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NIST NCSTAR1-3D

Federal Building and Fire Safety Investigation of the

World Trade Center Disaster

Mechanical Properties of Structural

Steels

William E. Luecke

J. David McColskey

Christopher N. McCowan Stephen W. Banovic^ Richard J. Fields

Timothy Foecke

Thomas A. Siewert

Frank W. Gayle

National Institute of Standards and Technology • Technology Adminisiralion •^ U S. DeparlmenI^ of^ Comm

Disclaimer (^) No. 1 Certain commercial entities, equipment, products, or materials are identified in this document 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. Nor (^) does such identification imply a finding of fault or negligence by the National Institute of Standards and Technology.

Disclaimer No. (^2) The policy of NIST is to use the International System of Units (metric units) in all publications. In this document, however, units are^ presented^ in metric^ units^ or^ the^ inch-pound^ system,^ whichever^ is^ prevalent^ in^ the^ discipline.

Disclaimer No. 3

Pursuant to section 7 of the National Construction Safety Team Act, the NIST Director has determined that certain

evidence received by NIST in the course of this Investigation is "voluntarily provided safety-related information" that is "not directly related^ to the building^ failure^ being investigated"^ and^ that^ "disclosure^ of that^ information^ would^ inhibit^ the voluntary provision of that type of information" (15 USC 7306c). In addition, a substantial portion of the evidence collected by NIST in the course of the Investigation has been provided to NIST under nondisclosure agreements.

Disclaimer No. 4 NIST takes no position as to whether the design or construction of a WTC building was compliant with any code since, due to the destruction of the WTC buildings, NIST could not verify the actual (or as-built) construction, the

properties and condition of the materials used, or changes to the original construction made over the life of the

buildings. In addition, NIST could not verify the interpretations of codes used by applicable authorities in determining

compliance when implementing building codes. Where an Investigation report states whether a system was

designed or installed as required by a code provision, NIST has documentary or anecdotal evidence indicating whether the requirement was met, or NIST has independently conducted tests or analyses indicating whether the

requirement was met.

Use in Legal Proceedings

No part of any report resulting from a NIST investigation into a structural failure or from an investigation under the

National Construction Safety Team Act may be used in any suit or action for damages arising out of any matter

mentioned in such report (^) (15 USC 281a; as amended by P.L. 107-231).

National Institute of Standards and Technology National Construction Safety Team Act Report 1-3D

Natl. Inst. Stand. Technol. Natl. Constr. Sfty. Tm. Act Rpt. 1-3D, 322 pages (September 2005)

CODEN: NSPUE

U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 2005

For sale by the Superintendent of Documents, U.S. Government Printing Office internet: bookstore.gpo.gov —^ Phone: (202) 512-1800^ —^ Fax:^ (202) 512- Mail: Stop SSOP, Washington, DC 20402-

Abstract

This report provides five types of mechanical properties for steels from the World Trade Center (WTC):

elastic, room-temperature tensile, room-temperature high strain rate, (^) impact, and elevated-temperature

tensile. Specimens of 29 different steels representing the 12 identified strength levels in the building as

built were characterized. Elastic properties include modulus, E, and Poisson's ratio, v, for temperatures up

to 900 °C. The expression for E{T) for T < 723 °C is based on measurements of WTC perimeter column steels. Behavior for T > 723 °C is estimated from literature data. Room temperature tensile properties

include yield and tensile strength and total elongation for samples of all grades of steel used in the towers.

The report provides model stress-strain curves for each type of steel, estimated from the measured stress-

strain curves, surv iving mill test reports, and historically expected values. With a few exceptions, the

recovered steels, bolts, and welds met the specifications they were supplied to. In a few cases, the

measured yield^ strengths^ of recovered^ steels^ were^ slightly^ lower^ than^ specified,^ probably^ because^ of^ a

combination of mechanical damage, natural variability, and differences in testing methodology. High-

strain-rate properties for selected perimeter and core column steels include yield and tensile strength, total

elongation and strain rate sensitivity for rates up to 400 s"'. Measured properties were consistent with

literamre reports on other structural steels. Impact properties were evaluated with Charpy testing.

