Federal Building and Fire Safety Investigat

of the World Trade Center Disaster

Project #3: Analysis of Structural St

Frank Gayle – Project Leader

Richard Fields – Technical Lead

Dave McColskey – Co-PI

Metallurgy Division 

Steve Banovic, Tim Foecke, Bill Luecke

Maureen Williams, Carrie Campbell, 

Dave Kelley, Sandy Claggett, Carlos Beauchamp

Materials Reliability Division (Boulder, Colorado)

Dave McColskey, Tom Siewert, Chris McCowan

Ray Santoyo, Lonn Rodine

Materials Science and Engineering Laboratory
Recovered WTC steel will be characterized to:

Compare tested properties to specified yield strengtlocation)Assess quality of the recovered steel Provide properties to be used in analysis of the build

3⁄4 deduce the baseline performance of the buildings und

gravity loads

3⁄4 determine how much damage was done to the buildin

especially the floors and core, by plane impacts

3⁄4 determine how the steel responded to the high tempe

the fires

3⁄4 determine the most probable structural collapse sequ
Project 3 interacti

other WTC Invest

Projects

Model: Building Aircraft Impact Damage Fires 

3. Analysis of 

Steel & Specs

5. Structu

Collaps

2. Baseline 

Performance & 

Impact Damage

4. Thermal & 

Tenability 

Environment
• Task 1 - Collect and catalog physical evidence

• Task 2 - Document failure mechanisms and dam• Task 3 - Metallurgical and mechanical propertie

determination (room temperature, high

temperature, high strain rate)

• Task 4 - Correlate specified properties with mea

properties

• Task 5 - Characterize thermal excursions of ste• Task 6 – Final report

Tasks
• Structural steel

• Design specifications 

• sections & minimum yield strengths• documents from:

• Port Authority of New York and New • Leslie E. Robertson Associates

• Material specifications (ASTM, etc)• Supplier production information

Yawata Steel (now Nippon Steel)Laclede SteelMany others

Task 1 - Collect and catalog physical evidence
Exterior Framed-Tube

Central CBox Colu

59 columns per side

Bar Joist

Concrete Slab

Floor

Covering

Trough

Decking

Air-condit

Electric DuctDuct

Technical Services

Technical ServicesTechnical ServicesTechnical Services

Skylobby

Skylobby

Express Elevators Express

Elevators

LocalElevators

Local

Plaza Level Elevators

WTC Tower Design

110 stories
Wind loads primary factor in p

column design.•14 different grades of steel

(36 to 100 ksi yield streng

•Arrangement of steel neither synor the same for the two towers

Gravity loads primary factor icolumn design.• 4 grades of steel

(99% are 36 and 42 ksi yield s

• Conventional (albeit massive!) beam construction

Simulated distribution of perimeter column yield strength

WTC 1, North Face
705

7

706

24

707

7

702

7

701

95

801

95

703

75

802

7

803

95

804

9

806

7

906

48

805

7

904

77

903

80

602

80

605

80

604

80

603

80

601

80

501

83

502

83

504

86

503

83

505

86

506

83

507

83

902

48

902

48

905

86

907

48

1001

80

1003

77

1004

86

1005

86

1006

77

100

77

1002

77

606

80

607

80

704

< 1

705

7

706

24

707

7

702

7

701

95

801

95

703

75

802

7

803

95

804

9

806

7

906

48

805

7

904

77

903

80

602

80

605

80

604

80

603

80

601

80

501

83

502

83

504

86

503

83

505

86

506

83

507

83

902

48

902

48

905

86

907

48

1001

80

1003

77

1004

86

1005

86

1006

77

100

77

1002

77

606

80

607

80

704

< 1

Core ColumnsTransition floors from welded box column to wide flange columColumns and orientations shown for 84th floor
Steel Search, Collection, Logging and Shipping t

SEAoNY – Dave Sharp, many othersNIST – John Gross (NIST- BFRL)

Dave McColskey (Matls Rel.)Steve Banovic (Metallurgy)

~ 1.5 million tons of deb1/4-1/3 steel
Salvaged Steel at NIST
DOCUMENTATION OF SAMPLE REMOVAL

Outside of building looking in at tree

Top

Bottom

C1 C3 C2

Middle

P21-C1M-1

P-21

Missing

CrushedSplitOnly part of column

Only IW left

A 130: 93-96

P21-C1B1

P21-C1B1

P21-C1B-1

P21-C2B-1

P21-C2B-1

P21-C2M-1

P21-C2M-1

P21-C2M-1

P21-C2B-1

1. Generic diagram2. Sample identification code3. Removal method that cause

disturbance to the surrou

Confidential and Pre-Decisional Document

A 130: 93-96(stenciled or stamped)
Catalog of Steel – according to class

7 Coupons from Bldg 5

32 Miscellaneous (bolts, other)

2 "Bowtie" pieces of exterior wall

25 Channels23 Trusses43 Wide Flange sections (WF)11 Box beams / Core Columns93 Perimeter panel sections

236 TOTAL PIECES
Catalog of Steel – identified perimeter panels

41 panels identified by serial number, 

other markings, or geometry

• Columns – all strengths from 50 to 100 ksi• Spandrels – all strengths from 36 to 70 ksi & 80 ksi

