Fahim Sadek, Emil Simiu, William Fritz, and 

Building and Fire Research Labo

National Institute of Standards and Tec

U.S. Department of Commerce

fahim.sadek@nist.gov

Federal Building and Fire Safety Inves

of the World Trade Center Disast

Baseline Structural Performance a

Impact Damage Analysi

December 2, 2003
Scope of Project Scope of Project

• Baseline Performance

 Develop reference structural models of the WTC  Establish baseline performance under design lo

(gravity + wind)

• Aircraft Impact Damage

 Simulate aircraft impacts into the towers to estim

damage to structural, mechanical, and architect

 Determine the response of towers immediately 

close to collapse were the buildings immediatelimpact?)
Baseline Performance Baseline Performance

• Tasks

 T1: Develop structural databases for each of  T2: Develop reference structural models of th

• Typical floor models• Models of the whole towers

 T3: Establish baseline performance under w

loads:

• Estimate load vectors• Determine demand/capacity ratios (utilization rat

and connections
Structural Databases Structural Databases

• WTC DB Development: Data Entry

 Scanned tables and stored in TIFF files Optical character recognition (OCR)

• Converts to text format file• Modified to filter unnecessary characters

 Raw text file split screen compared with image 

and cleaned

 Text file imported into MS Excel
Structural Databases Structural Databases

• WTC DB Development: Quality Control

 First check during OCR process Second check: random, but methodical check 

engineer

 Third check: ‘cross-check rectify’

• Programmatically compared with database develope

consultant of the leaseholder of the towers

 Final review
Structural Databases Structural Databases

• WTC DB Development: Modifications to DB

Modifications to Members of the WTC-DB 

Item Summary Tower Element Floor 

ElemenEffecte

1 

Core Column Reinforcing A and B Numerous 98-106 

Core Colum

2 

Fiduciary Bank Vault B 

Col. 508B and Col. 1008B 97-45 

Core Colum

3 

Bombing of 26 February 1993 - Repair A 

Col. 324, Bracing G313A and G304A 

B-2 Level 

PerimeColumand Bracin

4 EXCO Stair A Col. 901A 26 

Core Colum
Structural Databases Structural Databases

• Skidmore, Owings & Merrill (SOM) third-party rev

Random checks of digitized databases a

properties

• Results

No discrepancies were found
Structural Databases Structural Databases

• NIST In-house Review

 Line-by-line review of all files Random checks by project leader Calculate all cross section properties and compare 

database

• Results

 Minor discrepancies were identified between the L

drawing books

• Examples: Book 1, entry 207 should be 287

Book 3, entry 3010 should be 301D

 No discrepancies were found for cross section pro
Reference Structural Models Reference Structural Models

• Task objective: Use structural databases to deve

finite element structural models of the towers. Musing SAP2000 Version 8, and include:

Floor modelsWhole tower models
Reference Structural Models Reference Structural Models

Floor models:

 3-D models of typical floor systems:

• A truss floor system (96th floor of WTC 1)• A mechanical floor (75th floor of WTC 2)

 Models are used:

• To establish the baseline performance of the floor sy

loads 

• To estimate the in-plane stiffness of the floor diaphra

simplified models for the floor system that accurately of the floor diaphragms

• As a reference for other significantly more detailed m

developed in Projects 2 (impact analysis) and 6 (firewith proper modifications
Typical Floor Systems Typical Floor Systems

Type 1 - WTC Typical Truss Floor Panel Plan

Tower A Floors: 10 - 24 60 - 66 Tower B Floors: 14 - 24 60 - 74

26 - 40 68 - 74 26 - 40 84 - 9150 - 58 84 - 91 50 - 58 93 - 106

93 - 105

Note: All panel types within 1" length tolerance, except floors 10,11, 39, 40, 70, & 71 which are within 

FR1 ER1 DR1 CR1 BR1 A1 B1 C1 D1 E1 

Note: H1 = H6Note: HR1 = HR6 

F1 E1 D1 C1 B1 A1 BR1 CR1 DR1 ER1 

G1

CORE

G

HI H

J1 

HR1 

GR1
Floor Systems: Floor Systems: Floor 96 Floor 96-A Model
Floor Systems: Floor Systems: Floor 75 Floor 75-B Model
Reference Structural Models Reference Structural Models

Whole tower models:

 3-D models of the 110-story structure and b

each of the two towers. Models include all p

elements including exterior columns, interioand bracings in basement floors, spandrel b

and simplified models for floor systems 

 Models will be used to establish the baseline 

each of the two towers under gravity and wi

 Reference for other significantly more detail

developed in projects 2 (impact analysis) ananalysis)
Tower Structural System FE Models Tower Structural System FE Models 

(Global Model) (Global Model)Models include:

Core columns Exterior panels

• Foundation to floor 7• Trees (transition from 3’-4 to 10’-0 col. 

spacing)

• Floor 9 to 106• Floor 107 to roof

Hat trussRigid floor diaphragmsFlexible floor diaphragms
Modeling of Exterior Panels Modeling of Exterior Panels

Detailed shell model of exterior panel E
Modeling of Floor Diaphragm Modeling of Floor Diaphragm

Equivalent floor diaphragm to capture the in-plane stiffness of the floor system
Whole Tower Models: Below Floor 9 Whole Tower Models: Below Floor 9
Whole Tower Models: Hat Truss Whole Tower Models: Hat Truss 
Structural Models Review and Val

Structural Models Review and Va

SOM Third-party Review

 Consistency with original design

• Random checks

 Verification/validation of models

• Review assumptions and level of detail• Perform analyses using various loading conditions 

the models

NIST In-House Review

 Consistency with original design

• Models geometry / cross section properties• Material properties

 Verification/validation of models

• Review assumptions and level of detail• Perform analyses using various loading conditions 

the models
Structural Models Review and Val

Structural Models Review and Va

• Conducted workshop for NIST investigators and contra

28, 2003 to review the reference models developed by 

 Workshop Attendees (26 experts): 

• LERA (2 experts)• SOM (2 experts): third-party reviewer• Teng & Associates (1 expert): outside experts on pr

collapse

• Professor Kasper Willam: outside expert on thermal

• Professor David M. Parks: outside expert on compu

aircraft impact analysis

• ARA (2 experts): contractor on analysis of aircraft im

towers

• NIST (17 experts): all key investigators

 Members of the NCST Advisory Committee invited Minutes of the workshop are being prepared Feedback from workshop will be included in the fin

models (in progress)
Use of Reference Models in the In

Use of Reference Models in the In

• NIST-approved models will be used as a 

reference for other significantly more detailed models to be developed in the investigation

 Aircraft impact analysis Thermal-Structural Response Collapse Initiation Sequence Analysis

• Detailed models should be traceable back 

to the NIST-approved reference models.
Baseline Performance Analysis Baseline Performance Analysis

• Conduct linear static, structural analyses of eac

towers to establish their baseline performance uloads:

 Gravity loads:

• Dead loads• Live loads used in the original design of the t• Live loads according to either current New Y

code or current ASCE 7 standard

 Wind loads:

• Wind loads used in the original design of the • Wind loads based on the state-of-the art in w

and extreme climatological information from applicable standards 
Baseline Performance Analysis Baseline Performance Analysis

• Wind loads:

 Reviewed original estimate of wind effects us

WTC towers. The estimates are consistent wart in wind engineering in the early 1960’s: knunavailable at that time for more scientific est

Given the significant wind engineering advansubsequent years, those estimates can be im

 NIST is comparing the wind loads from wind 

loads specified in building codes.

 NIST has reviewed two sets of wind tunnel te

the insurance litigation in 2002. Both tests su

load effects may be higher than in the origina
Baseline Performance Analysis Baseline Performance Analysis

• Wind loads:

 NIST is developing framework for analyzing e

directionality on structural response to evaluain the two 2002 studies. 

 NIST is developing wind loads on the WTC to

two 2002 sets of wind tunnel tests for analysiperformance and comparison with the origina
Aircraft Impact Analysis: Methodo

Aircraft Impact Analysis: Methodo

• The impact analysis will include the followin

analyses:

Component Level AnalysisSubassembly AnalysisGlobal AnalysisSimplified/Approximate Analysis
Aircraft Impact Analysis: Methodo

Aircraft Impact Analysis: Methodo

• Component level analysis:

 Exterior and interior column impacted by 

aircraft engine

 Exterior column impacted separately by:

• Segment of an aircraft wing• Model of aircraft fuel tanks filled with fuel

 Analyses include

• Highly-detailed finite element models considering 

possible dynamic plastic fracture criteria of materials

• Coarser finite element models similar to those 

used in the global analyses
Aircraft Impact Analysis: Methodo

Aircraft Impact Analysis: Methodo

• Subassembly analysis:

Analysis of a strip from the exterior wall 

to the core impacted separately by:

• A segment of the same width of an aircraft 

wing

• An engine

Analysis objectives:

• Study the interactive failure phenomenon of 

the aircraft and building components

• Study sensitivity of damage estimates to 

various uncertainties

• Identify the most influential parameters that 

affect the damage estimates
Aircraft Impact Analysis: Methodo

Aircraft Impact Analysis: Methodo

• Global analysis:

Develop finite element models of the tow

• Dense finite element mesh in the impact• Coarser mesh below and above impacte

towers)

