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Technical Note (National Research Council of Canada. Division of Building Research), 1969-03-01
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Edmundston Forum - Report on Snow Loads
Plewes, W. G.
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NRC Publications Record / Notice d'Archives des publications de CNRC: https://nrc-publications.canada.ca/eng/view/object/?id=4569671a-1515-4445-afaf-1088a476bc55 https://publications-cnrc.canada.ca/fra/voir/objet/?id=4569671a-1515-4445-afaf-1088a476bc55
DIVISION OF BUILDING RESEARCH
No.
531
NOTIE
'1
E
C
JH[
N II CAlL
PREPARED BYW.
q.
Plewes CHECKED BY W. R. S. APPROVED BY R. F. L.ｾ March 1969
PREPARED FOR The Honourable B. Fernand ｎ｡､･｡ｵｾ ｍ｡ｹｯｲｾ City of Edmundston.
SUBJECT EDMUNDSTON FORUM- REPORT ON SNOW LOADS
Late in the afternoon of 7 February, and all day on 8 February 1969 the writer visited the site of the forum in Edmundston, New Brunswick, which had collapsed in the early m.orning of 4 February
1969. The visit was m.ade at the invitation of the Mayor of Edmundston,
The Hon. F. Nadeau, addressed to the Director of the Division of
Building Research. The purpose of the visit was to observe the wreckage,
to draw any lessons to be learned with regard to the application and enforcement of building codes and, also, to advise on any items falling within areas of competence of the Division.
The Division has been making a study of snow loads on roofs for many years, and the amount of snow load on the roof at time of
collapse seem.ed to be an item of special importance. Mr. Leger, City
Engineer for the City of Edmundston, said that he had obs erved the forum during the pr evious week and estimated that ther e had been about 18 inches of old snow with a slight ]mm.p at the ridge.
Snow measurem.ents on the wreckage were m.ade by Mr. Terriault, Mr. Leger and the writer on the morning of 8 February.
2
-Sampling Method
At fir st it was planned to us e a piece of 8 -in. diam.eter "Sonotube" as a vertical sampler. The old snow was too hard and crystalline to force the tube through it easily, so most of the sam-pling was done by weighing the snow in metal pails of known volume
and weight. In filling the pails and attempt was made to include a
proportionally correct amount of the profile of the snow at a
cross-section. (In tests carried out at the Division of Building Research,
this method has' been found to give results equivalent to the DivisionIs standard snow sampler when the snow is crystalline. )
Because of the collapse very little of the snow on the south side
of the roof was undisturbed. A few places were found where the layering
of the snow appeared as it had been on the roof before collapse and
m.easurements were made there. Figure I shows the location of the
measurements and Table I gives further notes on these. Unit Weight and Specific Gravity
Measurement at A gave the weight of the ice layer on the bottom.
of the snow accumulation. This ice was very porous and weighed 29. 6 lb
per cu ft. Since the layer in several places examined was only about
1/4 in. thick and was at the most 3-1/3 in. thick, it appeared to be a spotty accumulation from early in the season.
Measurements at B, C and D were essentially at the same
location, in a cluster about 1 ft apart. One weight m.easur em.ent was
obtained for new snow and two for the old. At E the depth of new sno;w
was merely observed. At F the new snow was observed and the old snow
weighed.
Measurements at G to J were taken in what appeared to be
undisturbed snow on the north slope of the collaps ed roof. In general
the old snow did not have any significant new snow on top, probably
because of the wind effects during the storm. Undisturbed fr esh snow,
most of which fell after the collaps e, was found, however, at G on a flat surface.
Weights taken with a Sonotube sampler tended to be sm.aller than
those taken in weighed pails. It is believed that the Sonotube measurements
are too low because of the difficulty in preventing loss of material with a
samples of old snow were measured by Messrs. Leger and Terriault
but the exact locations are unknown to the writer. In any case, including
all measurements. the average weight was 30. I (say 30) lb per cu ｦｴｾ
The specific gravity of the old snow on this basis was 30/62. 4 =
O. 48. It has been found in the Snow Survey of the Division that this is
in line with specific gravity of old snow found on roofs. Early in the
winter season the specific gravity of snow on a roof is usually low
(0.2 or less). Toward spring. the accumulated snow becomes more
and more compacted and changes to an accumulation of larger crystals
due to the sun and heat loss through the roof. The specific gravity
progressively increases and commonly exceeds O. 4.
Specific gravities from 0, 45 to O. 49 have been obtained by the Division and reported also by others in studies of ground accumulations. For these reasons a mean of O. 48 for the roof of the Edmundston forum
is not unreasonable. The mean deviation from the average is 0.05. '
Therefore, in order not to overestimate the snow load, it seems to be advisable to use O. 43 as a lower bound to the range in which the actual
specific gravity might reasonably lie. This corr esponds to a unit weight
of 27 lb per cu ft and this value is used in subsequent calculations. As there were only two values obtained for the weight of new snow, a simple average was taken, that is, 15 lb per cu ft.
Depth and Weight of Snow on Roof
The actual point -by-point distribution of the snow on the roof
is not known, but the average can be estimated. Twelve to fourteen
ft from the east edge of the south slcpe, the depth of old snow was about 8 in. Near the centre of the building, 4 to 5 ft south of the ridge, it
,measured 18 in. deep. Mr. Leger said that the week before he had
estimated the depth to be about 18 inc. .
