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>le- NBS TECHNICAL NOTE 909
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry,

and commerce. The Institute consists of the Office of Measurement Services, the Office of Radiation Measurement and the following Center and divisions:

Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Center

for Radiation Research: Nuclear Sciences; Applied Radiation — Laboratory Astrophysics 2 — Cryogenics ~ — Electromagnetics " — Time and Frequency 2

. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational institutions, and Government; provides

advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials, the Office of Air and Water Measurement, and the following divisions:

Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor

Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Insti- tute consists of the following divisions and Centers: Standards Application and Analysis — Electronic Technology — Center for Consumer

Product Technology: Product Systems Analysis; Product Engineering — Center for Building

Technology: Structures, Materials, and Life Safety; Building Environment; Technical Evaluation and Application — Center for Fire Research: Fire Science; Fire Safety Engineering.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing

equipment, techniques, and computer languages. The Institute consists of the following

divisions: Computer Services — Systems and Software —: Computer Systems Engineering — Information Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum

accessibility to the scientific information of the world. The Office consists of the following

organizational units: Office of Standard Reference Data — Office of Information Activities — Office of Technical

Publications — Library — Office of International Relations — Office of International Standards.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

3 Located at Boulder, Colorado 80302.
Investigation of Wind Damage in the

Metropolitan Washington, D.C. Area,

April 3-4, 1975

F. Y. Yokel, C. W. Yancey,

L. E. Cattaneo, and R. D. Marshall

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Sponsored by

Defense Civil Preparedness Agency

Department of Defense

Washington, D.C. 20301 ^T °F e«

0fH:AU Of

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

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued May 1976
Library of Congress Catalog Card Number: 76-600017

National Bureau of Standards Technical Note 909

Nat. Bur. Stand. (U.S.), Tech. Note 909, 65 pages (May 1976) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1976

For sale by llie Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:909). Price $1.05 (Add 25 percent additional for other than U.S. mailing).
TABLE OF CONTENTS

Page

Abstract and Key Words 1

1. Introduction 1

2. Meteorological Data 2

2.1 Wind Speeds . 2

Figure 1 3

2.2 Comparison with Recommended Design Speeds 4

3. Method and Scope of Survey 5

4. Failure Histories 5

4.1 Masonry Wall Failure in a 19 Story Apartment Building . 5

Figures for Section 4.1 8

4.2 Brick Veneer and Roof Failure in Wood Frame Buildings . 12

Figures for Section 4.2 14

4.3 Failure of a Brick Gable Wall and a Roof Overhang in

a Three-Story Wood Frame - Brick Veneer Apartment

Building 19

Figures for Section 4.3 20

4.4 Dynamic Response of Concrete Masonry Infill Walls in

a Penthouse 23

Figures for Section 4.4 25

4.5 Roof Damage in a Three-Story Garden Apartment Complex . 27

Figures for Section 4.5 28

4.6 Failure of a Roof under Construction 33

Figures for Section 4.6 34

4.7 Collapse of Concrete Masonry Walls During Construction. 39

Figures for Section 4.7 40

4.8 Miscellaneous Damage 44

Figures for Section 4.8 45

5. Discussion of Field Observations 54

5 .

1

General ' 54

5.2 Non-Loadbearing Masonry Walls 54

5.3 Overhanging Roofs 56

5.4 Unusual Building Configurations 56

5.5 Structural Skin (cladding and roofing) 57

iii
Page

5.6 Buildings under Construction 57

6. Summary and Recommendations 58

7. Acknowledgements 59

8. References 60

iv
Investigation of Wind Damage in the

Metropolitan Washington, D.C. Area, April 3-4, 1976

F.Y. Yokel, C.W. Yancey, L„E. Cattaneo, and R.D. Marshall

A limited investigation was conducted of wind damage that occurred

on April 3 and 4, 1975 in the Metropolitan Washington, D.C. area.

Meteorological data indicate that the winds were somewhat less severe

than those that should be anticipated by designers. Thus, most of the

observed damage reflects inadequacies in design or construction.

Damage was observed in occupied buildings, as well as in buildings

under construction. Damaged elements of occupied buildings included:

masonry curtain walls; masonry gable walls; masonry veneer; roofs with

overhangs; roofing; and cladding. Damaged elements of buildings under

construction included roofs and masonry walls.

Key Words: Building codes; design standards; masonry construction;

roofs; siding; structural engineering; wind; wind damage; wind

engineering.

1 . INTRODUCTION

On April 3 and 4, 1975, during a period of approximately 36 hours,

a large region in the Eastern United States was subjected to severe

winds. The area affected extended from Northern Pennsylvania to Southern

Virginia. The National Bureau of Standards conducted a limited study

of wind damage to buildings in, and adjacent to, the Metropolitan

Washington, D.C. area. The information presented in this documentation

and interpretation of the observed damage is intended to serve as

a reference for further studies, research and recommendations.
2 . METEOROLOGICAL DATA

2.1 Wind Speeds

Continuous records of wind speeds for the period April 3-4 were

obtained by the National Weather Service at stations located at Washington

National Airport, directly across the Potomac River from the District

of Columbia, and at Dulles International Airport, approximately 21

miles (34 Km) to the west. Both anemometers have typical airport

exposures and are located 20 feet (6.1 m) above ground level. The

locations of the observation points are shown in figure 1.

