HomeMy WebLinkAboutAPA959USACRREL
August 1973
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ARLIS
Alaska Resources
Library & Information SerVices
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ALASKAN SNOW LOADS
by
Wayne Tobiasson and Robert Redfield
Presented at the 24th Alaskan Science Conference
University of Alaska
August 1973
U. S. ARMY COLD REGIONS RESEARCH AND ENGINEERING LABORATORY
HANOVER, NEW HAMPSHIRE
INTRODUCTION
Very little specific information on snow loads is available for
Alaska even though for most of the state, snow loads are the maximum
climatic loads induced on structures. The Uniform Building Codel7 is
perhaps the most widely used building code in Alaska and it simply
states that "snow loads shall be determined by the Building Official."
A few large communities arm the Building Official with a specific snow
load. The Building Code of the City of Fairbanks9 adopts the Uniform
Building Code, replacing the q_uoted sentence above with "The snow load.
is hereby determined to be 40 pounds per sq_uare foot 11
• While on the
subject of Fairbanks it is interesting to note that current design
snow loads there vary from 30psf to 65psf depending on the reference
chosen.9,10,ll,l3 In most other Alaskan communities, lack of criteria,
not variations depending on data source, is the problem.
Few Building Officials have direct knowledge of appropriate snow
loads and snow load q_uestions are freq_uently referred to engineers and
architects who generally have never measured a snow load but through
experience, have suggestions for design. Many refer to the map "Design
Data for Military Construction in Alaska"10 prepared by the Alaska
District, Corps of Engineers in l958 or the ioint Army-Air Force _Tech-
nical Manual "Load Assumption for Buildings" 1 issued in 1966. Both
documents are currently under revision with significant changes anti~
cipated.
Until recently the 1955 American Standards Association publication
"Minimum Design Loads in Buildings and Other Structures"2 was used to
establish loads for the "lower 48" states. It did not contain loads
for Alaskan areas. In 1972 that document was superseded by the American
1?/
y95
.71P'I
;913
National Standards Institute (ANSI) puhlication·, "Building Code Req_uire-
ments ;for Minimum Design Loads in Buildings and Other structures"l but
again snow load criteria-for Alaska were absent. National Weather
Service records for Alaska were analysed for inclusion in the 1972 ANSI
standard but they were not published sine~ many of the ground snow load
measurements were of q_uestionable value.2
Most design snow loads currently in use in Alaska are essentially
opinions based on experience. The vast·majority of Alaskan structures
hold up well under snow loads and in that light there is a tendency to
believe that since structures are not collapsing~ the proper loads are
being used. Of course, wasteful overdesigns may also be occurring.
When a sound bui·lding fails under snow loads, authorities generally
react by increasing the design snow load for the region rather than
introducing provisions to account for special loading conditions. Failures
are seldom documented much beyond the point. of stating that "the building
collapsed under a heavy snow load".
Several snow load case histories and a. few well-documented building
failures indicate that most structures are significantly overdesigned.
but an occasional structure is underdesigned·because attention has not
been paid to special situations such as drifting and sliding snow.
To improve upon this situation new snow load criteria have been
developed based on a statistical analysis of weather records and review
of snow load case. histories. The elevation, the local site conditions
and the geometric, thermal and aerodynamic features of facilities have
been considered.
INFORMATION SOURCES
Except for· recent work by Isyumov18 who is developing a method of
predicting snow loads by considering daily snowfall, air temperature,
windspeed, wind direction and roof properties, it is generally accepted
that knowledge of the snow load on the ground is the first step in·.
developing snow load criteria for roofs. Much of the weather data
needed for Isyumov' s approach is not available for Alaskan stations.
There are only 18 National Weather Service stations in Alaska that
measure 1~oth the dept·h and load (i.e. water eq_ui valent) of snow on the
ground. Depths are measured in inches of ~ and gravimetric measure-
ments of load are presented in inches of water. By multiplying inches
of water by 5.2 the load can be converted to psf.
Unfortunately the ground snow load information collected at ten
of the eighteen stations is either an estimated value based on the
assumption that 10 inches of snow has a water eq_uivalent of one inch
(ie the specific gravity of the snow is 0.1) or is unreliable due
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to lack of trained observers.22 The eight remaining stations report
valuable ground snow load information but eight widely-separated stations
do not provide much of a base on which to develop state-wide criteria.
Fortunately a second source of snow load information is available.
USACRREL initiated and has co-sponsored a program of detailed snow ob-
servations at 17 stations in Alaska for periods from 7 to 17 years. The
United States Air Force, Air Weather Service and the United States
National Weather Service (formerly the U. S. Weather Bureau) participated
in the data collection.* The program aud several relationships developed
from the data are described by Bilello. ,5 By combining the 8 National
Weather Service stations and the 17 sites studied by USACRREL, 25 loca-
tions in Alaska with reliable ground snow load information were obtained .
Perhaps it would be possible to generate state-wide criteria using
only these 25 stations but a far better job can be accomplished by also
considering snow depth measurements available at these stations and at
112 additional smaller weather stations throughout the state. Peri.ods
of record range from 4 to 23 years depending on location. Where only
snow depths are measured, the density of that snow must be estimated
before snow loads can be computed.
The climatological series of the maximum annual depth of snow on the
ground at 137 locations has been analysed statistically. Then, using the
combined depth and load information available at 25 of these locations,
conversion densities were developed, regionalized and applied to all 137
locations to generate ground snow loads at each site.
STATISTICAL ANALYSIS OF DEPTHS
Extreme value statistical studies by Thom 2 3 indicate that climato-
logical series of annual maximum snow depths on the ground closely follow
log-normal distributions (ie when plotted on log-normal probability paper
the distribution is linear). He uses the mean and standard deviation of
the logarithms of the series to establish confidence intervals, then aids
the reader in visualizing his complex statistics by presenting results
using Blom plotting positons.6 Where a series consists of 10 or fewer
values Blo~'s positions are preferred over the more common positions of
Gumbell5 ,l . Both plotting positions are defined below:
Probability (Blom)
*We are indebted to Mr. Michael Bilello, Research Meteorologist, USACRREL,
for permitting us to utilize the snow load information collected under
his direction and to Mr. Roy Bates, Meteorological Technician, USACRREL,
for guidance in the reduction and analysis of this information.
