National Institute of Standards and Techno

Technology Administration

U.S. Department of Commerce

NIST Response to the World Trade Center Di

Federal Building and Fire Safety Investiga

of 

the World Trade Center Disaster

Part III – Baseline Performance

April 5, 2005
Baseline Performance Analysis Baseline Performance Analysis

• Standard fire resistance of WTC floor system

• Effects of scale, restraint condition, fireproofing thickness

• Quality and properties of structural steel

• Identification and location of recovered WTC steel• Mechanical properties of steel (room/elevated temperatures, • Comparison with testing and construction specifications

• Design wind loads

• Estimation of wind effects using wind tunnel testing• Comparison with prescriptive code requirements

• Response of WTC towers to design wind loads

• Estimation of drift ratios; comparison with limits used in pract

required by codes

• Estimation of component demand-to-capacity ratios
Innovative WTC Tower Structural System Innovative WTC Tower Structural System

• Innovative structura

when built; incorpornew and unusual fe

• Two features requir

consideration:

• Composite fl

system using open-web baspray-applied fireproofing

• Design for w

and control oinduced vibra
Fire Performance of Composite Floor Sy

Fire Performance of Composite Floor Sy

• Fire-protection of a truss-supported floor system with spray-o

fireproofing was innovative and not consistent with then-prevpractice.

• No evidence found of technical basis in the selection of 

thickness to meet 2 h fire rating:

• 1/2 in. specified when WTC towers were built to maintain 

(not 1-B) fire rating requirement of the NYC Building Cod

• 1-1/2 in. specified for upgrades some years prior to 2001• 2 in. for similar floor system in an unrestrained test (mode

evaluation service recommendation in June 2001, unrelabuildings)

• No evidence that full-scale fire resistance test of the WTC 

system was conducted to determine the required fireproofithickness; in 1966, the Architect of Record and, in 1975, the Engineer of Record stated that the fire rating of the WTC floocould not be determined without testing.
NYC Building Code Provisions NYC Building Code Provisions

(Fire Resistance in hours) (Fire Resistance in hours)

Floors

3 3/2 1-1

Columns

4 4/3

2

1938 1968* 200

* Building code governing original design and occupanc

(the architect/owner could choose either Class 1-A or 1

** Sprinklers required for buildings of unlimited height
Fireproofing Thickness in NIST Fireproofing Thickness in NIST-Sponso

Sponso

Tests at UL Tests at UL

As-Applied

Original Specified

Condition

3/4 in 3/8 in Overs1/2 in 0 in No Over

Metal D

Bridging Trusses Primary Trusses

• Three tests were conducted in the as-applied conditio

• 35-foot span; restrained test• 35-foot span; unrestrained test• 17-foot span; restrained test

• One test was conducted in the original specified cond

• 17-foot span; restrained test
Results From NIST Results From NIST-Sponsored Tests at 

Sponsored Tests at 

___________________________________________________(1) Imminent collapse(2) Vertical displacement exceeded capability to measure accurately(3) Did not occurThe end-point criterion that determined the rating is shown in matching color.

--- 58 66 58 (3) 120(1) 3⁄4 

17 ft, restrained, 1⁄2 in fireproofing 4

210 2 

(2) 180 157 86 76 (3) 17 ft, restrained, 3⁄4 in fireproofing 3

146 2 

(2) --- --- 76 62 (3)

35 ft, unrestrained,

3⁄4 in fireproofing

2

116 11⁄2 

(1) --- 111 66 62 (3) 35 ft, restrained, 3⁄4 in fireproofing 1

Rating

(hr)

Maximum

(1300oF)

Average

(1100oF)

Maximum

(Ambient

+325oF)

Average

(Ambient

+250oF)

ASTM

E 119-

Failure 61

to

Support 

Load

Steel Temperatures Temperature on Unexposed Surface

Standar

Test

Terminated

(min)

Times to Reach End-Point Criteria (min)

Test Description
Findings of Standard Fire Resistance Te

Findings of Standard Fire Resistance Te

• The test structures were able to withstand standard fire conditions for be

minutes and 2 hours. The floor system did not fail to support loads in an

• The 1968 New York City building code—the code that the WTC towers w

but not required to meet when they were built—required a 2-hour fire rati

system.

