Commentary on Part 9 National Building Code of Canada: 1990

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Commentary on Part 9

National Building Code

of Canada

1990

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NRCC 37818 I RC-P-3664 NR36-1 /1990-9-1 E ISBN 0-660-15427-7

Ottawa, Canada

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Preface

This is the first edition of a Commentary on Part 9, Housing and Small Buildings, of the National Building Code of Canada. Although many people have made inputs to the content of this

Commentary, it is primarily the work of Mr. A.T. Hansen, P. Eng. From 1965 to 1977, Mr. Hansen was Research Advisor on Residential Standards and from 1977 to 1986 was Technical Advisor to the Standing Committee on Housing and Small Buildings, the committee responsible for the techni-cal content of Part 9. He was therefore involved in the development of many of Part 9' s current require-ments, in many cases conducting the background research and drafting the requirements for Standing Committee approval. Because of the long period over which these requirements have been devel-oped, this Commentary is necessarily based to a large extent on Mr. Hansen's recollections of the Standing Committee's thinking and rationale when these requirements were discussed. Indeed, it is

Commentary on Part 9

doubtful if this Commentary could have been pre-pared if this resource had not been available. This Commentary has been reviewed by the current members of the Standing Committee on Housing and Small Buildings and the current Technical and Research Advisors to the Standing Committee. Comments on this document are welcome and should be sent to

-Secretary

Canadian Commission on Building and Fire Codes

Canadian Codes Centre

Institute for Research in Construction National Research Council of Canada Building M-24 Montreal Road Ottawa, Ontario K1A OR6 1 Copyright © NRC 1941 - 2019 World Rights Reserved © CNRC 1941-2019 Droits réservés pour tous pays

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Commentary on Part 9

Introduction Section 9.1 Section 9.3 Section 9.4 Section 9.5 Section 9.6 Section 9.7 Section 9.8 Section 9.9 Section 9.10 Section 9.11 Section 9.12 Section 9.13 Section 9.14 Section 9.15 ••••..••••...••.•••.•••. 5 General •••••••••••••••••••••••••••• 11 Materials, Systems and Equipment •••••••••••••••• 17 Structural Requirements 23 Room and Space

Dimensions •••••••••••••••••••••• 27 Doors ••••••••••••••••••••••••••••••• 31 Windows and Skylights ••• 35 Stairs, Ramps,

Handrails and Guards ••••• 39 Means of Egress •••••••••••••• 47 Fire Protection ... 57 Sound Control ... 91 Excavation •••••••••••••••••••••• 95 Waterproofing and Dampproofing ... 99 Drainage •••••••••••••••••••••••• 1 03 Footings and Foundations •••••••••••••••••• 105 Section 9.16 Siabs-on-Ground •••••••••••• 117 Section 9.17 Columns ... 119 Section 9.18 Crawl Spaces •••••••••••••••• 123 Section 9.19 Roof Spaces •••••••••••••••••• 127 Section 9.20 Above Grade Masonry ••• 131 Section 9.21 Chimneys and Flues •••••• 145 Section 9.22 Fireplaces ... 149 Section 9.23 Section 9.24 Section 9.25 Section 9.26 Section 9.27 Section 9.28 Section 9.29 Section 9.30 Section 9.31 Section 9.32 Section 9.33 Section 9.34 Section 9.35 Appendix A Wood-Frame Constnaction ... 153 Sheet Steel Stud

Wall Framing ••••••••••••••••• 181 Thermal Insulation and Control of Condensation •••••••••••••••• 183 Roofing •••••••••••••••••••••••••• 189 Siding •••••••••••••••••••••••••••• 201 Stucco ••••••••••••••••••••••••••• 209 Interior Wall and

Ceiling Finishes ••••••••••••• 211 Flooring ••••••••••••••••••••••••• 215 Plumbing Facilities ••••••• 219 Ventilation ••••••••••••••••••••• 223 Heating and Air-Conditioning •••••••••••• 229 Electrical Facilities ... 233 Garages and Carports ••• 235 Design Assumptions

for Span Tables ... 237

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Commentary on Part 9

Introduction

This commentary has been prepared to help users of the National Building Code (NBC) have a better understanding of the requirements in Part 9, Housing and Small Buildings. Knowing the objec-tives of the requirements, and the general principles on which they are based, should assist the user in applying them.

This commentary has no legal status and is not intended for formal adoption. Its purpose is solely informational. Sketches and diagrams illustrate the principles only. Other methods of satisfying the intent of the requirements may be equally valid. The primary objectives of Part 9 are to provide rea-sonable levels of health, fire safety and structural sufficiency in buildings. It applies to new buildings, and the demolition, relocation, alteration or change in use of existing buildings. The requirements nec-essary to achieve these goals are complex, and can present the Code user with a formidable challenge. Like all regulations, Part 9 is intended to be applied in a reasonable manner, taking into account the nature and objectives of the requirements. Sound judgement and common sense are therefore essential tools in applying Part 9. This is especially important in alterations to existing buildings, where new con-struction interfaces with old.

Part 9 is much more detailed than other Parts of the Code. This is partly due to the manner in which the requirements originated. A great number of specific construction details cover most of the operations that occur in building construction. Individual requirements, however, vary in contributing to the overall objectives of providing for the health and safety of occupants in buildings. Judgement is therefore required in determining the degree to

which departures from individual requirements should be permitted without affecting the health and safety objectives.

Historical Development

Part 9 is like no other Part of the Code. To some extent, it can be considered a code within a code. This is a concept unique to Canada and is the result of the evolution of the NBC down through the years. Before Part 9 existed as a separate part of the NBC, Central Mortgage and Housing Corporation (now Canada Mortgage and Housing Corporation [CMHCD issued its own standards for housing, and continued to do so until 1958, when the Corporation relinquished this role to the National Research Council (NRC) and the then Associate Committee on the National Building Code (ACNBC) - recently reconstituted as the Canadian Commission on Building and Fire Codes (CCBFC). Until that time, CMHC Building Standards co-existed with the National Building Code as separate and distinct requirements, even though there were similarities between the two. Subsequent to ACNBC involve-ment, the CMHC requirements were melded with those of the National Building Code, and the result-ing requirements reflected both sets of standards. Many of the detailed specification type requirements in Part 9 can be traced back to CMHC requirements, and this helps to explain the unique style of Part 9. In combining the NBC and CMHC requirements, the different objectives of the two were recognized. Although both shared a common goal of providing for health and safety in buildings, CMHC, as a guar-antor of mortgages, was also concerned with the resale value of the buildings. Many CMHC

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~ents went beyond normal building code provi-SIOns for health and safety and regula ted amenities as well. In combining the requirements for Code purposes, the health and safety implications of each had to be evaluated. Those intended for health and safety purposes were eventually included in the NBC, and formed the basis for Residential

Standards, which was issued for CMHC purposes. The latter Standards also included non-code require-ments, which were distinguished from the Code requirements by a lighter type face. The division of the requireme~ts into Code and non-Code types was somewhat arbItrary. A number of requirements that had only an indirect or tenuous relationship with health and safety were included as Building Code requirements. It was considered prudent in such cases to err on the side of caution. This is reflected in the current requirements, and partly accounts for the amount of detail included in Part 9.

