S. Shyam Sunder

Acting Deputy Director and Lead Invest

Building and Fire Research Labo

National Institute of Standards and Tec

U.S. Department of Commerce

sunder@nist.gov

NIST Response to the World Trade Ce

World Trade Center Investigation 

October 19, 2004
Outline Outline

• Background and Schedule• Investigation Objectives• Collapse Hypotheses for WTC 1 and 2 • Fire Performance of Floor System• Analysis of Recovered Steel• Baseline Performance Analysis

• Design Wind Loads• Structural Performance

• Evacuation and Occupant Behavior• Emergency Response and Communications• Issues Related to Practice, Standards, and C
Background Background

• Good, solid technical progress on investigation; drawing on talent fro

and contractors; $16 million investigation; $5.5 million awarded in con

• Two public updates issued (December 2002, December 2003); two te

issued (May 2003, June 2004).

• NYC Public Meeting (February 12, 2004) to solicit comments on (1) t

investigation, (2) additional information that NIST might consider, (3) recommendations.

• Second technical progress report released June 18, 2004; 1,054 pag

Web site http://wtc.nist.gov.

• Public release of information following June 2004 Advisory Committe

• Locations of WTC decedents at time of impact, July 20, 2004.• Fire endurance tests of WTC floor system at Underwriters Labo• Design wind loads for the WTC towers, September 17, 2004.

• Investigation is ongoing; current findings may be revised and addition

presented in final report.

• NIST is not making any recommendations at this time; all recom

in the final report.
Schedule for Completion of Invest

Schedule for Completion of Invest

• Most technical work is complete; major focus is on writ

reports.

• NIST plans to release a draft of the final report for publ

December 2004 or January 2005

• Public comment period of 4-6 weeks after release of th• NIST plans to release final Investigation report in April • WTC 7 report will be issued as a supplement to the ma

2005; final in July 2005.

• Decoupling of WTC 7 report necessary to accomm

staffing needs for work on WTC towers.

• This change affects mainly the collapse analysis; o

be reported with the other Investigation reports.

• WTC Conference: Putting Recommendations into Prac
Report Hierarchy Report Hierarchy

Final WTC

Investigation

Report

Program Report 1

Program Report 2

Program Report 3

Program Report 4

Program Report 5

Program Report 

Technical Topic Report 1Technical Topic Report 2Technical Topic Report 7Technical Topic Report 5Technical Topic Report 4Technical Topic Report 6Technical Topic Report 3

Technical Topic Report 1Technical Topic Report 2Technical Topic Report 5Technical Topic Report 4Technical Topic Report 3

Technical Topic Report 1Technical Topic Report 2Technical Topic Report 4Technical Topic Report 3

Technical Topic Report 1Technical Topic Report 2Technical Topic Report 7Technical Topic Report 5Technical Topic Report 4Technical Topic Report 6Technical Topic Report 3

TechnTopic ReporTechnTopic ReporTechnTopic ReporTechnTopic ReporTechnTopic Repor

Eight

reports 

totaling 

about 

1,500 

pages

About 30

reports 

totaling 

about 

8,000 

pages

WTC 7 

Some technical topic reports may be combined; others may be integra
NIST WTC Investigation Objectives NIST WTC Investigation Objectives

• Determine:

• why and how the WTC Towers collapsed follo

impact of the aircraft, and 

• why and how the 47-story WTC 7 collapsed

• Determine why the numbers of injuries and fatalit

high depending on location, including technical as

protection, occupant behavior, evacuation, and emresponse

• Determine the procedures and practices that were 

design, construction, operation, and maintenance buildings

• Identify, as specifically as possible, areas in n

and fire codes, standards, and practices that w
Some Specific Questions Some Specific Questions

 How and why did WTC 1 stand nearly twice as long as W

(103 min. vs. 56 min.) though they were hit by virtually ide

 What factors related to normal building and fire safety con

to the terrorist attacks of September 11, 2001, if any, could 

prevented the collapse of the WTC towers?

 Would the undamaged WTC towers have remained stand

large building fire scenario?

 What factors related to normal building and fire safety con

could have saved additional WTC occupant lives or could loss of life among the ranks of first responders?

 How well did the procedures and practices used in the des

operation, and maintenance of the WTC buildings conform practices, standards, and codes?
Context of Findings Context of Findings

 Buildings are not specifically designed to withstand the impact of fue

airliners. While documents from the Port Authority of New York and indicate that the impact of a Boeing 707 flying at 600 mph, possibly c

was analyzed during the design of the WTC towers in February/Marc

subsequent fires was not considered. Building codes do not require consider aircraft impact.

 Buildings are not designed for fire protection and evacuation under th

conditions similar to those caused by the terrorist attacks of Septemb

 The load conditions induced by aircraft impacts and the extensive fir

which triggered the collapse of the WTC towers, fall outside the norm considered in building codes.

 Prior evacuation and emergency response experience in major even

collapse of tall buildings such as the WTC Towers and WTC 7 that weveryday use; instead, that experience suggests that major tall buildconditions, not overall building collapse. 

 The PANYNJ was created as an interstate entity, under a clause of t

permitting compacts between states, and is not bound by the building local, state, or federal jurisdiction. The PANYNJ’s longstanding state

where appropriate, exceed requirements of local building and fire cod
Analysis of Probable Collapse Seq

Analysis of Probable Collapse Seq

NIST developed and used a comprehensive approathe probable collapse sequences, from aircraft impainitiation. The approach:

 Combined mathematical modeling, well-establis

probability-based analysis methods, laboratory eanalysis of photographic and videographic evide

 Allowed for evaluation and comparison of possib

sequences based on different damage states, fistructural load redistribution paths.

 Accounted for variations in models, input param

and observed events.
Analysis of Probable Collapse Seq

Analysis of Probable Collapse Seq

• NIST simulated highly-complex structural failures and fires at the compon

system levels to determine the probable collapse sequences. In many ilimits of current engineering software and computational platformsdeveloped by NIST include:

• A detailed structural model of a typical truss-framed floor of the WTC tow

elements and 166,000 degrees of freedom. 

• A detailed structural model of a typical beam-framed floor of WTC towers 

and 35,000 degrees of freedom.

• A detailed global model of WTC 1 structure with over 80,000 elements a

freedom (with 17 flexible and other rigid diaphragm floors).

• A similar detailed global model of WTC 2 structure with over 78,000 elem

freedom. 

• A structural model of a typical turbofan engine of the Boeing 767-200ER 

60,000 elements and 100,000 nodes.

• A comprehensive structural model of the Boeing 767-200ER aircraft, inc

landing gear, fuel tanks, passenger cabin, and cargo bay, with over 530,nodes. 

• A computational fluid dynamics model of the fires and thermal environme

floors and 1,200,000 cells in WTC 1 and 6 floors and 900,000 cells in WT

• A fire-structure interface model that maps the thermal environment onto 

model, comprising 500,000 floor elements and 300,000 column nodes pe

• The first four models described above were used to evaluate the baseline 

towers under design gravity and wind loads. They also served as referen

aircraft impact damage and response of the thermally-insulated WTC str
Leading Hypothesis for Collapse o

Leading Hypothesis for Collapse o

The following chronological sequence of major events led to the eventual collapsredistribution paths and damage scenarios are being refined to determine the pro

 Aircraft impact damage to perimeter columns, mainly on the North face, re

column loads, mostly to the adjacent perimeter columns and to a lesser ex

 After breaching the building’s perimeter, the aircraft continued to penetrate 

floor framing, core columns, and fireproofing. Loads on the damaged coluintact core and perimeter columns mostly via the floor systems and to a le

 The subsequent fires, influenced by the impact damaged fireproofing cond

• Softened and buckled the core columns and caused them to shorten

displacement of the core relative to the perimeter which led to the flo

columns inward, and (2) transferring vertical loads to the perimeter c

• Softened the perimeter columns on the South face and also caused 

increase significantly due to restrained thermal expansion.