The record at Washington National Airport shows a fairly steady

increase in sustained wind speed (1-minute average) from 10 knots

(kts)- (5.1 m/s) at 0400 hrs (4 A.M.) to 30 kts (15.4 m/s) at 1200

hrs on April 3. Peak gusts (2-3 second average) ranged from 25 kts

(12.9 m/s) at 0500 hrs to 53 kts (27.3 m/s) at 1200 hrs. Several

gusts in excess of 50 kts (25.7 m/s) were observed during the remainder

of the day with the maximum of 57 kts (29.3 m/s) observed at 1549

hrs when the wind direction was 310 degrees. The record for April

4 indicates sustained wind speeds of 20 to 30 kts (10.3 to 15.4 m/s)

with peak gusts ranging from 30 to 50 kts (15.4 to 25.7 m/s) throughout

the day. The maximum speed of 53 kts (27.3 m/s) occurred at 1247

hrs. The wind direction ranged from 280 to 350 degrees.

The records obtained at Dulles International Airport on April

3 and 4 are quite similar to those described above, the maximum gusts

being slightly less. The peak recorded gust on April 3 was 54 kts

(27.8 m/s) from 290 degrees at 1953 hrs and the peak for April 4 was

45 kts (23.1 m/s) from 300 degrees at 0816 hrs.

Most of the damage in the Washington, D. C. metropolitan area

is believed to have occurred on April 3 between 1200 and 1900 hrs.

The approximate wind direction at Washington National Airport during

this interval was 310 degrees and several gusts in excess of 50 kts

(25.. 7 m/s) were recorded.

As with similar storms, wind speeds far exceeding these quoted

above were reported, but could not be verified. A typical example — Meteorological data were reported in nautical miles per hour (kts)
so

COOoi
is a reported speed in excess of 100 mph (44.7 m/s) in Prince Georges

County, Maryland. A subsequent inquiry of the source of the information

revealed that the anemometer indicator in question had a double scale

with ranges of 0-25 and - 100 mph. The scale selection switch was

set on the low range, thus indicating 100 mph to a person not familiar

with the device. It is quite likely that wind speeds in the area extending from Frederick, Maryland north to Pennsylvania were higher

than the Washington National Airport observations due to valley channeling

or mountain lee wave effects.

2.2 Comparison with Recommended Design Speeds

In assessing the performance of buildings subjected to extreme

winds, it is necessary to compare the probable maximum speeds with

those which serve as a basis for the calculation of design wind loads.

For this purpose the record obtained at Washington National Airport

seems most appropriate since the exposure, while locally open, has

a long fetch extending over the Falls Church and Arlington, Virginia

area. The basic wind speed used in American National Standard (ANSI)

2/ A58.1 1972 [1]— is the annual extreme fastest mile speed at 30 feet

(9.1 m) above ground for a given mean recurrence interval. The associated

terrain is flat, open country. Using the 1/7 power law, the peak

gust at 30 feet (9.1 m) for Washington National Airport on April 3 was 70 mph

(31.3 m/s) which corresponds to a fastest-mile speed of approximately

56 mph (25 m/s) . Comparing this with the recommended basic wind speeds

for the Metropolitan Washington, D. C. area (ANSI A58.1), the associated

mean recurrence interval is roughly 5 years. It seems reasonable

to conclude, therefore, that the winds of April 3-4, 1975 were not

unusually severe from the viewpoint of recommended design speeds.

It is interesting to note that the recommended basic wind speed for

a 50-year mean recurrence interval is 75 mph (33.5 m/s) (ANSI A58.1)

which results in dynamic pressures some 80 percent higher than those

associated with the observed fastest-mile speed.

2/— Numbers in brackets indicate references listed in Section 7.
3

.

METHOD AND SCOPE OF SURVEY

A survey of limited scope was conducted. Thus no attempt was made to determine the ratio between the number of buildings damaged

and the total inventory of buildings in the affected area. There was

also no attempt to secure plans and specifications for the buildings

surveyed or to conduct an in-depth engineering analysis in order to

determine the causes of failure or to verify assumed failure mechanisms.

The survey was generally limited to visual observation of distress

in the field and to conversations with building occupants, construction

personnel and building officials.

At the outset, building inspectors' offices in the various jurisdictions in the Washington, D. C. metropolitan area were contacted

in order to locate the more important cases of damage to buildings.

Cooperation was subsequently received from building officials in gaining

access to damaged buildings.

In each case an attempt was made to document all known structural

damage by visual inspection. In some cases repair work was already

under way at the time of the survey team's arrival, and, as a consequence,

some o

c the damage that had occurred was difficult to assess. The survey included occupied buildings and buildings under construction. The general areas where building damage was surveyed are indicated

in figure 1. (hatched areas).

4

.

FAILURE HISTORIES

4.1 Masonry Wall Failure in a 19 Story Apartment Building

Figure 2 shows the rear elevation of a 19 story apartment building

complex. The prevailing wind direction is shown by the arrow at the

top of the figure. In this building a masonry wall fell from the

location marked 1 into a lobby (location 2) destroying the roof

of the lobby. Some of the debris from the wall and the lobby roof

resting on the floor of the lobby is shown in figure 3. Figure 4

shows the wall failure in more detail and figure 5 shows an elevation

of a similar wall in the opposite wing of the twin buildings which

did not fail. The buildings are reinforced concrete structures.

