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COMMENTARIES ON PART 4

OF THE NATIONAL BUILDING

CODE OF CANADA 1977

SUPPLEMENT No.4

TO THE NATIONAL BUILDING

CODE OF CANADA

ARCHIVES

Issued

by

the

Associate Committee on the National Building Code

National Research Council of Canada

Ottawa

Price $2.50

NRCC No. 15558

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A. G. Wilson (Chairman)

H. B. Dickens (Deputy Chairman)

S. D. C. Chutter D. E. Cornish S. Cumming R. F. DeGrace M.G. Dixon J. T. Gregg W. B. Guihan R. V. Hebert J. S. Hicks

M. S. Hurst (ex officio)

H. T. Jones P. M. Keenleyside J. Longworth J. A. McCambly C. J. McConnell R. C. McMillan Retired·

D. O. Monsen (ex officio)

A. T. Muir** F.-X. Perreault A. R. Pitt G. B. Pope H. R. Stenson R. A. W. Switzer A. D. Thompson J. E. Turnbull C. J. Ward

D. W. Boyd (Research Advisor-Meteorology)

R. S. Ferguson (Research Advisor)

R. H. Dunn (Secretary)

C. D. Carruthers (Chairman until November, 1975)

STANDING COMMITTEE ON STRUcrURAL DESIGN J. Longworth (Chairman) N. N.Aylon R. L. Booth L. H. Bush J. F. Cutler A. G. Davenport V. C. Fenton P. J. Harris D. J. Kathol D. E. Kennedy D. J. L. Kennedy H. Krentz N. C. Lind Retired· G. W. Elkington O. Safir C. Marsh W. McCarthy V. Milligan W. Paul B. G. W. Peter A. G. Stermac E. Y. Uzumeri H. P. Vokey

W. R. Schriever (Research Advisor)

R. H. Dunn (Secretary)

CSA/NBC JOINT LIAISON COMMITTEE ON LIMIT STATES DESIGN

D. J. L. Kennedy (Chairman) L. H. Bush A. G. Davenport J. L. deStein V.c. Fenton P. J. Harris N. C. Lind J. Longworth C. Marsh V. Milligan C. R. Wilson

D. E. Allen (Research Advisor and Secretary)

*Committee term completed during preparation of 1977 Code. **Deceased September 16, 1976.

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(3)

COMMENTARIES ON PART 4

OF THE NATIONAL BUILDING

CODE OF CANADA 1977

SUPPLEMENT No.4

TO THE NATIONAL BUILDING

CODE OF CANADA

Issued

by

the

Associate Committee on the National Building Code

National Research Council of Canada

Ottawa

NRCC No. 15558

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(4)

First Edition 1970 Second Edition 1975

Third Edition 1977

© National Research Council of Canada 1977 World Rights Reserved

Printed in Canada

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v

TABLE OF CONTENTS

Page

Preface. . . . . . . . . . . . . . .. vii

Commentary A

Commentary B

Commentary C

Commentary D

Commentary E

Commentary F

Commentary G

Commentary H

Commentary I

Commentary

J

Commentary K

Commentary L

Serviceability Criteria for

Deflections and Vibrations

1

Wind Loads . . .

7

Progressive Collapse and

Structural Integrity. . . . . . .. 37

Effects of Deformations in

Building Components ... . . . .. 45

Load Combinations for Structural Design. . . .. 51

Limit States Design . . . 55

Tributary Area . . . . . .. 63

Snow Loads . . . 69

Rain Loads . . . .. 85

Effects of Earthquakes . . . 89

Dynamic Analysis for the

Seismic Response of Buildings

109

Foundations. . . . . . .. 125

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PREFACE

The purpose of these commentaries is to make available to the designer detailed design informa-tion which will assist him in his use of the Nainforma-tional Building Code. The commentaries are pro-vided as background information and, in some cases, as suggested approaches to certain design questions, but not as mandatory requirements.

This edition contains an important new Commentary on foundations (Commentary L), which has been prepared to provide background material in support of Section 4.2 (Foundations) of the NBC.

Because the information provided in these commentaries cannot cover all conditions or types of structures that occur in practice, and also because new information may become available in the future, the designer should try to obtain the latest and most appropriate design information avail-able. For unusual types of structures it may be necessary to resort to specialized information such as theoretical studies, model tests or wind tunnel experiments to provide adequate design values.

CSA Standard S304-1976, "Masonry Design and Construction for Buildings" now replaces the design requirements for plain and reinforced masonry, formerly included as Part I of the 1975 edi-tion of this Supplement. This Standard is now referenced in Secedi-tion 4.4 of the 1977 NBC. It was produced under the auspices of a Joint CSA/NBC Committee and is essentially an updated ver-sion of Part I.

The reference to CSA S304-1976 in Part 4 of the Code is a continuation of the policy established in 1975 by which the design Standards for Timber, Concrete, Steel and Aluminum were removed from this document and simply referenced in Part 4. This action was necessary to avoid the need for major revisions to the Supplement whenever new revisions to these Standards were released.

To assist the user of the Supplement who intends to work in metric, a pamphlet has been prepared which gives the appropriate metric values for the imperial units of measure contained herein. The pamphlet, which is distributed with each copy of the Supplement, is intended to provide a basis for working in metric terms pending completion of a fully metric document in a subsequent edition.

These commentaries were prepared with the assistance of the following:

D. E. Allen D. J. L. Kennedy J. H. Rainer

W. A. Dalgliesh D. A. Lutes W. R. Schriever

A. G. Davenport W. G. Plewes D. A. Taylor

Commentary L (Foundations) was prepared with the assistance of a Task Group appointed by the Standing Committee on Structural Design and consisted of the following members: V. Milli-gan (Chairman), L. Brzezinski, D. Klajnerman, W. E. Lardner and E. Y. Uzumeri.

Comments and inquiries on aspects of these commentaries pertaining to the interpretation and use of the National Building Code should be addressed to the Secretary, Associate Committee on the National Building Code, National Research Council of Canada, Ottawa, Ontario KIA OR6. Requests for technical information of a non-Code nature are also welcome and should be directed to the staff of the Division of Building Research, who provide supporting services to the Code Committees.

Le Code national du batiment, ses supplements et les documents qui s'y rattachent sont disponi-bles en fran~ais. On peut se les procurer en s'adressant au Secn!taire, Comite associe du Code national du batiment, Conseil national de recherches du Canada, Ottawa, Ontario KIA OR6.

