Snow Load Canada

Ser TH1 National Research Conseil m b d N21d

ROOF SNOW LOADS IN CANADA

1 by D. A. Taylor

Reprinted from Canadian Journal of Civil Engineering Vol. 7, No. 1 , March 1980 pp. 1-18

1 DBR Paper No. 882 Division of Building Research

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Volume 7 Number 1 March 1980 Volume 7 numero 1 mars 1980

Roof snow loads in Canada 4

D. A. TAYLOR Building Structures Section, Division of Building Research, National Research Council of Canada, Ottawa, Ont., Canada KIA OR6

Received April 17, 1979 Revised manuscript accepted October 18, 1979

The National Building Code of Canada requires buildings to be designed tocarry uniformly and nonuniformlv distributed snow loads and the "Commentarv on Snow Loads" in S u ~ ~ l e m e n t No. 4 to the ~ A i o n a l Building Code of Canada gives detailed design information.-'This paper discusses the material given in the 1977 commentary and supplements it with examples and photographs.

Le Code national du bliment du Canada exige que les bitiments soient en mesure de supporter des surcharges de neige reparties egalement ou inegalement et le "Commentaire sur les surcharges de neige" dans le Suppldment No 4 du Code donne de I'inforrnation detaillee pour le calcul. Cette communication traite de la matiere dans le commentaire 1977 et I'enrichie d'exemples et de photographies.

Can. J . Civ. Eng., 7,l-18 (1980)

Introduction The Division of Building Research of the National

Research Council of Canada has conducted surveys of snow on buildings since 1956 to provide a basis for improving design snow loads in building codes. The first survey, carried out at approximately 10 locations across Canada, examined how snow accumulated on house roofs, flat and multilevel roofs, and the curved roofs of some aircraft hangars. On the basis of the survey results (Lutes 1970), a number of special case histories (Schriever et al. 1967; Lutes and Schriever 1971), and research and codes from other countries, a supplement and a "Commentary on Snow Loads" were provided for the 1965 National Building Code of Canada (NBC) (Schriever and Peter 1965; Schriever and Longworth 1965).

In 1967-1968 a survey of flat roofs, both simple and multilevel, was started at five locations across Canada. At the end of its twelfth year it has produced just enough data to begin statistical treatment. Surveys of the density of ground and roof snow and pilot surveys of snow on mobile homes and arched roofs are also in progress. In addition, a pilot study of the effect of roof slope and roofing material on accumulation of snow has been under way for 4

years. Further, P. Schaerer, of the British Columbia Regional Station of the National Research Council of Canada, has been collecting data on the specific gravity of ground snow in the mountains (Schaerer 1970) and on the effect of elevation on ground snow loads.

Accuracy of Design Loads The process of measuring snow loads is relatively

imprecise. Measurements of depth and density are taken with metre sticks and sampling tubes. Prior to 1978 densities were obtained by taking samples horizontally (from a vertical section cut through the full depth), using a short 3.4 in. (86 mm) tube of 250 mL volume. The samples were collected in closed containers and weighed in the laboratory. During the winter of 1977-1978 a vertical tube sampler, MSC type 1, was obtained. This 40 in. (1 m) long 2.78 in. (70 mm) diameter tube is inserted vertically into the snow with a careful twisting action, allowing the sharp cutting teeth to penetrate crust and ice layers. When the cutting edge reaches a plate inserted at the surface to protect the roof covering, the sam- ple is removed and weighed on location.

Measurements are often taken in bitter weather, with no control and sometimes little knowledge of

03 15-1468/80/010001-18SO1 .MI0 @I980 National Research Council of Canada/Conseil national de recherches du Canada

2 CAN. J. CIV. ENG. VOL. 7. 1980

FIG. 1. Removal of solid ice from Metcalfe Arena, Ontario, 9 February 1978 (g = 60 psf (2.9 kPa)). 5 in. (13 cm) of ice was left to protect the roof surface.

some of the factors of influence. As a result, the measured loads and design coefficients computed from these measurements are only approximate. On complex roofs where the designer estimates reasonable depths and computes loads from recommended densities, the results will be even less exact. I t is apparent, therefore, that at the best of times it is unreasonable to assume that design loads have any great precision.

National Building Code Commentary on Snow Loads

Many architects and engineers designing roofs in Canada use the "Commentary on Snow Loads1" (NBC 1977a) or other documents derived from it. This commentary contains useful information but it must be used sensibly. If the material is properly understood and common sense and knowledge of local conditions are applied, the designer will be reasonably well equipped to deal with snow and ice loads on many roof shapes.

Snow on the Ground Specijic Gravity The specific gravity of snow on the ground varies

over a wide range, depending primarily on the weather to which it has been subjected (air tempera- ture, humidity, sun, wind, rain), on ground tem- perature, depth of snow, and the manner in which the snow was deposited. Generally it ranges from 0.05-0.1 for light new snow to about 0.4 for old snow.

'Henceforth referred to as the commentary.

After a day or two the specific gravity of new snow may be as high as 0.2 and of wet snow even higher. On the Prairies and in the north snow in wind-packed drifts can be very hard and dense (SG = 0.35-0.4) immediately after wind storms of some days' duration.

