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CASE STUDYStructural Clay Tile Floor Failure
Background
“Sheffield” tile floor system
Simple span of 31 ft - 6 in
Constructed in 1961
Floor of grade school library/study area
Background
Cracking/popping noises heard by library occupants on normal day
Library and classroom below vacated
Floor cracks and sags became visible within next few hours
Floor appeared to stabilize after several inches of deflection
Site Observations
Proprietary clay tile/concrete composite slab system
No signs of pre-existing deterioration or distress in Library or other rooms
Live loading appeared relatively light
No signs of structurally significant modifications
Site Observations
Library
31.5 ft
One-wayslab span
Typicalloadbearingwalls
Beam
Plan viewshowingfloor layout
Site Observations
Isometric viewof typicalSheffield Tileinstallation
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Site Observations
8” unit
4”
8”
Concrete topping
1.25” dia. bar
Cross sectionshowing typicalSheffield Tileunit, topping, andreinforcing bar
Site Observations
Damaged plaster andexposed underside of floor system
Site Observations
Bucket of backhoe was pressed onto floor to assess short term stability before removing important items from library
Site Observations
Lower portionof floor system collapsed about 3 weeks after load test!
Site Observations
Looking up into coredhole from room below(Intact slab)
Site Observations
Cores showing large planardiscontinuities in tiles
Topping
Topping
Joint
Plaster
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Analysis
Identify critical stresses in tile units
Compare with test data
Typical Flexural Member
C
T
v
Solid web is primarily a shear element
Analysis
Portion of typical flexural member
shear flow=
reinf. steel
Shear flowmechanism intypical flexuralmember
Sheffield Flexural Member
C
T
v
Web is not solid. Discrete web elements sustain much more than shear.
ΔT
ΔT
mVertical tension and compression
ΔT
ΔT
Analysis
Shear flowmechanism intypical sectionof SheffieldTile beam
Analysis
TVsVs
Vc Vc
Tensile stress along top of tilewall due to the fact that:
T x d > 2 Vs x 6”
12” unit
d
Tile joints every 12”create significantbending tensilestresses acrosstile discontinuitiesthat exist in upperportions of walls
T
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Site Testing
Core penetration
Pull up until failure aftercoring to depth shown
Lift off testing on intactfloor areas to estimatetensile strength of tilein critical area
Site Testing
Failure surface afterlift off test showingextent of core waterpenetration oneventual failure plane
Site Testing
In-situ tensile testing indicated average strength of about 120 psi, with standard deviation of about 40 psi
Several specimens were loose after coring (these were not included in the statistics shown above)
Analysis
Load-induced tile wall tensile stresses were estimated using floor system self weight only
Self weight stresses were estimated to be nearly 100 psi in highest shear areas
These stresses would increase over time due to clay tile swelling and concrete topping shrinkage
Volume Change Effects
Topping
Tile Tile
Concrete shrinks
Clay tile expands Clay tile expands
Volume Change Effects
Topping
Tile Tile
Initial compressive stress distribution
Comp. centroid
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Volume Change Effects
Topping
Tile Tile
Long-term compressive stress distribution
Comp. centroid
Concrete shrinkage and tile expansion cause stresses to increase here Lower comp. centroid means
increased force needed for same moment. Larger force means more local (vertical) tension in tile
Conclusion
Floor system failed due to inadequate tensile strength in critical areas of tile units
Long term tile growth and accumulation of web failures caused situation to become critical almost 40 years after construction
Design/development of this system failed to recognize significance of closely spaced vertical joints in combination with inherent material weaknesses
etc.
Stamped writing reads:SHEFFIELD
LOAD BEARING TILE
etc.
