific trend is not clear.
simulations with tolerance of 10-8 are references

14. 14
Improving Convergence of IN:
Integration Time Step
Convergence failure time [sec] of
simulations for different integration
time steps for GM31 (Bridge A)
• Smaller time step during simulation does not necessarily improve convergence behavior.
• Reducing time step, only when needed, can be useful to overcome convergence issues.
• Again, specific trend is not clear.

15. 15
Lyapunov-based Nonlinear Solution
Algorithm: Lyapunov Stability Theory
   xx v
Considering a positive scalar function (norm) v(x)
• v = 0 at the equilibrium point
• Radially unbounded
v(x): Decreasing (non-increasing) along the dynamic trajectory
Global convergence (stability)Sufficient condition!
Mathematics
Control &
system
engineering
Solution
algorithms
[This study]
Comprehensive framework for stability
Bypass calculation of system trajectories for stability check

16. 16
Convergence of Nonlinear
Solution Algorithms
  0ug 
Regular Newton-Raphson (NR) algorithm and its variants:
- Convergence performance is not guaranteed
- It depends on the initial guess and the consistent Jacobian
Nonlinear Equation
   
  j
jjjj
ur
uguJuu
11
11

 
    r
uusignug 
Regular NR
Spectral radius 𝜌
Example: Memoryless nonlinear function
   ug
u
uJ




17. 17
Convergence of Nonlinear
Solution Algorithms
    0
r
uusignug
An algorithm with global convergence should be pursued,
i.e., the initial guess does not affect the convergence.
𝑟 = Τ1 3 , 𝜌 = 2 𝑟 = Τ1 2 , 𝜌 = 1
Nonlinear Equation

18. Lyapunov-Based Nonlinear Solution
Algorithm
State space
g(u) space
Lyapunov
(energy)
function
eq
i 1u
Region of attraction of NR & its
variants (unknown, in general)
     111
2
1
  ii
T
iv ugugu
)( iug
iu
  0ug 1iStructural Dynamics
  iv ug
18
Hypothetical dynamical system
(Path in the iteration domain)
    )( 0KKuguΚJu TT
dtd 
  01 iv u
 1iv u radially unbounded
GOAL
K=K0 F(J) affects convergence & accuracy; K0≈[10, 20]
in considered examples [Liang & Mosalam, 2016, 2017].
Solve ODE by explicit Dormand-Prince method.
Solution (equilibrium) point!
0ug  )( 1
eq
i
strictly decreasing

19. Lyapunov-Based Nonlinear
Solution Algorithm
𝑟 = Τ1 3 𝑟 = Τ1 2
Nonlinear Equation     0
r
uusignug
19

20. Application: 2-Story Shear
Type Building
Perfect matching!
%1u
Lyapunov-based algorithm
is being implemented in
OpenSees 20
Increase the scale
Wood (E≈2000 ksi) frame
Story height = 100”
(Beam: ∞-rigid; Columns: 7”×7”)
Displacement time history of DOF1

21. Application: 2-Story Shear
Type Building
DOF 1 DOF 2
Time Time
%1u %2u
NR fails while new algorithm can simulate through the entire response.
21

22. DOF 1 DOF 2
Application: 2-Story Shear
Type Building
Regular NR: Infinite bouncing between two points about the equilibrium point (1,1,1)
22

23. Application: 2-Story Shear
Type Building
Proposed new algorithm: Converges to the equilibrium point (1,1,1) smoothly
DOF 1 DOF 2
23

24. Progressive Collapse Simulation:
Direct Element Removal
24
[Talaat & Mosalam, 2007]
➢ Basis for a progressive collapse algorithm
➢ Removal criteria used to detect failure
➢ A detected failed element is directly removed from the structural model
➢ Requires corresponding removal of:
▪ Dangling nodes & associated constraints
▪ Floating elements
▪ Nodal & element forces of the removed element
Floating element
Dangling node
Dashed elements have been
removed during analysis
Intact structure
Nodal load
Distributed load
Kocaeli, 1999
Chi-Chi, 1999

25. 25
Advantage:
✓ Avoid numerical problems related to negative stiffness or residual strength
m2
m1
F

Numerically challenging!
m1
Collapsed
& removed
Progressive Collapse Simulation:
Direct Element Removal
Not only removing an element that does not exist anymore,
but also removing the numerical problem!

