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Forensic En ineerin =Forensic En ineerin =
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Causes of Distress in ConcreteCauses of Distress in Concrete
David W. FowlerThe University of Texas at Austin
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What is forensic engineeringForensic engineeringscience
concerned with the relations betweenengineering and the law.Forensic en ineerin is a
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specialized discipline which is arelatively new and unfamiliar field to
the public.More common definition: activitiesrelated to failure investigation
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Forensic engineers performautopsies on components and
materials or full-sized buildings,bridges, foundations, and otheren ineered constructed works in
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order to determine the cause andextent of failure.
A secondary purpose is todetermine methods ofrepair/rehabilitation/replacement.
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What are the qualifications of forensicengineers?
Must be expert. Must be ethical
M lik iv
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Must enjoy digging for the truth
Must think outside the box
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So if you want to be anengineering Sherlock Holmes,
lets go further
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How is failure defined?
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What is a failure?
When a facility, structure or material does
not perform in the manner it was intended.
It does not have to collapse or cause
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be a failure
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Failure Examples A floor vibrates when people walk on it.
One room in the building is not cooled by
the air conditioning system A parking area ponds water when it rains
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The foundation moves differentially,
causing cracking in the interior and
exterior wall.
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Famous Failures
Tower of Pisa, Italy
Tay Bridge, Scotland
Hyatt Regency Hotel Walkway, Kansas
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ty Big Dig (Boston) Tunnel Ceiling Panels
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Famous FailuresTower of Pisa (1173 to present)
200 ft (60 m) tall
Inclined 5.5
Extensive efforts to
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Many efforts tocorrect problem
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Famous FailuresTay Bridge, Scotland (1879)
Completed in 1828
Length: 10,321 ft. (3146 m) 85 simply supported iron lattice-truss spans
Failure occurred during a major storm
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Train was passing over bridge 75 people died
Cause of failure: improper estimation of wind
force in design 10 psf (0.5 kPa) instead of 50 psf (2.5 kPa)
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Famous FailuresTay Bridge, Scotland (1879)
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Famous FailuresHyatt Regency Hotel in Kansas City
(1981)
Hanging walkway supported by steel rods
Colla sed killin 113 eo le
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Cause: poor connection detail
Poor communication between engineer
and steel fabricator involved
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Famous Failures
Hyatt Regency Hotel in Kansas City(1981)
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Famous Failures
Hyatt Regency Hotel in Kansas City(1981)
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Famous Failures
Hyatt Regency Hotel in Kansas City(1981)
16Hanger rod box beamconnection
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Ultimate Capacity ofConnection
Original Design: Ultimate capacity of
connection only 60% of required capacity As Built: Ultimate capacity was just a little
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Either way, connection was bound to fail!
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Famous Failures
Big Dig Ceiling Panel Failure
On 11 pm, July 10, 2006, four sections of
precast concrete ceiling panels fell in aconnector tunnel in Boston.
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- - ,to Logan Airport with her husband, waskilled.
The epoxy anchors holding the ceilingpanels failed due to faulty construction anddesign.
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The Accident Scene
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Car Crushed by Ceiling Panel
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HoweverHowever
These are very dramatic butThese are very dramatic but
spectacular!spectacular!
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Why Did I Fail?
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Scientific Method for Solving
Forensics Problems State the problem
Make observations
State the likely causes of the failure
Test the likely causes by making moreobservations, performing tests, doingcalculations, doing a literature search, etc.
Develop the most likely cause or causes
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Often the anxwer will be yes, maybe orno, maybe.
There may be more than one contributing
factor
Developing a simple matrix is helpful in
com ng o e mos e y causes.
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Symptom Cause
Cracking shortly (minutes to hours)after placing (before setting)
Plastic shrinkage cracking (rate ofevaporation > rate of bleeding)
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Symptom Cause
Cracking shortly (hours to days)
after placing (after hardening)
Drying/thermal cracking (improper timing
of spacing of joints)
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Symptom Cause
Cracking occurring after structure
is in service
Structural cracking (improper placing or
insufficient reinforcement)
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Symptom Cause
Map cracking typically
occurring after 5 to 10 years
Volumetric expansion of concrete (in this
case ASR)
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Symptom Cause
Map cracking typically
occurring after 5 to 10 years
Volumetric expansion of concrete (in this
case ASR & DEF)
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Symptom Cause
Map cracking occurring
anytime from weeks to years
Excessive drying shrinkage
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Compare with map-cracking due to ASRwhere extrusion of
joint sealantprovides evidence ofexpansion
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Symptom Cause
Map cracking typically occurring
anytime from months to years
Freeze-thaw attack (non-air-entrained
concrete that was also hit by truck)
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Symptom Cause
Surface scaling occurring after
approximately 5 years
Abrasion by vehicular traffic (in wheel
path)
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Symptom Cause
Surface scaling occurring after decades Water abrasion
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Symptom Cause
Severe spalling of concrete
surface
Fire damage (petroleum tanker crashed
into bridge pier and ignited)
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Symptom Cause
Severe erosion of concrete
occurring after years
Sewer-pipe corrosion (chemical-biogenic
attack of paste by sulfuric acid)
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Symptom Cause
Spalling and disintegration
occurring after a few years
Freeze-thaw attack (non-air-entrained
concrete)
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Symptom Cause
Shearing of concrete column and
spalling of concrete cover
Shoving of foundation due to expansion
of adjacent pavement (ASR)
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Symptom Cause
Spalling of concrete over rebar Corrosion of rebar initiated by chlorides
(from roadway above)
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Symptom Cause
Spalling of concrete over rebar Corrosion of rebar initiated by chlorides
(from unwashed sea-dredged sand)
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Case of the
Ruptured Slab
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Background Residence located in South Texas
Construction date unknown: estimated tobe before 1950
Owners purchased in 1992 Construction
Slab on grade
Wood framing
Stucco exterior
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Observations: Owner Interview Owner discovered that no water pressure
existed on April 15, 2000 Water going through meter; meter turned off
n pr , p um er ocate ea eneatlaundry room slab.
