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Field instructions for observers at "F" stations of snow loads on flat
roofs
DIVISION OF BUILDING RESEARCH
NATIONAL RESEARCH COUNCIL OF CANADA
,
'Jr
EClHIN IICAJL
NOTE
No.
511
PREPARED BY D. A. Lutes CHECKED BY W. R. S. APPROVED BY R. F. L.
ｾ .November 1967
PREPARED FOR Inquiry and Record Purposes
SUBJECT FIELD INSTRUCTIONS FOR OBSERVERS AT "F" STATIONS
OF SNOW LOADS ON FLAT ROOFS
These instructions are intended for observers at "FI !
Stations ("F" indicates flat roofs) taking part in a new survey of snow loads on flat roofs in selected locations acros s Canada. The purpose of this new survey, being started in the 1967/68 winter,
superceding the "A" and liB" Station survey, is to provide information on high and low loads occurring on single -level and multi-level flat roofs which constitute the dominant type of roof on newer commercial and indus trial buildings.
The instructions are in three parts:
(A) List of equipment and instructions for choosing roofs and filling out Form F-l.
(B) Instructions for the field measurements.
(C) Instructions on filling out Form F- 2 and when to take measurements.
(A) List of Equipment and Instructions for Choosing Roofs Observers will be supplied with the following items:
(1) Pads containing forms on which to record snow measurements and other pertinent information, one pad to be used for each roof under obs ervation.
Form F-l: "Details of Roof and Other Relevant Information" and "Plan of Surrounding Area". Two copies are inCluded: both are to be filled out and one is to be returned to the Division of Building Research (envelopes are also provided). The other should be kept by the observer.
Form F-2: "Observation of Snow Depth and Density". There are 30 copies of this form on the pad. One of these should be completed each time and sent to the Division.
(2) Snow Depth Yardstick.
(3) Snow Density Kit. This kit includes the following: Snow sample cutter with back plate.
Knife. Buckets.
Container (graduated in cubic centimeters). Choosing the Roofs
The survey will be based on two types of flat roofs in selected locations. The observer should choose, if possible, flat roofs of the following types:
(i) Simple flat roofs (very large and medium sized roofs are of interest). (Figure 1).
(ii) Multi-level flat roofs: Buildings with at least two roof levels. A one-storey (9 - 12 ft) difference in roof levels only is wanted for this survey. (Figure 2). The observer should choose roofs with either no or low parapets, on relatively new buildings which can be used over a period of years. Permission to us e the roof for the survey should be obtained in advance. As many buildings as possible should be observed, including at least five of the multi-level flat roof type. Exposure
It is desirable to observe separately well sheltered and
exposed roofs in most locations. A sheltered roof is one surrounded by higher objects (buildings, trees, etc.) which protect the roof from high winds. An exposed roof is one not obstructed from the . wind by high objects. It may be surrounded by buildings of the . same height.
3
-(B) Instructions for Field Measurements
Depth Measurements on the Roof:
A yardstick is supplied for measurements of snow depth on the roof. When snow conditions are uniform one representative measurement should be taken for each 20 to 30 ft square section. but if the snow condition is non-uniform (drifts), then measure-ments should be taken at more closely spaced intervals.
Depth Measurements on the Ground:
The observer should take a number of readings with the yardstick of snow depth on the ground. around the buildings in an area completely free of drifts i. ,e. preferably in a treed area, at the same time as the roof measurements.
Density Measurements on the Roof and on the Ground:
The density (or unit weight) of the snow is required to enable an accurate calculation of the actual load of the snow cover per unit area to be made. The average density of the snow cover is measured by taking a set of samples of snow of known volume from a vertical cut in the snow and melting this snow in order to obtain, by volume measurement. the ｷ｡ｴｾｲ equivalent.
A measurement should ·be taken on each large flat surface of the roof. These locations should be shown on Form F-2. At the same time. a density measurement should be made on the ground in the area where the ground depth is measured.
Each observer is supplied with a set of instruments to measure density:
Snow Sample Cutter - The cutter shown in Figure 3 has a thin saw-tooth cutting edge for use in both hard and soft snow. The internal volume of each cutter is 250 cc. between the planes X-X and Y - Y.
The cutter is used horizontally and therefore a vertical cut should first be made through the snow with a flat-end shovel. The cutter is pushed into the snow while the cylinder is rotated around its axis to avoid com-pressing the snow sample. A knife is used to trim "the sample level with the forward and rear edges of the
·-e
slippery samples from sliding out of the cutterafter the sample has been isolated.
Bucket and Graduated Container - Each bucket (containing the snow samples from one location only} shall be covered, labelled, and taken indoors to melt the snow. The volume of the water should be measured in the graduated container as soon as
possible after the melting is complete in order to prevent exces s evaporation. Since one cubic centi-meter of water weighs one gram, the number of c. c. read on the graduated cylinder derived from all samples obtained will yield the average density
according to Table 1. It is essential, therefore, to record how many samples are in the bucket.
The average density of the snow cover is obtained from samples taken at regular vertical spacings (say 4-inch). If the snow depth is less than about I foot, more than one sample should be taken in each 4-inch layer to give at least four samples to the bucket, but there must be the same number of samples taken in each layer of the snow cover to give a correct average. If there are any icy layers in the snow cover the thickness of the ice should be recorded. The observer should use judgement in taking the measurements.
When cutting samples fFom a vertical section of snow, samples should be cut first from the top layer, then the next layer and so on, to the roof or the ground. This- could be don.e on a flat roof or the ground by cutting a s mall trench.
Samples of soft snow should be carefully cut in order to avoid compressing the snow. Snow samples that do not completely fill the internal volume of the cutter should be dis carded.