Soon after the initial publication of the NBC (1941)

it became evident that many municipalities lacked' the expertise to enforce its complex requirements. Smaller municipalities were chiefly concerned with small siI?ple buildings such as houses, and require-ments dIrected at larger complex buildings had little relevance to them. To accommodate these munici-palities, therefore, a separate, abridged form of the

~BC was publ~sh~d. By restricting its scope to rela-tIvely small buIldIngs and to less critical types of occupancies, the abridged or "Short Form" of the NBC was made much briefer and simpler than the unabridged form.

~~en the CMHC requirements became the responsi-bIlIty of the ACNBC, and were combined with those of the National Building Code, both a new edition of the Short Form (1965), and the first edition of the Residential Standards (produced for the benefit of CMHC) were published. The requirements in the Short Form were eventually included in the NBC as Part 9, Housing and Small Buildings (1970), and the Short Form was no longer published as a separate document. Part 9, in effect, replaced the Short Form and became the code within a code. The Residential Standards, however, continued to be published sep-arately as a service to CMHC until 1980.

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Recent Trends

Since many of the requirements in the Short Form were abridgements of those in the NBC, Part 9 dupli-cated some requirements from other parts.

As users of Part 9 became more familiar with other

pa~ts of the Code, however, the need for such dupli-catIon was reduced. The trend in recent editions has been to delete these duplications from Part 9 and replace them with cross references to similar

require-ment~ found elsewhere in the Code. Requirements affectmg only a small portion of the buildings within the scope of. P~rt 9, and intended primarily for larger, complex buIldmgs, have been gradually deleted in favour of cross references. Requirements for non-combustible construction, fire walls, atria and sprin-kler systems, for example, are no longer found in Part 9, having been replaced by appropriate refer-ences to Part 3.

Part 9 is currently considered to be primarily a rule-of-thumb document, intended for use without the need

fo~ p~ofessional as~istance. Where engineering design prInCIples are reqUIred to design a member, for exam-ple, the user is directed to the appropriate require-ments of Part 4. Requirerequire-ments for most structural loads have been removed from Part 9, and replaced by suitable references to Part 4. (In a few cases structural design requirements have been retained where the provisions in Part 4 are considered inappropriate for the simple buildings covered by Part 9.)

For somewhat similar reasons, most of the central heating and cooling requirements have been relocat-ed t.o Part 6, which deals with building services. The

desI~n of modern heating and cooling systems reqUIres expert knowledge, and their installations rely on complex standards. Requirements for such

~ystems were considered to be no longer appropriate m Part 9, and were moved to Part 6 in 1985.

However, this change has been criticized by Code users as reducing the ability of Part 9 to follow the "house as a system" approach and is currently being reconsidered.

Whi~e this current trend in reorganization may be a

partI~1 reversal of the original goal of having all reqUIrements for small buildings grouped together in a separate code (or code within a code), it represents

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a compromise. The inconvenience of having the requirements for small buildings distributed throughout the Code has to be weighed against the disadvantages of duplication, and with it the possi-bility of error and double standards.

Referenced Standards

Part 9 references 150 other codes and standards pub-lished by various standards writing organizations. These have a variety of objectives; some provide detailed installation instructions for materials, equipment or services; others define test methods, and still others describe the characteristics of prod-ucts ranging from elementary building materials to complex systems. Standards vary from simple one-or two-page statements to voluminous codes of practices that may rival the NBC in complexity. Some standards are more important than others in contributing to health and safety. To ensure compli-ance with standards that are considered to be partic-ularly critical, an administrative official may require proof of compliance before the product is permitted to be used. The official may require the product to be tested and certified by an independent third party, and to be identified by a label or stamp, even though this may not be required by the Code. Fire alarm and detection devices, prefabricated chim-neys, fire doors, heating, electrical and plumbing equipment are products for which independent cer-tification and labelling are the most practical means for a building official to ensure that a product plies with the prescribed standard, since such com-pliance cannot be determined by visual inspection. This certification and labelling is not required by the NBC; it is simply a convenient administrative arrangement between the building permit applicant and the building official.

In some cases, Part 9 requires products such as struc-tural plywood, waferboard, lumber and solid core wood doors to be identified by a label or stamp. The majority of materials, however, are not so identified, and the enforcing official (or the purchaser) usually relies on the integrity of the manufacturer. In many cases, manufacturers conform to national standards as a matter of policy, either to protect their public image, or to guard against legal challenges from building authorities or from purchasers. Should a

CommentlJlY

on

P'" 9

failure occur as a result of a non-conforming prod-uct, legal steps can be taken to have the situation corrected, and this also acts as a deterrent against non-conforming products. In the majority of cases, therefore, self-policing by manufacturers is responsi-ble for compliance to standards, where the product is not certified or identified by a label or stamp. The objectives of the NBC in terms of minimum standards for health and safety may differ slightly from those in referenced standards. This has led to differences in a limited number of cases between specific requirements in the NBC and those in the referenced standards. In most cases these differ-ences are intentional. For this reason, the require-ments in the NBC take precedence over those in referenced standards, where such differences exist. Normally the NBC does not duplicate requirements in a referenced standard unless there is a valid rea-son for doing so.

Alternatives

The requirements described in Part 9 may represent only one solution to a design problem. When a designer wishes to create an innovative design, how-ever, situations may arise that were unforeseen when the requirements were originally written. Commonly, more than one solution is possible to achieve levels of health and safety comparable to those implied in Part 9.

The evaluation of such alternative solutions is prob-ably one of the most difficult problems faced by the Code user, since it requires an appreciation of the intent of the original requirement, as well as a knowledge of the effectiveness of the proposed alter-native. Again, good judgement and common sense are the most effective tools available to Code users in making such determinations.

Administrators and designers alike should also make use of the experience of others. Consultations with more experienced Code officials can assist in assessing alternative proposals. Provincial code authorities in several provinces maintain competent staff who can provide valuable advice. Larger cities with experienced building department staff can also be a valuable source of information and advice for smaller communities without such resources.

