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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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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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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
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
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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MotivationCase Studies of Forensic FEA
Conclusions
Parking Structure Shrinkage CrackingIndustrial Structure on Non-Uniform BearingPedestrian Bridge Collapse
Parking Structure Shrinkage Cracking
Case Study # 1:
Parking Structure Shrinkage Cracking
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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MotivationCase Studies of Forensic FEA
Conclusions
Parking Structure Shrinkage CrackingIndustrial Structure on Non-Uniform BearingPedestrian Bridge Collapse
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:
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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MotivationCase Studies of Forensic FEA
Conclusions
Parking Structure Shrinkage CrackingIndustrial Structure on Non-Uniform BearingPedestrian Bridge Collapse
Parking Structure Finite Element Model Details
Model consisted of 24,000 shell elementsLoads: Gravity, temperature, shrinkage
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
k h k k
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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MotivationCase Studies of Forensic FEA
Conclusions
Parking Structure Shrinkage CrackingIndustrial Structure on Non-Uniform BearingPedestrian Bridge Collapse
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):
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
M i i P ki S Sh i k C ki
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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MotivationCase Studies of Forensic FEA
Conclusions
Parking Structure Shrinkage CrackingIndustrial Structure on Non-Uniform BearingPedestrian Bridge Collapse
Investigate Means of Relieving High Slab Stresses
Top: t = 2600 psi Bottom: t = 2800 psi
:
: .
:
:
Top: t = 1660 psi Bottom: t = 1968 psi
: , ,
:
:
Top: t = 715 psi Bottom: t = 845 psi
Relieve shrinkage stress 3.5x by adding expansion joints
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
Motivation Parking Structure Shrinkage Cracking
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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MotivationCase Studies of Forensic FEA
Conclusions
Parking Structure Shrinkage CrackingIndustrial Structure on Non-Uniform BearingPedestrian Bridge Collapse
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
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
Motivation Parking Structure Shrinkage Cracking
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
10/22
MotivationCase Studies of Forensic FEA
Conclusions
Parking Structure Shrinkage CrackingIndustrial Structure on Non-Uniform BearingPedestrian Bridge Collapse
Industrial Structure on Non-Uniform Bearing
Case Study # 2:
Industrial Structure on Non-Uniform Bearing
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
Motivation Parking Structure Shrinkage Cracking
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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MotivationCase Studies of Forensic FEA
Conclusions
Parking Structure Shrinkage CrackingIndustrial Structure on Non-Uniform BearingPedestrian Bridge Collapse
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
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
Motivation Parking Structure Shrinkage Cracking
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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MotivationCase Studies of Forensic FEA
Conclusions
Parking Structure Shrinkage CrackingIndustrial Structure on Non-Uniform BearingPedestrian Bridge Collapse
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
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
Motivation Parking Structure Shrinkage Cracking
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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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
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
Motivation Parking Structure Shrinkage Cracking
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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Case Studies of Forensic FEAConclusions
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.
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
Motivation Parking Structure Shrinkage Cracking
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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Case Studies of Forensic FEAConclusions
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
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
Motivation Parking Structure Shrinkage Cracking
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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Case Studies of Forensic FEAConclusions
Industrial Structure on Non-Uniform BearingPedestrian Bridge Collapse
Pedestrian Bridge Collapse
Case Study # 3:
Pedestrian Bridge Collapse
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
Motivation Parking Structure Shrinkage Cracking
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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Case Studies of Forensic FEAConclusions
Industrial Structure on Non-Uniform BearingPedestrian Bridge Collapse
Pedestrian Bridge Collapse
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
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
Motivationd f
Parking Structure Shrinkage Crackingd l f
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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Case Studies of Forensic FEAConclusions
Industrial Structure on Non-Uniform BearingPedestrian Bridge Collapse
Pedestrian Bridge Finite Element Model
Use FEA to investigate stability of structureModel details: 22,000 elementsAssume weight (but not stiffness) of concrete
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
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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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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Case Studies of Forensic FEAConclusions
Industrial Structure on Non-Uniform BearingPedestrian Bridge Collapse
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
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
MotivationCase Studies of Forensic FEA
Parking Structure Shrinkage CrackingIndustrial Structure on Non Uniform Bearing
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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Case Studies of Forensic FEAConclusions
Industrial Structure on Non-Uniform BearingPedestrian Bridge Collapse
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
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
MotivationCase Studies of Forensic FEA
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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Case Studies of Forensic FEAConclusions
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.
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
MotivationCase Studies of Forensic FEA
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8/10/2019 Lessons Learned from Forensic FEA of Failed RC Structures.pdf
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Case Studies of Forensic FEAConclusions
Thank YouContact: http://bendeaton.me
Deaton and Kahn Lessons Learned from Forensic FEA of Failed RC Structures
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