 Due to the combined effects of heating on the core and perimeter columns

bowed inwards, and highly stressed sections buckled.

 The section of the building above the impact zone began tilting to the Sout

perimeter columns buckled, and instability rapidly progressed horizontally 

and then across the adjacent East and West faces.

 The change in potential energy due to downward movement of building ma

exceeded the strain energy that could be absorbed by the structure. Glob
Leading Hypothesis for Collapse of 

Leading Hypothesis for Collapse of 

The following chronological sequence of major events led to the eventual collapse redistribution paths and damage scenarios are being refined to determine the pro

 Aircraft impact damage to perimeter columns mainly on the South face, res

loads, mostly to the adjacent perimeter columns and to a lesser extent to th

 After breaching the building’s perimeter, the aircraft continued to penetrate 

floor framing, core columns, and fireproofing. Loads on the damaged columintact core and perimeter columns mostly via the floor systems and to a les

 The subsequent fires, influenced by the impact damaged fireproofing condi

• Caused significant sagging of floors on the East side which induced t

columns inward on the East face.

• Softened and buckled the core columns on the East side and caused 

transferred significant additional load to the perimeter columns on the the floor system and to a lesser extent through the hat truss.

• Softened some of the perimeter columns that were exposed to high t

northern half of the East face.

 Due to the additional loads on the perimeter columns on the East face and 

perimeter columns, the East perimeter wall bowed inwards, and highly stres

 The section of the building above the impact zone began tilting to the East a

perimeter columns and the impact-damaged South perimeter columns buck

progressed horizontally across both faces and across the North face.

 The change in potential energy due to downward movement of building ma

exceeded the strain energy that could be absorbed by the structure. Globa
Working Hypothesis: Factors Eval

Working Hypothesis: Factors Eval

• Innovative structural system• Aircraft impact and subsequent fires• Post-impact condition of fireproofing• Quality and properties of structural steel• Relative roles of the perimeter and core co

composite floor system, including connectio

Working Hypothesis is consistent with all evidence cur

including photographs and videos, 

and emergency c
Inward Bowing of Perimeter Colum

Inward Bowing of Perimeter Colum

Minutes Prior to Collapse: WTC 1 

Minutes Prior to Collapse: WTC 1 
Inward Bowing of Perimeter Colum

Inward Bowing of Perimeter Colum

Minutes Prior to Collapse: WTC 2 

Minutes Prior to Collapse: WTC 2 
• Likely associated with inward pulling by hat truss o

provided by beam-framed upper floors due to core damage.

Gu

Kink Observed at 106 Kink Observed at 106th Floor of WT

Floor of WT
Tilting of Building Sections Tilting of Building Sections

Initiation of global collapse was first observed by the tilting of building sections above the impact regions of both WTC towers.

WTC 1 tilted to the soto the east and south counterclockwise mot
Critical Analysis Inter Critical Analysis Inter

-Dependencie

Dependencie

Colla

Reference 

Structural 

Models 

SAP 2000 

SAP to ANSYS 

Conversion

SAP to LS-DYNA 

Conversion

Compartment Damage

Debris and Fuel 

Distribution

SFRM Damage

Structural 

Damage

Ga

T

Tem

Resolution1-4 in.10-6 s

Aircraft Impact 

Damage

LS-DYNA

Fir

The

AN

Struc

and F

A

Time scale: 10 orders of magnitudeLength scale: 5 orders of magnitude

Baseline Performance Analysis
Determining the Probable Collapse 

Determining the Probable Collapse 

• Conduct extensive sensitivity analyses to determine most influential fac• Determine three sets of most influential factors for each analysis step: 

case, less severe case.

• The set of most influential factors is highly correlated for the different a

not independent). For example, the more severe case of aircraft impacmore severe case of fire dynamics, and they in turn lead to the more seanalysis, and together they led to the more severe case of structural re

• The first analysis sequence considers the set of factors for the realistic 

• A second analysis sequence is conducted to confirm the results for the 

• If the results for the realistic case suggest the possibility of more d

fire, the second analysis sequence considers the set of factors for 

each of the steps.

• If the results for the realistic case suggest the possibility of less da

the second analysis sequence considers the set of factors for the 

the steps.

• The analysis sequence is repeated with additional cases for the set of f

probable collapse sequence that best matches the observations.
Aircraft Impact Damage Aircraft Impact Damage 

to WTC 1 to WTC 1

Column DamaFireproofing and PartitionsFloors

More Severe More Severe Realistic Realistic
Aircraft Impact Damage Aircraft Impact Damage to WTC 2 to WTC 2

Realistic Realistic More Severe More Severe

Column DamaFireproofing and PartitionsFloors
Comparison of Damage to Core Co

Comparison of Damage to Core Co

Prior Studies Prior Studies

 WTC 1 MIT 4 - 12 Weidlinger 23 severed 

5 damaged 

NIST – Realistic Damage 3 severed 

10 damaged 

NIST – More severe Damage 6 severed 

11 damaged 

MIT study reported in Chapter 4 of a collection of essays by MIT researchers “The ToweWeidlinger study prepared for Silverstein Associates as part of insurance litigation involv
Column Demand/Capacity Ratios B

Column Demand/Capacity Ratios B

Aircraft Impact in WTC 1 (Floors 93 

Aircraft Impact in WTC 1 (Floors 93 

501 

1001 

N

= 1.0*

0.20

0.17

0.20

0.44 0.4

0.50

*Design intent is for columns with DCR < 1; plastic deformations can a
Column Demand/Capacity Ratios A

Column Demand/Capacity Ratios A

Impact in WTC 1 (Floors 93 to 98) Impact in WTC 1 (Floors 93 to 98)

501 

1001 

N

= 1.0

1.03

0.17

0.20 0.34

0.77

0.45 0.4
• The two WTC towers withstood the initial impact of virtually identical aircraft (B

terrorist attacks of September 11, 2001. 

• The robustness of the perimeter frame-tube system and large dimensional size 

buildings withstand the aircraft impact. 

• The WTC towers displayed significant reserve capacity, vibrating immediately 

amplitudes that were about 40 percent of the amplitudes for design wind condidesigners and an oscillation period nearly equal to that measured for the unda

• Calculations of demand to capacity ratios due to aircraft impact damage 

affected by the aircraft impacts, the majority of the core and perimeter co

continued to carry their loads after the impact. The loads from damaged 

taken over by nearby undamaged columns.

• The above finding supports the fact that the WTC towers withstood the in

finding that they would have continued to remain standing indefinitely buevent such as the subsequent fires.

Aircraft Impact Damage to WTC Tow

Aircraft Impact Damage to WTC Tow
Upper Layer Temperatures (WTC 1

Severe Case
Realistic Case More 

Combustible Load more critical in WT

Reconstruction of Fires (WTC 2, 8
Relative Roles of Aircraft Impact a

Relative Roles of Aircraft Impact a

• Fires played a major role in further reducing the structural capacity o

collapse. While aircraft impact damage did not, by itself, initiate buildcontributed greatly to the subsequent fires by:

• Compromising the sprinkler and water supply systems;• Dispersing jet fuel and igniting building contents over large areas;

• Creating large accumulations of combustible matter containing aircraf

• Increasing the air supply into the damaged buildings that permitted sig

release rates than would normally be seen in ventilation limited buildin

spread rapidly within and between floors; and

• Damaging ceilings that enabled “unabated” heat transport over the flo

and to structural components.

• The jet fuel, which ignited the fires, was mostly consumed within the 

impact. The fires that burned for almost the entire time that the build

were due mainly to burning building contents and, to a lesser extent, fuel.

