Modeling Seismic Isolators in SAP2000
Manish Kumar, Ph.D., P.E., M.ASCEPostdoctoral Associate
Civil, Structural and Environmental Engineering University at Buffalo, State University of New York (SUNY)
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Outline
• Introduction• Isolator Models• Modeling in SAP2000• Example of Base-Isolated Building
– Modeling– Nonlinear time-history analysis
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Introduction
Fixed-base
Base-isolated
Seismic isolator(lead-rubber bearing)
Shaking
Shaking
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Isolator Models
• Rubber isolator– Low damping rubber (LDR) bearings– Lead-rubber (LR) bearings
• Friction isolator• T/C friction isolator• Triple pendulum isolator
Internal section view of lead-rubber bearing
Friction (single pendulum) isolator
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Physical Model
• 2 Node, 12 DOFs• Connected by 6 springs
– Represents mechanical behavior in 6 directions
Physical model Discrete spring model
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Define in SAP2000
• Modeled as Link Element in SAP2000– Define>>Section Properties>>Link/Support
Properties>>Add New Property• Mass and moment of inertia of isolators
– Generally assigned zero• Directional properties: U1, U2, U3, R1, R2, R3
– Mechanical behavior in six direction– U1: axial, linear only– U2, U3: shear directions, linear and nonlinear– R1: torsion about U1, linear only– R2, R3: rotations about U2 and U3, linear only
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Define in SAP2000
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Axial behavior: rubber isolator
• U1 directional property– Always linear
• Effective stiffness: options• Make it fixed (check fixed box)• Assign a large value
– Note: don’t assign unrealistic large values
• Calculate from bearing properties– 𝐾𝐾𝑣𝑣 = 𝐴𝐴𝐸𝐸𝑐𝑐
𝑇𝑇𝑟𝑟(See Constantinou et al (2007) for details)
• Effective damping– Usually 0% is recommended– 2-3% can be used for natural rubber
Axial behavior
𝐾𝐾𝑣𝑣𝑐𝑐
𝐾𝐾𝑣𝑣𝑓𝑓
AXIAL FORCE
AXIAL DEFORMATION
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Axial behavior: friction isolator
• U1 directional property– Always nonlinear– Compression only (gap)
• Effective stiffness: options• Make it fixed (check fixed box)• Assign a large value
– Note: don’t assign unrealistic large values
• Calculate as 𝐾𝐾𝑣𝑣 = 𝐴𝐴𝐸𝐸𝑐𝑐𝐻𝐻
– 𝐴𝐴 =area, 𝐸𝐸𝑐𝑐 = elastic modulus, 𝐻𝐻 = height
Axial behavior
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Shear behavior: rubber isolator
• U2, U3 directional properties– Coupled bidirectional Bouc-Wen
• Mechanical properties– Calculate from bearing properties
• See Constantinou et al (2007) for details
• Stiffness = 𝐾𝐾𝑒𝑒𝑒𝑒• Yield strength = 𝐹𝐹𝑌𝑌• Post Yield Stiffness Ratio = 𝐾𝐾𝑑𝑑/𝐾𝐾𝑒𝑒𝑒𝑒
Shear behavior10/34Modeling of Seismic Isolators in SAP2000
Manish Kumar
Shear behavior: friction isolator
• U2, U3 directional properties– Coupled bidirectional Bouc-Wen
• Effective stiffness: options• Calculate from bearing properties
– See Constantinou et al (2007) for details
• Friction can be varied– 𝜇𝜇 = 𝜇𝜇𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 − 𝜇𝜇𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 − 𝜇𝜇𝑓𝑓𝑒𝑒𝑠𝑠𝑠𝑠 𝑒𝑒−𝑟𝑟𝑓𝑓𝑓𝑓𝑒𝑒×�̇�𝑢
– Use same 𝜇𝜇𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓 and 𝜇𝜇𝑓𝑓𝑒𝑒𝑠𝑠𝑠𝑠 for no variation
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Time-history analysis
• Modal – Linear & nonlinear
• Modal nonlinear analysis– Also called fast nonlinear
analysis (FNA)– Appropriate for limited
nonlinear in members at concentrated locations (e.g, structures with dampers and base-isolators)
• Direct integration– Linear & nonlinear– Appropriate to capture
large nonlinear response of structures (e.g, collapse simulation)
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Time-history analysis
• Modal – Linear & nonlinear
• Modal nonlinear analysis– Also called fast nonlinear
analysis (FNA)– Appropriate for limited
nonlinear in members at concentrated locations (e.g, structures with dampers and base-isolators)
• Direct integration– Linear & nonlinear– Appropriate to capture
large nonlinear response of structures (e.g, collapse simulation)
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Example: base-isolated building
