Manual for Screening of Buildings for Seismic Investigation

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Manual for Screening of Buildings for Seismic Investigation

Rainer, J. H.; Allen, D. E.; Jablonski, A. M.

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IaC-CIaC

Manual for

Screening

of Buildings for Seisll1ic

MANUAL FOR SCREENING OF BUILDINGS

FOR SEISMIC INVESTIGATION

Prepared

Institute for Research in Construction

National Research Council Canada

Ottawa

Funded by:

Department of National Defence

Canada Mortgage and Housing Corporation City of Vancouver

B.C. Buildings Corporation

Institute for Research in Construction

© National Research Council of Canada 1993 December 1993

ACKNOWLEDGMENT

The development of the present manual was supported financially by a contract from the Department of National Defence (DND), by the Canada Mortgage and Housing Corporation. the British Columbia Building Corporation. and by the City ofVancouver. The many discussions and

comments byMr. Vaidyanathan of DND and by A Geraghty of the City of Vancouver were most

helpful in the development of this manual. The encouragement and advice. as well as cooperation.

fromMr.U. Morelli of the Federal Emergency Management Agency. Washington. D.C.. and Dr. C.

Rojahn of the Applied Technology Council. Redwood City. California. is much appreciated. Most of the figures and photos in this manual were obtained from the document ATC-21 (Rapid Visual Screening of Buildings for Potential Seismic Hazards: A Handbook). courtesy of the Applied Technology Council. The map of Canadian seismicity was provided by the Geological 5mvey of Canada. and the seismic zoning maps were obtained courtesy of the National Building Code of Canada.

This version of the manual was prepared by the follOwing: J.H. Rainer. Principallnvestigator

D.E.Allen

AM. Jablonski

with the collaboration of: J.K. Blohm R DeVall F.Knoll D. Mitchell RG. Redwood IRC/NRC. Ottawa IRC/NRC. Ottawa

IRC/NRC. Ottawa (formerly)

Wayte. Blohm& Associates. Victoria. B.C.

Read Jones Christoffersen. Vancouver. B.C.

Nicolet. Chartrand&Knoll Ud. Montreal. Quebec

McGill University. Montreal. Quebec McGill University. Montreal. Quebec

NOTICE

This manual was prepared by the Institute for Research in Construction (IRC) for the

sponsoring organizations listed on the title page. The procedure described is intended as an initial

screening method for ranking buildingsinan inventoIY to determine the need for more detailed

seismic evaluation. The numerical resultsare not to be construed as indicating the presence or

FOREWORD TO THIS MANUAL

This manual presents a rapid screening procedure for ranking buildings in an inventory

for later. more detailed seismic evaluations. The procedure is intended to precede and be

compatible with the NRC document "Guidelines for Seismic Evaluation of Existing Buildings" which is compatable to the National Building Code of Canada. The methodology adopted in this

manualisbased on identifying themainfeatures of any building affecting risk of seismic hazards

and the importance of the building as determined by its use and occupancy. A numerical scoring system is used. which is related to the earthquake requirements of the National Building Code of

Canada Itmust be emphasized that this method is not an evaluation for seismic adequacy. but

merely a screening procedure to rankbuildings to find those that should be evaluated in more

detail.

This manual has made extensive use of the ATC-21 document ''Rapid Visual Screening of

Buildings for Potential Seismic Hazards: A Handbook". publishedbythe U.S. Federal Emergency

Management Agency in July 1988. Major changes with respect to the ATC-21 document are:

1. The procedure is adapted to Canadian seismicity and building practice.

2. The screening procedure is based on field inspection of the inside of each building as well

as the outside or an inspection of the building drawings. whereas the ATC-21 procedure is based on field inspection of the outside of each building only.

3. The scoring system was revised to include non-structural as well as structural hazards.

and alsoimportance of the buildings as determined by use and occupancy.

4. The data collection form was modifiedtosuit the procedure adopted in this manual.

5. The procedure is focussed on buildings that are generally covered by Part 4 of the National

Building Code of Canada.

The development of this manual began with the establishment of a method. The numerical

method adopted is based on a product of factors that are known to affect the behaviour of

buildings in earthquakes. and relating each of these factors to the seismic requirements of the 1990 National Building Code of Canada. This method was supplemented with explanatory text. largely taken from the ATC-21 document resulting in a 'Draft for Trial Use". This draft received

reviews by two different seismic consultants and from the staff of two different agencies. As a

. result of the comments received. the following major changes were then made to the "Draftfor

Trial Use":

1. Revisions were carried out to the Seismic Screening Form (see Appendix C), among them:

· the factor fortype of construction (C) was increased for concrete construction.

· the factor for building importance (E) was altered by increasing the bounds for N values of the various occupancies.

· the factors for non-structural hazards(F)were doubled to reflect their proper

2. The text was clarified throughout the manual.

3. New examples were includedinAppendix B.

4. The title was changed to better reflect the manual's role in the seismic investigation process. 5. A section was added on uncertain identification of building type.

The producers of the manual at

IRe

SUMMARY AND APPLICATION

A se1sm1c screening procedure is

presented which is based on a rapid

inspection of each building or its drawings. Information for each building is collected on

a standard$eismicScreening Form. which is

used to obtain a score for a seismic priority index for each bUilding. The scores are then

used torankall buildings of the inventory for

detailed seismic evaluation. i.e.. screening

out those that require further consideration.

This manual provides the inspector with

background information anddatarequired to

complete the form.

The methodology is based on the key factors that affect risk of seismic hazards for any building; seismicity. soil conditions. type of structure. irregularities of the structure. and the presence of non-structural hazards.

It is also based on the importance of the

building as affected by its use and

occupancy. since this affects the

consequences of seismic damage. Much of

the informationwillusually be obtained prior

to the building inspections. including

seismicity. soil conditions. and importance of the building. The key information on type and irregularities of the structure and the

presence of non-structural hazards will

usually be determined by inspection of each

building or its drawings. The informationis

used to select scoring factors on the Seismic Screening Form and combine them to

determine a score for the building; the

higher the score. the greater the priority for further consideration. The score relates to

the deviation of contributing factors to

earthquake requirements in the 1990

National Building Code of Canada. The score contains two components which may be treated separately. one related to behaviour

of the structure. the other related to

behaviour of non-structural components.

This separationisuseful for planning future

seismic evaluation and upgrading.

The buildings are then ranked

categories: those that are considered of low pI1ority. of medium pI1ority. and of high pI10rity for a more detailed investigation. The divisions into low. medium. and high pI1oI1ties are somewhat arbitrary and depend on local resources and pI10rities as well as

the kinds of buildings involved. The

following values of seismic priority index (SPI) are suggested as a st:arting basis: less than

10for low. between 10 and 20for medium.

and larger than 20 for high pI1oI1ty.

