CLIMATIC INFORMATION
for
BUILDING DESIGN IN CANADA
SUPPLEMENT No.
1
TO THE NATIONAL BUILDING CODE
OF CANADA
Issued
by
the
Associate Committee on the National Building Code
National Research Council of Canada
Ottawa
-
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i!i:,YfERIAL
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ASSOClATE COMMITTEE O N
THE NATIONAL BLllLDlNG CODE
A .
G. Wilson (Chuirniun)
H.
B.
Dickens
( D e p u ~ , '
C k u i r n i ~ ~ n )
B.
A .
Bonser
R . F.
Buckingham
S.
D. C . Chutter
D.
E.
Cornish
S.
C u m m i n g
R . F.
D e G r a c e
M. G . Dixon
J .
T . G r e g g
R.
V. Hebert
D ' A .
G . Helmer
J. S. Hicks
M .
S. H urst
(v.vo/j;c.io)
H . K . Jenns
H. T.
Jones
P.
M . Kettnleyside
J . Longworth
Retired*
J.
A.
McCambly
W.
M . McCance
R .
C . McMillan
J. M c Q u h a e
D. 0.
Monsen
(cn.v o[/~!licio)F.-X.
Perreault
G .
B.
Pope
R .
A .
W.
Switzer
R . T.
Tamhlyn
D.
L. Tarlton
A .
D. T h o m p s o n
J .
E.
Turnhull
N . G . Vokey
D. W .
Boy
J
(Re.srcrrr,h
A
dvisor-
Merc~orolog,,')
R . H .
D u n n (Sec.rerurlsj
*Committee term completed d u r i n g preparation
of
1977 Code.
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CLIMATIC
INFORMATION
for
BUILDING DESIGN IN
CANADA
1977
SUPPLEMENT No.
1
TO THE NATIONAL BUILDING CODE
OF CANADA
--
REFEFENCE
MATERIAL
i
1
DO
NOT
R E M O V E
I
1Issued
by
the
Associate Committee on the National Building Code
National Research Council of Canada
Ottawa
NRCC No.
15556
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First Edition 1953
Second Edition 1960
Third Edition 1965
Fourth Edition 1970
Fifth Edition 1975
Sixth Edition 1977
: . 'ONational Research Council of Canada 1977
World Rights ReservedPrinted in Canada
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TABLE
OF
CONTENTS
...
Preface
vii
...
Abbreviations
ix
...
January Design Temperatures
1
...
July Design Temperatures
2
...
Heating Degree-Days
3
...
Rainfall Intensity
3
...
One-Day Rainfall
4
...
Annual Total Precipitation
4
...
Snow Loads
4
...
Wind Effects
6
...
Seismic Zones
7
...
References
8
...
Table of Design Data for Selected Locations
9
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vii
PREFACE
The great diversity of climate in Canada has a considerable effect on the performance of build-
ings, consequently, their design must reflect this diversity. The purposes of this Supplement are to
explain briefly how the design weather values are computed and to present recommended design
data for a number of cities, towns and smaller populated places. It is through the use of such data
that appropriate allowances can be made for climate variations in different localities of Canada
and that the National Building Code can be applied nationally.
The design data in this Supplement are based on weather reports supplied by the Atmospheric
Environment Service, Department of the Environment (formerly the Meteorological Branch,
Department of Transport). They have been collected and analysed, where necessary, for the Asso-
ciate Committee on the National Building Code by Donald
W.
Boyd, Department of the Environ-
men t Meteorologist with the Division of Building Research, National Research Council of
Canada. Mr. Boyd has also devised appropriate methods and estimated the design values for all
the locations in the Table of Design Data for Selected Locations in Canada where weather obser-
vations were lacking or inadequate.
The weather data in this edition are provided in SI metric units in line with current practice of
reporting weather information and in recognition of the decision to work towards the adoption of
SI units in construction. These values may be converted to imperial units if required using the
appropriate conversion factors given in the National Standard of Canada CAN3-2234.1-76,
"Canadian Metric Practice Guide."
