NBSIR 76-1048 f R)

The Incidence of Abnormal

Loading in Residential Buildings

E. V. Leyendecker and E. F. P. Burnett

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

May 1976

Final Report

Prepared for

Division of Energy, Building Technology and Standards

Office of Policy Development and Research

Department of Housing and Urban Development

Washington, D. C. 20410

NBSIR 76-1048

THE INCIDENCE OF ABNORMAL

LOADING IN RESIDENTIAL BUILDINGS

E. V. Leyendecker and E. F. P. Burnett

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

May 1976

Final Report

This report is to be superseded by a future publication, Building Science Series 89, "The

Incidence of Abnormal Loading in Residential Buildings," which will receive general distribution and should be cited as a reference. Please consult the NBS Office of Technical

Information and Publications to obtain the proper citation.

Prepared for

Division of Energy, Building Technology and Standards

Office of Policy Development and Research

Department of Housing and Urban Development

Washington, D. C. 20410

U.S. DEPARTMENT OF COMMERCE. Elliot L. Richardson. Secretary

Dr. Betty Anck«r-John«on. Astittant Secntary for Science and Technohgy

NATIONAL BUR^U OF STANDARDS. Emest Ambler. Acting Director

SI Conversion Units

In view of present accepted practice in this technological area, U. S. customary units

of measurement have been used throughout this report. It should be noted that the U. S. is a signatory to the General Conference on Weights and Measures which gave offical status to

the metric SI system of units in 1960. Readers interested in making use of the coherent

system of SI units will find conversion factors in ASTM Standard Metric Practice Guide,

ASTM Designation E 380-74 (available from American Society for Testing and Materials,

1916 Race Street, Philadelphia, Pennsylvania 19103). Conversion factors for units used

in this paper are:

Length

1 in = 0.0254* meter

1 ft = 0.3048* meter

1 mile = 1609.344* meter

Area

1 in^ = 6.4516* x lO"^ meter

Force

1 lb (Ibf) = 4.448 newton

Pressure

1 psi = 6895 N/m^

*Exact value

li
Table of Contents

Page

SI Conversion Units

Abstract 1

1. INTRODUCTION 1

2. OBJECTIVE. AND SCOPE 4

3. ABNORMAL LOADS 4

3.1 General 4

3.2 Gas-Related Explosions 4

3.3 Bomb Explosions 7

3.4 Motor Vehicle Collision with Buildings 8

3.5 Sonic Boom 9

3.6 Aircraft Collision with Buildings 10

3.7 Hazardous Materials 11

4. EVALUATION OF DATA 12

4.1 Assumptions and Summary of Reviewed Data 12

4.2 Effect of Building Size 15

4.3 Comparison with other Data 15

4.4 Load Data 16

5. CONCLUSIONS AND CONCLUDING REMARKS 16

5.1 Conclusions 16

5.2 Concluding Remarks 18

6 . ACKNOWLEDGMENTS 18

7. REFERENCES 19

Table 1 22

Figures 23

ill
THE INCIDENCE OF ABNORMAL LOADING

IN RESIDENTIAL BUILDINGS

by

Edgar V. Leyendecker* and Eric F. P. Burnett**

ABSTRACT

The findings of an analysis of available U. S. statistics concerning the incidence

of abnormal loading events in residential buildings are presented. The study evaluates

natural gas explosions, bomb explosions, motor vehicle collision, sonic boom, aircraft

collision, and explosion of hazardous materials.

It is concluded that the gas related explosion, bomb explosion, and vehicular

collision are of significance in building design for progressive collapse. Of these, the

natural gas explosion is the most significant in terms of incidence. The gas explosion

causing severe damage occurs with an annual frequency of 1.6 per million dwelling

-3

units and approaches a probability of 1 x 10 per apartment building per year.

Key Words: Bombs; building codes; design criteria; explosions; gas; hazardous materials;

loads; progressive collapse; sonic boom; vehicular collision.

1. INTRODUCTION

There has been international concern that buildings, particularly multistory buildings,

may be susceptible to a progressive "house of cards" type of failure. This concern was

intensified with the Ronan Point apartment collapse in London, England, in 1968 (figure 1).

In this 22-story building of precast concrete panel construction, collapse was precipitated by a gas explosion in the kitchen of an apartment on the 18th floor. The explosion

blew out an exterior wall panel; the loss of support provided by the panel resulted in

a chain reaction of collapse to the roof. The collapse also progressed almost to the

ground as debris from above fell on successive floors below [23]. This type of chain

reaction, or propagation of failure, flowing damage to a relatively small portion of a structure, has been termed "progressive collapse."

*Dr. Leyendecker is a Structural Research Engineer at the National Bureau of Standards.

*Dr. Burnett is an Associate Professor of Civil Engineering at the University of Waterloo,

Ontario, Canada, and was a Guest Worker at the National Bureau of Standards, October 1971

to April 1972.

1
Allen and Schr lever [4] conducted a survey to determine the frequency of occurrence of

progressive type of collapses. They used two news sources - the Engineering News Record

(ENR) from 1968 to 1972 and newspaper clippings from Canadian newspapers from 1962 to 1972.

The results of the Allen and Schriever survey are shown in figure 2. Based upon the ENR

data, an estimate of 5 1/2 progressive collapses per year was obtained. This is about 20

percent of the total of 110 collapses recorded by ENR during the four-year survey period.

The newspaper survey over a ten-year period resulted in a slightly larger annual estimate of

progressive collapses, 7 1/2 per year, which constituted a smaller percentage, about 15

percent, of the total number of collapses. These are most likely lower bound estimates since

it is considered unlikely that the data sources included all collapses.

The initial failure which leads to a progressive collapse may be caused by an abnormal

loading, that is, a condition of loading with a small probability of occurrence which a designer does not consider in the normal analysis and design of a particular structure. The

gas explosion at Ronan Point is an example of such a loading. Using the previously described

data sources, Allen and Schriever also compiled the occurrence of abnormal loads (figure 3)

causing heavy damage. The results of their survey fell roughly into three categories;

explosions, impact, and foundation mishaps. They also pointed out that both fire and

abnormally low strengths due to faulty design or workmanship could be the initiating cause

of a progressive collapse.

In 1971 Ligtenberg [24] published a paper on structural safety and fire which included

data on sources of building damage in the Netherlands (figure 4) . The sources of damage

included explosions, impact, wind loads, overloading, material defects/faulty design, and

fire. These damage sources are similar to those described by Allen and Schriever.