Prediction of Depth Snow accumulates on the ground in uniform layers

where there is no wind and in drifts, usually against obstructions, where there is exposure to the wind. In Canada there are few well-sheltered locations near populated areas and drift-free accumulation of snow is usually found only in large forests or sheltered mountain valleys. Unfortunately this makes it difficult to find good sites on which to measure ground snow loads. Land around meteorological stations where readings of snow depths are sometimes taken is often quite exposed because many such stations are at airports and therefore not at all suited to the measurement of ground snow depths. Nevertheless, the snow depths measured at meteorological stations across Canada are the best measurements available. Boyd, meteorologist at the Division of Building Research (DBR) in Ottawa, used this ground depth data from 480 stations to make statistical predictions of the 1-in-30 year depths at these and other locations (Boyd 1961; NBC 19773). The l-in-30 year or 30 year return depth is the depth that is likely to be exceeded once in 30 years, on the average, or has a 1-in-30 chance of being exceeded in any one year.

The specific gravity assumed for the calculation of load was 0.2. This is somewhat lower than measured values, but it was considered justified because

TAYLOR

-. 3,s. 8 in. (10.2 cm) /' I

12 In. (30.5 cm) ICE ICE '

+ 3 ft (90 cm) SNOW

FIG. 2. Sketch of Metcalfe Arena showing ice thickness.

the maximum depth of snow would usually occur immediately after a deep layer of new, lighter snow had fallen on top of the old (Boyd 1961). This figure is not conservative, however, for snow on the slopes of mountain valleys in heavy snow regions of British Columbia where it could be about 0.30-0.35 or for areas where windpacked snow is common. Local experience will have to guide designers.

To the loads computed using a specific gravity of 0.2, Boyd added the load resulting from the maximum 24 h rainfall during the winter months, when snow depths are greatest, to obtain the NBC ground load. The contribution of rain loads was arbitrarily limited to 50x of the total load because it is unlikely that snow will retain any more water.

Snow on Buildings Wind is the dominant factor affecting distribution C; = 0 . 8 . 8

of snow on and around buildings. Though the con- tern here is primarily with snow on roofs, it is important to note as well that the effect of snow on the ground around buildings should be considered and . that orientation and landscaping may be critical in TYPICAL VALUES: keeping doors, exits, and loading ramps free of snow drifts and the cost of regular snow clearance to a minimum (Theakston 1961 ; Schaerer 1972).

For design purposes it has been the practice to use the ratio of snow on the roof to snow on the ground as a coefficient, C,, that depends mainly on building geometry. The design load coefficient for uniformly distributed load on a well-insulated or unheated roof in a perfectly sheltered location (which might be found in some mountain valleys of western Canada) is C, = 1.0. For most other sheltered locations C, may be reduced wind, and FIG. 3. Flat and shed roofs. For roofs exposed to the wind sublimation to about 0.8, and for buildings com- on all sides, all values of C, marked with an asterisk (*) may pletely exposed on all sides, to 0.6. The basic coeffi- be reduced by 25%.

s a

SHELTERED EXPOSED

0 " TO 30" 0 . 8 0 . 6

40" 0 . 6 0 .45

50" 0 . 4 0 . 3

60" 0 . 2 0 .15

70" TO 90" 0 0

4 CAN. 1. CIV. ENG. VOL. 7, 1980

FIG. 4. Roofs at DBR/NRCC (Division of Building Research)experimental site in Ottawa varying in slope from 20 to 60".

FIG. 5. Snow retained on 35" asphalt roof (second roof from foreground) at DBR/NRCC experimental site in Ottawa.

cient is 0.8, however, and should only be reduced to 0.6 if the structure is and will remain exposed to wind on all sides. To use C, equal to 0.6 the designer must be satisfied that the roof will not become sheltered by new buildings, trees, parapets, signs, etc., during its lifetime.

Statistical Roof Snow Loads Although the Division of Building Research has

been measuring snow on roofs since 1956, no surveys have lasted for more than 12 years; they are, therefore, not yet on a sound statistical footing. As a

result, roof snow loads are obtained by multiplying the 30 year return ground load by various coefficients (C,) that depend on external geometry and exposure to wind. There is, however, no statistical connection between roof and ground loads because the C, coefficients are not statistically derived.

So many factors such as heat transfer through the roof, slope, drainage, roof texture and colour, etc., can affect roof snow that a great many data (many more than are now available) are required for a reliable, direct index establishing the statistical connection between ground and roof loads.

TAYLOR 5

1

C A S E I 1 I

F O R a L 15" U S E C A S E I ONLY

FOR a > 15" U S E C A S E I A N D I 1

C A S E I

a C;

0" T O 70" 0 . 8 - f l

C A S E I1

FIG. 6. Gable or hip roofs. For roofs exposed to the wind on all sides, all values of C, marked with an asterisk (*) may - be reduced by 25%.

Density Density measurements have been taken on flat

roofs of industrial buildings over the last 12 years. Although analysis is not complete, the results indicate that the density of snow on flat roofs is usually higher than that of snow on the ground. There are a number of reasons for this. Melting due to heat loss through the roof, direct or reflected solar radiation, and radiation absorbed by the dark roof surface all cause an increase; in addition, poor drainage of melt- or rainwater as a result of the capillary or "sponge-like" action of the snow, inadequate roof drains, or drains blocked with ice and debris also contributes. A case illustrating high densities caused in part by poor drainage follows.

