NBSIR 73-221

Abnormal Loading on Buildings

and Progressive Collapse

Norman F. Somes

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

May 1973

Interim Report

Covering period November 1971 - August 1972

Prepared for Office of Research and Technology

Department of Housing and Urban Development

Washington, D. C. 20410

NBSIR 73-221

ABNORMAL LOADING ON BUILDINGS

AND PROGRESSIVE COLLAPSE

Norman F. Somes

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

May 1973

Interim Report

Covering period November 1971 - August 1972

Reprinted from National Bureau of Standards Building Science

Series 46, February 1973

Prepared for Office of Research and Technology

Department of Housing and Urban Development

Washington, D. C. 20410

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

SI Conversion Units

In view of present accepted practice in this country in

building technology, common U.S. units of measurement have

been used throughout this paper. In recognition of the

position of the United States as a signatory to the General

Conference on Weights and Measurements which gave official

status to the metric SI system of units in 196 0, we assist

readers interested in making use of the coherent system of

SI units by giving conversion factors applicable to U.S.

units used in this paper.

Length

1 in = 0.0254 meter (exactly)

1 ft = 0.3048 meter (exactly)

Force

1 lb (Ibf) = 4.448 Newton (N)

Pressure

1 psi = 6 895 N/m

Abstract

The document is an interim report of ongoing studies at

the National Bureau of Standards c It defines the several

aspects of abnormal loading on buildings and the problem of

progressive collapse. It docnments the extent to which

present UoS. Codes and Standards address the problem,

Abnomal loadings are identified^, classified and

discussed v/ith regard to their characteristics and frequencies

of occurrence. The report reviews the state of international

knowledge of the characteristics of abnormal loadings and the

response of buildings and building elements to these loadings

.

The latter includes discussion of several incidents in which

multistory buildings have collapsed progressively.

Using currently available statistics an estimate is made

of the combined frequency of abnormal loadings on residential

buildings in the U.S. For buildings susceptible to progressive collapse, the corresponding risk of fatality is compared

with the levels of risk that society will generally accept.

The risk is further compared with the risk of mortality

associated with fire in residential buildings, an area of

considerable public concern and expenditure

»

It is concluded that U.S. standards-writing bodies should

adopt appropriate rational criteria as soon as possible to

reduce the risks of progressive collapse. There are several

areas in which criteria might be introduced to reduce the

risk of progressive collapse. These are discussed; particular

attention is given to the philosophies behind the structural

criteria implemented in the USA and other countries

.

Key Words ; Abnormal loading, buildings, codes, design

criteria, multistory, progressive collapse, risk,

stability, standards, strength. United States.

Table of Contents

Page

1. Introduction 1

2u Problem Definition 7

3. Public Acceptance of Risk g

4. Classification of Abnormal Loadings 14

5. Studies of Abnormal Loadings 17

6. Summary of the Probabilities of Abnormal Loadings

and Their Implication for the USA 49

7. Alternative Approaches for Criteria 5C

8. Philosophies for Structural Criteria 53

9. Building Response to Abnormal Loadings 55

10. Conclusions 62

11. Acknowledgm.ents 64

12. References 64

Author ' s Note

This document is one of 15 state-of-the-art reports

that provided source material for a National Workshop on

Building Practices for Disaster Mitigation held at the

National Bureau of Standards, Boulder, Colorado, August 2 8 - September 1, 1972.

With other reports and findings of the Workshop, this

document was published in full in National Bureau of Standards

Building Science Series 46, February 1973 to which reference

may be made.

At the request of the project sponsor, the document is

now printed separately as an NBS Report.

i^NOEMAL LOADING ON BUILDINGS AND PROGRESSIVE COLLAPSE

By

Norman Fo Somes

1. Introduction

Since 196 8, there has been growing international

concern that multistory buildings are frequently designed

without explicit consideration for abnormal loading conditions

.

On May 16 of that year, there occurred the much-publicized

collapse of a portion of the Ronan Point apartment building

in London, The building has 22 stories of precast concrete

panel construction above a cast-in-place concrete podium » A

typical floor layout is shown in Figure 1 in which the

structural walls are shown solid. The collapse was triggered

by an accidental explosion of gas that leaked from the connection of a gas range located in an apartment on the 18th floor.

The Report of the Inquiry into the Collapse [1] states:

"The explosion blew out the non-load-bearing

face walls of the kitchen and living room, and

also^ unfortunately, the external load-bearing

flank [end] wall of the living room and bedroom

of the flat, thus removing the support for the

floor slabs on that corner of the nineteenth

floor J. which collapsed » The flank wails and

floors above this collapsed in turn, and the

weight and impact of the wall and floor slabs,

falling on the floors below caused a progressive

collapse of the floor and wall panels in this

corner of the block [building] right down to the
failure location

Figure 1 Typical floor layout of Ronan Point apartment

building (explosion occurred in a SE-corner

apartment). Reproduced from [1].

2
level of the podium."

The extent of the collapse is shown in Figure 2; an adjacent

building of identical construction shows the appearance of

the building prior to the collapse. From Figures 1 and 3 it

may be seen that collapse affected both the living room and

bedroom above the 16th floor, while below this level, the

collapse was limited to the living room. Four people were

killed in the collapse and seventeen people were injured.

"The loss of life and injury might well have been

very much worse. At 5:45 a.m., mercifully, most

tenants were in their bedrooms ..."

The Report also documents that, by a fortunate chance, of

the four apartments directly above the one in which the

explosion took place, only one was occupied at that time.

The Report of the Inquiry drew international attention

to several deficiencies in existing codes and standards,

particularly as they applied to multistory buildings. Interim

additional criteria [2] , having regard to the appraisal and

strengthening of existing buildings and the design of new

structures, were quickly implemented in the United Kingdom.

Several other countries in Europe introduced additional design

criteria to deal explicitly with the risks exposed by the

Ronan Point incident.

To date, with one exception, the U.S. codes- and

standards-writing bodies have not published criteria to provide

protection against abnormal loads and possible progressive

collapse. The exception is the 1972 American National

Standards Institute A58, Minimum Design Loads in Buildings

and Other Structures [3] which provides a short statement

drawing attention of the designer to the problem. Several

standards-writing bodies have established technical committees

to consider the problem, however.

The prime mover in the matter of progressive collapse in

the USA has, to date, been the Department of Housing and Urban

3
Figure 2 Ronan Point apartment building after the

collapse, with a second identical building

in the background. Reproduced from [1].
Figure 3 Layout of the SE-corner apartment on the 18th

floor where the explosion originated.

Reproduced from [1].

5
Ij ; lopment (HUD). In August 1969, provisions against progressive collapse were included for the first time in a

Structural Bulletin [4] issued to the producer of a precast

concrete housing system by HUD's Federal Housing Administration (FHA) . Criteria against progressive collapse were

included in HUD's Guide Criteria documents [5] prepared by

the National Bureau of Standards (NBS) early in 19 70 and

implemented in HUD's experimental housing program Operation

Breakthrough. In October of 1971, the FHA circulated its

draft document "Provisions to Prevent Progressive Collapse"

[6] for review by certain trade associations, standardswriting bodies and members of the design profession. This

document expressed criteria intended for application by FHA

in evaluating multistory buildings for which Federal mortgage

assurance is required. The document has not been circulated

in final form but the draft has served as a starting point

for much discussion. In each of these preceding instances,

the criteria reflected the United Kingdom requirements at

that time

In November 1971, the National Bureau of Standards, at

the request of HUD, began a detailed study of abnormal loading

on buildings in the USA and the problem of progressive

collapse. The result to date provide the basis for this

interim progress report, which firstly attempts to define

the parameters of the problem, to classify and discuss the

various sources of abnormal loading and to quantify their

frequency insofar as the currently available U.S. data

permits. The implication of these findings for the USA are

then discussed. The response of buildings and building

elements to abnormal loading is then reviewed, including

cases of progressive collapse. Several alternative approaches

for the introduction of criteria are presented and, finally,

conclusions are given with respect to the problem posed in the

USA.

6
2. Problem Definition

Progressive collapse may be defined as a chain reaction

of failures following damage to only a relatively small

portion of a structure . An abnormal loading may be defined as a condition of

loading which a designer, following established practice

does not include in the norm.al analysis and design of a

particular structure . It is a loading condition of

sufficient severity and probability of occurrence to be

a cause for concern, but still of such a relatively rare

nature as to be outside of normal design-life expectancy.

This definition goes beyond that of static and dynamic forces

and includes such conditions as the dislodgement of a bearing

wall panel, and the development of a "•/eld failure in a steel

connection.

Recent reports [7, 8] confirm the growing view that

studies of the problem of progressive collapse should deal

with multistory construction of all types and not be limited

to high-rise construction or simply to that using precast

concrete panels. However, the view is also widely held that

framed buildings are more tolerant of local damage and have

more resistance to progressive collapse than load-bearing

structures. It is reasoned that this is due to the fact that

continuity between members is more easily accomplished in

framed buildings than in load-bearing structures, and that

the former have greater ability for developing alternate

paths for forces in the event of the loss of a critical

member. This' viewpoint is supported in part by the documented

experience of engineers during World War II bombings [9]

,

Occupants of multistory buildings, whatever type of

construction is used, have a right to expect adequate and

consistent levels of safety. The user requirement may be

expressed as adequate protection from extreme loads .

7
Expressed as a performance requirement, this corresponds to

adequate strength , namely compliance with a specified load

capacity. Present U.S. design standards specify load

capacity in terms of combinations of severe dead, live, snow,

wind, or earthquake loads. They do not specify load capacity

with respect to abnormal loading conditions as defined herein.

In a general sense, adequate safety is achieved by

insuring that, at loads less than the specified load capacity,

there is no loss of static equilibrium resulting in:

Local Collapse, or

Extensive Collapse.