Ligtenberg described several of these loadings as "extreme situations" (i.e., fire,

collision, and explosion). He observed that the action (e.g., the direction of load application) of these "extreme situations" on a structure may be completely different from the

"normal situation" used for design purposes. Hence, attempts to modify customary design

methods by merely increasing the loads encountered in the "normal situation" by a "factor

of safety" in an effort to account for the "extreme situation" are not realistic and may

lead to erroneous results:

Somes [32] has attempted to classify the various abnormal loadings as follows:

A. Violent Change in Air Pressure

Sabotage bombings

Service system explosions

Other explosions within the building

Explosions external to the building

B. Accidental Impact

Highway Vehicles

Construction Equipment

Aircraft

2
C. Faulty Practice

Design Error

Construction Error

Misuse or Abuse by the Occupant

D. Foundation Failure

Unforeseen Settlement

Foundation Wall Failure

Burnett [13] has developed a more extensive classification system.

It Is likely that the greatest number of abnormal loadings occur during wartime.

McGulre [25] has summarized one British report on wartime experiences as follows:

In reviewing British experience in World War II, Baker, Williams, and Lax

observed: "One of the outstanding facts . . . of the air attacks on this country was

the very high resistance of framed buildings to the effects of high-explosive

bombs." [7]

For multistory steel framed buildings they found that, "in general the extent to

which the presence of the steel framework limits the destructive effect of the bomb on

the building as a whole is very remarkable, when compared with the typical effects in

multistory load-bearing wall buildings. The virtual absence of any progressive

collapse enables the floors and walls which are in quite close proximity to the bomb

to remain effective as screens for those portions of the building which are more remote,

with the result that the area of bad damage to the structure is very small. "

In multistory concrete framed buildings the conclusion was the same: "Obviously

nothing can be done to limit the zone of primary damage, but fortunately it is so small that the danger of resultant collapse is not serious. The continuity of most

reinforced concrete framing is such that the structure will bridge over the demolished

portion. If the only damage to the structure were that due to the primary effect of

the explosion, it is fairly safe to say that no serious collap>se would ever result. "

Findings such as these induced somewhat of a state of euphoria in structural

design. There seemed, until recently, to be a general feeling that if one followed

established practice and codes the resulting building would have inherent ability to

transmit forces around any area damaged by any abnormal incident.

This feeling persisted even though some of the practices responsible for the good

record were changing. Multistory structural forms were devised which do not have the

redundancy or ductility of conventional frames. Modem in-filing may not have the

incidental structural resistance provided by most of the older partitions and non-load

bearing walls. Many of these developments have been used successfully and economically

in compliance with established building codes.

3
The differenoe between some of the newer structures and the traditional ones

was brought out in a tragic way in the Ronan Point collapse.

In view of the changing practices mentioned by McGuire [25] and the differences between

extreme loads (e.g. abnormal loads) and normal loads pointed out by Ligtenberg, [24] It is

considered desirable to reassess abnormal loadings to determine whether or not they can

continue to be neglected in design. Accordingly, the Department of Housing and Urban

Development requested the National Bureau of Standards to prepare a study of the significance

of such incidents on residential buildings with regard to progressive collapse.

2. OBJECTIVE AND SCOPE.

The purpose of this report is to discuss sources of abnormal loading and to estimate

the risk of such loading in residential building design. The chief criterion for selection

of the loadings studied was plausibility. In this regard the data provided by Ligtenberg [24],

Allen and Schriever [4], and the various classifications by Somes [32] and Burnett [13] were

helpful in selecting the abnormal loadings described herein.

As has been indicated, these abnormal loadings occur rarely. Hence they are examined

for their possible effect on structural integrity rather than their effect on serviceability.

Only loadings on completed buildings are considered. Although very important, loadings

which might result as a consequence of faulty practice or during the construction phase are considered beyond the scope of this report. Fire is also excluded since fire protection is

required in current codes and is thus considered a normal design requirement.

The following are the loadings discussed in this report:

o Gas-Related Explosions

o Bomb Explosions

o Motor Vehicle Collision with Buildings

o Sonic Boom

o Aircraft Collision with Buildings

o Explosion of Hazardous Materials

3. ABNORMAL LOADS

3.1 General

The following sections contain information from documents which have been prepared as reference material for this report. These background documents are referenced in the section

titles. The census data [11, 12] used in determining annual frequencies are contained in

Table 1.

3.2 Gas-Related Explosions [17]

The explosions considered in this section are limited to incidents involving piped-gas

distribution systems. Explosions relating either to gas transmission or to gathering systems

4
or bottled or stored gas are not included. The most conmon forms of gas distributed by

pipeline are natural gas, manufactured gas, liquified petroleum gas (propane and butane),

and mixed gas (usually manufactured plus natural or liquified petroleum gas) . In terms of

residential consumption in 1970, 98.5 percent of all gas utility customers used natural

gas [22] and, for all practical purposes, the data for natural gas may be considered to be

representative of the industry.

Gas-related explosions result from the leakage, accumulation, and ignition of gas.

The Office of Pipeline Safety (OPS) [17] of the U.S. Department of Transportation reported that

between 1970 and 1972 leaks occurred at the average rate of about one leak for every mile

of gas distribution system (figure 5) . Considering the number of miles of gas distribution

pipeline, the incidence of leaks, the proportion of the population served directly with gas,

and the fact that a gas-related explosion can occur in buildings that are not directly

supplied with gas, it is evident that gas-related explosions could pose structural problems

for buildings in built-up or urban areas wherever a gas-distribution system exists. It

should be noted that a dwelling unit may be affected not only by an explosion within the

unit, but also by explosions occurring at other locations within or outside of the building.

Explosion data from the American Gas Association (AGA) [5] are shown in figure 6. The

available data indicate the annual incidence of gas-related explosions during the data

collection period 1957-1966. These data were broken down into explosions involving reparation (assumed to mean damage) greater than or less than $1000. No further breakdown of

reparation levels is available. There is also no indication of circumstances surrounding

the individual explosions. Note that 10 percent (151) of the 1508 incidents involved

reparation in excess of $1000.

The AGA data are taken to be representative of 1960. Based on the assumption of one leak per mile of the 391,440 miles of pipeline in 1960 [22], an explosion rate of 3.85

explosions per 100 leaks may be obtained for 1960. By further assuming this explosion rate

to be constant and applying it to the 573,868 leaks in 1970, a total of 2210 explosions may

be estimated for that year. Ten percent of the total, i.e. 221, are estimated to have

damage in excess of $1000. These data for 1960 and 1970 are summarized in figure 7.

Explosion data from the OPS are shown in figure 8. It is immediately apparent that the

total number of explosive incidents reported by the OPS is an order of magnitude lower than

the total number of explosive incidents reported by the AGA. Under the terms of the 1968

Natural Gas Pipeline Safety Act, all incidents of any consequence, that occur within gastransportation systems, must be reported to the OPS by the gas company involved. However,

the Act defines gas distribution systems as extending up to and including but not downstream

(on the customer's side) of the meter. Accordingly, gas-related incidents caused by leaks

that occur downstream of the customer's meter are not necessarily reported to the OPS.