5
Typically, reinforced concrete flat slabs, marked number 1 in figure

4, are structurally supported by reinforced concrete bearing walls

marked number 2 in figure 4. The roof slab, marked number 3 in figure

4 is a reinforced concrete slab which is also supported by the concrete

walls and is further stiffened by a reinforced concrete rim beam,

marked number 1 in figure 5 . The masonry walls are nonstructural

curtain walls consisting of one wythe of brick backed up by one wythe

of nominally 4 in (10 cm) thick concrete block. Ties between the

brick wythe and the concrete block wythe are provided by truss-type

horizontal joint reinforcement. There is no vertical reinforcement

and the field inspection showed no evidence that reinforcement ties

were provided between the masonry wall and the roof or between the

masonry wall and the floor slabs. Based on the visual inspection and

on occupant accounts the failure mechanism is envisioned as follows

First the window, marked number 4 in figure 4 (see the encircled area

on the right hand building) broke inward under the wind pressure,

the local wind direction being substantially different from the prevailing

direction. Subsequently, outward pressure acted on all the interior

surfaces of the room (the opposite window in the same room did not

break) and the accelerated flow between the buildings probably resulted

in very low pressures acting on the exterior of the wall. In addition

to this outward pressure, the roof was quite likely subject to upward

pressure on the overhanging part and to suction on the top. The combination of these pressures together with the probable absence of ties

between the wall and the roof caused the masonry wall to fail as a cantilever along the top of the floor slab of the uppermost story.

Figure 6 shows a portion of the failure surface. It should be noted

that the failure surface (near the edge of the floor slab) is clean

and smooth with no substantial remnants of the mortar joint above

it. This is taken as an indication that the bond between the mortar

joint of the concrete block wythe and the floor slab was probably

weak. Thus at this cross section, effective moment resistance was provided only by one 3-1/2 in (8.9 cm) wide brick wythe. There was

not enough moment resistance available to prevent the wall from failing

as a cantilever beam. The weight of the wall section that fell on

6
the lobby was considerable and could have caused loss of life. A

view from underneath the roof slab is shown in figure 7. Note the

concrete rim beam which stiffens the roof slab. The surface where

the wall separated is marked by an arrow. It can be reasonably assumed

that most of the weight of this portion of the roof slab is transferred

by the rim beam to the concrete columns and therefore only a minor

part of this weight rests on the masonry wall. Thus no substantial

frictional force would resist the outward movement of the top of the

masonry wall. Other views of the failure are provided in figures

8 and 9.
Figure 2 - Damaged areas of high-vise apartment building,

ing south.

LookFigure S - Partial view of the debris on the floor of the lobby.
Figure 4 - Structural details of the twin buildings. Looking

north.

* "','" *-' :'"X.- " ^

Figure 5 - Front elevation of similar wall located in undamaged

building. Looking west.
Figure 6 - Failure surface (between dashed lines) on top of

floor slab.

WAU SEPARATION SURFACE

Figure 7 - Surface of separation between rim beam and top of

wall. Looking at underside of roof slab.

10
Hgure 8 - View of upper-story damage. Looking north,

Figure 9 - Closeup of roof slab in damaged area. Looking

north.

11
4.2 Brick Veneer and Roof Failure in Wood Frame Buildings

Figure 10 is a rooftop view which shows a portion of a garden

apartment complex consisting of two and three-story, wood-frame, brick

veneer structures. The end walls of three buildings in this complex

were damaged as a result of the brick veneer flexing and separating

from the sheathing and wood framing. Figure 11 shows the most severe

of the three cases of endwall damage. The brick veneer failed over

a large area of the wall. Eyewitnesses reported a transverse deflection

of approximately 6 to 8 inches (15 to 20 cm) in the brick veneer before

the failure occurred. The walls in the other two similarly affected

buildings were shored (or braced) before substantial flexural failure

of the veneer occurred. The prevailing wind direction is shown by

the arrow in figure 11. The brick veneer was connected to the wood

framing by corrugated steel ties. Evidence of broken ties as well

as of the ties which were pulled out from the wood construction was

found in the field investigation.

The wind direction, which was parallel to the wall, probably

caused suction to act on the brick veneer. Two possible failure

mechanism are envisioned: 1. the outward deflection of the gable

wall could have exceeded the deflection tolerance of the masonry veneer;

2. the ties could have been too weak or too flexible to prevent a separation between the masonry veneer and the wood framing. The location

of the failure area relative to the floor diaphragms which restrain

the outward deflection of the wood framing seems to indicate that

the failure in this case was caused by separation between the masonry

veneer and the wood framing. There also seems to be evidence that

the brick veneer could have moved substantially outward before the

ties between the brick veneer and the wood framing were actually structurally engaged. This is illustrated by figure 12, which shows a tie

connected to a brick taken from the site. The only nail hole in that

tie through which a nail was driven is shown by the arrow. The distance

of this nail hole from the back face of the brick indicates that the

brick could have moved outward substantially before the tie was tight

enough to be structurally engaged. Figure 13 shows one of two endwalls

which was braced because it was flexing excessively. A total failure

12
of the veneer was prevented by the bracing. The two arrows show the

distance between the present location of the window and its original

location relative to the veneer (identified by the remnants of the

caulking). Note that the wall moved out considerably. The deflection

seen in the picture is actually the residual deflection that remained

after bracing. The maximum deflection during the wind storm probably

substantially exceeded the deflection seen here. Figure 14 shows

the second wall which had to be braced during the storm. It is evident

that the strength or stiffness deficiency in the veneer construction

was not limited to the building where complete veneer failure occurred.