3
Probability (Gumbel) = (n: 1 ) 100.
where: n = number of years of record
m = rank of a particular annual extreme value. For the
smallest value, m = 1; for the largest, m = n.
Probability is in percent.
All statistical results in this report are based on the use of
Blom's plotting positions since a third of the locations have fewer
than 10 years of snow depth records. Representative log-normal prob-
ability plots of the annual climatic series of maximum snow depths
on the ground are shown in Figure 1.
Return periods can be used to relate probabilities to the "design
life" of a facility. Return periods for annual series are defined as
follows:
100
Return period = 100 _ Probability
where probability is in percent and return period is in years.
A return period scale is superimposed on the probability scale of
Figure 1. The snow depth from a regression line at a specific return
period is the depth that can be expected to be equalled or exceeded
once during that period. For example in Figure 1 the snow depth for
a 25-year return period at Cape Lisburne would equal 3.1 ft.
The snow depth corresponding to 5, 10,
return periods have been determined for 137
a digital computer. The Fortran II program
is presented in Appendix A of this report.
lations of both the input data and results
Periodic updating of the data and revision
tively easy tasks.
25, 30, 50 and 100-year
locations in Alaska using
developed for this task
Samples of computer tabu-
are presented in Appendix B.
of results will be rela-
RELATING DEPTHS AND LOADS
An annual record of snow depth and load for two representative
locations is presented in Figure 2. In the simplest case, such as at
Cape Lisburne, both the depth and load maximize on the same date.
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Return Period, yrs
30
4.0 • I I I I I I I I I r 1P 2 r 1 s,o ~g.o I --"C 3.0
c:
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CAPE LISBURNE
0 ·5 I ~ ; ~~ do 3
1o 4
1o ~o ~o io ~o So 9~ I ~8 Ss ~9.5
96.7
Probability, %
Return Period, yrs
9.0 i I I I I I I I 1 1 7 1P 2 ?3
1° 5,0 1 ?~ 1
--
8.0
7.0
6.0
"C 5.0
; 4.0
0 ....
(!)
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..c.
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0
3: 2.0
0 c:
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..c. -~ ~ 1.0
UTOPIA CREEK
0.5 I . 2 5 10 20 30 40 50 60 70 80
Probability, %
90 96 1 9a 99 99.5
96.7
Figure 1. Log-normal probability plots for Cape
Lisburne and Utopia Creek
5·
However for many locations, such as Utopia Creek, the maximum load
occurs at a later date than the maximum depth. When the load and depth
maximize concurrently, the ratio of the maximum load, in psf, to the
maximum depth, in feet of snow, is equal to the density of the snow in
pcf on that date. When they maximize on different dates the ratio is
not a physically-measurable density, simply a convenient conversion
factor relating depth and load. This factor has been termed conversion
density. If the depth and load maximize on different dates, the
conversion density will always be less than the density measured at
the time of maximum load but greater than that measured at the time of
maximum depth.
6r 60 I CAPE LISBURNE -"' Q. -r~ ~4 g4o
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Figure 2. Record of snow depth and load for
one season for Cape Lisburne and Utopia Creek.
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DEVELOPING CONVERSION DENSITIES
For the 25 locations where both snow depth and water equivalent
are measured, each annual conversion density was determined. Perhaps
the most useful grpahical presentation, of the many investigated, was
the relationship between each annual conversion density and its
associated maximum annual snow load for a particular location. Two
such graphs are shown in Figure 3. The density trend varies but for
each station it is possible to extrapolate to a constant density for
heavy snow loads. Similar curves were produced for all locations where
ground snow loads were measured. The results indicated that a single
conversion density could be established for each location regardless of
the length of the return period. The conversion densities thus developed
are presented in Figure 4.
30~---r---
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CAPE LISBURNE
• • -----·---r---. _......-.r • ---
.r/ Trend • •
10 20 30 40 50
Ground Snow Load, psf
UTOPIA CREEK
• •
Trend • __ :__. ___ ...,._. _____
---;-. .
•
•
10 20 30 40 50
Ground Snow Load, psf
60
60
Figure 3. Graphs used to establish conversion densities for
Cape Lisburne and Utopia Creek.
7
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Figure 4. Conversion densities for 25 locations in Alaska.
Using information in "Snow Surveys for Alaska"14 , the U. S.
Geological Survey physiographic map of Alaska2 5, and Bilello's map of
average seasonal snow-cover densities5 as guides, conversion densities
were regionalized. The conversion density in each region was further
categorized according to site elevation and proximity to the coast.
Regionalized densities are presented in Figure 5. These values were then
applied to the design snow depths determined for 5, 10, 25, 30, 50 and
100-year return periods. '
GROUND SNOW LOADS
The place name for each location, its latitude, longitude and
elevation and the ground snow load in psf for 6 return periods are pre-
sented in Table I. For temporary facilities the 5-year return period
snow load should be selected. For permanent facilities the 25-year
return period is appropriate. However, as hazards to life and property
increase in the event of a failure, design loads should increase toward
the 100-year value. The snow load values for 23 locations are suspect
8
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coastal 25.0
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Conversion Density ( ;>cfl
Region Oto500' 500to ooove
1500' 1500'
A
B
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12.5
15.6
18.7
25.0
15.6
18.7
21.8
21.8
18.7
21.8
25.0
21.8
\
Note:
See mop for ccostol v:lues
;--~..,--c ~', ' (;~'-
000"0116.~~
coastal 15.6
Figure 5. Regionalized conversion densities.
and are followed by an asterisk in Table I. Sixteen of the suspect
locations have only 4 to 6 years of record. Since most of these
limited records cover the heavy snow year of 1970-71, excessive statis-
tical values generally result. Suggested 25-year return period values,
based on comparisons with other stations in the vicinity and considera-
tion of the 1970-71 snow loads as generally representing more than
15-year values, are shown in-parentheses in Table I. Suggested values
for other return periods can be obtained by multiplying the asterisked
value for the desired return period by the ratio of the suggested
25-year value (in parentheses) to the asterisked 25-year value.
