aref presentation

SEISMIC PERFORMANCE OF A NOVEL PRECAST SEGMENTAL

CONCRETE BRIDGE

Amjad J. Aref, Ph.D.Professor, Department of Civil, Structural,

and Environmental EngineeringUniversity at Buffalo – State University of New York

September 20, 2010

PROJECT TEAM MEMBERS

Petros SiderisPh.D. Candidate, Dept. of CSEE at UB

Myrto AnagnostopoulouSenior Structural Engineer, SEESL at UB

Amjad J. ArefProfessor, Dept. of CSEE at UB

Andre FiliatraultProfessor, Dept. of CSEE at UB

OVERVIEWINTRODUCTIONEXPERIMENTAL INVESTIGATION

Prototype BridgeExperimental Bridge SpecimenTest Motions – Seismic HazardTest Execution

NUMERICAL MODELINGModeling Approaches for Segmental SystemsMulti-Element Approach (as proposed herein)

Simplified 2D Model of Bridge Specimen in RuaumokoSimplified 3D Model of Bridge Specimen in SAP2000

Preliminary Comparison of Numerical with Experimental ResultsConclusion

Experimental InvestigationNumerical Modeling for Segmental Systems

INTRODUCTIONPrecast Concrete Segmental Bridges

Brief History:First cast-in-place segmental concrete bridge, Germany (1950) to cross the Lahn River First precast segmental concrete bridge, France (1962) to cross the Seine RiverFirst application in the United States: John F. Kennedy Memorial Causeway in Corpus Christi, Texas (1973).

Advantages:Higher construction quality (precast plants)Rapid construction (Accelerated Bridge Construction - ABC)

Concerns:Effects of segmental joint response on global system stabilityReliability of existing analysis tools in predicting the 3D dynamic response of such systems (Performance-Based Design)

Analytical Studies + Large-scale bridge experiments Investigate these concerns and propose feasible alternatives

Superstructure Section

EXPERIMENTAL INVESTIGATIONPrototype Bridge (Megally et al. 2002)

Single-cell box girder bridge consisting of 5 spansEach span is post-tensioned with a harped shape tendonThe piers are square hollow sections of ~30 feet heightThe ‘Span-by-Span’ construction method has been assumed

EXPERIMENTAL INVESTIGATION

Front Elevation

EXPERIMENTAL INVESTIGATIONExperimental Bridge Specimen

Single-span bridge - Both of its supports overhanging at equal lengths (25% of span)Large-scale model (SL=2.4)Consists of:

Post-tensioned Deck (12 tendons):8 segments

Post-tensioned Pier (8 tendons):5 segmentsCap beamFoundation block

Deck simply supported on cap beamsSand bags to simulate additional loads

All post-tensioned together


Dimensions:61.875 ft long (pier-to-pier distance: 41.875 ft)14.125 ft high including cap beam and foundation block (16.489 ft including deck as well)

Design according to:AASHTO LRFD Bridge Design Specifications (2007)PCI Bridge Design Manual (2003) (partial assistance)

Response modifications factors:Superstructure: R=2.5 Substructure: R=3.75 (=1.5*2.5)Cap beam and foundation block designed to remain elastic under any load combination (capacity design)


Components (out of scale)Deck Segment Pier Segment

Cap Beam Foundation Block

Structural Engineering and Earthquake Simulation Laboratory (SEESL) at UB


Lateral restrainer

Sand Bag

Cap BeamPier

Foundation Block

Deck

EXPERIMENTAL INVESTIGATIONPost-tensioning system (Deck and piers)

2'-4

3/8

"

20'-11 1/4"10'-0"

1'-1

0 1/

2"10

'-0"

2'-3

"

T. 6

T. 7

T. 8 T. 10

T. 9

T. 5

T. 4

T. 3 T. 1

T. 2T. 11

T. 12

T. 14

T. 13

5'-9 3/4" 8'-4 1/2" 8'-4 1/2"8'-4 1/2"

30'-11 1/4"

1'-2 7/8"

9"

1'-2 7/8"

9"9"

Y

X

9"

10'-0

"2'

-3"

Actuator bolt holes

Ducts

9'-0"

C.M.