As it is not practical to list values for all municipalities in Canada, recommended design
weather data for locations not listed can be obtained by writing to the Meteorologist, Division of
Building Research, National Research Council of Canada, Ottawa, Ontario K I A OR6. It should
be noted, however, that these recommended values may differ from the legal requirements set by
provincial or municipal building authorities.
The information on seismic zones has been provided by the Earth Physics Branch of the
Department of Energy, Mines and Resources. The table now includes the ground acceleration
ratio "A" as well as the seismic zone designation. Information for municipalities not listed may be
obtained by writing to the Seismology Division, Earth Physics Branch, Department of Energy,
Mines and Resources, Ottawa, Ontario K I A OE4.
The Charts included in previous editions of this Supplement have been omitted since they are
not intended to be used for design purposes and their inclusion may imply such use. Two of these
Charts, one showing seismic zones and the other permafrost distribution, have been included in
Supplement No. 4 to the 1977 National Building Code in Commentaries J and L, respectively.
Comments on this document are welcomed by the Associate Committee and should be for-
warded to the Secretary, Associate Committee on the National Building Code, National Research
Council of Canada, Ottawa, Ontario K1A OR6.
Le Code national du bitiment, ses supplements et les documents qui s'y rattachent sont disponi-
bles en franqais. On peut se les procurer en s'adressant au Secretaire, ComitC associe du Code
national du bitiment, Conseil national de recherches du Canada, Ottawa, Ontario K 1 A OR6.
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LIST OF ABBREVIATIONS
Abbreviations of words and phrases in this Supplement have the following meanings:
ACNBC
. . .
Associate Committee on the National Building Code
A
. . .
Acceleration ratio
ann.
. . .
annual
OC
. . .
degree(s) Celsius
. . .
OF
degree(s) Fahrenheit
dept.
. . .
department
gnd.
. . .
ground
Kg/m3
. . .
kilogram(s) per cubic metre
kN/m2
. . .
kilonewton(s) per square metre
min.
. . .
minute(s)
m/s
. . .
metre(s) per second
NBC
. . .
National Building Code of Canada
pcpn.
. . .
precipitation
psf
. . .
pound(s) per square foot
p
. . .
Page
Rain.
. . .
Rainfall
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CLIMATIC INFORMATION
for
BUILDING DESIGN IN CANADA
The choice of climatic elements tabulated in this Supplement and the form in which they are
expressed have been dictated largely by the requirements for specific values in several sections of
the National Building Code of Canada. Heating degree-days and annual total precipitation are
also included. The following notes explain briefly the significance of these particular elements in
building design, and indicate what observations were used and how they were analysed to yield
the required design values. To estimate design values for location where weather observations
were lacking or inadequate, the observed or computed values for the weather stations were plotted
on large-scale maps. Isolines were drawn on these working charts to show the general distribution
of the design values.
In the Table, design weather data are listed for over
600
locations, which have been chosen for a
variety of reasons. Incorporated cities and towns with populations of over 5 000 have been
included unless they are close to other larger cities. For sparsely populated areas many smaller
towns and villages have been listed. The design weather data for weather stations themselves are
the most reliable and hence these stations have often been listed in preference to locations with
somewhat larger populations. A number of requests for recommended design weather data for
other locations have been received, and where most of the elements were estimated, they were also
added to the list. In some cases the values obtained from the large-scale charts have not been
rounded off.
The Table of design values should not be expected to give a complete picture of the variations
of these climatic elements. If application is made to the Building Research Meteorologist as men-
tioned in the Preface, values will be estimated for locations not listed in the table using the list of
observed or computed values for weather stations, the large-scale manuscript charts and any other
relevant information that is available. In the absence of weather observations
at
any particular
location, a knowledge of the local topography may be important. For example, cold air has a tend-
ency to collect in depressions, precipitation frequently increases with elevation and winds are gen-
erally stronger near large bodies of water. These and other relationships affect the corresponding
design values and will be taken into consideration where possible in answering inquiries.