• Example from SAP2000 manual• Base-isolated building• Rubber isolator• Nonlinear time-history analysis
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Building model
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Material properties• Steel
– E=29000 ksi, Poissons Ratio = 0.3– Beams: W24x55; Columns: W14x90
• Rubber Isolator Properties– Vertical (axial) stiffness = 10,000 k/in– Initial shear stiffness in each director = 10 k/in– Shear yield force in each direction = 5 kips– Ratio of post yield shear stiffness to initial shear stiffness = 0.2
• Vertical Loading and Mass– Roof: 75 psf DL, 20 psf LL– Floor: 125 psf DL, 100 psf LL
• Time History– Apply ground motion simultaneously in X and Y directions
• Required– Obtain displacement histories at the floor level and the roof in X direction– Plot base-shear history vs displacement in Y direction
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Model development
• File>>New Model>>3D Frame
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Model development
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Define materials
• Define>>Materials– Default concrete and steel– Verify values
Concrete
Steel
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Define frame sections
• Define>>Section Properties>>Frame Sections– I/Wide Flange Sections>>SECTIONS.PRO– Import W24x55 and W14x90 sections by holding
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Define area sections
• Define>>Section Properties>>Area Sections– Shell>>Add New Sections>>Shell-Thin– Define two shell sections (note: change units to Kip, in, F)
• ROOF (membrane: 6”, Bending: 6”)• Floor (membrane: 10”, bending: 10”)
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Assign frame sections
• Select all beams and assign section W24x55• Select all columns and assign section W14x90
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Assign area sections
• Change to X-Y plane (top view)• Draw>>Quick Draw Area>>
– Click on each quadrant at level 3 of with ROOF section selected– Click on each quadrant at level 2 with Floor section selected
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Define Isolators• Define>>Section Properties>>Link/Support Property Data
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Assign Isolators• Draw>>Draw 1 Joint Link
– Select all the nodes at the bottom
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Define Load Patterns
• Define>>Load Patterns>>Add New Load Pattern– Add a LIVE load pattern
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Assign Loads• Change units to lb, ft, K• Select all objects at level 2
– Assign>>Area Loads>>Uniform (Shell): 125 DEAD and 100 LIVE in the gravity direction
• Select all objects at level 3 (Roof)– Assign>>Area Loads>>Uniform (Shell): 75 DEAD and 20 LIVE in the gravity direction
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Create Base Floor (level 1)
• Create base floor using Replicate command– Select everything at level 2 to be copied to level 1– Edit>>Replicate– Copies everything: frame, sections, load assignments etc.
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Assign Joint Constraints
• Create and assign 3 Diaphragm constraints for three levels– Select everything at level 1– Assign>>joint>>Constraints>>Add New Constraint (Diaphragm)
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Define Ground Motion
• Define>>Functions>>Time History>>>>Add New Function (From File)
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Define Load Case• Define>>Load Cases>>
– Modify existing MODAL load case to include ritz vector– Define a nonlinear gravity (GRAV) load case– Define a nonlinear time history (EQ) load case
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Run the analysis
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Post-processing
• Results can be obtained from DISPLAY menu– Show Response Spectrum Curves– Show Plot Functions
• To obtain time-history of response parameters
– Show Tables• To export data to excel/txt for post-processing
– Save Name Display Set• To obtain custom response parameters
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Thank You!Questions?
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