Figure 1 shows ice 12 in. (30 cm) deep being removed with chain saws from a completely exposed roof of an unheated arena just south of Ottawa. The

roof is flat with a gentle slope across the 110 ft (33.5 m) width of 1.7 ft (0.52 m), a 1.55% slope (Fig. 2). About 20 mm of rain during December plus one heavy rainstorm of 60 mm and a lighter one of about 20 mm during January, in combination with solar radiation, heat from lights in the arena, and snow on the roof, contributed to an amazing buildup that produced a load exceeding the NBC design value by a considerable amount. The total snowfall up to the fourth week in January had been above average, but between then and 9 February only 1-2 cm had fallen.

Results from the survey of multilevel flat industrial roofs in Ottawa show that the average specific gravity, SG, of snow in drifts formed where roofs change elevation varies approximately as shown below :

Snow only Mid-December SG = 0.20 + 0.05 Mid-January SG = 0.23 f 0.06 Mid-February SG = 0.27 f 0.06 Mid-March SG = 0.30 + 0.07

Snow plus ice, slush, and water at the roof surface Mid-December SG = 0.21 f 0.04 Mid-January SG = 0.25 + 0.08 Mid-February SG = 0.29 + 0.08 Mid-March SG = 0.33 f 0.08

The commentary recommends a specific gravity of 0.24 (i.e., density = 15 pcf (240 kg/m3)) for all roofs unless more specific local information is available. This value is supposed to correspond to the specific gravity of the maximum depth of snow on the roof, but the tabulated results indicate that it may not be conservative enough for drifts on multilevel flat roofs in the Ottawa area.

Snow Drifts Some roofs are geometrically so complex that it is

not appropriate to apply the various cases recommended in codes and commentaries directly in obtaining reasonable snow loads. In these situations a reversal of the usual way of arriving at the loads is helpful. The designer should consider dealing initially with volumes and depths of snow rather than with loads. Engineering judgement can then be applied to obtain reasonable profiles of the snow accumulations and loads can be computed from the depths, using a recommended or known density. This approach can also be used profitably in much simpler situations. For example, on a structure where snow slides from a larger upper roof to a much smaller, lower one, it may become evident that the lower roof is not large enough to retain the sliding volume from the upper and that appropriate adjustments in volumes and loads should be made. In the following

6 CAN. J . CIV. ENG. VOL. 7, 1980

FIG. 7. Unbalanced snow load on boathouse caused by wind (near Ottawa).

sections the design cases for the simpler roofs de- When a roof is sloped (Fig. 3), it has been observed scribed in Figs. H-l to H-7 of the commentary are that less snow usually accumulates during snowfall discussed in turn. and that it mav slide off all or art of the surface.

( I ) Flat and Shed Roofs (Fig. 3)-For design purposes snow on flat roofs is 80% of the 30 year return ground load, g, but may reach 100% in very sheltered locations or only 60% in a completely exposed en- vironment.'

On a very large flat roof the designer should consider that a greater depth of snow may be deposited and retained than on a smaller one (see Isyumov 1971; Isyumov and Davenport 1974); the average speed of the flow over a large roof is less than that over a smaller one because of the reduced over-all influence of the accelerated flow over the leading edge. If a large roof is reasonably well insulated, the design load should probably be higher than 60 or even 80% of the ground load, even if the roof is exposed. Some buildings now exist in Canada with dimensions of up to 1000 ft x 1000 ft (300 x 300 m) in plan, and the current provisions in the commentary were certainly not recommended with such roofs in mind.The designer should probably be very cautious where roofs exceeding about 400 ft (120 m) in the direction of the prevailing winter winds are involved.

especially if thk surface is smodth. The amount of accumulated snow is described by the product of the "slope-reduction" factor, P, and the basic roof load (Fig. 3). P is defined as follows: P = 1.0 when 0" I a < 30"; P = 1.0 - (a - 30°)/400 when 30" i a < 70"; and P = 0.0 when 70" I a < 90".

The author has been conducting a pilot experiment of snow on sloping roofs in a sheltered deciduous bush at the National Research Council of Canada to improve these slope-reduction equations (Fig. 4). Six 8 ft x 8 ft (2.44 m x 2.44 m) sloping roofs, three covered with green steel siding and three with green asphalt shingles, and one 8 ft x 8 ft (2.44 m x 2.44 m) flat roof covered with the green metal were built. There were one of steel at a 20" slope, one each of steel and asphalt at 35 and 50°, and one of asphalt at 60°, all unheated and facing north. As the study has been in operation for only five winters (since 1974-1975), not enough data are available yet to make predictions; but snow quite often slides off all except the 35" asphalt roof (Fig. 5). It is estimated that the lower bound for slide-off on a slippery roof may be about 10".

'In the 1980 commentary where SI units will be used, g will f2) Or Hip Roofs (Fig. 6)-1n the design of be the acceleration due to gravity and therefore a new symbol gable or hip roofs the influence on snow of such fat- SO will be introduced as the symbol for the basic ground load. tors as exposure to wind and sun, and slope and tex- The "weight density"? will be replaced by the "mass density" ture must be evaluated. Further, the possibility of p multiplied by the acceleration due to gravity; i.e., snow sliding off even gradually sloped roofs (as low

y = pg = (240 kg/m3)(9.81 m/s2) as 10") on pedestrians or lower roofs must be ex- = (2350 kg.m/s2) x l/m3 = 2.35 kN/m3 amined.

