Federal Building and Fire Safety Inves

of the World Trade Center Disast

Building and Fire Research Labo

National Institute of Standards and Tec

U.S. Department of Commerce

Technical Conference on the 

Building and Fire Safety Invest

the 

World Trade Center Disas

Building and Fire Codes and Practice

Richard W. Bukowski, P.E., FSFPE

H. S. Lew, Ph.D., P.E.

Nicholas J. Carino, Ph.D. (Retired)
Scope

• Document the design and construction of structural • Document the design and construction of fire protec

systems

• Compare then current building regulatory and code 

• And how these influenced the design an

• Document maintenance and modifications to structu

and egress systems

• And how these were influenced by chan

standards, and practices during the life o

• Document the fuel system for emergency power in 
WTC 1 and 2 WTC 1 and 2 

Architects:

Minoru Yamasaki & AssociatesEmery Roth & Sons (ARCHITECT-OF-RECORD 

Structural Engineers:

Skilling, Helle, Christiansen & Robertson (StructuRecord)
Documents Reviewed and Indexed Documents Reviewed and Indexed

• Complete set of design, construction and maintenance 

WTC 1 destroyed during collapse

• Construction documents were not retained by the Gene

(Tishman Construction Co.) after 7 years

• More than 8,000 documents from:

• Port Authority• LERA• Silverstein Properties• New York City (Fire Department, Police Department, 

Department of Buildings, Department of Design and Office of Emergency Management)

• Group of companies that insured the towers• Laclede Steel• Isolatek• FEMA

• Indexed database available for future access to docume
Support Contractors Support Contractors

-

Rolf Jensen & Associates, Inc.

-

S. K. Ghosh

Associates, Inc

-

Rosenwasser/Grossman Consulting 

Engineers P.C.
New York City Building Code New York City Building Code

• Building Code not based on any model code, bu

volunteer committees

• Adopted and modified by Local Laws passed • Interpreted by Building Commissioner throug

• Interpretations and explanation in Policy and Pro

and Rules issued by the Building Commissioner

• No Fire Code, but Rules of the City of New York 

Commissioner

• Reference Standards based on national consens

adapted and adopted for local conditions and pra
Origin of 1968 NYC Building Code Origin of 1968 NYC Building Code

• 1962 -

Brooklyn Polytechnic Institute wa

develop a draft code drawing from BOCApractice, and utilizing volunteer committe

• December 6, 1968 -

Local Law 76 (1968

1938 Code and adopted 1968 Code

• Between 1969 and 2002 there were 79 L

modified the 1968 Code
Rationale for the Selection of the 1968 N

Rationale for the Selection of the 1968 N

Code for the WTC Towers Code for the WTC Towers

• Jun

e 2

2, 1965 l

etter from Joh

n M. Kyle, Chi

ef Engi

n

e

er to Malcolm 

Division, WTC:

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

new New

York City Buildi

ng Code, I feel we shoul

d take advantag

• This deci

sion is based on the following:

1. The new code has received thorough review by inte

groups and representatives of the City and has beemajor objections.

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

approved well before we construct our buildings.

3. The Commissioner has stated that he favors the app

Port Authority in using advanc

ed techniques in the 

Trade Center. He also stated that the Port Authority is provisions of the Building Code.”
Rationale for the Selection of the 1968 N

Rationale for the Selection of the 1968 N

Code for the WTC Towers Code for the WTC Towers

• September 29, 1965 letter from Malcolm P. Levy, Chief, 

WTC to Minoru Yamasaki, Architect:

• “... Generally the tower core should be redesigned to elim

and to take advantage of the more lenient provisions reg

• May 19, 1966 memorandum from Guy F. Tozzoli, Directo

Department to John M. Kyle, Chief Engineer:

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

memorandum to Mr. Levy of June 22, 1965, based on the fa

Code had bee

n thoroughly review by interested technic

a

modified to meet all major objections; (2) it would probabwe constructed our buildings; and (3) the Commissioner faof using advanced techniques in the design of the World that the Port Authority, according to him, was not s

ubject t

o t

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, 1967, from Lester S

Structural Engineer, World Trade Department, PAN

Y

Linn (Deputy Director for Physical Facilities, WTD, PThe World Trade Center –

Towers and Plaza Buildin

NYC Building Code Revision Effective 12-6-68.