The National Building Code of Canada3 '8 '20 uses a single conver-
sion density of 12pcf (.19 gm/cm3) throughout Canada to convert 30-year
return period snow depths to ground snow loads. A surcharge equivalent
to the weight of a one-day spring rainfall is added. The rainfall
surcharge varies from less than 5psf in the Canadian prairies and Arctic
to 25psf on Vancouver Island.7 Such a surcharge is not needed for the
Alaskan loads presented in this report since spring rains are already
part of the maximum annual water equivalent observations.
9
Station name
Adak
Adult Conservation Camp
Allakaket
Alpine Inn
Alyeska
Anchorage
Angoon
Aniak
Annette
Annex Creek
Attu
Auke Bay
Barrow
Barter Island
Beaver Falls
Bethel
Bettles
Big Delta
Birch Road
Cape Decision
Cape Hinchinbrook
Cape Lisburne
Cape Newenham
9ape Romanzof
Cape Saint Elias
Cape Sarichef
Central
Chena Hot Springs
Chignik
Chitina
Circle Hot Springs
Clear
Clearwater
Cold Bay
College Magnetic Observatory
Cooper Lake Project
Cordova
Crooked Creek
Eagle
Eielson AFB
Table I. Ground snow loads.
Elevation,
Latitude Longitude ft
51°53•
61°42•
66°34•
61°43•
60°58'
6101Qo
57°30•
61°35•
55°02•
58°19•
52°50•
58°23•
71°18•
70°08•
55°23•
60°47•
66°54•
64°00•
61°08•
56°00•
60°14•
68°52•
58°38•
61°46•
59°48•
54°36'
65°34'
65°03•
56°18•
61°32•
65°29•
64°18•
64°03•
55°12'
64°52•
60°23'
61°58'
61°52•
64°46•
64°40•
176°38'
148°59•
152°40•
148°54•
149°08•
150°01•
134°35•
159°32•
131°34•
134°06•
173°11'E
134°38•
156°47'
143°38•
131°28'
161 °48•
151°31•
145°44'
149°46'
134°08•
146°39'
166°07•
162°04•
166°03•
144°36'
164°56•
144°49•
146°03•
158°24•
144°27•
144°38'
149°11'
145°31•
162°43•
147°50•
149°40•
145°19•
158°06•
141°12•
147°06•
15
825
600
455
251
114
15
81
110
24
70
42
31
39
35
125
666
1268
460
39
185
45
475
434
58
175
960
1195
30
575
935
580
1100
96
621
445
1000
125
821
558
10
Ground snow load, psf
'5yr. lOyr 25yr 30yr 50yr lOOyr
23 28
30 41
61 80
19 22
98 122
52 64
50 64
50 63
20 30
133 158
51 66
61* 79*
43 50
66 82
53 72
36 37
57 60
111 113
27 28
154 159
80 82
83 86
79 82
43 45
191 197
86 89
103* (100) 107*
59 60
104 108
101 105
42
71
131
30
180
92
98
92
54
216
102
123*
65
121
124
36 46 61
92 111 137
63 73
140 156
86 102 43 59 82
62 66 72 72 75
15 20 28 29 34
57 82 121
62 74 90
60 78 104
73 116 148
66 89 121
127 155
92 101
108 125
182 222
126 148
15
37 -
59
62*
38*
30
52
43
20
59
34
65*
42
61
62
21
39
77
83*
60*
33
65
58
24
71
39
79*
49
74
80
29 30
42 42
101 105
113* (80) 118*
100* (80) 108*
36 36
81 83
79 82
29 29
87 90
46 47
98* (110) 101 *
58 60
90 93
105 110
35
44
120
138*
139*
38
93
96
32
100
50
112*
65
104
126
48
86
156
34
206
105
114
106
67
240
119
168*
72
139
150
88
176
124
78
42
193
114
148
232
177
43
46
141
165*
187*
41
106
115
35
112
55
127*
72
116
147
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Station name
E klutna Lake
Elmendorf AFB
Fairbanks
Farewell
Five Finger Light Station
Flat
Fort Yukon
Galena
Gilmore Creek
Glacier Bay
Glennallen
Gulkana
Gunsight
Haines Terminal
Holy Cross
Homer
Hughes
Iliamna
Intricate Bay
Juneau
Kasilof
Kasitsna Bay
Kenai
Ketchikan
King Salmon
Kitoi Bay
Kobuk
Kodiak
Kotzebue
Lake Minchumina
Linger Longer
Little Port Walter
Mankomen Lake
Manley Hot Springs
Matanuska Agric. Exp. Sta.
McGrath·
McKinley Park
Moose Pass
Moses Point
Nenana
Table I (Cont'd).
E{evation,
Latitude Longitude ft
61°24'
61 °15•
64°49'
62°32'
57°16•
62°29•
66°33•
64°44'
64°59'
58°27•
62°07'
62°09•
61°54•
59°16•
62°10'
59°38•
66°04•
59°44•
59°34'
58°22'
60°19•
59°28•
60°34•
55°21•
58°41•
58°11'
66°54•
57°45•
66°52•
63°53'
59°26•
56°23•
62°59•
65°00•
61°34•
62°58•
63°43'
60°28•
64°43•
64°33•
149°09•
149°48•
147°52•
153°54•
133°37•
158°05•
145°12•
156°56•
147°31 1
135°53 I
145°32 1
145°27•
147°18•
135°27•
159°45•
151 °30•
154°15•
154°57 1
154°28•
134°35•
151°15 1
151°33 1
151°15•
131 °39•
156°39•
152°21 I
156°52•
152°31•
162°38•
152°17 1
136°17•
134°39 1
144°29•
150°39 1
149°16 1
155°37•
148°58 1
149°23•
162°04•
149°05 1
882.