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Arrangement

Part 9 is divided into 35 separate sections. The first eleven are of general application or relate to struc-tural design or use and occupancy, such as egress and fire safety. The 19 sections that follow relate to construction practices and, in general, follow a sequence similar to actual construction, starting with e.xcavation and ending with interior finishing opera-tions. An additional four sections relating to mechanical and electrical services, and a section on garages and carports for dwellings, complete the requirements.

The requirements in Part 9 can be divided into four basic types which relate to other parts of the Code. These can be considered "Use and Occupancy" requirements such as those in Part 3, "Building Structure" requirements that perform a similar role to Part 4, "Building Envelope" requirements that serve a similar function to Part 5, and "Building Services" requirements that are related to Parts 6 and 7. This breakdown is shown in Figure 1.

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Commentary on Part 9

Figure 1. Part 9 contents

Section

Use

and Building Building Building

Occupancy

Structure Envelope Services

9.1 9.2

9.3 Materials, _equipmentsystems &

9.4 Structural Design

9.5 Room & space

dimensions 9.6 Doors 9.7 Windows 9.8 Stairs, ramps, handrails, guards 9.9 Means of egress 9.10 Fire protection 9.11 Sound control 9.12 Excavation

9.13 Watefproofing _dampproofing&

9.14 Drainage 9.15 Footings & foundatiOns 9.16 Slabs on ground 9.17 Columns 9.18 Crawlspaces 9.19 Roofspacas

9.20 Above ground masonry

9.21 Chimneys & flues

9.22 Flr~

9.23 Wood frame _{constn.action}

9.24 Sheet steel stud

construction

9.25 Thermal _oontroI insulation &

of condensation

9.26 Roofing

9.27 SidIng

9.28 Stucco

9.29 Interior watt &

ceiling finishes

9.30 Flooring

9.31 Plumbing facilities

9.32 Ventilation

9.33

Heat"'

&

air-conditioning

9.34 Electricat facilities

9.35 Garages & carports

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Section 9.1

General

Part 9 applies to buildings up to 600 m2 _{in building}

area and not more than three storeys in building height, provided they are not classified as assembly, institutional, or high-hazard industrial buildings (Table 2). Those which contain such critical occu-pancies, or which exceed these size limits, require special fire and structural safety features and are beyond the scope of the simplified requirements in Part 9. These buildings are regulated by the require-ments in other parts of the Code.

Part 9 must be used in conjunction with Parts 1,2,7 and 8, which apply to all buildings regardless of size or occupancy. Part 1, Scope and Definitions, lists the defined terms that are used throughout the Code; these are distinguished by the use of italics. They have special meanings in the context of the Code that go beyond or are more specific than normal dic-tionary definitions. Part 1 also lists all of the abbre-viations used throughout the Code.

Part 2, General Requirements, includes quasi-admin-istrative requirements, such as the application of each part of the Code, information required to be shown as plans and specifications, and assessment of alternate materials or systems, and references cli-mate information for design purposes. A complete list of all standards referenced throughout the Code is also included.

Part 7, Plumbing Services, serves principally as a ref-erence to the Canadian Plumbing Code.(S) The latter provides detailed requirements for the installation of plumbing equipment, including drainage, venting and water systems.

Part 8, Safety Measures at Construction and Demoli-tion Sites, protects the general public during the con-struction or demolition of a building. This includes the use of fencing or barricades, protected walk-ways, waste chutes and waste disposal. Procedures to protect the public when operations are carried out

(5) Canadian Plumbing Code 1990, NRCC 30625, National

Research Council, Ottawa.

General

above the street or other public property are also included, along with measures to reduce the risk of fire during the construction and demolition stages. Many enforcement officials and designers refer to buildings within the scope of Part 9 as "Part 9 Buildings," while those regulated by other parts are generally referred to as "Part 3 Buildings" (or some-times Part 4 Buildings). However, this is a mislead-ing practice since buildmislead-ings which do not fall within the scope of Part 9 must satisfy the requirements of all other parts of the Code, not just those in Part 3. The general application of the various parts in rela-tion to size and occupancy is illustrated in Figure 2.

Occupancy Classification of

Buildings (9.10.2.)

Since Part 9 applies only to certain occupancies, one of the first things to be established in applying the Code to a building is the occupancy classification of the building. This is not only important in determin-ing whether a builddetermin-ing is within the scope of Part 9, but also for the proper application of many fire pro-tection and egress requirements.

A description of the various occupancies covered in Part 9 is provided in Table 1. A summary of the occupancies not covered in Part 9 is given in Table 2.

Figure 2. Application of parts

NON·PART9 BUILDINGS ali buildings not covered by Part 9 1. Scope and Definitions 2. General Requirements 3. Use and Occupancy 4. Structural Design 5. Wind, Water and

Vapour Protection 6. Heating, Ventilation and Air-conditioning 7. Plumbing Services 8. Safety Measures at Construction and Demolition Sites 9. Housing and Small Buildings PART 9 BUILDINGS up to 3 storeys, 600 m2 except: assembly, institutional, high hazard 11 Copyright © NRC 1941 - 2019 World Rights Reserved © CNRC 1941-2019 Droits réservés pour tous pays

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Table 1

Summary of occupancies covered in Part 9

Occupancy Designation Description of use Examples Residential GroupC Sleeping rooms Houses, Hotels,

for persons who are not Dormitories, Boarding or detained involuntarily or lodging houses, Motels, who do not need care or Apartments, Children's treatment custodial homes,

Convalescent homes* Business Group D Transaction of Banks, Barber shops, and Personal business or for Dental offices, Medical Services personal or offices, Offices, Tool

professional services renta1, Appliance service Mercantile Group E Display of Department stores,

merchandise or Mercantile stores, sale of retail goods Supermarkets, Shops Medium Group F, Making, repairing Warehouses,

Hazard Division 2 or storing goods Workshops,

Industrial or materials Salesrooms, Factories, Planing mills, Repair garages, Laboratories, Service stations Low Hazard Group F, Same as above but Creameries, Factories, Industrial Division 3 with low fire load La bora tories,

(combustible content Storage garages, less than 50 kg/m2) Salesrooms,

Warehouses, Storage rooms, Workshops

* Group C classification permitted only if the home is set up as a dwelling unit for not more than ten people, who must be ambulatory. Otherwise the building is classified as Group B (see Table 2).