Buildings with SPI scores greater than 30

can be considered potentially hazardous. The procedure presented in this manual is meant to be the preliminary screening phase of a multi-phase procedure

for identifying potentially hazardous

buildings. Buildings identified by this

procedure as needing further evaluation should be analyzed in more detail by a profeSSional engineer experienced in seismic design. Because this procedure is based on a rapid inspection. in some cases hazardous

details will not be evident. and some

seismically hazardous structureswillnot be

identified as such. Conversely. buildings

identified as potentially hazardous may prove to be adequate. Such cases should. however. be the exception rather than the rule.

Local governments may use the procedures outlined in the manual to begin

a local program. However, because this

manualisnot in itself a legal document. the

local program must be carefully planned and worked into the locality's legal framework. Before a local jurisdiction can adopt a

mitigation program, it should devise

consiStent, legally defensible procedures for the citing of individual hazardous buildings and set cI1teria so that detailed engineering

studies could eventually determine

TABLE OF

CONTENTS

ACKNOWLEDGMENT

FORWARD TO TInS MANUAL SUMMARY AND APPLICATION

Page 11 iv 1.

2.

3.

1.1 Scope and Purpose of This Manual

1.2 Survey Personnel Qualifications and Training 1.3 How to Use This Manual

1.4 The Nature of Earthquakes 1.5 Seismicity of Canada

EARTHQUAKE BEHAVIOUR OF BUILDINGS 2.1 Earthquake Effects

2.2 How Earthquake Forces are Resisted

2.3 Types of Building Structures and Typical Earthquake Damage 2.4 Configuration Problems

2.5 Non-Structural Hazards

2.5.1 Exterior Non-Structural Components 2.5.2 Interior Non-Structural Components

GENERAL SURVEY IMPLEMENTATION INSTRUCTIONS 3.1 Survey Implementation Sequence

3.2 Budget Development and Cost Estimation 3.3 Pre-Field Planning

3.4 Training of Personnel

3.5 Review of Seismic Screening Form 3.6 Survey Tools to be Taken into the Field 3.7 Information Sources 1

1

3

3

5

4. THE SCREENING PROCEDURE 39

4.1 Overview of Screening Procedure 39

4.2 Pre-Field Data 41

4.2.1 Identification of Building and Inspector 41

4.2.2 Number of Storeys and Total Floor Area 41

4.2.3 Year Built and Applicable NBC 41

4.2.4 Effective Seismic Zone 43

4.2.5 Soil Conditions 43

4.2.6 Use and Occupancy 44

4.3 Field Data 45

4.3.1 Sketches, Photos 45

4.3.2 Type of Structure 45

4.3.2.1 Distinguishing Frame Structures from Bearing

Wall Structures 47

4.3.2.2 Where to Look for Clues 50

4.3.2.3 Characteristics of Exposed Construction

Materials 50

4.3.2.4 Wood Light Frame (WLF) 52

4.3.2.5 Wood, Post, and Beam (WPB) 52

4.3.2.6 Steel Moment Frame (SMF) 58

4.3.2.7 Steel Braced Frame (SBF) 58

4.3.2.8 Steel Light Frame (SLF) 58

4.3.2.9 Steel Frame with Concrete Shear Walls (SCW) 58

4.3.2.10 Steel Frame with Infill Masonry Shear Walls

(SIW) 61

4.3.2.11 Concrete Moment Frame (CMF) 61

4.3.2.12 Concrete Shear Walls (CSW) 62

4.3.2.13 Concrete Frame with Infill Masonry Shear

Walls (CIW) 62

4.3.2.14 Precast Concrete Frame (PCP) 62

4.3.2.15 Precast Concrete Walls (PCW) 65

4.3.2.16 Reinforced-Masonry Bearing Walls with Wood

or Metal Deck Diaphragms (RML) 65

4.3.2.17 Reinforced-Masonry Bearing Walls with

Concrete Diaphragms (RMC) 68

4.3.2.18 Unreinforced-Masonry Bearing-Wall Buildings

5.

4.3.3 Irregularities

4.3.3.1 Vertical Irregularity

4.3.3.2 Horizontal Irregularity (Torsion) 4.3.3.3 Short Concrete Columns

4.3.3.4 Soft Storey 4.3.3.5 Pounding

4.3.3.6 Major Modifications 4.3.3.7 Deterioration

4.3.4 Non-Structural Hazards

4.3.4.1 Falling Hazards to Life 4.3.4.2 Hazards to Vital Operation 4.4 Scoring System

4.5 Data Confidence

RANKING AND SCREENING OF BUILDING INVENTORY 5.1 Ranking of Buildings According to Score

5.2 Cut-Off Score for Screening Out Low Risk Buildings

70

70

70

Appendix B: Examples

Appendix C: Seismic Screening Form

Seismic Screening Inventory Form

77

79 87 90

1

INTRODUCTION

Earthquakes and the resultant danger of building collapse or damage are hazards in many

parts ofthe world. In order to provide a tool to

evaluate the danger of building collapse or damage due to earthquakes, this manual presents a method whereby buildings can be rapidly identified via a "rapid survey" as seismically acceptable or potentially seismically hazardous. The method generates a score, which results from a quick site inspection of the building, both inside as well as outside, or from a quick

inspection of the architectural and structural

drawings.

The score is related to the degree to which the building is judged to deviate from

current (1990) seismic requirements. A high

score suggests that the building requires

additional study by a professional engineer experienced in seismic design, and a low score indicates that the building is probably adequate. The score is separated into two components, one for the structure, the other for non-structural components.

1.1 Scope and Purpose of ThisManual

1bis manual is intended as a rapid

screening method befitting buildings that are

covered by Part 4 of the National Building Code

of Canada (NBC). It is not intended for small

buildings covered by Part 9 of the NBC, such as single-family or small multi-family houses. The screening method presented here is compatible with and intended to be followed by the detailed

evaluation method presented in "Guidelines for Seismic Evaluation of Existing Buildings", IRCINRC, 1992.

Although newer buildings in a

community may have been properly designed and constructed to resist earthquake forces, there may be many older buildings that pose a threat to life safety or to the community as a whole if

subjected to an earthquake. The engineering

profession has addressed the problems associated with earthquakes by developing "seismic hazard zones" and design recommendations associated with those zones. Hazard zones are developed

by looking at the number, size and location of

past earthquakes, the locations of active faults, and the likelihood of future earthquakes in each region. Appendix A contains maps showing the

maximum expected earthquake motions. The

maps were used to determine the seismic hazard

zones intheNBC 1990.