Properties for perimeter and core column steels were consistent with other structural steels of the era. The

impact toughness at room temperature of nearly all WTC steels tested exceeded 15 ft lbf at room

temperamre. Elevated-temperature stress-strain curves were collected for selected perimeter and core

column and truss steels. The report presents a methodology for estimating high-temperature stress-strain

curves for the steels not characterized based on room-temperature behavior and behavior of other

structural steels from the literature. The measured elevated-temperature stress-strain behavior of WTC

steels is consistent with other structural steels from that era. For the truss steels, the report presents a

complete constitutive law for creep deformation based on experimental measurements. For the steels not

characterized, the report presents a methodology for estimating the creep deformation law.

Keywords: Creep, high strain rate, high temperature, impact, modulus, tensile strength, yield strength.

World Trade Center.

NISTNCSTAR 1-3D, WTC Investigation ill

Table of Contents

Abstract (^) iii List of Figures (^) ix

List of Tables xv

List of Acronyms and Abbreviations xvii

Preface (^) xix

Acknowledgments xxix

Executive (^) Summary xxxi

Chapter 1

Introduction 1

1. 1 Overview of Report 1

1.1.1 Elastic Properties (Chapter (^) 2) 1

1. 1 .2 Room Temperature Tensile Properties (Chapter (^) 3) 1 1.1.3 High-Strain-Rate (^) Properties (Chapter 4) 3
2. 1 .4 Impact Properties (Chapter (^) 5) 3 1.1.5 Ele\ ated-Temperature Properties (Chapter 6) 3

1 .2 Description of the Major Building Components 4

1.2.1 Perimeter Columns 4

1.2.2 Core Columns 6

1 .2.3 Flooring System 6

1 .3 Specimen Nomenclature 7

1 .4 Symbols and Abbreviations 8

Chapter 2

Elastic Properties 11

2.1 Introduction 1

1

2.2 Experimental Procedure 11

2.3 Elastic Properties (E, v, G) for 0<T<723 °C 1

1

2.4 Elastic Properties (E, v, G) for T>910 °C 13 2.5 Elastic Properties (^) (E, v, (^) G) for 723 °C<T<910 °C 13 2.6 Uncertainties 14 2.7 (^) References 14

NIST NCSTAR (^) 1-3D. WTC Investigation v

Table of^ Contents

Table of Contents

6.3.1 Tensile Tests 130 6.3.2 Creep Tests 130 6.4 Recommended values for steels 134 6.4. 1 A Universal Curve for Elevated-Temperature Tensile Properties 1 34 6.4.2 Analysis of Tensile Data 136 6.4.3 Estimating Elevated-Temperature Stress-Strain Curves 137

6.4.4 Analysis of Creep Data 149

6.4.5 Recommended Values for Bolts 155 6.5 Summary 157 6.6 References^158

Chapter? Summary and Findings 161 7.1 Summary 161 7.2 Findings 162

Appendix A Data Tables and Supplemental Figures 163

Appendix B

Effects of Deformation of Wide-Flange Core Columns on Measured Yield Strength 253

Appendix C

Provisional Analysis of High-Rate (^) Data 263

Appendix D

Deformation of Steels Used in WTC 7 273

Appendix E Specimen Geometry Effects on High-Rate Tensile Properties (^279)

vni NIST^ NCSTAR^ 1-3D,^ WTC^ Investigation

List (^) of Figures

Figure P-1. The eight projects in the federal building (^) and fire safety investigation of the WTC disaster xxi

Figure 1-1. Cross section of a perimeter column; sections with and without spandrels 4

Figure 1-2.^ Characteristic perimeter column panel illustrating the various components.