• Par

agraph 2. B. states:

•

“For office buildings there is no ec

advan

tage in using Class 1A Construction, and E

1B Construction for the WTC Towers and Plaza Bare Occupancy Group “E” (Business) with a fire iAs such, columns must have a three hour rating and with a 2 hour rating.”
Applicable Building Codes and Policies Applicable Building Codes and Policies

•Although not required to conform to NYC codes, the PANY

the provisions of the proposed 1968 edition of the NYC Builthree years before it went into effect. 

•The 1968 edition had less restrictive provisions compa

edition that was in effect when design began for the WTC to1968 code:

• Eliminated a fire tower as a required means of eg• Reduced the number of required stairways from 6 

the stair capacity from 30 to 60 per 22 inch unit of size of doors leading to the stairs from 44 in. to 36 

• Reduced the fire rating of the shaft walls in the bu

to 2 h;

• Changed partition loads from 20 ps

f

to loads base

partitions per unit length (that reduced such loads including the WTC

buildings);

• Permitted a 1 h reduction in fire rating for all struc

(columns from 4 h to 3 h and floor framing membe
Review of Drawings by New Yo

Review of Drawings by New Yo

Department of Buildings Department of Buildings

1975 Letter from Code Architect to Port Authority

- NYC/DOB reviewed W

TC 1 & 2 drawings in early 1

- Made 6 comments concerning the plans in relation dealt with egress issues)- Architect submitted responses to the comments to 

March 1968.

January 1968 letter from Solomon of Emory Roth to Levy 

1. Occupant load per floor is <365, stair capacity is 3902. Max travel distance is 140 ft, 200 ft permitted in unspr3. No fire tower required4. Cellar stairs are enclosed 2 h and lead to street or Co

underground street by PANYNJ)

5. Cellar stairs used for tenant storage are <10% of area

for egress requirements

6. Sprinklered

garage meets 150 ft travel distance requi
Port Authority Policies and Proced

Port Authority Policies and Proced

Design and Construction Design and Construction

•PONYA (PANYNJ after 1972) creat

operate transportation facilities (porairports)

•As an “interstate compact” organiza

the U.S. Constitution they are not suof any lower jurisdictions, including regulations

•1965 - Instructed architects and eng

follow 2nd and 3rd drafts of 1968 Cod

& 2, documenting differences from 1as variances justified as acceptable practice or proposed for the 1968 co
Port Authority Policies and Proced

Port Authority Policies and Proced

Maintenance and Modifications Maintenance and Modifications

•While not required, Local Laws were 

even where not retroactive

•LL5/73 and LL16/84

•Tenant manuals and review system

developed and implemented to manmodifications

•PANYNJ office•Outside consultants (LERA)•Due Diligence reviews

•Upgrades after 1993 bombing
Code Conformance Agreement be

Code Conformance Agreement be

Port Authority and NYC Building D

Port Authority and NYC Building D

1993 MOU between Port Authorit

y and NYC Bui

-

Reaffirmation of longstanding stated policy of 

conform to New York City Building Code. 

-

Each project reviewed and examined for code 

Port Authority.

-

Plans prepared and sealed by NY State licens

professional.

-

Port Authority design professional approving t

not have assisted in the preparation of the pla

-

Variances from code, acceptable to Port Auth

submitted to New York City Building Departmeconcurrence.
Code Conformance Agreement between Code Conformance Agreement between 

Port Authority and NYC Building Depart

Port Authority and NYC Building Depart

1995 Supplement to 1993 MOU 

- In lieu of any review by Port Authority, tenant malicensed design professionals to prepare and review to certify that plans conform to NYC building code.- Design professionals reviewing and certifying platenants should not be the same design professionacertification that the project had been constructed ithe plans and specifications. 