222
436
1499
30
309
443
120
959
50
1456
1570
2960
175
200
67
545
145
170
12
75
12
86
15
49
15
140
21
10
701
700
14
3025
300
150
344
2070
485
15
356
11
Ground snow load, J!.Sf
5 yr 10 yr 25 yr 30 yr 50 yr 100 yr
47
42
52
38
24
55
50
56
48
63
48
38
68
57
52 59
57* 78*
85* 107*
29 34
42 52
56* 77*
67 91
68 82
45 57
62 77
46 59
34 40
46 -60
61 72
82* 103*
55 68
27 37
67
56
78
60
64
85
65
69
57
80
62
69
87
66
69 70
109* (90) 114*
137* (100) 142*
41 43
66 68
108* (90) 113*
126 132
100 103
74 76
97 101
77 80
47 48
79 83
85 87
131* (90) 136*
84 86
53 56
76
62
89
70
89
97
71
76
135*
161*
47
77
135*
156
113
86
113
91
52
95
96
153*
96
67
22 26 31 31 34
43 60 86
87 106 132
91 109
136 152
31 38 49 51 58
56 65 77 79 86
52 60 70 72 77
119 137
102 134
103* 130*
50 59
19 24
51 60
71 87
78 98
105* 127*
45 58
160 163
179 186
167* (120) 173*
70 72
31 32
71 72
109 112
126 130
156* (125) 161 *
76 79
177
215
197*
78
37
79
125
148
179*
90
85
68
101
79
120
110
77
82
164*
187*
52
86
164*
199
127
100
129
107
57
112
106
176*
109
83
37
134
172
66
95
84
193
255
227*
86
43
87
142
171
202*
106
Station name
~ikiski Terminal
~orne
Northeast Cape
Northway
Nunivak
Oil Well Road
Palmer
Paxson Lake
Petersburg
Point Hope
Point Lay
Point Retreat Light Station
Port Alexander
Port Alsworth
Port Heiden
Puntilla
Richardson
Russian Mission
Saint Marys
Saint Paul Island
Seldovia
Seward
Shemya
Shishmaref
Sitka
Sitkinak
Skagway
Slana
Snowshoe Lake
Sparrevohn
Summit
Talkeetna
Tanacross
Tanana
Tatalina
Teller ·
Thompson Pass
Tin City
Tok
Tonsina Lodge
Table I (Cont'd).
Elevation, _ Ground snow load, psf
Latitude Longitude ft 5· yr 10 yr 25 yr 30 yr . 50 yr 100 yr
60°41•
64°30 1
63°17•
62°55•
60°23•
61°14'
61°36'
62°57•
56°49'
68°20•
69°45•
58°25•
56°15•
60°12•
56°57•
62°06•
64°17•
61°47'
62°04 1
57°09•
59°26•
60°07•
52°43•
66°14'
57°03•
56°33•
59°27'
62°43'
62°02•
61°06•
63°20•
62°18•
63°24•
65° 10•
62°54•
65°16•
61°08•
65°34•
63°21•
61°40•
151°23•
165°26•
168°41•
141°56•
166°12•
149°43'
149°06•
145°30•
132°57'
166°48'
163°03•
134°57'
134°39•
154°18•
158°37•
152°45'
146°22•
161°19•
163°11'
170°13•
151 °42'
149°27'
174°06•E
166°07'
135°20•
154°08•
135°19•
143°44'
146°40•
155°33•
149°09•
150°06•
143°19'
152°06•
155°58•
166°21•
145°45'
167°55'
143°02•
145°11'
110
13
38
1713
44
370
225
2750
50
15
10
50
18
260
92
1832
875
50
25
22
31
70
122
14
67
53
10
2200
2410
1580
2401
345
1549
232
964
10
2500
269
1620
1500
12
57* 75*
70 89
90 114
51 61
57 74
40 45
32 45
67 79
60 84
56* 101*
99* (80) 103* 118*
114 118 134
148 154 175
74 76 84
98 102 117
51 52 56
63 66 79
95 97 106
120 127 152
189* (90) 207* 284*
140*
155
204
94
137
60
97
118
186
410*
35 44 57 58 66 76
40 58 87 92 112 142
56 72 95 99 113 133
24* 29* 35* (75) 36* 39* 44*
15 21 30 31 38 47
79 90 102 104 112 121
57 73 95 99 113 132
52 70 95 99 115 137
19* 23* 27* (100) 28* 31 * 34*
37 48 63 65 75 88
70* 103*
50 63
16 20
38* 40*
34 44
25 37
11* 13*
42 60
43' 49
72 98
122 148
106 130
4'8 57
59 74
70 78
41 66
176 223
52 70
42* 46*
35 37
156* (90)
80
26
42* (90)
57
166* 204*
83 93
26 30
42* 43*
60 74
56 60 74
14* (100) 14* 15*
88 94 114
56 57 60
136 142 168
180 186 205
161 166 186
69 71 77
94 97 109
88 89 95
109 117 151
288 299 340
97 102 120
50* (70) 50* 53*
40 40 42
261*
107
34
44*
95
95
16*
142
65
204
230
210
86
126
101
203
394
144
55*
43
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Table I (Cont'd) .