26. 26
-0.02 -0.01 0
-2
-1
0
1
2
Displacement [inch]
Force[kips]
m=3
m=1
0 10 20 30 40
-0.1
-0.05
0
0.05
0.1
0.15
Time [sec]
Displacement[inch]
First Level Displacement
Upper spring force reduces to residual strength
NLTA w/ 1940
El Centro GM
0 10 20 30 40
-0.1
-0.05
0
0.05
0.1
0.15
Time [sec]
Displacement[inch]
without removal
with removal
T1=0.27 sec
T2=0.07 sec
1=2%
Advantage:
✓ Avoid numerical problems related to negative stiffness or residual strength
-0.04 -0.02 0
-2
0
2
Displacement [inch]
Force[kips]
X
Remove top
mass & spring
u1
Progressive Collapse Simulation:
Direct Element Removal

27. Example: Infill Wall Element Removal
27
Peak displacement profile from NLTA
under Los Gatos GM Loma Prieta, 1989
0 0.05 0.1 0.15 0.2
0
2
4
6
8
10
12
14
16
18
20
Displacement [m.]
Height[m.]
0 0.05 0.1 0.15 0.2
0
2
4
6
8
10
12
14
16
18
20
Displacement [m.]
Bare frame
Frame with infill walls
Longitudinal dir. Transverse dir.
3.65 m
4.90 m
Long.
Trans.
Building designed according to code w/o effect of infills
0 10 20 30 40 50 60
0
10
20
30
40
50
60
LongitudinalTransverse
Deformation @ peak roof displ.
http://opensees.berkeley.edu/wiki/index.php/Infill_Wall_Model_and_Element_Removal
[Mosalam & Günay, 2015]
IPdisplacement OOP displacement
Failure Curve

28. 28
Hybrid Simulation

29. 29
Analytical
Simulation
Hybrid
Simulation (HS)
=
Experimental
Simulation
+
Background

30. 30
Background
u2
u1
m2
m1
g
2
1
1
21
2
1
2221
1211
2
1
2
1
u
m
m
u
u
cc
cc
u
u
m0
0m
















 
























f
ff
Objective of analytical simulation: Solve the equations of motion using
numerical integration methods
pfucum  
m u c u f p
Analytical
Simulation: m, c, f and p modeled using
regular OpenSees commands

31. 31
Background
u2
u1
Bottom spring
replaced with a test
specimen (SIP)
Analytical
substructure
m2
m1
u2
u1
m2
m1
g
2
1
2
1
2221
1211
2
1
2
1
u
m
m
u
u
cc
cc
u
u
m0
0m
















 
























a
ea
f
ff
Experimental
substructure
Measured
(experimental)
Computed
(analytical)
(-)
▪ m, c, p & fa modeled using regular OpenSees commands
▪ Experimental elements in OpenSees simulate experimental
substructures where fe is acquired from a physical specimen
▪ OpenFresco allows experimental element to communicate
with computations

32. Application I*: Stiffened Wind
Turbine Blade
32
▪ While alternative energy sources still a small fraction of global energy
production, they provide a large % in some countries.
▪ In Denmark, 28% of total electricity demand is provided by wind
energy, and even allowing exporting energy to other countries.
▪ Excessive winds are useful for increased use of wind power if
structural integrity of wind turbines is maintained under such winds.
*In collaboration with Jacob H. Høgh, Technical University of Denmark
D-string stiffener as a
retrofit increases stiffness
& strength of wind
turbine blades

33. 33
Validation of the retrofit system by large scale testing is difficult:
➢ Size limitations of experimental facilities: Large size blades (> 80 m).
➢ Complex loading: Centripetal force from rotation, varying wind loads
and changing gravity loads due to rotation.
Full scale test of a 34 m wind
turbine blade (Jensen et al., 2006)
HS can overcome these problems!
Application I: Motivation

34. ≡
34
Application I: Loading
 Shear center does not coincide with the
center of gravity
 Torsion leads to opening of trailing edge
 D-string stiffener prevents this opening
Sparcaps
Leadingedge
Trailingedge
Shearwebs
Sandwichpanels
Internal forces
Sparcaps
Leadingedge
Trailingedge
Shearwebs
Sandwichpanels
Shear center
Center of gravity

35. 35
Application I: Hybrid Model
Spar caps
Leading edge
Trailing edge
Shear webs
Sandwich panels
Analytical substructure: The wind turbine
blade, a glass fiber laminate structure
with linear elastic properties under
operational conditions [OpenSees]
Experimental substructure: D-string
stiffener, with nonlinear viscoelastic
response [UTM in HS setup]
3D Printed
92 nodes (9 fixed in all translations), 102 ShellMITC4 (4 node bilinear shell elements with 6 DOF per node &
modified shear interpolation), nDMaterial ElasticOrthotropic with PlateFiber section Rayleigh damping ratio: 1%