The Ronan Point incident was clearly due to an abnormal

loading condition and the collapse was extensive. Had the

damage been confined to the apartment in which the explosion

originated, it is doubtful whether the accident would have

received more than local newspaper coverage. Such explosions

occur somewhere every day and arouse little reaction from

society at large. It is only when they produce extensive

collapse that they generate international attention.

The foregoing discussion serves to delineate the NBS

study which is concerned with abnormal loadings on multistory

buildings of any type of construction, and the need to prevent

progressive collapse, as a result of such loadings .

3. Public Acceptance of Risk

Risk is a function of the probability of occurrence and

the consequence of a particular event. Zero risk in the face

of all possible conditions and hazards can never be achieved.

By assessing the statistics of all foreseeable hazards and

evaluating their consequence, an acceptable level of safety

8
can be achieved, acceptable safety at an acceptable cost.

The acceptable risk to life and property is probably best

decided by representatives of the community at large.

Otway et al [10] have provided one basis for considering

the risks with which society is prepared to live. For this

purpose, they use the U.S. accidental death statistics for

1966. The probability of death per person per year is given

for a series of types of accidents in the following table:

Table 3.1

Motor Vehicle

Falling

Fire

Drowning

Firearms

Poisoning

Earthquake

Lightning

2. 7 X 10

-4

1. 0 X 10

-4

4. 0 X 10

-5

2. 8 X 10

-5

1. 3 X 10

-5

1. 1 X 10

-5

8. 0 X 10

-7

5. 5 X 10

-7

The paper points out that situations providing hazards

on the order of 10 deaths per person per year are uncommon.

When a risk approaches this level immediate action is taken

to reduce the hazard. This level of risk is unacceptable to

-4

everyone in society. At an accident level of 10 deaths per

person per year, people spend money, especially public money,

-5

to control the cause. Risks at the level of 10 deaths per

year are still significant to society. Accidents with a

~6 probability about 10 deaths per person per year are not of

great concern to the average person. He may be aware of them

but he feels that will never happen to him. There is a

general lack of concern about accidents having a mortality

9
risk of less than 10 per person per year.

Some qualification should be made with respect to this

last point and the statistic for earthquakes. In fact, a considerable amount of money is spent on earthquake protection

and this would appear to conflict with Otway's last general

coMmento However, Otway divided incidents by the total U.S.

population whereas the earthquake risk is geographically

concentrated c If, for example, the population living within

seismic Zone 3 were used, the probability of death per person

would be increased by a factor of approximately 5, to 4.0 x

10 °, thereby appearing to remove the above conflict. Another

factor contributes to the considerable U.S. expenditure on

earthquake protection; in addition to deaths and injuries,

serious earthquakes are accompanied by large property losses.

Public expenditure to provide protection against fire

in buildings is very considerable and this is in response to

-5

a probability of 4 x 10 deaths per person per year together

with heavy property damage. It would be useful to assess the

frequency of abnormal loading incidents that, for people

living in buildings susceptible to progressive collapse, would

constitute a risk of death from that cause equal to that from

fire. It is important to differentiate between buildings that

are and are not susceptible to progressive collapse. Figure

4 shows a speculative plot of the total number of U.S. housing

units ^ increasing with time, and having a value of Y units at

a particular point in time. There is a reason to believe

that, in the absence of new criteria to minimize the risk of

progressive collapse, the number of buildings that are susceptible, will otherwise increase. A second plot shows the

number of susceptible units increasing with time from a

relatively insignificant level to a very significant fraction

of the total number of housing units. Of course, it is with

the objective of stopping any such growth, that the NBS study

and related studies are underway.

10
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11
The following assumptions will be made in order to

carry out a comparison of the respective risks due to progressive collapse and fire:

la. The total number of abnormal loading incidents

affecting U.S. housing units in a year corresponding

to the selected point in time is N.

2o The probability of occurrence of an abnormal

loading on a housing unit is the same irrespective

of the type or location of the unit.

3 . The abnormal loadings in question are of such

severity that progressive collapse of susceptible

buildings could occur.

On the basis of assumption 2, the probability of an abnormal

loading per year in a housing unit susceptible to progressive

collapse is N/Y.

In what follows, the result will be seen to depend upon

the size and architectural arrangement of the multistory

building considered. Consider a 100 housing unit building

with a central service core, four housing units per story and

an average occupancy rate of 4 persons per unit. If, as in

the Ronan Point building, the collapse affected one quarter

of the building in plan, then an estimate of loss of life

would have to be based upon some fraction of the number of

occupants of that quarter. For part of each 24 hours, people

will be absent from the building e.g., at work or school or

in recreational activity. Furthermore, it is doubtful if

more than one-half of the casualties would result in death.

There would be a number of severely injured, less injured,

etc. When both factors are considered, a reasonable estimate

of the number of deaths would appear to be 25, namely 1/16 of

the total number of building occupants

The following discussion will be confined to units

12
susceptible to progressive collapse . The probability of an abnormal loading in a 100 unit

building per year is

^ X 100

The risk of death due to progressive collapse per person

per year in that 100 unit building is

fxlOOxji

Equating this risk of fatality to that due to fire

§ X 100 X = 4 X 10"^

Y ±5

If Y is taken to correspond to the U.S. housing unit total

as given by the 1970 U.S. Housing Census [11], namely 67.7

million

4 X 10"^ X 16 X 67.7 x 10^

100

= 433 incidents

This result states that, for the conditions considered,

an annual U.S. total of 433 abnormal loadings on housing

units would result in a risk of fatality that would correspond

to the general risk of fatality in fire. Clearly the result

is a function of the architectural layout of the building

considered and hence the estimation of the ratio of possible

deaths to the total number of occupants of the building.

It is shown that the risk of fatality due to progressive

collapse per year could be written

r7 X No. of units X TpT13
For a given architectural layout, and given values of N and

Y, the risk appears to be directly proportional to the number

of units, namely the number of stories.

The use of the figure of 433 incidents should be

qualified to account for the assumptions made and for its

dependence upon architectural layout and size of building.

Nonetheless, the figure of 433 incidents establishes an order

of magnitude that is useful in assessing the significance of

the statistics for abnormal loadings that are discussed in

Chapter 5 and summarized in Chapter 6. In Chapter 6 it will

be shown that a lower bound estimate of the number of abnormal

loadings per year on housing units is 702.

4. Classifications of Abnormal Loadings

Publications such as the Engineering News Record

regularly describe engineering failures as well as successes.

The authoritative work of Feld [12] has described many

building failures. Allen and Schriever [7] have summarized

reported failures of recent years in North America. Two

conclusions are drawn from works such as these : Only a small

minority of building failures occur due to a loading of a

type explicitly considered in the design. The great majority

of failures result from loading conditions for which current

codes and standards give little or no guidance and which, as

a consequence, are not considered in design. Such loading

conditions are termed abnormal loadings in this report. The

second conclusion is that there is a large variety of abnormal

loadings and no classification of them could reasonably be

expected to be complete. One important contributing factor

is thaty with ever-advancing technology, new sources of

abnormal loadings can be expected to be generated. With

these several thoughts in mind, the following classifications

14
are deliberately limited to abnormal loadings for which the

probability of occurrence seems significant. The first

classification is an overall generic one:

Violent change in air pressure

Accidental impact

Faulty practice

Foundation failure

These classifications will now be discussed in more detail.

A, Violent Change in Air Pressure

This includes

:

Sabotage bombings

Service system explosions

Other explosions within the building

Explosions external to the building

Sabotage, using explosives, is a very serious form of

abnormal loading. The motive for sabotage might concern

only one person, a family, or an organization resident in

the building, yet the bombing could affect many or all of

the occupants. Service system explosions can originate in

heating, cooling, and cooking systems, in high-pressure steam

pipes and in boilers. Sources of other internal explosions

include containers of liquified gases such as propane or

butane or containers of gasoline. There are a number of

sources of accidental explosion external to the building such

as the shipment of hazardous materials through urban areas

by truck, railroad, and waterway or by the rupture of gas

transmission and distribution systems.

B , Accidental Impact

This includes

:

Highway Vehicles

Construction Equipment

Aircraft

15
Trucks and automobiles leaving the highway out of

control are included in the first category. Accidents

involving cranes and lifting devices of all kinds are

included in the second category. In urban areas, construction

frequently takes place on congested sites that have relatively

small clearances from existing occupied buildings.

C . Faulty Practice

Past experience would indicate that when failures do

occur, they are frequently the result of faulty practice.

Whether or not local or extensive collapse results is

largely a function of the type of construction involved, i.e.

whether it can tolerate local damage without extensive

collapse

Design Error

Construction Error

Misuse or Abuse by the Occupant

Misuse or abuse by the occupant can include ill-considered architectural changes or cutting of the structure.

D. Foundation Failure

The ASCE Research Council on Expansive Soils has

documented [13] that building foundation failures and distress account for average annual property damage in the USA

valued at $740 million. While this figure is not broken down

into specific categories of failure, it is nonetheless indicative that present codes and standards may not provide adequate

requirements for foundation design. Feld [12] has documented

a number of instances in which foundation failure has produced

severe building distress and even total collapse. It is

apparent that foundation failures , including the following

specific cases, can pose severe abnormal loadings:

Unforeseen Settlement

Foundation Wall Failure

16
Scouring Action of Floods on Foundations

Adjacent Excavation

An important factor affecting the probability of

foundation failure is the growing scarcity of land in urban

areas which is causing more and more buildings to be located

on sites previously considered to be of marginal quality for

construction purposes.

5. Studies of Abnormal Loadings

The NBS studies to date have revealed that statistics

have been or are being compiled by appropriate authorities

with respect to sabotage bombings, gas explosions, explosions

of hazardous materials in transit, highway vehicle accidents

and aircraft accidents. Efforts to locate data regarding

other abnormal loadings will continue; however, it is

recognized that, for certain incidents, statistics either

may not be available or they may be collected in such a

fragmented manner as to make them of little value.

