BUILDING STRONG ® Analysis and Design of Large-Scale Civil Works Structures Using LS-DYNA® David...
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Transcript of BUILDING STRONG ® Analysis and Design of Large-Scale Civil Works Structures Using LS-DYNA® David...
BUILDING STRONG®
Analysis and Design of Large-Scale Civil Works Structures Using LS-DYNA®
David Depolo, M.S., P.E.Structural Engineer
Sacramento & Philadelphia Districts
Thomas Walker, P.E.Structural EngineerSacramento District
Eric Kennedy, P.E.Structural EngineerSacramento District
Ryan TomAmerican River Design
Sacramento District
PRESENTED BY THEU.S. ARMY CORPS OF ENGINEERS
NON-PRESENTING CO-AUTHORS
LSTC International Users’ ConferenceJune 7, 2010
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Introduction
Project overview
The JFP model► Properties► Troubleshooting & Lessons Learned
Designing from the model
Running the model
Seismic input
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The Folsom JFP LS-DYNA ModelOverview
Foundation(*MAT_ELASTIC, E = 3500ksi)
Shear Zone(*MAT_ELASTIC, E = 324ksi)
Backfill(*MAT_PSEUDO_TENSOR)
Reservoir(*MAT_NULL)
Control Structure
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The Folsom JFP LS-DYNA ModelControl Structure
Non-Flow Monoliths
Non-Flow Monolith
Flow-Through Monoliths
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The Folsom JFP LS-DYNA ModelFlow-Through Monoliths
Piers(Designed using LS-DYNA output)
Pier Struts(Designed using LS-DYNA output)
Invert Slab
Headwall
Radial Gates(Rigid, defined
individually)
Gate Arms
Trunnion Girders
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Rigid Bodies & SOFT
SymptomUnrealistic spikes in forces at the radial gate
Peak force/length along pier during earthquake
Corrected
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Rigid Bodies & SOFT
ReasonsGates defined using *MAT_RIGID
Reservoir is merged with the gate to obtain correct hydrostatic pressures
SolutionOptional Card A:SOFT = 0uses a penalty formulation, interface stiffness is based on the bulk modulus
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Reservoir Contacts
SymptomDuring an earthquake, some fluid elements lose pressure
ReasonStructure displacements created a free surface
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Reservoir Contacts
Solution1. Split the reservoirat monolith joints
2. Define a contact surface between reservoir parts
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Reservoir Contacts
For troubleshooting, split contacts so you can focus on problem areas
(each conduit has its ownset of contacts)
For verification, split contacts into pieces that are easily replicated with a calculator
HSF = 0.5*γ*H2*b
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Hydrostatic Pressures
Complex topography can cause incorrect pressures
Idealized geometry ensures the loads to the structure are more realistic
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Post-Tensioned Anchorage
Option 1: Constrained Nodes► Each trunnion girder is constrained to
nodes that represent the dead ends of the anchors
► *CONSTRAINED_EXTRA_NODE_SET
► Pros• Simple, easy to implement• Transfers all forces directly to the slab
► Cons• Ignores elastic behavior of anchors• Creates a rigid plane in the slab
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Post-Tensioned Anchorage
Option 2: Beam Elements► Hughes-Liu (Type 1) or Truss (Type 3)► Tied Node-to-Surface contacts at both ends► More realistic than constrained nodes – pressure between
trunnion girder and pier changes during the earthquake
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Post-Tensioned Anchorage
Hughes-Liu beams use *INITIAL_STRESS for post-tensioning
► 100% Applied initialization – no option to ramp with gravity loads
Truss elements require pressure loads on surfaces to simulate post-tensioning
► Stress in beam is the change from the post-tensioning stress
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Design
Nodal contact forces recorded at pier/slab interface and two higher contacts
Force and moment demands calculated for each nodal group at each output time (dt = 0.01sec)
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Design
Site constraints required an optimized reinforcing design
Generate an interaction diagram for each reinforcing pattern
Axial force determines moment capacity and affects shear capacity
This design would have been much more difficult without LS-DYNA
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Running the ModelStep 1
Run the model with gravity loads first► Use *LOAD_BODY_PARTS to apply gravity to