• The 45-minute fire resistance for the standard 17-foot test with the s

inch fireproofing did not meet the 2-hour requirement of the NYC buThis test had no fireproofing on the bridging trusses nor on the underside deck.

• The 2-hour fire resistance for the standard 17-foot test with the as-a

average 0.75 inch fireproofing met the 2-hour requirement of the NYcode. This test had half the fireproofing thickness on the bridging trusseoverspray on the underside of the metal deck.

• The difference in test results is not due to the fireproofing thickness 

trusses, but possibly due to moisture content differences in the conand the presence or lack of fireproofing overspray on the underside deck. 
Role of Fire Resistance Tests Role of Fire Resistance Tests

• The fire resistance tests cannot be used to determine the actual per

the floor systems in the collapse of the WTC towers, nor can the teswhether or not the actual floor systems as built met code requireme

the PANYNJ could have taken the highly unusual step of reclassifyistructure to Class 1-C, with a 1-1/2 hour required rating for floors an

rating for columns, when installation of the sprinkler system was coprior to September 11, 2001.

• The fire resistance tests provided valuable insights into the behavior of th

for use in analyzing the thermal response and collapse of the WTC towe

• The occurrence and spread of jet-fuel due to the terrorist attacks on Sept

2001 ignited multi-floor fires in the WTC towers. These fires were significfrom the fires to which floor systems in standard U.S. fire rating tests are Consider, for example:

• Combustible fuel load of the hijacked jets.• Extent and number of floors involved in fires.• Rate of fire spread across and between floors.• Ventilation conditions in the fire-affected floors.

• The probable collapse sequences for the WTC towers are based on the b

thermally weakened structural components that had extensive damage to gypsum board fire protection induced by the debris field generated by air
Failure at 

connection 

between floor 

system and 

exterior 

columns

Dr. John Fisher (Lehigh) and R

WTC steel columns Dr. 

Steel from WTC Towers

Clean weld fracture of Interior columns
WTC Steel at NIST

Ten different steel companies fabricated structural components foWTC towers, using steel from at least 8 different suppliers. Four supplied the major structural components of the 9th to the 107th flo
Inventory of Steel Pieces at NIST Inventory of Steel Pieces at NIST

• NIST has catalogued a total of 236 pieces of recovered WTC steel, inclu

• 94 perimeter columns panel sections• 44 wide flange sections• 11 built-up box columns• 23 floor trusses• 25 channels• 7 coupons from WTC 5• 2 “bow-tie” pieces• 30 miscellaneous (isolated bolts, floor hanger components, and other)

• Steel in NIST’s possession represents roughly 1/2 percent of the 200,00

structural steel used in the construction of the WTC towers.

• NIST inventory includes pieces from the impact and fire regions, perime

core columns, floor trusses, and other pieces such as truss seats and w

• Original, as-built locations of 42 recovered perimeter panels and 12 reco

columns were determined, based on markings and geometry of the colu

• A significant number of structural pieces were recovered from locations 

impact and fire damaged regions of the WTC towers, including 4 perimedirectly hit by the airplanes and 3 core columns located within these are
Analysis of Recovered WTC Steel Analysis of Recovered WTC Steel

 NIST’s collection of 236 WTC steel pieces 

was adequate for determining the quality and properties of steel for the investigation (reconstruction of the impacted/fire zone was not attempted).