• Typical office furnishings were able to sustain intense fires for at leas

floor. The typical floor had on average about 4 psf of combustible ma

aircraft solid combustibles was significant in the immediate impact region o
Results of Thermal Analysis for W

Severe Case Realist
Results of Thermal Analysis for W

Severe Case Realist
Column Shortening in WTC 1 Meas

Column Shortening in WTC 1 Meas

of Plastic of Plastic

-to

-Elastic Strain (Ductility

Elastic Strain (Ductilit

N

5011001 

12.8 3.1 0.9 5.8 2.9 

10.6 6.4 0.8 0.2 2.1 10.4 10.5 1.6 0.4 1.8 0.3 

11.6 14.5 0.5 11.1 0.4 

1.0 31.9 0.4 5.0 21.0 1

0.2 22.7 23.0 22.0 29.6 

Creep and nonlinear buckling will increase core 
Findings on Leading Collapse Hypo

Findings on Leading Collapse Hyp

• The time delay between the collapses of the WTC towers was due prima

• The asymmetric structural damage due to aircraft impact in WTC 2 • The time it took for heat to soften and buckle, and thereby shorten

fireproofing dislodged by debris impact. 

• The structure’s ability to redistribute loads as the core columns sh

• The time it took for the fires to traverse from their initial location to 

where perimeter columns were bowing inwards, as seen only minueach tower (the traverse time of fires to the South face of WTC 1 w

East face of WTC 2 where the fires already existed).

• The time it took for heat to soften and buckle those perimeter colu

• The composite trussed floor system pulled the perimeter columns inwa

WTC 1 and East face of WTC 2. The inward pulling by the floor system was 

core columns and, augmented in WTC 2, by significant sagging of the floor s

• The heating of the structural members was more sensitive to the effect 

due to debris from aircraft impact than to episodic regions of missing f

fireproofing.

• The debris field generated by aircraft impact removed significant fireproofing 

well as some of the walls. The structural components that became therm

generally determined by the debris field. Had the fireproofing not been 

field, the temperature rise of structural components would likely have b

global collapse.

• The time to destructive heating was determined by the fires, whose exte

determined by the jet fuel and the nature and (rather low) loading of combust

initial or surviving building partitions, and the ease with which windows 
Innovative WTC Tower Structural S

Innovative WTC Tower Structural S

Exterior Framed-Tube

Floor Slab

Central CoreBox Columns

59 columns per side

Bar Joist

Concrete Slab

Floor

Covering

Trough

Decking

Air-conditioning

Electric DuctDuct

• Innovative 

when builtnew and u

• Two featur

considerat

• Com

sysopesprafirep

• Des

and indu
Fire Performance of Composite Flo

Fire Performance of Composite Flo

• Fire-protection of a truss-supported floor system wit

fireproofing was innovative and not consistent with tpractice.

• No evidence of technical basis in the selection o

thickness to meet 2 h fire rating:

• 1/2 in. specified when WTC towers were built• 1-1/2 in. specified for upgrades some years prior • 2 in. for similar floor system in an unrestrained te

evaluation service recommendation in June 200buildings)

• No evidence that full-scale fire resistance test of 

system was conducted to determine the required fthickness; in 1966, the Architect of Record and, in 1Engineer of Record stated that the fire rating of the Wcould not be determined without testing.
NYC Building Code Provisions NYC Building Code Provisions

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

Floors

3

2 

Columns

4

3

1938 1968* 

* Building code governing original design and oc** Sprinklers required for buildings of unlimited h
Results From NIST Results From NIST

-Sponsored Tes

Sponsored Tes

___________________________________________________(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(

17 ft, restrained, 1⁄2 in fireproofing

4

210

(

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

3

146

(

--- --- 76 62 (3)

35 ft, unrestrained,

3⁄4 in fireproofing

2

116 

(

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

Maximum

(1300oF)

Average

(1100oF)

Maximum

(Ambient

+325oF)

Average

(Ambient

+250oF)

Failure 

to

Support 

Load

Steel Temperatures Temperature on Unexposed Surface

Test

Termi

ated

(min

Times to Reach End-Point Criteria (min)

Test Description
Findings of Standard Fire Resistan

Findings of Standard Fire Resistan

• The test structures were able to withstand standard fire conditi

minutes and 2 hours. The floor system did not fail to support lo

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

but not required to meet when they were built—required a 2-ho

system.

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

inch fireproofing did not meet the 2-hour requirement of thThis test had no fireproofing on the bridging trusses and on the 

deck.

• The 2-hour fire resistance for the standard 17-foot test wit

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

• The likely cause of the difference in results is not the firep

the trusses themselves, but the presence or lack of fireproof the metal deck. 
Role of Fire Resistance Tests Role of Fire Resistance Tests

• The fire resistance tests cannot be used to determin

performance of the floor systems in the collapse of can the tests determine whether or not the actual flomet code requirements.

• The fire resistance tests provided valuable insights into 

floor systems for use in analyzing the thermal response WTC tower structures.

• The fire conditions in the WTC towers due to the terroris

September 11, 2001 were far more extreme than those in standard U.S. fire rating tests are subjected. Conside

• 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 leading collapse hypotheses for the WTC towers ar

behavior of thermally weakened structural components damage to fireproofing or gypsum board fire protection field generated by aircraft impact. 
Analysis of Recovered WTC Steel Analysis of Recovered WTC Steel

 NIST has a total of 236 WTC steel pieces in 

its possession; collection was adequate for determining the quality and properties of steel for the investigation.

• Impact/fire damage region emphasized

• All 14 grades of steel for perimeter columns• 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 Cor

Comparison of Specified Propertie

Comparison of Specified Propertie

Measured Properties at Room Tem

Measured Properties at Room Tem
Comparison of Specified Propertie

Comparison of Specified Propertie

Measured Properties at Room Tem

Measured Properties at Room Tem

Floor Truss Components Tru
• Approximately 87 percent of the tested steel specimens e

minimum yield strengths specified in design documents.

• Approximately 13 percent of test results on the damaged 

required minimum yield strength specified in design docuare not unexpected since:

• Change in test procedure from mill tests could account f• Loss of yield point due to damage to steel accounts for 

• The distribution of wide flange core column properties is lower 

historical data; the distributions for other components are consdata.

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

affected by the fraction of steel that, according to NIST testhe required minimum yield strength. The typical factors ostress design can accommodate the measured property vminimum.

Findings on Mechanical Properties 

Findings on Mechanical Properties 

Steel
Use of Wind Tunnel Testing to Est

Use of Wind Tunnel Testing to Est

• Wind loads were a major governing factor in the 

components that made up the WTC tower structhe perimeter frame-tube system.

• Wind loads are relevant to evaluating:

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

unanticipated events such as major fires or 

• Design practices and procedures that were use

• NIST has completed an independent analysis to es

performance of the WTC towers under the original and has compared those results with then-prevailinrequirements.

• In July 2004, the designer provided NIST with inter

information in original source documents to determloads for the WTC towers.
Design Wind Loads for the WTC To

Design Wind Loads for the WTC To

Wind loads being considered include:

 Original WTC design wind loads d

from wind tunnel testing in the 1960’

 Wind loads based on two recent sta

practice wind tunnel studies conduand RWDI for insurance litigation, 20

 Wind load estimates developed by N

state-of-the-art considerations app

RWDI data, and reviewed by SOM uto NIST, 2004
Wind Load Estimates for WTC 2

1960’s 13.1 10.1 16.5 

Original WTC Design (Clarified by designer in July 2004)

2004 12.2 14.0 15.6 1

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

CPP / ASCE 7-98 2002 15.1 15.3 17.1 1

*

2002 NA NA NA 

CPP / NYC Building CodeRWDI / ASCE 7-98 2002 10.6 12.2 13.5 1

2002 9.7 11.1 12.3 1

RWDI / NYC Building Code

9.3 9.3 

1968 - 2001 NYC Building Code

5.3 5.3 

Prior to 1968 NYC Building Code

Ab

N-S E-W Resultant

Base Shear 10 

3 kips

Source Year

* Using ASCE 7-98 sections 6.5.4.1 and 6.6
Findings on Design Wind Loads Findings on Design Wind Loads

 The original design wind loads on the towers excee

established by the New York City building code prioWTC towers were designed) and through 2001 (whewere destroyed).