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COMMENTARY A

SERVICEABILITY CRITERIA FOR

DEFLECTIONS AND VIBRATIONS

TABLE OF CONTENTS

1

Page

Deflections .••...•...••.•...•... 3

Vibrations . . . . • • . . . • • . . . • . . . . .. 3

References ...•..•...

~

. . . • • • . . . .. 5

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COMMENTARY A

Serviceability Criteria for Deflections and Vibrations

I. The advent of stronger materials, lighter more rigid cladding, smaller damping and more accurate strength calculations taking account of interaction of components means that excessive deflections and vibrations now have a greater influence in structural design than before. Excessive deflections and vibrations are usually controlled in codes by limiting the member deflection under specified load to some ratio of the span (L), for example, L/360 (for cantilevers, L may be taken as twice the length of the cantilever). Table A-I summarizes deflection criteria in this form in various standards pertinent to the National Building Code of Canada 1977. These deflection criteria depend on the types of construction and materials and on the conditions of use. As an aid to the designer, the problems associated with excessive deflection and excessive vibration are briefly dis-cussed and references are given.

DEFLECTIONS

2. Excessive structural deflections can create a variety of problems: cracks or crushing in non-structural components such as partitions, lack of fit for doors, walls out of plumb or eccentric-ity of loading caused by rotation, unsightly droopiness and ponding. Cracks, besides being unsightly, may transmit unwanted sound through partitions, or water and cold air through exte-rior surfaces, and thus promote corrosion. Control of cracking in structural concrete is separately covered in CSA A23.3-1973, "Code for the Design of Concrete Structures for Buildings."

3. There are usually a number of alternative design solutions to problems caused by excessive deflection. Partition cracking, for example, can be avoided either by making the supporting struc-ture stiff enough or by providing flexible joints in the partitions. Similarly, to avoid cracking, plas-tered ceilings should be hung from the floor beams, not rigidly attached to them.

4. The deflection criteria in Table A-I apply to conventional forms of construction under conventional conditions of use. The most severe deflection requirement, 1/480, for members sup-porting plastered ceilings or partitions,(1) may not be sufficient for cracking of plaster or rigid partitions.(3) As an aid to the designer for new or unusual cases, more detailed deflection criteria are suggested in Reference (2); case histories of damage due to excessive deflections (including also differential settlement and temperature movements) are given in References (4) to (7).

VIBRATIONS

5. Two types of vibration problems arise in building construction: continuous vibrations and transient vibrations. Continuous vibrations arise due to the periodic forces of machinery or certain human activities such as dancing; these vibrations can be considerably amplified when the peri-odic forces are synchronized with a building frequency-a condition called resonance. Transient vibrations are caused by footsteps or other impact and decay at a rate which depends on the avail-able damping.

6. The undesirable effects of continuous vibrations caused by machines can be minimized by special design provisions,(8),(9) such as locating machinery away from sensitive occupancies, vibra-tion isolavibra-tion or alteravibra-tion of the frequency of the structure. Human beings can create periodic forces in the frequency range of approximately 1-4 Hz, and floor frequencies less than about 5 Hz should be avoided for light residential floors, schools, auditoria, gymnasia and similar occupan-cies. For very repetitive activities such as dancing, it is possible to get some resonance when the beat is on every second cycle of floor vibration, and it is therefore recommended that the fre-quency of such floors be 10 Hz or more, unless there is a large amount of damping.

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Table A-I SUMMARY OF MAXIMUM DEFLECTION/SPAN RATIOS IN NBC 1977 AND PERTINENT CSA STANDARDS(/) CSA 086-1976, CSA A23.3-1973, CSA SI6-1969; CSA SI57-1969, NBC 1977-Part 9, Wood Concrete SI6.1-1974, Structural Aluminum Residential Standards Structural Steel Roof or floor members I 1 (3) 1(3) I I I supporting plastered 360 480 or 240 360 360 360 ceilings, partitions, etc. Floor members not 1(2) (4) I (5) I 1(7) I supporting plastered 180 320 200 240 or 360 ceilings, partitions, etc. -Roof members not 1 (2) I I (6) I (6) 1 I (8) 1 supporting plastered 180 180 180 or 240 180 180 or 240 ceilings, etc. Wall members 1 (2) 1 180 -180 -Column 1 2 3 4 5 6 Notes to Table A-I: (I) Deflection under live load only unless otherwise noted. (2) Modulus used for calculation based on short term test but there is a warning clause on creep deflection. (3) Deflection which occurs after attachment of non-structural elements, including creep deflection due to sustained load plus immediate deflection due to additional live load. The lower figure applies when non-structural elements are not likely to be damaged by large deflections. (4) Immediate live-load deflection. There is a warning on ponding for roof members. (5) There is a warning clause on vibrations. (6) 1/180 applies to sheet metal or elastic membrane roof cover and 11240 to asphaltic built-up roofs. There is a warning clause on ponding. (7) For bedrooms only. (8) If there is no ceiling. .&;.

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(10)

7. Transient vibrations in floor systems due to foot impact may cause discomfort or annoy-ance to the occupants as a result of, for instannoy-ance, china rattling. In Table A-I the deflection criteria of 11360 for wood floors(J) and 1/320 for steel floors which do not support brittle materials attempt to control such vibration effects. These criteria apply only to conventional floors with spans less than approximately 20 ft and frequencies greater than about 10 Hz. They do not apply to long span floors, especially for those without partitions, or for floors for special purposes; Reference (10) contains further information and criteria on these cases. References (I) and (II) contain fur-ther information for light residential floors with wood decks.

REFERENCES

(1) Russell, W. A. Deflection Characteristics of Residential Wood-Joist Floor Systems. Housing and Home Finance Agency, Housing Research Paper 30, Washington, D.C., April

1954.

(2) Allowable Deflections. Subcommittee I, ACI Committee 435. Journal, Am. Concrete Inst., VoL 65, No.6, June 1968, p. 433.

(3) Plewes, W. G. and Garden, G. K. Deflections of Horizontal Structural Members. National Research Council of Canada, Division of Building Research, Canadian Building Digest No. 54, Ottawa, June 1964.

(4) Mayer, H. and Riisch, H. Bauschaden als Folge der Durchbiegung von Stahlbeton-Bauteilen (Building Damage Caused by Deflection of Reinforced Concrete Building Compo-nents). Deutscher Ausschuss fUr Stahlbeton, Heft 193, Berlin 1967. National Research Council of Canada Technical Translation TT1412, 1970.