A. Sabotage Bombings

Sabotage bombings are generally classified as explosive

or incendiary, but it is the former that are of particular

relevance to progressive collapse,

A.l Characteristics of Loading

There is no shortage of published technical information

relating to the pressures, rise time, and distribution

relationships for explosive charges. Organizations such as

the U.S. Department of the Army have developed and distributed

materials that serves as a guide in the use of explosives in

the destruction of military obstacles and in certain construction projects. The Army Field Manual [14] provides information

17
r = 0-2m

r = 0 5m

r=1 Om

Igi^g-a-mixture

r= 2 Om

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Time - ms

Figure 5 Pressure-time curves at selected distances (r)

from a 2.2 lb (1 kg) charge of TNT. The

ordinates give the pressures in atmospheres

above atmospheric pressure. The horizontal

dashed lines correspond to a total vacuum,

g

= gas, a = air. (Due to Granstrom, reference

1.5, reproduced from [70]).

18
including type, characteristics, and uses of explosives and

auxiliary equipment^ preparation^ placement and firing of

charges, and charge calculation formulas.

Figure 5 due to Granstrom [15] expresses pressure-time

curves at selected distances from a 1 kg (2.2 lb) charge of

TNT. The ordinates express the static peak pressure in

atmospheres. As an illustration, at a distance of 1 meter

(39.4 in) from the detonating charge the peak static pressure

would be approximately 11 atmospheres corresponding to

approximately 160 psi. This positive pressure pulse would

have a duration significantly less than 1 millisecond, A

pressure of 160 psi is so large in comparison with the normal

resistance of walls and floors as to make their destruction

a certainty. At a distance of 10 meters (32.8 ft) from the

charge the peak static pressure would be approximately 1.5

psi while the positive pressure pulse would last approximately

4 milliseconds. Pulses of these durations are so short, in

comparison to the natural period of building elements such as

walls and floors (20-40 milliseconds) as to require any

structural analysis of the element in question to be a

dynamic one.

The characteristics of explosive charges differ considerably from those of flammable gases. According to

Rasbash [16]

"As a rule, gas and vapor explosions take place

substantially more slowly than explosions involving

high explosives such as TNT. The most explosive

3 mixture of a fuel vapor and air in a volume of 30m

(1050 ft^) will contain about 2.5 kg (5.5 lb) of

fuel. The energy potential of this will be

equivalent to that of 20 kg (44 lb) of TNT, but

the pressure pulse with the gas explosion would

last several hundred milliseconds and with TNT only

about 1 millisecond."

19
For comparison, Figure 6 is included to show the pressuretime curves for typical vented gas explosions.

A. 2 Probability of Explosive Bombings

Two organizations have gathered nationwide statistics

for sabotage bombing in the USA, namely the Federal Bureau

of Investigation and the International Association of Chiefs

of Police (lACP)

The Federal Bureau of Investigation (FBI) commenced its

program to collect and classify bombing incidents at the

start of 1972. The Bureau issues monthly bulletins [17]

summarizing the data reported by its Field Agency; it is

understood that the first annual report, containing

statistics for 1972, will be issued in the spring of 1973.

Figures for the 10 month period, January through October

1972, are given in Table 5.1. The term "actual" is used to

denote that detonation of an explosive or ignition of an

incendiary material actually occurred, whereas "attempted"

denotes that detonation or ignition of the bomb did not take

place

Because reports continue to trickle in long after the

reporting period has passed, the FBI cautions the user of the

data that the figures are subject to revision (in an overall

upward sense) . The figures for the most recent months are

likely to change most. The monthly totals should therefore

not be used in an attempt to define trends.

Referring firstly to Lines 1 through 5 of Table 5.1, it

is seen that, during the 10-month period, there was a total

of 608 actual explosive bombings out of a total of 1689

incidents of all categories. The ratio of actual explosive

bombings to actual and attempted explosive and incendiary

incidents is 608/1689, namely 0.36.

The FBI uses a number of categories with which to

describe the target. For brevity, only four main categories

20
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/ ^ /A

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100 200 300 400

o

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a

sou /

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ve

atn / /a

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Figure 6 Pressure-time curves for typical vented gas

explosions. = pressure at which vents fail.

Broken line A denotes the subsequent pressure

rise in the absence of vents. Upper curve shows

the effect of smaller vents. Time scale is only

approximate. (Due to Mainstone, reference 70).

21
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22
are given; those having the most bearing on the subject in

hand. Lines 6 through 9 show figures respectively for the

main categories: commercial buildings, office buildings,

autom.obiles and residences . It is important to note that the

listed category of target is that in which the incident took

place. Thus, if an office unit within a residential building

were the target of an incident, the occurrence would be listed

under category "Office Building" not "Residences." If the

unit had been a store rather than an office, the entry would

be under "Commercial Building." Similarly, if an automobile

parked in a basement garage to the residential building was

the object of an attack, the incident would be listed under

"Automobiles." This suggests that inasmuch as they could

constitute abnormal loadings for the building as a whole,

certain incidents listed on Lines 6 through 8 might contribute

to the figures defining the probability of explosive loading o: residences i.e., line 9 might constitute a lower bound to the

frequency of abnormal loadings on residences. At this time

however, the readily available FBI data do not permit these

incidents to be identified, and for this reason, only the

figures on line 9 will be used further.

The FBI uses the term "Residences" to cover private

residences, apartment houses and other private property such

as sheds, and garages adjoining the residential building or

in its vicinity. The figures for other private property are

not included in the table; however the 10-month total for

this subcategory is 26

.

At the present time, the FBI makes no attempt to report

the num±)er of stories in the building that was subjected to th

attack, or the severity of the attack in terms of structural

damage to the building. Such information would add greatly to

the value of the statistics, at least as far as building safet

is concerned. It would also be valuable to have a clear

definition of the difference between a private residence and
an apartment house with respect to the FBI's classification

of incidents.

Probably the most useful figure to use, in order to

assess the frequency of abnormal loadings due to sabotage

bombing^ is the 10-month total for residences, 471. To arrive

at an estimate of the number that were actual explosive

incidents this number is multiplied by 0.3 6 giving 170 « If

this is converted to a yearly estimate, the result is 204.

It appears therefore, that at the present time, 566 sabotage

bombing incidents per year occur where the direct target is

a housing unit. Of these, an estimated 204 can be expected

to involve explosives with detonation taking place. For

reasons discussed earlier, this figure may be a lower bound;

it is also conceivable that not all incidents were recorded

by the FBI during this initial 10-month period of their data

collection.

Corroborating evidence to support the FBI data is

provided by the ICAP [18] and, on a State basis, by the

California Department of Justice [19] , In the period July

1971 through February 1972, the lACP operated the National

Bomb Data Center under the auspices of the U.S. Department

of Justice. Since that time, the functions of the Center

have been transferred to the FBI . The monthly summary reports

of the Center, during the period July 1971 through February

1972, provide a brief description of each incident in

addition to a statistical treatment of all bombings for the

month in question. One description, taken from the January

1972 report [20] , serves to illustrate the possible scale of

the risk:

"January 5, Las Vegas, Nevada. An explosive

device, consisting of 40 sticks of dynamite, was

placed in the laundry room of an apartment building.

The blasting cap detonated, but improper assembly

of the device, and the age of the dynamite combined

24
to produce only partial detonation. No damage

resulted.

Had the explosion occurred and had the building been of a design similar to that used at Ronan Point, the United States

may have had its first "Ronan Point" incident, with comparable

political and professional repercussions.

B . Gas Explosions

This subsection deals with loadings arising from

explosions occurring accidentally as a result of the distribution and use of gas. Because of Ronan Point, the dynamics

of gas explosions and their interaction with building construction of various forms have received considerable study

in the United Kingdom. The greater part of U.K. gas is still

manufactured, yet a rapidly increasing percentage is of

natural origin from North Sea sources. In contrast to the

U.K., the U.S. production of manufactured gas amounts to only

a few percent of the total, natural gas providing almost all

the needs

.

B.l Time-Magnitude-Distribution of Load

Rasbash documented [16] the maximum fundamental burning

velocities of some gas-air mixtures under atmospheric

conditions. Table 5.2 shows manufactured gas to burn

approximately three times as rapidly as natural gas. Such

data has caused certain members of the engineering profession

to make some distinction between the two types of gas when

considering the risk of progressive collapse.

Alexander and Hambly [21] discussed various means of

reducing the consequences of gas explosions in buildings;

such as, complete removal of the explosive source, forced

ventilation, and control of the maximum flow of gas from the

supply. They provided a qualitative description of the nature

of a gas explosion in terms of gas/air mixture, presence of

25
26
venting, pressure rise, turbulence, and other related

paramenters . They also provided a method of analysis of

structures subjected to these dynamic loads.

In another paper, Alexander and Hamley [22] developed

a method for design of structures to withstand dynamic loading

from gaseous explosions similar to that which caused the

collapse at Ronan Point. The loading pressure pulse was

described only qualitatively; a precise description could

not be given at that time due to the absence of relevant

experimental data. The response of the structure to such

dynamic loads was discussed and a method of design presented^

with examples for floor, slabs, and load-bearing walls.

Proposals were given for research that should be carried out

to determine the design pressure pulse and to check the

validity of the assumptions made.

Stretch [23] examined how explosions, caused by vaporphase reactions between common inflammable solvents or sources

of energy and air are controlled by particular features of

domestic buildings, and consequently strain or damage the

structure during their passage. He concluded that the

general use of materials such as gas is a safe and convenient

element in contemporary society, so that, today, homes are

considered fit for human occupancy only if they are designed

to withstand and contain, within tolerable limits, the risks

incumbent on a high standard of living. He showed how

inherent features of established structural systems have

obscured the necessity of special precautions in more recent

systems. Stretch gave simplified forms of the pressure waves

and used these to analyze the behavior of buildings of traditional brick design, framed construction, and concrete panel

construction, respectively.