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General

Table 2

Summary of occupancies not covered in Part 9

Occupancy Designation Description of use Examples

Assembly Group A Gatherings or Theatres, Auditoriums, meetings of people for Bowling alleys,

particular functions or Churches (or similar events, or for dining places of worship), and drinking Stadiums, Dance Halls,

Gymnasiums, Pubs, Restaurants, Schools, Swimming pools Institutional Group B Housing people for Jails, Prisons, Hospitals,

correctional or medical Nursing homes, purposes, or special Orphanages, care because of age Reformatories, or mental condition Convalescent homes High Hazard Group F, Making, repairing or Feed mills, Flour mills, Industrial Division 1 storing goods or Distilleries, Spray

materials with painting operations, highly flammable or Paint plants, Chemical explosive properties plants, Grain elevators Buildings are classified according to their "major

occupancy." This is defined as the principal use of a building. It is not unusual, however, for a building to contain a number of different occupancies that are ancillary or subsidiary to the principal occupancy.

cies was not regulated under Part 9 (Table 2), the entire building would be outside the scope of Part 9 and would be regulated elsewhere in the Code.

A store, for example, i~ a major occupancy classified as "mercantile" (Table 1). It may have an ancillary office area (which would be classified as business and personal services) and a storage area (which would be classed as industrial). These ancillary occupancies do not affect the major occupancy clas-sification of the building.

On the other hand, a building may be classified as having two or more "major occupancies" if the activ-ities in each are unrelated to the other. For example, a building may consist of a portion intended for office rental space (business and personal services) and a portion rented separately as apartment units (residential). Each activity is separate from the other. The building would then be classified for each major occupancy. If one of the major

occupan-In most small buildings the classification is straight-forward, and only one major occupancy is involved. In some cases, however, the distinction between a major occupancy and a subsidiary occupancy is not clear cut, and judgement is required. The ramifica-tions for fire safety must of course be weighed in making the decision. (Multiple major occupancies are also discussed in Section 9.10 of this

Commentary).

Building Size (9.10.4.,

Since Part 9 applies only to buildings up to three storeys in "building height" and not more than 600 m2 _{in ''building area," these size limits must be} determined to see if a building falls within the scope of Part 9. 13 Copyright © NRC 1941 - 2019 World Rights Reserved © CNRC 1941-2019 Droits réservés pour tous pays

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Building Height (1.1.3.2.)

The elevation of "grade" must first be established, since this is the datum from which the building height is measured. First, the average ground eleva-tion adjacent to each face of the building is deter-mined. "Grade" is defined as the lowest of these average levels (Figure 3).

The first storey, by definition, must have its floor level not more than two metres above "grade"; oth-erwise it is designated as the second storey. The first storey and the storeys above it determine whether a building is designated as one, two or three storeys in building height (Figure 4).

Figure 3. Determination of grade

Figure 4. First storey

E CI>

~

second storey first storey II I I ~::::::::::::::::::I

(a) building height - 2 storeys

~I

E

-grade N ....- level ... average ground elevation on each face assumed grade

third storey

second storey

first storey

west

(b) building height - 3 storeys

The designation of "grade" as the lowest average ground level can lead to anomalies in determining building height, particularly on sloping sites. A rela-tively long building with a series of vertical fire sep-arations (such as in row housing) could be

designated as 4 or more storeys in building height, even though the building may have, for example, only three storeys between any two vertical fire sep-arations. This could cause a significant upgrading of both fire protection and structural requirements even though, for all practical purposes, the building is only three storeys high (Figure 5).

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Figure 5. Buildings on sloping sites

"Grade" for each stepped portion

1 h fire separation at each stepped portion

To avoid this problem, residential buildings divided by vertical fire separations (1 h minimum), can have grade determined separately for each segment, pro-vided the distance from the street to the entrance door of each segregated portion is not more than 45 m. The separation has to be complete, however, extending through any basement or crawl space up to the underside of the roof deck.

Not all floor assemblies create storeys in the deter-mination of building height. Penthouses for enclos-ing service equipment or stairs, for example, are not counted as storeys.

If a mezzanine is essentially open to view so that a fire either on or below it would be apparent to all occupants on the floor level below, it could occupy up to 40% of the floor area on that storey and not be counted as a storey (Figure 6).

Figure 6. Open mezzanine

section view

no partitions } Visually unobstructed

~ T Not more than 1070 mm

,,'-no partitions Visually unobstructed

r

r-Not more than 1070 mm

')7'/AV,I,: 'Y~V#

Building would.be considered a 3 storey building if

the mezzanine is not more than 40% _{of the total floor}

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p

If, however, the conditions for visual openness are not met (either above or below the mezzanine) the maximum aggregate area is permitted to be only 10% of the floor area before it is considered an addi-tional storey (Figure 7).

Figure 7. Mezzanines enclosed or visually obscured

(a) enclosed top and bottorr (b) enclosed on bottom

up to 10"10 of floor area

floor area

\

(c) enclosed on top

The floor areas in Figures 6 and 7 are the total areas on the entire storey (excluding stair and service shafts) and not the area of the room only.

Sometimes "lofts" are used in dwelling units to cre-ate special design effects. These are also considered mezzanines. When used in row housing, they are obscured from other dwelling units by fire separa-tions between units. They would also be limited to 10% of the floor area or else be considered a storey. In this case, the limiting area is based on the aggre-gate area of mezzanines in the different units, and the floor area is for the entire building, not just an individual dwelling unit.

Building Area (1.1.3.1.,

Once the building height is known, the building area must also be determined to see if the building is within the scope of Part 9.

"Building area" by definition is the maximum area of anyone storey above grade. It is measured to the outside wall surfaces and, where the building is divided by a firewall, to the centreline of the fire-wall. Although the maximum building area usually occurs on the first storey, it can also occur on the sec-ond or third storey in some designs. In the building shown in Figure 5, it is the sum of the areas for the individual segments.

Most walls that separate one property from another are required to be firewalls (walls between semi-detached and row houses are the only exceptions to this rule). Firewalls are special kinds of fire separa-tions that have certain features to ensure their

stabil-Genera.

ity in fire (see 3.1.10.) so that, should the building on one side be destroyed, the firewall will remain. Firewalls may be voluntarily used by an owner to reduce the building area to take advantage of certain fire and structural concessions permitted for smaller buildings. They are frequently used to create build-ings of 600 m2 _{or less in order to be within the scope}

of Part 9. In row housing, for example, ordinary fire separations may be used between most units, but "firewalls" may be employed at 600 m2 _{intervals in} order to be within the scope of Part 9.

Sometimes several buildings are built over a com-mon basement used as a parking garage. This may occur with certain housing designs where separate clusters of low-rise housing units are built over a common parking garage (Figure 8).

Figure 8. Buildings separated from garage by a 2 h floor slab

A B

3·storey superstructures

Ordinarily, this would be considered a single build-ing, and the building area would be the area of the basement. This is generally a fairly large area, and the entire building would normally fall outside of Part 9. If the parking garage is separated from the building (or buildings) above it by a concrete slab providing adequate fire resistance (2 h or more), the buildings above the slab can be considered separate buildings. In Figure 8, building A would have its own building area and would be considered sepa-rate from building B. The garage portion, however, because of its large area, would probably be outside of the scope of Part 9. The separating slab can be considered equivalent to a firewall in creating sepa-rate buildings. The walls of the parking garage pro-jecting above the ground would also have to provide a 2 h fire separation and have no openings that might spread fire to the units above.