(5) Pfeffermann, O. Les Fissures dans les Constructions Consequences de Phenomenes Physi-ques Naturels. Annales de l'Institut Technique du Batiment et des Travaux Publics, No. 250, October 1968.

(6) Skempton, A. W. and MacDonald, D. H. The Allowable Settlements of Buildings. Proc., Institution of Civil Engineers, VoL 5, Part III, 1956, p. 727.

(7) Khan, F. R. and Fintel, M. Effects of Column Exposure in Tall Structures-Design Consid-erations and Field Observations of Buildings. Journal, Am. Concrete Inst. VoL 65, No.2, February 1968, p. 99.

(8) Thomson, W. T. Vibration Theory and Applications. Prentice-Hall.

(9) Steffens, R. J. Some Aspects of Structural Vibration. Building Research Current Paper Engi-neering Series 37, Building Research Station, Ministry of Technology, Great Britain. (10) CSA Standard CSA SI6.l-1974. Steel Structures for Buildings-Limit States Design.

Appen-dix Guide on Floor Vibrations.

(II) Onysko, D. M. Performance of Wood·Joist Floor Systems. Forest Products Laboratory Information Report OP-X-24, Canadian Forestry Service, Department of the Envi-ronment, January 1970, Ottawa.

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(11)

7

COMMENTARY B

WIND LOADS

TABLE OF CONTENTS

Page

Reference Wind Speed,

v,

and Pressure, q ...•... 7

Exposure Factor, C

e • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

10

Gust Effect Factor, C

g • • • • • • • • • • • • • • • • • • • • • • • '.' • • • • • • • • • • • • • • •

11

Vortex Shedding . . . . • . • • . . . • . . . • . • . . .

14

Pressure Coefficients . . . • . . . .. 15

Lateral Deflection of Tall Buildings Under Wind Loading. . . • . . .

16

Construction Stages ...•..•...•...••... 19

References ...•...•..•.•...•....•...•...•• 20

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COMMENTARY B

Wind Loads

I. Three different approaches to the problem of determining design wind loads on buildings are mentioned in Subsection 4.1.8., "Effects of Wind" of the 1977 edition of the National Building Code.(lj

2. The first approach, called "simple procedure," is similar to that in the 1960 and 1965 edi-tions of the NBC. It is supplied along with numerical values for all the factors involved, except for climatic data (given in Reference 2) and pressure coefficients (given in this Commentary). This simple procedure gives approximately the same wind pressures and suctions as the earlier editions, and is intended for the majority of buildings for which wind loading does not have a major effect on the structural design.

3. The 2 other approaches to wind load analysis are referred to in Article 4.1.8.2. of the 1977 NBC, where the designer is required to use either (a) special wind tunnel tests or other experimen-tal methods, or (b) a dynamic approach to the action of wind gusts to be called "detailed procedure," whenever the building is likely to be susceptible to wind-induced vibration. This may be true, for example, of tall and slender structures or doubly cantilevered canopies for which wind loading plays a major role in the structural design. Background information on the need for, and development of, new and more accurate methods of predicting wind loading effects on structures can be found in References (3), (4) and (5).

4. Special wind tunnel tests in which the relevant properties of the building plus immediate surroundings and of the oncoming flow must be adequately represented are recommended when-ever the cost, the unusual nature of the building or site or other such considerations can justify the expense involved. For many cases for which the simple procedure is inadequate, however, there is still no clear indication of the need for a special wind tunnel test.

5. The third approach, the "detailed procedure," was devised(6) specifically for this intermedi-ate cintermedi-ategory of wind loading problems, although it can be used in other situations if its scope and limitations are recognized. The detailed procedure consists of a series of calculations involving (i) the intensity of wind turbulence for the site as a function of height and of the surface roughness of the surrounding terrain, and (ii) properties of the building such as height, width, natural frequency of vibration and damping. The end-product of the calculations is the gust effect factor Cg, which is multiplied by the reference wind pressure, q, the exposure factor, Ce, and the pressure coefficient Cp' to give that static design pressure which is expected to produce the same peak load effect as the actual turbulent wind for the appropriate probability leveL The format of the simple procedure in the NBC has been arranged to permit an easy transition to this more detailed consideration of wind effects.

REFERENCE WIND SPEED,

V,

AND PRESSURE, q

6. The reference wind speed, Y, is determined by extreme value analysis of meteorological observations of hourly mean wind speeds, taken at sites (usually airports) chosen in most cases to be representative of a height of 30 ft in an open exposure. The reference wind pressure, q, is deter-mined from Y by the following equation:

q(in pst)

=

C \f2 (1)

7. The factor C depends on the atmospheric pressure and the air temperature. The atmos-pheric pressure in turn is influenced mainly by elevation above sea level, but also varies somewhat in accordance with changes in the weather.

8. The following value of C is chosen to represent Canadian conditions: ifY is in miles per hour, C=0.OO27

ifY is in feet per second, C=0.OOI25

I

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10

9. The NBC Supplement No. I, "Climatic Information for Building Design in Canada 1977" contains a description of the procedures followed in obtaining the reference wind pressures, q, for 3 different levels of probability of being exceeded per year (1/10, 1130 and 1/100), that is, the ues commonly referred to as having return periods of 10, 30 and 100 years, respectively. These val-ues of q are tabulated in Supplement No.1 for many Canadian locations along with other climatic design data. A reference giving more detail on the choice of the conversion factor, C, from wind speed to pressure and a table for converting from pressure in pounds per square foot to speed in feet per second are also supplied in Supplement No. I to the NBC.

EXPOSURE FACTOR, Ce

Simple Procedure

10. In the simple procedure of the 1977 Code(1) the exposure factor, Ce• is exactly the same as

the old height factor, Ch, in the earlier editions of the Code. The name has been changed to

describe better the function of this factor when applied in the detailed analytical procedure where it reflects not only changes in wind speed with height, but also the effects of variations in the sur-rounding terrain. For the simple procedure, Ce' is based on the lis power law which is appropriate for wind gust pressures in open terrain (Ilio power law for wind gust speeds). The wind gust refer-red to is thought to last about 3 to 5 sec. and to represent a "parcel" of wind which is assumed effective over the whole of most ordinary buildings.

Detailed Procedure

II. For the detailed procedure the exposure factor, Ce, is based on the mean wind speed

profile, which varies considerably depending on the general roughness of the terrain over which the wind has been blowing before it reaches the building. This dependence on terrain is much more significant than is the case for the gust speed profile, i.e. variation of gust speed with height, and hence 3 categories have been established as follows:

Exposure A (open or standard exposure): open level terrain with only scattered buildings, trees or other obstructions, open water or shorelines thereof. This is the exposure on which the reference wind speeds are based.