Rasbash [16] , in a paper accompanying that presented by

Stretch [23] , discussed the influence of potential relief of

explosion pressures provided by external windows and doors

27
during gas and vapor explosions. He provided a quantitative

approach for estimating these pressures.

To define experimentally those data needed in the

mathematical modeling of gas explosions in buildings, Rasbash,

Ralmer, Rogowski and Ames [24] carried out experiments in

which they exploded mixtures of air and manufactured gas or

natural gas, respectively. These explosions took place in a

strong chamber with partitions simulating the division of a

building into rooms

Further experiments were carried out by Astbury, West,

and Hodgkinson [25] to investigate the effect of different

gas layering conditions in a pair of rooms. During the tests,

different layers of both gas and a gas/air mixture were used.

In two of these experiments, the most explosive (stoichiometric) mixtures of manufactured gas and air were obtained

and the resulting explosion caused, in one case, minor, and

in the other case, major damage to the 3 1/2-story building

which was of load-bearing brick. These experiments were a

repeat of earlier tests in which an attempt was made to

demonstrate the effects of turbulence. Turbulence has the

effect of increasing the pressures developed when an explosion

proceeds from one room to another filled with gas namely, the

"cascade" effect. The test demonstrated the effectiveness of

venting in limiting the maximum pressure developed in an

explosion. Despite suffering the extensive damage, the brick

building could be safely propped and no progressive collapse

occurred.

West [26] carried out tests involving the effect of gas

explosions on windows of various details in order to study

their effectiveness in providing venting. Specimens included

single-glazed windows of 32 oz glass and double-glazed units

of the same thickness of glass. Because of their higher

strength, the double-glazed windows provided ineffective

venting. Further, the resistance of glass to short-term

28
loads (defined as lasting 3 seconds) is more than twice that

under sustained loadings. Repeated explosions that do not

break the glass may be the cause of eventual failure at a

lower pressure. The strength of glass decreases with time.

For example, glass 18 months old failed at loads some 20

percent less than those obtained with newly-manufactured

glass. Finally, while the failure of glass may give an

indication of the pressure developed in a real incident, care

is necessary in interpreting test results since a distortion

of the frame can cause the glass to break at a pressure less

than its actual breaking strength.

Mainstone [27] reviewed the existing experimental data

on the strength of glass under loads of very short duration.

He provided the basis for a graphic presentation of likely

breaking pressures, under gas-explosion loading, for particular

sizes and thicknesses of window panes. An earlier review by

Rasbash of data on the venting of gas explosions was then

used as a basis for extending the graphic presentation to

cover also the possible rise in pressure after the glass is

broken by an explosion. The graphic presentations can be

used directly for estimating the pressure reached in actual

explosions from observations on the damage to glass windows;

and they were prepared primarily to be used in the design of

glazing as an explosion vent.

On the basis of a review of the above studies, it is

concluded that there is sufficient data available to allow

satisfactory prediction of the characteristics of gas

explosions in buildings, providing the gas mixtures can be

defined.

B.2 Probability and Consequence of Load

The probability of gas explosions has been studied both

in the United States and the United Kingdom. Whereas the

Ronan Point incident did not occur unit 1968, the American

29
Gas Association, which represents approximately 85 percent of

the U.S. gas industry, had compiled statistics of gas incidents

some years earlier. Before reviewing the results of the AGA

studies, it will be useful to consider the studies in the U.K.

to gain perspective.

In the Report of the Inquiry into the collapse at Ronan

Point, Griffiths, Pugsley and Saunders [1] assembled the data

available at that time dealing with the probability of gas

explosions in the U.K. Table 5.3 is taken from the report of

Griffiths et al and contains an analysis of explosions in

housing for each of the years 1957 through 1966.

Structural damage is defined as damage to the structure

over and above the mere blowing out of windows and window

frames. It will be seen from Tables 5.3 and 5.4 that, of the

known causes of explosions, manufactured gas is the principal

hazard. In the year 1966, there were approximately 18 million

housing units (apartments and houses) in the United Kingdom

and, of these, approximately 12,260,000 were supplied with

manufactured gas. The 1966 figures show that the frequency

of explosions involving manufactured gas in premises supplied

with gas is approximately 8 per million dwellings, of which

3.5 per million will be of sufficient violence to cause

structural damage. Griffiths et al assessed the chance of a

gas explosion in a high-rise apartment building. In a

building the size of Ronan Point, with 110 apartments and a

life of 60 years, there is slightly more than a 2 percent

risk that a gas explosion causing structural damage will

occur in one of the apartments during the lifetime of the

building, i.e., 3.5 x 10~^ x 110 x 60 x 100 = 2.31 percent.

In other words, the chances are that of every 50 such

buildings, one will experience structural damage as a result

of a gas explosion in its lifetime. They pointed out that,

whereas it may be argued that it is cheaper to prohibit the

use of gas in tall apartment buildings than to make the

30
Table 5.3

Frequencies of Explosions in Housing Units Estimated from Samples

of Fire Department Reports in the United Kingdom - Damage and

Explosive Material Reproduced from [Ij

Year

1966

65

64

63

62

61

60

59

58

57

Toial

Sampling

factor

I/l

1/1

1/2

1/6

1/2

1/2

1/4

1/4

1/4

l/I

Taul

explosive

213

181

168

216

234

198

144

148

192

195

1,889

Manufactured Gas j Liquefied Petroleum

1 Gases

Liquids

Total

97

76

80

84

70

46

72

88

64

70

747

Superficial

j StrKClurol

I 55

40

28

54

20

28

36

24

28

41

354

42

36

52

30

50

18

36

64

36

29

3<33

Total

14

14

8

6

8

10

16

12

96

Superficial Structural i Total

25

6

6

8

6

6

6

16

12

5

71

33

29

18

18

34

38

12

24

48

44

298

Superficial

25

20

16

12

28

30

8

20

' 32

35

226

Slruct'irat

8

9

2

6

6

8

4

4

16

9

72

Oilier and unkmmi

69

62

62

lOH

122

104

44

36

f.8

73

748

31
Table 5.4

Explosions in Domestic Premises

Reproduced from

Total

Explosions Explosive Material Damage Fault

Z i J

yianuf actured gas 97

Superficial 55

Installation 35

User 20

Unknown 0

Structural 42

Installation Zo User 9 Unknown 7

L. P.G. 14

(Liquefied petroleum

gases)

Superficial 8

Installation 3 User 4

Unknown 1

Structural 6 Installation 5 User 0

Unknown 1

Liquids 33

Superficial 25

Installation 6 User 17 Unknown 2

Structural 8

Installation 0

User 7 Unknown 1 Dther and

[Jnknown

in the United Kingdom

[1]

32
structures free from the risk of progressive collapse, they

did not accept this argument. They reasoned that gas is

justifiably regarded as a safe and acceptable fuel in domestic

premises generally [28] . Furthermore, the banning of gas

would not completely eliminate the risk of damage to che

structure of a tall apartment building, resulting in progressive collapse, although admittedly it V70uld remove the

m.ost likely cause. There would remain the possibility of

explosions caused by substances other than manufactured gas,

for example, gasoline or other volatile inflammable liquids,

butane gas cylinders, electrical apparatus, and so on; as

well as other foznns of accidental damage.

Prompted by the Ronan Point incident, the U.K.

Construction Industry Research and Information Association

(CIRIA) established a pilot survey [29] to establish the

procedures for future and wider surveys on the frequency of

gas explosions and the structural damage they can cause

c

Newspapers were used to obtain reports of explosions in

residences and, where appropriate, visits were made to

dam.aged properties and comprehensive investigation of the

circuiristances carried out. Findings showed tha-c gaseous

explosions in U.K. housing causing significant structural

damage occur at the rate of less than one per week.

Fry [3 0] examined U.K. fire incidents involving

explosions of manufactured gas in dwellings during the 13

years, 1957 to 1969. The average annual incidence was

shown to be approximately 9 0 but appeared to be increasing

as the consumption of gas increases. The average rate of

8 ojicidents is about 5.0 per 10 therms of gas sold. Approximately 4 8 percent of the incidents cause som.e structural

damage and in 4 0 percent of these it was considered "severe."

From reports involving manufactured gas and natural gas in

1S69, it appeared that natural gas was more likely to cause

explosions but that the explosions were of sim.ilar violence

33
for the two types of gas.

A later report [31] of the field survey of damage,

caused by gaseous explosions in the U.K. contained the

conclusion that roughly one severe or very severe explosion

occurs every two weeks. Furthermore, there is some evidence

that the frequency is increasing.

It is now useful to discuss the probability of gas

explosions within the Continental USA. The two principal

sources of information on the gas industry are the American

Gas Association and the Office of Pipeline Safety (OPS) of

the U.S. Department of Transportation. The following is an

attempt to evaluate information from each of these sources.

Within the Continental USA there are 915,000 miles of

gas pipeline [32]. This total is composed of 288,000 miles

of pipeline in gathering and transmission systems and

627,000 miles of pipeline involved in subsequent distribution

of gas [33] . Since the former are located largely in rural

areas, it is with gas distribution systems that this study is

primarily concerned. It is significant that, within the gas

distribution system, gas leaks are reported to occur at an

annual rate of 1 per 1.1 mile of gas line; e.g., a total of

more than 560,000 leaks in 1972. More than 300,000 of these

leaks occurred as a consequence of normal wear and tear; i.e.,

corrosion, fatigue, material failure, etc. Leaks are reported

[33] to occur with comparable frequency in both the mains and

service lines and about two-thirds of all leaks were associated

with the pipe itself, the remainder being associated with the

fittings and other attachments.