TAYLOR 7

-.I.------..--..,.....-.-P-.~,.~a - , -. . . . - . -. . . - . . . . . ,- . ,, . . . . .. . . ,,-, , ,," ' -'-

FIG. 8. Unbalanced snow loads on industrial buildings, caused by wind (Ottawa).

FIG. 9. Unbalanced load on garage at Glacier, B.C.

There are two basic load distributions to be considered. The first is the uniformly distributed load, 0.8g (case 1, Fig. 6), that will be deposited in the absence of wind, and the second is unbalanced load. This second case, with snow on one side and none on the other, arises as a result of two factors that sometimes act together. The first and most obvious is wind. Wind blowing over a peaked roof is accelerated

by being deflected upwards on the windward side. In the wake, on the leeward side of the peak, velocities drop and snow entrained in the wind and scoured from the other side will be deposited, as shown in Figs. 7 and 8.

The second cause of unbalanced load is slide-off. A uniformly loaded roof may be heated on one side by the sun, or perhaps by internal heat, then the P C

CAN. J. CIV. ENG. VOL. 7, 1980

/=--- .. . W I N D -

I C A S E I I I

C A S E I 1 1 ; A 2 . O

h 1 F O R - - U S E C A S E I O N L Y

L 10

h 1 F O R - > - U S E C A S E 1 A N D I 1

L 10

FIG. 10. Design snow loads on cylindrical afch roofs in the 1975 commentary. For roofs exposed to the wind on all sides, all values of C, marked with an asterisk (*) may be reduced by 25%.

CASE l l 1977 COMMENTARY

1975 COMMENTARY

FIG. 11. Comparison of case I1 loadings from 1975 and 1977 NBC snow load commentaries for a small arch (g = 60 psf (2.9 kPa)).

isotherm moves up into the insulating snow layers; the surface friction decreases as a thin film of meltwater is formed at the roof-snow interface and the snow slides off. This can result in a heavy, unbalanced Ioad as shown in Fig. 9.

There is another point to consider in the design of very gradually sloped roofs, gable or shed, whose neglect contributed to the mllapse of buildings in the Edmonton area in the 1973-1974 winter. If there is nonuniform heating of such roofs, owing to un-

I C A S E 11

W I N D W A R D S I D E C, = 0

y h x L E E W A R D S I D E C =- y = 1 5 pcf

9 ( 2 . 3 5 k ~ l r n ~ )

W H E N C > 2 . 0 U S E C = 2 . 0

T H E N C, = C - U S E C A S E I O N L Y FOR - h -

L 1 0

h 1 F O R - > -

L 1 0 U S E C A S E I A N D 11

FIG. 12. Snow loading on cylindrical curved roofs in the 1977 commentary, cases I and 11. For roofs exposed to the wind on all sides, all values of C, marked with an asterisk (*) may be reduced by 25%.

even heat loss -or melting caused by solar radiation during the day, snow on the upper areas may begin to melt. If the roof is colder farther down the slope or if the snow acts as a sponge, retarding drainage, the meltwater may refreeze instead of running off. In this way a heavy layer of ice may build up on the lower parts of the surface, including the eaves. In Edmonton the ice thickness formed was as much as 5 or 6 in. (13-15 cm), 24-28 psf (1.1-1.3 kPa).

(3) Cylindrical Curved Roofs-In the 1975 commentary (NBC 1975) two design cases were recommended for snow loading on curved roofs, as shown in Fig. 10.

Case I: a uniformly distributed load of 0.8g not reduced due to slope but reduced up to 25% if the building is exposed on all sides to wind.

TAYLOR 9

FIG. 13. Case I1 loading on arch showing long plateau at a load of 2g.

Case 11: a triangular load varying from zero at the crown to twice the 30 year return ground snow load (2g) at the edge of the roof, also not reduced due to slope.

The treatment of case I was not entirely satisfac- tory, for there should probably have been a reduc-

I tion allowed for slopes above 30". There were two major problems with case 11:

(a) The first was inconsistency in the use of a slope reduction formula for all sloped roofs but this one. Was it reasonable to expect a load of 2g to adhere to a steeply sloped surface, perhaps even 90", as would be recommended for a semicircular arch on vertical side walls? In a sense the load was largely indepen- dent of geometry.

(b) In moderate to heavy ground load areas the profile of the recommended design snow protruded far above the crown of the arch, as shown in Fig. 11, an apparently unreasonable situation.

Case I1 should clearly be reconsidered to accommodate the geometry of the curved roof. The adopt1:d distribution was based on a pilot study of snow on arches in and around Ottawa, on a case study of the collapse of Quonset-type buildings, on tests of models in a water flume at The University of Western Ontario by Isyumov (1971) and, of course, on engineering judgement.

The two distributions, cases I and 11, used in the Y 1977 edition of the commentary are shown in Fig. 12

and described in the following: Case I: The uniformly distributed load was modi-

v fied to make it consistent with the slope reduction formula, allowing a reduction in load on slopes over 30".