Some of the roof damage in the same apartment complex is shown

in figure 15. It occurred mostly near the windward edges of the roofs.

It should be noted that not only were roof shingles ripped out, but

also, in some instances, the supporting plywood sheets failed. Failure

of the plywood sheets tended to occur at the overhanging parts of

the roofs. Figure 16 shows a damaged roof under repair. The original

projecting purlins which faced the direction of the wind were ripped

out and had to be replaced. A measure taken to strengthen the new projecting purlins is shown in figure 17; the timber cleat identified

by the arrow was added and nailed to the truss and the projecting

purlin respectively. In this manner resistance against uplift pressure

was provided.

13
Figure 10 - Rooftop view of garden apartments damaged by wind.

Looking northwest.

WIND DIRECTION

Figure 11 - Damaged brick veneer endwall in a 3-story building.

Looking north.

14
Figure 12 - Two views of a brick and connected tie taken from

the endwall debris.

15
Figure IS - A brick veneer endwall braced because of

excessive flexing.

Figure 14 - A second endwall braced because of excessive

flexing.

16
M~

Figure 15 - Typical roof damage occurring along the windward

edges. Looking north.

Figure 16 - Partial view of a damaged roof under repair.

17
PROJECTING PURLIN

Figure 17 - The addition of a vertical wood eleat for increased

uplift resistance along windward edge of roof.

18
4.3 Failure of a Brick Gable Wall and a Roof Overhang in a Three-Story

Wood Frame - Brick Veneer Apartment Building

An overall view of the damaged building under repair is shown

in figure 18. Figure 19 shows a closer view of the damaged wall and

roof overhang. Figure 20 shows a similar undamaged wall in an adjacent

building.

In this case the wind acted normal to the plane of the gable

wall, pushing the wall into the building. The following failure mechanism

is envisioned: First, the overhanging part of the roof consisting

of asphalt tile roofing, plywood sheathing and wood purlins was uplifted

by the wind and failed; subsequently, the wall was deprived of its

lateral support at the top and failed as a cantilever.

It should be noted that although the gable wall was actually

a brick veneer wall up to the upper ceiling level, the wood frame

structure was not continued beyond that level. Rather, wood strips

were nailed to a fiberboard sheathing which was backed up against

the brick veneer. This detail is shown in figure 21. Purlins were

then nailed to these wood strips which are shown by the arrow. Lateral

support for the top of the cantilevering brick wall was provided by

these purlins which were framed into the first roof truss (following

the gable wall) and by the plywood sheathing resting on top of these

purlins. When the purlins and the plywood sheathing were lifted up

by the wind, the brick wall lost its lateral support and was subsequently pushed in. The cantilevering roof section was completely

broken off and came to rest on the roof of the building as shown in

figure 22.

19
Figure 18 - Partial view of a damaged roof overhanging a gable

wall. Looking northwest.

20
Figure 19 - A damaged brick veneer gable wall and roof overhang

under repair.

Figure 20 - Elevation view of an undamaged gable wall.

21
Figure 21 - Details along the back side of the damaged gable

wall. Looking west.

Figure 22 - View of a portion of the damaged overhanging roof.

22
4.4 Dynamic Response of Concrete Masonry Infill Walls in a Penthouse

Figure 23 shows part of a 13 ft (4.0 m) high penthouse which

is built on top of a 12-story office building. The office building

has a flat roof and the penthouse, which houses mechanical equipment,

is located about 15 ft (4.6 m) from the edge of the flat roof. The

structure of the penthouse consists of a concrete frame with infill

masonry walls. As can be seen in figure 23, most of the masonry walls

are covered by decorative stucco. Typically, the infill wall is 8

in (20.3 cm) thick hollow concrete block. The columns are 30 in x

22 in (76.2 cm x 55.9 cm) and the masonry panels are 13 ft (4.0 m)

wide. The roof overhang shown in the figure projects from a 12 in

(30.5 cm) deep beam which rests on top of the concrete block panels,

thus making the panels infill panels inside a concrete frame. There

is also a concrete beam along the bottom of the masonry panels. The

prevailing wind direction was normal to the plane of the wall and

the loads acted inward as shown by the arrow in figure 23. The effect

of the wind was to cause a violent dynamic response of the masonry

infill walls. Eyewitnesses reported that "the wall was wavering visibly

in an inward and outward fashion, apparently pivoting about its bottom."

It is difficult to interpret this statement exactly but evidently

the wall was flexing in and out with an amplitude large enough to

be clearly distinguishable by an observer. Inadequate restraint at

the top of the wall may have contributed to the large deflections.