(2)

Exposure B: suburban and urban areas, wooded terrain or centres of large towns. ( Z

)0.50

Ce

=

0.6 ,Ce>0.5

60 - (3)

Exposure C: centres of large cities with heavy concentrations of tall buildings. At least 50 per cent of the buildings should exceed 4 storeys.

( Z

)0.72

Ce

=

0.4 100 ,Ce~O.4 (4)

In Equations (2) to (4), Z is the height above ground in feet.

12. Exposure B or C should not be used unless the appropriate terrain roughness persists in the upwind direction for at least I mile, and the exposure factor should be varied according to the terrain if the roughness differs from one direction to another. Abrupt changes in ground slope near the building site may result in significantly higher wind speeds than over level ground, and thus exposure A may have to be applied in such situations even though the surface roughness may seem appropriate for B or C.

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(14)

Use of Exposure Factors

13. Exposure factors can be calculated from Equations (2) to (4) or obtained directly from the graphs in Figure B-1. They should be applied to the wind pressure rather than to speed; where it is necessary to determine the hourly mean wind speed at height, h, use the square root of C •.

14. The exposure factor is needed in 3 different capacities in the detailed procedure. First, the square root of C. is needed to determine the hourly mean wind speed at the top of the structure being designed, V H:

(5)

15. The reference wind speed, V, can be obtained from the reference wind pressure and the conversion table in Supplement No. lor by applying Equation (I).

16. Secondly, C. appears in Equation (7) used for calculating the gust effect factor, Cg• Here

again, Co is calculated using the height H of the structure.

17. Thirdly, Co is used in the calculation of pressure coefficients for the windward and lee-ward faces of tall, slender buildings. For the windlee-ward face, Co varies continuously with the height, Z, to the elevation in question; for the leeward face, Co is evaluated once at 'h the height, H, of the building.

GUST EFFECT FACTOR, Cg

Simple Procedure

IS. The implied gust effect factor of the earlier editions of the Code varied from 2.04 at 60 mph design gust wind speed to I.S4 at 120 mph design gusi wind speed, and was the same whether the whole structure was being designed, or some part of it such as a window or a wall panel. In the 1977 NBC(i) the gust effect factor for the simple procedure is 2.0 for the structure as a whole, and 2.5 for cladding or windows. On the other hand, the consequences of wind damage to cladding are less serious than structural damage, and the risk may be considered acceptably small if a probabil-ity of 1/10 is used for cladding design wind pressures rather than the 1130 or 1/100 specified for design

of the structure. The net result is that, although smaller, more severe gusts can be expected over small areas (and hence a larger gust effect factor of 2.5 is specified) the use of a more probable (and hence lower) reference wind pressure gives approximately the same design pressure for a panel or window as for the structure.

Detailed Procedure

19. The calculation procedure for the gust effect factor, Cg, is given in detail below, including

a sample calculation of Cg worked out in complete detail. In the detailed procedure the gust effect

factor is the ratio of the expected peak loading effect to the mean loading effect. Cg therefore

makes allowances for the variable effectiveness of different sizes of gusts and the load magnifica-tion effect caused by gusts in resonance with the structure vibrating as a single-degree-of-freedom cantilever. Cg is defined as follows:

(6)

where

a standard deviation of total loading effect,

Ii mean value of total loading effect, g peak factor of total loading effect.

The standard deviation divided by the mean, a/Ii, is the "coefficient of variation" for the total loading effect (7)

I

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(15)

'I

I

l

12

where K s

=

F

p

=

a factor related to the surface roughness coefficient of the terrain 0.08 for Exposure A

0.10 for Exposure B

0.14 for Exposure C,

exposure factor, previously defined, obtained from Figure B-1,

background turbulence factor, obtained from Figure B-2 as a function of height, H, and width, W, of the windward face of the structure,

size reduction factor, obtained from Figure B-3 as a function of the ratio of width, W, to height, H, of the windward face of the structure and the reduced frequency,

gust energy ratio at the natural frequency of the structure, obtained from Figure B-4 as a function of the wave number (natural frequency (cycles/sec.) divided by mean wind speed (fUsec.) at height, H, of structure),

critical damping ratio.

20. Suggested values for buildings are 0.01 for steel frames and 0.02 for reinforced concrete frames. On the other hand, the critical damping ratio for welded steel stacks may go as low as 0.001 for moderate amplitudes of displacement. Prestressed concrete structures having no micro-cracks due to tension may also have very low values for structural damping.

21. The peak factor, g, in Equation (6) gives the number of standard deviations by which the peak load effect is expected to exceed the mean load effect, and is given in Figure B-5 as a func-tion of the average fluctuafunc-tion rate. The average fluctuafunc-tion rate, v, can be estimated as follows:

where

v = no

V __

sF _ _

sF +

PB

no = natural frequency of vibration, cycles/sec. s, F,

p,

B as defined for Equation (7).

Explanatory Notes Regarding (Jlp. and g

(8)

22. The response of a tall, slender building to a randomly fluctuating force can be evaluated rather simply by treating it as a rigid, spring-mounted cantilever whose dynamical properties are specified by a single natural frequency and an appropriate damping value. The variance of the output quantity or loading effect is the area under the spectrum of the input quantity (the forcing function) after it has been multiplied by the transfer function. The transfer function is the square of the well-known dynamic load magnification factor for a one-degree-of-freedom oscillating mechanical system.

23. In the case of wind as the random input, the spectrum of the wind speed must first be multiplied by another transfer function called the "aerodynamic admittance function," which in effect describes how the turbulence in the wind is modified by its encounter with the building, at least insofar as its ability to produce a loading effect on the structure is concerned.

24. For the purposes of calculating (J/p., the spectrum of the wind speed is represented by an algebraic expression based on observations of real wind. The aerodynamic admittance function is also an algebraic expression, computed on the basis of somewhat simplified assumptions but appearing to be in reasonable agreement with the limited experimental evidence at present avail-able. The spectrum of wind speed is a function of frequency having the shape of a rather broad hump (Figure B-4). The effect of the aerodynamic admittance is to reduce the ordinates of the curve to the right of the hump more and more as the frequency increases. This is partly a reflection of the reduced effectiveness of small gusts in loading a large area. The effect of the dynamic load magnification factor or mechanical admittance is to create a new peak or hump centred at the nat-ural frequency of the structure, usually well to the right of the broad peak, which represents the maximum density of input power of the wind.

