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BUILDING STRONG ® Analysis and Design of Large-Scale Civil Works Structures Using LS-DYNA® David Depolo, M.S., P.E. Structural Engineer Sacramento & Philadelphia.

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Presentation on theme: "BUILDING STRONG ® Analysis and Design of Large-Scale Civil Works Structures Using LS-DYNA® David Depolo, M.S., P.E. Structural Engineer Sacramento & Philadelphia."— Presentation transcript:

1 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 Engineer Sacramento District Eric Kennedy, P.E. Structural Engineer Sacramento District Ryan Tom American River Design Sacramento District PRESENTED BY THE U.S. ARMY CORPS OF ENGINEERS NON-PRESENTING CO-AUTHORS LSTC International Users Conference June 7, 2010

2 BUILDING STRONG ® Introduction Project overview The JFP model Properties Troubleshooting & Lessons Learned Designing from the model Running the model Seismic input

3 BUILDING STRONG ®

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5 The Folsom JFP LS-DYNA Model Overview Foundation (*MAT_ELASTIC, E = 3500ksi) Shear Zone (*MAT_ELASTIC, E = 324ksi) Backfill (*MAT_PSEUDO_TENSOR) Reservoir (*MAT_NULL) Control Structure

6 BUILDING STRONG ® The Folsom JFP LS-DYNA Model Control Structure Non-Flow Monoliths Non-Flow Monolith Flow-Through Monoliths

7 BUILDING STRONG ® The Folsom JFP LS-DYNA Model Flow-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

8 BUILDING STRONG ® Rigid Bodies & SOFT Symptom Unrealistic spikes in forces at the radial gate Peak force/length along pier during earthquake Corrected

9 BUILDING STRONG ® Rigid Bodies & SOFT Reasons Gates defined using *MAT_RIGID Reservoir is merged with the gate to obtain correct hydrostatic pressures Solution Optional Card A: SOFT = 0 uses a penalty formulation, interface stiffness is based on the bulk modulus

10 BUILDING STRONG ® Reservoir Contacts Symptom During an earthquake, some fluid elements lose pressure Reason Structure displacements created a free surface

11 BUILDING STRONG ® Reservoir Contacts Solution 1. Split the reservoir at monolith joints 2. Define a contact surface between reservoir parts

12 BUILDING STRONG ® 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 calculator HSF = 0.5* γ *H 2 *b

13 BUILDING STRONG ® Hydrostatic Pressures Complex topography can cause incorrect pressures Idealized geometry ensures the loads to the structure are more realistic

14 BUILDING STRONG ® 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

15 BUILDING STRONG ® 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

16 BUILDING STRONG ® 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

17 BUILDING STRONG ® 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)

18 BUILDING STRONG ® 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

19 BUILDING STRONG ® Running the Model Step 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

20 BUILDING STRONG ® Running the Model Step 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 an unbounded condition

21 BUILDING STRONG ® Running the Model Step 3 Apply the seismic loads *LOAD_SEGMENT_SET_NONUNIFORM Each direction of motion has its own load curve

22 BUILDING STRONG ® 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 MCEs (3 records/per direction) Probabilistic OBEs (3 records/per direction)

23 BUILDING STRONG ® 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

24 BUILDING STRONG ® Seismic Input Seismic Input Location and Minimum Foundation Size Plane within foundation (*NODE_SET) Deconvolved ground motions Methods used to Deconvolve (Typ. 2D) NR Note: If model is too narrow seismic energy will exit through side of model.

25 BUILDING STRONG ® 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)

26 BUILDING STRONG ® 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

27 BUILDING STRONG ® 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.

28 BUILDING STRONG ® 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

29 BUILDING STRONG ® Seismic Input Example of Spectrally Matched Ground Motions

30 BUILDING STRONG ® Seismic Input Example of Spectrally Matched Ground Motions


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