1bis manual describes a method to rapidly and easily identify those buildings that might pose a risk of loss of life or injury, or severe curtailment of community services in the event of a damaging earthquake. 1bis method is meant to be fast and relatively inexpensive, so that a community or authority can develop a list of potentially hazardous buildings without the high cost of a detailed analysis of every building.

Ideally,ifa building receives a high score and is

thus identified tobepotentially hazardous on the

basis of this initial rapid screening procedure, that building would receive further study by a professional engineer having experience or

training in seismic design. On the basis of a more detailed inspection and review of structural

drawings, and engineering analyses, and other

detailed procedures, a fmal determination about the degree of seismic hazard can then be made.

1bis screening method can be employed as a tool for assigning relative priorities to the need for more detailed evaluation of a given

inventory of buildings. Thus, a group of

buildings, which are spread over a large region or indeed all across the country, can be ranked along relative levels of risk to the building

occupants andto the community as a whole. On

the other hand, the same ranking can be achieved for a group of buildings in a given community.

The seismic screening procedure

presented· in the manual is designed to be a

procedure in which no structural analysis

calculations are performed. The user will be

making decisions based on a scoring system

discussed later inthemanual. Inthis procedure,

the inspection of the building or its drawings will typically take an average of approximately one hour per building, more for large complex structures, less for smaller repetitive building.

Prior to the inspections, supplemental data will

be obtained from such other sources as assessor or building department files or previous studies. Such information should be reviewed and collated before beginning the field survey.

1bis procedure is intended to be

applicable nationwide for all conventional

building types. Non-building structures, such as bridges and large towers, are excluded Seismic screening is meant to be the first phase of a

multi-phase procedure in which selecteddataare

obtained and used to arrive at a preliminary decision regarding the potential hazard of the building. The subsequent phase consists of more detailed analyses of those buildings that were identified by seismic screening as being in need of further evaluation.

Although the seismic screening

procedure presented in this manual applies to

buildings of all types, due to budget or other

constraints some users may wish to restrict the survey to building types that they consider the most hazardous, such as unreinforced masonry or

non-ductile concrete. However, it is

recommended, at least initially, that all

conventional building types be considered, and that elimination of certain building types be well-documented and supported with both office

calculations andfield survey data. It is possible

that in some cases even buildings designed to

modem codes could pose life-safety hazards, particularly with regard to non-structural hazards.

1.2 Survey Personnel Qualifications and Training

The seismic screening procedure was developed and written for a target user consisting of the following groups:

- local building officials - professional engineers - registered architects - building owners - emergency managers

Any or all of these people might be involved in

the effortstoidentify a community's seismically

hazardous buildings and to mitigate the

associated hazard. Due to the varied

backgrounds of the members of this target group,

an efforthasbeen madetodefine technical terms

and, where possible, to provide rules that assist

in making judgments where engineering

experience would otherwise be required.

Before attempting to perform the

screening procedure, all survey personnel should

read and be familiar with this manual. As a

training exercise, it is recommended that all

personnel, . in conjunction with a professional engineer experienced in seismic design, initially

serve two purposes:

1. assure a more uniform interpretation of

the seismic screening form and scoring procedure; and

methodology. Suggestions are made as to how scores could be used to set priorities for subsequent evaluations. But as stated, these are only suggestions and should therefore be thoroughly reviewed by the community before adoption.

2. help to identify construction practices

unique to a particular jurisdiction and perhaps not specifically discussed in the manual.

1.4

1.3 How to Use this Manual

1llis manual is divided into several sections. The first sections contain background material; the later sections, material that may be referred to in the field. All sections should be read, however, before performing the survey.

Chapter 2 is an overview of the

behaviour of buildings in earthquakes. 1llis

includes a basic description of the structural elements needed to resist earthquake forces and a discussion of the most common damage that

occurs in different structural types. Chapter 3

provides guidance on how to plan and prepare for the survey, including training of persons,

survey tools required, and sources of

information. Chapter 4 provides. guidance to the

people carrying out the survey, including

data-gathering before the inspections (pre-fielddata),

what to look for during the inspections (field

data),and how to determine the scores. Chapter 5 provides post-survey guidance for ranking the

buildings according to the scores obtained and

selecting a cut-off score for screening out

buildingsthatdo not require detailed evaluation.

The key to the screening procedure is the

Seismic Screening Form at the end of this

manual (Appendix C) with filled-out examples given in Appendix B.

1llis manual does not include detailed information on how a particular community

should fund the survey, manage data, choose

personnel, or otherwise implement the survey

In a global sense, earthquakes result from motion between plates comprising the Earth's crust These plates are driven by the convective motion of the material in the Earth's mantle,

which, inturn,is driven by heat generated at the

Earth's core. That is, just as in a heated pot of water, heat from the Earth's core causes material to rise to the Earth's surface. Forces between the rising material and the Earth's crust cause the

plates to move. The resulting motions of the

plates relative to one another generate

earthquakes. Where the plates spread apart,

molten material emerges to fill the void. An

example is the ridge under the middle of the Atlantic Ocean. Initially molten, this material quickly cools and then, over millions of years, this material is driven by newer material across a major portion of the Earth's surface.

These large pieces oftheEarth's surface,

termed tectonic plates, move very slowly and irregularly. Forces may build up for decades or centuries at the interface between plates (termed

faults), until a large movement occurs all at

once. These sudden violent motions produce the

shaking that is felt as an earthquake. The

shaking can cause direct damage to buildings, roads, bridges, and other man-made structures, as well as triggering fIfes, landslides, tidal waves (tsunamis), and other damaging phenomena.

A fault is like a "tear" in the Earth's

crustand may be from one to over one hundred

kilometres deep. In some cases, faults are the

physical expression of the boundaries between adjacent tectonic plates and thus may be

hundreds of kilometres long. Inaddition, there may be thousands of shorter faults parallel to or

branching out from a main fault zone.

Generally, the longer a fault the larger the

earthquake it can generate. Beyond the main

tectonic plates, there are many smaller sub-plates, "platelets" and simple blocks of crust which occasionally move and shift due to the "jostling" of their neighbours or the major plates. The existence of these sub-plates means that smaller but still damaging earthquakes are possible almost anywhere, although perhaps with less likelihood.

Besides the seismic sources at the tectonic boundaries, such as on the west coast of North and South America, seismic activity can

also occur within thetectonic plates due to local

faults and local buildup of stresses. Such intra-plate seismic sources occur along the St Lawrence Valley and the region extending from north of Ottawa to Boston.