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25. The area under the loading effect spectrum, the square root of which is the coefficient of variation olp., is taken as the sum of 2 components: the area under the broad hump, which must be integrated numerically for each structure, and the area under the resonance peak, for which a single analytic expression is available. These components are represented in Equation (7) by Band

sFI{3, respectively. The factor K/C. can be thought of as scaling the result for the appropriate input turbulence level. If resonance effects are small, then sF 1 {3 will be small compared to the background turbulence B and vice versa. Note that although C. is normally a function of height, in Equation (7) it is evaluated at a particular height (usually H, the height of the building), and is treated as a single-valued parameter for calculating Cg•

26. If this method for calculating Cg is used for buildings or parts of buildings that are not

well represented by the simple model of a rigid cantilever oscillating about a spring-mounted base, additional sources of error will be introduced, although these are perhaps not very important when resonance effects are small. In the absence of a more precise analysis, the present method can serve as a guide to the peak gust loading on buildings that are not tall and slender, or even for win-dows or wall panels located on the windward sides of buildings. In considering a portion of the windward face, use the dimensions of the element for D and H in Equations (7) and (8), the natu-ral frequency of the element itself for no and velocity V z (where Z is the height of the element above ground) rather than V H' the velocity at the top of the structure. Similarly, Co should in this

case be evaluated at height Z for Equation (7).

27. The peak factor, g, depends on the average number of times the mean value of the load-ing effect is crossed durload-ing the averagload-ing time of I hr (3,600 sec.). The functional relationship in Figure B-5 was shown by Davenport(7) to hold when the probability distribution of the mean load-ing effect was normal (Gaussian).

28. As stated in Article 4.1.8.3. of the 1977 Code, structures must be able to withstand partial or unbalanced loading as well as the full design load. All structures, particularly those susceptible to unbalanced loading due to wind, such as double overhang girders and canopies, members sub-ject to stress reversal and structures with broad frontal area, should be capable of withstanding the effects of a reduced dynamic factor equal to 0.75 Cg, acting over any portion of the structure.

Sample Calculation of Cg

29. To illustrate the calculation of a gust effect factor the following sample problem will be worked in detail:

Required: To obtain the gust effect factor for a building with the following properties:

Height -600 ft

Width -100 ft

Depth -100 ft

Fundamental natural frequency ---4>.2 Hz

Critical damping ratio ---4>.015

Terrain for site -Exposure B

Reference wind speed at 30 ft open terrain 90 ft/sec.

Step I: Calculate required parameters

Mean wind speed at top of building V 600' from Equation (5)

=90x

V

1.90= 123 ftlsec. (Figure B-1) Aspect ratio W/H= 100/600=0.17

Wave number for calculation of F: no/V 600 = 0.00163

Reduced frequency for calculation of s: noH/V 600=0.975

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L

14

Step 2: Calculate alll, from Equation (7) (I) K=O.IO for Exposure B

(2) Ce= 1.90 (from Figure B-1) (3) B = 0.62 (from Figure B-2) (4) s = 0.11 (from Figure B-3) (5) F=0.28 (from Figure B-4) (6) ,B=O.oJ5 (given)

YO.IO

(7)alll = -1.90 ( 0.11 x 0.28 ) 0.62 + = 0.375 0.015

Step 3: Calculate v, from Equation (8) (I) no = 0.2 Hz (given)

Y

0.11 x 0.28

(2)v = 0.2 = 0.175/sec. 0.11 X 0.28 + 0.015 X 0.62

Step 4: Obtain peak factor g: (I) g=3.75 (from Figure B-5)

Step 5: Cg (from Equation (6»= 1 +3.75xO.375=2.41

VORTEX SHEDDING

30. Slender exposed structural elements and tall slender cylindrical structures such as chim-ney stacks, observation towers and in some cases, high-rise buildings, should be designed to resist the dynamic effects of vortex shedding. When the wind blows across a slender prismatic or cylin-drical body, vortices are shed alternately from one side and then the other giving rise to a fluctuating force acting at right angles to the wind direction along the length of the body. A struc-ture may be considered slender in this context if the ratio of height to diameter exceeds 5. The fre-quency, n. of the vortex shedding and of the force fluctuations is given by

where

n the frequency, Hz,

S the Strouhal number given below,

V H= the mean wind speed at the top of the structure as defined in Equation (5), ftlsec.,

o

= the diameter, ft.

For circular cylinders S = 0.18 for Re<2X 105, S 0.25 for Re>2x 105,

VHO where. Reynolds' number Re = X 105

16

For bodies with angular sections such as a rectangular, rolled-steel shape, S=0.15.

(9)

31. If the structure is free to oscillate in the plane normal to the wind, large oscillations will develop when the vortex shedding frequency is resonant with the natural frequency of the struc-ture. The dynamic influence will be approximately equivalent to the influence of a static force per unit height, FL , acting in the direction of oscillations

0.5 FL -CLOqcr (10) ,B

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the critical damping ratio as defined for Equation (7), 0.2 for circular cylinders,

velocity pressure for the mean wind speed which produces resonance.

15

32. For tapered stacks there is some reduction in the effective length over which the vortex shedding forces act. If the diameter of a section of the stack at height Z is Dz, then the velocity at which vortices are shed from this section resonant with the structure is given by the Equation (9), where n is set equal to the resonance frequency of the stack. The height over which these resonant eddy shedding forces then act is determined by the height of stack over which the diameter only changes by ± 5 per cent from the value Dz. Thus, on tapered stacks the vortex excitation can take place over a range of wind velocities corresponding to the variation in diameter of the stack. For each velocity the fluctuation force only acts over a limited section of the stack.

PRESSURE COEFFICIENTS

33. Pressure coefficients are the non-dimensional ratios of wind-induced pressures on a build-ing to the dynamic pressure (velocity pressure) of the wind speed that would be measured (usually) at the top of the building in the undisturbed air stream. Pressures on the surfaces of structures vary considerably with the shape, wind direction and the profile of the wind velocity. Pressure coefficients are usually determined from wind tunnel experiments on small-scale building models, although in a few recent instances measurements on full-scale buildings have been used directly. It

is essential in most cases that these pressures be measured in a wind tunnel in which the correct velocity profile is simulated; experiments in uniform flow can be highly misleading.(8).(9)

34. The pressure coefficients given in Figures B-6 to B-20 are all time-averaged values, that is, they refer to the mean value of the pressure on a surface. In addition, all pressure coefficients except the local pressure coefficients, Cp *, usually represent a spatially averaged pressure. The local maximum and minimum pressures acting over a small area are designated by Cp

*

and are appropriate for cladding design.