Including both single and multiple dwelling units,

almost 34.7 x 10 housing units in the USA used gas for house

heating in 1970 [32] . It is also estimated [5] that natural

gas serves about 55 percent of all housing units as a fuel

for residential space heating, and it is significant that,

of all new customers for house heating in 1970, 37 percent

34
were conversions to a gas service system. Because a single

customer or meter or furnace may involve more than one

dwelling unit, these figures may not be fully representative

of the total number of dwelling units to which gas is supplied.

The AGA estimates that, including appliance usage, natural gas

is supplied to upwards of 60 percent of the total residential

market

o

A prime source of information on gas leak incidents is

the summary to the research report "Public Safety and Gas

Distribution" [34] prepared for the American Gas Association

by Arthur D. Little, Inc., and dated December 1967. This

was a survey of the gas distribution industry in which 140

companies and systems participated, representing 83.3 percent

of all gas distribution meters. The yearly averages for

gas-related incidents are based on the 10-year period 1957-66.

As far as incidents involving payment of compensation are

concerned, the yearly averages are representative of the

7-year period 1957-1963. Both in terms of the time periods

and the number of companies involved, this survey constitutes

the most comprehensive, if not the only, study of the overall

safety of the gas distribution system within the continental

U.S.

With reference to Table 5.5, assembled from information

contained in the A. D. Little, Inc., summary, it is evident

that If 508 gas explosions can be expected in an average year

and, of these, 329 will require monetary compensation. A

total of 151 (60 percent of 253) explosions will involve

payments, of more than $1,000. It is recognized that only a

portion of these explosions could have been severe enough to

cause structural damage, but unfortunately, there is no record

of:

1 . the relative proportion of personal and property

damage

,

2. whether or not one or more buildings were involved

35
Table 5.5

Gas Related Incidents. Their Nature, Annual Incidence and Consequence

CTaJcen from A. D. Little Summary Report to American Gas Association [34]

10 YEAR AVERAGE 7 YEAR AVERAGE - 1957-63 INCLUSIVE

INCIDENT

REPORTED

INCIDENTS

INVOLVING

PAYMENT*

SIZE

- $1,000

1

OF ^AYMI

1 < $1,000

>$ 25.

-IN J.

^ $2 5.

NUMBER % NLfMBER %^

' % % %

EXPLOSION 1508 12.8 329 13.3 60 10 4

FIRE 1835 15.6 274 11.1 36 9 7

PRODUCTS OF COMBUSTION 1228 10.4 109 4.4 7 5 3

UNIGNITED GAS 3118 26.5 1498 60.7 12 66 69

FLASHBACK 4122 35.1 217 8.8 6 9 ——

11

OTHER 344 2.9 167 6.8 3 7

—!

s

103.3'*' 105.1'^

4- 124

" " r f 106^

1 102^

j TOTAL NUMBER

OF INCIDENTS 11753 2466

253

X65.1% / 24. 6:^

Total Payments by Gas Company exclusive of Company repair costs.

Some incidents may involve more than one of the listed phenomena.
and, if so, the type of building, and

3 . the relative severity of each of the incidents

involving payment greater than $1,000.

It may be presumed that of the explosions incurring

payment of less than $1,000, very few, if any, were likely

to have involved significant structural damage. In order to

evaluate a probability based upon the AGA statistics, it will

be assumed that only one building was involved in each of the

151 incidents and that each suffered significant structural

damage. This assumption can be seriously questioned as the

following illustration from an OPS report [3 5] would indicate

but, in the absence of more specific data, it at least provides

the basis of a conservative estimate:

. "Mobile Oil Corporation High Pressure Natural

Gas Pipeline

Houston, Texas, September 9, 1969 - Synopsis

At 3:40 p.m. on September 9, 1969, the 14-in pipe- • line carrying natural gas at a pressure of more than

780 psig ruptured in a newly constructed residential

subdivision 3 1/4 miles north of Houston, Texas. The

escaping gas created a dust storm like condition and

sounded like a jet engine. Electrical and telephone

utility servicemen working in the area, with the

help of local residents, immediately commenced to

evacuate all residents in the vicinity of the

rupture. About 8 or 10 minutes later, the escaping

gas exploded violently. Thirteen houses, ranging

from twenty-four ft to 250 ft from the rupture were

destroyed by the blast. The leaking gas caught fire

and continued to burn to a height of 12 5 ft for

1-1/2 hours until valves on the other side of the

37
leak were closed by Mobile workmen dispatched to

the valve locations. The fire abated at that

time, but some gas burned for another five hours.

In all, 106 houses were damaged and property

damage was estimated at $500,000. Miraculously,

there were no deaths but nine people were injured,

two seriously."

Office of Pipeline Safety figures for 1971 [35] indicate

that, of the total number of reported explosions, 50 percent

occurred in residential buildings and 7 percent in commercial

buildings. The remainder occurred in manholes, regulator

pits, etc., and it is unlikely that these involved compensation in excess of $1,000 since, in most instances, these

incidents occurred on or within gas company property and it is

unlikely that they involved the gas company in claims for

payment. Accordingly, it is presumed that 50/57, namely,

87.5 percent of the 151 incidents, namely 131, were likely to

have involved residential buildings.

The period over which the AGA statistics were gathered

centers approximately on 1960. According to the 1960 Census

of Housing [36], the occupied housing in the U.S. totaled

58,314,7 84 units. Accordingly, a crude estimate of the

annual probability of occurrence of a structurally significant

gas-related explosion in a residential unit is of the order

of 131/58,314,784 x 100 = .00022 percent or 2.2 structurally

significant explosions per million dwelling units per year.

During the period 1957-1963, the average annual natural

g gas sales to residential customers totaled 300 x 10 therms.

In terms of energy, the annual rate of structurally signifig cant gas-related explosions is 0.44 for every 10 therms of

gas supplied. If all gas-related explosions and the total

amount of natural gas sales are considered, the probability

g of explosion is 1.67 explosions per 10 therms per year.

38
In accordance with the provisions of the National Gas

Pipeline Safety Act of 1968 [33] , a nationwide reporting

system for gas-related incidents was initiated in 1970 by

the Office of Pipeline Safety (OPS) . Detailed reports are

required of individual incidents that involved one or more of

the following criteria:

1. Caused a death or a personal injury requiring

hospitalization;

2. Required any segment of transmission pipeline to

be taken out of service;

3. Resulted in gas igniting;

4 . Caused estimated damage to the property of the

operator, or others, or both, of a total of

$5,000 or more;

5. Required immediate repair and other emergency

action such as evacuation of a building, blocking

off an area, rerouting of traffic to protect the

public; and

6. Were deemed significant but did not meet the

criteria of 2 , 3 or 4

.

All gas companies with more than 100,000 customers (over

85% of the total number of gas customers) are required to

submit reports within 3 0 days of the incident. Records for

11 months of 1970 and all of 1971 are publicly available

[33].

In an NBS Technical Note entitled "Residential Buildings

and Gas-Related Explosions" [37] the AGA and OPS statistics

are fully discussed. It is shown that at this point in time

the AGA and OPS statistics are in conflict and of the two,

the AGA statistics would appear to be the more representative

since:

1. They attempt to cover all incidents, both upstream

and downstream of the meter in distribution systems

39
(see Table 5.6),

2. They represent a 10-year average, whereas the first

complete year of OPS statistics is 1971.

In that report, it is concluded that, though due regard

has to be taken for the limitations inherent in the available

statistics, the probabilities of the occurrence:

1. of a gas explosion capable of causing significant

structural damage is 2.2 per million housing units

per year.

2. of a gas explosion capable of causing significant

structural damage in a hundred-unit apartment

building during a 50-year service life is 0.011 or

1,1 percent.

C . Explosions of Hazardous Materials

The National Bureau of Standards is now gathering

statistics of explosive incidents arising from the normal

transportation of hazardous materials in urban areas by road,

rail, and waterway. Such materials include petroleum and its

products, chemicals, explosives, and liquified gases.

Approximately 20 percent of all hazardous materials transported in the USA are moved on waterway [3 8] . Waterway

explosions of such a nature that buildings are damaged in

the course of the incident are rare, yet the potential for

such an incident is not only present but increasing, particularly as the tonnage of petroleum and petrochemicals shipped

by water increases. Another source of potentially abnormal

loading exists where fuel transportation terminals are

located. Such terminals provide a storage facility for large

volumes of petroleum which have been transported on waterways

Storage tanks are now located in many densely populated areas

including Providence, Boston, Staten Island, and Philadelphia

more are planned.

40
Table 5.6

w^J^""!

L?cation of the Basic Cause of the Various Gas -Related

10 YEAR AVERAGE \ 7 YEAR AVERAGE

LOCATION OF BASIC

CAUSE OF INCIDENT

TOTAL NUMBER OF

INCIDENTS

TOTAL NUMBER OF

INCIDENTS

INVOLVING PAYMENT

NUMBER % NUMBER %

BEFORE OR AT THE METER

I.E., UPSTREAM

3,122 26.5 1 , 780 ' 72.1

AFTER THE METER

I.E., DOWNSTREAM

8,296 70.6 639 25.9

OTHER 335 2.9 47 2.0

TOTAL NUMBER OF

INCIDENTS PER YEAR

11,753 (0 -i'/o 2,466

41
It would appear that the shipment of hazardous materials,

by road and rail, poses a problem that is not negligible.

Strehlow [39] contends that the explosion of unconfined vapor

clouds, produced by the dispersion of flammable liquid or

vapor spills, is becoming a serious problem. He points out

that this is mainly because of the increased size of the

spills in recent years. Illustrating this point is the

accident that occurred in East St. Louis, Missouri on

January 22, 1972. Since the National Transportation Safety

Board is currently investigating the incident, and the report

is not yet released, the following information was obtained

from a file of newspaper clippings maintained by the Safety

Board; it should not be considered official:

The accident occurred in an East St. Louis railroad

yard where a process known as "humping" was taking

place. In this process, a railroad car is allowed

to roll freely toward other coupled cars with

sufficient momentum to allow the couplers to fasten.