With the present understanding of the earthquake-generating mechanism, the times, sizes, and locations of earthquakes cannot be

reliably predicted. Generally, earthquakes are

concentrated near faults, and certain faults are

more likelythanothers to produce a large event,

but the earthquake-generating process is not

understood well enough to predicttheexact time

of earthquake occurrence. Therefore,

communities must be prepared for an earthquake to occur at any time.

Four major factors canaffectthe severity

of ground-shaking and thus potential damage at a site. These are the size of the earthquake, the

typeof earthquake, the distance from the source

of the earthquake to the site, and the types of

soil at the site. Larger earthquakes shake longer and harder, and thus cause more damage.

Experience has shown that the ground motion

can be felt for several seconds to a minute or

longer. In preparing for earthquakes, both

horizontal (side to side) and vertical shaking

Generally, the farther from the source of an earthquake, the less severe the motion. The rate at which motion decreases with distance is a function of the regional geology and inherent characteristics of the earthquake and its source. The underlying geology of the site can also have a significant effect on the amplitude of the ground motion. Soft loose soils tend to amplify

the ground motion andinmany cases also make

it last longer. In such circumstances, building

damage can be accentuated. In the San

Francisco earthquake of 1906, damage was

greater in the areas where buildings were

con-structed on loose man-made fill and less at the

tops of the rocky hills. Even more dramatic was the 1985 Mexico City earthquake. 1bis earth-quake occurred 400 kilometres from the city, but very soft soils beneath the city amplified the ground-shaking enough to cause weaker mid-rise

buildings to collapse. Similar instances of

motion amplification with soft soil deposits were also observed during the 1988 Saguenay earth-quake in central Quebec and the 1989 Loma Prieta earthquake near San Francisco. Sites with rock close to or at the surface will be less likely

to amplify seismic motion. The typeof motion

felt also changes with distance from the earth-quake. Close to the source the motion tends to be violent rapid shaking, whereas farther away

the motion is normally more of a swaying. As

expected, buildings will respond differently to

the rapid shakingthanto the swaying motion.

There are many ways to describe the size

and severity of an earthquake and associated

ground-shaking. Perhaps the most familiar is

Richter magnitude. Richter magnitude is a

numerical description ofthe maximum amplitude of ground movement measured by a seismograph (adjusted to a standard setting). On the Richter

scale,thelargest recorded earthquakes havehad

magnitudes of about 8.5. It is a logarithmic

scale, and a unit increase in magnitude corres-ponds approximately to a thirty-fold increase in total energy of the earthquake.

intensity at a site is the force or acceleration

caused by the ground motion. In this manual,

the peak horizontal ground acceleration (or velocity) is employed as the measure of seismic

intensity. The peak horizontal ground

acceleration (or velocity) likely to occur during

the life of a building for any particular region of

Canada has been estimated (see Appendix A),

and the country divided into 7 seismic hazard

zones, 0 to 6. In conducting a survey of

seismically hazardous buildings for a specific city, generally only one seismic zone will be involved, while the real-estate holdings of a particular owner may extend over many seismic zones.

I.S Seismicity ofCanada

Itis evident from Figure 1-1 that some

parts of the country have experienced more and

larger earthquakes than others. The boundary

between the North-American and Pacific tectonic

plates occurs along the west coast ofthe United

States, Mexico, and Canada. The San Andreas fault in California, the Juan de Fuca subduction fault off Vancouver Island and the Aleutian Trench off the coast of Alaska are part of this

boundary. These active seismic zones have

generated earthquakes with Richter magnitudes

greater than 8. There are many other smaller

fault zones throughout western Canada and the

United Statesthat are also helping to release the

stress thatisbuilt up as the tectonic plates move

past one another. Because earthquakes always occur along faults, the seismic hazard will be greater for those population centres close to active fault zones.

In the eastern part of Canada and the

United States, the cause of earthquakes is less well understood. There is no plate boundary and

very few locations of faults are known.

Therefore, it is difficult to state where

earthquakes are most likely to

occur.

Several significant historical earthquakes have occurred, such as a major earthquake in New Madrid, Missouri, in 1811 and 1812, indicating that there is potential for very large earthquakes. Smaller, but still damaging, earthquakes, up to Richter magnitude 7, have occurred near the St Lawrence and Ottawa rivers (recently

La Malbaie in 1926, Cornwall-Massenain1944,

and Saguenay in 1988). However, most

earthquakes in the eastern part of the continent

are events of smaller magnitude. Because of

regional geologic differences, easternandcentral

North-American earthquakes are felt at much

greater distancesthanthose in the western part,

sometimes upto 1000km away. In the central

part of Canada, no significant earthquake risk

exists; while in the Arctic regions, some

moderate seismic activity is predicted. However,

both the recorded historyand instrumental data

of seismic activity in the Arctic is only very

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2

EARTHQUAKE BEHAVIOUR OF BUILDINGS

Many different types of damage can occur in buildings. Damage can be divided into

two categories: structural damage and

non-structural damage, both of which can be

hazardous to building occupants. Structural

damage means degradation of the building's structural support systems (Le., vertical and lateral force-resisting systems), such as the

building frames and walls. Non-structural

damage refers to any damage that does not affect the integrity of the structural support system. Examples of non-structural damage can be asso-ciated with failure of parapets, ornamentations, masonry partitions, heavy equipment, such as elevators, lifelines in critical facilities, etc. The type of damage to be expected is a complex

issue that depends, among other factors, on the

structural type and age ofthe building, seismic

ground motion, ground conditions, the proximity of the building to neighbouring buildings,

condition of the building, and the type of

non-structural elements. These possible contributions to the hazard of the building will be discussed in more detail below.

2.1 EarthquakeEtTeets

When earthquake-shaking occurs, a

building gets thrown from side to side or up and

down. That is, while the ground is violently

moving from side to side, the building tends to stay at rest, similar to a passenger standing on a bus that accelerates quickly. Once the building starts moving, it tends to continue in the same

direction, but by this timethe ground is moving

back in the opposite direction (asifthe bus

driver first accelerated quickly, then suddenly

braked). Thus the building gets thrown backand

forth by the motion of the ground, with some

parts of the building lagging behind and then

moving in the opposite direction. TIle force (F)

that the building sustains is related to its

mass(m) and the acceleration (a), according to Newton's law, F=ma. The heavier the building,

the more force is exerted. Therefore, a tall,

heavy, reinforced-concrete building will be

subject to much more force than a lightweight

one-story wood-frame house, given the same acceleration. Damage can be due to structural members (beams and columns) being overloaded or due to differential movements between

different parts of the structure.Ifthe structureis

sufficiently strong to resist these forces or differential movements, little damage will result. If the structure cannot resist these forces or differential movements, structural members will be damaged, and collapse may occur.