While the PANYNJ entered into agreements with the N

Buildings in the 1990s with regard to conformance buildings constructed in New York City to the NYC PANYNJ did not yield jurisdictional authority foenforcement oversight to the NYC Department o
Fire and Life Safety Systems Fire and Life Safety Systems
Building Codes Reviewed Building Codes Reviewed

(Fire Protection and Egress Systems) (Fire Protection and Egress Systems)

• NYC Building Code - 1968 edition• NY State Building Code - 1964 edition• Chicago Building Code - 1967 edition• BOCA Basic Building Code -

1965 edition

(National Model Building Code)

• NYC Building Code - 2001 edition • NFPA 101 - 1966 edition
National Fire Safety Standards Adopted b

National Fire Safety Standards Adopted b

1968 NYC Building Codes 1968 NYC Building Codes

• Sprinkler systems

• RS 17-2:NFPA 13 (1966) 

• Fire alarm systems

• RS 17-5:NFPA 72 (1967)

• Smoke management

• RS 13-1:NFPA 90A (1967)

• Egress systems 

• Covered in the Building Code
Review of Code Provisions Review of Code Provisions

• Construction Classes –

Unsprinklered

• Class 1A and 1B: NYC 68, NYS 64, BOCA 65 (Un• Class 1A and 1B: NYC 01 (Height limited to 75 ft. • Class 1A only: Chicago 67 (Unlimited height)

• Fire Resistance Rating (all codes, except NYC 01)

• Class 1A 

Columns: 4 hours (supporting more than one floBeams : 3 hours (floor construction)

• Class 1B

Columns: 3 hours (supporting more than one floBeams : 2 hours (floor construction)
NYC Building Code Provisions NYC Building Code Provisions

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

1938 1968* 

Columns

4 3 

Floors

3 2 

* Building code governing original design and o** Sprinklers required for buildings of unlimited h
Review of Code Provisions Review of Code Provisions

• Codes establish minimum requirements and 

rationale for selecting a higher Construction 1B)

• BCNYC permits 100 ft dead ends 

enclos

ures and 50 ft with 1 h.

• Selection of Construction Class is at discret

Owner/Architect
Review of Code Provisions Review of Code Provisions

• NYC Local Law 5 (1973) for New and 

Buildings (Full compliance by 1988)

• Applicable to new office buildings and ret

existing office buildings over 100 ft in hei

• Compartmentation required for unsprinkle

greater than 7500 ft

2

• Complete sprinkler protection could be pr

compartmentation

• Automatic stair door unlocking or unlocke• Approved evacuation plan and drills
Review of Code Provisions Review of Code Provisions

• NYC Local Law 5 (1973) for New and 

Buildings (Full compliance by 1988)

• Fire safety director and fire wardens• Fire command station in lobby• Voice communication system• Elevator recall• Zone smoke control and stair pressurizat• Stair pressurization not required if fully sp
Review of Code Provisions Review of Code Provisions

• NYC Local Law 16 (1984)

• Required sprinklers for buildings t

ft for new business, new and exis

• Emergency lighting in exits and co• Remoteness of exits (30 ft or 1/3 

distance)

• Full compliance by 1987
Code Provisions and Contemporary Pr

Code Provisions and Contemporary Pr

for Fire Protection and Egress Systems for Fire Protection and Egress Systems

• Sprinklers mainly in industrial and storage sp

• Rare even in high-rise buildings (except under

• Fire alarm systems

• Manual initiation for occupant notification• Control of fans and dampers to prevent smoke • Coded audible to indicate location of fire

• Smoke management not yet developed

• Required by insurance to mitigate property los• “smoke proof towers” and top vented stairs

• Egres

s system design based on 22 in. units o

• Occupant loads and capacities consistent• Scissor stairs common in NYC, remoteness ad
Due Diligence Review by RJA and 

Due Diligence Review by RJA and 

November 22, 1996 November 22, 1996

• Category A (no plans to correct)

