Performance-Based Seismic Assessment and Retrofit of Buildings

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Performance-Based Seismic Assessment and Retrofit of Buildings

By Murat Saatcioglu

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Seismic Risk

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 A large proportion of current infrastructure consists of non-ductile reinforced concrete frames and/or masonry buildings, especially those;

 built prior to the enactment of modern seismic codes or;

 built in areas where code enforcement can not be ensured.

 It is economically not feasible and practical to replace a large inventory of seismically deficient infrastructure with new and improved structures.

 Therefore, seismic retrofitting (upgrading) remains to be the only viable approach to seismic risk mitigation.

 Seismic risk mitigation is a multi-phase process that consists of i) seismic evaluation and ii) seismic retrofitting. Seismic evaluation includes seismic screening and detailed analysis for vulnerability assessment.

Infrastructure Mitigation

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Performance-Based Design and Assessment

Hazard Levels

Performance Levels

Performance Objectives

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Seismic Hazard Levels

Hazard Level Probability of

Exceedance Mean Return Period in Years

BSE-1E 20% in 50 years 225

BSE-2E 5% in 50 years 975

BSE-1N 10% in 50 years 475

BSE-2N 2%in 50 years 2475

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Structural Performance Levels (ASCE-41)

0

Immediate

Occupancy Life

Safety Collapse Prevention

Damage

Control Limited Safety

Q

Drift Ratio

More accurate

deformation limits are available in the form of strains, rotations and chord angles at the element level.

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Immediate Occupancy Performance Level describes the damage state where structure is safe to be re-occupied having suffered minor damage to the structural elements. For

reinforced concrete buildings, inter-storey drift of 1% and 0.5% provide approximate but reasonably accurate estimates of this performance level for frame and shear wall buildings, respectively.

Life Safety Performance Level describes the damage state where significant damage

occurs to the structure with extensive cracking and hinge formation in primary structural elements of reinforced concrete buildings while maintaining life safety of the occupants.

For these buildings 2% and 1% lateral drift ratios provide reasonably accurate estimates of life safety performance level for frame and shear wall buildings.

Structural Performance Levels

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Collapse prevention Performance Level describes the damage state occurs

immediately before the onset of partial or total collapse with extensive cracking, hinge formation and reinforcement buckling in concrete structural elements.

Though 4% and 2% drift ratios are often associated with this level of

performance for reinforced concrete frame and shear wall buildings, stability failure may sometimes occur prior to reaching these deformation limits.

Structural Performance Levels

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 Operational Performance Level describes negligible damage to non-structural elements, such as building cladding, windows and masonry partitions, so that the occupants continue using the building during and after the earthquake.

 Position Retention Performance Level describes more extensive damage. Yielding and distortions of connections, as well as extensive cracking and damage is expected.

However the non-structural elements, including suspended ceilings remain in their position.

 Life Safety Performance Level describes sever cracking and distortions in elements.

Windows crack, pilasters spall off in local areas, masonry elements dislodge, local

crushing and spalling of concrete and masonry occurs. However, the damage does not cause life safety hazard.

Nonstructural Performance Levels

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 It is generally accepted that the cost of achieving higher safety is often disproportionate to the incremental benefits achieved.

 Existing buildings often have less number of years left in their economic life, as compared to new buildings, which are designed based on a 50-year economic life.

 Meeting the new code requirements may render the building unsafe shortly after it is built if the code requirements were to change.

 Reduced or Enhanced Performance Objectives may be adopted.

Performance Objectives for Existing Buildings

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 Basic Performance Objective (BPOE): Buildings meeting the BPOE are expected to experience little damage from relatively frequent, moderate earthquakes but

significantly more damage and potential economic loss from the most severe and infrequent earthquakes that could affect them. The level of damage and potential

economic loss experienced by buildings rehabilitated to the BPOE likely will be greater than that expected in similar, properly designed and constructed new buildings or

existing buildings evaluated and retrofit to the Basic Performance Objective Equivalent to New Building Standards (BPON).

Performance Objectives for Existing Buildings

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Performance Objectives for Existing Buildings

 Enhanced Performance Objectives: A seismic evaluation that demonstrates

compliance with or a retrofit that provides building performance exceeding that of the BPOE.

 Reduced Performance Objectives: A seismic evaluation or a retrofit that addresses the entire building structural and nonstructural systems, but uses a lower selected Seismic Hazard Level or lower target Building Performance Level than the BPOE.

 Basic Performance Objective Equivalent to New Building Standards (BPON): For new buildings or existing buildings retrofitted to perform similar to new buildings.

This is a special case of “Enhanced Performance Objective.”