Building damage is related to the duration and the severity of the ground motion.

Larger earthquakes tend to shake longer and

harder and therefore cause more damage to

structures. Earthquakes with Richter magnitudes

less than 5 rarely cause significant damage to

buildings, since acceleration levels and duration of shaking for these earthquakes are relatively

small. In addition to damage caused by ground

shaking, damage can be caused by buildings pounding against one another, ground failure that

causes the degradation of the building

foundation, landslides, fires, and tidal waves (tsunamis). Many of these "indirect" forms of

The level of damage that results from a major earthquake depends on how well a

buildinghasbeen designed and constructed. The

exact type of damage cannot be predicted,

because no two buildings undergo identical motion. However, there are some general trends that have been observed in many earthquakes. Post-earthquake investigation teams have found

that steel buildings perform significantly better

than those built of unreinforced masonry, for

example. New buildings generally sustain less

damage thanolder buildings designed to earlier

codes. The collapse of load-bearing walls that

supporttheentire structure is a common form of

damage in unreinforced masonry structures.

Roofs have collapsedin many older precast

tilt-up buildings. From a life-safety perspective, vulnerable buildings need to be clearly identified and strengthened or removed.

Each building has its own vibrational

characteristicsthatdepend on building height and

structural type. Similarly, each earthquake has

its own characteristics that depend on the

geology ofthesite, distance from the source,and

the type and site of the earthquake source

mechanism. Sometimes the earthquake motion causes a sympathetic response with the building,

termed resonance. Resonance, which occurs

when the frequency of earthquake excitation is

equal to the

natUral

will cause an increase in the amplitude ofthe

building's vibration and consequently increase

the potential for damage. Resonance was a

major problem in the 1985 Mexico City

earthquake, which resulted in the total collapse of many mid-rise buildings.

2.2 How Earthquake Forces are Resisted

Buildings experience horizontal distortion

when subjected to earthquake motion. When

these distortions get large, the damage can be

catastrophic. Therefore, most buildings are

designed with lateral-force-resisting systems (LFRS) to resist the effects of earthquake forces.

In many cases, LFRS make a building stifferand

thus minimize the amount of lateral movement

andconsequently the damage. LFRS are usually

capable of resisting only forces that result from ground motions parallel to them. However, the combined action of LFRS along the width and length of a building can typically resist

earthquake motion from any direction. LFRS

differ from building to building, because thetype

of system is controlled to some extent by the basic layout and structural elements of the

building. Basically, LFRS consist of

axial-(tension or compression), shear- ,and bending-resistant elements.

In wood-frame stud-wall buildings,

plywood siding is typically used to prevent excessive lateral deflection. Without the extra strength provided by the plywood, walls would

distort excessively or "rack", resultinginbroken

windows and stuck doors. Inolder wood-frame

buildings, this resistance to lateral loads is provided by either wood or steel bracing.

The earthquake-resisting systems in

modem steel buildings take many forms. Many

types of diagonal bracing configurations have

been used. Examples of the use of single

diagonal braces, cross-bracing, and K-bracing are

shown in Figure 2-1. Moment-resisting steel

frames are also capable of resisting lateral loads. In this type of construction, the connections

betweenthebeams andthecolumns are designed

to resist the rotation of the column relative to the

beam. Thus, the beam and the column work

together and resist lateral movement by bending. This is different from the braced frame, where

loads are resisted through tension and

compression forces in the braces. Steel buildings are sometimes constructed with moment-resistant

frames in one directionandbraced frames in the

other.

In concrete structures, shear walls are sometimes used to provide lateral resistance, in

addition to moment-resisting frames. Ideally,

these shear walls are continuous reinforced-concrete walls extending from the foundation to

SINGLE DIAGONAL DOUBLE DIAGONAL BEAM SPECIALLY REINFORCED AT JOINT ｾ ECCENTRIC II

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the roof of the building, and can be exterior walls or interior walls. They are interconnected

with the rest of the concrete frame, and thus

resistthemotion of one floor relative to another.

Shear walls can alsobeconstructed of reinforced

brick or reinforced hollow-concrete block. 2.3 Types of Building Structures and Typical

Earthquake Damage

are described and illustrated here to provide an

overview of existing structural systems. The

model building types listed in Table 2-1 are

classified primarily according to the vertical

elements of the structural system of the particular building. The typical earthquake performances and expected type of damage of the different systems are also discussed.

Different types of common construction

my

Dominant Material Description of Type of Structure Symbol

WOOD Wood, tight Frame WLF

Wood, Postand Beam WPB

S1EEL Steel Moment Frame SMF

Steel Braced Frame SBF

Steel tight Frame SLF

Steel Frame with Concrete Shear Walls

sew

Steel Frame with Infill Masonry Shear SIW

Walls

CONCRElE Concrete Moment Frame CMF

Concrete Shear Walls CSW

Concrete Frame with Inflll Masonry CIW

Shear Walls

Precast Concrete Frame PCP

Precast Concrete Wall PCW

MASONRY RM Bearing Walls with Wood or Metal RML

Deck Diaphragms

RM Bearing Walls with Concrete Diaphragms RMC

Unreinforced-Masonry Bearing-Wall URM

Buildings

Note: KM= KemIorcea Maso

Table 2-1: List of Building Types

Wood Light Frame (WLF): WLF

buildings are typically apartment, commercial, or office buildings with a total floor area greater

than 600 m2or exceeding three storeys in height

(They are covered by Part 4 ofthe NBC.) The

vertical framing is of stud-wall construction, braced with diagonals, plywood, or equivalent.

Stud walls are typically constructed of 5- by ID-cm (2- by 4-in.) wood members verti-cally set about 40 em apart (Figure 2-2). These walls are braced by plywood or waferboard or by diagonals made of wood or steel.

Stud-wall buildings have performed very

well in past earthquakes because of inherent

qualities of the structural system and because

they are lightweight and low-rise. Cracks in the

plasterand stucco may appear, but these seldom

degrade the strength of the building. In fact, this

type of damage dissipates a lot of the

earthquake-induced energy. The most common

type of structural damage in older buildings

results from a lack of connection between the

superstructure and the foundation. Houses can

slide off their foundations if they are not

properly bolted to the foundation, resulting in

major damage to the building as well as to plumbing and electrical connections. In many municipalities, modem codes require wood structures to be bolted to their foundations.

However, the yearthatthis practice was adopted

will differ from community to community and should be checked

Many of the older wood-stud frame

buildings have either DO foundations or weak

foundations constructed ofunreinforced masonry or poorly reinforced concrete. These foundations have poor shear resistance and can fail.