This is a process regularly used to make up trains.

Alledgedly, in this instance not 1 but 4 already

coupled cars were being humped. The lead car,

filled with 500,000 lb (30,000 gallon) of propylene,

a derivative of liquid petroleum gas (LPG), was

travelling at approximately 15 mph, instead of the

recommended 6-7 mph, when it bumped a hopper car at

the end of the partly-assembled train. The total

momentum of the impact was such that the hopper

car coupler jumped over the coupler of the lead

rail car and punctured its propylene tank. The

cars continued to move for several hundred feet.

The released propylene formed an unconfined vapor

cloud which exploded 500 ft. from the location of

the leaking car.

The explosion shook a 4 square mile area and

shattered windows 8 miles away; the concussion

was felt up to 20 miles from the scene of the

accident. The explosion caused roofs and walls

to collapse as much as 6 blocks away. At least

3 separate fires followed the explosion and

involved 30 additional railroad cars. Apparently

none of these additional cars contained hazardous

materials, for no further explosions were reported.

42
Early reports estimated that 1,000 buildings had

been damaged including 650 homes and 350 business

buildings. Later information indicated that 868

buildings had been reported as damaged. One

hundred families were left homeless. In all 176

people were injured and total non-railroad property

damage was estimated to be 6.5 million dollars.

It is estimated by the National Transportation Safety

Board that the report will be completed by January 1973.

Strehlow cites a 1962 accident in the State of New York that

involved a truck:

July 26, 1962, New Berlin, New York [40]. 7,000

gallon tank truck in a single truck accident.

Tank ruptured catastrophi cal ly in town. Vapor

cloud covered 200,000 square feet and was 80 ft

deep before ignition. Explosion and following

fire killed 10, caused $200,000 damage.

Strehlow states that the characteristics of the initial

fire or explosion, which follows the ignition of a spill,

depend on four things:

1. The nature of the fuel.

2. The rapidity of the spill coupled with the wind

conditions, terrain and/or location of nearby

buildings.

3. The delay before an ignition source is found.

4. The nature of the ignition source.

He concludes that current theoretical results are unable to

predict the observed pressure-time behaviors.

Strehlow [3 9] has tabulated 108 accidental unconfined

vapor cloud explosions that have been documented over the

past 42 years. The list is not complete because it is

limited almost entirely to explosions that have occurred in

the USA and Germany, because the documentation of individual

explosions has often been fragmentary, and because information

43
about many on-site plant explosions are not accessible to

the general public. Using Strehlow's data, Figure 7 has been

plotted showing the average number of explosions per year

over periods of 2, 5 or 10 years. It is seen that the number

per year is increasing rapidly. In approximately two years,

1970 through January 1972, a total of 15 incidents occurred,

causing damage estimated at 23-27 million dollars. It

appears from Strehlow's paper that, with one exception, these

occurred in the United States.

How many housing units were subjected to severe blast

loading is not known with any certainty, but it is clear from

the incident at East St. Louis, January 22, 1972, described

earlier, that the number would be significant. The reports

of the incident, which has been previously discussed, estimated that 650 homes were damaged and 100 families were left

homeless. It would appear that a lower bound to the number

of housing units subjected to abnormal loading due to this

explosion was 100. The non-railroad loss in this incident

was 6.5 million dollars, out of an estimated total of 23-27

million dollars for the last two year period shown in Figure

7. If it is assumed that the number of housing units

affected in two years can be arrived at by prorating the

figures for St. Louis then this number would be approximately

23 100 X -g—^ = 354 housing units

On this basis, albeit a crude one, it could be expected

that 354/2, namely 177 incidents occur per year unless some

change in the incidence of accidental unconfined vapor clouds

were to take place.

C.l Highway Vehicle Impact

Sanders [41] has developed relationships to aid the

structural designer in estimating the probability of a

44
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45
structural component being struck by an errant motor vehicle.

An example of this type of incident would be a motor vehicle

straying from its normal path and hitting a building column.

The probability of failure of the column is derived from

combining the probabilities: (a) of a vehicle striking the

column and (b) of the vehicle having the mass, velocity, and

stiffness to cause failure of the column. The probability of

a motor vehicle striking an object located near a traffic

path is derived as a function of (a) the distance of the

object from the traffic path and (b) the volume of traffic

flow.

For the USA in 1970, the total number of motor vehicle

accidents of all types was approximately 16 million [71]

Highway accident statistics are compiled by each State and

the categories of information on the reporting forms frequently

total as many as 60 items. In spite of this, incidents in

which vehicles collide with buildings receive scant coverage.

Indeed, it is the exception for the report to identify the

object struck when a vehicle leaves the highway. The U.S.

Department of Transportation has assisted NBS in identifying

two states in which building strikes were considered in the

data collection. These States are Oklahoma and Illinois, the

former having relatively little urban area while the second

has large urban concentrations. In each case the data

analyzed were those of 1970.

Throughout the year 1970, there were 65,183 motor

vehicle accidents in Oklahoma. In 50 of these, a vehicle

was reported to have collided with a building. In Illinois

in 1970, there were 4 09,174 motor vehicle accidents and, of

these, the number in which a vehicle was reported to have

collided with a building is 1229.

If the following assumptions are made: 1) the Oklahoma

data, added to the Illinois data, is representative of the

USA as a whole; 2) the number of vehicle collisions with

46
buildings (C) is propertional to the population (P) , then,

based on 1970 data:

'us

'US

c + c

ILL OKLA

P + P

ILL OKLA

1279

13,674,000 X 203,212,000

'US 19,000 (approximately)

This estimate is for the number of vehicle collisions

with buildings for the USA as a whole in 1970, based upon

combined Oklahoma and Illinois data.

It is interesting to differentiate between urban and

rural situations, where urban is used to denote an incorporated area having a population of 2500 or more.

URBAN:

'US

1171

10,970,000 X 149,325,000

= 16,000 (approximately)

RURAL:

'US

108

2,704,000 X 53,887,000

Cyg = 3,000 (approximately)

On this basis, it is estimated that, nationally, the

1970 figure for vehicle building collisions is 16,000 in

urban areas and 3,000 in rural areas.

So far the discussion has dealt with reported collisions

of vehicles with buildings, with no consideration for what is

meant by 1) vehicle collision, and 2) building. Vehicles vary

widely in mass, and collisions can range from a gentle nudge

to an impact capable of removing a structural element. The

47
term building covers a range of structures from barns to

multistory apartment houses. As far as this study is

concerned, all standard reporting forms are unsatisfactory

in these respects. Efforts to break out the particularlyrelevant data are continuing; however, it seems unlikely

that the probability of abnormal loading of housing units

due to highway vehicle impact will be defined with any

clarity unless: 1) existing data recording procedures used

by the States are slightly modified, or 2) a separate detailed

study is made (this could be based on statistical sampling

and involve relatively small areas of the country)

However, with due regard to the uncertainties inherent

in the data available, a lower bound estimate can be made of

the number of abnormal loads per year on residential structures subjected to highway vehicle impact. From a study of

what detailed information is available, it is assumed that

at the national level at least one tenth of the incidents

will involve residences and, of these, one tenth will involve

sufficiently large to be considered an abnormal loading.

On this basis a conservative estimate of the number of

abnormal loadings on residential buildings per year due to

highway vehicle impact is believed to be 190 or, on the

average, four per State.

C.2 Aircraft Collisions With Buildings

In comparison with other forms of abnormal loading the

probability of such incidents is trivial. Using data furnished

by the U.S. Federal Aviation Authority, the NBS study was shown

that for buildings located further than 3 miles from the end

of an airport runway the probability of collision with build-

— 8 ings per year is approximately 10 per building.

48
6. Summary of the Probabilities of Abnormal Loadings

And Their Implications for the USA

Chapter 5 discussed the available statistics from which

preliminary estimates have been made of the annual incidence

of five sources of abnormal loading on residential buildings.

Chapter 4 reveals several abnormal loadings for which data

has not been presented. Efforts to quantify the probability

of occurrence of these other loadings will continue; however,

it should be recognized that for several reasons this

quantification may never be satisfactorily accomplished.

In arriving at estimates in Chapter 5, it has been the

author's intention to err on the low side in cases of doubt

about the available data. For this reason, and because not

all sources of loading are included, it is believed that the

following summation provides a lower bound result.

Number of Abnormal

Source Loadings Per Year

Explosive Bombing 204

Gas Explosion 131

Explosion of Hazardous

Materials 177

Highway Vehicle Impact 190

Summation 702

Chapter 4 discussed the national risk due to fire as a

basis for assessing the incidence of abnormal loadings in

housing units. It was shown that architectural layout and

building size affected the number of abnormal loadings on

housing units per year that, in buildings susceptible to

progressive collapse, would pose the same risk as fire.

For a 100-unit building, with 4 apartments per story, an

49
average of 4 persons per apartment, and a central service

core, this number was 433, while for a 50-unit building of

similar arrangement the corresponding number would be 866.

Comparing these figures with the lower bound estimate

of 702 abnormal loadings a year, it appears that, for buildings

susceptible to progressive collapse, abnormal loadings do pose

a risk comparable to that of fire. In the case of the fire

risk, specific design criteria are currently implemented.

The United Kingdom Authorities introduced criteria to

minimize the risk of progressive collapse upon concluding

that the frequency of explosions, involving gas in housing

units supplied with gas, was approximately 8 per million

housing units per year, of which, 3.5 million were of

sufficient violence to cause structural damage. It should

be noted that this figure relates to only one source of

abnormal loading.