Designations in parentheses refer to the specimen nomenclature of Table 1-1^5

Figure 1-3. Typical welded box columns and rolled wide-flange shapes used for core columns

between the 83rd and 86th floors 6

Figure 1^. Schematic diagram of a floor truss 7

Figure 2-1. Young's modulus as a function of temperature 15

Figure 2-2.^ Young's modulus, EiT). and shear modulus, G(T) for 0°C <^ T< 725°C. Young's modulus was measured on the WTC steels summarized in Table 2-1. Solid line is

Eq. 2-2.^ Shear modulus, G, calculated from and v via Eq. 2-4^16

Figure 2-3. Poisson's ratio (v) as a function of temperature. The sohd line is the fit of a 4th order

polynomial (Eq. 2-3) for 0 °C <^ r< 725 °C 16

Figure 2—4. Fractional error in representing the Young's modulus data for the three specimens of

perimeter column steel (Table 2-1) using Eq. 2-2^17

Figure 3-1. Flat tensile specimen typically used for standard room-temperature quasi-static tensile tests 21

Figure 3-2. Flat tensile specimen typically used for elevated-temperature tensile tests 2

1

Figure 3-3. Flat tensile specimen used for room and elevated-temperamre tensile tests 22 Figure 3^. Flat tensile test specimen used for some creep tests 22

Figure 3-5. Flat tensile test specimen used for some creep and elevated-temperature tests 23

Figure 3-6.^ Round tensile specimen used for room-temperature and elevated-temperamre tensile

tests ; 23

Figure 3-7. Round tensile specimen used for room-temperature tensile testing 24

Figure 3-8.^ Flat tensile specimen typically used for tensile testing of all-weld metal specimens^24

Figure 3-9.^ Heat affected zone tensile test specimen with flange/web weld^ intact.^ The^ flange^ is

the specimen portion that is in tension 25 Figure 3-10. Heat affected zone tensile specimen with weld and web machined flush^ to^ the^ flange

surface. The flange is the specimen portion^ that^ is^ in^ tension^25

Figure 3-1 1. Notched tensile specimens 26

Figure 3-12.^ The resistance weld shear strength test (a) before loading,^ (b)^ after^ failure^27

NISTNCSTAR 1-3D, WTC Investigation ix

List of Figures

Figure 4-6.^ Examples of tensile high-rate stress-strain curves for F, =^50 ksi perimeter column steel M26-C1B1-RP (^) ; 87

Figure 4-7. Example stress-strain and strain rate-strain curves for Kolsky tests 87

Figure 4-8.^ Quasi-static compression stress-strain curves for the tests summarized in Table 4-4 89

Figure 4-9.^ Strain^ rate^ sensitivity^ of yield and tensile strength as a function of specified minimum yield strength (^92)

Figure 4-10.^ Flow stress as a function of strain rate for Kolsky tests 93

Figure 4-11.^ Flow stress as a function of strain rate evaluated at different strains for Kolsky tests

on the A 325 bolt 94 Figure 4-12. Comparison of strain rate sensitivities of NIST WTC steels to values for structural

steels from the literature 96

Figure 4-13. Total elongation, El„ as a function of strain rate for high-strength, perimeter column

steels and high-strength steels in the literature 98

Figure 4-14. Total elongation, El„ as a function of strain rate for low-strength, core column steels

and low-strength steels in the literature 99

Figure 5-1^. An^ example^ transition^ curve^118

Figure 5-2. Charpy impact specimen geometries and orientations with respect to the plate rolling

direction 118

Figure 5-3.^ Longitudinal^ and transverse^ Charpy impact data of samples from the flange^ and

adjacent HAZ of perimeter column N8-C1M1, the flange and adjacent HAZ of perimeter column C 1 0-C 1 M 1 119

Figure 5-4. Longitudinal and transverse Charpy impact data for the spandrel associated with

perimeter column N8 and the web of wide-flange core column C-80 120 Figure 5-5.^ Transverse Charpy impact data from samples from perimeter column truss seats^ M4,

N13, and N8, and from floor truss components 121

Figure 5-6.^ Longitudinal Charpy impact data for A 325 bolts 122 Figure 5-7. Summary plot of the dependence of absorbed energy on test temperature for all

perimeter and core column steels. The absorbed energy values of the sub-size

specimens have been corrected using Eq. 5-2^ to compare them to data from full-size

(10 mm^ by^10 mm)^ specimens^123 Figure 5-8.^ Summary plot of the dependence of absorbed energy on test temperature for all truss

component and truss seat steels. The absorbed energy values of the sub-size

specimens have been corrected using Eq. 5-2 to compare them to data from frill-size

(10 mm by 10 mm) specimens 124 Figure 5-9.^ Strength-toughness relationships for several types of structural steels^ from^ the^ WTC construction era, after Irvine (^) (1969) 125