35. The internal pressure coefficients, Cpi, define the effect of wind on the air pressure inside the building and are necessary for the design of cladding and secondary supporting members for wall and roof systems. Like the external pressure coefficients, the Cpi are time-averaged values, but unless there are large openings joining the interior to regions of extreme wind speed, pressure or suction (windward and side walls), the maximum instantaneous internal pressures will not be appreciably different from the time-averages. On the other hand, if the permeability of the build-ing is gradually increased, the gustiness in the wind will have an increasbuild-ing effect in causbuild-ing peaks and lulls in the internal pressure. At present it must be left to the designer to decide in doubtful cases whether or not the gust effect factor, C.I.' should be applied to internal pressure coefficients (formula (b) in Sentence 4.1.8.1.(2) of the NBc).

36. Values of pressure coefficients sufficient for general purposes for 2 classes of structures are given in Figures B-6 to B-8. The pressure coefficients, unless otherwise noted, are based on the velocity pressures at the top of the building. Pressure coefficients for various other structures that have been tested in turbulent shear flows may be obtained from Reference 8.

37. Figures B-9 to B-20 are based on wind tunnel experiments in which the correct velocity profile and wind turbulence were not simulated, and should therefore be regarded with a certain

measure of caution. These figures are the same as in Tables 20 to 31 in the 1961 and 1965 editions of Supplement No.3 for use with the 1960 and 1965 National Building Codes, respectively, except for some deletions and a few corrections. They are based on the Swiss Association of Engineers and Architects Standards, S.I.A., No. 160, published in 1956.(10)

Rounded Structures

38. For rounded structures (in contrast to sharp-edged structures) the pressures vary with the wind velocity, depending on the Reynolds' number, ~. (defined following Equation (9». In

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[

16

ures B-ll, B-12, B-15 and B-20, which have been translated and reproduced from the Swiss tables(IO), the Reynolds' number is expressed by dVq where d is the diameter of the sphere or cyl-inder in feet and q is the velocity pressure in pounds per square foot. To convert to

Re,

multiply dVq by 1.8 X lOS.

39. The roughness of rounded structures may be of considerable importance. Common well-laid brickwork without parging can be considered as having a "moderately smooth" surface (Fig-ure B-II). Surfaces with ribs projecting more than 2 per cent of the diameter are considered as "very rough." In case of doubt, it is recommended to use those Cn values which result in the greater forces. For cylindrical and spherical objects with substantial stiffening ribs, supports and attached structural members, the pressure coefficients depend on the type, location and relative magnitude of these roughnesses.

Icing

40. In locations where the strongest winds and icing may occur simultaneously, structural members, cables and ropes must be calculated assuming an ice covering based on climatic and local experience. For the iced condition, values of Cn given in Table B-15 for thick wire cables for a "rough" surface should be used.

Structural Members

41. In Figures B-16, B-17, B-19 and B-20 pressure coefficients with the subscript 00 are used to indicate that they apply to structural members of infinite lengths and this is multiplied by a reduction factor, k, for finite lengths of members. If a member projects from a large plate or wall, the reduction factor, k, should be calculated for a slenderness based on twice the actual length. If a member terminates with both ends in large plates or walls, the reduction factors for infinite length should be used.

Shielding

42. For members that are located behind each other in the direction of the wind, the shield-ing effect may be taken into account. The windward member and those parts of the leeward mem-ber that are not shielded should be designed with the full pressure, q, whereas the shielded parts of the leeward member should be designed with the reduced pressure, q" according to Figure B-18.

43. For constructions made from circular sections with dVq<2.5 and

Ai

A~O.3, the shield-ing factors can be taken by approximation from Figure B-18. IfdVq>2.5, the shieldshield-ing effect is small and for a solidity ratio

A/

A~0.3, it can be taken into account by a constant shielding fac-tor k, =0.95.

LATERAL DEFLECTION OF TALL BUILDINGS UNDER WIND LOADING

44. Lateral deflection of tall buildings under wind loading may require consideration from the standpoints of serviceability or comfort criteria. There is a general trend toward more flexible structures, partly because adequate strength can now be achieved by using higher strength materi-als that may not provide a corresponding increase in stiffness.

45. One symptom of unserviceability may be the cracking of masonry and interior finishes. Unless precautions are taken to permit movement of interior partitions without damage, a maxi-mum lateral deflection limitation of 11250 to 1/1000 of the building height should be specified. According to Sentence 4.1.1.5.(4) of the 1977 NBC, 1/500 should be used unless a detailed analy-sis is made.

Wind-Induced Building Motion

46. While it is generally found that the maximum lateral wind-loading and deflection are in

the direction parallel with the wind (along-wind direction), the maximum acceleration of a

build-ing leadbuild-ing to possible human perception of motion or even discomfort may occur in the direction perpendicular to the wind (across-wind direction). Across-wind accelerations are likely to exceed

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along-wind accelerations if the building is slender about both axes, that is if YWD/H is less than Y), where Wand 0 are the across-wind and along-wind plan dimensions and H is the height of the building.

47. Although treatment of this subject is somewhat tentative, the following guidelines may be of assistance. On the basis of a wide range of turbulent boundary layer wind tunnel studies, it has been found that the peak acceleration in the across-wind direction at the top of the building can be found from the following:

(II)

48. In less slender structures or for lower wind speeds, the maximum acceleration may be in the along-wind direction and can be found from the expression

2 2 '"

f"i<s"F (

.l ) aD = 477 nD g

V

-=--::---Ce

f3D

Cg (12)

where

W, 0 across-wind and along-wind building dimensions, ft.

aW,aD peak acceleration in across-wind and along-wind directions, ftlsec2, a, = .0005 [Vu/(nw YWD)V3,

Y B = average density of the building, Ib/ft3,

f3w,f3D = fraction of critical damping in across-wind and along-wind directions, nw, nD fundamental natural frequencies in across-wind and along-wind directions, Hz,

.l maximum wind-induced lateral deflection at the top of the building in along-wind direction, ft,

g, K, s, F, Ce, C9-, as defined previously in connection with Equation (7). Note that f30

=

p and no

=

no in terms of previous definitions.