36. 36
Application I: HS Setup
OpenSees / OpenFresco
Actuator
Controller
DAQ / Digital Signal
Processor (DSP)
PI660HybridSim PC
Load Cell
D-stringstiffener
Displacement
Force
Relaxation plot
of the D-string
Testtime

37. 37
✓ All transient integrators require the solution of a linear system of equations
✓ An efficient linear system solver in OpenSees:
✓ Factorizes the coefficient matrix only once at the beginning of the simulation.
✓ Apply only for non-iterative integrators, e.g. Explicit Newmark (EN) & Operator Splitting (OS).
eff1eff puk  i
A Note about Computation Speed
0 1000 2000 3000 4000 5000 6000 7000
0
10
20
30
40
# of DOF
ComputationTimeperTimeStep[ms]
OpenSees: Multiple Factorization
OpenSees: Single Factorization
algorithm Linear <-initial/secant> <-factorOnce>
effeff pum 1i
    Iβtγt kcmmeff  11 2
cmmeff γt
OS:
EN:
Used in the RTHS of substation electrical
equipment with multiple shaking tables
Table 1 Table 2
Equip.1
Equip.2
Connecting Cable

38. 38
Application I: External Loading
≡
NG
NGy
NGx
 Forces are assigned to the nodes (NG) according to the tributary areas
 NGx and NGy are computed as dynamic loading:

➢ NGx = NG sin = NG sint
➢ NGy = NG cos = NG cost
 = 4.36 rad/sec (~0.7 Hz)  T = 1.44 sec
 depends on wind velocity

39. 39
Application I: HS Results
Time (sec)
Displacement(mm)
HS test at 0.713 Hz
Force(N)
Displacement (mm)
Stiffener reduces max. opening
to 25% of unstiffened value
Spar caps
Leading edge
Trailing edge
Shear webs
Sandwich panels
HS results for opening of trailing edge

40. Application II**: HS of a Curtain
Wall Façade System
Main structure of the Shanghai Tower and Its Components
40
**In collaboration with Yangling Wang, Tongji University, China

41. Application II: Simplified Modeling
Simplified model
(OpenSees)
Plane view
Hanging design
Simplification of components
Note: stiffness ratio β = Ksi / Ki
41

42. Application II: Pre-Test Analysis
Pseudo-acceleration response spectra of floor motions for CW system
of Zone 8 (most critical)
Determine the floor accelerations from NLTA of the
Shanghai tower main structural system
42
4 different
input GMs

43. Application II: Pre-Test Analysis
Acceleration Amplification Index (AAI=PFA/PGA) Normalized maximum element deformation
Determine the critical stiffness ratio configuration β
β=0.1 is the most critical case
43
β = Ksi / Ki

44. Application II: HS Model
(OpenSees)
44

45. Application II: HS Results
Node 2 acc. Node 2 displ.
MinorEQRareEQ
45
Tested

46. Application II: HS Results
46
a) FW-US_Minor b) FW-US_Medium c) FW-US_Rare
Force-displacement relationships of the experimental element
from HS test & pure analysis
Tested

47. Concluding Remarks
47
 OpenSees is an integral part of the PEER Center:
▪ Refer to PEER Report: Overview and Activities, PEER Report No. 2017/01,
University of California, Berkeley, CA, June 2017
▪ You are welcome to attend the PEER Annual Meeting, UCB, Jan. 18-19, 2018
▪ Feel free to contact PEER via email, peer_center@berkeley.edu, or via the
message board, http://opensees.berkeley.edu/community/index.php, for
benchmark examples, simple or advanced GUIs or questions to be shared with
OpenSees community
 Expending OpenSees activities include:
▪ New developments, e.g. solution algorithms
▪ New applications in analytical & hybrid simulations

48. Future Directions
48
 User-friendly GUIs for increased use of OpenSees among practitioners
 Faster computations using OpenSees parallel versions locally and in the cloud
 Data-driven Structural Health Monitoring (SHM) using robust OpenSees
analysis & machine learning tools
 A new Python interpreter for OpenSees*
 A new integrated development environment for OpenSees*
 A new framework developed as part of the NHERI SimCenter that will use
OpenSees to develop applications for UQ estimation and Performance Based
Engineering for Earthquake, Wind and Tsunami events*
*Already discussed by Dr. Frank McKenna