Develop Boeing 767 aircraft modelAircraft impact initial conditions

• Aircraft speed, location of impact, and or

Examine tower stability after impact
Aircraft Impact Analysis: Methodo

Aircraft Impact Analysis: Methodo

• Simplified/approximate analysis:

Objective:

• Serves as check on global FE Simulatio• Supports the sensitivity and probabilistic 

Approach:

• Simplified models of aircraft and towers.• Aircraft breakup and fragment loading.• Evaluation of damage to the tower struct• Debris cloud effects.
Aircraft Impact Analysis: Methodo

Aircraft Impact Analysis: Methodo

• Expected outcomes:

 Estimates of the damage to structural system

• Exterior and interior columns: number and lo

columns:

 severed, severe damage, moderate dam

• Floor systems: floor portions and trusses tha

 Estimates of deformations and accelerations

• Estimates and contours of components of ac

deformations, including localized effects, as a 

• Deformations and accelerations will be comp

mechanical properties of spray-on fire resistadeveloped in an experimental and analytical 

NIST to estimate the extent of damage to the columns and floor systems.
Aircraft Impact Analysis: Methodo

Aircraft Impact Analysis: Methodo

• Expected outcomes:

Estimates of the aircraft fuel dispersal dEstimates of debris field:

• Database of major fragments of aircraft a

structural components of towers.

 Spreadsheet format defining mass, ap

speed, and trajectory. 

 Trajectory of each fragment consists o

point of exit or resting place. 

 If fragment is deflected dramatically by 

obstruction, its path is broken up into s
Aircraft Impact Analysis: Methodo

Aircraft Impact Analysis: Methodo

• Probabilistic Assessment:

Objectives:

• Sensitivity analysis to assess the effects 

associated with various parameters and influential parameters that affect the dam

 Orthogonal factorial design Component level and subassembly an

• Probabilistic analysis to determine the p

associated with different damage estima

 Event tree and Monte Carlo technique Global level analysis
Aircraft Impact Analysis: Status Aircraft Impact Analysis: Status

• Aircraft Model Development: Boeing 767-200E

 Documentary aircraft structural information Data from measurements on 767 aircraft
Aircraft Impact Analysis: Status Aircraft Impact Analysis: Status

• Aircraft Engine Model Development

Detailed FE 
Aircraft Impact Analysis: Status Aircraft Impact Analysis: Status

• Component level analyses

 Investigate detailed response and modeling 

Exterior Column Analyses Evaluation of F

(Lagrang

Evalu

(she
Aircraft Impact Analysis: Status Aircraft Impact Analysis: Status

• Component level analyses: Preliminary engine 
Aircraft Impact Analysis: Status Aircraft Impact Analysis: Status

• Tower model development:

 Using structural databases and reference 

tower models as a reference

 Impact analyses have different modeling 

requirement

A

Reference models 

LS-DYN

Models for multiple panels
Simplified Stability Analysis of WT

Simplified Stability Analysis of WT

How many failed columns are needed 

of stability? Where do the failed columbe located?

How many floors need to be removed 

of stability?
Simplified WTC tower models Simplified WTC tower models

 Bolted connection between exterior 

columns represented with equivalent rotational springs

 Load redistribution between columns 

occurs via hat truss 

 Floors below the region of fire and 

impact (85th floor for WTC 1) 

represented with equivalent vertical springs

 Gravity loads represent service 

conditions and are estimated from detailed floor model
Effect of Spandrel Stiffness Effect of Spandrel Stiffness 

North wall loads from a single fl

Stiffness Included Stiffnes

 Spandrel stiffness plays a key role in transferring floor loads 

must be included in the model

02468

101214

1

4

7

10

13

16

19

22

25

28

31

34

37

40

43

46

49

52

55

58

Columns Reactions (Kips)

02468

101214

1

4

7

10

13

16

19

22

25
92 64 62 43 43 64 

40 41 27 18 24 28 

37 31 24 6 10 24 

38 25 12 13 8 

40 35 20 11 18 28 

92 64 62 43 36 64 

Core Column Loads (Kips) from a S

Core Column Loads (Kips) from a S
Status of Simplified Stability Analy

Status of Simplified Stability Analy

Work is ongoing using three methods:

Linear static demand-capacity analys

service loads

Linear stability analysis to determine c

loads and shapes

Nonlinear stability analysis accounting 

and geometric nonlinearities
Summary Summary

 Completed the development of structural data

WTC towers. Final approval shortly.

 Completed the development of reference stru

the WTC towers. Models are under review.

 Baseline performance analysis will start upon 

reference models. Progress made in defining 

towers.

 Aircraft impact analysis: Progress made in de

aircraft model, component level analysis, and 

 Simplified stability analysis of the towers is on