Garages often have a very large door

opening in one wall with little or no bracing,

creating what is generally referred to as a soft

storey. This wall has almost no resistance to

lateral forces, which is a problem if a heavy load such as a number of storeys sits on top of the garage. Apartment buildings built over garages

have sustained significant amounts of damagein

past earthquakes, with many collapses.

Therefore, this type of configuration should be

examined more carefully and perhaps

strengthened

Some wood-frame structures, especially older buildings, have masonry veneers that may

consists of onewytheof brick (a wythe is a term

denoting the width of one brick) attached to the stud wall. In older buildings, the veneer is either

insufficiently attached orhaspoor quality mortar,

which often results in the veneer peeling off during moderate and large earthquakes.

Unreinforced masonry chimneys may also represent a life-safety problem. 'They are often inadequately tied to the building and

therefore fall when strongly shaken. On the

other hand, chimneys of reinforced masonry generally perform well.

Wood, Post, and Beam (WPB):

Post-and-beam wood construction is often used in

commercial and industrial buildings. These are generally larger buildings, such as warehouses, offices, churches, theatres, fire stations, or even

large gas stations. This type of construction

consists of larger rectangular - 15 by 15 cm (6

by 6 in.) and larger - or sometimes circular

wood columns framed together with large wood beams or trusses. 'The structure is encased by different types of external walls, including masonry and stone veneers.

TImber pole buildings are a less common

form of construction. They are more often

subject to wood deterioration due to the exposure of the columns, particularly near the ground surface. Together with an often-found soft

storey in this building type, this deterioration

may contribute to unsatisfactory seismic

performance.

Post-and-beam buildings tend to perform well in earthquakes, if adequately braced. However, walls often do not have sufficient

bracing to resist horizontal motionandthus they

may deform excessively.

These types of construction often have

different types of external walls including masonry or fake stone veneers, which represent another hazard.

Rooflfloor span systems: 1. wood joist and rafter 2. diagonal sheathing 3. straight sheathing

Wall systems:

4. stud wall (platform or balloon framed) 5. horizontal siding

10

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9

Foundatiorv' connections: 6. unbraced cripple wall 7. concrete foundation 8. brick foundation

Bracing and details:

9. unreinforced brick chimney 10. diagonal blocking

11. let-in brace (only in later vintages)

If this type of building has substantial plywood shear walls, it should be classified as light wood frame (WLF).

Steel Frame Buildings: Steel frame

buildings tend to be more satisfactory in their earthquake resistance compared to other structure types because of their strength, flexibility and lightness. Collapse in earthquakes has been very rare, although steel frame buildings did collapse,

for example, in the 1985 Mexico City

earthquake. In the earthquakes in the United States, these buildings have performed quite well, and probably will not collapse unless subjected to extremely severe ground-shaking. Possible damage includes:

Typical steel moment-resisting frame structures usually have similar bay widths in both the transverse and longitudinal direction,

around 7 to 10 m. The load-bearing frame

consists· of beams and columns distributed throughout the building. The floor diaphragms are usually concrete, sometimes over steel decking. Moment-resisting frame strucwres built since 1950 tend to incorporate prefabricated panels hung onto the structural frame as the exterior finish. These panels may be precast concrete, stone or masonry veneer, metal, glass, or plastic. This structural type is used for commercial, institutional, and other public

buildings. It is seldom used for low-rise

residential buildings. 1.

2.

Non-strucwral damage to elements such as interior partitions, equipment, and

exterior cladding, resulting from

excessive deflections in frame structures; Cladding and exterior finish material can fall if insufficiently or incorrectly connected;

Old steel-frame strucwres are usually clad or infilled with unreinforced masonry, such as bricks, hollow clay tiles, and terra cotta tiles (see type SIW for a detailed discussion). Other frame buildings of this period are encased in concrete. Both wood and concrete floor or roof

diaphragms are common for these older

buildings.

3. Permanent displacements caused by

plastic deformation of structural

members;

4. Pounding with adjacent structures can

occur.

Steel Moment Frame (SMF): This type

has a frame of steel columns and beams (see

Figure 2-3). In some cases, the beam-column

connections have very small moment-resisting capacity; in other cases, some of the beams and columns are fully developed as moment frames to resist lateral forces. Usually the structure is concealed on the outside by exterior walls, which can be of almost any material: curtain walls, brick masonry, or precast-concrete panels; and on the inside, covered by ceilings and column furring.

Steel Braced Frame (SBF): Braced

steel-frame structures (Figure 2-4) have been built since the late 1800s with similar usageand exterior finish as the steel moment frame buildings. Braced frames are sometimes used for long and narrow buildings because of their stiffness. Although these buildings are braced with diagonal members, the bracing members usually cannot be detected from the building exterior.

In concentric braced frames, the lateral forces or loads are resisted by the tensile and compressive strength of the bracing (see Figure 2-4). A recent development in seismic bracing

is the eccentric brace. Here the bracing is

slightly offset from the main beam-to-column connection, and the short section of beam is expected to deform significantly under major

seismic forces and thereby dissipate a

Steel Light Frame (SLF): Most

light--frame buildings existing today were built after

1950 (Figure 2-5). They are used for

agricultural structures, industrial factories, and

warehouses. TIley are typically one storey in

height, sometimes without interior columns, and

often enclose a large floor area. Construction is typically of steel frames spanning the short dimension of the building and resisting lateral

forces as moment frames. Forces in the long

direction are usually resisted by diagonal steel--rod bracing. These buildings are usually clad with lightweight siding.

Because these buildings are low-rise, lightweight, and constructed of steel members,

they usually perform relatively well in

earthquakes. Collapses do not usually occur.

Some typical problems:

1. Insufficient capacity of tension braces

can lead to their elongation and, inturn,

to building damage;

2. Inadequate connection to the foundation

can allow the building columns to slide;

3. Loss of cladding can occur.

1. Shear cracking and distress can occur

around openings in concrete shear walls;

2. Wall construction joints can be weak

planes, resulting in wall shear failure below expected capacity;

3. Insufficient lap lengths in vertical

reinforcing steel can lead to wall

bending failures.

Steel Frame with Innll Masonry Shear

Walls (SIW): This constructiontype(Figure

2-7) consists of a steel structural frame and walls

infilled with unreinforced masonry (URM). In

older buildings, the diaphragms are often wood.

More recent buildings have reinforced concrete

floors. Because of the masonry infill, the

structure tends to be very stiff. In major

earthquakes, the inf111 walls may suffer

substantial cracking and deterioration, thus

reducing their stiffness. This, in turn, puts

additional demands on the frame. Some of the walls may fail while others remain intact, which may result in torsion or soft-storey problems. The hazard from falling masonry is significant as

these buildings can be taller than 20 storeys.