49. References
49
1. Mosalam, K.M. and Günay, S. (2015). “Progressive Collapse Analysis of Reinforced
Concrete Frames with Unreinforced Masonry Infill Walls Considering In-Plane/Out-
of-Plane Interaction,” Earthquake Spectra, 31(2), 921-943.
2. Mosalam, K.M., Günay, S. and Takhirov, S. (2016). “Response Evaluation of
Interconnected Electrical Substation Equipment Using Real-time Hybrid Simulation
on Multiple Shaking Tables,” Earthquake Engineering & Structural Dynamics,
45(14), 2389-2404.
3. Liang, X. and Mosalam, K.M. and Günay S. (2016). “Direct Integration Algorithms
for Efficient Nonlinear Seismic Response of Reinforced Concrete Highway Bridges,”
Journal of Bridge Engineering, 21(7), 04016041.
4. Liang, X. and Mosalam, K.M. (2016). Performance-Based Robust Nonlinear Seismic
Analysis with Application to Reinforced Concrete Highway Bridge Systems, PEER
Report No. 2016/10, Pacific Earthquake Engineering Research Center, University of
California, Berkeley, CA, Dec. 2016.
5. Liang, X. and Mosalam, K.M. (2017). “Lyapunov-Based Nonlinear Solution
Algorithm for Structural Analysis,” Journal of Engineering Mechanics (under review).

50. 1. Lu, Wensheng; Huang, Baofeng; Chen, Shiming; Mosalam, Khalid M., Acceleration demand of the
outer-skin curtain wall system of the Shanghai Tower, STRUCTURAL DESIGN OF TALL AND SPECIAL
BUILDINGS, Volume: 26, Issue: 5, Article Number: e1341, Published: APR 2017.
2. Bakhaty, Ahmed A.; Govindjee, Sanjay; Mosalam, Khalid M., Theoretical Evaluation of Hybrid
Simulation Applied to Continuous Plate Structures, JOURNAL OF ENGINEERING MECHANICS, Volume:
142, Issue: 12, Article Number: 04016093, Published: DEC 2016.
3. Liang, Xiao; Mosalam, Khalid M., Lyapunov Stability Analysis of Explicit Direct Integration Algorithms
Applied to Multi-Degree-of-Freedom Nonlinear Dynamic Problems, JOURNAL OF ENGINEERING
MECHANICS, Volume: 142, Issue: 12, Article Number: 04016098, Published: DEC 2016.
4. Mosalam, Khalid M.; Gunay, Selim; Takhirov, Shakhzod, Response evaluation of interconnected
electrical substation equipment using real-time hybrid simulation on multiple shaking tables,
EARTHQUAKE ENGINEERING & STRUCTURAL DYNAMICS, Volume: 45, Issue: 14, Pages: 2389-2404,
Published: NOV 2016.
5. Liang, Xiao; Mosalam, Khalid M., Lyapunov Stability Analysis of Explicit Direct Integration Algorithms
Considering Strictly Positive Real Lemma, JOURNAL OF ENGINEERING MECHANICS, Volume: 142,
Issue: 10, Article Number: 04016079, Published: OCT 2016.
6. Liang, Xiao; Mosalam, Khalid M., Lyapunov Stability Analysis of Explicit Direct Integration Algorithms
Considering Strictly Positive Real Lemma, JOURNAL OF ENGINEERING MECHANICS, Volume: 142,
Issue: 10, Article Number: 04016079, Published: OCT 2016.
7. Liang, Xiao; Mosalam, Khalid M.; Gunay, Selim, Direct Integration Algorithms for Efficient Nonlinear
Seismic Response of Reinforced Concrete Highway Bridges, JOURNAL OF BRIDGE ENGINEERING,
Volume: 21, Issue: 7, Article Number: 04016041, Published: JUL 2016.
8. Lu, Wensheng; Huang, Baofeng; Mosalam, Khalid M.; Chen, Shiming, Experimental evaluation of a
glass curtain wall of a tall building, EARTHQUAKE ENGINEERING & STRUCTURAL DYNAMICS, Volume:
45, Issue: 7, Pages: 1185-1205, Published: JUN 2016.
Published Bibliography (1/3)
50