The flexing of the wall did not lead to a failure, although there

is substantial evidence of local distress. Figure 24 shows a view

of the bottom of the wall at the interior of the penthouse. Note

the crack between the bottom course of the masonry and the concrete

beam at the bottom of the wall. Figure 25 shows a similar crack between

the top of the masonry wall and the top concrete beam. The picture

was taken in the interior of the penthouse in a part which is not

covered by a roof (the cooling tower area) . Figure 26 shows bracing

which was installed in the interior of the penthouse during the storm

in order to stabilize the infill panels. The building was evacuated

during the storm because of fear that collapse of infill walls might

23
cause a breakdown in the mechanical system of the building, and the area around the building was roped off to protect passersby from the

hazard of falling debris.

i

24
WINd DIRECTION

•ftFigure 23 - Exterior view of the damaged masonry watts of a mechanioat equipment penthouse. Looking south.

Figure 24 - View of the bottom of a damaged wait at the

interior of the penthouse.

25
Figure 25 - Mortar crack between top of masonry wall and bottom

of concrete beam.

Figure 26 - Typical timber bracing used along the interior

side of damaged wall.

26
4.5 Roof Damage in a Three-Story Garden Apartment Complex

Figure 27 shows a front elevation of a typical three-story unit

of a garden apartment complex with flat mineral surface built-up roofs.

Figures 28 -31 show overall views of the roof damage suffered by the

apartment complex. The prevailing wind direction is shown in figure

28. The figure also shows that the roofs overhanging the balconies

were ripped off by the wind. The view through the window on the upper

left of figure 29 shows that part of the roof and ceiling over the

living unit is removed. Another front view of the damage is given

in figure 30 and a side view in figure 31. Figure 32 shows some of

the debris of the collapsed roof. The roof construction consisted

of 2 x 8 (5 x 20 cm nominal) wooden joists 24 inches (61.0 cm) on

centers with plywood on both sides of the joists. This flat roof was then covered with a composite roofing. No information was available

on the anchorage between the joists of the roof structure and the

masonry walls of the building and no evidence of anchorage could be

found in the field examination. Apparently, the wind exerted substantial

uplift on the overhanging portions of the roof and in particular on sections of the roof covering the balconies. Evidence of this uplift

can be seen in Figure 33. Note the separation between the roof (which

is in this case still in place) and the masonry wall. Figure 34 shows

that the portion of the roof overhanging the balcony was supported

by a beam which in this case stayed in place while the roof was demolished,

In figure 35 a case is shown where the beam was also pulled out from

its bearing socket. An overall view of the roof under repair is shown

in figure 36. The areas under repair in this figure give an indication

of the extent of the damage suffered.

27
Figure 27 - Front elevation of a typioal three-story apartment

bui Iding

.

s&% ,-ROOF BLOWN 0FF-,

Figure 28 - Front elevation of a building which sustained roof

damage. Looking east.

28
Figure 29 - Closeup view of damage showing loss of roof over

living unit and baleony.

Figure 30 - Closeup view of damage above third-story balcony,

29
Figure 31 - Side view of damaged building showing partial

destruction of roof.

Figure 32 - Ground-level view of some wood framing and

sheathing from a damaged roof.

30
Figure 33 - Residual separation between soffit and masonry

wall caused by uplift on roof overhang.

Figure 34 - Girder which supported portion of roof

that covered balcony.

31
BEAM S0CKE1

Figure 35 - A balcony above which the roof and supporting

girder were removed.

fc REPWR AREA

'<~~ * : -"

Figure 36 - Overall view of roof under repair.

32
4.6 Failure of a Roof under Construction

Figure 37 shows a three-story garden apartment building. This

is one of the completed units which suffered no damage. Figure 38

shows a row of the same type of units under construction. The prevailing

wind direction is indicated by the arrow. Note that in the completed

unit (figure 37) the truss structures which support the roofs are closed by siding, giving the appearance of a mansard. Figure 38 shows

part of the roofs which were removed by the wind and deposited in

front of the buildings under construction. A closer look at an undisturbed

roof assembly in place is shown in figure 39. Figures 40 and 41 show

details of the roof assemblies which were lifted up, overturned and

deposited on the ground. The truss structure had wooden top and bottom

chords and galvanized steel tubing as diagonal truss members. The

top of the roof was covered by 1/2 in (1.3 cm) thick plywood sheathing.

Although this plywood sheathing was in place, the built-up roofing

had not yet been installed. The span length of the trusses was 49

ft (14.9m) and spacing between trusses was 24 in (61.0 cm). Connections

of the roof trusses to the masonry walls are shown in figures 42,

43 and 44. Figure 44 shows that not only was the roof truss structure

removed, but also a steel beam which served as a lintel over a window

opening. One of these lintels, in place, is marked by the arrow in

figure 45. Two of the displaced lintels are seen, still attached

to the trusses in figure 38. The failure of these roofs is probably

attributable to two causes: 1. the sides of the trusses were open

and the windows were not completed, thus providing openings which

caused pressure buildup on the underside of the roof sheathing; this

pressure, together with suction acting on the top of roof surface

exerted a considerable upward force; 2. the built-up roofing was

not yet in place to add weight which could have contributed to the

resistance to uplift pressure. Figure 45 shows the details of roof

trusses on units which were under construction and which were not

damaged by the wind.