All the weather records that were used in preparing the table were, of necessity, observed at
inhabited locations, and hence interpolations from the charts or the tabulated values will apply
only to locations at similar elevations and with similar topography. This is particularly significant
in mountainous areas where the values apply only to the populated valleys and not to the moun-
tain slopes and high passes, where, in some cases, very different conditions are known to exist.
JANUARY
DESIGN TEMPERATURES
A building and its heating system should be designed to maintain the inside temperature at
some pre-determined level. To do this it is necessary to know the most severe weather conditions
under which the system will be expected to function satisfactorily. Failure to maintain the inside
temperature at the pre-determined level will not usually be serious if the temperature drop is not
great and if the duration is not long. The outside conditions used for design should, therefore, not
be the most severe in many years, but should be the somewhat less severe conditions that are occa-
sionally but not greatly exceeded.
Winter design temperature is based on an analysis of winter air temperatures only. Wind and
solar radiation also affect the inside temperature of most buildings, but there is no convenient way
of combining their effects with that of outside air temperature. Some quite complex methods of
taking account of several weather elements have been devised and used in recent years, but the use
of average wind and radiation conditions is usually satisfactory for design purposes.
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The winter design temperature is defined as the lowest temperature at or below which only a
certain small percentage of the hourly outside air temperatures in January occur. In previous edi-
tions of this Supplement the January design temperatures were obtained from a tabulation of
hourly temperature distributions for the ten-year period 1951 to 1960 for 11
8 stations prepared by
the then Meteorological Branch of the Department of Transport. Hourly data summaries(') (which
include temperature frequency distributions) based on the 10-year period 1957 to 1966 have been
published for several stations each year since 1967 and are now available for 109 stations. They
provide a second set of January design temperatures. For the 69 stations that appeared in both
lists the current design temperature is the average of these 2, and is, therefore, based on the 16-
year period 1951 to 1966 with a 4-year overlap. For the 89 stations that appeared in only
1
of the
lists, the design temperatures were adjusted to make them more consistent.
The January design temperatures for all the other locations in the table are estimates. The esti-
mates in earlier editions of this Supplement have all been considered and, where necessary,
adjusted to make them more representative of the 16-year period. Most of the adjustments were
less than one Celsius degree and only about 16 exceeded one and a half degrees.
The 2 tabulations and the calculations above were all in Fahrenheit degrees. These were con-
verted to Celsius and rounded off to the nearest degree.
The adjustments mentioned above are an indication of the variation in the design temperature
from one decade to another. The design temperatures for the next 20 or 30 years may differ from
the tabulated values by one or two Celsius degrees and, of course, the year to year variation will be
much greater. Most of the temperatures were observed at airports. Design values for the core areas
of some large cities could be a degree or two milder but values for the fringe areas are probably
about the same as for the airports. No adjustments have been made, therefore, for the city effect.
The 2% per cent January design temperature is the value ordinarily used in the design of heating
systems. In special cases when the control of inside temperature is more critical, the 1 per cent
value may be used.
JULY DESIGN TEMPERATURES
A building and its cooling and dehumidifying system should be designed to maintain the inside
temperature and humidity at certain pre-determined levels. To do this it is necessary to know the
most severe weather conditions under which the system will be expected to function satisfactorily.
Failure to maintain the inside temperature and humidity at the pre-determined levels will usually
not be serious if the increases in temperature and humidity are not great and if the duration is not
long. The outside conditions used for design should, therefore, not be the most severe in many
years, but should be the somewhat less severe conditions that are occasionally but not greatly
exceeded.
The summer design temperatures in this Supplement are based on an analysis of July air tem-
peratures and humidities only. Wind and solar radiation also affect the inside temperature of most
buildings and may in some cases be of more importance than the outside air temperature. It
seems, however, that no method of allowing for variations in radiation has yet become generally
accepted. When requirements have been standardized, it may be possible to provide more com-
plete weather information for summer conditions, but in the meantime only dry-bulb and wet-
bulb design temperatures can be provided.
