Post on 29-May-2022
Federal Building and Fire Safety Investigation of the World Trade Center Disaster
Building and Fire Research LaboratoryNational Institute of Standards and Technology
U.S. Department of Commerce
Condition of Thermal Insulation:Methodology
June 23, 2003
Nick Carino John Gross
Kuldeep PrasadFahim Sadek
Monica StarnesJiann Yang
DisclaimerDisclaimer
Certain commercial entities, equipment, products, or materials are identified in this presentation in order to describe a procedure or concept adequately or to trace the history of the procedures and practices used. Such identification is not intended to imply recommendation, endorsement, or implication that the entities, products, materials, or equipment are necessarily the best available for the purpose.
ThermalThermal--Structural ModelingStructural Modeling
• Analysis of undamaged buildings exposed to postulated fires
• Analysis of damaged buildings exposed to 9/11 fires• Condition of fireproofing needs to be characterized
OverviewOverview
• Sensitivity study• In-place conditions
History of sprayed fire resistive material (SFRM)MeasurementsStatistical analysis
• Equivalent thickness• Thermal properties• Response to impact• Summary
Sensitivity StudySensitivity Study
• Effects of thickness variability and “gaps” in SFRM• Simplified finite-element analysis
1 in. thick steel plate, 60 in. longAverage thickness: 0 to 2.0 in.Standard deviation: 0 to 1.0 in.Gap length: 0 to 30 in.
• Exposure: 1100 ºC fire• Compute temperature history of steel
ModelModel
2 in.
2 in.
60 in.
1 in. thick steel plate Fireproofing
1 2 3 4 5
ModelModel
• Use psuedo-random number generator to select SFRM thickness at cross-section
Based on average thickness and standard deviation
SFRM
High conductivity material
ExampleExample
Uniform Thickness Standard Deviation = 1 in.
0.5 in. 1.75 in.
Effect of Gap in SFRMEffect of Gap in SFRM
1 2 3 4 5
With Gap
No GapTime = 50 min
Sensitivity StudySensitivity Study
• Variability in thickness of SFRM reduces its effectiveness
• Gap in SFRM:Local heatingPath for heat flow into member
History of SFRM in WTC TowersHistory of SFRM in WTC Towers
• 1969: Decision to use ½ in. CAFCO® BLAZESHIELD ® Type D• 1970: CAFCO® BLAZESHIELD® Type D discontinued at 38th floor,
replaced with CAFCO® BLAZESHIELD® Type D/CF• 1994: Thickness measurements on floor trusses on 23rd and 24th
floors• 1995: PA initiated study of SFRM thickness during tenant alterations• 1999: PA established guidelines for SFRM replacement and repair• Late 90s: SFRM upgraded to 1 ½ in. with CAFCO® BLAZESHEILD®
Type II
Specified ThicknessSpecified Thickness
• October 30, 1969 PA CorrespondenceColumns < 14WF228: 2 3/16 in.*Columns ≥ 14WF228: 1 3/16 in.Beams, spandrels, floor trusses: ½ in.
• Alcoa Drawings (Note 11)3 h on spandrels ½ in. 1/2 in. V.A.**4 h on columns (heavy) 1 3/16 in. 7/8 in. V.A.
• 1995 Upgrade StudyFloor trusses 1 ½ in.
*CAFCO® BLAZESHIELD® Type D**V.A. = Vermiculite aggregate plaster
Thickness MeasurementsThickness Measurements
• 1994 measurements on 23rd and 24th floors of WTC 1• Analysis of photographs taken in the 1990s• Thickness measurements in PA Construction Audit
Reports from late 1990s• 1999 measurements of beams and columns in WTC 1
shaft 14/15
Floors 23 and 24 Trusses (1994)Floors 23 and 24 Trusses (1994)
• 16 randomly selected trusses per floor• 6 replicate measurements on “flanges and web”
0
0.2
0.4
0.6
0.8
1
1.2
22 23 24 25
Aver
age
Thic
knes
s, in
.
Floor
Grand Average = 0.74 in.St. Dev. of Avg. = 0.16 in.
Normal Probability Plots Normal Probability Plots
• Thickness appears better described by log-normal distribution
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
.01 .1 1 5 10 2030 50 7080 90 95 99 99.9 99.99
y = 0.74187 + 0.1607norm(x) R= 0.97209
Ave
rage
Thi
ckne
ss, i
n.