WTC 1

• 25 panels• 21 near impact floors

- 3 hit directly by plane

WTC 2

• 16 panels• 4 near impact floors

3⁄4 samples of all 14 grades specified in structural 

steel drawings are available for test
WTC 1

• 8 columns

• 5 wide flange• 3 built-up box columns

• 1 from impact zone

WTC 2

• 5 columns

• 2 wide flange• 3 built-up box columns

• 2 from impact zone

Catalog of Steel – identified core columns

Core 

Co

(

3⁄4 samples are available of 2 grades (36 and 42 ksi) 

of both box and wide flange columns, configurations which represent 99% of core columns in the towers.
North Tower

North Face

Identified

perimeter

panels
Specifications and Steel Supplier Documents

• Port Authority contracts – allowable steels

• ASTM steel designations• certain proprietary steel

• Structural Steel design drawings

• provide minimum yield strength for all steel compo

• ASTM specifications for individual steel types

• composition• mechanical properties (room temperature only)

• Supplier production information

• grade substitutions (always to higher strength)• typical properties for proprietary steels• Laclede Steel and Nippon Steel (Yawata) extreme

Analysis of these documents allow estimation of 

properties when specified minimum yield strength 
Truss Properties – Laclede Steel

Top ChordA 242 (50 ksi minimum Y

Bottom ChordA 36 specified 

(but mostly higher stren

supplied instea

WebA 36 (36 ksi minimum Yie

Specified properties for 

Supplier of approximately 50 truss variants of 60', 35', and bridging trusse

• Supplier documents show where substitutions we
Estimated Properties - Perimeter C

Pacific Car & Foun

Seattle, WA

55 800 tons, 36 ks

Plate 3: BethlehemTruss seat 36 ksi

Plate 1,2: Yawata

Spandrel (4) Yawata

s

s

Exterior wall column plates 1, 2, 4 

42 57 (1,3) Yawata "A 441 45 57 (3) Yawata "A 441 

50 58 Yawata "A 441 55 65 for plates with t<=1.5" Yawata "A 441 55 66 for plates with t> 1.5" Yawata WEL-TE60 70 for plates with t<=1.25" Yawata "A 441 60 71 for plates with t>1.25" Yawata WEL-TE65 76 for plates with t>0.5" Yawata WEL-TE65 76 for plates with t<=0.5" Yawata WEL-TE70 81 Yawata WEL-TE75 86 Yawata WEL-TE80 91 Yawata WEL-TE90 105 (2) Yawata "A 514 

100 105 Yawata "A 514 

Grade Estimated 

YS YS * Notes Steel source / 

(ksi) (ksi)

* Actual yield strength estimated by NIST based on supplier d
Task 2 Document Failure Mechanisms and Dam

• Analysis in progress• Examination of local damage and failure mechanisms• Requires deciphering of post-collapse damage from pre-colla

• image analysis, comparison of pre-collapse images with salvage

• Wiss, Janney, Elstner contractors

• Experts in structural failure analysis• Will provide observations and statistics of repeated patterns of p

impact failures/fractures of bolts, welds, truss seats, spandrel spand column splices, & fire damage described as function of locaor away from impact zone or fires)

• Identify any structural elements that might have been especially

sensitive to the fire

• These observations will help us estimate energy absorbed du

impact, and performance in fire.

Task 3 – Mechanical Property Determination

• Room Temperature Tensile

• Analysis of baseline structural performance• Comparison with specified properties

• High Strain Rate

• Analysis of aircraft impact damage• Analysis of most probably structural collapse se

• High Temperature

• Analysis of structural response to fires• Analysis of most probable structural collapse se
Room Temperature Mechanical Properties

NIST Tensile tests• Yield and ultimate strength, ductility and 

workhardening behavior (per ASTM A370 and E8)

• for comparison with specified properties• for analyzing baseline structural performanceSpecimens• perimeter columns (12 +) and spandrels (10)• core box columns (2) and wide flange columns (2) • truss components (3 +) and inner & outer seats (2 +)• channels, splice plates, welds, bolts
Mechanical properties 