• Impact/fire damage region emphasized

• All 10 grades of steel used for perimeter panels 

(12 grades were specified)

• Both grades for steel trusses

• Two grades (representing 99%) for core 

columns

 Based on stampings on steel and mechanical 

tests, analysis indicated that the correct specified materials were provided for specified elements; when these data are combined with available pre-collapse photographs, aircraft impacted pieces from WTC 1 were in precise locations specified in design drawings
Perimeter Columns Core Colu

Comparison of Specified Properties with Comparison of Specified Properties with Measured Properties at Room Temperat

Measured Properties at Room Temperat

The safety of the WTC towers on September 11, 2001 was most likely not affected by the fraction of stto NIST testing, did not meet the specified minimum yield strength. The typical factors of safety in allowdesign can accommodate the measured property variations below the minimum.
Comparison of Specified Properties with Comparison of Specified Properties with 

Measured Properties at Room Temperat

Measured Properties at Room Temperat

Floor Truss Components Truss Se

The safety of the WTC towers on September 11, 2001 was most likely not affected by the fraction of stto NIST testing, did not meet the specified minimum yield strength. The typical factors of safety in allowdesign can accommodate the measured property variations below the minimum.
• Approximately 87 percent of the tested steel specimens exceeded t

minimum yield strengths specified in design documents.

• Approximately 13 percent of test results on the damaged steel did n

required minimum yield strength specified in design documents. T

are not unexpected since:

• Change in test procedure from mill tests could account for 2-3 ksi

• Loss of yield point due to damage to steel accounts for 2-4 ksi in s

• The distribution of wide flange core column properties is lower than expe

historical data; the distributions for other components are consistent with data.

• The safety of the WTC towers on September 11, 2001 was most like

affected by the fraction of steel that, according to NIST testing, did the specified minimum yield strength. The typical factors of safety stress design can accommodate the measured property variations minimum.

Findings on Mechanical Properties of WT

Findings on Mechanical Properties of WT
Enhancements added by NIST.

Broken 

Column Longitu

Panel JuObscure

Calculated frequencies and periods without P-∆ effects for the WTC to

to

Torsion

6 0.417 2.4

6 0.417 

E–W

5 0.263 3.8

4 0.238 

N–S

4 0.233 4.3

5 0.263 

Torsion

3 0.192 5.2

3 0.192 

E–W

2 0.093 10.7 1 0.088 

N–S 1 0.088 11.4

2 0.093 

(Hz) (s) (Hz) 

Mode Frequency Period Mode Frequency 

Direction of WTC 1 WTC 2

Motion

Calculated frequencies and periods with P-∆ effects for the WTC tower

towe

Torsion

8 0.455 2.2

8 0.455 

E–W

5 0.250

4

4 0.227 

N–S

4 0.227 4.4

5 0.250

Torsion

3 0.189 5.3

3 0.192 

E–W

2 0.088 11.3 1 0.083 

N–S 1 0.083 12.1

2 0.089 

(Hz) (s) (Hz) 

Mode Frequency Period Mode Frequency 

Direction of WTC 1 WTC 2

Motion
Comparison of Measured and Calculated Na

Frequencies and Periods of WTC 1

N-S E-W Torsion N-S E-W 

October 11, 1978 11.5 mph, E/SE 0.098 0.105 0.211 10.2 9.5 January 24, 1979 33 mph, E/SE 0.089 0.093 0.203 11.2 10.8 

March 21, 1980 41 mph, E/SE 0.085 0.092 0.201 11.8 10.9 

Decmber 11, 1992 - 0.087 0.092 - 11.5 10.9 

February 2, 19931 20 mph, NW 0.085 0.093 0.204 11.8 10.8 

March 13, 19931 32 mph, NW 0.085 0.094 0.199 11.8 10.6 March 10, 19941 14 mph, W 0.094 0.094 0.196 10.6 10.6 

December 25, 19942 N 0.081 0.091 - 12.3 11.0 

Average - 0.088 0.094 0.202 11.4 10.6 

Theoretical Value - 0.084 0.096 - 11.9 10.4 

Reference Global Model

LERA/NIST - WTC 1 

without P-Delta 0.088 0.093 0.192 11.4 10.7 

LERA/NIST - WTC 1 

with P-Delta 0.083 0.088 0.189 12.1 11.3 

Notes:

1Reported frequency value is the average of the SW corner, NE corner and center core frequency m2Reported frequency is based on center core data only.