51. 9. Liang, Xiao; Mosalam, Khalid M., Lyapunov Stability and Accuracy of Direct Integration Algorithms
Applied to Nonlinear Dynamic Problems, JOURNAL OF ENGINEERING MECHANICS, Volume: 142, Issue:
5, Article Number: 04016022, Published: MAY 2016.
10. Moustafa, Mohamed A.; Mosalam, Khalid M., Substructured Dynamic Testing of Substation Disconnect
Switches, EARTHQUAKE SPECTRA, Volume: 32, Issue: 1, Pages: 567-589, Published: FEB 2016.
11. Gunay, Selim; Mosalam, Khalid; Takhirov, Shakhzod, Real-time hybrid simulation in a shaking table
configuration for parametric studies of high-voltage equipment and IEEE693 development, NUCLEAR
ENGINEERING AND DESIGN, Volume: 295, Pages: 901-909, Published: DEC 2015.
12. Moustafa, Mohamed A.; Mosalam, Khalid M., Seismic response of bent caps in as-built and retrofitted
reinforced concrete box-girder bridges, ENGINEERING STRUCTURES, Volume: 98, Pages: 59-73,
Published: SEP 2015.
13. Drazin, Paul L.; Govindjee, Sanjay; Mosalam, Khalid M., Hybrid Simulation Theory for Continuous
Beams, JOURNAL OF ENGINEERING MECHANICS, Volume: 141, Issue: 7, Article Number: 04015005,
Published: JUL 2015.
14. Mosalam, Khalid M.; Gunay, Selim, Progressive Collapse Analysis of Reinforced Concrete Frames with
Unreinforced Masonry Infill Walls Considering In-Plane/Out-of-Plane Interaction, EARTHQUAKE
SPECTRA, Volume: 31, Issue: 2, Pages: 921-943, Published: MAY 2015.
15. Gunay, Selim; Mosalam, Khalid M., Enhancement of real-time hybrid simulation on a shaking table
configuration with implementation of an advanced control method, EARTHQUAKE ENGINEERING &
STRUCTURAL DYNAMICS, Volume: 44, Issue: 5, Pages: 657-675, Published: APR 2015.
16. Lee, Hyerin; Gunay, Selim; Mosalam, Khalid M., Comparison of the seismic response of reinforced
auger pressure grout and concrete columns, ENGINEERING STRUCTURES, Volume: 87, Pages: 139-
152, Published: MAR 2015.
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52. 17. Mosalam, Khalid M.; Gunay, Selim, Seismic performance evaluation of high voltage disconnect switches
using real-time hybrid simulation: I. System development and validation, EARTHQUAKE ENGINEERING
& STRUCTURAL DYNAMICS, Volume: 43, Issue: 8, Pages: 1205-1222, Published: JUL 2014.
18. Gunay, Selim; Mosalam, Khalid M., Seismic performance evaluation of high-voltage disconnect switches
using real-time hybrid simulation: II. Parametric study. EARTHQUAKE ENGINEERING & STRUCTURAL
DYNAMICS, Volume: 43, Issue: 8, Pages: 1223-1237, Published: JUL 2014.
19. Mosalam, Khalid M.; Hube, Matias A.; Takhirov, Shakhzod M.; Gunay, Selim, Teaching Innovation
through Hands-on-Experience Case Studies Combined with Hybrid Simulation, JOURNAL OF
PROFESSIONAL ISSUES IN ENGINEERING EDUCATION AND PRACTICE, Volume: 139, Issue: 3, Pages:
177-186, Published: JUL 2013.
20. Talaat, Mohamed; Mosalam, Khalid M., Modeling progressive collapse in reinforced concrete buildings
using direct element removal, EARTHQUAKE ENGINEERING & STRUCTURAL DYNAMICS, Volume: 38,
Issue: 5, Pages: 609-634, Published: APR 2009.
21. Elkhoraibi, Tarek; Mosalam, Khalid M., Towards error-free hybrid simulation using mixed variables,
EARTHQUAKE ENGINEERING & STRUCTURAL DYNAMICS, Volume: 36, Issue: 11, Pages: 1497-1522,
Published: SEP 2007.
22. Pan, P; Tomofuji, H; Wang, T; Nakashima, M; Ohsaki, M; Mosalam, KM, Development of peer-to-peer
(P2P) internet online hybrid test system, EARTHQUAKE ENGINEERING & STRUCTURAL DYNAMICS,
Volume: 35, Issue: 7, Pages: 867-890, Published: JUN 2006.
23. Mosalam, KM; White, RN; Ayala, G, Response of infilled frames using pseudo-dynamic experimentation,
EARTHQUAKE ENGINEERING & STRUCTURAL DYNAMICS, Volume: 27, Issue: 6, Pages: 589-608,
Published: JUN 1998.
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52