49. Although many additional factors such as visual cues, body position and orientation and state-of-mind are known to influence human perception of motion, it appears that when the ampli-tude of acceleration is in the range of 0.5 per cent to 1.5 per cent of the acceleration due to gravity, movement of the building becomes perceptible to most people.(ll) to (13)

50. Based on this and other information, a tentative acceleration limitation of I to 3 per cent of gravity once every 10 years is recommended; for use in conjunction with Equations (II) and (2) the lower value might be considered appropriate for apartment buildings, the higher value for office buildings. The application of Formulas (II) and (12) tend to give conservative results insofar as they assume that the wind always comes from the most sensitive direction, and this factor has also been considered in setting the above limitation. If the designer has available more detailed information he can make suitable allowances.

51. Owing to the relative sensitivity of the two expressions (II) and (2) to the natural fre-quency of vibration, and in (12) to the corresponding building stiffness, it is recommended that these be determined using fairly rigorous methods, and that approximate formulas be used with caution. For example, the adoption of a natural frequency of WIN where N is the number of sto-reys may not be consistent with the assumption that the displacement under wind loading is as large as H/500.

52. If a more rigorous analysis is not available, the maximum deflection resulting from the equivalent static wind loading can be related to the fundamental building frequency using modal representation of the building motion. The following assumptions may be acceptable:

(I) Use first mode only, assumed linear

</>(Z) = rlZ (13)

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18

(2) Uniform distribution of building mass

WDYB

m(Z) =

-go

As a consequence of modal representation

H

f

cp(Z)m(Z)cp(Z)dZ = I

o

u(Z) r 2CP(Z) H W P(Z) 4w2ni> =

f

CP(Z) - - CP(Z) dZ

o

u(Z)

where CP(Z) = fundamental eigenvector, rl , r2 = constants,

m(Z) = distribution of building mass, with height Z, 0

<

Z

<

H, slugs/ft, go acceleration due to gravity 32.2 ftlsec.2

,

u(Z) = displacement at height Z, 0

<

Z

<

H, ft,

P(Z) = distribution of equivalent static wind pressure with height Z, 0

<

Z

<

H,psf.

Other symbols are as defined earlier. From Equations (13), (14) and (I 5)

$(Z) =

CV

w~:

H' )Z

From Equations (16), (17) and (I 8)

Substituting Equation (19) into Equation (16), the deflection at height H becomes

H 3go

f

Z P(Z)dZ ~ = 0 4w2ni> D YB H2 (14) (15) (16) (17) (I8) (19) (20)

One possible expression for P(Z) assumes a power law variation of a maximum at the top of qCeCgCp

(21)

where Cp 0.8 (-.5) = 1.3 and a is the appropriate exponent from Equations (2), (3) and (4).

Substituting Equations (20) and (21) into (12)

aD/go g

V

KsF

(~)(

Ceq) (22) C.flD 2

+

a DYB

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53. Sample Calculation of aw and aD- A detailed calculation for aw and ao using Equations (II) and (12) will be made for the sample problem worked earlier to illustrate the calculation of a gust effect factor:

assume that nw

=

no 0.2 Hz f3w =

f30

= 0.015 Illb/ft3 'YB Step 1: Calculate ar : ar = .0005 (123/ (0.2 X 100W 3 .201 Step 2: :alculate aw ( 0.201 ) aw -0.2

x

0.2

x

3.75

x

100 ,~

=

2.24 ft/sec.2 11 v.015

awl&:> = 6.9 per cent Step 3: Calculate q

q = 0.00126 X 90 X 90

= 10.2 psf Step 4: Calculate aD/&:>

V

0.10 X 0.11 X 0.28 ( 3.9 ) (1.90 X 10.2)

ao/&:> = 3.75

-1.90 X 0.015 2.50 100 X 11

= 3.4 per cent

54. In this example clearly the across-wind accelerations overshadow the along-wind acceler-ations. Table B-1 gives the results of calculations for 5 sample buildings (generally for wind along both axes) for 3 different reference wind pressures and the 3 different terrains. Case 5 is in fact the building treated in the above example, and the reference wind pressures are appropriate for cities like Montreal, Toronto and Vancouver, respectively.

Pressure Differences Across Interior Walls and Partitions

55. Considerable pressure differences can result across interior walls and partitions in high-rise buildings and in low-high-rise buildings in exposed locations if windows are broken during a storm. In certain locations this could result in almost the full pressure difference between the windward and leeward sides of the building being applied across the interior wall or partition. This could happen for example if a large window on the windward side were broken by flying debris and the full positive pressure were to act on the walls of a small room located at this broken window. Simi-lar conditions could prevail in an apartment building with operable windows or doors. This pres-sure difference could be aggravated by stack effects in a tall building in the winter time. On the other hand, general experience does not indicate many failures of interior walls due to this cause, and thus it is not always considered necessary to design interior walls and partitions for the maxi-mum possible pressure difference. A design pressure difference of the order of 10 psf may be appropriate.

CONSTRUCTION STAGES

56. It should be noted that the shape of a structure may change during erection. The wind

loads, therefore. may be temporarily higher during erection than after completion of the structure.(l3) These increased wind loads should be taken into account using the appropriate coefficients from Figures B-6 to B-20.

REFERENCES

(I) National Building Code of Canada 1977. National Research Council of Canada. Associate Committee on the National Building Code, Ottawa, NRCC No. 15555.

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l

20

(2) Climatic Information for Building Design in Canada. Supplement No. I to the National Building Code of Canada 1977. National Research Council of Canada. Associate Committee on the National Building Code, Ottawa, NRCC No. 15556.

(3) Dalgliesh, W. A. and Schriever, W. R. Recent Research on Wind Forces on Tall Buildings. Proc., Canadian Structural Engineering Conference, Toronto, 19/20 February 1968, University of Toronto Press.

(4) Davenport, A. G. New Approaches to the Design of Structures Against Wind Action. Proc., Canadian Structural Engineering Conference, Toronto, 19/20 February 1968, Uni-versity of Toronto Press.

(5) Proceedings, International Research Seminar on Wind Effects on Buildings and Structures. Ottawa, 1967-published September 1968 by University of Toronto Press.

(6) Davenport, A. G. Gust Loading Factors. Journal Structural Division, Proc., Am. Soc. Civ. Engrs., Vol. 93, June 1967, pp. 12-34.

(7) Davenport, A. G. Note on the Distribution of the Largest Value of a Random Function with Application to Gust Loading. Proc., Institution Civil Engineers, Vol. 28, June 19~4,

pp. 187-196. London.