Case 11: (a) The windward side of the roof was assumed to be clear of snow; (b) the upper surface of the snow drift on the leeward side was assumed to be parallel to the ground and no higher than the crown of the arch; (c) the maximum load at any point on the surface was to be not greater than twice the 30 year return ground load; and, (d) the slope reduction factor, p, as previously described, was applied at all points on the surface after application of (c).

On a circular roof with a radius greater than 15gl-y there is, using case 11, an area extending some

F O R - & L U S E C A S E I O N L Y L 1 0

h 1 F O R - > - U S E C A S E S 1 A N D I 1

L 1 0

C A S E L ! 0 I

C A S E 1 1 ' 0

, ~ = 1 5 pc f L E E W A R D S I D E C = - 9 ( 2 . 3 5 k ~ l r n ~ )

y h x > 2 . 0 U S E C = 2 . 0 W H E N - 9

T H E N C , = C B

I F THE T O T A L S N O W L O A D P E R U N I T L E N G T H O F B U I L D I N G ( P E R P E N D I C U L A R T O THE S P A N ) I N C A S E I 1 E X C E E D S g L12, C A S E S I 1 1 A N D IV M A Y BE U S E D I N S T E A D O F C A S E I 1

C A S E I 1 1 0 -

L 1 2 . L I Z

C A S E IV

0

FIG. 14. Modified snow loading on arches. For roofs exposed to the wind on all sides, all values of C, marked with an asterisk (*) may be reduced by 25%.

distance along the span on which this load will remain at the maximum of 2g (see Fig. 13). The extra conservatism of this plateau at 2g, absent in the loads of the 1975 and previous commentaries, has an un- favourable effect when some structures (e.g., arenas) are examined for upgrading. Because available data on arenas do not indicate a need for more conservative loads (Taylor 1979), consideration is being given

10 CAN. J. CIV. ENG. VOL. 7, 1980

FIG. 15. Unbalanced load on arch of 38.5 ft (11.7 m) span, Ottawa, 23 January 1979.

FIG. 16. Unbalanced load on arch of 42 ft (12.8 m) span, Ottawa, 13 January 1977.

by the Associate Committee on the National Building horizontally from the base of the arch, will probably Code to a modification of case 11, using a scheme be one to two times the maximum drift height. These something like that shown in Fig. 14. and unbalanced drifts on Quonset-type buildings

As snow may slide off an arched surface or drift are shown in Figs. 15-18. (Further discussion and over it, accumulations may form at the base on either examples of snow on curved roofs are contained in a or both sides if the curved surface intersects the draft paper, Snow Loads for the Design of Cylindrical ground or another roof. It is apparent that the maxi- Curved Roofs in Canada 1953-1980, by D. A. Taylor.) mum heights will correspond approximately to a C, In designing a flexible arch, it is important to con- of about 1.5 on the windward and 1.75-2.0 on the sider the overall stability of the structure under the leeward sides. The width of these drifts, measured design snow and wind loads. The structure will

TAYLOR 11

FIG. 17. Side view of arch shown in Fig. 16.

FIG. 18. Ground drift on Quonset arch of 32 ft (11.9 m) span 30 km south of Ottawa, 13 January 1977.

behave in a nonlinear fashion if it is flexible, and hence a second-order nonlinear stability analysis of the whole building may be necessary.

(4) Valley Areas of Two-span Roofs-The treatment of this case (Fig. 19) has always appeared somewhat unsatisfactory. The simple but rather unrealistic distributions used in the National Building Code were taken from Soviet work carried out prior to

1965 (Schriever and Otstavnov 1967). Valley areas of two-span roofs have not been studied extensively in the field so that there are few observations of the mechanisms at work. One exception is a survey of a National Research Council of Canada building in Ottawa (Fig. 20).

Calculations of the accumulations forming in valleys, in general on the basis of a sliding mech- anism, were carried out by the author. Very large

12 CAN. J . CIV. ENG. VOL. 7. 1980

c ; = 0 . 8 - 8, C; = 0 . 8 -bl

C A S E I

C A S E I1

F O R B O T H a l A N D a 2 L 10" U S E C A S E I O N L Y

O T H E R W I S E U S E C A S E I . 1 1 , A N D 111

FIG. 19. Valley areas of two-span and multispan curved or sloped roofs. For roofs exposed to the wind on all sides, all values of C, marked with an asterisk (*) may be reduced by 25%.

snow loads were obtained in these areas if a triangular distribution (with equaI leg lengths measured up each slope from the low point of the valley) was assumed. Such high loads seemed unrealistic, however, for there appeared to have been few problems associated

with the present distributions and the Associate Committee decided to leave the distributions un- changed for the time being.

Two factors are apparently active: (a) Wind: snow is scoured from the windward

face and dropped with other entrained snow into the valley, as in Fig. 20.

(b) The second factor is probably creep rather than sliding. As the snow creeps down the slope it becomes denser at the bottom where it meets the snow in the valley; it may possibly wrinkIe and layer, but the stiffness of the snow cover on the slopes will prevent it from slipping completeIy. Hence, a buildup larger than that in Fig. 19, case 111, caused by sliding, wiH probably not occur.