The frequency distribution of combinations of dry-bulb and wet-bulb temperatures for each
month from June to September have been tabulated for 33 Canadian weather stations by
Bo~ghner.'~)
If the summer dry-bulb and wet-bulb design temperatures are defined as the tempera-
tures that are exceeded 2Y2 per cent of the hours in July, then design values can be obtained
directly for these 33 stations.
The dry-bulb design temperatures in previous editions of this Supplement were based on the
values for these 33 stations and a relationship between the design temperatures and the mean
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annual maximum temperatures. Hourly data summaries(') (which include temperature frequency
distributions) based o n the 10-year period 1957 to 1966 are now available for 109 stations. They
provide a second set of July dry-bulb design temperatures. For the 109 stations the current dry-
bulb temperatures are the averages of the values in these 2 sets. For all the other locations in the
table the previous values have been adjusted to make them consistent with the calculated values.
The adjustments exceeded one Celsius degree in only about 20 cases. All values were converted to
degrees Celsius and rounded off to the nearest degree.
The July wet-bulb design temperatures have been obtained in the same way, with one excep-
tion. The previous values were obtained directly for the
33
stations in Boughner's p ~ b l i c a t i o n , ' ~ )
and all the rest were estimated from these 33 without using any intermediate statistic. The current
values for the 109 stations with hourly data summaries are averages between the previous values
and the values from the hourly data summaries. For all the other locations the previous values
have been adjusted to make them consistent. The adjustments exceed one Celsius degree in only 6
cases. All wet-bulb values were converted to degrees Celsius and rounded off to the nearest degree.
HEATING DEGREE-DAYS
It has long been known that the rate of consumption of fuel or energy required to keep the inte-
rior of a small building at about 70°F (2 1.1 "C) when the outside air temperature is below 65°F
(18.3"C) is roughly proportional to the difference between 65°F and the outside temperature.
Wind speed, solar radiation, the extent to which the building is exposed to these elements and the
internal heat sources also affect the heat required, but there is no convenient way of combining
these effects. For average conditions of wind, radiation, exposure and internal sources, however,
the proportionality with the temperature difference still holds. Heating degree-days based on tem-
perature alone are, therefore, still useful when more complex methods of calculating fuel require-
ments are not feasible.
It has been decided that, for Canada, heating degree-days in the future will be the degree days
below 18°C. This is slightly below 65°F but for practical purposes the difference is not important.
Since the fuel required is also proportional to the duration of cold weather, a convenient
method of combining these elements of temperature and time is to add the differences between
18°C and the mean temperature for every day in the year when the mean temperature is below
18°C. It is assumed that n o heat is required when the mean outside air temperature for the day is
18°C or higher.
Degree days below 18°C have been computed day by day for the 30-year period 1941 to 1970
for about 92 stations. The averages of the annual totals for these stations are given in the table to
the nearest degree day.
For all the other locations in the table the degree-days below 65°F in the previous edition of this
Supplement were converted to degree-days below 18°C and rounded off to the nearest 10 degree-
days. Adjustments ranging from 80 to 120 Celsius degree-days were made to allow for the differ-
ences between 65°F a n d 18°C.
A difference of only one Celsius degree in the annual mean temperature will cause a difference
of 250 to 350 in the Celsius degree-days. Since differences of half a degree in the annual mean tem-
perature are quite likely to occur between
2
stations in the same city or town, it is obvious that
heating degree-days can not be relied on to a n accuracy of less than about 100 degree-days.
RAIN
FALL
INTENSITY
Roof drainage systems are designed to carry off the rainwater from the most intense rainfall that
is likely to occur. A certain amount of time is required for the rainwater to flow across or down the
roof before it enters the gutter or drainage system. This results in the smoothing out of the most
rapid changes in rainfall intensity. The drainage system, therefore, need cope only with the flow of
rainwater produced by the average rainfall intensity over a period of a few minutes which can
be
called the concentration time.
