Performance Based Design and Evaluation of Concrete Dams Yusof - - PowerPoint PPT Presentation

Performance‐Based Design and Evaluation of Concrete Dams

Yusof Ghanaat

Quest Structures, Inc.

Orinda, California, USA

Performance‐based Design

Standard Procedures

• Are based on simple stress

checks and limit equilibrium analysis to avoid collapse and protect life

• Performance and hazard are

not well defined

• May be used for non‐critical

dams and screening purposes

Performance‐based Procedures

• Design/evaluate a dam to have

a predictable performance for specified levels of seismic hazard

• Acceptance based on

substantiated performance level in terms of damage thresholds, permanent displacements, not FoS

Presentation Outline

• Design/Evaluation Criteria

– Ground motions – Performance levels – Performance goals – Design requirements – Analysis and evaluation procedures

• Performance evaluation of a gravity dam

– Damage thresholds with linear/nonlinear analysis

• Performance evaluation of an arch dam

– Damage thresholds with linear/nonlinear analysis

• Summary

Design/Evaluation Earthquakes

• OBE – Operating Basis Earthquake

– 50% probability of exceedance in service life of structure – Serviceability performance

• MDE – Maximum Design Earthquake

– 1/3,000 to 1/10,000 AEP recommended by USSD – For dams it is taken equal to MCE – Damage control performance

• MCE – Maximum Credible Earthquake

– Largest EQ on a specific fault or seismic source zone – Background seismicity, “random EQ”, or “floating EQ” – Collapse prevention

Design/Evaluation Earthquakes

84th % Cascadia Interface has a 3000‐yr RP 50th % Cascadia Interface has a 900‐yr RP

Design/Evaluation Earthquakes

• Local fault 84th % ground

motion exceeds 10,000‐year UHS

• 84th % Cascadia has a RP ≈

3,200 years

Design/Evaluation Earthquakes

84th% New Madrid ground motion at the site has a RP of just less than 2,475 years

Design/Evaluation Earthquakes

Median “Random EQ” MCE in EUS exceeds 10,000‐year UHS

Performance Levels

• Serviceability Performance

– Operable immediately after OBE level earthquakes

• Damage Control Performance

– Damaged but not lose load resistance capability

• Collapse Prevention Performance

– Damage and movements could be severe but not lead to collapse

Damage Control Performance

(Non-linear Strain Hardening Range)

Collapse Prevention

(Non-Linear Strain Softening Range)

Displacement

Serviceability Performance

(Linear Elastic Range)

Load

Performance Requirements

Displacement

Serviceability Performance

(Linear Elastic Range)

Load

Ground Motion

Elastic Deformation

Performance Requirements

Damage Control Performance

(Non-linear Strain Hardening Range)

Displacement

Serviceability Performance

(Linear Elastic Range)

Load

Ground Motion

High Tension Regions

Tensile Overstressing & Cracking

Performance Requirements

Damage Control Performance

(Non-linear Strain Hardening Range)

Collapse Prevention

(Non-Linear Strain Softening Range)

Displacement

Serviceability Performance

(Linear Elastic Range)

Load

Sliding & Rotational Stability

Performance Goals

• Both strength and

serviceability should be considered

– Strength requirement ensures that failure in shear, flexure, tension, or compression will not occur – Serviceability requirement is to ensure the project will function without interruption, with little

• r no damage.
• Limited ductile behavior

– Characterized by an elastic range and limited inelastic range followed by a complete

• r significant loss of strength

– Dams exhibit limited‐ductile behavior in flexure and brittle behavior in shear – But some residual capacity may still exist due to dead load effects that contribute to shear‐friction resistance and to

• verturning resistance

Design Requirements

• Serviceability design

– Reduce the probability of damage to an acceptable level – Select an appropriate OBE event in combination with appropriate design/evaluation procedures and loading combinations

• Strength design

– Reduce the probability of collapse to an acceptable level – Select an appropriate MDE event in combination with specific design/evaluation procedures and loading that ensure the structure will perform as intended

QS = QD + QL + QOBE QDC = QD + QL + QMDE

Tiered Analysis and Evaluation

• Estimate peak responses
• Evaluate using DCR’s

Response Spectrum Analysis

• Identify potential failure modes
• Evaluate using damage control Thresholds

Linear Time‐History Analysis

• Model/analyze major nonlinear mechanisms
• Estimate permanent offsets/compressive stresses
• Perform post‐earthquake stability

Nonlinear Time‐History Analysis

Allowable DCR Values for RS

Action In terms of Stresses Performance Objectives Serviceability (OBE) Damage Control (MDE) Tension due to flexure 1.0 1.5 Diagonal tension due to shear 0.8 0.9 Shear due to sliding 0.8 1.0

Gravity Dams

Damage Threshold

DCR < 1, Minor or No Damage

• Nearly linear‐elastic response

DCR < 2, Damage Acceptable If

• Overstress < 15%
• CID < 0.3(2‐DCR)

DCR > 2 , CID > Threshold

• Damage is Significant
• May need nonlinear analysis or retrofit

Example Gravity Dam

Height = 74 m Crest Thickness = 9.75 m Base Thickness = 53.83 m

Earthquake Ground Motions

Response Spectra and Acceleration TH’s

Linear Time‐History Stresses

• Max. = 855 psi (5.9 MPa)
• Max. = 1,800 psi (12.4 MPa)

Comparison with Thresholds

Nonlinear Analysis Stress Contours

200 psi = 1.4MPa Gap‐friction Elements

Nonlinear Response Histories

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 5 10 15 20 Time (sec) D isp la ce m e n t (in )

20 mm

Arch Dams

Damage Threshold

DCR < 1, Minor or No Damage

• Nearly linear‐elastic response
• Minor joint opening

DCR < 2, Damage Acceptable If

• Overstress < 20%
• CID < 0.4(2‐DCR)

DCR > 2 , CID > Threshold

• Damage is Severe
• May need nonlinear analysis or retrofit

Arch Dams

Nonlinear Time History Evaluation

Overstressing

• Contraction joint opening
• Increased cantilever stresses
• Unstable cantilever blocks

Sliding Stability

• Along dam‐foundation interface
• Foundation/abutment rock wedges

Acceptance Criteria

• Compressive stresses
• Amount of joint opening
• Non‐recoverable block movements

Example Arch dam

Height = 468 ft (142.6 m) Crest Length = 724 ft (220.7 m) Crest Thickness = 12 ft (3.7 m) Base Thickness = 52 ft (15.8 m)

Arch Dam Linear Response

Exceeds Damage Threshold

Arch Dam Nonlinear Response

Contraction and lift joints repeatedly open and close

Summary

• A performance‐based approach

was introduced for seismic design and evaluation of concrete dams

• How to design/evaluate dams to

have a predictable performance for specified levels of seismic hazard was outlined

• Three seismic performance

levels: serviceability, damage control, and collapse prevention were defined

• Analysis and evaluation

procedures for each performance level with corresponding acceptance criteria were discussed with examples

• These guidelines and provisions

were developed for the USACE and are being used by Federal, States, and dam engineering community in the United States and abroad

Linear Time History

Damage Control Threshold