At the time of the writerts visit it was noticed that the roof
deck of the south side was tipped toward the inside of the building and had deposited all its snow in an almost uniform long row or pile along
the building length. A cross -section was measured across this, as
shown in Figure 2. The widths shown across the pile were measured
with a tape m,easure. The average depth was estimated to be 2. 5 ft
ｾ｢ｯｶ･ the top of the rink boards, which are themselves 4 ft high. The pile of snow was on top of the 2-in.., roof deck, which in
4
-twisted under the mass. To allow for these,
6
in. was deductedgiving a net depth of
6.
0 ft. Although the snow in the pile was firm, the old snow may have bulked somewhat in sliding off the roof deck. To allow for this and possible errors in estimating the depth, afurther 6 in. was deducted. The final depth used for calculation was
5. 5 ft.
Using this depth and the dimensions shown in Figure 2, the average depth on the roof at the time of collapse is calculated to be
2. 3 ft. As the values used to calculate this were taken on the low
side to avoid an overstatement of snow condition, it is thought that
the average snow on the roof was at least this deep. During a storm
the snow on a roof of this size and shape would blow off the windward
side of the gable onto the other. Weather records show that about
8 in. of snow fell during the day prior to the collapse, and therefore, more than 8 in. could easily have accum.ulated on the south slope, having been augmented by snow blowing onto it over the ridge from the north slope.
In calculating the weight on the south slope one of two assumptions can be made.
(1) Mr. Leger mentioned that 18 in. of old snow had been on
the roof before the collapse and 18 in. was actually measured
4 ft from the ridge near the centre of the building. Assuming
that this prevailed over most of the south slope, then the total snow weight would be:
Old snow: 18/12 x 27
=
New snow: 0.8 x 15
=
40.5 12.0
52. 5 (say 52) lb per sq ft.
(2) If 18 in. was the maximum depth of the old snow and varied
from. that down to 8 in. as measured at the east end, then the total snow weight would be about:
Old snow: 8
+
18 x 27 = 29.22 x 12
New snow: 1. 2 x 15 = 18.0
On the north slope most of the new snow appeared to have
blown off during the storm. From measurements and observations
ｾｴ appeared that there had been a relatively uniform layer of old
snow about 8 in. deep on that side, giving ｾ load of 8/12 x 27 = 18 1b
per cu ft. Conclusion
The snow load on the roof of the Edmundston forum at the
time of the collapse appears from the evidence to have been unbalanced. An estimate of the average on the south slope gives a possible range of
47 to 52 Ib per sq ｦｴｾ On the north slope the average load was about
18 Ib per sq ft.
Bearing in mind that (the weights are calculated on the actual area of roof rather than the projected area) and that the estimate was based on the lowest probable value for the specific gravity of the old
ｳｮｯｷｾ 0.43, as compared with the measured mean (specific gravity
=
O. ＴＸＩｾ the above weights are considered to be an underestimate rather
.,
TABLE I
SNOW SAMPLES - EDMUNDSTON FORUM
I
Depth of Depth of Net Wt. of Net Wt. of
Location Old Snow, New Snow, Old Snow, New Snow, Notes
in. in. cu ft cu ft A
-
-
-
-
Underlain by 3 -1/211 of porous ice (net wt. ) 29. 6 Ib/cu ft B 9-1/4 13.0 By Sonotube C 7-3/8-
32.8-
1/4" ic e on bottom D 8-1/4-
29.6-
1/41 ' ice on bottom E 4-1/2-
Not weighed -F 18-3/4 5-1/2* 23. 2-
By Sonotubee
F(a)-
4-
17.5 Fell after collapseG
-
6 -1/2*-
Not weighed Depth only. Icebelow 2-3/4" thick.
H 7-1/2 None 23. 2 1/2" ice on bottom
1/2" crust on top
I 7 None Not weighed l/Z" crust on top
J 9 None 32.8 1/2" crust on top
Three other samples of old snow taken by Messrs. Terriault and Leger - location unknown.
K 32.1
L 36. I
M 31. 2
Average Weight 30. 1 15. 3
(say) 30 15
Specific Gravity: Old Snow = O. 48; New Snow
=
O. 24>:<From measurement F(a) and other observations about 4 in. of new snow fell after the collapse.
Nominal
Di rection
ｳｾ
-
LI'\ ('f'\ I-4-'
-C>3D'
('f'\ｾｇ
Note:
Dimensions Refer to
Estimated Position on
Roof Before Collapse
®
i-A..
ｅＧ｜ｊＢｴＭｾＭＺＭＺＢＱＴｲＫＭＭＮD
-.:::r - ./
B
lR
lI ' ... <.0C"
ur
L L. " " - - - . . , r - - - l 0 1-1- - i l 0 t----.r---I.----JL--.J,-..L. o 0 o 0.@
FIGURE 1Eave
I
ｾ
I
ｾｾ
651Roof Di men sion s
9
14
1n
Ridge
__ IH
Est."6.
5" 321 231 231 Pill e Volume 23x5.5 - lx3.l1x5. 5 + 2+:.5 x 9 126.5 - 8.7 + 33.7 = 151.5 Cu FtLength of Roof Deck Covered by Snow in Pile = 64.5 Ft . _151.5_
.. Average Depth - 64.5 - 2.35 Ft
FIGURE 2
EDMUNSTON FORUM - CALCULATION OF SNOW VOLUME AND DEPTH