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In Canada it has been customary to use the 15-min. rainfall that will probably be exceeded on
the average once in 10 years. The concentration time for small roofs is much less than 15 min. and
hence the design intensity will be exceeded more frequently than once in 10 years. The safety fac-
tors included in the tables in the ACNBC Canadian Plumbing Code will probably reduce the fre-
quency to a reasonable value and, in addition, the occasional failure of a roof drainage system will
not be particularly serious in most cases.
dThe rainfall intensity values tabulated in the previous edition of this Supplement were based on
measurements of the annual maximum 15-min. rainfalls at 139 stations with 7 or more years of
record. They were the 15-min. rainfalls that would be exceeded once in 10 years on the average, or
the values that had 1 chance in 10 of being exceeded in any 1 year. They were computed or esti-
mated to the nearest tenth of a n inch. The current values in millimetres were obtained by a direct
conversion, and hence almost all the values end in 0 , 3 , 5 or
8.
It is very difficult to estimate the pattern of rainfall intensity in mountainous areas where precip-
itation is extremely variable. The values in the table for British Columbia and some adjacent areas
are mostly for locations in valley bottoms or in extensive, fairly level areas. Much greater intensi-
ties may occur on mountain sides.
ONE-DAY RAINFALL
If for any reason a roof-drainage system becomes ineffective, the accumulation of rainwater
may be great enough in some cases to cause a significant increase in the load on the roof.
Although the period during which rainwater may accumulate is unknown, it is common practice
to use the maximum 1-day rainfall for estimating the additional load.
For most weather stations in Canada the total rainfall for each day is published. The maximum
"I-day" rainfall (as it is usually called) for several hundred stations has been determined and pub-
lished by the Atmospheric Environment Ser~ice.'~)
Since these values are all for predetermined 24-
hr periods, beginning and ending at the same time each morning, it is probable that most of them
have been exceeded in periods of 24-hr including parts of 2 consecutive days. The maximum "24-
hr" rainfall (i.e. any 24-hr period) according to Hershfield and Wilson is, on the average, about
1 13 per cent of the maximum
"
1 -day" rainfall.(4)
Most of the 1-day rainfall amounts in the table have been copied directly from the latest edition
of Climatic Normals'j) where the record maximum values are tabulated in millimetres. Values for
the other locations have been converted to millimetres from the estimated values in the previous
edition of this Supplement. These maximum values differ greatly within relatively small areas
where little difference would be expected. The variable length of record no doubt accounts for part
of this variability, which would probably be reduced by an analysis of annual maxima instead of
merely selecting the maximum in the period of record.
ANNUAL TOTAL PRECIPITATION
The total amount of precipitation that normally falls in 1 year is frequently used as a general
indication of the wetness of a climate. As such it is thought to have a place in this Supplement.
Total precipitation is the sum in millimetres of the measured depth of rainwater and
1/10of the
measured depth of snow (since the average density of fresh snow is about
' / l othat of water).
Most of the average annual total precipitation amounts in the table have been copied directly
from the latest edition of Climatic Normals(j) where averages for the 30-year period 1941 to 1970
have been tabulated in millimetres. For all other locations the estimates in the previous edition of
the Supplement have been converted to millimetres and rounded off to the nearest 10 mm.
SNOW LOADS
The roof of a building should be able to support the greatest weight of snow that is likely to
accumulate on it. Some observations of snow loads on roofs have been made in Canada, but they
are not sufficiently numerous to form the basis for estimating snow loads throughout the country.
Similarly, observations of the weight or water equivalent of the snow on the ground are inade-
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3n
nd
LC-re-
rill
ip-
:as
ter
'of.
ice
Im
lb-
24-
em
24-
3ut
Ion
for
3 U Seas
)art
l
of
:ral
:nt.
the
try.
I
Lde-
quate. The observations of roof loads and water equivalent are very useful, as noted below, but the
basic information for a consistent set of snow loads must be the measured depth of snow on the
ground.