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Factory-built Houses

The National Building Code applies the same requirements to site-built ~n~ factory-bu~lt .houses. However, it can often be dIffIcult determmmg whether a factory-built house complies with these requirements once it has been delivered to its con-struction site because many of the wall, roof and floor assemblies are closed in and their components cannot be inspected. CSA Standard CAN / CSA ~

A277 "Procedure for Certification of Factory-BuIlt Houses" was developed to address this problem. It

describes a procedure whereby an independent cer-tification agency can review the quality cOJ;tr~1 pro-cedures of a housing factory and make perIOdIC, unannounced inspections of its products and thus, through suitable labelling, provide assurance to authorities at the final site that those components which cannot be inspected on site comply with the code indicated on the label. CSA A277 is not a building code, only a procedure for ce~tifying.co~

pliance of factory-built components WIth a buIldIng code or other standard. If a factory-built house bears a label of a creditable certification agency

indi-cating that compliance with the National Building Code has been certified using the A277 procedure, the accepting authority will have some assurance that the hidden components do not need to be inspected again on site.

On the other hand, portions of the CSA Z240 se~ies

of standards on mobile homes do resernble a buIld-ing code. These portions contain requirements in many areas where the NBC also has requirements and frequently the requirements are differ~nt.

Because it would be illogical to have two dIfferent sets of requirements for houses, one set w~ich

applies to site-built houses and one set whIch applies to factory-built houses, the NBC does not make reference to these portions of the Z240 stan-dards. Other portions of the Z240 standards deal with special requirements for mobile homes related to the fact that these houses must be moved over roads. The NBC does not have requirements in this area. Therefore, labelling which indicates that a fac-tory-built house complies with the Z240 standards can NOT be taken as an indication that the house complies with the NBC.

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Section 9.3

Materials, Systems and

Equipment

Most common materials, systems and equipment used in buildings are covered by standards prepared by various standards writing organizations. These are referenced throughout the different sections of Part 9. Section 9.3, however, was included in Part 9 to provide additional requirements for the three basic construction materials: wood, concrete and metal. These requirements apply generally to the succeeding sections and avoid the unnecessary repe-tition of similar requirements throughout Part 9. Additional requirements for these materials also appear in subsequent sections when they are specific to those sections.

Concrete (9.3.1.)

Although requirements for mixing and placing con-crete are covered in CSA Standard CAN3-A438, cer-tain basic requirements are repeated in this Section, along with a reference to this Standard. They are included to emphasize the essential requirements needed to produce good concrete. Section 9.3 requires that concrete be made with clean, well-graded aggregate, clean water and an appropriate amount of portland cement. The resulting strength, durability and resistance to water penetration depend on these constituents.

Cement, in combination with water, forms a paste that coats the individual aggregate particles, and binds them together in a solid mass when the cement reacts with the water (hydration). The resulting strength of the mixture depends on the amount of water in relation to the amount of cement (water-cement ratio). Theoretically, only a small amount of water is necessary to complete the hydra-tion of the cement. Water in excess of this amount will produce voids in the cement paste when it evap-orates and will reduce the concrete strength. The water-cement ratio, therefore, controls the final strength of the concrete. In designing concrete mixes, a sufficient quantity of water-cement paste must be provided to have a workable mix so that it

Afateria'sJ

Systems and

Equipment

will fill the forms without leaving voids or honey-combs, while maintaining the necessary

water-cement ratio to produce the required strength. To obtain the greatest strength from a given quantity of cement, the water must be reduced to the amount that will just provide a workable mix. (Workability is measured as the slump of the con-crete. Slump is determined by filling a standard cone-shaped form with concrete and measuring the amount of settlement or slump when the cone is removed).

An air-entraining agent is commonly added to con-crete to increase its slump or workability. This cre-ates minute bubbles in the concrete, making it more fluid. It also increases its resistance to damage from de-icing salts. Air-entraining agents must be con-trolled, however, since excessive amounts will reduce concrete quality.

Well-graded aggregate is necessary for strong, dense concrete. Insufficient fine material to fill the voids between the coarse aggregate will result in weaker concrete with less resistance to the passage of water. An excess of fine material, on the other hand, requires additional cement paste to bind the individ-ual aggregate particles together. Such concrete requires more cement than one with well-graded aggrega te to yield the same strength.

Concrete strength has been the traditional means by which concrete quality is measured. The strength values required in Part 9 are minimum levels, and they make no allowance for poor site practices such as over-watering the mix to increase the workability of the concrete. Concrete strength is measured by testing small concrete cylinders (150 mm dia.) in compression. These are prepared when the concrete is placed, and cured for the required time (usually 28 days), before being tested.

The hydration of concrete takes place over a relative-ly long period of time. Although the initial harden-ing (or set) takes place in 48 hours or less, dependharden-ing on the temperature, the chemical reaction continues for several months. If freezing takes place before the initial set occurs, the resulting strength can be seri-ously reduced. For this reason, protection is required during cold weather (defined as less than 1 DoC for three days). If concrete dries out before

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I

hydration is well advanced, weaker concrete will also result. The longer the concrete is kept moist, therefore, the higher will be its final strength. Normally about seven days of curing is recommend-ed. In most areas ready-mixed concrete is used in

preference to site-mixed concrete because of conve-nience and superior quality control that is provided.

If additional water is requested in the mix when it is delivered to the site, a lower quality of concrete will result. A table of concrete mixes is provided in Section 9.3 for those who mix on site.

Lumber (9.3.2.)

Lumber is a variable product, subject to many defects, including knots, decay, worm holes, split-ting, warping, twisting and cupping. Its serviceabil-ity will be affected by the severserviceabil-ity of such defects, depending on its end use. Lumber is therefore

sort-ed into grades which have different limits on such

defects.

The variability of defects between species and between regions, as well as the wide variation in the end use of lumber, has led to a complicated set of grading rules. In some uses, appearance may be an

important factor, while in others it may not. Bending strength may be critical in some instances,

while compressive strength may be the overriding consideration in others. The grading order must therefore take into account the intended end use of the product.