33
Figure 37 - A completed three-story garden apartment building.

otfS!»

s

LINTELS

Figure 38 - Front elevation of several buildings under construction

from which the roofs were lifted and dropped on the ground.

34
Figure 39 - Closeup view of the underside of a typical roof

assembly. J

Figure 40 - Side elevation of the roof assemblies lifted uv and deposited on the ground.

35
Figure 41 - Closeup view of overturned roof assemblies,

Figure 42 - A front masonry wall with projecting bolt for

anchoring the roof assembly

.

36
ANCHOR BOLT

Figure 43 - A portion of a wood bearing plate and a connecting anchor bolt remain atop masonry

wall.

u&**

LINTEL BEARING PAD

f£0j55fl

Figure 44 - Elevation of a rear wall from which the roof assembly

and lintel beam have been removed. Looking northwest.

37
LINTEL

REAR OF BUILDING

Figure 45 - Rear view of roof truss support in an undamaged

building. Looking east.

38
4.7 Collapse of Concrete Masonry Walls During Construction

Figure 46 shows the end unit of a four-unit concrete masonry

row house under construction. The exterior side wall of this unit

collapsed and fell into the street causing two fatalities. (See the

debris in the foreground of the photograph) . Figures 47 and 48 are other views of the same structure taken from another street at right

angles to the one into which the wall collapsed. The direction of

the prevailing wind is shown by the arrow in figure 47. Note the

partial collapse (figure 48) of cross-walls in other portions of the

structure which were parallel to the exterior wall that collapsed.

Details of the pulled-out joint reinforcement in one of the damaged

cross-walls is shown in figure 49. The construction was 8 in (20

cm) unreinforced hollow concrete block with joint reinforcement.

The incident illustrates the vulnerability of masonry walls in the

construction stage before they are fully braced at their tops.

Another instance of masonry wall failure during construction

is shown in figures 50 and 51. These walls were braced prior to the

storm.

39
Figure 46 - End living unit of a four-unit masonry row

house complex damaged during construction.
wall collapseoccurred on

This end

Figure 47 - Front elevation of three of the living units in row house complex.

Figure 48 - Closeup front elevation of damaged masonry structures

,

41
Figure 49 - Details of pulled-out joint reinforcement in a damaged cross wall.

Figure 50 - Masonry wall failure during construction.

42
Figure 51 - Closeup view of collapsed wall showing horizontal

joint reinforcement.

43
4.8 Miscellaneous Damage

Figure 52 shows an approximately 40 ft (12.2 m) high church steeple

which was observed to sway considerably. Figure 53 is a picture of

the ceiling at the base of the church steeple. Note the damage that was caused to the cladding by the movement of the steeple. The brittle

gypsum ceiling spalled as a result of the movement of the steel truss

members

.

Figure 54 is an overall view of a church which suffered considerable

roof damage. Figures 55, 56 and 57 show more detail of the roof damage.

Figure 58 shows gaps in the roof which were caused by uplift forces.

Damage to the flashing is shown in figure 59. The roofing is rigid

asbestos shingles.

The remaining figures show damage to buildings and structures

to the north and west of Washington, D. C. where the wind speeds are believed to have been slightly higher then the NWS observations described

in Section 2. Figures 60 and 61 show damage to asphalt shingle roofs

in a subdivision. Damage to a barn roof is shown in figure 62. Damage

to sheet metal siding in row-house units is shown in figures 63 and

64. Damage to signs and billboards is shown in figures 65 and 66.

Figure 67 shows a severely damaged barn.

44
Figure 52 - Elevation of a church steeple affected

by wind.

45
Figure 53 - Damaged ceiling cladding at the base of church

steeple.

Figure 54 - Overall view of a church which suffered roof damage,

Looking west.

46
Figure 55 - View along church roof showing extent of roof

damage. Looking north.

Figure 56 - Roof damage at rear (west side) of ohuroh.

47
Figure 57 - Closeup view of roof damage at rear of church.

Figure 58 - Uplift of asbestos roof shingles along north side

of church.

48
FLASHING

DAMAGE

Figure 59 - Damage to roof flashing on gable overhang.

Figure 60 - Damage to asphalt shingle roof located in a new

subdivision.

49
Figure 61 - Uplift of asphalt shingles along ridge and over

other roof areas.

Figure 62 - Roof damage sustained by a barn located in an open area.

50
Figure 63 - Damaged sheet metal siding on an end town'house unit.

Figure 64 - Damaged sheet metal siding on a townhouse unit.

51
^igure 65 - Overturning failure of a service station sign,

Figure 66 - Overturning failure of a roadside billboard.

52
Figure 67 - Partial oollapse of a barn located in rural area.

53
5 . DISCUSSION OF FIELD OBSERVATIONS

5.1 General

Based on available wind speed measurements, the winds causing

the damage surveyed in this report had a mean recurrence interval

of approximately 5 years. In accordance with prevailing design practice,

residential buildings should be designed to resist dynamic pressures

approximately 80 percent higher than those associated with the measured

wind. Thus it is postulated that the observed damage reflects inadequacies in the design or construction, some of which may be attributable

to deficiencies in building codes and design standards.