11'-0 3/8"

4'-2

7/8

"

Stopper against lateral sliding

EXPERIMENTAL INVESTIGATIONNovelties:

Internal unbonded tendons (super- and substructure): Strain distributed over larger lengths

Higher ductility Enhanced self-centering capabilities

Moment arm is maintained StabilitySegmental joints (super- and substructure):

Simple plane surface-to-plane surface contact Negligible/No tensile strength gap opening is allowedRelative segment sliding

(i) Multi-level seismic “isolation”(ii) Restoring force provided by the tendons (dowel effect)

Vertical seismic design spectrumConsidered in MCEER/ATC Joint Venture (MCEER-03-SP03, 2003), but not in AASHTO 2007Vertical design spectrum 2/3 of Horizontal

EXPERIMENTAL INVESTIGATIONTest Motions – Seismic Hazard

Ground motion (GM) ensemblesSubset of FEMA P695 Far-field GM set (5 out of 22 motions)Subset of FEMA P695 Near-field GM set (6 out of 28 motions – 3 with and 3 without pulse)Subsets were selected to be “representative” of the full sets

Both GM sets scaled to Seismic Hazard Levels:MCE (2% in 50 yrs) DBE (10% in 50 yrs)Intermediate DBE (R=2.5 – 57% in 50 yrs) Low DBE (R=3.75 – 88% in 50 yrs)

Asynchronous base excitationTime delay between the time instants that the seismic wave reaches each piers

Δt1=0.05 sec (based on soil profile at the site)Δt2=0.5 sec (amplified)

EXPERIMENTAL INVESTIGATIONTest Motions – Seismic Hazard

AASHTO 2007: Sa(T) for 10% in 50 yrs (DBE)Different hazard levels using FEMA 356 (at short periods)Scaled subset geometric mean spectra fitted to:

Real System Domain:Far-Field Subset fitted to AASHTO Spectra

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

T (sec)

Sa (g

)

2% in 50 yrs

10% in 50 yrs

88% in 50 years (4.1% per yr)57% in 50 years (1.6% per yr)

Model Domain:Far-Field Subset fitted to AASHTO Spectra

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

T (sec)

Sa (g

)

2% in 50 yrs

10% in 50 yrs88% in 50 years (4.1% per yr)

57% in 50 years (1.6% per yr)

Real Structure Domain:Near-Field Subset fitted to AASHTO Spectra

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5T (sec)

Sa (g

)

2% in 50 yrs10% in 50 yrs88% in 50 years (4.1% per yr)57% in 50 years (1.6% per yr)

Model Domain:Near-Field Subset fitted to AASHTO Spectra

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

T (sec)

Sa (g

)

2% in 50 yrs

10% in 50 yrs



Link: GM Ensemble

Tables

EXPERIMENTAL INVESTIGATIONTest Execution

Instrumentation:~ 220 channels

~ 40 Load Cells Post-tensioning forces~ 70 accelerometers Accelerometer~ 90 LVDTs gap opening and relative sliding~ 20 String Pots global system displacements

Achieved Test Protocol142 seismic tests176 identification tests (white noise: PGA~0.1g)


Achieved Test Protocol:Unloaded Specimen – no lateral restrainers:

Far-field ensemble: Low DBEIntermediate DBEDBE – Vertical onlyMCE – Vertical onlyUltimate loading: MCE for Vertical + 62.5% DBE (R=1.6) for horizontal

Near-field ensemble: Intermediate DBE

Unloaded specimen with Lateral restrainers in contact with deck:Far-field ensemble:

Low DBEFully Loaded specimen – Lateral restrainers at distance from deck:

Far-field:DBE – Vertical onlyMCE – Vertical onlyLow DBELow DBE – Asynchronous motion: Time delay of Δt=0.05 secLow DBE – Asynchronous motion: Time delay of Δt=0.5 sec

Partially Loaded specimen – Lateral restrainers at distance from deck:Far-field:

DBE – Vertical only


Recorded Dynamic Response1979 Imperial Valley Earthquake

Delta – UNAMUCSDMw=6.5Appropriately scaled in accordance with similitude assumptionsFar-Field Motion 2 – Ultimate Loading – Unloaded Specimen

VideosGeneral view - Test ABC_S1_SC_M2_XYZBase of west pier - Southwest corner - Test ABC_S1_SC_M2_XYZ_V05Base of east Pier - Northwest corner - Test ABC_S1_SC_M2_XYZ_V11

Test ExecutionDynamic Response

Far-Field Motion 2 – Ultimate Loading – Unloaded Specimen:

General view - Test ABC_S1_SC_M2_XYZBase of west pier - Southwest corner - Test ABC_S1_SC_M2_XYZ_V05Base of east Pier - Northwest corner - Test ABC_S1_SC_M2_XYZ_V11

Far-Field Motion 4 – MCE (Vertical only) – Unloaded specimen:

General view - Test_ABC_S1_FF4_M4_Z_eDeck mid-joint - Test_ABC_S1_FF4_M4_Z_V13

Far-Field Motion 4 – MCE (Vertical only) – Fully loaded Specimen:

General view – Test_ABC_S3b_FF4_M4_Z_eDeck mid-joint - Test_ABC_S3b_FF4_M4_Z_V13


Link: Accelerograms

for SC_M2

Link: Accelerogram

for FF4_M4


Recorded Dynamic Response1979 Imperial Valley Earthquake

Preliminary Conclusions:Maximum acceleration appears to be “bounded”Negligible permanent displacements

Total Acceleration - Longitudinal Direction (X) - West Pier

-1.5

-1

-0.5

0

0.5

1

1.5

0 10 20 30 40 50 60

Time

Acc

eler

atio

n (g

)

FoundationDeck

Total Acceleration - Lateral Direction (Y) - West Pier

-1.5

-1

-0.5

0

0.5

1

1.5

0 10 20 30 40 50 60

Time

Acc

eler

atio

n (g

)

FoundationDeck

Relative Displacement - Lateral Direction (Y) - West Pier

-6

-4

-2

0

2

4

6

0 10 20 30 40 50 60

Time

Dis

plac

emen

t (in

)

Cap-beamDeck


Recorded Dynamic Response1992 Landers Earthquake

Coolwater - SCEMw=7.3Appropriately scaled in accordance with similitude assumptionsFar-Field Motion 4 – MCE (Vertical only) – Unloaded specimen:


Recorded Dynamic Response1992 Landers Earthquake

Preliminary Conclusions:Resonance, but response is “bounded”Negligible permanent displacements

Vertical Total Acceleration - Mid-span

-6

-4

-2

0

2

4

6

0 5 10 15 20

Time

Acc

eler

atio

n (g

)

DeckFoundation

Vertical Total Acceleration - Mid-span (Zoom in)

-6

-4

-2

0

2

4

6

6 7 8 9 10 11 12 13 14

Time

Acc

eler

atio

n (g

)

Deck

Vertical Relative Displacement - Mid-span

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

0 2 4 6 8 10 12 14 16 18 20

Time

Dis

plac

emen

t (in

)

Deck

NUMERICAL MODELINGModeling Approaches for Segmental Systems

Finite Element Method Approach

3D solid elements and 2-node elementsNonlinear material propertiesContact interfaces – frictionSequential loading

Good accuracy, if convergence can be achievedExcessive computational resources

Analysis time - Storage capacityDifficult to use

Beam-Column Elements:Approach:

Equivalent plastic hinges (Px-My-Mz Interaction) to model joint opening and slidingSequential loading

Easy to useMay provide “fairly” good results, but cannot provide a general framework

NUMERICAL MODELINGModeling Approaches for Segmental Systems:

Beam-column elements with zero-length hertzian contact springs

Placed at the ends of beam-column elementDistributed the edge of the end cross-sectionsEasy to useMay provide “fairly” good results in some cases It cannot provide a general framework, since concrete crushing is not considered

Macro-element approach:It could provide an optimum balance amongst: Desired accuracyComputational resourcesSimplicityCurrently under development… (not presented herein !)

NUMERICAL MODELINGModeling Approaches for Segmental Systems:

Multi-Element Approach (as proposed herein)Concept:

Use elements from existing structural analysis softwareAvailable to practicing engineers and small/medium construction firms

Easy to useSequential loadingBetter performance than beam-column element approach (???)

This presentation will illustrates some partial findings of this research for the multi-element approach:

Structural analysis software:Ruaumoko 2D model SAP2000 3D model

NUMERICAL MODELINGMulti-Element Approach (as proposed herein)

Segment:Beam-column element for interior part of a segment

End plastic hinges with Px-My-Mz Interaction must be considered for consistency (not for joint opening and/or sliding!!!)