(8) Jensen, M. and Franck, N. Model Scale Tests in Turbulent Wind, Part II. Danish Technical Press, Copenhagen, 1965.

(9) Leutheusser, H. J. and Baines, W. D. Similitude Problems in Building Aerodynamics. Jour-nal of Hydraulics Division, Proc., Am. Soc. Civ. Engrs., Vol. 93, May 1967, pp. 35-49.

(to) Normen fur die Belastungsannehmen, die Inbetriebnahme und die Uberwachung der Bau-ten. (Standards for Load Assumptions, Acceptance and Inspection of Structures). Schweizerischer Ingenieur und Architekten Verein (Swiss Association of Engineers and Architects), No. 160, Zurich, Switzerland, 1956.

(II) Chen, P. W. and Robertson, L. E. Human Perception Thresholds of Horizontal Motion. Journal of Structural Division, Proc., Am. Soc. Civ: Engrs., Vol. 98, August 1972, pp.1681-1695.

(12) Chang, F. K. Human Response to Motions in Tall Buildings. Journal of Structural Division, Proc., Am. Soc. Civ. Engrs., Vol. 99, June 1973, pp. 1259-1272.

(13) Hansen, R. J., Reed, J. W. and Van Marcke, E. H. Human Response to Wind-Induced Motion of Buildings. Journal of Structural Division, Proc., Am. Soc. Civ. Engrs., Vol. 99, July 1973, pp. 1587-1605.

(14) Walshe, D. E. Measurements of Wind Force on a Model of a Power Station Boiler House at Various Stages of Erection. National Physical Laboratory, NPL Aero Report 1165, September 1965, Teddington, England.

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21

Table B·I

WIND INDUCED BUILDING MOTIONS: EXAMPLES OF CALCULATED PEAK ACCELERATIONS

(a) IN ALONG·WIND (D) AND ACROSS·WIND (W) DIRECTIONS AT TOP OF BUILDING (H)

Expo- q

=

6.6 psf (1/ I 0 basis) q

=

8.1 psf (I / 10 basis) q = 9.3 psf (1/10 basis)

Zone sure Factor V(H) aD aw V (H) V(H) aw Ce in fps in %g in%g in fps in g in g in fps in g in %g Case I: 400 x 175 x 100 ft building(l) OPEN 2.07 104 1.26 3.19 115 1.71 4.47 124 2.11 5.62 SUBR 1.55 90 0.95 1.98 100 1.30 2.78 108 1.61 3.49 CITY 1.09 76 0.72 1.10 84 0.98 1.55 90 1.22 1.94 Case 2: 400 x 100 x 175 ft building(1l OPEN 2.07 104 2.72 1.98 115 3.70 2.77 124 4.55 3.48 SUBR 1.55 90 2.06 1.23 100 2.82 1.72 108 3.48 2.17 CITY 1.09 76 1.55 0.68 84 2.13 0.96 90 2.63 1.21 i Case 3: 500 x 175 x 100 ft building!!; OPEN 2.20 108 1.90 3.18 119 2.56 4.46 128 3.13 5.60 SUBR 1.73 95 1.53 2.15 106 2.07 3.01 113 2.54 3.77 CITY 1.27 82 1.22 1.29 91 1.66 1.81 97 2.05 2.27 Case 4: 500 x 100 x 175 ft buildinglll OPEN 2.20 108 2.41 3.75 119 3.29 5.25 128 4.05 3.60 SUBR 1.73 95 1.92 2.53 106 2.63 3.55 113 3.25 4.45 CITY 1.27 82 1.52 1.52 91 2.08 2.14 97 2.58 2.68 Case 5: 600 x 100 x 100 ft building(1) OPEN 2.31 110 2.15 4.89 122 2.92 6.85 130 3.57 8.61 SUBR 1.90 100 1.80 3.52 III 2.44 4.94 119 3.00 6.21 CITY 1.45 87 1.49 2.27 97 2.03 3.18 104 2.50 3.99 Case 6: 600 x 100 x 150 ft building\li OPEN 2.31 110 1.62 2.66 122 2.21 3.72 130 2.72 4.68 SUBR 1.90 100 1.35 1.91 III 1.84 2.68 119 2.27 3.37 CITY 1.45 87 1.11 1.23 97 1.52 1.73 104 1.88 2.17 Case 7: 800 x 250 x 125 ft building(') OPEN 2.51 115 0.93 3.01 127 1.26 4.22 136 1.54 5.30 SUBR 2.19 107 0.83 2.41 119 1.13 3.38 128 1.38 4.24 CITY 1.79 97 0.74 1.72 107 1.00 2.42 liS 1.23 3.03 Case 8: 800 x 125 x 250 ft buildingm OPEN 2.51 lIS 2.12 1.97 127 2.88 2.76 136 3.52 3.47 SUBR 2.19 107 1.89 1.57 119 2.57 2.21 128 3.15 2.77 CITY 1.79 97 1.68 1.12 107 2.28 1.58 liS 2.81 1.98 Col. I 2 3 4 5 6 7 8 9 10 II Note to Table B-1:

(I 'Full dimensions and properties of Cases I to 8 are given in the Table at the top of p. 22.

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22

Note to Table 8-1 (Cont'd)

Height H. Density. in D Direction in W Direction

Case

ft Ib/cu ft

Dimen. Frequency Damping Dimen. Frequency Damping

I 400 9.0 175 0.250 0.015 100 0.200 0.010 2 400 9.0 100 0.200 0.010 175 0.250 0.015 3 500 10.0 175 0.175 0.010 100 0.200 0.010 4 500 \0.0 100 0.200 0.010 175 0.175 0.010 5 600 I \.0 100 0.200 0.015 100 0.200 0.015 6 600 11.0 100 0.200 0.020 150 0.200 0.020 7 800 12.0 250 0.150 0.015 125 0.125 0.010 8 800 12.0 125 0.125 0.010 250 0.150 0.015 Column I 2 3 4 5 6 7 8 9

II

L

111

L~~ -.l ~

Imt

/

I'-tt,

,

"

L I I I L I I I I I .

1000

800

600

500

L

lL 1 V

Il

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Figure 8-1 Exposure factor as a function of terrain roughness and heigh t above ground

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- 2019

World

Rights

Reserved

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CNRC

1941-2019

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Figure 8-2 Background turbulence factor as a function of width and height of structure

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- 2019

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1941-2019

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