lessons learned from forensic fea of failed rc structures.pdf

8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf

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MotivationCase Studies of Forensic FEA

Conclusions

Motivation Tools for Structural Analysis

Problem StatementStructural failure continues to be a reality because critical limitstates are often undetected by engineering analysis.

Nonlinear Finite Element AnalysisState-of-the-art: Concrete compression crushing, tensilecracking, tension stiffening, steel reinforcement plasticity,

steel-concrete bond-slip, geometric nonlinearity, etc.Powerful tool but expensive, time-consuming, and largelyunavailable for practicing engineers

Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
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Conclusions

Motivation Tools for Structural Analysis

Linear Elastic Finite Element AnalysisAvailable to every practicing engineerCANNOT describe distribution of force, stress, &displacements at ultimate limit state ... but

CAN indicate existence of serious problems

Goal of PresentationDemonstrate key practical techniques:

3 case studies of real structural failureEvaluation using linear elastic FEAFeatures common to all structural engineering softwareDemonstration of failure to meet key performance criteria

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Conclusions

Parking Structure Shrinkage CrackingIndustrial Structure on Non-Uniform BearingPedestrian Bridge Collapse

Parking Structure Shrinkage Cracking

Case Study # 1:

Parking Structure Shrinkage Cracking

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Conclusions


Overview of Parking Structure Serviceability Failure

3-story parking deck, 95 meters 20 metersExtensive early-age cracking of slabsProbable cause of cracking: shrinkage

High w/c ratio + no expansion joints

Representative photograph:

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Conclusions


Parking Structure Finite Element Model Details

Model consisted of 24,000 shell elementsLoads: Gravity, temperature, shrinkage


k h k k
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Conclusions


Application of Shrinkage via Temperature Load

T sh = sh

sh = specied shrinkage strain

= coeff. of thermal expansion

For sh = 0 .0005 in in and = 5.5 10 6 / F T sh = 90 .9 F

Displacement Graphic (Red = deformed, Blue = undeformed):


M i i P ki S Sh i k C ki
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Conclusions


Investigate Means of Relieving High Slab Stresses

Top: t = 2600 psi Bottom: t = 2800 psi

:

: .

:

:


: , ,

:

:


Relieve shrinkage stress 3.5x by adding expansion joints


Motivation Parking Structure Shrinkage Cracking
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Conclusions


Parking Structure Shrinkage Analysis Conclusions

Shrinkage easily incorporated via temperature load in FEAShrinkage analysis would have suggested:

A spacing of expansion joints at 30 meters (vs. 95 meters)Construction sequence that would have reduced restraintShrinkage performance criteria in mix design


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Conclusions


Industrial Structure on Non-Uniform Bearing

Case Study # 2:

Industrial Structure on Non-Uniform Bearing


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Conclusions


Overview of Tall Industrial Structure

Cylindrical industrial structure on mat foundationSuperstructure: 550-ft tall; Mat: 100-ft wide and 8-ft thickSignicant displacements occurred during construction

Presence of non-uniform geological structure below mat:

Superstructure

Mat foundation

Rock Soil


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Conclusions


Model Characteristics

38,000 shell elementsLoads: Gravity, Wind, Seismic

P- effects neglectedCompression-only springs tosimulate supportSubgrade condition, compare:

Uniform subgrade modulus(neglect rock prole)Variable subgrade modulus


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Case Studies of Forensic FEAConclusions

g g gIndustrial Structure on Non-Uniform BearingPedestrian Bridge Collapse

Response Increase: Uniform vs. Variable Subgrade

Response Gravity+WindTip Lateral Displacement 73% increaseFoundation Settlement Displacement 46% increaseArea of steel required by Wood & Armer 58% increaseShear force through foundation section 395% increase


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g g gIndustrial Structure on Non-Uniform BearingPedestrian Bridge Collapse

Vertical Displacements in Mat Foundation

Gravity AloneMax uplift: 0.15 in.Max settlement: 1.91 in.

Gravity + WindMax uplift: 1.80 in.Max settlement: 3.74 in.


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Industrial Structure on Non-Uniform BearingPedestrian Bridge Collapse

Lateral Displacement at Top of Structure

Max Lateral DisplacementGravity: 11.5 in.Gravity + Wind: 34.8 in.

Contributions to Drift 81.8% Rigid body rotation 18.2% Flexure


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Pedestrian Bridge Collapse

Case Study # 3:



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Bridge collapse during placement of concrete deck in 200252 meter long, single steel tub girder bridgeFailure mode: global lateral torsional bucklingFEA conducted for Dr. Donald White at Georgia Tech


Motivationd f

Parking Structure Shrinkage Crackingd l f
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Pedestrian Bridge Finite Element Model

Use FEA to investigate stability of structureModel details: 22,000 elementsAssume weight (but not stiffness) of concrete


MotivationC St di f F i FEA

Parking Structure Shrinkage CrackingI d t i l St t N U if B i
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Stability During Placement of Deck Concrete

Goal: Determine when placement of deck causes instabilityFor each load combination SW Steel + LC1 LC9, performelastic stability analysis & compute buckling load multiplier.

SW Steel

Slab LC1

Slab LC2

Slab LC3

Slab LC4

Slab LC5

Slab LC6

Slab LC7

Slab LC8

Slab LC9

+

10 0.2 0.4 0.6 0.8

1.2

0

0.2

0.4

0.6

0.8

1

Fraction of Concrete Deck Placed

P / P c r

P/Pcr = 1.0

LC2LC3

LC4

LC5

LC6LC7

LC8 LC9

~68% ofconcretedeckplaced



Parking Structure Shrinkage CrackingIndustrial Structure on Non Uniform Bearing
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Global Lateral Torsional Buckling Conrmed

Instability occurs when deck was placed over 2/3 of lengthBuckling mode shape matches observed failure modeIf only considered LC9 (full deck), limit state was identied


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Conclusions

Linear elastic FEA points to failure modes not captured insimplied analysesStraightforward and inexpensive to generate

Commonly ignored structural behaviors can be modeled:ShrinkageNon-uniform bearing conditionsEvaluation of structural stabilityConstruction sequence

While approximate, analysis contributes signicant value todesign and construction process.


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Thank YouContact: http://bendeaton.me

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