5
However, the AGA [5] reports that about 70 percent of all gas-related incidents and about 26

percent of all incidents involving payment arise from problems originating downstream of

the customer's meter. This system of reporting (see reference [17] for a further discussion

of the reporting requirements) may also account for some of the variations between the AGA

and OPS data shown in figures 7 and 8 respectively. Because of the different reporting

systems, the OPS figures are considered by the authors as lower bound data for the frequency

of gas-related explosions. The AGA figures are considered more representative than OPS

figures for the total frequency of explosive incidents and the total incidence of explosions

causing damage in excess of $1000. However, the OPS data (figure 8) do to a limited extent,

identify the number of explosions involving residential buildings and which of these involved

damage in excess of $1000 and $10,000. It should be pointed out that in some cases data were

not available in terms of dollars of damage, in these cases the authors estimated an amount

of damage.

The data for residential buildings are summarized in figure 9. Except for the 1508

and 151 reported explosions in 1960, the data in this figure are estimates. The number of

incidents involving residential buildings was obtained by multiplying the total number of

AGA reported incidents by the ratio of the OPS-reported incidents involving residential

buildings to the OPS-reported total number of incidents. The number of residential buildings

in the two damage categories was obtained by multiplying the AGA-reported incidents causing

damage over $1000 by the ratio of the OPS-reported incidents in the appropriate damage

category to the OPS-reported incidents causing damage over $1000. The estimates obtained

by this procedure are thought to be reasonable ones for obtaining "order of magnitude"

probabilities for gas-related explosions involving residential units.

Assuming each incident to involve only one residential unit and using a 1970 total of

67,700,000 residential units (see appendix), the data in figure 9 may be used to obtain

the following annual frequencies of occurrence:

(1) 18 gas-related explosions per million dwelling units

(2) 2.5 gas-related explosions per million dwelling units causing damage in excess

of $1000

(3) 1.6 gas-related explosions per million dwelling units causing damage in excess

of $10,000

The $10,000 structural damage level is thought to be reasonably indicative of explosions

resulting in severe damage. Based upon this assumption, it can be estimated that about two severe gas explosions occur per week in the U.S.

It should be noted that these statistics are independent of the number of the dwelling

unit in a building. Hence, the more units there are in a building serviced with gas, the

greater is the probability of a gas-related explosion occurring somewhere in the building.

6
3.3 Bomb Explosions [16]

The strategically-placed explosive bomb is judged to be the most potentially hazardous

type of abnormal loading on a building. The determined, knowledgable saboteur could cause

extensive damage to a structure with little chance of being deterred. Fortunately this

type of severe bombing incident is apparently rare in the United States at this time.

Bombings do, however, occur in the U. S. and these are examined in this section.

A record of bombing activity within the U. S. has been maintained since mid-1970

following the establishment of the National Bomb Data Center (NBDC) [9, 10, 26, 27, 28].

During the period July 1970 through February 1972, the International Association of Chiefs

of Police (lACP) operated the NBDC under the auspices of the U. S. Department of Justice.

Subsequent to that time, the functions of the Center have been transferred to the Federal

Bureau of Investigation (FBI). In addition, the California Department of Justice maintains

records for that State [8].

Both lACP and FBI data for July 1970 through June 1974 are shown in figure 10 [9, 10, 26,

27, 28]. The total bombing activity is shown in figure 10(a) and residential bombing activity

is shown in figure 10(b) for both explosive and incendiary bombing. The lACP data are most

readily available for the entire 18-raonth period, July 1970 through December 1971, without a

complete breakdown between 1970 and 1971. FBI data are available on an annual basis as

shown. The lACP data may also be broken down as shown for total bombing activity, which

is useful to observe trends. Based on the data shown, it appears that bombing activity

decreased in 1974; although not to the 1970 level. The years 1971 and 1972 might have been

influenced by the Vietnam conflict and the Presidential election. However, over this

30-month period only 2.6 percent of all incidents were attributed to antiwar motives. About

22 percent of the incidents were considered politically related but the bulk of these were

attributed to motives not necessarily peculiar to the years 1970, 1971, and 1972. Moreover,

the majority of politically-related incidents involved incendiary rather than explosive

devices

.

There were an average of 1929 explosive and incendiary bombing incidents during the

reporting period (figure 10a) with an average of 492 of these involving residential units

(figure 10b). The majority of these incidents involved incendiary bombs; however, this report

considers only explosive bombings in which detonations actually occurred. It is important to

point out that between 80 and 90 percent of the bomb attacks on residences were attributed

to either personal animosity or malicious destruction. These motives are thought to be

independent of whether a residence is a single-family dwelling or in a high-rise building.

The FBI data provide no information on damage levels to individual residences as a

result of a bombing incident. On the other hand, the lACP data provide brief descriptions

of most of the reported incidents. These descriptions may include dollar estimates of

damage or descriptions such as minor damage or building destroyed. As a consequence, the

lACP data are used in this report to determine the significance of bomb incidents.

7
The explosions were categorized in the following manner:

Moderate: Windows blown out, doors blown off hinges, and partitions damaged and/or

a damage estimate of less than $1000.

Intermediate: Fairly extensive structural damage, such as walls blown down; and/or

a damage estimate between $1000 and $10,000.

Severe: Extensive structural damage or dwelling unit destroyed; and/or a damage

above $10,000 and/or a death (a death was used only if no other data were

available)

Both damage descriptions and dollars of damage were rarely available, thus the above

descriptions involved some arbitrary judgement on the part of the authors.

The results of the examination of the lACP case records are shown in figure 11(a) [9, 10,

26, 27, 28] for the 18-month period July 1970 through December 1971. Some 83 of the 134

reported incidents caused moderate, intermediate, or severe damage. Each of the three damage

categories constituted the percent of the total number of incidents as shown.

It is assumed that the percent of the total number of incidents for each damage category

is valid for the average of the entire data period July 1970 through June 1974. Based on

this assumption, the estimated annual data shown in figure 11(b) were obtained. Only 8

incidents fell into the intermediate damage category while 15 were estimated in the severe

damage category.

Using a 1970 total of 67,700,000 residential units, the following frequencies of occurrence are obtained:

(1) 2.1 explosions per million residential units

(2) 0.92 explosions per million residential units causing moderate damage

(3) 0.12 explosions per million residential units causing intermediate damage (more

than $1000 but less than $10,000)

(4) 0.22 explosions per million residential units causing severe damage (more than

$10,000).

As in the case of gas explosions, these statistics on bombing incidents are indpendent

of the number of dwelling units in a building. Thus the probability of an explosion occurring

in a building increases with its size.

3.4 Motor Vehicle Collision with Buildings [20]

Some 16 million motor vehicle accidents of all types occurred in the United States

during 1970 [2]. Although the various states collect a copious amount of records con- cerning these accidents, only a few states identify vehicle collision with a building as

a particular category of data. Possibly because of this, there are no national statistics

on vehicular collision with buildings. Fribush, et al [20], using data from two states,

have attempted to estimate such data.