 The original design wind load estimates were higher th

other selected building codes of the era (Chicago, New including the relevant national model building code (BO

 Resultant wind load estimates developed by NIST base

considerations are higher by as much as about 15% thoriginal design wind loads for WTC 1, and lower by aboresultant original design loads for WTC 2. 

 Estimated wind-induced loads on the towers vary b

between two wind tunnel/climatological studies conindependent laboratories, voluntarily provided to Ninsurance litigation concerning the WTC towers; thknowledge in wind engineering is evolving.
Results of Baseline Analysis Results of Baseline Analysis

 Analysis completed by LERA under contract 

presented reviewed by NIST and SOM unde

 Demand/Capacity ratios (DCRs) for structur

estimated using Allowable Stress Design (A

 Calculated drift (maximum sway at roof) due 

wind loads:

• WTC 1: 4 ft – 8.6 in. (~ H/300)• WTC 2: 5 ft – 5.4 in. (~ H/260)

 Typical drift ratios considered in practice (no

building codes):

• H/500 in Chicago (~ 2 ft – 8.9 in.)• H/400 in New York City (~ 3 ft – 5.0 in.)
Results of Baseline Analysis for W

Results of Baseline Analysis for W

 DCRs for Structural Components under Original WT

North East South 
Results of Baseline Analysis for W

Results of Baseline Analysis for W

Mean 

DCR 

% members 

with DCR>1

% members 

with DCR>1.0

Exterior Columns (Floor 9-106) 

Original WTC Design Loads Lower Bound SOP Case 

 SOA Case 

0.76 0.78 1.10 

1.1 

2 

72 

0.4 0.9 

60 

Spandrel Beams (Floor 9-106) 

Original WTC Design Loads Lower Bound SOP Case 

 SOA Case 

0.31 0.32 0.52 

0 0 

0.5 

0 0 

0.3 

Core Columns 

Original WTC Design Loads Lower Bound SOP Case 

 SOA Case 

0.86 0.86 0.84 

10 

9.9 8.9 

5.3 5.3 5.2 

Hat Truss (Columns) 

Original WTC Design Loads Lower Bound SOP Case 

 SOA Case 

0.47 0.45 0.52 

0.4 0.4 3.8 

0.4 0.4 0.8 

* Number of members includes columns with 1⁄2 floor height due to the presence 
Findings of Baseline Performance 

Findings of Baseline Performance 

• Normal design practice is intended to ensure that d

capacity.

• DCRs estimated from the original design case are in g

obtained from the lower bound state-of-the practice c

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

• DCRs from the state-of-the-art case exceed those from 

and state-of-the-practice cases due to the following rea

• SOA wind loads are higher than those used in the 

case by about 25 percent. Note that SOA wind loasmaller than those obtained by CPP (an upper bou

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

1/3 increase of allowable stresses under wind load
Findings of Baseline Performance 

Findings of Baseline Performance 

• The allowable stress design method has an inherent factor of 

components. For example, the safety factor for yielding and b

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

SOA case.

• 1.26 and 1.44 for perimeter columns in the original design and SO

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

• After reaching the yield strength, structural steel components 

significant reserve capacity, thus allowing for load redistributiothat are still in the elastic range.

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

considered to be approximately 25 percent of the design live l

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

reserve capacity for the exterior walls (demand on exterior cotheir capacity).

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

the fraction of members for which the demand exceeded 
Evacuation and Emergency Respo

Evacuation and Emergency Respo

Based on interviews of over 1,000 WTC surviving occupants:• It is estimated that 17,400 occupants (± 1,200) were prese

on the morning of September 11, 2001. The initial populatiosimilar: 8,900 (± 750) in WTC 1 and 8,500 (± 900) in WTC 2. September 11, 2001, 16 percent were also present during the 

• About 6 percent of the surviving occupants reported a pr

their mobility. These limitations included obesity, heart condassistance to walk, pregnancy, asthma, being elderly, chronic surgery or injury, and other.

• About 7 percent of the surviving occupants reported havi

about the building. These included fire safety staff, floor wabuilding maintenance, and security staff. Searchers assist the facilitating evacuation.
Decedent Analysis Decedent Analysis

September11Victims.com: This site is dedicated to 

the victims of September 11, 2001 tragedy.

Portraits: 9/11/01: Published by the New York Times 

in 2003, this book includes short interviews with family members of many decedents.

CNN.com In-Depth Special 

(http://www.cnn.com/SPECIALS/2001/memorial/index.html): Tribute site for people to write remembrances of decedents.

Badge List maintained by Port Authority of New York 

and New Jersey: Includes name, employer, building, and floor for all occupants with badgeaccess to WTC 1 or WTC 2.

Numerous memorial sites maintained by companies 

which lost employees: Includes names and remembrances of decedents. Examples include the Port Authority, Fire and Police Departments, Marsh & McLennan Companies, EuroBrokers, Fiduciary Trust, and others.

Newsday.com: Includes short stories written about 

specific decedents.

NIST Interviews with occupants and family members* Where possible, eyewitness accounts were used to place individuals. 

Where no specific accounts existed, employer and floor information was used to place individuals. Total 

No Information United Flight 175 American Flight 11 Bystander/Nearby Building 

Volunte

Hosp

First Responders (Total) Unknown Location Inside WWTC 2Confirmed Below Impact in WTC 2

At or 

World Trade Center 2 Occu

At or 

World Trade Center 1 Occu

Likely Location at Time of 
Background Information on Evacu

Background Information on Evacu

• Two-thirds of surviving occupants reported having partic

the 12 months prior to September 11, 2001, while 17 percereceived no training during that same period. Of those papercent were instructed about the location of the nearest stair

over half of the survivors, however, had never used a stairwelSeptember 11, 2001.

• Two thousand one hundred forty-six building occupants (1,466 

2, and 56 in either WTC 1 or 2) and an additional 421 first ressecurity guards but not aircraft passengers and crew or bystahave lost their lives on September 11, 2001.

• Approximately 87 percent of the WTC tower occupants, inc

percent of those below the floors of impact, were able to 

• Rough estimates indicate that about 20 percent or more of t

occupants and first responders who were in the WTC tow

may have been alive in the buildings just prior to their coincludes 118 occupants below the floors of impact who died bunknown number of occupants above the floors of impact who prior to collapse.
Evacuation Findings: First Evacuation Findings: First

-Person 

Person 

• Overall, about 7,900 survivors evacuated WTC 2 in 73 min (i.e., from the instant the W

WTC 2 collapsed); while about 7,500 survivors evacuated WTC 1 in 103 min. The ov

(108 survivors per min) was about 50 percent faster than that in WTC 1 (73 survi

• Functioning elevators allowed many (roughly 3,000) survivors to self-evacuate W

prior to aircraft impact. All but one of the 99 elevators in WTC 1 were not functioninthe stairways. 

• The stairwells, with partition wall enclosures that provided a 2 h fire-rating but little stru

the region of the aircraft impacted floors. 

• One of the stairwells in WTC 2 (Stairwell A on the North side) was passable 

for some period of time after WTC 2 was attacked.

• All three stairwells in WTC 1 and the two other stairwells in WTC 2 were rendered 

aircraft impact.