Orginal Design - Predicted Values

Direction of Motion

Frequency (HZ) Period 

Direction of 

Data Source/ 

Event Date Wind Speed & 

Direction

Historical Data

Average of Measured Data
Use of Wind Tunnel Testing to Estimate 

Use of Wind Tunnel Testing to Estimate 

• Wind loads were a major governing factor in the design 

components that made up the WTC tower structures, esthe perimeter frame-tube system.

• Wind loads are relevant to evaluating:

• The baseline performance of the WTC towers.• The reserve capacity of the structures to withstand unan

events such as major fires or impact damage.

• Design practices and procedures that were used.

• NIST has completed an independent analysis to establish t

performance of the WTC towers under the original design wand has compared those results with then-prevailing code requirements.

• In July 2004, the designer provided NIST with interpretation 

information contained in original source documents that wadetermine the design wind loads used for the WTC towers. has a better understanding based on this information.
Design Wind Loads for the WTC Towers Design Wind Loads for the WTC Towers

Wind loads being considered include:

 Original WTC design wind loads determin

from wind tunnel testing in the 1960’s

 Wind loads based on two recent state-of-th

practice wind tunnel studies conducted by 

and RWDI for insurance litigation, 2002

 Refined wind load estimates developed by 

based on the RWDI data, and reviewed by Sunder contract to NIST, 2004
Baseline Performance Analysis Baseline Performance Analysis

• Load Combinations

 Original WTC design loads case:

• WTC design gravity (dead and live) loads• Original WTC design wind loads (98 mph 20-min aver

1,500 ft above ground; equivalent to 67-75 mph fastesft above ground). 

 Lower-bound state-of-the-practice case:

• Current New York City Building Code (NYCBC) live lo• RWDI wind loads with wind speed scaled to the curre

wind speed (80 mph fastest mile at 33 ft above ground

 Refined NIST estimate case:

• Current ASCE 7 Standard (a national standard) live lo• Wind loads developed by NIST based on refinements 

consider current state of the art in wind engineering (8fastest mile at 33 ft above ground). 
Wind Load Estimates for WTC 2

1960’s 13.1 10.1 16.5 8.8 12

Original WTC Design (Clarified by designer in July 2004)

2004 12.2 14.0 15.6 12.8 11

NIST / third-party SOM review / ASCE 7-02

CPP / ASCE 7-98 2002 15.1 15.3 17.1 15.5 14

*

2002 NA NA NA NA N

CPP / NYC Building CodeRWDI / ASCE 7-98 2002 10.6 12.2 13.5 11.1 10

2002 9.7 11.1 12.3 10.1 9

RWDI / NYC Building Code

9.3 9.3 7.6 7

1968 - 2001 NYC Building Code

5.3 5.3 4.2 4

Prior to 1968 NYC Building Code

Ab

E

About NS N-S E-W Resultant

Base Mome

Base Shear 10 

3 kips

Source Year

* Using ASCE 7-98 sections 6.5.4.1 and 6.6
Wind Load Estimates for WTC 1

1960’s 9.8 10.6 14.0 10.3 9.1 

Original WTC Design (Clarified by designer in July 2004)

2004 14.1 13.0 16.1 12.4 13.

NIST / third-party SOM review / ASCE 7-02

CPP / ASCE 7-98 2002 NA NA NA NA NA 

2002 NA NA NA NA NA 

CPP / NYC Building CodeRWDI / ASCE 7-98 2002 12.3 11.3 14.0 10.8 11.

2002 11.4 10.5 13.0 10.1 10.