Elevati.on, Ground snow load, LJ.Sf
Station name Latitude Longitude [t 5 yr 10 yr 2s rr. 30 yr 50 yr 100 yr
Trims Camp 63°26' 145°26 1 2408 226* 299* 404* (200) 421 * 491* 585*
Umiat 69°22' 152°08• 337 49 57 68 70 76 84
Unalakleet 63°53 1 160°48' 15 62 82 110 115 134 159
University Experiment Station 64°51' 147°52• 475 46 56 68 70 78 87
Utopia Creek (Indian Mtn.) 65°59' 144°29' 1220 66 85 111 116 133 155
Valdez 61°08• 146°15• 49 136 159 18'7 191 207 228
Wainwright 70°40' 159°50• 315 26 29 33 34 36 39
Wales 65°37• 168G03• 9 49 64 85 88 102 121
Wasilla 61"37• 149°24• 500 51* 74* 108* (60) 115* 140* 176*
West Fork 65°28• 148°40' 425 39* 45* 54*(80) 55* 60* 66*
Whittier 60°47' 148°4l' 15 190 250 334 349 404 478
Wild Lake 67°33' 151°33' 1190 46* 49* 54*(120) 55* 57* 60*
Willow Trading Post 61''47' 145°11' 1400 82* 99* 119* (90) 123* 135* 151*
Wiset~mn 67"'26• 150"13' 1286 84 103 128 132 148 167
Wrangell 56°28• 132°23' 37 32 46 67 70 85 106
Y:1kata~~a 6o··or)· 1:12":10• 27 80 102 132 138 157 182
Yakutat G\l'':1 1 • l:l9"40• 28 122 149 18·1 190 212 2·10
The conversion densities developed for Alaska vary from a low of
12pcf (0.19 gm/cm3) in the Copper River lowland to a high of 28pcf
(0.45 gm/cm3) along the west coast below the Seward Peninsula. In the
interior, on the Alaska Peninsula, on the Aleutians and on the "Panhandle,"
densities vary between 16 and 25pcf (.26 and .40 gm/cm3) depending on
location and elevation. For the wind-driven snow on the North Slope a
density of 24pcf (.38 gm/cm3) is used •
Maps of Alaska overlaid with ground snow load isolines have not
been made since they tend to obscure local variations and may result
in hasty generalizations. To determine ground snow loads for sites not
listed in Table I the loads reported at several sites in the vicinity
should be inspected with attention paid to elevation and other geographical
features. Such an approach will not only produce more meaningful cri-
teria but will also alert the user to the extent of local variations in
the vicinity. In some areas a 50-mile change in location has little
influence on loads but in other areas loads might triple in the same
distance.
13
Local variations are particularly significant in southeastern
Alaska. The significant variation in the depth of snow on the ground
for four locations all within an 11-mile radius of Juneau is shown in
Figure 6.
\__
7r------.-----.------~----~------~----~
..... 6 -~
'U
c:
::l 5 0 ......
(!)
Cl)
-!: 4
c:
0
== g 3
(f) -0
:: 2
a.
Cl)
0
----Annex Creek
--Auke Bay
........ Juneau
-·-Pt. Retreat
Light Sta.
{'\... A /f\ .'\ I~
/: ... \. f:.\ . \ '-\ .. \ . . ..
1:\ ):~. . .. \, .. . . .. \ . . . . . . ·.. . ~ . . . . . I = ·.. . ~ .
I : . ... : ~ \ . . . . .. ~. ·. . . . / .......... . . . ·-.. . 1\ . --..... . . . . ···~ _J, .. · ·......:...··-.. :/· .
-.· ~······...,.(.•' ---.... . .....,
0 I ~··"· -.i. ¥ r· .. Jan I Feb I Ma~·-·r ... \.A~;"""I
---.1965 1966-------
Figure 6. Depth of snow on the ground for four locations
all within an 11-mile radius of Juneau.
Most snow depths reported for mountainous regions were measured
in the populated valley areas. Such depths are often significantly
less than those at high elevations in the area.
A few measurements of the depth or water equivalent of snow on the
ground are of little direct value in establishing design loads. However,
if such measurements are also obtained at one or more of the locations
listed in Table I at the same time, the ratios can aid in the transfer
of criteria from one location to another. Such measurements should be
taken at intervals during the late winter and early spring when the snow
load is near its seasonal high.
14
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ROOF LOADS
Ground-to-roof conversion
Snow loads on roofs are affected by local winds and temperatures,
the exposure of the structure, its thermal characteristics and geometry
and its aerodynamic position. Factors have been developed to consider
these variables. The factors are based on "Commentary No. 2 Snow Loads"
in the Canadian Structural Design Manua1 20 with modifications based on
snow load observations in Alaska by the U.S. Army Engineer District
Alaska and USACRREL and in Canada by the National Research Council.l9,2l
The following equation has been developed to relate the "basic roof
snow load" to regional winds, exposure of the structure, roof thermal
characteristics, and ground snow loads.
pr = C C Ctp r e g
where:
"
pr
c r
= basic roof snow load in psf
= regional ground-to~roof conversion factor which
considers local winds and temperatures
C = exposure of the structure e
ct = thermal characteristics of the roof
pg = ground snow load in psf for the appropriate
return period •
Appropriate values for Cr' Ce, Ct and p are listed in Tables II,
III, IV and I respectively. g
The "basic roof' snow l.o~," p , requires further modification to
account for nonuniform and 'unbalan~ed loads., roof slope, extra snow
collected in valleys, sliding of snow onto lower·roofs and wind drifting
of snow onto .roofs located in areas of aerodynamic shade.
M:Lnimum loads
All roofs should be designed to sustain a minimum uniform live
load of 20psf except that a 15psf minimum load can be used for unobstructed
metal roofs with a slope greater than 6 on 12 and fabric roofs with the
vertical angle from the eave to the crown greater than 34°, since such
roofs will shed snow by sliding.
15
Table II. Regional groun~-to-roof conversion factor, Cr.
~
"3
t!J?..P
Region
Arctic Slope
Northwest
Inland
Coastal and mountainous
Yukon
Southwest
Mountainous
Other areas
South Central
Coastal
Other areas
Southeast
Yukon
l6
cr
.4
.5
.4
.5
.5
.4
.5
.6
.5
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Table III. Exposure factor, C e.
Siting of structure
Windswept
Suburbs with few trees
Near some trees or other windbreaks
In among trees
Table IV. Roof thermal factor, Ct.
Thermal condition
Heated building with unventilated roof having
conventional insulation (R < 15)
As above. but ventilated
Heated building with unventilated, well-
insulated roof (R > 15)
As above but ventilated .
Building kept just above freezing
Unheated building
Full and zero loads
ce
1.0
1.1
1.2
1.3
ct
1.0
1.1
1.1
1.2
1.3
1.4
For all roofs the effect of removing the load from any portion of
the loaded area should be investigated. In some instances, unloading
certain areas will induce heavier stresses in the roof than with the
entire roof loaded. Cantilevered roof joists are a good example:
removing the load from the cantilevered portion will increase the bending
stress and deflection at center span. In other situations undesirable
stress reversals may result.
Roof slope
All loads acting on sloping surfaces should be considered to act
on the horizontal projection of that surface.