8
There are two classes of motor vehicle-building collision; namely (a) those involving

a vehicle operating within its normal domain, e.g., an automobile colliding with a column in

a parking garage, and (b) those collisions that occur as a consequence of the vehicle

leaving its normal operating domain. Only the last class of accidents is considered in

this report.

Fribush used two approaches to obtain estimates for the number of motor vehicle-building

collisions. A crude proportionality model was used to project the accident records of the

two data states based on both population and motor vehicle registration. This approach

resulted in an estimate of about 20,000 vehicular collisions with buildings during 1970. An

alternate approach, based on regression analysis, resulted in an estimate of 40,500 collisions

distributed between urban and rural areas as shown in figure 12. The estimates obtained

from this second approach are used in this report since the regression analysis model con- sidered more variables than the proportionality model.

No statistics were available which indicated the type of building struck or the extent

of damage caused. However, data were available on the estimated dollars of damage for about

one-half of the collisions in the two sample states. Based on these estimates, the data

shown in figure 12 were obtained. About 5800 of the collisions are estimated to have caused

non-auto property damage in excess of $1000 and, of these, about 530 collisions caused damage

in excess of $5000.

Assuming each incident to involve only one residential unit and using a 1970 total of

67,700,000 residential units the following annual frequencies of occurrence are obtained:

(1) 600 collisions per million dwelling units.

(2) 86 collisions per million residential units causing non-auto property damage in excess of $1000.

(3) 7.8 collisions per million residential units causing non-auto property damage in excess of $5000.

The last frequency based on $5000 property damage which is an order of magnitude less

than the preceding one based on $1000 property damage is thought to be the more meaningful

estimate for severe building collisions.

3.5 Sonic Boom [15]

An aircraft flying at supersonic speeds generates shock waves which result in the

acoustic phenomenon known as sonic boom. The shock wave created is propagated behind the

aircraft in the shape of a cone (figure 13). The surface of intersection of the cone with

the ground, as shown in figure 13, may be referred to as the swath. The sonic boom is

hear at all points within the swath and structures within this area are subjected to an

overpressure. Both the swath and overpressure are affected by parameters such as:

o aircraft specifications i.e., size, shape, length, wing span, etc.,

o aircraft motion i.e., altitude, speed and nature of maneuver,

o atmospheric conditions i.e., temperature, ambient pressure, turbulence, etc.

o nature of the terrain.

9
Existing data on damages caused by sonic boom have been generated, both accidentally

and experimentally, in the U. S. almost exclusively by military aircraft. Some 41,617 sonic

boom damage claims [18] totaling more than $20,000,000 [38] were filed against the U. S.

between 1956 and 1970 for damages allegedly caused by the military. This represents about

one-third of the total number of complaints made but not all of which were filed. Payments

totaling $1,716,639 were made against 37 percent of the filed damage claims [18].

At least 25 percent of these damage claims arose from monitored supersonic flights

over built-up areas. For example, during a six-month period in 1964 a total of 1253 booms

were deliberately caused along a straight flight track passing over a northwest portion of

Oklahoma City. These investigative test flights were sponsored by the Federal Aviation

Administration and were undertaken by the Department of the Air Force. Peak overpressures

between 1 and 4.4 psf with a median value of 1.2 psf resulted in 4901 damage claims being

filed but only 287 payments totaling $123,061 [18] were made.

An accidental boom occurred over Albuquerque during maneuvers in a 1969 air show by

an F - 4E aircraft. Reed [30] estimated that overpressures must have averaged more than

20 psf over about one square mile of the city. In spite of this relatively high overpressure,

damage, although extensive, was apparently limited to glass breakage and cracked plaster.

A study [29] involving about 3000 complaints resulted in the damage breakdown shown in

figure 14. Wiggins [38] reached similar conclusions in a study of damage payments arising

from four separate accidental sonic boom incidents. Experimental programs such as the

White Sands tests which, during 1964 and 1965, involved 1493 booms and overpressures as high as 38 psf [31, 39] produced only relatively minor damage in the buildings involved.

In a follow-up study Wiggins [39] reported that some 7 years and 802 booms (56 of which were

in the 8-20 psf range) later, any cumulative damage due to sonic boom was far outweighed

by the effects of natural deterioration.

The available data indicate that the sonic boom damage which has occurred in the past

has been largely "cosmetic" in nature. Thus, barring changes in aircraft size, type, and

operating procedures, it is concluded that the sonic boom need not be considered as far as structural Integrity is concerned.

3.6 Aircraft Collision with Buildings [14]

Aircraft traffic may be categorized as General Aviation, Certified Route and Supplemental Air Carrier, or Military aircraft. The category of General Aviation includes a wide

variety of aircraft ranging in use from personal to crop dusting and accounts for about 80

percent [33] of all non-military flying time. The category of Certified Route and Supplemental Air Carrier includes aircraft such as scheduled and non-scheduled commercial aircraft.

The category of Military aircraft is self explanatory. Accident statistics [1, 6, 33] for

the two categories of civilian aircraft are given in figure 15 for the years 1968 through

1971.

10
Upon request the National Transportation Safety Board analyzed all Civil Aviation

accidents over the 7-year period 1964 to 1970 in order to establish the number of aircraftbuilding collisions, to ascertain the type of building or buildings involved and to obtain

some idea of the nature of non-aircraft property damage. The results of this analysis [19]

indicate an average of five incidents involving residences; i.e., single unit, double unit,

or row-type housing. There were an average of eight incidents involving "buildings." This

category of "buildings" includes residential buildings other than in the preceding category

as well as buildings such as churches, schools, and office buildings.

During the 7-year reporting period there were two collisions involving high-rise apartment buildings [14]. In one instance a single engine plane struck a glancing blow at the

sixteenth story level of a 30-story apartment building. Damage to the building was of a "cosmetic" nature. In the second instance a small aircraft impacted the air-conditioning

equipment at the top of a 16-story apartment building. In both cases the buildings were

shrouded in fog and none of the building occupants were killed.

Collisions with buildings at airports occur more frequently than the off-airport

accidents discussed here; however, these are considered beyond the scope of this report.

Available data from the U. S. military indicate that during the period 1962 to 1972

the Air Force recorded three accidents involving collisions with buildings. The Army

recorded three accidents involving collisions with buildings during the period 1969 to

1972. This averages out to less than one collision per year.

NTSB [3] figures also Indicate that of all accidents involving large fixed wing civil

aircraft over the 6-year period 1964 to 1969, approximately 55 percent occurred within (on

or over) the confines of the airport and an additional 15 percent within 5 miles of the

airport. The remaining 30 percent of the accidents occurred at distances of more than 5 miles

from an airport. More than half, i.e., 55 percent, of the accidents occurring at distances

greater than 5 miles from airports resulted in damage of some consequence. Thus the probability of aircraft accident and hence building collision is dependent upon location relative

to the airport and the size (primarily height) of the building.