Snow-Storm Records:
When des cribing new snow, thes e des criptive ter ms should ·be used where they apply: wet, dry, light, heavy, drifting,
non-drifting, rain, hail, ice-pellets (sleet), snow, granular-snow (e. g. "heavy, wet snow later mixed with ice pellets and rain!I).
If meteorological records cannot be obtained, the observer should, if possible, des cribe wind speed and direction as indicated
5
-in Table I I. This Table contains the Beaufort scale to help in assessing wind speed.
(C) .Filling Out Form F-2 and When to Take Measurements
The snow depth record sheets should be filled out after significant snowfalls at the time in the winter when snow depth is probably at a maximum for the winter.
Density measurements should be made as well as depth measurements on the roof and ground, when there is an average depth of 5 inches or more.
Snow-storm records should be included for each storm yielding a significant snowfall.
Where much drifting occurs, a general description of the snow on the roof by means of sectional diagrams should accompany depth measurements. Photographs accompanying observations will often be mce t us eful.
CONCLUSION
The purpose of this investigation is to improve still further and to refine existing snow load requirements for buildings with flat roofs in Canada. It is hoped that ultimately the records obtained will produce such definite information that recommendations can be made that will achieve still further economy in building. This project must, of necessity, be carried on for a number of years before any recommendations can be made. Actual measurement of snow on roofs is the only way in which a basis for determining snow load requirements can be achieved. It must be stressed that the value of the project is directly related to the care taken by each observer in the taking of his measurements.
A Final Personal Note:
In order to keep the foregoing instructions as brief and concise as possible, they have had to be drafted in a necessarily impersonal manner. Will all those who use this note and follow these instruc-tions please be assured that the members of the Division of Building Research who are concerned with the snow load project realize
which the taking of the records herein described calls for. All observers may rest assured that their efforts will be greatly appreciated and that the combined results should lead to a sig-nificant advance in knowledge of snow loads and therefore to the improvement of building in Canada.
TABLE I
Weight Read on Scale in Grams Number of Samples Density 4 5 6 7 8 9' 10 11 12 gm/cc 80 100 120 140 160 180 200 220 240 0.08 90 113 135 158 180 203 225 248 270 0.09 100 125 150 175 200 225 250 275 300 O. 10 110 138 165 193 220 248 275 303 330 O. 11 120 150 180 210 240 270 300 330 360 O. 12 130 162 195 228 260 293 325 358 390 O. 13 140 175 210 245 280 315 350 385 420 O. 14 150 188 225 263 300 348 375 413 450 0.15 160 200 240 280 320 360 400 440 480
o.
16 170 213 255 298 340 383 425 468 510 0.17 180 225 270 315 360 405 450 495 540 O. 18 190 238 285 333 380 428 475 523 570 0.19 200 250 300 350 400 450 500 550 600 O. 20 210 262 315 368 420 473 525 578 630 0.21 220 275 330 385 440 495 550 605 660 0.22 230 288 345 403 460 518 575 633 690 0.23 240 300 360 420 480 540 600 660 720 0.24.
, 250 313 375 438 500 563 625 688 750 0.25 260 325 390 455 520 585 650 715 780 0.26 270 338 405 473 540' 608 675 743 810 0.27 280 350 420 490 560 630 700 770 840 0.28 290 363 435 508 580 653 725 798 870 0.29 300 375 450 525 600 675 750 825 900 0.30 310 388 465 543 620 698 775 853 930 0.31 320 400 480 560 640 720 800 880 960 0.32 330 413 495 578 660 743 825 908 990 O. 33 340 425 510 595 680 765 850 935 1020 0.34 350 438 525 613 700 808 875 963 1050 0.35 360 450 540 630 720 . 810 9 00 990 1080 0.36 370 463 555 648 740 833 925 1018 1110 0.37 380 475 570 665 760 855 950 1045 1140 0.38 390 488 585 683 780 878 975 1073 1170o.
'39 400 500 600 700 800 9 00 1000 1100 1200 0.40 410 513 615 718 820 923 1025 1128 0.41 420 525 630 735 840 945 1050 1155 0.42e
430 538 645 753 860 968 1075 1183 0.43 440 5'50 660 770- 880 990 1100 0.44 450 588 675 788 900 1013 1125 0.45Beaufort No.
e
Title
TABLE 2
BEAUFORT SCALE OF WrnD VELOCITIES
Miles per hour
Term for Use in Form A-2 •
o
1 2 3 4 56 .
7. 8 9 10 11 12-calm light air slight breeze gentle breeze moderate breeze fresh breeze strong breeze high wind gale strong gale whole gale storm hurricaneCalm, smoke rises vertically. Direction of wind shown by smoke drift, but not by wind vanes.
Wind felt on face; leaves rustle ordinary vane moved by wind. Leaves and small twigs in con-stant motion; wind extends light flag.
Raises dust and loose paper; small branches are moved.
Small trees in leaf begin to sway. Crested wavelets form on inland waters.
Large branches in motion: whist-ling heard in telegraph wires, umbrellas used with difficulty. Whole trees in motion; incon-venience felt when walking against wind.
Breaks twigs off trees; generally impedes progress.
Slight structural damage occur s (cl:irnney pots and slate removed) Seldom experienced inland; trees uprooted; considerable structural damage occur s.
Very rarely experienced, accom-panied by wide spread damage.
1
1 -
3
4 - 7 8 - 12 13 - 18 19 - 24 25 - 31 32 - 38 39 - 46 47 - 54 55 - 63 64 - 75 Above 75 calm 10 mph 20 mph 30 mph 45 mph extreme wind•