Typical damage: Steel Frame with Concrete Shear

Walls (Sew): The construction of this

structural type (Figure 2-6) is similar to that of the steel moment-resisting frame in that a matrix of steel columns and girders is distributed

throughout the structure. The joints, however,

are not designed for moment resistance, and the lateral forces are resisted by concrete shear walls. The shear walls can be part of the elevator/service core, exterior walls or interior walls. This type of structure performs as well in

earthquakes as other steel buildings. Some

typical types of damage:

1.

2.

3.

1nf111 walls tend to buckle and fall

out-of-plane when subjected to strong lateral forces. Because the infill walls are

non-load-bearing, they tend to be thin

(around 23 cm or 9 in. maximum) and

do not have additional shear strength

because of compression from above. Veneer masonry around columns or beams is usually poorly anchored to the structural members and can disengage and fall.

Interior infill partitions and other non-structural elements can be severely damaged and collapse.

CORRUGATED METAL SKIN: LIGHTWEIGHT PURLINS TYP.

DIAPHRAGM TIE-ROD BRACING TRANSVERSE STEEL MOMENT-RESISTANT FRAMES LONGITUDINAL TIE-ROD BRACING TYP.

Roof/floor span systems:

1. steel framing with concrete colier 2. wood floor joist and diaphragm

(diagonal and straight)

6

Details:

5. unreinforced and unbraced parapet and cornice 6. solid party walls

Wall systems:

3. non·load·bearing concrete wall 4. non-load-bearing unreinforced

masonry cover wall

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Openings and wall penetrations: 7. window penetrated front facade 8. large openings of street level shops

1. Large tie spacings in columns can lead to a lack of concrete confinement or shear failure.

2. Placement of inadequate rebar splices at

the same location

can

failure.

3. Insufficient shear strength in columns

can lead to shear failure prior to the development of moment hinge capacity. Concrete Moment Frame (CMF): Two construction subtypes fall under this category:

(a) non-ductile reinforced-concrete frames

without reinforced infill walls, and (b) ductile

reinforced-eoncrete frames. TIle most prevalent

of these are non-ductile reinforced concrete frame structures without reinforced infill walls built between about 1950 and 1972 (Figure 2-8). In many regions of Canada, this type of construction continues to the present. This group

includes large multi-storey commercial,

institutional,andresidential buildings constructed

using flat-slab frames, waffle-slab frames, and the standard girder-eolumn-type frames. These

structures generally are more massivethan

steel-frame buildings, are under-reinforced (i.e., have insufficient reinforcing steel embedded in the

concrete) and display low ductility. Some

typical problems:

Lack of continuous beam reinforcement can result in hinge formation during load reversal.

Insufficient shear tie anchorage can lead to premature brittle failure in shear or compression.

The relatively low stiffness of the frame can lead to substantial non-structural damage.

Inadequate reinforcing of beam-column joints or location of beam bar splices at columns can lead to failures.

6.

5.

7.

8. Pounding damage with adjacent

buildings can occur.

Concrete Shear Wall (CSW): This

category consists of buildings with a concrete wall structural system or frame structures with

shear walls (Figure 2-9). TIle entire structure,

along with the usual concrete diaphragm, is typically cast-in-place. Such "box" systems were often used in schools, churches and industrial buildings. Frame buildings with shear walls tend

to be commercial and industrial. A common

example of the latter type is a warehouse with

perimeter concrete walls.

This building type generally tends to

Recently built concrete moment frames are required to have special reinforcing details in order to achieve satisfactory ductility. This has been required in the high seismic zones in Canada since the mid-1970s.

Concrete shear-wall buildingsconstructed

since the early 19508 tend to'be institutional,

commercial, and residential buildings, ranging

from one to more thanthirtystoreys. Residential

buildings of thistype are often mid-rise towers.

The shear walls in these newer buildings canbe

located along the perimeter, as interior partitions, or around the service core.

Soft storey, where infill walls exist in the upper storeys but not at the ground floor. The difference in stiffness creates a large demand at the ground floor columns, causing structural damage. When the earthquake forces are very

high, the steel frame itself can fail

locally. Connections between members are usually not designed for high lateral loads (except in tall buildings) and can

lead to damage. Although complete

collapse has seldom occurred, it cannot be ruled out.

4.

Root/floor diaphragms: 1. concrete waHle slab 2. concrete joist and slab

.3. steel decking with concrete topping

Curtain walV non-structural intill: 4. masonry in/illwalls

5. stone panels 6. metal skin panels 7. glass panels

8. precast concrete panels

Structural system:

9. distributed concrete frame

Details:

10. typical tall first floor (soft story)

Roof/floor span systems: 1. heavy timber rafter roof 2. concrete joist and slab 3. concrete flat slab

1

Wall system:

4. interior and exterior concrete

bearingwaJ1s

5. largewindowpenetrations of school and hosphal buildings

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perform better than concrete frame buildings. They are quite heavy relative to steel frame buildings, but they are also stiff due to the presence of shear walls. Some types of damage commonly observed in taller buildings are caused by vertical discontinuities, pounding, or irregular

configuration. Other damage specific to this

building type:

1. Shear cracking and distress can occur

around openings in concrete shear walls . during large seismic events.

height. Beams are often Ts and double Ts, or

rectangular sections. Pre-stressing of the

members, including pre-tensioning and

post-tensioning, is often used.

The earthquake performance of this structural type varies greatly and is sometimes poor. This type of building can perform well if the details used to connect the structural elements have sufficient strength and ductility (toughness). Some of the problem areas specific to precast frames:

Concrete Frame with 10011 Shear

WaIls (CIW): This category consists of

buildings with a concrete frame with

unreinforced-masonry or reinforced-masonry

infill walls (Figure 2-10). These buildings tend to have larger members, although the amount and detailing of reinforcement is more in question. These concrete frames have been used for commercial as well as industrial structures, and

are generally more than three storeys tall. The

hazards of these older buildings are similar to and perhaps more severe than those of the newer

frames. Where unreinforced-masonry (URM)

infill is present, a falling hazard exists. The

failure mechanism of URM infill in a concrete frame is the same as the steel frame infill.

5. Connections at bases of precast columns

and wall panels may be inadequate.

4. Corrosion of metal connectors between

prefabricated elements can occur.

Structures of this type which employ cast-in-place concrete shear wall for lateral load resistance should be treated as concrete shear walls (CSW).

Poorly designed connections between prefabricated elements can fail.

Accumulated stresses or gaps can result because of shrinkage and creep.

1.

3. Loss of vertical support can occur due to

inadequate bearing area or insufficient connection between floor diaphragm

elements, beams, wall panels, and

columns.