“Fiber Springs” to model end regions of segmentsCombination of 2-node friction/contact elements and hysteretic elementsAppropriately distributed over segment cross-section in parallel with each other and the neutral axis of the segment

Post-tensioning Strands:Tension-only truss elements with inelastic behavior Post-tensioning is applied as: initial stress or initial strain or initial temperature change

Appropriate inter-element boundary conditions:Rigid links:

Connect beam-column with fiber springsKeep tendons at proper position

Link: Schematic derivation of multi element modeling


Criteria for Distribution of “Fiber Springs”Equivalent Cross-Section moduli:

Maximum distance between fiber springs Resultant axial force should be able to run “smoothly” over the area of the cross-section – Partial contact should be allowedSymmetric distribution for symmetric sections

2 2

1

2 2

1

1

N

y i i iiAN

z i i iiA

N

i i iiA

EI Ez dA z E A

EI Ey dA y E A

EA E dA E A

=

=

=

⎧= ≈⎪

⎪⎪⎪ = ≈⎨⎪⎪

= ≈⎪⎪⎩

∑∫

∑∫

∑∫

Recommendation:Acceptable Error < 1%

10

Ni i

ii i

E AyL=

∑1

0N

i ii

i i

E AzL=

∑and


Length of fiber springs:~0.9d (from section depth), or ~1-1.5 t (from wall thickness)

Further calibration of “Fiber Springs”Axial Properties:

Stiffness:

Yield Force:

Shear Properties: According to friction properties at the segment-to-segment interface and segment shear properties

Bending Properties: Correspond to global segment properties as if is was modeled as a beam-column element

Torsional Properties: Correspond to global segment properties as if is was modeled as a beam-column element

,1

NBeam Column fiber

y y ii

F F−

=

=∑

1

NBeam Column fiber

ii

K K−

=

=∑


Simplified 2D Model of Bridge Specimen in RuaumokoLateral directionRelative joint sliding is not consideredShear is transferred by a pinned connection between the two segments. Pin is located at segmental interface

Rigid links

Contact element

“Fiber Springs”

Tendons

No sliding

½ Deck mass

Beam-Column element


Simplified 2D Model of Bridge Specimen in RuaumokoShort example - Fully loaded specimen

Segment interior: Beam-column elements with end plastic hinges (Px-My-Mz Interaction)Segment End discretization: 9 compression-only bilinear hysteretic “Fiber springs” of length (2x6”=12”)Tendons: Tension-only bilinear elements with slackness and initial loadingApplied motion:

N-S Component of 1940 El Centro recordProperly scaled (similitude) in time (x 1/2.388) and amplitude (x 2.388) PGA=1.62g (versus the original 0.34g)

Example resultsModal analysis: T1=0.224sec, T2=0.018sec and T3=0.009sec


Simplified 2D Model of Bridge Specimen in RuaumokoExample results

Dynamic analysis (ξ=3%):

Preliminary Conclusions:Total deck acceleration seems to be limited at 0.5 -0.6 gSystem, after minor concrete crushing at pier base, returns to its original position

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

0 2 4 6 8 10 12

t (sec)

Tota

l Acc

eler

atio

n (g

)

Base

Deck

-2.50-2.00-1.50-1.00-0.500.00

0.501.001.502.002.50

0 2 4 6 8 10 12

t (sec)

Rel

ativ

e D

eck

Dis

plac

emen

t (in

) .


Simplified 3D Model in SAP2000Segment ends:

Definition of “Fiber Spring”:Two 2-node Nonlinear Links in series

(i) Friction Isolator (lateral response)(ii) Multi-linear Plastic Spring with Kinematic Hardening (axial response)

Length: 2x6”=12” for piers, and 2x7.5”=15” for deckCross-section discretization:

Deck Pier

Link: Schematic derivation of multi element modeling


Simplified 3D Model in SAP2000Segment Interior:

Beam-column element with end plastic hinges (Px-My-MzInteraction)

PT Tendons:Beam-Column Steel Element, with initial strain to induce PT forces (Iyy=Izz=J=0 ~ truss element)

Material properties:Concrete: fc=6000 psi (unconfined)Steel: Fy=50 ksi

Segmental Joint and Deck-to-Cap beam interfaceFriction: μ=0.3

Model development:Sequential load application (post-tensioning, dead loads, live loads)

Beam Element passing through the center line of the deck cross-section –Extrude View

Beam Element passing through the center line of the pier cross-section –Extrude ViewSegment-to-segment

contact

Segment-to-segment contact


Simplified 3D Model in SAP2000

Friction ElementHysteretic Element

Beam - Column Elements (for tendons)