In the USA, the frequency of abnormal loadings on housing

g units per year appears to be in excess of 702/(67.7 x 10 )

approximately 10 per million. These data appear to provide

adequate justification and need for standards-writing bodies

to adopt criteria to deal explicitly with abnormal loadings

and progressive collapse.

7. Alternative Approaches for Criteria

Each of the following approaches, or some combination,

may be used in the preparation of criteria to reduce the

level of risk from abnormal loadings:

Eliminate Source

Reduce Magnitude

Limit Extent of Structural Damage

Resist Local Structural Damage

50
The banning of gas from buildings would illustrate an

attempt to partially implement the first of these approaches.

The second is illustrated by regulations that might call for

buildings to be sited at distances from a highway and with

such barriers that impact from vehicles become unlikely. The

provision of alternate paths for loads in the event of the

loss of a critical member illustrates the third approach.

The fourth approach is the basis of the U.K. criterion, in

which the element is designed to resist 5 psi pressure in any

direction, applied to the element and to those elements

attached to it

.

For the most part, the first two approaches are nonstructural.

A. Non-Structural Design Criteria

It could also be feasible to consider introducing

criteria in non-structural areas including the following:

Zoning, Siting, Planning

Regulations for Service Systems

Transportation Regulations

A.l Zoning, Siting, and Planning Criteria

The following are examples of areas in which criteria

could be considered for development or modification:

1. The location of gas mains in urban areas.

2. The location of multistory buildings with respect

to highways, railroads, and waterways.

3 . The location of multistory buildings with respect

to similar buildings.

4. The zoning of land for residential use, taking into

account the possibility of local external explosions.

A. 2 Regulations for Service Systems

The following are features of service systems that might

51
be studied with a view to reducing risk:

1. The location of high pressure gas riser mains in

multistory buildings.

2. The location of boiler rooms and furnaces and HVAC

plant in general.

3. Ventilation provisions for gas mains, boilers, and

furnaces.

4. Specifications for pipe work and fittings.

5 . Maintenance and inspection procedures for plumbing

installations.

As an illustration, French regulations [4 2] prohibit the

use of gas in buildings higher than 50 meters (164 feet) , and

this law was enforced before the Ronan Point collapse.

Furthermore, when gas is installed in buildings, French codes

have requirements for ventilation that are more stringent than

most codes; e.g., the gas supply pipes are enclosed in a

ventilated duct space.

A. 3 Transportation Regulations

Considerable volumes of hazardous materials are transported through urban areas in the U.S. by means of truck,

railcar, and waterway traffic. Minimization of the risk

incurred might come about from new or improved regulations

following a study of:

1. The type and volume of hazardous materials

transported as one cargo.

2. The engineering specifications used in the regulation

of the design and maintenance of vehicles used in

this transportation, the standard procedures for the

operation of these vehicles, and the procedures for

the supervision of operatives.

3 . The routes over which this transportation is

permitted to take place and, in particular, the

52
proximity to urban areas.

4. The statistics for explosions of hazardous cargoes

in shipment.

8. Philosophies for Structural Criteria

Criteria to minimize progressive collapse are being

implemented in several countries including the Nordic

Countries, Countries of Eastern Europe, France, Italy, United

Kingdom, the United States and Canada. A detailed review of

the contents of these various criteria is beyond the scope of

this paper. It is sufficient and more appropriate to identify

the various philosophies that have been used in the preparation of these criteria

j

1. The Cautionary Note ; The attention of the structural

engineer is drawn to the risk of progressive collapse

and he is urged to take precautionary measures to deal

with it, as in the National Building Code of Canada,

1970 Edition [43] which states that: "Buildings and

structural systems shall provide such structural

integrity that the hazards associated with progressive

collapse, due to local failure caused by severe overloads or abnormal loads, not specifically covered in the

section, are reduced to a level commensurate with good

engineering practice." The 1972 American National

Standards Institute A58, Minimum Design Loads in

Buildings and Other Structures [3] , also gives a

cautionary note as follows : " Progressive Collapse - Buildings and structural systems shall provide such

structural integrity that the hazards as soc lated with

progressive collapse such as that due to local failure

caused by severe overloads or abnormal loads not

53
specifically covered herein are reduced to a level

consistent with good engineering practice."

The Alternate Path Approach ; Criteria of this form call

upon the designer to consider successively the removal

of structural elements or combinations thereof from the

building in a systematic manner, and insure thereby

through analysis and design, that the building remains

capable of withstanding a specified combination of loads

albeit with a small overload factor.

Specified Abnormal Loadings ; In this approach, an

equivalent static loading consisting of a uniform

pressure is assumed to envelope the loading states that

might be produced by the various abnormal loadings on

buildings . A structural element or combination of such

elements , that may not be removed in accordance with the

previous approach, is required to withstand the application of this pressure to its surface and those surfaces

of elements attached to it (subject to the strength of

their connections) . An example of this approach is

provided by the United Kingdom Regulations [44]

Specifications for Reinforcement and Reinforced

Connections ; This approach removes from the structural

designer the responsibility for considering successively

the application of abnormal loadings to various portions

of the structure. It does so by specifying reinforcement

for elements and for their connection in such a way as to

insure an adequate strength and ductility in the event of

abnormal loadings. An example of this is provided by the

CEB Regulations [45] used in France and in the soon-to-be

ratified Uniform Code in the United Kingdom [46]
5. Deemed to Satisfy Clauses ; Finally, there have emerged

a number of documents in which it is stated that design

in accordance with certain national standards is deemed

to satisfy the intent of other standards dealing

explicitly with the risk of progressive collapse.

Examples of this include BS 44 9, Design for Steel

Structures, in the United Kingdom [47].

In the United States, there have been two cases where

criteria have been developed explicitly to deal with progressive collapse, albeit applied in a limited manner. One

of these involved the preparation of the Guide Criteria [5]

for use in the evaluation of the housing systems demonstrated

as a part of the Operation BREAKTHROUGH program. These

criteria were prepared by the National Bureau of Standards

on behalf of the Department of Housing and Urban Development.

Expressed in performance language, they were an adaptation of

the Fifth Amendment to the U.K. Building Regulations 1970

[48] , and expressed both the alternate path approach and the

use (where absolutely necessary) of the specified abnormal

loading approach (5 psi) . These criteria were used in the

evaluation of multistory systems in the BREAKTHROUGH program.

The second set of criteria [6] was prepared by the

Federal Housing Administration of the Department of Housing

and Urban Development for use in the preparation of Structural

Engineering Bulletins for those industrialized systems being

designed and built with a FHA mortgage guarantees. Essentially, FHA based these criteria on the British Standard Code

of Practice 116, Addendum No. 1 for the Design of Large

Concrete Panels [44] . There is no shortage of literature discussing the

reaction of the building community to the introduction of

criteria against progressive collapse in high-rise buildings.

The reader is referred to Collins [49] , Short and Miles [50]

,

55
Rodin [51, 52], Creasy [53], Ferahian [54, 55], Lewicki [56,

57], and a collection of the discussions of structural

engineers at a special meeting on the subject [58, 59] . It

is a subject which has generated a great deal of interest

among structural engineers.

t 9, Building Response to Abnormal Loadings

This section will deal with the response of buildings

to abnormal loadings from three standpoints: cases of

progressive collapse, experimental studies, and analyses.

A. Cases of Progressive Collapse

A number of people including Griffiths [1] , Rodin [52]

and Ferahian [54] have provided considerable insight into

the behavior of the Ronan Point building. Slack [6 0] , has

discussed two other instances of explosions in reinforced

concrete buildings, albeit factory buildings. The first

case study is concerned with an explosion in a 4-story castin-place framed structure, while the second illustrates the

nature of blast damage within a single-story precast concrete

framed building. In the cast-in-place structure the progression of the explosion pressure wave is traced and the

resulting damage to the external cladding as well as fire

damage are described. The precast concrete frame building

suffered column damage due to the confinement of the primary

explosion. In both of the buildings, large areas of cladding,

or their connections, were weaker than the main structural

frame, resulting in limited overall damage to the buildings.

From the first case, it appears that the framed cast-in-place

structure had a greater inherent resistance to damage than

would be indicated by a straight-forward analysis taking

account of structural continuity. In the precast frame

56
structure, it was found that a precast frame can have a greater resistance to collapse than would seem apparent from

a basic reinforced concrete design analysis. However, it is

Slack's recommendation that full or partial continuity at the

cast-in-place joint be provided to give individual members

greater resistance to damage.

A short report [61] was given on the bombing of an

Army Officer's building in Aldershot near London in February

1972. The Irish Republican Army claimed to have carried out

the bombing using 280 pounds of gelignite. The building did

not collapse. The walls of one-half of the building were

blow away completely, but the structural frame of reinforced

concrete remained intact.

A gas explosion destroyed a row of 22 single-story

shops in Clarkston, Glasgow, Scotland. Below the shops,

were a series of basement voids into which gas seeped from

a nearby broken main, and concentrated until it was ignited.

The explosive pressure has been estimated to be at least

7.3 psi (and possibly as high as 14.6 psi) . The maximum

damage occurred about half way along the row. At the location

of the explosion, floor slabs were lifted and the collapse of

the front row of columns was attributed to the failure of 12

in by 18 in reinforced concrete beams at ground level.

On the night of March 6, 1972, an explosion occurred in

an 11-story apartment building in Barcelona, Spain, resulting

in the collapse of a portion of the building (See Figure 8)

and the deaths of 18 people and many injuries. It was concluded [8] that the collapse was the result of a conventional

explosive. Basically, the building is a load-bearing brick

structure. From evidence available, it would appear that the

explosion occurred in an apartment on the fourth floor.,

destruction of this unit then leading to the progressive

collapse of all stories above. Falling debris caused

destruction of the rooms directly below the apartment. The
Figure 8 Apartment building at Calle Santa Amelia,

Barcelona, Spain following a progressive collapse

on March 6, 1972.