Broke slab and found copper water supply
pipe had completely separated Pipe was repaired.
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Observations: Owner Interview
Owner had found N-S crack through slabearlier, before loss of pressure occurred.
,
door to bedroom stuck. Shortly after leak was found, owner heard
loud noise and found that front of househad a severe crack and stucco had fallenoff house.
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Observations: Owner Interview
Another noise was heard and crack was
was broken.
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Visual Observations
N-S slab crack extending from north wallthrough house to utility
Crack was in line with location of leaklocation
Numerous other cracks in floor and walls
N S Slab Crack: Looking down on
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N-S Slab Crack: Looking down on
crack after carpet removed
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Location of Pipe Separation
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Cut Out Section of Pipe
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Repaired Water Pipe
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Separation of Cabinet at Ceiling
Door Frame Planed due to
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Door Frame Planed due toSticking Door
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Wall-Ceiling Crack
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Severe Wall Cracking on North Side
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Cracking at NE Corner
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Stucco Cracking at Window
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Findings: Floor Level Survey
Floor levels were taken in June 2000b defendants ex ert showed a
mound in vicinity of leak. Floor levels taken by me in December
2000 were similar to those taken bydefendants expert.
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LeakLeakLeakLeak
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Findings: Water UsageMonth BilledMonth BilledMonth BilledMonth Billed 1999199919991999 2000200020002000
JanuaryJanuaryJanuaryJanuary 18,00018,00018,00018,000 23,00023,00023,00023,000
,,,, ,,,,
MarchMarchMarchMarch 18,00018,00018,00018,000 17,00017,00017,00017,000
AprilAprilAprilApril 21, 00021, 00021, 00021, 000 18,00018,00018,00018,000
MayMayMayMay 20,00020,00020,00020,000 26,00026,00026,00026,000
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Defendant Expert Findings Plumbing leaks did not contribute to
cracking. Causes of cracking attributed to:
Differential soil movement due toseasonal climatic effects
Inadequate compaction of soil
Infrequent water pattern around house
Trees near foundation
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OutcomeOutcomeOutcomeOutcome
Initially, no money offered
After my report was presented, $39,000 After some serious negotiation, $140,000
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A 12-STORY BUILDING LYING
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ON THE GROUND in CHINA
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Heavy rains resulted in waterseeping into the ground.
The building began to shiftand the concrete pilings
were snappeddue to the uneven lateralpressures.
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The building began totilt.
And thus came the eighthwonder of the world
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On Piers and Still Problems
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An educational building for a church was
constructed on expansive clay. The structural engineer had a soil
piers to go about 20 ft. into the clay.
Pier as designed
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Pier as built
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Design of Pier Foundations
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When the drought broke the moistured lli f th il th l i th
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When the drought broke, the moisturecaused swelling of the soil, the clay in theactive zone created skin friction on the
surface of the pier and lifted it up. But how could the skin friction uplift break
the concrete?
How much force would it take to break thepier?
How much stress would it take to breakthe concrete?
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The total uplifge providec by the soil on
the pier is over 100 000 lbs
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the pier is over 100,000 lbs. Had the steel been placed to the bottom of
the bell, it would not have stopped thecrack from forming but it would have keeph r k fr m nin wi .
The bell would have served as an anchorpreventing the pier from moving upward.
But this illustrates the force clay can exert
in lifting up a structure.
Foundation Solution
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AlternateDesign
A f fi l f il f
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A few final failures for your
viewing pleasure.
104
2.
Where does the stair go?
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105
5.
How do I get up these stairs?
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106
6.
Escalator for very short people
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107
9.
Someone forgot about the light pole
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108
11.
We need to grow taller people
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And the winner is . . .
110
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You may be the next engineering
Sherlock Holmes!
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Does it pay well?
2001 Salaries
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General Civil $73,000
Civil (Structural) 75,000
Geotechnical 68,000
Forensic 143,500
View these as relative, but they arehigher than in 2001.
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Questions?
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Winner: Not My Job Award