17
The basic roof snow load, p , should be used without modification
for simple shed, gable and hip r~ofs having a slope of 3 on 12 (14°)
or less and for domed or vaulted roofs where the centerline rise is
less than one tenth the span.
For unobstructed metal roofs where snow can slide off the eave,
the flat roof snow load can be reduced by the roof slope factor, C ,
determined using the dashed line in Figure 7. s
4
on
12
3 6 8 12 on on on on
1.0 12 12 12 12
0.81 \
\ ~II Other
Surfaces
I
0.6.
cs I \
I
0.4, ·\
Metal or Other\
0.2t-Slippery Surface \
0 40° 60° 80°
Roof Slope
Figure 7. Graph for determining the roof slope factor, C s
18
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For other roofs that cannot be relied on to shed snow loads by
sliding, the solid line in Figure 7 should be used to determine the
slope factor, C . s
For curved roofs with the rise greater than one tenth the span,
the roof slope factor, Cs, should be determined from the appropriate
curve in Figure 7 basing the slope on the vertical angle from the eave
to the crown. The dashed line in Figure 7 should be used for fabric
structures, including inflatables.
Unbalanced loads
For hip or gable roofs with a slope exceeding 4 on 12 (18°) the
structure should also be designed to sustain an unbalanced uniform
load on the lee side equal to 1.25 times the balanced load. In the
unbalanced situation the windward side shall be considered clear of
snow.
For domed and arched roofs with a rise greater than one tenth
the span the lee side unbalanced load should be a triangular distribu-
tion increasing from zero at the crown to 2.0 times the balanced load
at the eave.
Lower roofs (aerodynamic shade)
Snow drifts will accumulate on roofs in the wind shadow of high~r
roofs. The affected roof may be influenced by a higher portion of
the.same structure or by another structure nearby if the separation is
20 ft or less. When a new structure is built near existing structures
drifting possibilities should be investigated for the existing struc-
tures and the new structures.
The drift load will 2 ~ecrease.as ~he spacing between structures
increases. The factor -spa~Ong ln feet can be used to decrease
drift loads for spacings to 20 ft. For spacings greater than 20 ft
drift loadings need not be considered;
The surcharge load due to drifting can be approximated by a
triangular distribution assuming that the drift rises to the level
of the higher roof at the edge of the lower roof and tapers to zero
at a distance along the roof equal to 2 times the clear height between
the two roofs. The clear height is measured from the top of the
balanced snow load on the lower roof to the closest point on the upper
roof. If the clear height exceeds 5 ft the values for a 5-ft clear
height will be used to establish the extent and magnitude of the sur-
charge load. To determine the thickness of the balanced snow load and
19
the weight of snow in the drift, a density of 20pcf can be assumed.
The drift load should be div~ded by the exposure factor, Ce, since the
potential for drifting will also be influenced by site factors.
Multiple folded plate and barrel vault roofs
Such roofs collect excess snow in the valleys by wind drifting and
by sliding. To account for the excess no reduction in load is allowed
for slope as for shed, gable, hip and curved roofs.
The redistribution of load toward valleys also requires consideration
of nonuuniform loading for all such roofs having a slope or equivalent
slope exceeding 2 on 12 (10°).
The nonuniform load should decrease uniformly from a value two
times the flat roof load Pr' at the valley to zero at the ridge. The
total weight of snow on the roof is the same for the uniform and tri-
angular loadings.
For sawtooth and similar roofs with one surface vertical or nearly
so, uniform loads and nonuniform loads are developed in a similar manner.
Sliding snow
Snow may slide off metal, plastic or fabric surfaces but usually
remains on wood, composition shingle or built-up surfaces unless the
slope exceeds 45°. Situations which permit snow to slide onto lower
roofs should be avoided. Where this is not possible the extra load
added by sliding snow should be considered.
The final resting place of the sliding snow will depend on the
size, position, and orientation of each roof. Distribution of sliding
loads might vary from a uniform load 5 feet wide if a significant
vertical offset exists between the two roofs, to a 20-foot wide uniform
load where a low slope upper roof slides its load out over a second roof
only a few feet lower much like a flowing glacier. For conditions where
a portion of the sliding load is expected to also slide clear of the
lower roof an appropriate percentage of the upper roof load should be
used in the calculation. The Canadian Code 20 suggests using 50% of the
upper roof load for the general case. Where all the upper roof load
can be expected to remain on the lower roof after sliding, the full
load should be considered.
If the upper roof surface is metal, the upper roof load for the
sliding load calculation should be based on the solid line in Figure 7
not the dashed line.
20
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-,
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Roof projections
A continuous obstruction longer than 15 ft may produce a significant
drift on a roof. The loads causeO. by such a drift can be considered
triangularly distributed on either side of the obstruction with a peak
intensity 16 times the clear height of the projection. This value is
based on the assumption that the drift reaches a maximum height equal
to 80% the clear· height and that the drift snow has a density of 20pcf.
This load should be divided by the exposure factor, Ce, since the
potential for drifting will also be influenced by site factors.
The lateral extent of the drift from a rectangular obstruction
can be assumed equal to four times the clear height. L-or U-shaped
obstructions (plan view) will increase snow loads to distances of 6
to 8 times the clear height respectively. For a four-sided obstruc-
tion such as a perimeter parapet the affected. distance should be taken
equal to 10 times the clear height.
Example·
The following example is included to illustrate the snow load
calculation technique.
21
KENAI EXAMPLE
Example: Determine snow loads for a large. conventionally insu-
lated flat roof warehouse in an open area at Kenai. Also determine
loads on the covered loading dock.
Kenai Solution
Pr = C~ Ce Ctpg
For warehouse Pr = (.5)(1.0)(1.0)(84) = 42 psf.
For loading dock Pr = (.5)(1.0)(1.4)(84) = 59 psf.
59 psf Depth of snow on dock roof = --= 3 ft.
20 pcf
Clear height = 6 ft - 3 ft = 3 ft.
Drift .load at wall = (20)(clear height) 20(3) c = -1-= 60 psf.
e
Total load at wall= 59+ 60 = 119 psf.
Lateral extep.t of drift = (2)(clear height) = 2(3) = 6 ft.