everything except the foundation Apply Single Point Constraints (SPCs) at all
boundaries► *DATABASE_SPCFORC
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Running the ModelStep 2
Apply the equilibrium forces to the model► *LOAD_NODE_POINT with output in the spcforc database► Ramp these forces on the same load curve as the gravity
loads► *BOUNDARY_NON_REFLECTING
should replace all SPCs• This allows the seismic
waves to exit the model, simulating anunbounded condition
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Running the ModelStep 3
Apply the seismic loads ► *LOAD_SEGMENT_SET_NONUNIFORM► Each direction of motion has its own load
curve
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Seismic Input Selection of Time Histories
► Characterize Design Earthquake Magnitude► Distances from source to site► Subsurface conditions► Duration of Strong Shaking► Available Records or Simulated Time Histories► Deterministic and Probabilistic
Deterministic MCE’s (3 records/per direction) Probabilistic OBE’s (3 records/per direction)
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Seismic Input Seismic Input Methods
► Displacement Time History► Velocity Time History► Acceleration Time History► Force (or Stress) Time History (preferred)
Non-Reflecting Boundary
DAM
HORIZONTAL PLANE FOR GROUND MOTIONS
Ground
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Seismic Input Seismic Input Location and Minimum Foundation Size
► Plane within foundation (*NODE_SET)► Deconvolved ground motions► Methods used to Deconvolve (Typ. 2D)
3H C 3H
2L
H
2H
C/2
L
45° 45°
NR
NR = Non-Reflecting Boundary
NR
H = Dam Height
C = Crest Length
L = Length and Depth of Foundation Model
___ = Horizontal Plane to Apply Ground Motion
NR Note: If model is too narrow seismic energy will exit through side of model.
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Seismic Input Modifying Time Histories to Develop Design Records
► Simple (Uniform) Scaling• Determine Natural Period of Structure• Deconvolved earthquake applied to foundation model w/o
structure to develop response spectrum • Compare recorded and smooth design spectrums• Apply single factor so that response spectrum of scaled motion
is a close match to design spectrum at the natural period► Disadvantages
• More EQ records required (min. of 3)• Natural Period of structure must be determined• Agreement of response spectrums could vary significantly at
other periods• Scaling for different directions of motion (1 factor for all
directions vs. different factors for each direction)
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Seismic Input
► Spectral Matching (preferred method)• Modifying frequency content of input motion so that recorded
response spectrum is a close match to the design response spectrum at all periods
• Deconvolved vs. Free Field Motion► Advantages
• Sufficient to have one time history for each direction• Multiple structures at a site with varying periods would not need
scaling for each structure• The energy of the time history is not greatly altered
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Seismic Input► Precautions
• Ensure the character of the scaled record in the time domain is fairly similar in shape, sequence, and number of pulses with respect to the original time history.
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Seismic Input Spectral Matching Procedure
► Outcrop acceleration time history for each component ► FFT of Outcrop acceleration time history► Apply Outcrop motion at depth in model as force time
history and record acceleration of node on surface of foundation model
► FFT of computed acceleration time history► Compute correction factor in Frequency Domain as the ratio
of the Outcrop to Computed motion amplitudes► Apply correction factor to the input motion in the frequency
domain► Inverse FFT of corrected motion to return to time domain► Compute corrected force time history► Repeat procedure if necessary
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Seismic InputAcceleration Response Spectra - Target vs. Computed: won-95, 5% Damping
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.01 0.1 1 10
Period (sec)
Pse
ud
o A
bs.
Acc
. (g
)
w on95 Free Field
Original Run
1st Iteration
Horizontal DesignSpectrum
Example of Spectrally Matched Ground Motions
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Acceleration Repsonse Spectra - Target vs. Computed: cpe-237, 5% Damping
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.01 0.1 1 10
Period (sec)
Pse
ud
o A
bs.
Acc
. (g
)
cpe-237 Free Field
Original Run
1st Iteration
Horizontal DesignSpectrum
Seismic Input
Example of Spectrally Matched Ground Motions