The estimation of the design snow load on a roof from snow depth observations involves the fol-
lowing steps:
1. The depth of snow on the ground which will be equalled or exceeded once in 30 years, on
the average, is computed.
2.
A density is assumed and used to convert snow depths to loads.
3. An adjustment is added to allow for the increase in the load caused by rainwater absorbed
by the snow.
4. Because the accumulation of snow on roofs is often different from that on the ground, cer-
tain adjustments should be made to the ground snow load to provide a design snow load on
a roof.
These steps are explained in more detail in the following paragraphs.
The annual maximum depths of snow on the ground for periods ranging from
5
to 3
1
years are
now available for about 480 stations. Many of these have such short records that they cannot be
considered reliable, but on the other hand they cannot be ignored. About a quarter of the stations
have records of at least 20 years which is much more information than was used for previous esti-
mates of snow loads. These data were assembled and analysed using Gumbel's extreme value
method as explained by Boyd.(5) The resulting values are the snow depths which will probably be
exceeded once in 30 years on the average, or which have a probability of
1
in 30 of being exceeded
in any one year.
The specific gravity of old snow generally ranges from 0.2 to 0.4 times that of water. It is usually
assumed in Canada that 0.1 is the average specific gravity of new snow. The 30-year maximum
snow depth will almost certainly occur immediately after an unusually heavy snowfall, and hence
a large proportion of the snow cover will have a low density. It therefore seemed reasonable to
assume a mean specific gravity under these unusual circumstances of 0.2 for the whole snow cover.
This is slightly higher than the 0.192 which was previously used for the sake of convenience when
working with inches and psf.
Because the heaviest loads in Canada frequently occur when early spring rain adds to an
already heavy snow load, it was considered advisable to increase the snow load by the load of
rainwater that it might retain. It is convenient to use the maximum 1-day rainfall in the period of
the year when snow depths are greatest. Boyd has explained how a 2- or 3-month period was
sele~ted.'~)
The results from a survey of several winters of snow loads on roofs indicated that average roof
loads were generally much less than loads on the ground. The conditions under which the design
snow load on the roof may be taken as 80 or
60
per cent of the ground snow load are given in Sec-
tion 4.1 of the National Building Code 1977. The Code also permits further decreases in design
snow loads for steeply sloping roofs, but requires substantial increases for roofs where snow accu-
mulation may be more rapid. Recommended adjustments are given in NBC Supplement No.
4,
"Commentaries on Part
4
of the National Building Code of Canada 1977."
The ground snow loads computed in kilonewtons per square nietre were all plotted on maps as
an aid in estimating values for the other locations listed in the table. All values are tabulated to the
nearest tenth of a kilonewton per square metre but some may be in error by 10 per cent.
Tabulated values cannot be expected to indicate all the local differences in ground snow loads,
even where these are known to exist. The values in the table are intended to apply only to the area
within a town or village and not necessarily to extended areas such as townships. This fact is par-
ticularly important in mountainous areas where much higher snow loads often occur on mountain
slopes or high passes.
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WIND EFFECTS
All structures should be built to withstand the pressures and suctions caused by the strongest
gust of wind that is likely to blow at the site in many years. For many buildings this is the only
wind effect that needs to be considered, but tall or slender structures should also be designed to
limit their vibrations to acceptable levels. Wind induced vibrations may require several minutes to
build up to their maximum amplitude and hence wind speeds averaged over several minutes or
longer should be used for design. The hourly average wind speed is the value available in Canada.
The provision of "velocity pressures" for both average wind speeds and gust speeds for estimat-
ing pressures, suctions and vibrations involves the following steps:
1. The annual maximum hourly wind speeds were analysed to obtain the hourly wind speeds
that will have
1 chance in l0,30 and 100 of being exceeded in any 1 year.
2. An average air density was assumed in order to compute the "velocity pressures" for the
hourly wind speeds.
3.
A value of 2 was assumed for the "gust effect factor" to compute the "velocity pressures"
for the gust speeds.