Prior to 1970, Canadian lumber associations pub-lished their own grading rules, which varied from association to association. In the latter part of the 1960s, the lumber industry began to move towards

uniformity in grading rules. Such uniformity was

needed to facilitate the marketing of lumber

throughout the USA and between the USA and Canada. The National Lumber Grades Authority (NLGA) was established, and in 1970 issued the Standard Grading Rules for Canada. While market pressures led to general industry acceptance of uni-form standards for framing lumber, a similar con-sensus was not reached for board lumber grades. Thus Table 9.3.2.A., Minimum Lumber Grades for Specific End Uses, lists three different grading rules for such material.

18

To identify board grades, the paragraph number of the NLGA rules under which the lumber is graded must be shown in the grade mark. Paragraph 113 is equivalent to Western Wood Products Association (WWP A) rules, and Paragraph 114 is equivalent to West Coast Lumber Inspection Bureau (WCLIB) rules. When the lumber is graded in accordance with WWPA or WCLIB rules, the grade mark will not contain a paragraph number.

Although grades for framing lumber are uniform throughout North America, different end use requirements have led to three different ways for grading it. "Stud" grade is intended for vertical framing members up to 140 mm wide. "Light fram-ing" grades (construction, standard, utility and economy designations) are intended for general light framing, provided the members are 89 mm or less in width (2 x 4 or less). "Structural Joist and Plank" grades (No.1, No.2 and No.3) are intended for hor-izontal members where bending strength and stiff-ness are prime considerations.

Most lumber in Canada is graded visually only. This is the basis for the grade designations in the end use table in Section 9.3. A professional grader visually evaluates each piece of lumber and stamps it with the appropriate grade designation.

Certification by the Canadian Lumber Standards (CLS) Administrative Board applies to the inspec-tion, grading and grade marking of lumber, includ-ing mill supervisory service, in accordance with CSA Standard 0141, "Softwood Lumber."

Grade marks denote the moisture content of lumber

at the time that the rough sawn lumber was planed or dressed to its finished size. liS-DRY" in the mark indicates the lumber was dressed at a moisture con-tent not exceeding 19%. "MC 15" indicates a mois-ture content not exceeding 15%. "S-GRN" in the

grade mark signifies that the lumber was dressed at a moisture content higher than 19% at a size to allow for natural shrinkage during seasoning.

Each mill or grader is assigned a permanent number. The point of origin of lumber is identified in the grade mark by use of a mill or grader number or by the mill name or abbreviation. The CLS certified agency under whose supervision the lumber was

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• Materia's, Systems and Equipment

Table 3

Species designations and abbreviations

Commercial Designation of Species or Species Combination

Abbreviation Permitted on Grade Stamps

Species Included

D Fir-L (N) Hem-Fir (N)

Douglas Fir, Western Larch Douglas Fir-Larch

Hemlock-Fir

Spruce-Pine-Fir S-P-F or Spruce-Pine-Fir

Western Hemlock, Amabilis Fir White Spruce, Engelmann Spruce, Black Spruce, Red Spruce, Lodgepole Pine, Jack Pine, Alpine Fir, Balsam Fir Northern Species North Species

grade marked is identified in the mark by the regis-tered symbol of the agency.

Although some species of lumber are marketed indi-vidually, they are generally grouped for marketing into the species combinations in Table 3. The maxi-mum allowable spans for these combinations are listed in the span tables for joists, rafters and beams. Since the allowable span for a commercial species combination is based on the weakest species in the combination, the use of the span is permitted for any individual species included in the combination. Lumber is also graded by machines that automati-cally flex each piece and stamp it with the appropri-ate allowable bending stress and stiffness

designation (liE" value). Some visual grading is also necessary to obtain better correlation between the machine-measured characteristics (which are mea-sured perpendicular to the width) and the actual direction of loading (parallel to the width). Most stress-graded lumber is used in roof and floor truss construction rather than for general framing. Part 9 does not contain span tables for machine stress-rated (MSR) lumber because of its relatively low usage. Span tables for MSR lumber are available from the Canadian Wood Council. (6)

(6) Machine Stress-Rated Lumber (including span tables), CWC

Datafile, WP-5, Canadian Wood Council, 55 Metcalfe Street, Ottawa KIP 6L5.

Any Canadian softwood covered by the NLGA Standard Grading Rules

The control of grading and grade stamps for MSR lumber is similar to that required for visually graded lumber. The information provided on the stamp is also similar except that the stamp contains the words "machine rated," and the stress and stiffness desig-nations appear on the stamp in place of the visual grade designations. Grade mark facsimiles for grad-ed lumber are shown in Figure 9.

Figure 9. Grade mark facsimiles for MSR lumber

Facsimile of Grade Mark Association

A.F.P.A~oo S·P·F Alberta Forest Products Association

MACHINE RATED S·DRY _{204-11710 Kingsway Avenue}

2100f 1.BE _{Edmonton, Alberta T5G OX5}

I

lmH

O S-P-F

I

Interior Lumber Manufacturers Association

S • DRY

203-2350 Hunter Road

00 MACHINE RATED

2400 f 2.0E Kelowna, British Columbia V1X 6C1

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Lumber Sizes

(9.3.2.6.,

Lumber sizes have traditionally been expressed in full inch sizes. At one time lumber was sawn to full inch sizes and was subsequently dressed to a slight-ly smaller size. The resulting undersized product continued to be referred to by the full inch size des-ignation, and this then became a nominal size. Over time, further size reductions occurred to obtain addi-tional"nominal size" members from each tree. The last general size reduction occurred in 1970.

The standard that regulates finished dimensions of lumber (CSA 0141) allows for the fact that lumber shrinks as it dries. For that reason, the grade stamp designates the condition of the wood at the time of dressing. When Canada converted to metric mea-surements, it became impractical to use nominal des-ignations for lumber, and precise metric size

designations were substituted. These were exact conversions of the 1970 inch sizes. Table 4 shows a comparison of the nominal inch designations and actual sizes both in inches and millimetres.

Table 4

Comparison of lumber sizes

Nominal Size Exact Dry Size Exact Dry Size (in.) (in.) (mm) 1 by2 3/4 by 11/2 19 by 38 2by3 11/2 by 2 1/2 38 by 64 2by4 11/2 by 3 1/2 38 by 89 2by6 11/2 by 5 1/2 38 by 140 2by8 11/2 by 7 1/4 38 by 184 2 by 10 11/2 by 9 1/4 38 by 235 2 by 12 1 1/2 by 11 3/4 38 by 286

The NLGA "Standard Grading Rules for Canadian Lumber" permit lumber to be dressed to sizes below these standard sizes, provided the grade stamp shows the reduced size. Undersized lumber that is not less than 95% of the corresponding standard size may be used under Part 9 without special engineer-ing analysis; however, the allowable spans in the span tables must be reduced by 5%.

