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