• Occupant preparedness:

• Occupants were often unprepared for the physical challenge of full building 

occupants required one or more periods of rest during stairwell descent or t

the stairwells strenuous.

• Occupants were often unprepared to encounter transfer hallways during the 

evacuees occasionally hesitated or debated a course of action upon encou(Note, however, that NYCLL 5 prohibits requiring occupants to practice stai

• Mobility challenged occupants were not universally identified or prepared fo

occupant, for example, reported being ‘left’ on their floor by colleagues, calland was eventually assisted by strangers (occupants). 
Evacuation Rates in the WTC Towe

Evacuation Rates in the WTC Towe

• After the first airplane struck WTC 1 and before the second ai

the survivors in WTC 2 were twice as likely as those in WTC 1 the building (41 percent versus 21 percent). The rate of evacWTC 2 was twice the rate in WTC 1 during that same period.

• Approximately 75 percent of WTC 2 occupants above the 

successfully descended below the 78th floor prior to the a

am.

• Soon after WTC 2 was struck by the airplane until about 20 m

collapsed, the survivors in WTC 2 and WTC 1 exited at about 

prior evacuation rate of WTC 1).

• During the last 20 min before each building collapsed, the eva

buildings slowed to about one-fifth the immediately prior evacsuggests that for those seeking and able to reach and use and stairways, the egress capacity (number and width of was adequate to accommodate survivors.

• Based on a report by a first responder from a floor in the 20s 

occupants was making its way down the stairwell some minutcollapse. Among other possibilities, this group may have com

floors of impact by finding a way through the only passable sta
Evacuation Rates in the WTC Towe

Evacuation Rates in the WTC Towe

• Even though a percentage of evacuees reported that they perceive

ingress) to be problem, it was found not to be a significant factor in 

of occupants in WTC 1 when compared to other factors, including 

evacuation interruption, and encountering obstacles in the evacuacues) such as smoke, water, or debris.

• Based on use of existing egress models and actual evacuation time on 

estimated that a full capacity evacuation of each WTC tower with 25the number present on September 11, 2001—would have required aa significantly faster total evacuation at full capacity would have required 

capacity (number and width of exits and stairways).

• In WTC 1, the average surviving occupant spent 48 seconds per floor d

This translates to approximately 0.2 m/s (0.65 ft/s), which is about 50% 

measurement presented in the SFPE Handbook of Fire Protection Engi

evacuations.

• In WTC 1, each stairwell door exited approximately 37 people per minut

minutes, which is comparable to the slowest measurement presented in 

Protection Engineering for non-emergency evacuations.

• Thus the average surviving occupants moved slower down stairs a

than previously reported for non-emergency evacuations.
Roof Evacuation Roof Evacuation

• The PANYNJ’s standard occupant evacuation procedures and drill

stairways to exit at the bottom of the WTC towers. The standard pthe doors to the roof locked with a key being required to gain roof a

reports that it never advised tenants to evacuate upward.

• There were at least two decedents who had tried to get to the roof 

locked to both the WTC towers. In addition, a PANYNJ employee WTC 2 was unable to walk down the stairs, or go to the roof as insanother PANYNJ employee (PANYNJ Channel Y).

• The NYPD and FDNY policies for roof operations were focused 

first responders with access into the building above the fire floconventional rescue, and comforting occupants. Roof rescue measure of last resort to be used, for example, to assist occupemergencies.

• The NYPD aviation unit arrived at the WTC site soon after WTC 1 w

repeated attempts to examine the possibility of roof access/rescueconditions at the top of the WTC towers prevented the conduct of s

• Considering the capacity of typical helicopters and travel time

fraction of the large number of occupants could have been evatowers prior to their collapse had roof rescue been possible o
Public Address System Announce

Public Address System Announce

• Damage to the 22nd floor communication closet likely disabled th

announcement capability in WTC 1. Many announcement attemlobby command station.

• Announcements in WTC 2 were heard by occupants building-wid

aircraft struck at 9.03 am. Announcements were also heard in a(including above the impact area) after the second aircraft struck 

• At 9:00 am an announcement stated “There is a fire condition 

secure. Please return to your offices.”

• At 9.02 am an announcement stated “May I have your attentio

is in Building 1. However, if conditions on your floor warrant, y

an orderly evacuation.”

• At 9.20 am an announcement was made updating occupants 

building and progress of the evacuation and informing occupa

to leave, they could then use the concourse.

• Prior to 9.37 am an announcement instructed occupants to “g
Emergency Communications to WT

Emergency Communications to WT

• Occupants called 9-1-1 and the Port Authority s

assistance and advice.

• Opportunities to improve occupant’s situatio

were often lost. Specific knowledge about loand impact damage was only occasionally c

occupants who requested the information anapparent coordination.

• Some operators advised sheltering (e.g., many 

while others advised evacuation (e.g., many PA operators); some permitted window breaking whinstructed occupants not to break windows.
Role of Emergency Responders Role of Emergency Responders

FDNY - Established operational control and the Inc

Post for the WTC operations, conducted evacuatoperations, and fought fires at the disaster.

PAPD - Established security at the WTC and cond

and rescue operations.

NYPD - Established traffic control, perimeter secur

security for command posts, and conducted evacrescue operations inside the WTC. The aviation 

observation capabilities and assessed the potentrescue.

OEM - Functioned as a multi-agency command res

provided support for all agencies and departmen

disaster.
FDNY Operations at the WTC FDNY Operations at the WTC

Three operational strategies:Outside Command Posts & Inside Command Commthe Outside Command Post - Fires in the buildings wwere located too high in the buildings to accomplish fire that could save the lives of occupants above the fires. Tto evacuate and rescue all below the fires.Command Officers for Inside Operations - The fires extinguish. The objective was to get enough personnel upstairs to cut a path through the fire to rescue occupanand also evacuate and rescue all below the fires.Company Level Command - They saw this as a convhigh-rise fire. The objective was to get up to the fire floothe fires. In some cases, firefighters were persuaded b

officers to switch from the idea of fire fighting to evacuatoperations. 

No first responder interviewed by 

the WTC tower
Situational Awareness:

• Emergency responders working outside of the WTC view building conditions and communicate over radios situational awareness.

• Situational awareness for personnel that observed thand fires from outside the buildings before entering exp

maintaining their awareness after entering the buildings• Emergency responders working inside of the WTC bunot see what was happening outside and had poor radhad poor situational awareness. 

• Emergency responders working inside of the WTC busee what was happening outside and had good radio cbetter situational awareness than those with poor radio 

Emergency Responder Operations Emergency Responder Operations 
• After aircraft impact, only two elevators out 

operating inside the two WTC towers. WTC 

lobby to the 16th floor. WTC 2, from the lobby to 

• The stairways were filled with occupants evacu

buildings. FDNY personnel and other emergeresponders reported difficulty attempting to stairs due to this counterflow.

• Counter flow in the staircases made it difficult fo

responders to carry equipment up the stairways

• Counter flow in the staircases caused teams of 

responders to become separated, causing deladisrupting team operations.

FDNY Access to the WTC Towers FDNY Access to the WTC Towers
Emergency Responders & High Emergency Responders & High

-Rise B

Rise B

• First responding FDNY units took from 4 to 10 minutes 

complex. They then got their equipment and received aanother 3 to 5 minutes. Time to begin operations 7 to 

• Of the 27 emergency responders interviewed that were 

maximum floor height achieved before WTC 2 collapsehour 13 minutes.

1 – A police officer carrying no extra equipment and in a patro

uniform climbed to the 44th floor.

8 – Emergency responders (FDNY, PAPD, NYPD) climbed to 

Two FDNY took an elevator to the 16th floor.

16 – Emergency responders (mostly FDNY) climbed to the 20’2 – Emergency responders (NYPD) climbed to the teens.