• Stairs A and C discharge at mezzanine (PA position 

“public way”)

• Exit stair venting (PA position is that the shaft height 

compliance)

• Category B (remedied or in progress)

• Upgrading of fireproofing• Tenant separation and corridor walls being extended • Individual fire command stations and additional manu

speakers

• Category C (remediation plans in process)

• Doors to mechanical spaces in stairways• Standpipes exceed 145 foot limit on some floors• Egress issues at Mall and B-1

• Category D (Miscellaneous)

• Smoke control at PATH mezzanine• Code compliance oversight• Lack of Certificate of Occupancy
Fire Resistance Ratings Fire Resistance Ratings

WTC 1 and 2 WTC 1 and 2

• Construction Class 1B (3 hour protected)

• 3 hours on columns• 2 hours on floors• 2 hours on exit access corridors 

• 1 hr permitted but 2 hr used to permit 100 foo

• 2 hours for shaft enclosures• 1 hour for tenant separation (demising) walls
Compartmentation Requirements Compartmentation Requirements

• The NYC Building Code and PANYNJ practice required partitio

spaces from each other and from common spaces such as tthe elevators, stairs and other common spaces in the building co

• Local Law 5 (1973) required compartmentation of unsprinklered 

buildings over 100 ft in height “having air-conditioning and/or mesystems that ser

ve more than the floor on which the equipment 

subdivided by 1 hour fire separations into spaces or compa7,500 f

t

2. Floor areas could be increased up t

o 15,000 ft

2 if p

resistive construction and smoke detectors. 

• Shortly after the adoption of LL 5 (1973), the PANYNJ began to 

compartmentation as a part of new tenant layouts as evidenced alteration contracts at this time. 

• Following the 1975 fire a fire safety consultant report recomme

the buildings be retrofit with sprinklers to address possible sthe PANYNJ realized that this would also obviate the need for permit the unobstructed views for which the buildings were knowsprinkler the buildings left the arrangement again with only part

spaces from each other and from exit access corridors or ccore, and with shaft enclosures.
Means of Egress Means of Egress

WTC 1 and 2 WTC 1 and 2

•Occupant load (per floor) 390

•39000 sq ft net @ 100 sq ft pe

for group E

•Units of exit width required, 6 1⁄2

•390 @ 60 persons per 22 in u

•Min clear width of stairs 44 in (2•Door widths 36in (min) to 48 in 
Egress from Windows on the Worl

Egress from Windows on the Worl

• Assembly occupancy located on the 106th and 107

• Windows on the World Restaurant (original to th• Greatest Bar on Earth• Multiple function rooms and support spaces• Occupancy of 1130 per floor

• Closed following the 1993 bombing, re-opened in • Dec. 1994 meeting between PANYNJ and DOB re

• 2-h separation into areas of refuge containing o

elevator (27-367 NYCBC

)

• 2 such spaces doubles exit capacity, 3 or more 

(27-372 NYCBC)

• Agreed in Jan. 1995 memo 

• Approach extended to Top of the World in WTC 2
Egress from Top of the World Egress from Top of the World

• Observation Deck located on the 107th floor of WT

building) 

• Tenant/operator notified to follow agreed approach

designer

• Divided floor into three areas of refuge (1751 total

• Area 1 – 935• Area 2 – 343• Area 3 – 473• Did not include occupant load of roof deck whic

floor

• PANYNJ informed tenant to limit occupancy to 113

• Reported by PANYNJ that turnstiles were not e

• Monitored by staff
Required Fourth Stairway Required Fourth Stairway

• Building codes (including NYCBC) require 2 stairs, 

• 3 where occupant load >500, and • 4 where occupant load >1000

• NYCBC 27-367 states additional load not counted if space is <

use.