Percent
(c)
-0.8
-0.6
-0.4
-0.2
0
0.2
.01 .1 1 5 10 2030 50 7080 90 95 99 99.9 99.99
y = -0.32122 + 0.21269norm(x) R= 0.98754
Ln (T
hick
ness
)
Percent
(e)
Thickness Natural Logarithm of Thickness
Lognormal DistributionLognormal Distribution
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 0.5 1 1.5 2 2.5
Freq
uenc
y
Thickness, in.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
-2 -1.5 -1 -0.5 0 0.5 1
Freq
uenc
y
Ln (Thickness)
Analysis of PhotographsAnalysis of Photographs
• Original SFRM: WTC 1 (22, 23, 27) in mid 1990s• Upgraded SFRM, WTC 1 (below 31) in 1998
Bar Radius + Fireproofing Thickness
Center Line
Reference Length
Results of Photo AnalysisResults of Photo Analysis
0
0.5
1
1.5
2
2.5
.01 .1 1 5 10 2030 50 7080 90 95 99 99.9 99.99
MainBridgingStrutMain-Upgrade
Thic
knes
s, in
.
Percent
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
.01 .1 1 5 10 2030 50 7080 90 95 99 99.9 99.99
Ln MainLn BridgingLn StrutLn Main-Upgrade
Ln T
hick
ness
Percent
• Original SFRM thickness distribution appears to be lognormal
• Upgraded SFRM thickness distribution appears to be normal
Summary of Photo AnalysisSummary of Photo Analysis
0.50.2 in.0.4 in.Original
Strut
0.20.4 in.1.7 in.Upgraded Main
0.60.25 in.0.4 in.Original Bridging
0.50.3 in.0.6 in.Original
Main
Coeff. of VariationStd. Dev.Average
ThicknessFloor Truss
Construction Audit ReportsConstruction Audit Reports
• 18 data sets for WTC 1 (93, 95, 98, 99 and 100)• 14 data sets for WTC 2 (77, 78, 88, 89, 92)
1
1.5
2
2.5
3
3.5
4
4.5
5
0 1 2 3
Thic
knes
s, in
.
Tower
Combined MeasurementsCombined Measurements
• Lognormal distribution appears more appropriate• Overall average = 2.5 in.• Overall standard deviation = 0.6 in.
0.4
0.6
0.8
1
1.2
1.4
1.6
.01 .1 1 5 10 2030 50 7080 90 95 99 99.9 99.99
y = 0.90833 + 0.2266norm(x) R= 0.99261
Ln (T
hick
ness
)
Percent
1
1.5
2
2.5
3
3.5
4
4.5
5
.01 .1 1 5 10 2030 50 7080 90 95 99 99.9 99.99
y = 2.5449 + 0.57398norm(x) R= 0.98583
Thic
knes
s, in
.
Percent
Core Beams and ColumnsCore Beams and Columns
• April 1999 measurements in shaft 14/15 of WTC 1 (1st to 45th floor)
Average = 1.0 in.St. Dev. = 0.2 in.
Average = 0.8 in.St. Dev. = 0.2 in.
0
0.5
1
1.5
2
0 10 20 30 40 50
IndividualAverage
Thic
knes
s, in
.
Floor
Beams
0.75 in.
0.0
0.5
1.0
1.5
2.0
0 10 20 30 40 50
IndividualAverage
Thic
knes
s, in
.
Floor
Columns
0.5 in.
0.20.25
0.2 in.0.2 in.
1.0 in.0.8 in.
Core BeamsCore Columns
PA Measurements
0.20.6 in.2.5 inTruss UpgradedPA Measurements
0.50.60.50.2
0.3 in.0.25 in.0.2 in.0.4 in
0.6 in.0.4 in.0.4 in.1.7 in.