RWDI / NYC Building Code

9.3 9.3 7.7 7.7 1968 - 2001 NYC Building Code5.3 5.3 4.2 4.2 Prior to 1968 NYC Building Code

Abo

E-W About N-S N-S E-W Resultant

Base Mome

Base Shear 10 

3 kips

Source Year
Base Shears and Base Moments Due to W

Loads from Different Building Codes

4.2 7.7 7.6 8.5 

Base Moment(106 kips-ft)

5.3 9.3 9.5 9.8 

Base Shear(103 kips)

1965

BOCA/BBC

1964

NY State Code

1968-2001

NYC Code

1938

NYC Code
Findings on Design Wind Loads Findings on Design Wind Loads

 The original design wind loads on the towers exceeded thos

established by the New York City building code prior to 196WTC towers were designed) and through 2001 (when the Wwere destroyed).

 The original design wind load estimates were higher than those 

other selected building codes of the era (Chicago, New York Staincluding the relevant national model building code (BOCA). 

 Refined estimates of the resultant wind loads developed by NIST 

by as much as about 15% than the resultant original design wind 

WTC 1, and lower by about 5% than the resultant original design WTC 2. 

 Estimated wind-induced loads on the towers vary by as mu

between two wind tunnel/climatological studies conducted independent laboratories, voluntarily provided to NIST by pinsurance litigation concerning the WTC towers; the state-oknowledge in wind engineering is evolving.
Sources of Major Differences in Wind Lo

Sources of Major Differences in Wind Lo

Estimation Methods Used in Current Pra

Estimation Methods Used in Current Pra

• Design wind speed (codes, standards, site-specific e• Hurricane wind profile (whether or not hurricane wind 

are flatter than the profiles for extratropical windstorm

• Estimation of “component” wind effects with a specifi

recurrence interval by integrating wind tunnel data wispeed and direction information (e.g., up-crossing mesector-by-sector method, storm passages approach)

• Estimation of “resultant” wind effects using load comb

methods (e.g., principle of companion loads, companin-time loads)
Baseline Performance Analysis Baseline Performance Analysis

 Analysis completed by LERA under contract to NI

Results presented reviewed by NIST and SOM uncontract to NIST.

 Lateral drift ratios estimated and compared with d

considered in practice.

 Demand/Capacity Ratios (DCRs) for structural co

estimated using Allowable Stress Design (ASD).
Results and Findings of Drift Analysis Results and Findings of Drift Analysis

70.6 H/244 83.9 H/205 75.6 H/227 71

Refined NIST caseSOP case 56.8 H/303 68.1 H/253 59.7 H/287 56

56.6 H/304 55.7 H/309 51.2 H/335 65

Original design case

Tot

Dr

(in

Drift 

Ratio

Total 

Drift 

(in.)

Drift 

Ratio

Total 

Drift 

(in.)

Drift 

Ratio

Total 

Drift 

(in.)

E–W N–S E–W 

WTC 1 WTC 2

Loading Case

 The calculated drift ratios correspond to a damping ratio of 2.5% in estimated wind  Typical drift ratios considered in practice for serviceability, stability, and safety (not 

building codes):

• H/500 in Chicago (~ 32.9 in.)

• H/400 in New York City (~ 41.0 in.)

 Under the original design wind loads, the WTC towers would need to ha

between 1.2 to 1.5 times stiffer to achieve a H/400 drift limit, and betweetimes stiffer to achieve a H/500 drift limit; this could be efficiently achieved exterior column areas in lower stories and/or significant additional damping.
Maximum Inter-story drift for WTC 1 and 

Refined NIST case h/180 h/150 h/160 SOP case h/225 h/185 h/200 Original design case h/225 h/230 h/230 

E–W N–S E–W 

WTC 1 WTC 2

Loading Case

0

200400600800

1000120014001600

0.00 0.20 0.40 0.60

Interstory Drift/Story Height [%]

Height above B6 Level [ft.]

E–W direction

0

200400600800

1000120014001600

0.00 0.20 

Interstory D

Height above B6 Level [ft.]