4

(5) Lower of Multilevel Roofs-Snow drifts occur- ring at changes in elevation of multilevel roofs are of current interest. The 1978-1979 winter was the twelfth of a survey of snow on horizontal multilevel roofs at five locations across Canada: Halifax, Arvida, Ottawa, Saskatoon, and Edmonton, with occasional reports from Vancouver, about 40 roofs in all. The analysis is in progress, but data collected over such a period are not entirely adequate for statistical analysis of snow loads and the survey will continue.

The commentary deals with multilevel roofs in the manner illustrated in Fig. 21. When the change in roof elevation corresponds to a height of about 3gl.y (i.e., C, = 3) or less, then under average wind conditions snow may pile up as high as the upper roof in a triangular distribution, approximately as shown in Figs. 22-27. As the difference in elevation, h, increases from 3 to 4 or 5 times g/y, the maximum observed drift coefficient C, has seldom been found to exceed 3.0 (Fig. 24), with a few notable excep- tions. For h greater than about 4-5g/y the drift I

against the wall will probably be less severe because the snow from the upper roof will spread over a larger area (Fig. 22b).

In Canada's north and in other very cold areas I

where the prevailing winds are strong and regular, there may be an unlimited amount of ground snow available for drifting. Snow drifts against obstacles on 'the ground have exceeded C, = 3.0 by a large margin in these areas. In fact, "steady state" conditions have been achieved and drifts observed ramping up to the top of two-storey buildings, apparently in areas where the I-in-30 year ground snow is only about 30 psf (1.5 kPa). More research on snow on buildings in the north is planned.

(6) Multilevel Roofs with Upper Roof Sloped-A multilevel roof with the upper roof sloped towards the leeward side, as shown in Fig. 28, differs from

TAYLOR 13

I I 1 36 ft 36 A % f f (11 m) 111 m) (11 ml

FIG. 20. Snow accumulations on valley roofs in Ottawa, March 1971 (measured ground load was 70 psf (3.3 kPa) and the specific gravity of snow on the roof, 0.30).

by judgement. It was assumed that 50% of the total J design volume of snow on the upper roof would slide

off, but the 50% figure was clearly an estimate based on the reduced probability that both upper and lower roofs will carry the design load over their en-

_-- tire surfaces simultaneously when sliding occurs. As long as a slope is greater than 10-15" some sliding will probably occur; it is also possible that the full design load will slip off, especially on steeper roofs. a In any case, and depending on the design ground snow load, the size of the upper and lower roofs, and the height of the drop, it may not be possible to accommodate 50% of the design volume of snow

c s L b * Ti* from the upper roof. If there is some question, the snow depth profile should be plotted to scale, as sug- gested before, to verify that the final distributions are reasonable.

w = 2 h ul' Designers should also be alert to the fact that sliding snow is a potential danger to pedestrians,

h vehicles, and lower roofs, as well as an expensive C , = 7 - . 1 = 15 p c f 1 2 . 3 5 k ~ l m ~ l

9 nuisance when it blocks entrances and loading h ramps.

WHEN 7- < 0 . 8 ' USE C S = 0 . 8 ' 9

h (7) Areas Adjacent to Roof Projections-Projec- * WHEN 7- > 3 . 0 U S E C , = 3 . 0 9 tions extending above the design depths of uniformly

distributed snow on a roof accumulate about two w = 2 h thirds of the maximum depth of snow that would

8 W H E N h < 5 f t ( 1 . 5 m l U S E w = L O t t 1 3 m ~ be deposited behind an equal "step" or change of h > 15 t t 14 .5 m ) U S E w = 30 t t 19 m ) elevation on a multilevel flat roof (Fig. 29). The

h = D I F F E R E N C E OF ROOF H E I G H T S I N R ( m ) accumulation on the leeward side is reduced to this g = GROUND S N O W L O A D I N p s f 1 k P a ) apparently because the top of the projection is not w = w 1 D T H OF DR l F T F R O M H I G H E R B U I L D l N G I N f t I m ) large enough to act as a reservoir of snow available a = D I S T A N C E B E T W E E N B U I L D I N G S < IS f t 15 m ) for drifting and because the obstruction acts as a

solid fence, causing blowing snow to accumulate on FIG. 21. Snow accumulation on the lower level of a two-level

roof. For roofs exposed to the wind on all sides, all values of both sides. The total volume within a short distance C, marked with an asterisk (*) may be reduced by 25%. of either side of a projection, however, is more than

it would be if the top of the projection were an upper - -

the flat multilevel roof (Fig. 21) in that sliding be- level roof. comes a major factor. Snow that slides from the The requirements for areas adjacent to roof pro- upper roof will drop to the lower, adding to any jections, as shown in Fig. 29, were based on experi- drifts already there. Because there are very limited ence and the judgement of the code committees rather data, the amount likely to slide has been determined than on field observations across the country. A

14 CAN. J. CIV. ENG. VOL. 7, 1980

FIG. 22. Snow drifts on the lower of two adjacent flat roofs (g = 60 psf (2.9 kPa)). (a ) From Fig. 21. (b) Two possible distributions: - and - -.