Assuming that each of the 13 or so aircraft with building collisions per year involved

one residential unit and using a 1970 total of 67,700,000 residential units, the probability

of about 2 collisions per 10 million residential units per year is obtained. Thus, in

general, it is concluded that aircraft collision need not be specifically considered as an

abnormal loading. However, buildings in close proximity to an airport may require a specific

site analysis to be made.

3.7 Hazardous Materials [35]

Explosive incidents may occur in urban areas during the normal transportation of

hazardous materials or during storage of such materials. The magnitude of the problem is

illustrated by an accident that occurred in East St. Louis, Missouri on January 22, 1972 [32].

11
During a railroad yard operation, a tank car filled with propylene, a derivative of liquid

petroleum gas, was punctured by another car. The released propylene formed an unconfined

vapor cloud which subsequently exploded. Some 650 homes were damaged, 100 families were

homeless, and an estimated 6.5 million dollars of damage occurred. Strehlow [36] tabulating

108 such explosions over a 42-year period contends that incidents such as the one described

above are increasing.

Steele, et al, [35] studied data on five different handling procedures for hazardous

materials, i.e., (1) pipeline (except natural gas), (2) water carriers, (3) motor vehicles,

(4) railroad, and (5) storage. On the whole they were unable to develop empirical evidence

that determined whether or not such operations pose a threat to buildings. The trend

toward increased volumes of materials shipped in proximity to structures raises the prospect

of future Incidents. However, it is thought that this source of abnormal loading need not

be considered at this time.

4. EVALUATION OF DATA

4.1 Assumptions and Summary of Reviewed Data

The following abnormal loads have been considered in this report:

o Gas-Related Explosions

o Bomb Explosions

o Motor Vehicle Collision with Buildings

o Sonic Boom

o Aircraft Collision with Buildings

o Explosion of Hazardous Materials

These loadings do not constitute a complete list of abnormal loadings which may occur. However, on the basis of the various surveys described earlier (see section 1), these loads

are plausible potential sources of the initial failure which might lead to progressive

collapse.

It has been concluded earlier that the sonic boom, aircraft collision, and hazardous

material handling do not usually constitute a problem for buildings. However, it is possible

that certain building locations may require a site study.

The data for the gas explosion, bomb explosion, and vehicular collision may be discussed

in terms of probability theory. The probability of structural failure due to abnormal loads,

P(F)^, may be stated as

P(F)^ = P(f|aB) X P(AB) (1)

where

12
P(f|aB) = Probability of failure given an abnormal load event

P(AB) = Probability of occurrence of an abnormal load event

The probability of occurrence of an abnormal load event, P(AB), is dependent on factors

such as building location or type of service system but is independent of any particular

type of building construction or procedure used in design. On the other hand, the probability

of failure given an event, P(f|ab), is a function of building type as well as the particular

codes and standards used in its design and construction. Thus, statistics on failure due to

abnormal loadings should include the frequency of occurrence of an event as well as the

frequency of failure of a particular building type as a consequence of the event.

The total number of events, required to estimate the probability of occurrence, P(AB),

is usually available for the loadings described in this report. However, the probability

of failure given the event, P(f|ab), is not available. Instead, the number of incidents

causing damage in excess of a specific level (as measured in dollars) or a brief description

of damage is usually provided. There is rarely complete information on the type or number

of structures affected or an adequate description of their damage. There are also no available U. S. data which provide a correlation between a specified damage level and a corresponding damage loss in dollars. Finally, there are no data on the load characteristics

of the actual events.

In order to compare the data for the gas explosion, bomb explosion, and vehicular

collision, the following definitions of damage (similar to those for bomb explosions) are

adopted for the remainder of this report:

(1) Total incidents - All incidents involving a particular abnormal load.

(2) Intermediate or greater damage - Damage in excess of $1000 for gas explosions,

$1000 for vehicular collision, or described as intermediate or greater for bomb

explosions. This level implies fairly extensive damage such as walls blown down.

(3) Severe damage - Damage in excess of $10,000 for gas explosions; $5000 for vehicular

collision; or described as severe for bomb explosions. This level implies extensive structural damage, such as unit destroyed.

These definitions involve some arbitrary judgement because of the limitations on the

available data. However, it is interesting to note that Taylor and Alexander [37] collected

data in which some 122 gas explosions caused damage judged as significant (similar to the

category of severe used in this report) with an estimated total loss of repair i£ '500,000.

This is an average of £ 4100 per incident or about $10,200 (at an exchange rate of $2.50) in

terms of the 1971, 1972 collection period. Although circumstances differ considerably, this

lends further substance to the judgments made above which correlate damage to dollar amounts.

The estimated annual frequencies for the categories defined above are summarized in figure 16.

The annual frequencies shown in figure 16 may be discussed in terms of equation 1. The

probability of occurrence, P(AB), is readily obtained from the data in the figure. Since data

13
on P(f|ab) and are not available, data have been presented in a form resulting in

the probability of damage above a specified level, P(D) ,

P(D) = P(D IaB) X P(AB)

P P

(2)

where

P(D AB)

P

P(AB)

P

Probability of damage above a specified level given an abnormal

load event

Probability of occurrence of an abnormal load event

Subscript indicating the specified damage level, e.g. intermediate

or severe.

The terms P(D) and P(AB) are obtainable from the data in figure 16 and P(Dp|AB) may be

computed using equation 2 rewritten as

P(D)^

(2A)

Using this equation, values for PCD^lAB) were computed and are shown in table 1.

Table 1 - Calculated Estimates of P(D Iab)

P

P(D) Calculated P(D AB)

Abnormal Based On P Based On Based On '^Based On

Loading P(AB) Int . Damage Severe Damage Int. Damage Severe Damage

Gas. Expl. 18x10"^ 2.5x10"^ 1.6x10'^ 0.14 0.089

Bomb Expl. 2.1x10"^ 0.34x10"^ 0.22x10"^ 0.16 0.11

Veh. Coll. 600x10"^ 86.0x10"^ 7 . 8x10"^ 0.14 0.013

NOTE: The probabilities listed are per dwelling unit.

The probabilities of any one of the abnormal events resulting in intermediate damage is

on the order of 15 percent. However, for severe damage, the probabilities for the gas and

bomb explosions are about 10 percent but only about 1 percent for the vehicular collision.

This merely indicates that , given one of the three abnormal loadings , the gas or bomb explosion is more likely to result in severe structural damage than the vehicular collision.

The probability of failure given a normal load, P(f|n), is frequently taken to be of the

order of IxlO" or less. Assuming ^ (^^^^^^^ | AB) = P(f|aB), it may be seen that the P(f|aB)

of about 0.10 compares unfavorably with the 1x10 ^ used for P(F|n). This is a crude comparison, but it does provide a "feel" for the meaning of the numbers. It should also be noted

that the statistics do not differentiate between building types or the regulations used in

their design.