58
opinions given in the report [8] are based upon evidence

available at the time of its preparation in July 1972, and

may not be the final word as to the cause. If it is assumed

that the apartments in question were occupied at the rate of

4 persons per apartment, then the death toll represents 18

out of 44 affected by the incident, 41 percent.

It was reported [62] that a car crashed into a fivestory tenament building in New York dislodging a column and

sending tenants and portions of the structure crashing into

the street.

A considerable discussion of cases of progressive

collapse in Canada and the U.S. in the years 1968 to 1972

has been given by Allen and Schriever [7]

.

B. Experimental Studies

It is appropriate to discuss these studies from the

standpoint of the materials concerned.

B.l Reinforced Concrete Panels

Ronan Point provided considerable impetus for construction companies in the United Kingdom to expand their already

large experimental efforts and to develop improved procedures

for design. Accordingly, the large construction companies

marketing concrete panelized systems carried out evaluations

of their systems to determine whether they met the new

criteria [2]

.

Much of the testing has been carried out in the U.K. , by

the Building Research Establishment and Imperial College,

concerning the ability of concrete panel structures to bridge

over the loss of structural elements.

Outside the United Kingdom, considerable experimental

work has been carried out by Hanson and Olesen [63] in

Denmark. Tests were made to determine the strength and

stiffness of vertical-keyed shear joints between wall

59
elements of prefabricated concrete. It is known that

Olesen is continuing with a series of laboratory tests, in

which two stories and two bays of a building of precast

concrete panel construction are simulated. In these tests,

wall and floor panels will be removed successively to

determine the distress in the remaining structure.

Granstrom [64] , in Sweden, has reported model tests,

to scale of 1:20, involving studies of joint forces and

framework deformations of buildings that have sustained

local damage. The tests relate mainly to domestic and office

buildings of precast concrete. The results show that providing joint connections, even of moderate strength, can reduce

dramatically the probability of collapse of buildings. The

shear resistance of vertical joints in large concrete panel

construction, and the ability of the joints to transmit

horizontal in-plane tensile forces, has been studied extensively and the various contributions are too numerous to list

in this report. The same is true of horizontal connections

between floor panels and the top of wall panels. The reader

is referred to the considerable volume of information presented

in March 1970, at a Symposium on joints in Precast Concrete

Components held in the United Kingdom [65]

B.2 Load-Bearing Masonry

Wilton, Gabrielsen, Edmunds, and Bechtel [66] reported

the early progress on a long range program with the objective

of developing improved methods of predicting the structural

response, failure modes, and debris characteristics of masonry

wall panels. This information is required by the Office of

Civil Defense in order to develop improved shelter systems.

The study combines both analytical and experimental work and,

in the period reported, the authors have developed a statistical failure theory for brick structures, a failure theory

for wall panels, and the development of an analytical program

60
whereby wall panel failure predictions may be used in the

design of an experimental test program. Their research

included testing of brick wall panels, interior gypsum

board wall panels, concrete wall panels and a small number

of tests to investigate the effect of air blast on shelter

ventilating equipment. A later report by Wilton, Gabrielsen

and Morris [67] describes the results of the continued

investigation of the response to blast loading of full-scale

wall panels of relatively brittle materials, notably nonreinforced brick. Information such as element-failure times

energy transmitted to a building frame, and the influence of

support conditions and other geometric factors were obtained

from the tests. Loading tests were carried out on walls

which completely closed a test tunnel, on walls with 17.5

percent doorway openings, and on walls with 16.7 and 27

percent window openings

.

In the United Kingdom, it is now a requirement under

the Building Regulations [4 8] , that structures of five

stories and more shall remain stable after the removal of

a specified length of load bearing wall, although at a

substantially reduced safety factor. Sinaj and Hendry [68]

describe three experiments that had the objective of confirm

ing that this could be achieved in a simple five-story brick

cross-wall structure. In each experiment, a section of the

main load bearing wall was removed at ground level to test

the stability of the structure in a damage condition, as

might occur following an internal explosion. Measurements

were made of the applied loads, deflections and strains.

The theoretical conclusion that the structure would remain

stable under these conditions was confirmed and some informa

tion was obtained concerning the strength of 114 mm. (4.5

inches) thick wall panels subjected to lateral loadings.

61
C . Analysis

The most extensive work in the area of analysis of

building response to overpressure loading is that of Newmark

[69] who prepared a state-of-the-art report presented at the

August 1972 Conference on the Planning and Design of Tall

Buildings. In this paper, he discussed the effects on buildings of external loadings of a transient or impulsive nature

where these loadings can arise from the detonation of

explosives, including gas or other sources, from sonic boom

from aircraft, or from accidental impact. Fundamental

relations for developing blast resistance design procedures

are also presented based upon the work of Newmark and others

over a period of many years.

Other analytical procedures have been reviewed earlier

in this paper in the context of their application.

10. Conclusions

This report is in the nature of a progress document,

coming at the end of the first year of a study of abnormal

loadings and progressive collapse. This study is being

carried out by the National Bureau of Standards on behalf of

the Department of Housing and Urban Development.

The report has discussed the state of knowledge with

respect to abnormal loadings on buildings, the response of

buildings and building elements to these loadings, and

criteria by which the risk of progressive collapse might be

minimized.

Probably very few buildings of the types constructed in

the past would be susceptible to progressive collapse in the

event of an abnormal loading. To date, few incidents of

progressive collapse have occurred in the USA, and none

approach the magnitude of the Ronan Point incident in the

62
United Kingdom.

The patterns of siting, design, and construction of

buildings are changing, however, and the frequency of occurrence of certain abnormal loadings on buildings is increasing.

The NBS study is concerned not so much with U.S. buildings of

the types constructed in the past, but with those types

constructed now, and in the future. There is reason to

believe that, in the absence of new criteria to minimize the

risk of progressive collapse, the number of buildings that

are susceptible will increase.

It should not be assumed that conventional systems are

free from the risk of progressive collapse. Rather, studies

should be made to determine the general degree of susceptibility to progressive collapse of multistory buildings of all

construction types. There is also need to study certain

abnormal loadings on buildings for which little knowledge is

available at present, such as unconfined vapor cloud explosions

and vehicular impact.

Several other countries have implemented criteria to

minimize the risk of progressive collapse. The data presented

and discussed in Chapters 5 and 6 of this report are believed

to express a lower bound to the probability of abnormal loadings in the USA. Yet these data indicate that, for buildings

susceptible to progressive collapse in the USA, the risk

substantially exceeds that which prompted other countries to

implement criteria. Furthermore, the risk of fatality appears

to be on a par with that for fire.

It is concluded, therefore, that U.S. standards-writing

bodies should adopt appropriate rational criteria as soon as

possible.

63
11 . Acknowledgments

Several of the author's colleagues and associates have

contributed valuable advice during the preparation of this

report, in particular C. P. Siess, Professor of Civil

Engineering, University of Illinois and John E. Breen,

Professor of Civil Engineering, University of Texas.

Dr. Eric F. P. Burnett, Visiting Researcher at NBS from the

University of Waterloo, Canada, made important contributions

to this study. These contributions are gratefully acknowledged.

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67
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70
n... a FORM NJ3S-114A (<-7n

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

. SHEET

1. PUBLICATION OR REPORT NO.

N3SIP "3-221

2. Gov't Accession No.

TITLE AND SUBTITLE

Abnormal Loading on Buildings and Progressive Collapse

3. Recipient's Accession No.

5. Publication Date

6. Performing Organization Code

NBSIR 73-221

7. AUTHOR(S)

Kcrman F. Somes

i. Performing Organization

10. Project/Task/ Work Unit No. 9. PERFORf.ilNG ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

Office of Research and Technology

Department of Housing and Urban Development

Washington, D.C. 20410

13. Tyf>e of Report- & Period

Covered _ . Interim

Nov 1971 - Aug 1972

14. Sponsoring Agency Code

isr SUPPLEMENTARY NOTES This document is 1 of 15 reports that provided source material for

a National Workshop on Building Practices for Disaster Mitigation held at NBS, Boulder,

Aug^28-Sep^l., 72 . With other reports and findings of the Workshop, this document was p

iiShed in full m NBS Building Science Series 4b, Feb 73, to wnicn reference may be maa

At the request of the project sponsor, the document is now printed separately as an NBS

ub16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant Report

.

bibliography or literature survey, mention it here.) The document is an interim report of ongoing studies at the National Bureau of

Standards. It defines the several aspects of abnormal loading on buildings and the

problem of progressive collapse. It documents the extent to which, present U, S,

Codes and Standards address the problem.

Abnormal loadings are identified, classified and ^discussed with regard to their

characteristics and frequencies of occurrence. The report reviews the state of

international knowledge of the characteristics of abnormal loadings and the response

of buildings and building elements to these loadings. The latter includes discussion

of several incidents in which multistory buildings have collapsed progressively.

Using currently available statistics an estimate is made of the combined

frequency of abnormal loadings on residential buildings in the U. S. For buildings

susceptible to progressive collapse, the corresponding risk of fatality is compared

Vflth. the levels of risk that society will generally accept. The risk, is further

cocqpared with the risk of mortality associated with fire in residential buildings,

an ajrea of considereible public concern and expenditure.

It is concluded that U. S. standards-writing bodies should adopt appropriate

rational criteria as soon as possible to reduce the risks of progressive collapse.

There are several areas in which criteria might be introduced to reduce the risk

of progressive collapse. These are discussed; particular attention is given to the

philosophies behind the structural criteria implemented in the USA and other countries.

17. KEY WORDS (Alphabetical order, separated by semicolons) Abnormal loading; building; codes ; design

criteria; multistory; progressive collapse; risk; stability; standards; strength;

United States

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