Design Snow Loads
Warehouse 42 psf ·
Loading dock
j_ T
Edge of
1
59 psi
Warehouse " _j_
22
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.
REFERENCES
l. American National Standards Institute Inc. (1972) Building Code
Requirements •· for Minimum Design Loads in Buildings .and Other
Structures ANSI A 58.1-1972 •
2. American Standards Association (1955) American Standard Building
Code Requirements·f'or Minimum Design Loads in Buildings and
Other Structures ASA A 58.1-1Sl55.
3. Associate Committee on the National Building Code (1971) National
. Building Code of Canada, 1970. National Research Council of
Canada. NRC 11246, Ottawa, Canada.
4. Bilello, Michael ( 1969) Surf' ace Measurements of Snow and Ice for
Correlation with Aircraft and Satellite Observations.
USACRREL SpecialReport 127.
5. Bilello, Michael (1969) Relationships Between Climate and Regional
Variations inSnow-cover Density in North America. USACRREL
Research Report 267.
6. Blom, Gunnar (1958) Statistical Estimates and Transformed Beta
Variables, John Wiley and Sons, New York.
7. Boyd, D. W. (1961) Maximum Snow Depths .and Snow Loads on Roofs in
Canada. Proceedings of' the Western Snow Conference April
l96l. Also issued as DBR Research Paper 142 (NRC 6312).
8. Boyd, D. W. ( 1970) Climatic Information for Building Design in
Canada; Supplement No. l to the National Building Code of
Canada.
9. City of Fairbanks; Alaska (1971) City of Fairbanks Building Code
Ordinance No. 2070 •
10. Corps of Engineers (1958) Design Data for Military Construction in.
Alaska. Map prepared by the Office of the .District Engineer,
Anchorage, Alaska.
ll. Departments of the Army and Air Force (1966) Load. Assumption ·for
Buildings TM-5-809-l.
12. Department of Commerce, Weather Bureau (ESSA) and National Oceanic
· and Atmospheric Administration ·(NOAA) Climatological .Data,
Alaska. Asheville, N. C. (monthly records during the past
23 years).
23
13. Department of' the Navy, Naval Facilities-Engineering Command
(1967) Des·ign Manual, Cold Regions Engineering NAVFAC DM-9.
14. Grant , Kenneth E. ( 1971) Snow Surveys f'or Alaska. Soil Conser-
vation Service, Washington, D. C.
15. Gumbel, E. J. (1954) Statistical Theory of Extreme Values and
Some .Practical Applications. Nat-ional Bureau of' Standards,
Applied Mathematics Series No. 33.
16. GumbeJ,., E. J. (1958) Statistics of Extremes.< Columbia University
Press, New York.
17. International Conference of Building Off'icials (1970) Uniform
Building Code, 1970 Edit!on Volume I.
18. Isyumov, Nicholas (1971) An Approach to the Prediction of Snow
Loads. Ph.d. thesis submitted to the Faculty of Graduate
Studies. The University of' Western Ontario, London, Canada.
19. Schriever, W. R. ; Faucher, Y. ;· and Lutes' D, A. (1967) Sno.w
Accumulations in Canada, Case Histories: I. Division of'
Building Research Technical Paper 237, National Research
Council NRC 9287, Ottawa, Canada.
20. Schriever, W. R•; Lutes, D. A.; and Peter, B. G. W. (1970)
"Snow Loads, Commentary No. 2 11 in Canadian Structural Desig:n
Manual which is Supplement No. 4 to the National Building
Code of Canada.
21. Schriever, W. R. and-Lutes, D. A~ (1971) Snow Accumulations in
Canada, Case Histories: . II. National Research Council,
NRC 11915, Ottawa, Canada.
22. Sear by, Harold W. (1972) Regional Climatologist, U. S. Dept. of
Commerce, NOAA, Anchorage Alaska; Personal communications.
23. Thom, H. C .• -S. (1966) Distribution of Maximum.Annual Water
Equivalent of Snow on the Ground. Monthly Weather Review,
Vol. 94,-No. 4, April 1966.
24. Thom, H. C, S. ( 1971) Environmental Data Service , ESSA (now NOAA) ,
WasJtington, D. c. Personal communications.
25. Wahrhaftig, C. (1965) Physiographic Divisions of Alaska. u. S.
Geological Survey Professional Paper 482. (Map referred to
is-. plate I of this reference. )
24
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APPENDIX A: FORTRAN II PROGRAM FOR ANALYSIS OF SNOW DEPTHS
C ·LEAST SQUARES ANALYSIS OF LOG•NORHAL PROBABILITY PLOT
C INCLUDES CALCULATION OF CORRELATION COEFFICIENT AND
~ COEFFICIENT OF DETERHINATION FOR THE RE~RESSION LINE
C USING BLOM 1 S PLOTTING POSITIONS
C READ IN LIST OF VARIATES USED IN LAYING OUT PROBABILITY
C PAPER AS THE FIRST SET OF DATA.
DIMENSION Y!lOOl~ P!10QI.H!100l.XSCALEI100l.V199l.YLOGI100)
READ PAPE.R_ TAPE 101. V
101 FORMATIE20.0l
C 20 ANUM CHARACTERS PROVIDED FOR DATA LOCATION
4 READ PAPER TAPE. 103. NAl• NA2. NAJ. NA4. NA5• IDUH
J03 FORMAT f5A4• 1101
C 8 ANUM CHARACTERS PROVIDED FOR DATA SOURCE
READ PAPER TAPE 104. lSOl. ISQ2. IDUM
104 FORMATC2A4•I101
C 8 ANUH CHARACTERS PROVIDED FOR DATA TYPE
READ PAPER TAPE 104. ITYPE1• ITYPE2o~. IDUH
N=O
J N= N+ 1
t Y VALUES READ !N IN ORDER "FRO~ SMALLEST TO LARGEST.
C LAST ENTRY MUST BE 9999.
C IF THERE IS ANOTHE~ DATA SET• A NUMBER LESS THAN 99999999.
C MUST BE ENTERED FOLLOUING "THE 9999. ENTRY DESIGNATING-THE
C END OF THE PREVIOUS DATl SET~ IF NO MORE DATA SETS FOLLOU.