• PANYNJ interpr

etation (2005) 

“Based on PA's meeting on exits from the Windows on the W

December 6, 1994, it's PA's understanding that "20% of the 27-367 is intended to be the total floor area in the building oprincipal use.”DOB interpretation (2005)

“The idea behind the 20% allowance is the fact that the PA [N

Public Assembly] space is used by the same tenant employfamiliar with the building and regularly participate in the fir

e 

about a roof top restaurant or an observation deck that is opublic and tourists, the concept of exit reduction shall not be 

NIST concludes that four stairways were required. Never mentio

discussions on egress from Assembly spaces or in due diligen
Elevators (99 in each tower) Elevators (99 in each tower)

WTC 1 and 2 WTC 1 and 2

•By Code not utilized for fire service acce

occupant egress during an emergency

• Elevators were used for occupant egre

after 1 was hit and before 2 was hit

•Three elevator “zones” with sky lobbies (

and 78th floors)

• 8 express concourse to 44• 10 express concourse to 78• 24 locals per zone in groups of 6• 7 freight elevators (1 serving all floors) • All retrofit with Firefighters Emergency 

ASME A17.1)
Fire Suppression Fire Suppression

(original) (original) 

•Standpipes

• Located within 145 feet of any point• Outlets located within stairways• Tanks on mechanical floors and cross c

standpipes

• Standpipe communication system

•Sprinklers only required below grade prior 
Fire Alarm System Fire Alarm System

(original) (original)

•Installed in corridors above minimum requ•Smoke detectors

• 4 per floor at air return grilles to prevent 

smoke

• No manual initiating devices
Miscellaneous Requirements Miscellaneous Requirements

(original) (original) 

•Emergency power not required•Stair pressurization not required•Smoke purge system not required•Some requirements for exit signs and ligh

of egress
Modifications Modifications

• LL5-1973 and LL16-1984 resulted in major changes to 

were incorporated into initial WTC 7 design because o

• Compartmentation 

• 7500 sq ft with 1-hr• 10,000 sq ft with 2-hr• 15,000 sq ft with 2-hr and smoke detectors• Not required if sprinklered

• Class E alarm and voice communication system• Fire command station in garage (later moved to lob• Emergency power for exit signs and egress lighting• Sprinkler system• Smoke control/purge• Controlled inspection of Spray Applied Fireproofing • Carpet in exit access corridors• Stair re-entry every four floors
Findings Findings

• While not subject to the building and fire regulations of New York City the PA gener

design stage and as they evolved over the life of the buildings through the adoption 

• LL5-1973• LL16-1984

• The fire and life safety systems were generally consistent with the requirements of n

accepted practices 

• PA developed detailed tenant manuals and alteration procedures, and conducted re• PA addressed some problems over an extended period of time 

• Adequacy of and upgrades to fireproofing• Tenant separation walls• Sprinkler retrofits• Inspection by independent third parties

• Recommendation 25. Nongovernmental and quasi-governmental entities that own or lease buildings an

safety code requirements of any governmental jurisdiction are nevertheless concerned about the safety responding emergency personnel. NIST recommends that such entities should be encouraged to provexceeds the level of safety that would be provided by strict compliance with the code requirements of ajurisdiction. To gain broad public confidence in the safety of such buildings, NIST further recommends and as-built safety be certified by a qualified third party, independent of the building owner(s). The procode enforcement in areas including interpretation of code provisions, design approval, product accepconstruction, and post-occupancy inspections over the life of the buildings.

• Recommendation 26. NIST recommends that state and local jurisdictions should adopt and aggressive

building codes to ensure that egress and sprinkler requirements are met by existing buildings44. Furth

modified where needed (such as when there are assembly use spaces within an office building) to meecodes.

