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Analysis and Design of Large-Scale Civil Works Structures Using LS-DYNA®

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## Analysis and Design of Large-Scale Civil Works Structures Using LS-DYNA®

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**PRESENTED BY THE**U.S. ARMY CORPS OF ENGINEERS Analysis and Design of Large-Scale Civil Works Structures Using LS-DYNA® Eric Kennedy, P.E. Structural Engineer Sacramento District Ryan Tom American River Design Sacramento District David Depolo, M.S., P.E. Structural Engineer Sacramento & Philadelphia Districts Thomas Walker, P.E. Structural Engineer Sacramento District NON-PRESENTING CO-AUTHORS LSTC International Users’ Conference June 7, 2010**Introduction**• Project overview • The JFP model • Properties • Troubleshooting & Lessons Learned • Designing from the model • Running the model • Seismic input**The Folsom JFP LS-DYNA ModelOverview**Reservoir (*MAT_NULL) Control Structure Foundation (*MAT_ELASTIC, E = 3500ksi) Backfill (*MAT_PSEUDO_TENSOR) Shear Zone (*MAT_ELASTIC, E = 324ksi)**The Folsom JFP LS-DYNA ModelControl Structure**Non-Flow Monoliths Flow-Through Monoliths Non-Flow Monolith**The Folsom JFP LS-DYNA ModelFlow-Through Monoliths**Headwall Piers (Designed using LS-DYNA output) Trunnion Girders Pier Struts (Designed using LS-DYNA output) Radial Gates (Rigid, defined individually) Invert Slab Gate Arms**Rigid Bodies & SOFT**• SymptomUnrealistic spikes in forces at the radial gate Corrected Peak force/length along pier during earthquake**Rigid Bodies & SOFT**• ReasonsGates defined using *MAT_RIGIDReservoir 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**Reservoir Contacts**• SymptomDuring an earthquake, some fluid elements lose pressure • ReasonStructure displacements created a free surface**Reservoir Contacts**• Solution1. Split the reservoirat monolith joints 2. Define a contact surface between reservoir parts**Reservoir Contacts**• For troubleshooting, split contacts so you can focus on problem areas(each conduit has its own set of contacts) • For verification, split contacts into pieces that are easily replicated with a calculatorHSF = 0.5*γ*H2*b**Hydrostatic Pressures**• Complex topography can cause incorrect pressures • Idealized geometry ensures the loads to the structure are more realistic**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**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**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**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)**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**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**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_REFLECTINGshould replace all SPCs • This allows the seismicwaves to exit the model, simulating anunbounded condition**Running the ModelStep 3**• Apply the seismic loads • *LOAD_SEGMENT_SET_NONUNIFORM • Each direction of motion has its own load curve**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)**Ground**DAM HORIZONTAL PLANE FOR GROUND MOTIONS Non-Reflecting Boundary Seismic Input • Seismic Input Methods • Displacement Time History • Velocity Time History • Acceleration Time History • Force (or Stress) Time History (preferred)**NR**Seismic Input • Seismic Input Location and Minimum Foundation Size • Plane within foundation (*NODE_SET) • Deconvolved ground motions • Methods used to Deconvolve (Typ. 2D) Note: If model is too narrow seismic energy will exit through side of model.**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)**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**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.**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**Seismic Input**Example of Spectrally Matched Ground Motions**Seismic Input**Example of Spectrally Matched Ground Motions