Figure 5-10. Scanning electron micrographs of the fracture surface of a Charpy V-notch

longitudinal (^) specimen orientation from an N8-C1M1 perimeter column (WTC 1-142-

97-100). (a) ductile dimples (oval feamres) and general surface morphology, (b)^ low

magnification view of large and small ductile dimples on the^ fracture surface,^ (c)

higher magnification view 125

NISTNCSTAR (^) 1-3D, WTC Investigation xi

List of Figures

Figure 5-11.^ Scanning electron micrographs of the fracture surface of an N8-C1M1 perimeter

column sample showing ductile tearing features that form due to fracture initiation

and growth at elongated inclusions and pearlite on planes parallel to the rolling plane.

The "ductile dimples" in this case are linear features with a peak-valley morphology 126

Figure 5-12.^ Perspective view of the fracture surface of sample N8-C1M1 showing the long peak- valley features characteristic of the fracture surface for transversely oriented impact

specimens. The green line indicates the topography of the fracture surface 126

Figure 5-13. A gray-scale image (a) and compositional maps from fracture surface of an N8- ClMl perimeter column Charpy V-notch specimen. The relative concentrations of

(b) iron,^ (c)^ manganese,^ and^ (d) sulfur. The surface of the "ductile dimple" is littered

with the remnants of manganese sulfide inclusions 127

Figure 5-14. The fracture surface of a perimeter truss seat, N13-C3B1 that was tested at room temperature shows cleavage facets, which indicate a brittle fracture mode 127

Figure 6-1. Elevated-temperature stress-strain curves. Specimen N8-C1B1 A-FL is from a Fy 60 ksi perimeter column flange plate from WTC 1 column 142 between floors

97-100. Annotations refer to individual test specimen numbers 131

Figure 6-2. Creep curves of A 242 truss steel from specimen C-132 at 650 °C. Dashed lines

represent the fit from Eq. 6-14 using the parameters in Eqs. 6-16, 6-17, and 6-18.

Experimental curves are graphically truncated at e =^ 0.05 132 Figure 6-3. Creep curves of A 242 truss steel from specimen C-132 at 600 °C. Dashed lines

represent the fit from Eq. 6-14^ using the parameters in Eqs. 6-16, 6-17, and 6-18.

Experimental curves are graphically truncated at s =^ 0.05 132 Figure 6-4.^ Creep curves of A 242 truss steel from specimen C-132 at 500 °C. Dashed lines

represent the fit from Eq. 6-14 using the parameters in Eqs. 6-16, 6-17, and 6-18.

Experimental curves are graphically truncated at e =^ 0.05 133 Figure 6-5.^ Creep curves of A 242 truss steel from specimen C-132 at 400 °C. Solid lines

represent measured creep strain. Dashed lines represent the fit from Eq. 6-14 using

the parameters in Eqs. 6-16, 6-17, and 6-18.^ Experimental curves are graphically

truncated at s =^ 0.05 133

Figure 6-6.^ Ratio,/ of room- to high-temperature yield strength (F,.) for all steels characterized.

The spread of data at room temperature exists because for a given steel, the

individual tests are normalized to the mean room temperature yield strength. The

solid line is the expression, Eq. 6-1, developed using literature data on structural

steels, which^ are denoted by the smaller symbols 135

Figure 6-7. Ratio of room- to high-temperature tensile strength (TS) for the steels in Table 6-1.

The spread of data at room temperature exists because for a given steel, the

individual tests are normalized to the mean room temperature yield strength. The

solid line is the expression developed for literature data on structural steels, Eq. 6-2,

denoted by the smaller symbols 135

Figure 6-8.^ K(T), Eq. 6-5,^ for the A 36 steel of Harmathy (1970), used to model the behavior of

steel withF, =^36 ksi 140

Figure 6-9. n(T), 6-6, for the A 36 steel of Harmathy (1970) used to model the behavior of steel with =36ksi 140

xii NIST^ NCSTAR^1 -3D,^ WTC^ Investigation

List of Figures

This page (^) intentionally left blank.