Hereafter is a brief discussion of the types of construction

that suffered damage.

5.2 Non-loadbearing Masonry Walls

Within the range of loads which masonry walls are designed to

support, their lateral-load capacity increases when the vertical load

they support is increased. Thus a wall which supports heavy vertical

loads is much less vulnerable to wind load than is a non-loadbearing

wall. This important fact should be recognized by designers and reflected

in building codes and design standards. Even though other structural

members do not depend on nonloadbearing masonry walls for their support,

the collapse of such walls, because of the weight of the debris, is a serious safety hazard and the impact loads from falling debris may

trigger further structural collapse. This is illustrated in the case reported in Section 4.1.

To be structurally safe, non-loadbearing masonry walls must be

capable of resisting wind-induced moments without depending on the

strengthening effect of vertical loads transferred from other members.

The absence of substantial vertical loads also reduces the lateralload capacity of connections, since no restraint can be derived from

frictional resistance. Thus it is critically important to provide

adequate ties to intersecting structural elements. A wall panel which

is inadequately tied at its top becomes a cantilever wall with a corresponding reduction in lateral-load capacity.

54
The preceding discussion which generally applies to all types

of non-loadbearing walls is applicable to some of the following cases which were encountered in this survey:

(a) Curtain walls

Curtain walls are continuous over more than one story and can

be very vulnerable if the connections to the structure are inadequate.

In the case in Section 4.1 no evidence was found of ties between the

curtain wall and the roof, or any of the floor diaphragms. Since in

this case only the 4 in (10 cm) thick block wythe was supported at

each floor level, the only connection to the building was provided

by the mortar joints between the block wythe and the concrete slabs.

The resistance to wall separation, provided by these joints, would

depend on the bond strength of the mortar and on frictional forces.

As previously noted, frictional forces are very small in non-loadbearing

walls, and the bond strength of the mortar can not be relied on, since,

in many cases, separation may occur by shrinkage of the block wythe,

expansion of the brick wythe, or volume changes and bending induced

by temperature gradients. In absence of an effective connection, any

part of the 70 ft (21.3 m) high curtain wall described in Section 4.1

could separate from the structure under extreme loading conditions.

In this particular instance the upper-story portion acted as a cantilever.

Both, adequate ties to the structure and adequate strength of the wall

to resist the horizontal load without the assistance of a verticalload component are necessary to prevent such a failure.

(b) Gable Walls

The upper portions of gable walls become free-standing cantilevers

when their connection to the roof fails. The upper portion of these

walls does not support any substantial vertical load, and may even

be subjected to a net upward tensile force if substantial uplift acts

on the roof. Thus the lateral-load capacity of this portion of the

gable wall is small. In the case presented in Section 4.3 the joint

between the gable wall and the roof failed as a result of the uplift

force on the roof and subsequently the wall could not resist the wind

force.

55
(c) Masonry Veneer

Masonry veneer derives most of its lateral-load resistance from

horizontal ties to structural members. Its tolerance to horizontal

deflections is small when compared to other types of exterior skin.

Thus, the horizontal ties to the structure must have adequate strength

to transmit the wind load (pressure or suction) , and have adequate

stiffness to prevent excessive deflection of the veneer. Even though

this was not definitely proven the evidence in the case presented in

Section 4.2 seems to indicate that the primary cause of failure was excessive slack in the ties, which can be viewed as a form of "inadequate

stiffness."

5.3 Overhanging Roofs

Overhanging portions of roofs are subjected to greater net uplift

forces than other parts of the roof and thus are vulnerable to wind

damage. The observed instances (Sections 4.2, 4.3, and 4.5) illustrate

several aspects of the problem:

(a) Major structural failure may be triggered when the wall depends

on lateral support from the roof (Section 4.3).

(b) The force exerted on the overhanging portion of the roof may

also trigger failure in other portions of the roof (Section 4.5).

The most frequently observed failure mechanism is initial failure

of connections between various portions of the roof construction, such

as nailing of the sheathing and roofing to the purlins, the connection

between purlins and trusses, or the connection of the roof structure

as a whole to the building. These initial connection failures subsequently trigger failure of the connected elements. Most roof failures

have a tendency to propagate because of leverage exerted by partially

disconnected elements.

5.4 Unusual Building Configurations

The geometric configuration of a structure or the effect of adjacent

structures may cause responses of unanticipated severity. In the following instances this may have been the case:

56
(a) The configuration of the twin apartment towers with the narrow

passage between the two buildings may have played some role in the

failure reported in Section 4.1.

(b) The dynamic response of the masonry infill wall in Section 4.4

may have been caused by turbulence induced by the building con- figuration since this type of construction would normally not have

experienced distress under the reported wind conditions.

(c) The church steeple of Section 4.8 apparently experienced

displacements in excess of those anticipated in the design.

5.5 Structural Skin (cladding and roofing)

Section 4 documents several cases of separation of structural

skin, mostly on roofs, but in some instances on sidewalls. The prevalence of roof damage was generally attributable to the suction acting

along the leading edges of roofs. No attempt was made to determine

whether damage in any particular case was attributable to construction

or design defects. In any case, the great number of instances observed

suggests that present practice with respect to the attachment of structural skin may be inadequate in locations where high local suction

can be expected.