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Seismic Hazard

Level Operational

Performance Immediate

Occupancy Life Safety Collapse Prevention

50% in 50 Years a b c d

20% in 50 Years

BSE-1E e f g h

5% in 50 Years

BSE-2E i J k l

2% in 50 Years

ASCE-7 MCE_g m n o p

Basic Performance Objective for Existing Buildings (BPOE): g and l Enhanced Objectives: 1) g and i, j, m, n, o, or p; 2) l and e or f

3) g and l and a, or b; 4) k, m, n, or o alone Limited Objectives: 1) g alone; 2) l alone

3) c, d, e, or f

Performance Objectives for Existing Buildings

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Seismic Evaluation Methodologies used in Canada

In Canada a two-step approach is used:

 Seismic Screening: A large inventory of seismically deficient buildings exist in Canada.

Buildings designed prior to the enactment of modern seismic codes of post late 1970 and early 1980 era are vulnerable to seismic damage. It is not practical to conduct structural analysis of a large number of older buildings. Hence, a quick seismic screening procedure is employed to establish priorities for further evaluation of fewer select buildings.

 Detailed Structural Analysis: Those buildings found to have a high score of Seismic

Priority Index established by screening are analyzed further by either static or dynamic procedures, following either elastic or inelastic approaches.

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Screening for Seismic Risk

Seismicity, A=1.0 to 4.0

Soil Conditions, B=1.0 to 2.0 Type of Structure, C=1.0 to 3.5 Building Irregularities, D=1.0 to 4.0 Building Importance, E=1.0 to 3.0 Structural Index (SI) = A B C D E

Non-Structural Hazard, F= larger of F1 and F2 Falling Hazards to life, F1=1.0 to 6.0 Hazards to Vital Operations, F2=1.0 to 6.0 Non-Structural Index (NSI) = B E F

Seismic Priority Index (SPI) = SI + NSI

Rock, Stiff Soil, Soft soil,

Liquefiable Soil, Unknown Soil Effective Seismic Zone

Wood, Steel Concrete, Precast, Masonry Infill, Masonry

Vertical, Horizontal, Short Column, Soft Story, Pounding, Modifications, Deterioration, None

Occupancy and Operational Requirements

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(1) SCREENING

( Seismic Priority Index, SPI ) SPI< or = 10 Evaluation Priority “Low” 10<SPI< or = 20 Evaluation Priority “Medium” SPI >20 Evaluation Priority “High”

(2) EVALUATION

Upgrading

Needed ? Inventory

(3) Design and construction for Upgrading

YES NO

“Low”

“Medium” or “High”

Screening for Seismic Risk

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Seismic Screening Software Developed by uOttawa

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Fuzzy-Logic Based Seismic Risk Assessment Tool; CanRisk Developed at uOttawa

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An Overview of Seismic Retrofit

Techniques Developed @ uOttawa

By Murat Saatcioglu

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Seismic Retrofit Strategy Employed

 Retrofit of individual non-ductile elements for increased strength and deformability.

 Bracing the structure to increase its seismic resistance while controlling lateral drift so that non-ductile members are not forced beyond their elastic limits.

 Research was conducted to develop strategies in both categories:

 Column jacketing with FRP

 External transverse prestressing (RetroBelt) of columns

 Column retrofit with high-strength steel straps

 Shear wall retrofit with steel strips

 Lateral bracing of non-ductile frames with BRB

 Lateral bracing of frames with progressively engaging steel strands

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FRP Jacketing of Columns

Examples of field applications

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FRP Jacketing of Columns

Square and circular flexure-dominant and shear dominant columns were tested

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-800 -600 -400 -200 0 200 400 600 800

-8% -6% -4% -2% 0% 2% 4% 6% 8%

Drift Ratio

Moment, M (kN-m)

BR-SS-R

Short Square Column L = 1500 mm

Continuous bars

3 CFRP plies (wrapping) Axial Load = 1160 kN (15% P )

h = 500 mm

M F P

M=FL+P L

BR-SS-R

Short Square Column L = 1500 mm

Continuous bars

3 CFRP plies (wrapping) P = 1160 kN (15% Po)

FRP Jacketing of Columns

Shear critical square columns

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Retrofit for Enhancement of Concrete Confinement

δ = 0.040 for ductile moment resisting frames (R

_d

= 4 )

δ = 0.025 for moderately ductile moment resisting frames (R

_d

= 2.5) k

_c

= 1.0 for circular and oval columns

k

_c

= 0.4 for square and rectangular columns

CSA S806-12 Requirement

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Hardware required

External Prestressing (RetroBelt)

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1

2

3 4

Installation on Circular and Square Columns

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Shear Critical Columns with and without RetroBelt

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𝑉𝑉_𝑛𝑛 = 𝑉𝑉_𝑐𝑐 + 𝑉𝑉_𝑠𝑠 + 𝑉𝑉_𝑝𝑝