14
4.2 Effect of Building Size

The probability of an abnormal loading event occurring in a building is likely to

increase with the building size. Thus the probability of a gas or bomb explosion is assumed

to increase in proportion with the number of units in a building. In the case of the

vehicular collision, it is assumed that only the ground story is usually accessible to a vehicle and that the probability of collision is proportional only to the number of units

at ground level.

The effect of building size on the probability of a gas explosion occurring in the

building is shown in figure 17 by plotting probability versus the number of dwelling units

(the probabilities were obtained by multiplying the appropriate data in figure 16 by the

number of residential units). Auxiliary axes are provided to indicate the number of stories

when there are 5, 10, 15 or 20 dwelling units per story.

A data point for an explosion in a building the size of Ronan Point (see section 1) is

shown based on an annual frequency of 2.2 severe gas-related explosions per million dwelling

units (Great Britain data, see reference [17]) and a 22-story high building with 5 dwelling

units per story. Note that the point is very close to the U. S. data for explosions causing

severe damage. Similar plots are presented in figures 18 and 19 for bomb explosions and

vehicular collisions, respectively. The "Ronan Point" estimate is included for reference.

The variation of the probability for the severe abnormal loading events of figures 17,

18, and 19 are summarized in figure 20 for comparison. Since the data for vehicular collisions were taken proportional to the number of units at the ground level rather than the

number of units in the building, comparisons can only be made at discrete points. Thus, the

horizontal lines shown in figure 20, which indicate probability of severe collision, are maximums for buildings with the number of units per story shown encircled in the figure. For

instance, assuming 10 units/story and reading the appropriate axis, it may be seen that the

probability for a gas explosion exceeds a vehicular collision for buildings greater than about

5 stories in height. It can be shown by reading the appropriate axis, that this height of

5 stories is independent of the number of units per story. Similarly bomb explosions exceed

vehicular collisions for buildings over 35 stories in height. This height of 35 stories is

independent of the number of units per story.

4.3 Comparison with Other Data

Taylor and Alexander [37] and Fry [21] have conducted surveys in Great Britain on building damage due to gas explosions. Taylor and Alexander also studied other abnormal

loading events but the data were more limited.

Fry's survey consisted of examining fire brigade reports during the period 1957 through

1970. Taylor's survey consisted only of 1971 and 1972 but was more thorough than Fry's in

15
that a better data base, which included some field investigations, was available. Direct

comparison between these surveys and the data in this report are made on the basis of the

assumption that Fry's total number of gas explosions causing severe damage and Taylor's

incidents causing significant damage are comparable to incidents reported herein which caused

severe damage.

On this basis, 18 percent (241 out of 1353) of Fry's incidents were severe. Taylor and

Alexander reported 20 percent (75 out of 386) incidents as severe. In this survey 9 percent

(109 out of 1217) were considered severe. It is interesting to note that if the intermediate

damage level is used rather than severe damage level, the result for this study is 14

percent (170 out of 1217). While the agreement is better, the number is not thought to be

as significant.

Taylor and Alexander also found that at least 58 of 98 incidents attributed to piped

gas originated from circumstances beyond the user's control. These were sources such as

external main or service pipe as well as meter or installation pipework.

4.4 Load Data

The previously cited report by Taylor and Alexander presented data on peak pressure

reached in 122 gas explosion incidents judged as severe. Each of the incidents was investigated and a low and high estimate for pressure was assigned based on characteristics of

physical damage. Their results are shown in figure 21.

As expected, "large peak pressures occur only rarely. In particular, the pressure of

2 35kN/m (5 psi) , which is mentioned frequently in the literature on progressive collapse, was exceeded only four times (at most) in the two years of their survey.

5. CONCLUSIONS AND CONCLUDING REMARKS

5.1 Conclusions

The purpose of this report has been to discuss plausible sources of abnormal loading

events and to estimate the risk of the loadings in residential building design with regard

to progressive collapse.

A number of assumptions (section 4.1) have been necessary in order to analyze the data

presented in this report. These are related primarily to the lack of detailed descriptions

of damage accompanying the various abnormal loading events and the lack of detailed descriptions of the buildings involved in the incidents. In spite of these limitations the following

conclusions are considered valid. Where numerical data are presented they are thought to be

the correct order of magnitude.

16
1. At this time it appears unnecessary to include sonic boom, aircraft collision, and

hazardous material accidents in design for progressive collapse for typical building locations. It may be necessary in some instances to prepare site studies, as for buildings

located adjacent to airports.

2. Gas explosions occur with an annual frequency of 18 events per million dwelling

units with an annual frequency of 1.6 events per million dwelling units causing severe

damage. Based upon the assumption that severe damage is equivalent to structural failure,

there is a 9 percent probability of local structural failure, given that the event occurs.

3. Bomb explosions occur with an annual frequency of 2.1 events per million dwelling

units with an annual frequency of 0.22 events per million dwelling units causing severe

damage. Based on the assumption that severe damage is equivalent to structural failure,

there is an 11 percent probability of element failure given that the event occurs.

4. Vehicular collisions with buildings occur with an annual frequency of 600 events

per million dwelling units with an annual frequency of 6.8 events per million dwelling units

causing severe damage. Based on the assumption that severe damage is equivalent to structural

failure, there is a 1 percent probability of element failure given that the event occurs.

5. The probabilities for the preceding three events increase with increasing building

size as shown in figures 17, 18, 19, and 20. For the case of severe damage, the gas explo-

-3

sion event approaches an annual probability of 1x10 and exceeds the bomb explosion event by

an order of magnitude and is greater than that for vehicular collisions for buildings in

excess of five stories in height.

6. No load data for actual events are available except for Taylor and Alexander's

survey in the United Kingdom (figure 21) which considered primarily data for gas explosions.

2 At the most, 3 percent of these events had peak pressures in excess of 35kN/m (5 psi) . This

pressure has been frequently mentioned in progressive collapse literature.

7. The statistical reporting of abnormal loading events needs to be considerably

improved in order to obtain load data and damage data in particular types of building

construction.

5.2 Concluding Remarks

In dealing with events which have a low probability of occurrence but potentially

severe consequences it is necessary to consider what society is willing to accept. Starr [34]

has suggested that social activities consist of "voluntary" (consciously assumed) and

"involuntary" (unconsciously assumed) activities. His data indicated that the public will

accept a "voluntary" type of risk which is on the order of 1000 times greater than

"involuntary" risks. He finally concluded that "we are loathe to let others do unto us

17
what we happily do to ourselves." There is a parallel here with progressive collapse if it

is accepted that living in one's home and, being exposed to death falls into the category of

"involuntary" action. The thought that a person in one dwelling unit should be exposed to

death as a consequence of an abnormal loading event in another dwelling unit appears to be

unacceptable. However, the risk involved should be kept at a level associated with the

necessary or "involuntary" activities.

The probability of failure P(F) as expressed in equation (1) consists of two terms.