Typical inter-story drift values considered in practice for serviceability (not required by building codes) range from h/300 to h/400.Inter-story drifts for WTC 1 under original design wind loads 
Results of Baseline Analysis for WTC 1 Results of Baseline Analysis for WTC 1

 DCRs for Structural Components under Original WTC Design 

North East South W
Results of Baseline Analysis for WTC 1 Results of Baseline Analysis for WTC 1

* Number of members includes columns with 1⁄2 floor height due to the presence of column splices.

The safety of the WTC towers on September 11, 2001 was most likely not affected by the fraction of members for which the demand exceeded allowable capacity.

Mean 

DCR

% members 

with DCR>1

% members 

with DCR>1.05

Appro

memb

DCR

Exterior Columns (Floor 9-106)

Original WTC Design LoadsLower Estimate SOP CaseRefined NIST Estimate Case

0.760.781.10

1.1

2

72

0.40.9

60

12

18

Spandrel Beams (Floor 9-106)

Original WTC Design LoadsLower Estimate SOP CaseRefined NIST Estimate Case

0.310.320.52

00

0.5

00

0.3

1

Core Columns

Original WTC Design LoadsLower Estimate SOP CaseRefined NIST Estimate Case

0.860.860.84

10

9.98.9

5.35.35.2

222

Hat Truss (Columns)

Original WTC Design LoadsLower Estimate SOP CaseRefined NIST Estimate Case

0.470.450.53

0.40.43.8

0.40.40.8
Findings of Baseline Performance Analy

Findings of Baseline Performance Analy

• Normal design practice is intended to ensure that demand is 

capacity.

• DCRs estimated from the original design case are in general c

obtained from the lower bound state-of-the practice case. Fo

loading cases, a small fraction of structural components had DCthan 1.0. These were observed around the corners of the exteriocolumns and spandrels as well as the core columns.

• DCRs from the refined NIST estimate case exceed those from 

design and state-of-the-practice cases due to the following reaso

• The refined NIST estimate wind loads are higher than those 

lower bound SOP case by about 25 percent. Note that the r

wind loads are 20 percent smaller than those obtained by Cupper bound SOP case).

• The current national standard for loads (ASCE 7-02) does n

1/3 increase for the allowable dead load induced stresses.
Findings of Baseline Performance Analy

Findings of Baseline Performance Analy

• The allowable stress design method has an inherent factor of safety for 

components. For example, the safety factor for yielding and buckling is

• 1.67 and 1.92 for core columns in the original design and SOP cases, and for 

refined NIST estimate case.

• 1.26 and 1.44 for perimeter columns in the original design and SOP case (dis

1/3 increase in allowable stress under wind loads).

• After reaching the yield strength, structural steel components continue to 

significant reserve capacity, thus allowing for load redistribution to other that are still in the elastic range.

• On September 11, the towers were subjected to in-service live loads, wh

considered to be approximately 25 percent of the design live loads.

• On September 11, the wind loads were minimal, thus allowing significan

reserve capacity for the exterior walls (demand on exterior columns was their capacity).

• The safety of the WTC towers on September 11 was most likely not 

the fraction of members for which the demand exceeded allowable 
Web site Web site http:// http://wtc.nist.gov wtc.nist.gov

Email to Email to wtc@nist.gov wtc@nist.gov

Facsimile to (301) 975 Facsimile to (301) 975-6122

Regular mail: Regular mail:

WTC Technical Information Repository, Stop 8

Technical Information Repository, Stop 8

100 Bureau Drive, Gaithersburg, MD 20899 Bureau Drive, Gaithersburg, MD 20899-86
Background Slides Background Slides
No burn

faint cracks from drying of paint

700 deg C for an hourmud cracks partially obscured by formation of whiteish “sandy” phase on surfacedelamination easy with finger

1000 deg C for an hourwholescale spalling of paint, falls offthick scale forms between paint and steel

250 deg C for an hour 1 mm mud cracks much more visible

Burn Tests on Primer Paint Burn Tests on Primer Paint

Observaticondition paint coudetect pienot excee

and those exceeded did not ex