xiv (^) NISTNCSTAR (^) 1-3D, WTC (^) Investigation

List (^) of Tables

Table P-1. Federal building and fire safety investigation of the WTC (^) disaster xx Table P-2. Public meetings and briefings of the WTC Investigation xxiii

Table 1-1^. Specimen nomenclature for perimeter column specimens 9

Table 1-2.^ Specimen nomenclature for core box and wide-flange shapes, trusses, and all other

specimens (^10) Table 1-3. Mechanical testing definitions used in this report 10

Table 2-1^. Specimen^ data^ for^ Young's^ modulus^ (E) determination 15

Table 3-1^. Results of tensile tests on bolts 3

1

Table 3-2.^ Room-temperature^ weld^ properties^ as^ measured^32

Table 3-3. Results of transverse tensile tests on welds from specimen N-8 (WTC 1, column 142, floors 97-100, specified F, =^60 ksi) 32

Table 3—4. Fillet weld sizes for various plate thicknesses in the core box columns 32

Table 3-5. Summary of mechanical properties and chemical compositions for steels from

low-strength perimeter columns 40

Table 3-6. Summary of mechanical properties and chemical compositions for steels from high-

strength perimeter columns 43

Table 3-7. Summary of mechanical properties, chemical compositions, and relevant ASTM and

Yawata specifications for steels from high-strength perimeter columns 45

Table 3-8.^ Summary of mechanical^ properties^ and^ chemical^ compositions^ for^ steels^ from^ core

column wide-flange shapes 49

Table 3-9.^ Summary of mechanical properties and chemical compositions for steels from core box

columns 51

Table 3-10. Common truss component dimensions and standards 57 Table 3-11.^ Summary of mechanical properties and chemical compositions,^ and^ specifications for truss steels tested 58 Table 3-12. Summary of mechanical properties, chemical compositions for truss seat steels tested 61 Table 3-13.^ Estimated static yield strengths and work-hardening^ parameters, Eq. 3-5,^ for^ perimeter

column steels 66

Table 3-14.^ Estimated static yield strengths and work-hardening^ parameters, Eq.^ 3-5,^ for^ core^ column

and truss steels 68

Table 3-15. Room-temperature weld metal properties as designed^74

NIST NCSTAR 1-3D. WTC Investigation xv

List of Acronyms and Abbreviations

Acronyms

AISC American^ Institute^ of^ Steel^ Construction

AISI American^ Iron^ and^ Steel^ Institute

ASTM ASTM International AWS American Welding^ Society BPS Building Performance Study CVN Charpy V-notch FATT fracture appearance^ transition^ temperature FEMA Federal Emergency Management^ Agency HAZ heat-affected zone HSR high-rate style HSLA high-strength, low-alloy JIS Japan Industrial Standard LERA Leslie E. Robertson Associates LRFD load and resistance^ factor^ design METT mid-energy transition^ temperature NIST National Institute^ of^ Standards^ and^ Technology PANYNJ Port^ Authority^ of^ New^ York^ and^ New^ Jersey PC&F Pacific Car^ and^ Foundry PONYA Port^ of^ New^ York^ Authority SEAoNY Structural^ Engineers^ Association^ of^ New^ York SHCR Skilling, Helle,^ Christiansen,^ &^ Robertson SMA shielded metal^ arc SRS strain^ rate^ sensitivity use United States^ Code WF wide-flange (a^ type^ of^ structural^ steel^ shape^ now^ usually^ called^ a^ W-shape) WTC World^ Trade^ Center WTC 1 World Trade Center^1 (North^ Tower) WTC 2 World Trade Center^2 (South^ Tower)

NISTNCSTAR 1-3D. WTC Investigation XVll

List of Acronyms^ and Abbreviations

WTC7 World Trade Center 7

Abbreviations

°C degrees^ Celsius

°F degrees^ Fahrenheit

|im micrometer

ft foot

Fy yield strength

gal gallon GPa gigapascal;^ 1x10^^ N/m"

h hour

in. inch L liter

lb pound

Ibf pound force

kip a force equal to 1 000 pounds

ksi 1 ,000 pounds per square inch

m meter

min minute

mm millimeter Mn magnesium

min minute

MPa megapascal; 1x10^ s second

xvni NISTNCSTAR 1-3D, WTC Investigation