Wood Shrinkage

When green lumber dries, no shrinkage takes place until the fibre saturation point is reached (about 30% moisture by weight). At this point, all the free water has evaporated and only the cell walls are saturated. As the wood continues to dry, it shrinks almost in direct proportion to the moisture loss. Eventually, it reaches a moisture level in equilibrium with the moisture in the surrounding air. In summer this is about 8 to 10% moisture content (MC) for most regions and in winter about 12 to 14%.

On average, therefore, wood in service has a mois-ture content of 10% in most regions. The maximum moisture content permitted at the time of installation is 19%. At this point about half of its normal shrink-age will have occurred. Additional drying and shrinkage will usually occur before the interior vapour barriers and finishes enclose the wood.

If lumber is installed with a moisture content in excess of its fibre saturation point, little or no shrink-age may take place before the wood is enclosed. When the wood eventually dries, wallboard cracks, nail pops, and floor squeaks may result. Where a significant amount of moisture is trapped between vapour-resisting materials, the wood may retain excess moisture long enough to allow decay to start.

Waferboard, Strandboard and

Plywood

Most waferboard, strandboard and plywood used in construction is made with waterproof adhesives, and will not deteriorate very rapidly when exposed to the weather. Occasionally, however, substandard material may find its way to the construction site. In some cases, imported material may be made with non-waterproof adhesives. Because of unfortunate past experiences from the use of such material, structural plywood, strandboard and waferboard is required to be face stamped so that it can be easily identified after it is installed. By identifying the pro-ducing mill, the standard to which it is produced, and the type of glue bond, compliance can be quick-ly determined, and should future problems develop, the material can be traced to the manufacturer.

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p

Strandboard is a relatively new product to Canada.

It differs from waferboard in that its fibres are ori-ented principally in the longitudinal direction, mak-ing it much stronger in this direction than in the transverse direction. Waferboard, on the other hand, has randomly oriented fibres, which gives it similar properties in both directions.

Because of the directional properties, strandboard is required to be marked to show its principal fibre ori-entation so that when it is applied as subflooring or roof sheathing, the strongest direction will be trans-verse to the supports. This is the same as required for plywood, which is also much stronger in the direction of the grain of the surface plies.

Materials,

Systems and Equipment

Metal (9.3.3.)

Metal thicknesses have customarily been specified in a variety of ways, depending on the type of metal and its intended use. Several different gauge num-ber systems have traditionally been used, each with its own manufacturing tolerance limits. Some met-als are met-also specified in terms of weight per unit area. To simplify designating metal thicknesses, the NBC uses exact minimum metric values that take into account the allowable negative manufacturing tolerances. Except in the case of galvanized studs, the metal thicknesses specified in Part 9 include the zinc coating.

Where galvanized sheet metal is specified in Part 9, it must meet the requirements for a "G 90"

designa-tion for galvanizing. This means that it must have at least 0.90 ounces of zinc per square foot of sheet (275 g/m2). This is the total amount of zinc on both faces and is determined under laboratory conditions by measuring the weight or mass of sample speci-mens before and after the zinc coating is removed by a suitable reagent. 21 Copyright © NRC 1941 - 2019 World Rights Reserved © CNRC 1941-2019 Droits réservés pour tous pays

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p

Section 9.4

Structural

Requirements

Structural requirements in Part 9 are presented in a different format from those in Part 4. Those in Part 4

require engineering analyses and evaluation to deter-mine compliance with the NBC. Those in Part 9, on the other hand, are presented as rule-of-thumb solu-tions, generally without the need for further calcula-tions or evaluacalcula-tions. This format can be of

considerable advantage since it spares the Code user the expense of professional assistance in the con-struction of a building. Such requirements, however, can also be restrictive in that only one design solu-tion is normally provided although others may be equally valid. For this reason Code users are given a choice between the rule-of-thumb requirements in Part 9 or the design approach in Part 4.

Most rule-of-thumb requirements have evolved over the years through trial and error. They are generally based on trade practices that experience has shown to

be satisfactory. Some, however, have developed as a result of engineering analyses and in effect are pre-engineered solutions. (The span tables for joists, rafters, trusses and beams are representative of the latter type). Since rule-of-thumb structural requirements are gen-erally based on experience with small residential buildings, they cannot be applied safely to all build-ings within the general scope of Part 9. Additional limits are therefore specified throughout Part 9 to restrict the application of the requirements to the size or type of building for which past experience (or in some cases, calculation) applies. When these limits are exceeded, the designer is required to use the design procedures in Part 4.

The design solutions found in Part 9 can be different from those determined by calculations in Part 4.

Rule-of-thumb practices that have evolved through trial and error are based on loads that have actually occurred. Conventional engineering calculations, on the other hand, tend to be conservative both in load assumptions and in allowable design stresses, and may not take into account the full effect of load

shar-Structural

Requirements

ing on overall strength. Such rule-of-thumb solu-tions therefore may be somewhat more lenient or less conservative than those determined by Part 4. On the other hand, where the requirements in Part 9 are based on pre-engineered solutions, they can be more conservative than those in Part 4. Some calcu-lations may be based on conservative assumptions in order for them to be applied to a wide range of conditions. Footing sizes, for example, are based on very conservative soil bearing values so that the footing sizes listed can be applied safely to a wide range of soils. By using more exact soil bearing val-ues for the specific soil conditions, the procedures in Part 4 could result in smaller footing sizes for certain types of soil than would be determined by the rule-of-thumb approach in Part 9.

Snow Loads (9.4.2.)

Design snow loads for roofs are determined from the maximum snow depths on the ground observed over a thirty year period, taking into account region-al variations in snow density. The roof snow load is calculated as 60% of the ground load for buildings over 4.3 m wide, and 50% for smaller buildings, such as typical mobile homes. To this is added an

allowance for rain that could be expected to occur, also once in thirty years. The design roof load is assumed to be uniform over the entire roof, with no allowance for drifting. The snow load factor reduc-tions are based on the premise that a greater portion of the snow is likely to blow off small roofs than larger roofs.

The ground snow load is listed separately from the rain load portion in Chapter 1 of the NBC

Supplement. This is a departure from previous edi-tions of the Supplement and was done to allow vari-ous drift factors to be applied to the snow load portion that would be inappropriate for the rain load portion. These drift load factors are not used for small buildings within the scope of Part 9 but have a significant effect on buildings covered in Part 4. While the design loads in Part 9 are less conservative than those required in Part 4, they are restricted to wood frame assemblies with closely spaced mem-bers (up to 600 mm o.c.) and relatively short spans (up to 12.2 m). 23 Copyright © NRC 1941 - 2019 World Rights Reserved © CNRC 1941-2019 Droits réservés pour tous pays

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Experience over the past 30 years has demonstrated the adequacy of wood frame roof structures when constructed to these simplified design procedures. Similar experience does not exist for other struc-tures, however. Steel and concrete roof structures are therefore required to be designed to the more conservative requirements in Part 4.