The structural designer has very little control over the probability of an abnormal event,

P(AB). The probability of the event may be reduced by other action such as improvements of

pipeline safety in the case of gas explosions. However, the structure can be designed so

that the probability of failure given the occurrence of an event P(f|aB), is acceptably

small. This approach of designing the structure to resist specified loads has been referred

to as "specific local resistance." Since the frequency of a gas explosion exceeds the other

abnormal loading events by an order of magnitude, it is suggested that this loading may be

used as the design basis.

A second approach, when it is infeaslble to design the critical elements with sufficient

strength, is to design the structure in such a way that if an element fails, the resulting

forces may be redistributed without inducing progressive collapse. This approach has been

referred to as the "alternate path" method.

6 . ACKNOWLEDGMENTS

The cooperation of the following individuals and organizations in gathering the

statistics used in this report is gratefully acknowledged:

o Jane P. Morton, International Association of Chiefs of Police

o Federal Bureau of Investigation

o American Gas Association

o Mr. Starke Jett, National Transportation Safety Board

o Major William M. Price, U. S. Army Agency for Aviation Safety

o Colonel Ray C. Staley, Department of the Army

o Commander R. A. Eldridge, Naval Safety Center, Department of the Navy

The authors would like to thank Dr. B. R. Ellingwood, Dr. C. Culver, Mr. S. R. Hastings,

and Dr. J. G. Kowalski of the Center for Building Technology, National Bureau of Standards;

Mr. L. A. Issen of the Center for Fire Research, National Bureau of Standards; and

Mr. W. J. Werner of the Office of Policy Development and Research, Department of Housing

and Urban Development for their review of the manuscript.

18
7 , REFERENCES

1. Accident and Fatality Rates, 1962-1972. National Transportation Safety Board,

Washington, D. C, (Analysis dated 23 January 1973).

2. "Accident Facts," National Safety Council, Chicago, Illinois, 1971.

3. Airport Proximity Study, U. S. Civil Aviation 1964-1969. National Transportation

Safety Board, Washington, D. C.

4. Allen, D. E., and Schriever, W. R. , "Progressive Collapse, Abnormal Loads, and Building

Codes," Structural Failures : Modes , Causes , Responsibilities , American Society of

Civil Engineers, New York, 1973.

5. Arthur D. Little Inc., "Public Safety and Gas Distribution," Research Report prepared

for the American Gas Association, Washington, D. C, December 1967.

6. Aviation Forecasts Fiscal Years 1973-1984, Department of Transportation, Federal

Aviation Administration, September 1972.

7. Baker, J. F., Williams, E. L., and Lax, D., "The Design of Framed Buildings Against

High Explosive Bombs," The Civil Engineer in War , Institution of Civil Engineers,

Vol. 3, London, 1948.

8. "Bomb Accidents in California during 1971," Division of Law Enforcement, Organized Crime

and Criminal Intelligence Branch, California Department of Justice, Sacramento, California.

9. "Bomb Summary," Uniform Crime Reports , Federal Bureau of Investigation, National Bomb

Data Center, 1972.

10. "Bomb Summary," Uniform Crime Reports , Federal Bureau of Investigation, National Bomb

Data Center, 1973.

11. Bureau of the Census, "1960 Census of Housing. Detailed Housing Characteristics,

United States Summary Report," U. S. Department of Commerce, Washington, D. C,

U. S. Government Printing Office.

12. Bureau of the Census, "1970 Census of Housing. Detailed Housing Characteristics,

United States Summary Report," U. S. Department of Commerce, Washington, D. C,

U. S. Government Printing Office.

13. Burnett, E. F. P., "Abnormal Loadings and the Safety of Buildings," Indus tralization

in Concrete Building Construction , Publication SP 48, American Concrete Institute,

Detroit, 1975.

14. Burnett, E. F. P., "Aircraft Accidents and Building Safety," NBS-GCR 76-67,

Center for Building Technology, National Bureau of Standards, Washington, D. C. 20234,

August 1973.

15. Burnett, E. F. P., "Sonic Boom and Considerations of Building Safety," NBS-GCR 76-68,

Center for Building Technology, National Bureau of Standards, Washington, D. C,

20234, August 1973.

16. Burnett, E. F. P., "The Structural Implications of Explosive Bombing Incidents,"

NBS-GCR 76-66, Center for Building Technology, National Bureau of Standards,

Washington, D. C. 20234, August 1973.

17. Burnett, E. F. P., Somes, N. F., and Leyendecker, E. V., "Residential Buildings and

Gas-Related Explosions," NBSIR 73-208 , Center for Building Technology, National Bureau

of Standards, Washington, D. C. 20234, June 1973.

19
18. Clarkson, B. L. and Mayes, W. H., "Sonic Boom Induced Building Structure Response

Including Damage," Journal of the Acoustical Society of America, Vol. 51, No. 2,

(Part 3), February 1972.

19. Collisions with Buildings, U. S. Civil Aviation 1964-1970, National Transportation

Safety Board, Washington, D. C. (Analysis Dated March 1972).

20. Fribush, S. L., Bowser, D. , and Chapman, R. , "Estimates of Vehicular Collisions with

Multi-Story Residential Buildings," NBSIR 73-175 , National Bureau of Standards,

Washington, D. C, April 1973.

21. Fry, J. E., "Gas Explosions Attended by Fire Brigades in Dwellings," Journal of the

Institute of Fuel , England, August-September 1971.

22 . Gas-Facts - A Statistical Record of the Gas Utility Industry in 1970 , American

Gas Association, Washington, D. C, 1970.

23. Griffiths, H., Pugsley, A., and Saunders, D., Report of the Inquiry into the Collapse

of Flats at Ronan Point , Canning Town, Her Majesty's Stationary Office, London,

England, 1968.

24. Ligtenberg, F. K. , "Structural Safety and Fire (1.3)," Build International , London,

Vol. 4, No. 5, September /October 1971.

25. McGuire, W. , "Prevention of Progressive Collapse," ASCE-IABSE Regional Conference on Tall Buildings, Bangkok, January 1974.

26. "Monthly Summary Reports, January through December 1971," National Bomb Data Center,

International Association of Chiefs of Police, Gaithersburg, Maryland.

27. Morton, J. P., "Bombing Incident Target Analysis, July 1970 - December 1971,"

International Association of Chiefs of Police, Gaithersburg, Maryland, 1973.

28. Morton, J. P. and Persinger, G. S., "Bombing in the United States, July 1970 - June 1971," The National Bomb Data Center, International Association of Chiefs of

Police Inc., Gaithersburg, Maryland, 1971.

29. Nixon, C. W. and Hubbard, H. H. , "Results of USAF-NASA-FAA Flight Program to Study

Community Responses to Sonic Booms in the Greater St. Louis Area," NASA TN D-2705,

May 1965.

30. Reed, J. W. , "The Thunderbird Boom Incident," Albuquerque, November 16, 1969.

Sandia Laboratories Technical Memorandum, SC-TM-70-592 , January 1971.