C 99999999. OR GREATER MUST "BE ENTERED.
READ PAPER TAPE 102. Y(N)
.102 FORMATCE20.0)
IF (Y(Nl•9999.) 1.2.2
2 N&N-1
DO 10 1 ,.1. N
PCII=IfFlOATIJl·.375)/(FLOATIN)+.25))•100.
MC I )=Pr 11+0.5
XSCALEIJI:VIMII))
10 YLOGIIJ~FtOGtOFCYCJl)
SUHX=O.O
SUHY"O.O
SUHXSO•O.O
SUHYSQ•O.O
SUHXY=o.o
DO 11 1•1. N
SUHX=SUMX+XSCALEII)
SUMY=SUMY•YLOGIIJ
SUMXSQ•SUHXSO+XSCALEIII•XSCALEIJ)
SUMYSO•SUMYSO+YLOG!li•YLOGIJ)
Al
0001
0002
0003
0004
coos
0006
0007
0008·
0009
0010
0 011
0012)
0013
0014
0015
0016
0017
0018
0019
0020
0021
0022
0023
0024
0025
0026
0027
0028
0029
0030
0031
0032
003J
0034
OOJS
0036
0037
0038
OOJ9
0040
0041
0042
004J
0044
ll
U3
105
106
107
108
J
109
uo
111
112
50
APPENDIX A
SUHXY=SUHXY+XSCALEIII•YLOGIJ)
ZN•N
AVEX:rSUHXJlN
AVEY 11 SUHY JZN
XYBIG=SUMXY-ZN•AVEX•AVEY
XBIG 2 SUMXSO-ZN•AVEX••2.
B•XYBIG/XBlG
AaAVEY-B•AVEX
COCOR=XYBJG/!!SUHXS0-2N•IAVEX••2.Jl•ISUHYSQ-2N•(AVEY••2.)JI
1••0.5
CO DEl =CDC QRu2.
Y101•10.••IA+8•2.674J
YS•lO.••!A+B•5.842J
Y30=10.••<A+B•6.817l
Y50•10.••(A+8•7.054l
Y100•10.••fA+B•7.J26l
Y10=10.••(A+B•6.282l
Y25=10.••(A+B•6.750l
PRINT 113
FORHATI1Ht,59HlEASl SQUARES ANALYSIS OF LOG NORMAL
!PROBABILITY PLOT IBLOH!l
PRINT 105, NAl~ NA2, NA3•NA4• NA5
FORMA111H0~9HLOCATION ,2X.5A4)
PRINT 106. IS01• 1502
FORHAT11H0,19HlNFORMATION SOURCE ,2X.2A4)
PRINT 107. JTYPEl•ITYPr2
FORMAT!lH0•9HDAlA USED//2X,2A4,JX,4H?LOTJ
DO J I •1• N
PRINT 108, Y!J), H(Jl
FORHAT!F8.2.J&l
CONTINUE
PRINT 109 .. 8• A
FORMAl(lH0·6HSLOPE•·FlO.S,2X•10HINTERCEPT•,F10.5)
PRINT 110, ITVPE1•ITYP[2•Y10l.YS•Y10•Y25•Y30•YSO•V100
FORHAT!1H0•13HRETURN PERlOD,2X.2A4.2X.8HPLOTTING/4X,
15HYEARS.16X~8HPOSITION//7X.4H1.Q1.5X.F6.2~7X.1Hl/
17X.tHS-8X.F6.2.6X.2H80/6X.2H10•8X.F6.2•6X.2H90/6X .. 2H25•
18X.F6.2,6X.2H96/6X.2H30.6X,f6.2,6X.4H96.7/6X.2H50,8X.
1F6.2.6X.2H98/5X.JHl00•8X•F6.2.6X.2H09).
PRINT 111.COCQR.CODET .
FORMAT11H0,24HCORRELAliON COEFFICIENT=•F10.5•5X~29HCOEFF
!ICIENT OF DETERMINATION••FlO.Sl
READ PAPER TAPE 112• ZoOM
FORMATIE20.0l
IFlZOON-99999999.1 4.So~50
STOP
END
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APPENDIX B: COMPUTER PRINT-OUTS FOR CAPE LISBURNE AND UTOPIA CREEK
·-~ .
-,
_,_,
lEAST SQUARES ANALYSIS OF LOG NORHALPROBABILITY PLOT (BLOH)
l
LOCATION CAPE LISBURNE ALASKA
INFORMATION SOURCE BILELLO
DATA USED _.
DEPTH PLOT
l s.oo 6
12.00 16
~ 15.00 26
15.00 3!)
17.00 45
d 19.00 55
19.00 65
•27.00 74
29.00 84
_.. 29.00 94
....., SLOPE.= .t&3t6 INTERCEPT., .33194
RETURN PERIOD DEPTH PLOTTING
YEARS POSITI:ON
1.0! 6.63 t
5 25.24 80
10 ~0.39 9o
25 37.02. 96
30 38.0& 96.7
-~ 50 42.0& 98
lOO 47.20 99
CORRELATION COEFFICIENT• .96621
__; COEFFICIENT OF DETERMINATION• ~93744
-= ,J
--'
-'
Bl
APPENDIX B
LEAST SQUARES ANALYSIS OF LOG NORMALPROBABILITY PLOT (BLOMI
LOCATION UTOPIA CREEK. ALASKA
lNFORHATION SOURCE BILELLO
DATA USED
DEPTH PL·Ol
_9.00 4
13.00 11
14.00 17
19.00 24
20-00 30
20:00 37
21-00 43
27-00 50
28-00 57
32-00 63
36.00 70
42.00 76 4s.oo 83
ss.oo 89
69.00 96
SLOPE= .25555 INTERCEPT., .13644
RETURN PERIOD DEPTH PLOTT INC
YEARS POSITION
1.01 6.60 1
5 42.60 80
10 55.18 9o
25 72~68 96
JO 75.60 96.7
50 86.92 96
too 102.00 99
CORRELATION COEffiCIENT• .99498
COEFFICIENT OF DETERMINATION~ .98998
B2
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