The actual wind pressure on a structure increases with height and varies with the shape of the
structure. The factors needed to allow for these effects are tabulated in Section 4.1 of the National
Building Code of Canada 1977 and in Supplement No. 4. The other 3 steps are discussed in more
detail in the following paragraphs.
Until recently the only wind speed record kept at a large number of wind-measuring stations in
Canada was the number of miles of wind that pass an anemometer head in each hour, or the
hourly average wind speed. Many stations are now recording only spot readings of the wind speed
each hour, and these may have to be used for design at some future time. For the present, how-
ever, the older hourly mileages are the best data on which to base a statistical analysis. The annual
maximum hourly mileages for over 100 stations for periods from 10 to 22 years were analysed
using Gumbel's extreme value method to calculate the hourly mileages that would have one
chance in 10,30 and 100 of being exceeded in any 1 year.
Values of the
"1
in
30"
hourly mileages for the additional 500 locations in the table have been
estimated. To obtain the "1 in 10" and "1 in 100" values for these locations it was necessary to
estimate the value of the parameter l / a which is a measure of the dispersion of the annual maxi-
mum hourly mileages. The 100 known values were plotted on a map from which estimates of l / a
were made for the other locations. Knowing the "1 in 30" hourly mileages and the values of l / a ,
the "
1
in 10" and
"
1 in 100" values could be computed.
Pressures, suctions and vibrations caused by the wind depend not only on the speed of the wind
but also on the air density and hence on the air temperature and atmospheric pressure. The pres-
sure, in turn, depends o n elevation above sea level and varies with changes in the weather systems.
If V is the design wind speed in miles per hour, then the velocity pressure, P, in pounds per square
foot is given by the equation
P
=cv2
where C depends on air temperature and atmospheric pressure as explained in detail by Boyd.@)
The value 0.0027 is within 10 per cent of the monthly average value of C for most of Canada in the
windy part of the year. This value (0.0027) has been used to compute all the velocity pressures cor-
responding to the hourly mileages with annual probabilities of being exceeded of 1/10, 1/30 and
1/100. The pressures were then converted from psf to kN/m2 and are shown in the table in col-
umns headed only by the numerical values of the probabilities.
The National Building Code requires the design gust pressures for structural elements to be
twice the corresponding hourly pressures in the table. Because wind speeds are squared to get
pressures, this statement is equivalent to saying that the gust factor is the square root of 2.
For buildings over 12 m high, the gust velocity pressures and suctions must be increased accord-
ing to a table in Section 4.1 of the National Building Code 1977 which is based on the assumption
Copyright
©
NRC
1941
- 2019
World
Rights
Reserved
©
CNRC
1941-2019
Droits
réservés
pour
tous
pays
the
the
nal
ore
;in
the
zed
)W-ual
sed
me
sen
'to
txi-
1
/ a
/ a ,
ind
res-
ms.
are
d.(6)
the
:or-
ind
:ol-
that the gust speed increases in proportion to the
'/lopower of the height. The average wind speeds
used in computing the vibrations of a building are more dependent on the roughness of the under-
lying surface. A method of estimating their dependence on roughness and height is given in Sup-
plement No. 4.
The calculations for building vibrations in Supplement No. 4 have been drawn up for wind
speeds measured in metres per second. The equation
P
=cv2
could be used to convert the tabulated pressures to wind speeds provided the constant C was con-
verted to SI units. If
P
is in newtons per square metre and V in metres per second, the value of C
would be 0.64689. In SI units, however, the equation can be written in the form
P
=X P V 2
where
pis the air density in kg/m3. The density of dry air at O°C and the standard atmospheric
pressure of 101.325 kPa is 1.2929 kg/m3. Half this value, or 0.64645, is very close to the converted
value of C. The difference (less than
1
in
1
000) is negligible and therefore the density of air at O°C
and standard atmospheric pressure has been adopted for converting wind pressures to wind
speeds. The following table has been arranged to give speeds to the nearest m/s for all pressures
appearing in the main table.
Note to Table:
( I )