31. Sharpe, R. L. and Garrison Kost, "Structural Response to Sonic Boom," Proc. A.S.C.E.,

Journal of Structural Division, Vol. 97, No. ST4, April 1971.

32. Somes, N. F., "Abnormal Loadings on Buildings and Progressive Collapse," a paper in

"Building Practices for Disaster Mitigation," National Bureau of Standards , Building

Science Series 46 , U. S. Government Printing Office, Washington, D. C. 20402,

February 1973.

33. Statistical Handbook of Aviation , Department of Transportation, Federal Aviation

Administration, U. S. Government Printing Office, Washington, D. C, 1970.

34. Starr, C, "Social Benefit versus Technological Risk," Science , Washington, D. C,

Vol. 165, No. 3899, September 19, 1969.

35. Steele, W. A., Bowser, D., and Chapman, R. E., "The Incidence of Hazardous Material

Accidents During Transportation and Storage," NBSIR 73-412 , National Bureau of Standards,

Washington, D. C. 20234, November 1973.

20
36. Strehlow, R. A., "Unconfined Vapor Cloud Explosions - An Overview," Report AAE TR 72-1,

UILU-ENG. 72 0501, Aeronautical and Astronautical Engineering Department, University

of Illinois, Urbana, Illinois, February 1972.

37. Taylor, N. and Alexander, S. J., "Structural Damage in Buildings Caused by Gaseous

Explosions and Other Accidental Loadings," CP 45/74, Building Research Establishment,

England, March 1974.

38. Wiggins, Jr., J. H., Effects of Sonic Boom , J. H. Wiggins Co., Palos Verdes Estates,

California, 1969.

29. Wiggins, Jr., J. H., "Sonic Boom and Natural Deterioration: Effects on Buildings.

White Sands, N. M. Structural Survey," Department of Transportation, F.A.A. Report

No. FAA-RD-72-25, February 1972.

21
Units per Building

1960 Housing

Units (x 10 )*

1970 Housing

Units (x 10 )*

Owner Occupied

1 detached 28.436 34.396

1 attached 1.526 1.112

2 1.443 1.706

3 and 4 0.456 0.454

5 or more (7 Ave.) 0.258 0.463

Mobile Home 0.677 1.751

32.797 39.882

Renter Occupied

1 detached 7.891 7.736

1 attached 1.860 0.794

2 2.683 3.402

3 and 4 2.343 2.816

5 to 9 1.770 2.284

10 to 19 1.141 2.219

20 to 49 1.283 1.873

50 or more 1.165 2.115

Mobile home 0.090 0.321

20.227 23.560

Vacant for Rent

1

2 to 4 (3.5 Ave.)

5 to 9 (7 Ave.)

10 or more (10 Ave.)

Summed Total

Total Number of Housing

Units In U. S.

0.603

0.378

0.154

0.291

1.426

0.571

0.407

0.194

0.494

1.666

54.450 65.108

58.326 67.699

* Taken from 1960 and 1970 Census of Housing, Detailed Housing Characteristics,

United States Summary, Tables 5 and 22 Respectively. [11] [12].

Table 1. Housing Data for 1960 and 1970

22
Figure 1 Ronan Point Apartment Building after the collapse, with a second identical building

in the background. (Courtesy of London Express News and Feature Services)
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GAS BOMB VEHICULAR

EXPLOSIONS EXPLOSIONS COLLISIONS

Figure 16. SUMMARY OF ANNUAL PROBABILITIES

OF ABNORMAL LOADINGS FOR 1970
lOTJ \ I I I—I I I I I I I I I \ I I I }

I 100 200 300 400 500 600 700 800 900

TOTAL NUMBER OF UNITS IN BUILDING

ES

Figure 17. VARIATION OF PROBABILITY OF GAS-RELATED

WITH INCREASING SIZE OF RESIDENTIAL BUILDING
Figure 18. VARIATION OF PROBABILITY OF BOMB EXPLOSIONS

WITH INCREASING SIZE OF RESIDENTIAL BUILDING
Figure 19. VARIATION OF PROBABILITY OF VEHICULAR COLLISION

WITH INCREASING SIZE OF RESIDENTIAL BUILDING
. RONAN POINT

SEVERE GAS EXPLOSION

SEVERE

VEHICULAR

COLLISION*

SEVERE BOMB EXPLOSION

/

— I

"I I I L J J L i_i L J L

I 100 200 300 400 500 600 700

TOTAL NUMBER OF UNITS IN BUILDING

NUMBER OF UNITS PER STORIC

10 15 20 25 30 35 40

10 15

10

20

15

25 30 35

20 25

J I I I I I I I Il6

10

10"

10

800 I 10 20 30

NUMBER OF UNITS

> PER STORY

NUMBER

> OF

STORIES

Figure 20. VARIATION OF PROBABILITY OF SEVERE ABNORMAL LOADINGS

V/ITH INCREASING SIZE OF BUILDING
PEAK PRESSURE (psi)

1 23456789 10

120 I

1 1 1 1 1 1 1 1 1 n

10 20 30 40 50 60 70

PEAK PRESSURE (kN/m^)

Figure 21. FREQUENCY-SEVERITY CURVE FOR THE SIGNIFICANT

EVENTS IN TAYLOR AND ALEXANDER'S SURVEY (38)
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

^BSIR76-1048

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

THE INCIDENCE OF ABNORMAL LOADING IN RESIDENTIAL BUILDINGS

5. Publication Date

May 1976

6. Performing Organization Code

7. AUTHOR(S)

FHaflT" V T.pvpnH prlf PT and F.Tir F P Riimett

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 4618488

11. Contract/Grant No. IAA-H-37-72

Task 408

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Division of Energy, Building Technology and Standards

Office of Policy Development and Research

Department of Housing and Urban Development

Washington, D.C. 20410

13. Type of Report & Period

Covered

Final

14. sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.)

The findings of an analysis of available U.S. statistics concerning the incidence of

abnormal loading events in residential buildings are presented. The study evaluates

natural gas explosions, bomb explosions, motor vehicle collision, sonic boom, aircraft

collision, and explosion of hazardous materials.

It is concluded that the gas ifelated explosion, bomb explosion, and vehicular collision

are of significance in building design for progressive collapse. Of these, the natural

gas explosion is the most significant in terms of incidence. The gas explosion causing

severe damage occurs with an annual frequency of 1.6 per million dwelling units and

approaches a probability of 1 x 10~3 per apartment building per year.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Bombs; building codes; design criteria; explosions; gas; hazardous materials; loads;

progressive collapse; sonic boom ;vehicular collision

18. AVAILABILITY Unlimited

[X^ For Official Distribution. Do Not Release to NTIS

I ! Order From Sup. of Doc, U.S. Government Printing Office

Washington, D.C. 20402, SD Cat. No. CH

I I Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

22. Price

USCOMM-DC 29042-P74