• Estimated climbing rate based on a 60 minute climb

maximum height: 1.4 to 2 minutes/floor
High-Rise Buildings & Emergency 

Rise Buildings & Emergency 

Example: Fire department response to a 60 story high-ris

trapped above fires on the 58th floor and no opera

Lo

Firefighters begin to climb 10 minutes

Fire department arrival 4 minutes 

Firefighters carrying equipment and

wearing PPE ~ 70 minutes

Firefighters carrying no equipment and not

wearing PPE ~ 50 minutes

Fi

Firefighters carrying equipment and wearing PPE ~ 125 minutes

Firefighters carrying no equipment and not

wearing PPE ~ 90 minutes
• All three of the responding departments, FD

PAPD experienced difficulties with radio com

• Each of the departments was aware of the s

associated with their radio communications they related to operations in high-rise buildin

• Two basic issues with radio communication

1. Normal function of the radio equipment 

environments. (Radio signal attenuation 

concrete buildings)

2. The volume of radio traffic

Radio Communications Radio Communications
Emergency Communication Recor

Emergency Communication Recor

• NIST reviewed audio communications tapes recorded by the P

recording of the FDNY’s city-wide high-rise Channel 7 (Port ADepartment’s [PAPD] Channel 30) radio repeater that was loc

• NIST reviewed audio tapes copied from original NYPD comm

including NYPD internal department operations.

• FDNY communications recordings from the WTC location 

2001, are not available because the primary field communthe shop for repairs. A back-up field-communications vawhich did not have a recording capability—was destroyed towers collapsed.

• The best record of radio communications reflecting fire d

came from the FDNY Channel 7/PAPD Channel 30 and firsprovided by FDNY personnel during their interviews. 

• The PANYNJ installed the radio repeater system for use by F

bombing.
Analysis of Emergency Communic

Analysis of Emergency Communic

• After the first aircraft struck WTC 1, there was an approximat

increase in traffic level over the normal level of emergencommunications, followed by an approximate factor of 3 ste

level of subsequent traffic.

• A surge in communications traffic volume made it more 

the flow of communications and delivery of information.

• Roughly a third to a half of the radio messages transmitted d

traffic surge conditions were not complete messages nor u

• FDNY’s city-wide high-rise Channel 7 (PAPD Channel 30) 

the WTC site was operating. 

• NYPD aviation unit personnel reported critical information ab

collapse of the WTC towers several minutes prior to their cohas been found to suggest that the information was furthall emergency responders at the scene.
• Analysis of the FDNY City-wide, high-rise, channel 7 (PAP

repeater recording indicates that the World Trade Center was operating.

• At approximately 9:05 a.m. the repeater’s recording system re

Lobby Command Post attempts to check repeater operations. talkie radio communications were recorded.

• It is possible that one or both of the following conditions comp

that took place at the WTC 1 Lobby Command Post:

• The radio repeater handset earpiece was broken.• The radio repeater handset volume was not turned up.

• It is unlikely that the repeater’s antenna was broken or m

since radio signals were received during the radio check and the communications that followed from inside WTC 2

• The repeater’s system recorded radio communications that to

several different firefighters and several different FDNY officeinside WTC 2.

WTC High WTC High-Rise Radio Repeater Sy

Rise Radio Repeater Sy
Command and Control Command and Control

• First responders— including key incident commanders—

adequate information (voice, video, and data) on, nor an of, the conditions in the WTC buildings and what was haat the WTC site. Interagency information sharing was in

• FDNY command and control was seriously affected by th

communications.

• Large numbers of fire fighters were dispatched to the WT

adequate command posts and staff could be assembled 

• Self-dispatch complicated command and control at the s

command and control was affected by many self-dispatched ambulance units that contributed to clogging the streets so th

assigned to the WTC had difficulty getting through.

• FDNY apparatus had to be moved to allow some ambulance

exit the site with victims.
• FDNY’s system for maintaining records of unit assignmen

post was not capable of managing the numbers of units anassigned to the incident. 

• FDNY, NYPD, and PAPD: there was no means to back-up t

records generated at the command posts.

• A significant amount of evidence (first person interviews, 

photographic data) shows that the different agencies were during the WTC disaster.

• Inter-agency operations were detrimentally affected with t

command center that was located inside WTC 7. First persphotographic data show that OEM functions became dispersedand other supporting systems were lost, and the unified operatdiminished. OEM personnel were working with different emergdepartments and were located at the various department comm

• First responder interviews suggest that inter-agency competitio

operations at the WTC complex on the morning of September suggest that some of the problems experienced were due to peunderstanding the operating practices of the other agencies.

Command and Control Command and Control
Mobility Impaired Occupants Mobility Impaired Occupants

• As the emergency responders started evacuating WTC 

of WTC 2, they found mobility impaired occupants still in down.

• Ambulatory mobility impaired occupants typically walked 

one hand on each hand rail and took one step at a time addition, they were typically accompanied by one persoor an emergency responder. This blocked others behinmore rapidly down the stairs.

• FDNY and PAPD personnel found 40 to 60 mobility i

on the 12th floor of WTC 1 as they went down and at

each floor on their way out. These impaired individua

on this floor in an attempt to clear the stairways.

• Emergency responders were assisting approximate

mobility impaired people down the staircase just prof WTC 1. It is unknown how many fatalities occurred w
Active Fire Protection Systems Active Fire Protection Systems

• The smoke management systems in the WTC towers were not activated 

September 11, 2001. It was determined that the likelihood of these syst

very low due to the damage inflicted by the aircraft impacts.

• Analysis indicates that the disruption of the HVAC (heating, ventilation

system, particularly the aircraft impact rupture of large return air sductwork created a major path for vertical smoke spread in the bui

• Analysis indicates that stair pressurization systems would have prov

to the passage of smoke in WTC 1 and WTC 2 had they been instal

2001. 

• The fire alarm system in WTC 7 sent only one signal (at 10:00:52 a.m. s

collapsed) to monitoring company indicating a fire condition. The signaspecific information about the location of the fire within the building.

• The resistance to failure of the fire alarm system communications 

command station and occupied WTC tower floors could have been cable had been used instead of copper lines, which were not permcode at the time of building construction.

• Although the fire sprinkler system was damaged by aircraft impact, the w

lacked redundancy, and the potential existed for single point failure of 

connection on each floor.
Example of Vertical Smoke Spread Example of Vertical Smoke Spread
Applicable Building Codes Applicable Building Codes

• Although not required to conform to NYC codes, the PANYNJ e

provisions of the proposed 1968 edition of the NYC Building Co

years before it went into effect. 

• The 1968 edition had less restrictive provisions compared w

was in effect when design began for the WTC towers in 1962. T

• Eliminated a fire tower as a required means of egress;• Reduced the number of required stairwells from 6 to 3 and 

to the stairs from 44 in. to 36 in.;

• Reduced the fire rating of the shaft walls in the building cor

• Changed partition loads from 20 psf to one based on weigh

length (that reduced such loads for many buildings includin

• Permitted a 1 h reduction in fire rating for all structural com

h to 3 h and floor framing members from 3 h to 2 h).

• The NYC Department of Buildings reviewed the WTC tower 

provided comments to the PANYNJ concerning the plans in relaBuilding Code. The architect-of-record submitted to the PANYN

comments, noting how the drawings conformed to the 1968 NY
Applicable Building Codes Applicable Building Codes

• In 1993, the PANYNJ and the NYC Department of Buildings entered int

understanding that:

• Restated the PANYNJ’s longstanding policy to assure that its 

New York meet and, where appropriate, exceed the requiremeCode.