CASE STUDYBillboard Shaft Failure
Background
Original structure completed in late 1988
Sign board extended about 5.5 ft shortly thereafter
Overall height about 150 ft
Sign board dim. (with apron) 48’ x 23’
Step-tapered steel pipe shaft; diameters ranging from 36” to 60”; wall thicknesses from 0.43” to 0.63”
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Background
Elevation and typicalconnection detail fromoriginal design drawings
Background
Photo of recently completedsign, before extensionof sign board
Background
Steel shaft failed in late 1992 (less than four years in service)
Failure occurred on relatively calm day
Failure involved fracture of steel pipe shaft at splice location
Background
Photos of structure shortly afterfailure
Site Observations
Fracture surface clearly included substantial areas of fatigue cracking
Fatigue cracking located on opposite sides of pipe
Fatigue crack location consistent with sign displacements normal to faces of sign boards
Two other joints had cracked through entire wall thickness
Site Observations
Clear fatiguecracking in areasof fracture surfaceshown by arrows
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Site Observations
Fatigue cracks justbelow fracturesurface, in weld ofring plate to lowerpipe
Site Observations
Through wall crackat joint above fracturelocation; seen frominside fallen sectionof shaft
Site Observations
Cracking at toe offillet weld in anothershaft joint
Analysis
Plan view
Oscillationcaused by vortex shedding
Vortex sheddingat ends of signs
Winddirection
Sign
Sign structure prone to oscillationdue to vortexshedding
Analysis
Plan view
Oscillationcaused by vortex shedding
Vortex sheddingat ends of signs
Winddirection
Sign
Direction of oscillationconsistent with fatiguecrack orientation
Area of fatigue cracking
Plan at fracture location
Analysis
Plan view
Oscillationcaused by vortex shedding
Vortex sheddingat ends of signs
Winddirection
Sign Fundamental periodof vibration about 2 sec
Wind speed causingresonance about 15 to20 mph
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Analysis
Plan view
Oscillationcaused by vortex shedding
Vortex sheddingat ends of signs
Winddirection
Sign
Stress range associatedwith resonance estimatedto be between 10 and 20ksi
Analysis
Transverse weld on pipe sections similar to Category C detail
Recommended stress range limits for Category C details: 500,000 to 2,000,000 cycles: 13 ksi
Resonant conditions produce 500,000 cycles in 278 hours; which is less than one percent of structure’s lifespan
Conclusion
Shaft failed due to wind-induced fatigue that should have been anticipated
Additional Efforts
Microscopic examination of failure surface to quantify stress ranges that caused fatigue crack growth and number of cycles prior to failure
CASE STUDYRoof Failure
Background
Structure consisted of steel trusses supporting roof deck
Structure about 40 years old
Recently renovated, including truss modifications
Building expanded a few years before collapse
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Background
Collapse occurred under accumulation of snow
Recent snow fall accompanied by westerly winds preceded event
Background
1980’s
Cross Section Through Building
1950’s
Collapse drift sourceOriginal drift source
Collapse area
Background
Typical framing athigh/low rooftransition
low roof truss
high roof truss
column
Background
high roof
low roof
Site Observations
20 ft
6 f
t 15 pcf
20 pcf
Typical drift conditions
Site Observations
First web compression element of many trussesbuckled as shown (element nearest high/low step)
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Analysis
Compare capacity of endcompression element todemands associated withdrift measurements
Analysis
Eccentricallyloaded singleangle:
actual
modelMeasuring rotationalrestraint at ends oftypical angle
Analysis
Accounting for eccentricity of load, ultimate axial capacity was slightly less than 11 kips
Calculated load due to measured drift parameters was 11.1 kips
Calculated demand due to code specified service loads was about 14.4 kips
Other Observations
Top chord was reinforced as part of renovation (still deficient with respect to code required loading)
Single angle compression element had sufficient strength if loaded concentrically
Conclusions
Trusses failed well below required nominal strengths
Single angle compression element had sufficient strength if loaded concentrically
Structure survived for decades due to small snow drift source CASE STUDY
Guyed Tower
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Background
1980 ft tall, steel framed tower
3 leg mast
guy cables at nine elevations on each leg
Legs consist of 6” dia. solid rounds
X-bracing consists of 0.75” dia. rounds being replaced with 0.875” dia. rounds
Background
Tower fell on very calm day
Brace replacement work was in progress
Background
Typical mast framing
Site Observations
All guy cables andtheir anchorageswere intact
Site Observations
Old bracing
New bracing
Site Observations
Temporarybracing frameused whenreplacingbrace rods
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Site Observations
Partial elevation
Typical towersegment
Temporarybracing framebolted to legs
Temporary bracingframe allowed for saferemoval of brace rodswithin frame boundaries.
This work was underwaywhen tower fell.
Typicalbracerod
Site Observations
In bay abovetemporaryframe, bothconnectionplates lookedlike this for oneof the rods
No bolts, no hole distress
Analysis
15 ft bay above temporary frame had one brace disengaged
Evaluate stability of unbraced bay
Axial load due to gravity = 330 kips/leg at unbraced location (below lowermost guy point)
Analysis1
5 f
t
Leg segments above andbelow unbraced segmentsprovide rotational restraintat ends of buckling legs.
The extent to which such restraint is provided dependsupon whether or notintermediate horizontalmembers provide lateralrestraint at mid bay.
Rods run thruU-bolt
Which bowedshape right?
330 k/leg
Analysis
Assume initial misalignment of L/500 between ends of unbraced segments
Axial load causing yielding to begin in leg rods with mid bay horizontal fully effective = 320 kips
Axial load causing yielding to begin in leg with ineffective mid bay member = 290 kips
Conclusions
Ultimate capacities of unbraced legs in range of gravity load demands
Tower fell because brace was removed without other means of lateral bracing in place
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CASE STUDYA Near Miss Analysis Breakdown
74
75
Rear legs
Front leg
Drive leg
Underbridge
76
Rear legs
Front leg
Underbridge
77
Front leg Drive leg
Underbridge
Rear legsFront leg
Drive leg
Underbridge
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79
Fully extended,
suspended underbridge
Maximum load on extended front leg
Extended front leg
R
Initial pier check
method:Simply analyze pier as
a free standing column
with an axial load.
Substantially
overestimated
capacity.
Modified pier check
method:Apply gantry reaction via
rigid; pinned-pinned link.
Pier bracing required at
the taller piers would not
have been identified
otherwise
System Schematic
R
R
Underbridge weight
Gantry weight
Pier (not laterally braced)
R
Vertical and lateral support
on completed deck
Front leg; pinned at each end
82
83 84
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85 86
87 88