• Recommendation 28. NIST recommend that the role of the “Design Professional in Responsible Charg

(1) all appropriate design professionals (including, e.g., the fire protection engineer) are part of the descare when designing buildings employing innovative or unusual fire safety systems, and (2) all appropre.g., the structural engineer and the fire protection engineer) are part of the design team providing the sstructure to resist fires, in buildings that employ innovative or unusual structural and fire safety system
Structural Design, Modifications a

Structural Design, Modifications a

Inspections Inspections
Objectives Objectives

• Comparison of structural provisions in selected c

• 1968 NYC Code• 1964 NYS Code• 1965 BOCA Basic Building Code• 1967 Chicago Code• 2001 NYC Code
Objectives Objectives

• Document the design, construction, modification, 

of the structural systems

• Provisions used to design and construct the bu• Design criteria• Methods used to proportion structural member• Tests performed to support the design• Innovative features of structural systems• Deviations granted by PANYNJ• Inspection protocols during construction• Maintenance of and modifications to structural 
Reports Reports

• NCSTAR 1-1 “Design, Construction and Mainten

and Life Safety Systems”

• NCSTAR 1-1A “Design and Construction of St• NCSTAR 1-1B “Comparison of Building Code 

Requirements”

• NCSTAR 1-1C “Maintenance and Modification

Systems”

• NCSTAR 1-1F “Comparison of 1968 and Curre

York city Building Code Provisions”
Building Codes Used in Design Building Codes Used in Design 

Initial design:• May 1963- Port Authority instructed architect & e

the1938 edition of New York City Building Code

Final design:

• September 1965- Port Authority instructed archite

revise design in accordance with the second and code (adopted Dec

ember 1968) 

Port Authority adopted the provisions of the propoYork City Building Code more than 3 years before effect.
Changes to 1938 Code Related to 

Changes to 1938 Code Related to 

Design 

• Uniform partition load based on weight of partition 

1938 required 20 psf

minimum load

• Permitted wind tunnel tests using models to estab

for wind load
Comparison of Code Structural Require

Comparison of Code Structural Require

• Dead loads• Liv

e loads

• Liv

e load reduction

• Lateral loads (wind and earthquake)• Progressive collapse resistance• Design standards
Dead Loads Dead Loads

• Permanent loads

• Structural members, permanent partitions, finis• Codes have similar values for partition loads a

finishes

• Building codes provide unit weights (densities) of 

calculating design dead loads 

• Codes have similar densities for building mate

e.g.,steel = 490 pcf, concrete = 150 pcf
Live Loads Live Loads

• Loads produced by use and occupancy• Codes specify basic live load values for design• Code values are based on experience and load s• Expressed as equivalent uniform loads

• Codes have similar values, 

e.g., Office = 50 psf, Lobby = 100 psf

• Concentrated loads treated separately

• Placed so as to produce maximum stresses
Live Load Reduction Live Load Reduction

Codes allow reduction of basic live load in proportio

members because it is unlikely that:

• All floors are fully loaded simultaneously

• Affects design loads for columns and walls

• Entire floor area is loaded to design value

• Affects design load for girders and beams
Live Load Reduction Methods Live Load Reduction Methods

• Percentage method (columns and walls)

Liv

e load reduction increases with increasing num

to a limit)

• Tributary area method

Liv

e load is reduced as the accumulated tributary 

maximum permitted reduction depends on ratio o
Reduced Live Load for Columns 

Reduced Live Load for Columns 

(Percentage Method) (Percentage Method)

100%

85%80%75%70%65%60%55%50%

80%

80%80%75%70%65%60%55%50%

Roof

1st floor below

2nd floor below

3rd floor below

4th floor below

5th floor below6th floor below7th floor below

8th and subsequent

floor below

NYC Building Code

(Alternative M

ethod)

Chicago Code

NY State

Building Co
Reduced Live Load for Beams, Girders 

Reduced Live Load for Beams, Girders 

(Tributary Area (Tributary Area Method) Method)

Contributory Area

(sq ft) 

NYC B

uildi

ng Code* Chicago Code N

100 or less 100 100 

100 - 150 100 95 150 - 200 80 - 85 95 200 - 300 80 - 85 90 300 - 450 60 - 75 85 450 - 600 50 - 70 85 

600 - and mor

e 40 - 65 

Also dep

ends 

on the 

DL/LL ratio

85 

* For colum

ns, limited to 80% ** Limited to 40% or value based on D
Live Load Reduction Used Live Load Reduction Used 