• Provided specific commitments to the NYC Department of Building

be undertaken by the PANYNJ to assure that buildings owned or o

in conformance with the Building Standards contained in the NYC 

• In 1993, the PANYNJ adopted a policy providing for implementation of f

made by local government fire departments after a fire safety inspection 

for the prior review by local fire safety agencies of fire safety systems to facility. Later in 1993, the PANYNJ entered into an agreement with FDNpolicy adopted by the PANYNJ and set forth procedures to assure that nsystems are in compliance with local codes and regulations.

• While the PANYNJ entered into agreements with the NYC Department o

with regard to conformance of PANYNJ buildings constructed in New YoCode, the PANYNJ did not yield jurisdictional authority for regulatooversight to the NYC Department of Buildings. The PANYNJ was crunder a clause of the U.S. Constitution permitting compacts between stathe building and fire codes of any local, state, or federal jurisdiction.
Rationale for the Selection of the 1

Rationale for the Selection of the 1

Building Code for the WTC Towers Building Code for the WTC Towers

June 22, 1965 letter from John M. Kyle, Chief Engineer 

Chief, Planning Division, WTC:

“This will confirm my advice that, in view of the more 

the proposed new New York City Building Code, I feel advantage of its provisions.

This decision is based on the following:

1. The new code has received thorough review b

technical groups and representatives of the Cmodified to meet all major objections.

2. It is scheduled to be officially submitted to the City 

should be approved well before we construct our 

3. The Commissioner has stated that he favors th

by the Port Authority in using advanced technof the World Trade Center. He also stated that t

not subject to the provisions of the Building Code.
Rationale for the Selection of the 1

Rationale for the Selection of the 1

Building Code for the WTC Towers Building Code for the WTC Towers

September 29, 1965 letter from Malcolm P. Levy, Chie

WTC to Minoru Yamasaki, Architect:

“... Generally the tower core should be redesigned to e

tower and to take advantage of the more lenient provisstairs...”

May 19, 1966 memorandum from Guy F. Tozzoli, Dire

Department to John M. Kyle, Chief Engineer:

“The decision to follow the new Code was, as you poin

memorandum to Mr. Levy of June 22, 1965, based on the new Code had been thoroughly review by interested tand was modified to meet all major objections; (2) it wadopted before we constructed our buildings; and (3) the favored the approach of using advanced techniques iWorld Trade Center and that the Port Authority, accordinsubject to the provisions of the Building Code.”
Rationale for the Selection of the 1

Rationale for the Selection of the 1

Building Code for the WTC Towers Building Code for the WTC Towers

Memorandum dated January 15, 1987, from Lester 

Structural Engineer, World Trade Department, PAJ. Linn (Deputy Director for Physical Facilities, WTSubject: The World Trade Center – Towers and PFire Rating per NYC Building Code Revision Effec

Paragraph 2. B. states:

“For office buildings there is no economi

using Class 1A Construction, and ER&S used CConstruction for the WTC Towers and Plaza Buare Occupancy Group “E” (Business) with a firhours. As such, columns must have a three hour construction with a 2 hour rating.”
Approach to Recommendations Approach to Recommendations

• In the United States, state and local governments are r

promulgating and enforcing building and fire regulation

• With some exceptions, the state and local regulations a

model building and fire codes developed by private sec

• The model codes, in turn, reference voluntary consens

developed by a large number of private sector standardorganizations (SDOs) accredited by the American Natio

Institute (ANSI).

• NIST does not set building codes and standards, but p

support to the private sector and other government agedevelopment of U.S. building and fire practices, standa

• NIST recommendations are given serious consideratio

organizations that develop national standards and mod

provide minimum requirements for public welfare and s
Approach to Recommendations (2

Approach to Recommendations (2

• The NIST building and fire safety investigation of the WTC disa

formulated recommendations. However, in formulating its rewill consider the following:

• Findings from the first three independent investigation 

building performance, evacuation and emergency responspractices.

• Whether findings relate to the unique circumstances s

attacks of September 11, 2001, or to normal building an

considerations, including evacuation and emergency 

• What technical solutions are needed, if any, to addres

buildings, occupants, and first responders, considering and the consequences of those hazards.

• Whether the risk is in all buildings or limited to selecte

distinct height and area or type of structural system), builddesign features, iconic/signature buildings, or buildings tha
Issues Related to Practice, Standa

Issues Related to Practice, Standa

Codes

• Based on the Investigation findings, NIST has identified issue

standards, and codes that will provide the basis for formulatinrecommendations.

• Issues arising from the investigation are grouped under the fo

• Increased Structural Integrity• Enhanced Fire Protection

• Passive Fire Protection• Active Fire Protection

• Improved Building Evacuation 

• Egress System Design• Emergency Communications to Occupants• Occupant Preparedness• Egress Technology

• Improved Emergency Response

• Access and Firefighting• Emergency Communications• Command and Control
Categories of Issues Categories of Issues

Level 1  Practices

 Standards, codes and regulations Adoption and enforcement R&D/further study Education and training

Level 2  All tall buildings (buildings over 10 stories in hei

 Selected tall buildings (buildings over 10 stories 

risk due to design, location, use, iconic status, c

 Selected other buildings (buildings that are at ris

location, use, historic/iconic status, contents, etc

Level 3  Related to 9/11 outcome

 Unrelated to 9/11 outcome
Issues – Increased Structural Integ

Increased Structural Integ

9 9 9 

Availability of protocols for selection of site-specific wind speed and directionality.

9 9 

Adequacy of prescriptive wind load standards for very tall buildings.

9 9 

Availability of standards for wind tunnel tests and for methods to estimate wind effects from test results for design purposes. 

9 

Availability of analytical methodologies for prediction of complex failure phenomena of structural systems under abnormal loads.

9 9 9 

Availability of explicit standards, code provisions, methodology, analytical design tools, and practical design guidance for designing structures to resist progressive collapse in the event of abnormal loads.

Standards, Codes, 

R&D/study Further

Regulations

Practices

Lev

Issue
Issues - Enhanced Fire Protection Enhanced Fire Protection

9 9 

Adequacy of ASTM E 119 to provide guidance on:Criteria for determining structural limit states, including failure, and means for measurementScale of test assembly versus prototype applicationEffect of end restraint conditions on test results, including influence of stiffnessStructural connectionsCombination of loading and exposure (temperature profile) represent expected conditionsProcedures to analyze and evaluate data from fire resistance tests of other building 

components and assemblies to qualify an untested building element

Repeatability and reproduceability of test resultsRelationships between prescriptive ratings and performance of the assembly in realistic building fires

9 9 9 

Technical basis for construction classification and fire rating requirements for tall buildings (fire rating, sprinkler-tradeoff, compartmentation)

9 9 9 

Conformance of applied passive fire protection to conditions in actual or equivalent tests used to establish fire resistance rating of the building component or assembly.

9 

Availability of regulations that would adopt code provisions using the “structural frame”approach to fire resistance ratings, which requires structural members to be fire protected to the same rating as columns.

9 9 

Standardized design-basis fire scenarios for design of fire protection systems and analysis of structural and thermal response do not account for building-specific conditions.

9 9 9 

Availability of standards, codes, methodology, analytical design tools, and practical design guidance to permit considering fire as a design condition for the structure as a whole system.

g

Standards, Codes, 

R&D/study Further

Regulations

Practices

Level 

Issue
Issues – Enhanced Fire Protection Enhanced Fire Protection

9 9 9 

Available means to transmit outside a building the status of emergency conditions in the building from information in the fire alarm system and other monitored building systems. 

9 

Survivability of fire alarm system information records of alarms during emergency events for investigation purposes.

9 9 

The quantity and reliability of information available to emergency responders at the fire/emergency command station. 

9 9 

Ability of fire alarm and communications systems to provide continuous, reliable, accurate, and sufficient information on conditions, so building fire emergencies including the evacuation process, if needed, can be managed using that information.