Main TrussBrdg. TrussStrutMain Upgraded
Photos
0.2*0.2 in.*0.7 in.TrussPA Measurements
Coeff. of Variation
St. Dev.AverageElementData Source
Summary of SFRM ThicknessSummary of SFRM Thickness
*Variability of averages
Equivalent ThicknessEquivalent Thickness
• Variability of thickness reduces effectiveness of SFRM• Not practical to include variable thickness in thermal
modeling• Establish “equivalent uniform thickness” that provides
thermal protection equivalent to variable thickness• Approach
Bar model with variable thickness SFRMCompute elongation under 12,500 psi tensile stress as a function of timeCompare with elongation obtained with uniform thickness SFRM
Cases Considered (Floor Truss)Cases Considered (Floor Truss)
• Lognormal distribution• Original SFRM: tavg = 0.75 in., st. dev. = 0.3 in.• Upgraded SFRM: tavg = 2.5 in., st. dev. = 0.6 in.• 3 sets of psuedo-random numbers• 100 elements for 60 in. bar• 5-point smoothing
55--point Smoothingpoint Smoothing
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
0 10 20 30 40 50 60
Rough TextureSmooth Texture
Thic
knes
s, in
.
Bar Position, in.
Results for Original SFRMResults for Original SFRM
0.00
0.20
0.40
0.60
0.80
0 5 10 15 20 25 30
Rough 1Rough 2Rough 3Smooth 1Smooth 2Smooth 30.4" Uniform FP0.5" Uniform FP0.6" Uniform FP
Def
orm
atio
n (in
)
Time (min)
Load = 9800 lbs
Results for Upgraded SFRMResults for Upgraded SFRM
0.00
0.20
0.40
0.60
0.80
1.00
0 20 40 60 80 100
Case 1Case 2Case 32.0 in. Uniform2.2 in. Uniform
Def
orm
atio
n, in
.
Time, min
Fireproofing Thickness in Thermal ModelingFireproofing Thickness in Thermal Modeling
• Floor trusses (Original):Main: T = 0.6 in.Bridging (Two-way): T = 0.6 in.Bridging (One-way) and struts: T = 0.3 in.Saddle and damper: T = 0 in.
• Floor trusses (Upgrade):T = 2.2 in., except dampers T = 0 in.
• Other elements: Specified thicknessAverage tends to exceed specified thicknessVariability reduces effectiveness
One-way
One-way
Two-way
Core
Thermal and Physical Properties of SFRMThermal and Physical Properties of SFRM
• MaterialsCAFCO® BLAZESHIELD® Type DC/F (Original)CAFCO® BLAZESHIELD® Type II (Upgrade)Monokote® MK-5® (WTC 7)
• Properties as function of temperatureThermal conductivitySpecific heat capacityDensityCoefficient of thermal expansion
Example ResultsExample Results
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0 200 400 600 800 1000 1200 1400
Monokote MK-5BLAZE-SHIELD IIBLAZE-SHIELD DC/FTh
erm
al C
ondu
ctiv
ity (W
/mK)
Temperature, oC
600
800
1000
1200
1400
1600
0 200 400 600 800 1000 1200 1400
Monokote MK-5BLAZE-SHIELD IIBLAZE-SHIELD DC/FS
peci
fic H
eat,
J/(k
g. K)
Temperature, oC
Thermal Conductivity Specific Heat Capacity
Impact DamageImpact Damage
• SFRM was dislodgedDebris fieldLocalized accelerations and deformations
• Estimate extent of dislodged SFRMMeasure static adhesive and cohesive tensile strengthDevelop “failure criteria”Impact analysis and engineering judgment to estimate extent of dislodged SFRM
Tensile PullTensile Pull--off Testoff Test
SFRM
Aluminum Plate
Adhesive
¼ in. Steel Plate
Simple ModelsSimple Models
afb
fbρ t
a =
t
Planar Substrate
ft
fb
a
dodi
4 ft (do + (α – 1) di
(do2 – di
2) ρπa =
α = ftfb
Encased Substrate
Impact TestsImpact Tests
• Determine acceleration to dislodge SFRM
Accelerometer
Accelerometer
Plate Encased Bar
SummarySummary
• Methodology for assessing condition of SFRM has been reviewed
• Variability of SFRM thickness is taken into account by use of “equivalent thickness”
• Available data used to obtain rational thickness valuesFloor trusses: original T = 0.6 in.Floor trusses: upgraded T = 2.2 in.Others: Specified thickness
• Temperature dependence of thermal properties established
• Mechanical damage to be estimated on the basis of tests and analysis