Outside of Core

Inside of Core
Live Load Reduction Live Load Reduction

Dra

wing used with permission of PANYNJ.
Live Load Reduction Live Load Reduction

Dra

wing used with permission of PANYNJ
Wind Load Wind Load

• Effect of wind is considered as a lateral pressure 

faces of the building

Base Shear

Overturning m
Wind Pressure Distributions of Wind Pressure Distributions of 

Different Building Codes Different Building Codes

Ground L

e

v

el

100 ft

300 ft

600 ft

1000 ft

1200 ft

20

25

30

35

40

NYC Building

Code 

(68 and 01)

12

28

32

35

37

N

Y State 

Code (64)

20

42.5

Chicago 

Code (67)
Design Wind Pressure: Design Wind Pressure:

1968 NYC Building Code 1968 NYC Building Code

Alternative method• “Suitably conducted model tests”• Fastest-mile wind speed of 80 mph at 30 ft above • Include all factors involved in consideration of win

• Pressure and suction effects• Shape factor

s

• Gust ...
Earthquake Load Earthquake Load

• NYC Building Code (1968) - No provisions• NY State Building Code (1964) - No provisions• Chicago Building Code (1967) - No provisions• BOCA Basic Building Code (1965)

Based 1961 UBC (Base shear V = ZKCW)

• NYC Building Code (2001)

Based on 1988 UBC (Base shear V = ZIKC
Provision for Progressive Collaps

Provision for Progressive Collaps

Resistance Resistance

• NYC Building Code (1968)

-

No provision

• NYC Building Code-Rules of the City of New Yor

• Alternate path method• Specific local resistance method• August 7, 1973 clarification: applicable to struc

connections that rely on friction
Design Loads for WTC 1 and 2 Design Loads for WTC 1 and 2

• Dead load: 

• Greater than or equal to design loads specified 

Building Code

• Liv

e load:

• Greater than or equal to design loads specified 

Building Code

• Liv

e Load Reduction:

• Equal to or more stringent than 1968 NYC Bui

requirements

• Wind load:

• Wind tunnel test results were significantly grea

prescriptive pressures in 1968 NYC Building C
Wind Load Design Values Wind Load Design Values 

1968

NYC Building Code

T

Base Shear

(kips) 9,250 

Overturning

Moment

(103 ft kip)

7,621 
National Structural Design Standa

National Structural Design Standa

• NYCBC (68)

1963 AISC Specifications, 1963 ACI 318 Co

• NY State BC (64)

Appropriate national standards

• Chicago BC (67)

1963 AISC Specifications, 1963 ACI 318 Co

• BOCA Basic Building Code (65)

1963 AISC Specifications, 1963 ACI 318 Co

• NYC Building Code (01)

1989 AISC Specifications, 1989 ACI 318 Co
Tests Performed to Support Desig

Tests Performed to Support Desig

• Exterior wall panel tests• Wind effect tests• Tests of viscoelastic dampers• Tests of floor trusses• Stud shear connector tests
Tests Performed to Support Desig

Tests Performed to Support Desig

Exterior Wall Panel Model Tests• University of Western Ontario• Linear-elastic tests using plastic models• Load-deflection characteristics • Examine “most effective construction” for 

wall panels 

• 15 different tests; replicating the 20th, 47th, 

and 4th floor exterior walls

Dra

wing use
Tests Performed to Support Desig

Tests Performed to Support Desig

0°

Grid System 

Wind Effects Tests• Meteorological Program• Wind Tunnel Program• Structure Damping Program• Physiological Program

Dra

wing used w
Tests Performed to Support Desig

Tests Performed to Support Desig

Viscoelastic Damping Unit Tests• Conducted to confirm effectiveness of the dampin

controlling building motion due to wind

• Two sets o

f t

ests

• 1967: Minnesota Mining and Manufacturing Com• 1968-1969: Massachusetts Institute of Technolog
Damping Units Damping Units

Source: FEMA WTC Report
Tests Performed to Support Desig

Tests Performed to Support Desig

Floor Truss Tests• Full-scale flexural tests • Interior panel connection tests

(between floor truss and bridging truss)