2.

Shear failure can occur at wall

construction joints usually at a load level below the expected capacity.

Bending failures can result from

insufficient chord steel lap lengths. 2.

3.

Precast-Concrete Frames (PCF):

Precast concrete-frame construction, first

developed in the 19308, was not widely used

until the 19608. Theprecast frame (Figure 2-11)

is essentially a post-and-beam systeminconcrete

where columns, beams, and slabs are

prefabricated and assembled on site. Various

types of members are used. Vertical

load-carrying elements may be Ts, cross shapes, or arches and are often more than one storey in

PrecastConcrete Walls (PCW): Most

of these use the tilt-up construction method. In

traditional tilt-up buildings' (Figure 2-12),

concrete wall panels are cast on the ground and

then tilted upward into theirfmalposition. More

recently, wall panels are fabricated off site and trucked in. The wall panels are welded together

or heldinplace by cast-in-place columns or steel

columns, depending on the region. The floor

and roof beams are often glue-laminated wood or steel open-webbed joists attached to the

UNREINFORCED-MASONRY INFILLWALL ｾＬＬｾ :.ｾ

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Roof/floor span systems: 1. structural concrete T-sections 2. structural double T-sections 3. hollow-core concrete slab

Wall systems:

4. load-bearing frame components (cross) 5. muhi-story load-bearing panels

Curtain wall system: 6. precast concrete panels 7. metal, glass, or stone panels

Structural system:

8. precast column and beams

Rooflfloor span systems: 1. glue Iiminated beam and joist

2. woodtruss 3. light steel -web joist

Rooflfloor diaphragms: 4. plywood sheathing

7

Details:

5. anchor-bolted wooden ledger for roof/floor support

Wall systems:

6. cast-in-place columns--square, T-shape, and H-shape

7. welded-steel-plate-type panel connection

Figure 2-12: Precast Concrete Walls (PCW) (Tilt-up

construction may incorporate steel frame with precast wall systems)

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tilt-up wall panels; these panels may be load-bearing or non-load-load-bearing depending on the

region. These buildings tend to be low-rise

industrial or office buildings.

Many tilt-up buildings do not have

sufficiently strong connections or anchors

between the walls and the roof and floor

diaphragms. During an earthquake, weak

anchors pullout of the walls, causing the floors or roofs to collapse. The connections between

concrete panels are also vulnerable to failure.

Without these connections, the building loses much of its lateral-force-resisting capacity. For these reasons, many tilt-up buildings were damaged in the 1971 San Fernando earthquake. Since 1975, tilt-up construction practices have changed in seismic areas of Canada, requiring positive wall-diaphragm connection. However, a large number of these older, pre-I97Os vintage tilt-up buildings still exist and have not been retrofitted to correct this wall-anchor defect.

Damagetothese buildings was observed again in

the 1987 Whittier, California, earthquake. These buildings are a prime source of seismic hazards.

In areas of low or moderate seismicity,

inadequate wall anchor details continue to be

employed. Severe ground-shaking in such an

area may produce major damage in many tilt-up buildings.

Reinforced-Masonry Bearing Walls

with Wood or Metal Deck: Diaphragms

(RML): Reinforced-masonry buildings are

mostly low-rise perimeter bearing-wall structures, often with wood diaphragms, although steel deck is sometimes used Floor and roof assemblies usually consist of timber joists and beams,

glue-laminated beams or light steel joists. The

bearing walls consist of grouted and reinforced hollow or solid masonry units. Interior supports, if any, are often wood or steel columns, wood-stud frames or masonry walls. The occupancy of

this building type varies greatly from small

commercial buildings to residential and industrial

buildings. Generally, they are less than five

storeys in height, although many mid-rise

masonry buildings exist.

Reinforced-masonry buildings can

perform well in moderate earthquakes, if they are adequately reinforced and grouted and if sufficient diaphragm anchorage exists. A major problem is control of the workmanship during

construction. Poor construction practice can

result in ungrouted and unreinforced walls. These conditions led to several collapses in the

1964 Alaska earthquake. Where construction

practice is adequate, insufficient reinforcement can be responsible for heavy damage of the

walls. The lack of positive connection of the

floor/roof diaphragm to the wall is also a problem.

Reinforced-Masonry Bearing Walls

With Concrete Diaphragms (RMC):

Reinforced-masonry buildings also have concrete diaphragms (floor and roof construction). The

rest of the information is similar to that

presented for RML. The poor anchorage and

connections of precast concrete diaphragms can be responsible for earthquake-related damage.

Unreinforced-Masonry Bearing-Wall

ｂｵｩｬ､ｩｮｾ (URM): Most unreinforced-masonry

(URM) bearing-wall structures in western

Canada and Quebec were built before the 19408 (Figures 2-13 a,b,c), although this construction

type was built in some jurisdictions having

moderate or high seismicity until the late 19408

or late 19508. These buildings usually range

from one to six storeys in height and typically

function as commercial, residential, and

industrial buildings. The construction varies

according to the type of use, although wood

floor and roof diaphragms are 'common in older

buildings. Smaller commercial and residential

buildings usually have light wood floor/roof joists supported on the typical perimeter URM wall and interior load-bearing wood partitions. Larger buildings, such as industrial warehouses, have heavier floors and interior columns, usually of wood. The bearing walls of these industrial buildings tend to be thick, often as much as

Roof/floor span systems:

1. wood post and beam (heavy timber)

2. wood post, beam, and joist (mill construction) 3. wood truss-- pitch and curve

Rooflfloor diaphragms: 4. diagonal sheathing 5. straight sheathing 3 1

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6. typical unbraced parapet andcomice

7. flat arch window opennings

Wall systems:

8. bearing wall-four or more wythes ofbrick 9. typical long solidpartywaD

Root/floor span systems:

1. wood post and beam (heavy timber)

2. wood post. beam, and joist (mill construction) 3. wood truss-- pitch and curve

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9. bearing wall- four to eight wythes of brick Details:

6. typical unbraced parapet and cornice 7. flat arch window opennings

8. small window penetrations(ifbldgisoriginally a warehouse)

Rooflfloor span systems:

1. wood post and beam (heavy timber)

2. wood post. beam, and joist (mill construction)

3

7

Details:

5. typical unbraced parapet and cornice 6. flat arch window opennings

7. typical penetrated facade of residential buildings 8. large opennings of ground floor shops

Rooflfloor diaphragms:

3. diagonal sheathing 4. straight sheathing

WaHsystems:

9. bearing wall- four to eight wythes of brick

10. typicallong sofldpartywall

11. UghtlVentilation wells In residential bldg 12. non-structuralwoodstudpartition walls