9 9 9 

Sensitivity of the level of performance of active fire protection systems (sprinklers, standpipes/hoses, fire alarms, smoke management) to the size and height of the building, compartmentation, building population, activities, transient fuel loads, fire department response, and threat profile.

9 9 

Evaluation of presently available fire-resistant steels; comparison with conventional steels; and test protocols and acceptance criteria. 

Standards, Codes, 

R&D/study Further

Regulations

Practices

Level 1 

Issue
Issues – Improved Building Evacu

Improved Building Evacu

Egress System Design Egress System Design 

Lack of adequate egress models and performance-based egress design 9 methodology accounting for human behavior during evacuation.

9 

Stairwells can be physically proximate yet considered remote by a "walking path" measurement; does not adequately meet separation requirements under non-fire conditions.

9 9 

Required professional training or accreditation for egress system designers (often architects).

9 9 9 

Mobility-challenged occupants: areas of refuge create a delay in evacuation; insufficient procedures for identifying those with challenges and assisting them. Some mobility-challenged occupants are not capable of effecting their own escape.

9 9 9 

Building egress systems are not designed to accommodate full building evacuation. Full building evacuation is foreseeable under conditions of widespread power outage, earthquake, fire, or terrorist attack.

9 9 

The egress path may be compromised prior to evacuation of the affected population due to a variety of scenarios (e.g., wind, earthquake, overpressure, impact, fire).

Education & Training

Standards, Codes, 

R&D/study Further

Regulations

Practices

Level 1 

Issue
Issues - Improved Building Evacua

Improved Building Evacua

Emergency Communications to Oc

Emergency Communications to Oc

9 

The electro-mechanical systems (e.g., elevators) integral to life safety may be compromised by a single event

9 9 

Use of emergency broadcast system for major emergencies such as that which occurred on September 11, 2001

9 

A building may have no capacity for public address announcements or instructions through the floor warden system

9 9 9 

Inadequate situational awareness from missed opportunities to better communicate information among occupants, 911 operator dispatch, fire department dispatch, and emergency management dispatch and site security

Education & 

Training

R&D/study 

Further

Standards, 

Codes, 

Regulations

Practices

Level 1 

Issue
Issues – Improved Building Evacu

Improved Building Evacu

Occupant Preparedness Occupant Preparedness

9 

Layouts (i.e., transfer floors) can be confusing to unprepared occupants.

9 

Need for floor wardens.

9 9 

Emergency plans are filed to achieve regulatory compliance, but are not adequately implemented in practice. In addition, occupants are often unprepared to evacuate a building. Preparedness includes adequate knowledge of the evacuation procedures and systems, and adequate means for pathway illumination.

Education & 

Training

R&D/study 

Further

Standards, 

Codes, 

Regulations

Practices

Level 1 

Issue
Issues – Improved Building Evacu

Improved Building Evacu

Egress Technology Egress Technology

9 

The electro-mechanical systems (e.g., elevators) integral to life safety may be compromised by a single event

9 9 

Egress systems do not enable all occupants an equal opportunity for evacuation. Hardened elevators, exterior escape devices, orstairwell navigation devices not considered.

9 

Elevator door restrictor plate can entrap building occupants in the event of an emergency.

Education & 

Training

R&D/study 

Further

Standards, 

Codes, 

Regulations

Practices

Level 1 

Issue
9 

Distance (i.e., remoteness) between stairwells where standpipes are located.

9 9 

Adequacy of capacity for egress and firefighter access during full evacuation of fully occupied tall buildings (access to tall buildings by first responders is hindered by counter flow, egress capacity, and lack of available elevators).

9 9 9 

Physiological impact on firefighters with equipment climbing more than 10 to 12 floors during an emergency. 

Standards, Codes, 

R&D/study Further

Regulations

Practices

Lev

Issue

Issues – Improved Emergency Res

Improved Emergency Res

Access and Firefighting Access and Firefighting
9 9 

Lack of communications network architecture (interoperability) and operational protocols for intra- and inter-agency communication at all levels of organizational hierarchy. This includes:

•Overall network architecture that covers local networking at incident sites, 

dispatching, and wide-area urban and rural networks.

•Scalability in terms of the number of first responders using the system 

and in providing radio coverage in large buildings with challenging radiofrequency propagating environments.

•Interoperability with existing legacy emergency communication systems.•Localization techniques to identify first responders within indoor building 

environments.

•Conventional two-way versus wireless network systems

9 9 

Performance requirements for emergency communication systems in high-rise buildings (i.e., design, testing, certification standards and maintenance and inspection requirements).

9 9 

Lack of rigorous pre-emergency inspection and testing of radio communications systems within high-rise buildings to identify performance gaps and inadequacies. 

Standards, Codes, 

R&D/study Further

Regulations

Practices

Leve

Issue

Issues – Improved Emergency Res

Improved Emergency Res

Emergency Communications Emergency Communications
Secure location of state and local emergency operation centers (EOCs). 9 

Rapid adoption and execution of a unified emergency response mission by all first 9 responder ranks.

9 9 

The dispatch of large numbers of personnel and apparatus and the ability of management to maintain accountability in a timely manner associated with arrival and deployment of personnel and the ability of the incident site to effectively accommodate large numbers of personnel and apparatus.

9 9 9 

Availability of effective codes and protocols for establishment and uninterrupted 

operation of the incident command and control system and for preservation and dissemination of information managed by command posts.

•Command posts established within the collapse zone of buildings that received serious 

structural damage and contained large multi-floor fires.

•Establishing the command post prior to dispatching needed units.•Effects of self-dispatch and free-lancing of first responders and ambulances, especially 

teams lacking protective clothing and medical equipment.

•Robustness of assignment and tracking (accountability) system for large scale 

emergencies.

9 9 9 

Availability of detailed procedures and methods for gathering, processing, and delivering situational information to all first responders, including 911 operators, wardens, incident commanders, etc.; this covers voice, video, and data integration.

Standards, Codes, 

R&D/study Further

Regulations

Practices

Leve

Issue

Issues – Improved Emergency Res

Improved Emergency Res

Command and Control Command and Control
Issues – Other Major Issues Other Major Issues

Structural principles education for fire protection engineers and fire protection principles education for structural engineers; structural and fire protection principles education for architects.Creation of broad training opportunities for rigorous use of computational fire dynamics and thermostructural analysis tools. 

9 9 

Availability of regulatory requirements for retention of documents related to the design, construction, operation, maintenance, andmodifications of buildings, including retention offsite and accessibility of building plans for emergency response.

•Maintenance and storage of documents.•Accessibility of building plans for emergency response.

g

Standards, Codes, 

R&D/study Further

Regulations

Practices

Level 

Issue
Approach to Recommendations (3

Approach to Recommendations (3

• NIST urges organizations responsible for building a

levels to carefully consider the findings and issues

• In its final report, a draft of which is expected to be rele

2004 or January 2005, NIST will recommend appropriathe way buildings are designed, constructed, maintainebe important for those recommendations to be thorougconsidered by the many organizations responsible for bsafety. 

• NIST will welcome comments from the public on the dr• As part of NIST’s overall WTC response plan, the Instit

reach out to these organizations to pave the way for timconsideration of recommendations stemming from this will hold a major conference in June 2005 to reinforce tfindings and recommendations from the Investigation aimplementation in practice. NIST also has expanded itof high priority need.
Disclaimer Disclaimer

Certain commercial entities, equipment, producand non-commercial entities are identified in thin order to describe a procedure or concept adetrace the history of the procedures and practiceidentification is not intended to imply recommenendorsement, or implication that the entities, prmaterials, or equipment are necessarily the besthe purpose. Nor does such identification imply fault or negligence by the National Institute of STechnology.
Web site Web site http:// http://wtc.nist.go

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

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

Regular mail: Regular mail:

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