• Bearing capacity (at ends of truss) tests • Shear knuckle tests
Tests Performed to Support Desig

Tests Performed to Support Desig

Shear Knuckle Tests

Dra

wing used with 
Guidelines for Inspection, Repair and Mo

Guidelines for Inspection, Repair and M

Structural Systems Structural Systems

• Tenant Construction Review Manuals• Architectural and Structural Design Guidelines• Structural Integrity Inspection Program• Facility Condition Surveys
Modifications to Structural System

Modifications to Structural System

Tenant Construction Review Manuals

• 1971, 1979, 1984 (revised 1990), 1997• Technical criteria to be used by tenants of all Por

• Applications: scope of work, design criteria, an• Structural requirements: design by registered a

professional engineer in accordance with appli

• Inspection program
Modifications to Structural System

Modifications to Structural System

Architectural and Structural Design Guidelines (• Requirements for tenant alterations in WTC 1 and • Structural design guide

• Minimum floor loads, core hole sizes, HVAC eq

• Structural specifications• Standard details
Structural Integrity Inspection Pro

Structural Integrity Inspection Pro

• Structural integrity inspections conducted in 1991

1995, 1996, 1997, 1998, 1999, and 2000

• Detect, record and correct distresses and deterio

lead to structural problems
Structural Integrity Inspections Structural Integrity Inspections

• Inspection/monitoring of 16 items, including:

• Space usage• Accessible columns• Bracing below grade• Hat trusses• Floor framing over mechanical areas• Floor framing over tenant areas• Natural frequency measurements• Natural frequency of floors• Damping units
Structural Integrity Inspection Structural Integrity Inspection

• Space usage – identify possible overloading

• Distribute granite slabs on floor106 over a larg

• Accessible columns –

check condition of columns 

• Missing thermal insulation on columns (1996, 1

• Bracing below grade

• No priority recommendations (1991, 1995) 

• Hat trusses –

check overall condition 

• No priority recommendations (1992, 1995) 

• Floor framing over mechanical areas –

check con

(1992, 1996, 1999) 

• Repair thermal insulation
Structural Integrity Inspection Structural Integrity Inspection

• Floor framing over tenant spaces 

–

check overal

framing system (1992, 1995, 1997, 1999) 

• Missing fireproofing on columns (1996, 1998) 

• Natural frequency measurements –

WTC 1 only 

• Measure and computed values are similar (1• No analysis made since 1998 

• Natural frequency of floors –

WTC 1 & 2 (1996)

• No appreciable difference between analysis a
Structural Integrity Inspection Structural Integrity Inspection

• Viscoelastic damping units –

WTC 1 & 2

• Damping units were in good condition

• Post 1993 explosion –

pre and post repair inspec

• Six inspections were made by LERA, U.S. Arm

Proto Mfg, and Lucius

Pitkin
Facility Condition Survey Facility Condition Survey

• Conducted by Engineering Quality Assurance D

PANYNJ

• Based on sampling plan and NDT test methods• Surveys in 1990, 1991, 1997, 2000

• Exterior wall system• Core columns• Hat tru

s

s

• Floor system• Damping system
Findings Findings

• Building codes and standards lack explicit struct

provisions to mitigate progressive collapse

• Recommendation 1 

• Progressive collapse should be prevented in b

the development and adoption of consensus scode provisions

• Standard methodology should be developed t

failure in structural systems
Findings Findings

• Building codes allow determination of wind press

tunnel tests for use in design

• Recommendation 2

• Performance standards should be developed f

tunnel testing and estimating wind loads and thbuildings for use in design
Findings Findings

• Building Codes lack minimum structural integrity 

means of egress in the building core that are criti

• Recommendation 18 (2)

• Egress systems should be designed to mainta

integrity
Findings Findings

• State and local jurisdictions do not require retenti

related to design, construction , maintenance, anbuildings

• Recommendation 27

• Building codes should incorporate a provision 

building owners to retain documents over the l