30 60 90 120

Specified Yield Strength, ksi

30

60

90

120

M

e

a

s

u

r

e

d

Y

i

e

l

d

S

t

r

e

n

g

h

t

,

k

s

i

Truss SeaTruss RodT1 Angle

Right FlanOutside WN8 AngleLeft FlangInside We

COMPONE

N8 Inside WebL and T

Prelimi

pr

Note: Specified minimum values apply to plate tested at the mill, not s

Measured yield strength vs specified minimum yield stre
Measured Truss Properties

Chemistry

FY (ksi) 57.6 60.3 60.0

UTS (ksi) 78.3 76.6 80.5N 0.008 0.0066 0.008

Mechanical Properties

0.044

0.29

0.0280.016

0.860.18

A242 Angle

Elongations acceptable

V 0.036 0.038Cu 0.26 0.08

S 0.032 0.019

P 0.009 0.008Mn 0.77 0.79C (wt %) 0.20 0.21

Element A36 angle A242 round

Tested at NIST from truss specimens recovered from WTC. Location in building unknown

Observations* A36 componexceed minim* No substanti(chemistry/micmechanical) bspecified as A * Truss steel wpresent day A minimum yield 
High Strain Rate Mechanical Properties

NIST High Strain Rate tests

• High strain rates cause significant 

increase in strength of steel

• Higher strength and energy absorption 

slow aircraft to greater extent

• Accurate modeling must account for 

the associated decrease in damageto internal structure

• Aircraft impact lead to strain rates estimated at 100 to 100Specimens• perimeter columns and spandrels • core box columns and wide flange columns• bolts
Conventional tensile vs Kolsky Bar (compre

Kolsky B

High Strain Rate Mechanical Properties

HSR tensile tests

• Yield and ultimate strength, ductility 

and workhardening behavior 

• Strain rates up to 500 per second

(50,000% elongation per second)

HSR compression tests• Yield strength and workhardening

behavior 

• Strain rates from 500 per second to 5000 per 

second (500,000% elongation per second)

• Specialized Kolsky Bar equipment at NIST
Bolts 1 1 1

Core columns 2 2 2

Spandrels 2 2 2

Outer Columns 10 10 10

900 s-1 700 s-1 500 s Compression -1

Core columns 2 2 2

Spandrels 2 2 2

Outer Columns 10 10 10

500 s-1 300 s-1 100 s-1

Tensile test Strain rate

High Strain Rate Test Matrix

Grades of steel (duplicates) tested at each condition
High Strain Rate Test Data

0.0

20.040.060.080.0

100.0120.0140.0160.0

0 50 100 150 200 250 300 350 400 450 

Strain Rate, 1/s

Stress, ksi Yield UTS

Linear (Yield)Linear (UTS)
0.0

20.040.060.080.0

100.0120.0

0 50 100 150 200 250 300 350 400 

Strain Rate, 1/s

YieldUTSLinear (UTS)Linear (Yield)

Stress

ksi

High Strain Rate Test Data
• Hot Tensile Tests

– Yield, ultimate, ductility, workhardening– 400, 500, 600, 650 oC– Strain rate is critical– Provides guidance for creep tests

• Creep

– “run-away strain” in A36 occurs 100 oC lower if cree

included in standard fire

– 400, 500, 600, 650 oC @ 2 stress levels– constant load, measure strain vs. time– creep ductility

• Elastic Modulus (buckling concerns)

Elevated Temperature Properties

• The fires did not melt the steel structure• Steel loses strength and modulus (stiffness) at temperatu

in a fire 
High Temperature Tensile Test Results

Truss Properties

Ultimate tensile

strength

Yield Strength

Truss – upper chord
0.67 YSRT

0.33 YSR

High Temperature CreepDeformation under constant load

Testing under way for:

Core columnsPerimeter columnsFloor truss

measured

predicted

Total strain under constant load during heatin

Figure 11. Measured and predicted strains during a non-li

for stresses of 1/3 and 2/3 of the room temperatur

400 500 600 700 

Temperature, Degrees C 

Strain

%

643210
High Temperature – Elastic Modulus (stiffness)

Young's Modulus

120130140150160170180190200210220

0 200 400 600 800

Temperature, C

Modulus, GPa Range of NIST measurements 

on WTC steels

Equation based on 

NIST measurements

and other steel data
Current Status of Mechanical Test Pr

Bolts (tensile & High Strain Rate) 75% 9Welds 30% 12High Strain Rate (compression) 50% 10High Strain Rate (tensile) 75% 10Creep 10% 12High temperature tensile tests 40% 10RT tensile tests (quasistatic) 95% 9

% complete Est. comp
Finally,

Comments are welcome on:• Goals and scope of test program• Test methods• Analysis of data• Other areas of interest

The Materials Science and Engineering Labo

is proud to contribute its knowledge and exper

the investigation.