Bow-string and arch type trusses are much more sensitive to the effects of unbalanced loads than typi-cal pitched roof trusses. Because of this, such trusses are also required to be designed to the more rigor-ous requirements in Part 4, which make allowance for drifting and unbalanced loads if the trusses are greater than 6 m in span.

Deflections (9.4.3.)

A deflection limit for members supporting plastered ceilings of 1/360 of the span at design load has been a

traditional design rule since long before the NBC existed. Its purpose was to prevent plaster cracks when the assembly deflected under load. The same limit was applied to gypsum board when it was introduced as an alternative to plaster. Maximum deflections have also been a useful limit to apply to the design of floors to limit springiness, particularly in living and dining areas.

In the 1990 Code, recognition has been given to the dynamic properties of wood floors as well. All such floors are now required to be designed to limit their vibrations by a design procedure described in the Appendix to the Code. This will be discussed in fur-ther detail when considering the new span tables introduced in this edition.

Ceiling finishes less prone to cracking, such as tiled ceilings, can accept larger deflections without dam-age. An increased deflection of 1/2 of the span is therefore permitted for ceiling joists, provided they do not also serve as floor joists in other than bed-rooms.

In the case of rafters that support no ceiling, such as in unfinished attics, even greater deflection is per-mitted (1/180 of the span) since rafter deflections are

not as readily noticed as in those of members sup-porting interior finishes.

Soil Bearing Capacity (9.4.4.)

Footings are required to be designed in conformance with Section 9.4 if the building consists of concrete or steel frame construction, or if the bearing strength of the soil is below that specified in Section 9.15. If

the soil consists of "soft clay," "loose sand" or "loose gravel," for example, the footings are to be designed in conformance with this Section, even for wood frame construction.

Clay and silt may be classified as "stiff" if difficult to indent by thumb pressure, "firm" if indented by moderate pressure, and "soft" if easily indented by thumb pressure. The indentation test should be car-ried out on an undisturbed sample such as on the wall of a test pit.

Sand or gravel may be classified by means of a pick-et test in which a 38 mm by 38 mm pickpick-et bevelled at the end at 45° to a point is pushed into the soiL Such material is classified as "dense or compact" if a man of average weight cannot push the picket more than 200 mm into the soil and "loose" if the picket pene-trates 200 mm or more.

Footing settlement would have little effect if it occurred uniformly for the entire building. Usually, however, it does not. Some portions of the building are more heavily loaded than others and will conse-quently settle more. In addition, the properties of the soil are usually not consistent over the entire building site. Consequently, settlement is not likely to be uniform. Significant differential settlement can lead to foundation cracks and damage to the build-ing superstructure. For this reason limits are placed on the allowable bearing pressure for different types of soil.

Although soil conditions directly beneath the foot-ings may be known, it is also important to know conditions for a reasonable depth (Le., a depth equal to twice the width of the footings). If a weak layer of soil is close to the footing level, the footing pressure may be transferred down to the weak layer and cause the weaker soil to be over-stressed. Section 9.4 assumes that the pressure is distributed down-ward from the footings at an angle of 60° to the hori-zon, and is uniformly distributed in a horizontal plane. In the case of strip footings, therefore, the

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>

w

pressure at any depth beneath the footings can be calculated as

PW

PI;::: W + 1.15 h

where P the pressure directly beneath the footing

W;::: the width of the footing

h

=

the depth at which the pressure PI is to be calculated.

Example: A strip footing 1.0 m wide exerts a pres-sure of 200 kPa on the soil on which it rests, and is underlain by a soft clay 0.5 m below the footing; the pressure, PI' on the clay layer would be calculated as

200 x 1.0 ;::: 127 kPa 1.0 + (1.15 x 0.5)

This would exceed the allowable value of 75 kPa list-ed in Table 9.4.4.A. for soft clay and the footing design would not be permitted.

In the case of square pad footings, the pressure P2 at a depth, h, below the footing can be calculated as

PW2

Structural

Requirements

The load distribution from the footing to the soil presumed in Section 9.4 differs from the theoretical distribution determined on the basis of more precise calculations. Figure 10 shows a comparison between the pressures determined from such calculations and the distribution presumed in Section 9.4. The com-parison shows that the presumptions in Section 9.4 are conservative and are therefore on the safe side. The allowable soil bearing values listed in Section 9.4 can be affected by ground water conditions. If the soil is granular (such as gravel, sand or silt), the allowable values should be half of those listed in Table 9.4.4.A. if the ground water is close to the bot-tom of the footing (a depth less than the width of footing).

Soil Shrinkage (9.4.4.4.,

Footing movement can be caused by soil moisture changes as well as loads. Clay soils shrink when they lose moisture and swell when they gain mois-ture, although the movement may not be completely reversible in certain types of clay such as Leda clay. Leda clay (found in the Ottawa and St. Lawrence valleys) and certain prairie clays exhibit greater sen-sitivity to soil moisture changes than others.

Figure 10. Lines of equal vertical stress caused by surface loads

Load 100 kPa

•••••••••

(a) Actual distribution of load

I

3

l

Load - 100 kPa

(b) Load distribution assumed in Section 9.4

Commentary on Part 9 National Building Code of Canada: 1990

Archives

Commentary on Part 9

National Building Code

of Canada

1990

•

I

Preface

Commentary on Part 9

II

Commentary on Part 9

Table of Contents

I

Commentary on Part 9

Commentary on Part 9

Introduction

Historical Development

Recent Trends

Referenced Standards

CommentlJlY

on

P'" 9

Alternatives

I

Other Documents

Arrangement

I

Commentary on Part 9

Section

Use

Occupancy

Heat"'

Section 9.1

General

General

Occupancy Classification of

Buildings (9.10.2.)

I

General

Building Size (9.10.4.,

Building Height (1.1.3.2.)

~

~I

r

\

Building Area (1.1.3.1.,

stabil-Genera.

Factory-built Houses

Section 9.3

Materials, Systems and

Equipment

Concrete (9.3.1.)

Afateria'sJ

Systems and

Equipment

I

Lumber (9.3.2.)

•

Materia's, Systems and Equipment

I

lmH

I

Lumber Sizes

(9.3.2.6.,

Wood Shrinkage

Waferboard, Strandboard and

Plywood

Materials,

Systems and Equipment

Metal (9.3.3.)

p

Section 9.4

Structural

Requirements

shar-Structural

Requirements

Snow Loads (9.4.2.)

Deflections (9.4.3.)

Soil Bearing Capacity (9.4.4.)