Simplified 3D Model in SAP2000Modal analysis:

Difference in the fundamental mode of the SAP2000 model with the Ruaumoko Model, mainly due to:

2D versus 3D modelInfinite shear stiffness at segmental joint in 2D model

Mode T (sec) Mode Shape / Deformation Characteristics

1st 0.284 Uniform lateral – Pier bending

2nd 0.163 Anti-symmetric lateral

3rd 0.150 Longitudinal – Pier and deck bending

4th 0.084 Vertical – Deck Bending

5th 0.040 Lateral – Lateral Deck Bending / Torsion of piers


Simplified 3D Model in SAP2000Dynamic analysis:

Applied motion: Motion 3 from FEMA P695 Far-Field GM Subset (Nishi-Akashi components - 1995 Kobe earthquake)

DBE Hazard LevelProperly scaled in time (x 1/2.388) and amplitude (x 2.388), due to similitude requirementsAfter all scaling: PGAx=251g, PGAy=2.47g, PGAz=1.81g

Rayleigh damping of ξ=3% assigned to the 1st and 4th

mode



Computed Response

-3

-2

-1

0

1

2

3

0 5 10 15

Time (sec)

Tota

l Acc

eler

atio

n X

(g)

Base AccelerationDeck

-3

-2

-1

0

1

2

3

0 5 10 15

Time (sec)

Tota

l Acc

eler

atio

n Y

(g)

Base AccelerationDeck

-5

-4

-3

-2

-1

0

1

2

3

4

5

0 5 10 15

Time (sec)

Rel

ativ

e D

ispl

acem

ent (

in)

LongitudinalLateralVertical



Preliminary conclusions:Total acceleration seems to be limited, mainly due to:

(i) Concrete crushing and relative segment sliding at joints

(ii) Sliding of deck on cap beamsLateral total acceleration does not exceed 0.3g –0.5gLongitudinal total acceleration does not exceed 0.9g -1.1gSystem returns to its original position

NUMERICAL MODELINGPreliminary Comparison of Numerical with Experimental Results

Modal analysisFundamental frequency comparison (Loaded???)

Deviation may be mainly attributed to Silicone sealant used at the segmental joint.If shear spring included:

T1=0.35 sec, T3=0.31 sec, T4=0.12 sec

NUMERICAL MODELING

SAP2000 Experiment

Mode T (sec) T (sec) Mode Shape / Deformation Characteristics

1st 0.284 0.36 Uniform lateral – Pier bending

3rd 0.150 0.27 Longitudinal – Pier and deck bending

4th 0.084 0.14 Vertical – Deck Bending

CONCLUSIONSExperimental Investigation

A novel bridge system consisting of post-tensioned superstructure and substructure was testedSystem was subjected to severe ground motions:

Deck: Survived with minor concrete crushing several MCE motionsPiers: Survived severe Far-field and Near-field motions

General characteristics of the response of segmental systems (associated with the novel structural concepts of this study), which originally observed from numerical analyses, proved to be valid experimentally as well:

Segmental joint opening and relative sliding have been observed to provide the system with enhanced self-centering capabilities and higher ductility capacityDamage on concrete segments was mainly spalling of the rebar cover and some crushing of concrete at base segments

CONCLUSIONSNumerical Modeling for Segmental Systems

A method to model efficiently segmental systems using existing structural analysis software widely available to practicing engineersSimilar approaches have been used in literature; however in this study, a general framework is attempted to be established consisting of rules and recommendations based on general principles of classical structural analysisThe proposed framework was used with two structural analysis programs: Ruaumoko and SAP2000. Both models appeared to capture the general trends of the response (which also observed experimentally)A comparison of the 3D SAP2000 model with some modal experimental results clearly showed the need for further refinement of this technique – Especially to capture sliding

ACKNOWLEDGEMENTSFederal Highway Administration of the U.S. Department of TransportationBodossaki FoundationSEESL Personnel (University at Buffalo)Joe Salvadori (DSI)Curt Haselton (California State University, Chico)David Welch (University at Buffalo)

Thank you!!!Questions?

aref presentation

Documents

d model of bridge specimen

pci bridge design manual

singlecell box girder

posttensioned deck

segments posttensioned

span construction method

ft long pier

ft high including cap