𝑉𝑉_𝑐𝑐 = 0.2 𝑓𝑓_𝑐𝑐^′𝑏𝑏_𝑤𝑤𝑑𝑑 𝑉𝑉_𝑠𝑠 = 𝐴𝐴_𝑣𝑣𝑓𝑓_{𝑦𝑦𝑦𝑦}𝑑𝑑 𝑠𝑠 𝑉𝑉_𝑝𝑝 = 2𝐴𝐴_𝑝𝑝(𝑓𝑓_{𝑝𝑝𝑝𝑝} + 0.002𝐸𝐸_𝑝𝑝) ℎ

𝑠𝑠_𝑝𝑝

50 𝑀𝑀𝑀𝑀𝑀𝑀 < 𝑓𝑓_{𝑝𝑝𝑝𝑝} ≤ 0.5𝑓𝑓_{𝑝𝑝𝑝𝑝}

Transverse Prestressing of Columns (RetroBelt) Design for Shear

Design for shear involves computing the contributions of concrete, internal shear reinforcement and external prestressing.

𝑠𝑠_𝑝𝑝 ≤ ℎ 4 𝑉𝑉_𝑛𝑛 ≤ 𝑉𝑉_𝑐𝑐 + 0.66 𝑓𝑓_𝑐𝑐^′𝑏𝑏_𝑤𝑤𝑑𝑑

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Confinement Deficient Columns with and without RetroBelt

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𝐴𝐴_{𝑝𝑝𝑠𝑠} = 2.1𝑠𝑠_𝑝𝑝ℎ 𝑓𝑓_𝑐𝑐^′

𝑓𝑓_{𝑝𝑝𝑝𝑝} + 𝜀𝜀_𝑦𝑦𝐸𝐸_𝑝𝑝 𝑀𝑀_𝑝𝑝

𝑀𝑀_𝑜𝑜 𝛿𝛿 𝜀𝜀_𝑦𝑦 ≤ 0.003.

Transverse Prestressing of Columns (RetroBelt) Design for Confinement Enhancement

Design for confinement is based on an earlier “displacement-based design expression developed by Saatcioglu and Razvi and adopted in ACI-ITG4 document for confinement of columns. It involves the computation of active and passive lateral confining

pressures.

𝐴𝐴_𝑔𝑔

𝐴𝐴_𝑐𝑐 − 1 ≥ 0.3 𝑀𝑀_𝑝𝑝

𝑀𝑀_𝑜𝑜 ≥ 0.2 𝑠𝑠 ≤ ℎ_𝑐𝑐

4 𝑜𝑜𝑜𝑜 150 𝑚𝑚𝑚𝑚 𝛿𝛿 = 0.025

𝛿𝛿 = 0.04

For moderately ductile columns For fully ductile columns

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Field Application

Transverse Prestressing of Columns (RetroBelt)

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Column Retrofit with Steel Straps

Column retrofit with high-strength steel

straps

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Precast concrete raiser units were manufactured to complete square and rectangular sections to circular and elliptical shapes.

Column Retrofit with Steel Straps

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Retrofit of a circular column with single straps

@ 7% Drift

Column Retrofit with Steel Straps

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Retrofit of a square column with double straps

@ 7% Drift

Column Retrofit with Steel Straps

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Development of a New Buckling Restrained Brace (BRB)

Innovative end units

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Development of a New Buckling Restrained Brace

(BRB)

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Bracing Through Diagonal Prestressing

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Unretrofitted capacity : 270 kN

250 kN

Bracing Through Diagonal Prestressing

While diagonal

prestressing proved to control drift, the

capacity increase was limited due to the yielding of the cables.

Subsequent dynamic analysis of buildings indicated limited

benefits, because the increase in stiffness shortened the period, which in turn attracted higher seismic forces.

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Lateral Bracing Through Progressively Engaging Cables (PEC)

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Lateral Bracing Through Progressively Engaging Cables (PEC)

Preliminary Analytical Investigation

 Case (a): First cable to engage at 1% drift and the other two at 1.5%.

 Case (b): First cable to engage at 1.5% drift and the other two at 2.0%.

 Case (c): First cable to engage at 2.0% drift and the other two at 3.0%.

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(a): 1% and 1.5%. (b): 1.5% and 2.0%.

(c): 2.0% and 3.0%.

Preliminary Analytical Investigation

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Lateral Bracing Through Progressively Engaging Cables (PEC)

Test 1 – Single 7-wire strand in each diagonal engaging at 1% drift

1% Drift

1% Drift

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Lateral Bracing Through Progressively Engaging Cables (PEC)

Test 1 – Single 7-wire strand in each diagonal engaging at 1% drift

At 2% Drift

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At 2% Drift

At 4% Drift

Test 2 – Single 7-wire strand engaging at 1.5% drift and additional double strands engaging at 2% drift

Lateral Bracing Through Progressively Engaging Cables (PEC)

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Lateral Bracing Through Progressively Engaging Cables (PEC)

Test 2 – Single 7-wire strand engaging at 1.5% drift and additional double strands engaging at 2% drift

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Questions and Comments…