5.6 Buildings under Construction

At various stages of construction, buildings are vulnerable to

wind damage. It would not be reasonable to require that a building

under construction be able to resist the design wind load which has

a 50-year mean recurrence interval. However, the wind reported herein,

which had an estimated 5-year mean recurrence interval, should not

have caused hazardous failures in buildings under construction. The

failures discussed hereafter are hazardous to workmen, bystanders,

and adjacent property,

(a) Masonry walls under construction:

At various construction stages masonry walls are free standing

cantilevers. Their vulnerability to wind pressures is further increased

by the uncured mortar which did not achieve its full tensile strength.

Because of the great weight of the debris, the collapse of such walls

57
endangers human life and adjacent property, and may even trigger progressive structural collapse of large portions of a building. Thus adequate

bracing should be in place during all the construction stages. Two

instances of masonry wall collapse are reported in Section A. 7.

(b) Roofs under construction:

Roofs may be vulnerable for the following reasons:

1. If roofing is only partially installed the roof may be subjected

to the full uplift forces while part of the weight counted on to

resist this force is not yet in place;

2. If windows or cladding are only partially completed the temporary

position of openings may give rise to internal wind pressures in excess of those assumed in the design. These pressures, combined

with suction forces acting on the roof, may cause overall roof

failure.

3. Connections may be unfinished or their grouting may be

insufficiently cured.

Roof failures could endanger human lives and missile effects associated with such failures could cause secondary damage. Some or all

of the conditions previously discussed may have contributed to the

roof failure reported in Section 4.6.

6. SUMMARY AND RECOMMENDATIONS

Available meteorological data indicate that the high winds associated with the damage reported herein were less severe than those that

should be anticipated by designers. Thus most of the observed damage

is probably attributable to deficiencies in design, construction, or materials. Even though the instances of damage observed represent

only a small percentage of the total inventory of building structures

in the affected area, the nature of the damage and its probable causes raise serious questions about the adequacy of some aspects of present

design and construction practice.

The following types of construction were found to be vulnerable

to wind damage: non-loadbearing masonry walls, including curtain walls,

veneer, and the upper portion of gable walls; overhanging roofs; some

58
unusual building configurations; exterior skin; and masonry walls and

roofs under construction.

It is recommended that building codes, design standards, FHA minimum

property standards, and conventional construction practice be reviewed

by appropriate standard-writing bodies or professional committees in

order to determine their adequacy in the following areas: • Connections for curtain walls and masonry veneer

• Connections between gable walls and roofs

• Roof construction with particular emphasis on overhangs and

connections between the various elements.

• Attachment of siding and roofing

• Temporary bracing of masonry walls during construction

• Precautions against roof failures during construction

• Design requirements and guides for unusual building configurations.

7 . ACKNOWLEDGEMENTS

The meteorological data were provided by the National Climatic

Center of the Environmental Data service of the National Oceanic and

Atmospheric Administration.

The following persons provided assistance in the collection of

information which forms the basis of this report:

Prince George's County, Maryland

James" R. Novak, Chief Building Inspector

David S. Robinson, Deputy Chief Building Inspector

Joseph B. McKay, Building Inspector

District of Columbia

Francis C. Boarman, Acting Chief, Construction Section,

Inspection Division

Arlington County, Virginia

P. 0. Shepherd, Jr., Director of Inspections

Richard B. Bier, Asst. Director of Inspections

Edward V. Bethel, Chief, Building Inspections Division

Alexandria, Virginia

Charles 0. Eberly, Director, Mechanical and Building Inspections

Dalton Getz, Field Building Inspector

59
The authors are also indebted to Betram M. Vogel from the Center for

Building Technology, for information on the building regulatory system

in the Washington D. C, Metropolitan area.

8 . REFERENCES

[1] American National Standard Institute A58. 1-1972, Building Code

Requirements for Minimum Design Loads on Buildings and Other

Structures, New York, N. Y. 10018.

60
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. TN 909

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Investigation of Wind Damage in the Metropolitan

Washington, D.C. Area, April 3-4, 1975

5. Publication Date May 1976

6. Performing Organization Code

7. AUTHOR(S)

F.Y. Yokel, C.W. Yancey, T..K. Caft-aneo, R.D. Marshall

8. Performing Organ. Report No. TN 90 9

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 4615132. 4615364 r 46111Jl

11. Contract/Grant No.

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

Defense Civil Preparedness Agency

Department of Defense

Washington, D„C„ 20301

13. Type of Report & Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 76-600017

16. ABSTRACT (A 200-word or less tactual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) A limited investigation was conducted of wind damage that occurred on April 3 and 4,

1975 in the Metropolitan Washington, D.C. area. Meteorological data indicate that

the winds were somewhat less severe than those that should be anticipated by

designers. Thus, most of the observed damage reflects inadequacies in design or construction. Damage was observed in occupied buildings, as well as in buildings

under construction. Damaged elements of occupied buildings included: masonry

curtain walls; masonry gable walls; masonry veneer; roofs with overhangs; roofing;

and cladding. Damaged elements of buildings under construction included roofs

and masonry walls.

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)

Building codes; design standards; masonry construction; roofs; siding; structural

engineering; wind; wind damage; wind engineering.

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