FIG. 23. Triangular load distribution on a two-level flat roof in Moncton, N.B., 30 December 1970 (g = 80 psf (3.8 * kPa)).

study of the drifts adjacent to projections of various widths is required to determine for design purposes a practical limiting width differentiating between a projection and an upper level roof.

Full and Partial Loading The requirements for full and partial loading form

part of the National Building Code, not just the commentary, and must be followed by designers using the code. The intention is that the requirement for consideration of full load (the specified load per unit area) on any one portion of the area and half on the remainder, whichever produces the greatest effect on the members concerned, will cover any

uneven accumulation of snow that might occur as a I

result of wind; heat loss, solar radiation, slipping, snow clearing, etc. It also applies to drift loads, for example case 11, Fig. 6: if wind blows across a roof at an angle to either side, diagonal drifts crossing a number of structural supports (purlins, trusses, arches, etc.) may form. In addition, snow cover on a sloping roof may slide partially off the roof, starting from either the high or low side. Then, if new snow falls, the resulting accumulation can be very close to full and partial. When conditions lead the designer to suspect that "full and zero" loading will be more likely, this should be considered as well.

The requirements for full and partial loading

TAYLOR 15

FIG. 24. Triangular drift on a two-level flat roof: elevation difference > 5g/y-Gander Airport (g = 70 psf (3.3 kPa)). Courtesy of H. M. Chafe, Gander.

should not be applied as checkerboard loading (full, partial, full, etc.) because such patterns are very unlikely.

Rain Plus Snow Loading It has been noted that in deriving the 30 year re-

turn ground loads the 24 h rainfall at the time of greatest snow depths was added to the load produced by the 30 year return snow depth at a specific gravity of 0.2 (Boyd 1961). In areas where winter rain ac- counts for a significant part of the ground snow load (up to 50%) this procedure creates problems. The drift load coefficients C,, which are multipliers used to convert ground to roof loads, effectively "drift the rain" in addition to the snow because it is in- cluded in the 30 year return ground loads. The possibility of separating the rain load from the basic ground load and adding it again to the drifted snow is under consideration.

Summary and Comments It is generally difficult to record good snow load

data on roofs and to isolate the influence of the many factors that affect them. This is doubly so in Canada because the country is so large and the climate so diverse. Much of the material on snow loads in codes has been determined by the judicious use of a few data and a large portion of engineering judgement. More data will be collected as financial support and

time allow, but years may pass before the questions arising today can be answered. In the meantime, recommendations in the commentary are subject to periodic review and adjustment.

The following is a summary of important points: (1) Snow data and hence design loads are approxi-

mate because of measurement error and variability of climate, geometry, and topology.

(2) In evaluating the effect of complex roof geometry or unusual conditions the designer should estimate and plot volumes and depths first, then convert them to loads, or at least check the depth profile computed from the loads to ensure that it is reasonable.

(3) For important roofs with complex geometry designers should consider wind tunnel or water flume tests but should realize that the particulate matter used may not be able to simulate certain snow overhangs, cornices, steep-sided drifts, and the adherence of snow to steep slopes, all of which are due to the cohesiveness of real snow.

(4) Drainage should always be provided on a roof but may be ineffective because of clogging by ice or debris or sponge-like retention of meltwater by capillary action of the snow.

(5) When a roof has a slope greater than about lo0, there is potential danger from sliding snow.

(6 ) In some mountain valleys of British Columbia where there is complete shelter, roof snow loads may

16 CAN. J . CIV. ENG. VOL. 7. 1980

be the same as those on the ground. The snow density may be higher than has been found adequate for design in the rest of the country. Designers should also be aware that the ground snow load increases from about 3 to 9 psf per 100 ft (0.14 to 0.42 kPa per 30 m) increase in elevation above the location for which the ground load is given.

All those who have photographs or case histories of significant snow accumulations on roofs are encouraged to contact the author. The Division of Building Research is grateful for any assistance (photographs, estimates of depths and densities, etc.), but would prefer photographs accompanied by measured depths and densities and description of the building shape, its surroundings, shelter from the wind, and notes on rainfall.

Individuals or agencies wishing to help with measurements are encouraged to contact the author for assistance and information on density samplers and depth gauges for installation on flat and sloping roofs. On a more casual basis, for those who wish to record a snow distribution but have no equipment, a tin can with its ends removed can be used to take samples of known volume through the full snow depth; the thickness of ice layers not sampled can be noted. On occasion a shovel can be used to isolate a rectangular column of snow through the full thickness; the volume can be determined and, after transfer to a container, the weight measured on a scale or

FIG. 25. Front view of triangular drift I1 ft (3.4 rn) deep on by the of the the lean-to roof adjacent to higher main roof, Quebec City, March method, all new data will contribute to the improve- 1969 (g = 80 psf (3.8 kPa)). Courtesy of M. Drouin. ment of snow loading recommendations in the

FIG. 26. Triangular drift on two-level flat roof in Ottawa, mid-January 1978 ( g = 60 psf (2.9 kPa)).

TAYLOR

FIG. 27. Heavy drift load on the lower of two adjacent flat roofs in Lethbridge, Alta., April 1960 (g = 30 psf (1.5 kPa)). Courtesy of R. E. Peacock, Lethbridge.

U P P E R R O O F

L O W E R R O O F

f i : - J - * - - - - - -- -- - -1

L O A D F R O M S L I D I N G S N O W

( F I G U R E 211 h 3

C = 213 y - . y = 15 p c f ( 2 . 3 5 k N l m I 9

h FIG. 28. Lower of two-level roofs with the upper roof sloped W H E N 213 y - < 0 . 8 ' U S E C S = 0 . 8 "

9 towards the lower.

h W H E N 213 y - > 2 . 0 U S E C S = 2 . 0

'3

2 . 5 g W H E N L < - U S E C S = 0 . 8 "

"Commentary on Snow Loads" of the National Y

Building Code. w = 2 h

Acknowledgements W H E N h < 5 f t ( 1 . 5 m l U S E W = 1 0 f t ( 3 m l W H E N h > 1 5 f t ( 4 . 5 m ) U S E W = 3 0 f t (9 m )

The author is grateful for the assistance of W. R. h = H E I G H T OF P R O J E C T I O N I N I t ( m l

Schriever, P. J. Daly, and all those who have recorded g = G R O U N D S N O W L O A D I N p s f I k P a l

case histories of snow accumulations and sent them w = W I D T H O F S N O W D R I F T I N f t ( m ~

to the Division. This paper is a contribution from the L = L E N G T H OF P R O J E C T I O N I N f t ( m )

Division of ~uilding Research, National Research FIG. 29, Areas adjacent to roof projections (NBC 19770).

Council of Canada, and is published with the ap- For roofs exposed to the wind on all sides, all values of C, proval of the Director of the Division. marked with an asterisk (*) may be reduced by 25%.

18 CAN. J. CIV., ENG. VOL. 7, 1980

BOYD, D. W. 1%1. Maximum snow depths and snow loads on roofs in Canada. Proceedings, Western Snow Conference, Spokane, WA, April 1960, pp. 6-16. (Division of Building Research, National Research Council of Canada, Ottawa, Ont., NRCC 6312. I1 p.)

ISYUMOV, N. 1971. An approach to the prediction of snow loads. Ph.D thesis, The University of Western Ontario, London, Ont., Research Report BLWT-9-71.534 p.

ISYUMOV, N., and DAVENPORT, A. G. 1974. A probabilistic approach to the prediction of snow loads. Canadian Journal of Civil Engineering, 1, pp. 28-49.

LUTES, D. A. 1970. Snow loads for the design of roofs in Canada. Proceedings, Western Snow Conference, Victoria, B.C., pp. 61-67. Division of Building Research, National Research Council of Canada, Ottawa, Ont., NRCC 1191 1.

LUTES, D. A., and SCHRIEVER, W. R. 1971. Snow accumulation in Canada- case histories: 11. Division of Building Research, National Research Council of Canada, Ottawa, Ont., NRCC 11915. 17p.

NATIONAL BUILDING CODE. 1975. Commentary H, snow loads. Supplement No. 4 to the National Building Code of Canada 1975. National Research Council of Canada, Ottawa, Ont., NRCC 13989, pp. 119- 130.

1977a. Commentary H, snow loads. Supplement No. 4 to the National Building Code of Canada 1977. National Re- search Council of Canada, Ottawa, Ont., NRCC 15558, pp. 69-83.

19776. Climatic information for building design in Canada 1977. Supplement No. 1 to the National Building Code of Canada. National Research Council of Canada, Ot- tawa, Ont., NRCC 15556. 19p.

SCHAERER, P. A. 1970. Variation of ground snow loads in British Columbia. Proceedings, Western Snow Conference, Victoria,

B .C., April 2 1-23., pp. 44-48.(Division of Building Research, National Research Council of Canada, Ottawa, Ont., NRCC 11910.)

1972. Control of snow drifting about buildings. Division of Building Research, National Research Council of Canada, CBD 1 4 6 . 4 ~ .

SCHRIEVER, W. R., and LONGWORTH, J. 1965. Commentary on loads due to snow. Section 4.1. Structural loads and proce- dures of the 1965 National Building Code of Canada. National Research Council of Canada, Ottawa, Ont., NRCC 9187, pp. 5-9.

SCHRIEVER, W. R., and OTSTAVNOV, V. A. 1967. Snow loads-preparation of standards for snow loads on roofs in various countries with particular reference to the U.S.S.R. and Canada. International Council for Building Research ? Studies and Documentation (CIB), Report No. 9, pp. 13-33.

SCHRIEVER, W. R., and PETER, B. G. W. 1965. Coefficients for snow loads on roofs. Supplement No. 3 to the National Building Code of Canada. Associate Committee on the Na- 4 tional Building Code, National Research Council of Canada, Ottawa, Ont., NRCC 8331, p. 23.

SCHRIEVER, W. R., FAUCHER, Y. and LUTES, D. A. 1x7. Snow accumulations in Canada-case histories: I. Division of Building Research, National Research Council of Canada, NRCC 9287.29 p.

TAYLOR, D. A. 1979. A survey of snow loads on the roofs of arena-type buildings in Canada. Canadian Journal of Civil Engineering, 6, pp. 85-%.

THEAKSTON, F. H. 1961. The science of snow accumulation in and around farm structures. Presented at the 1%1 Annual Meeting of the American Society of Agricultural Engineers, Iowa State University, Aimes, IA, Paper No. 61-421.7 p.

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