108. Allowance For Snow

In making an allowance for snow, one's judgment must be exercised.to a considerable degree, as the maximum snow fall varies widely in different localities, and the amount of snow that may lodge upon a roof will depend in a great measure upon the inclination of the roof and its exposure to the wind, also somewhat upon the roof covering and whether or not snow guards are used.

The weight of dry, freshly fallen snow is commonly given at 8 pounds per cubic foot, while saturated snow or snow mixed with hail or sleet may weigh as much as 32 pounds per cubic foot. Dry snow may attain a depth of 3 feet and possibly more in some localities, but snow weighing as much as 32 pounds per cubic foot will hardly ever be found more than 16 inches in depth, even on a flat roof.

It is generally assumed that a sloping roof cannot be exposed to the maximum snow load and wind pressure at the same time, but as a high wind may follow a sleet storm, some allowance should be made for sleet, under any method of computing stresses, i. e., for roofs in the Northern States. For flat roofs, and for such others for which it may be deemed desirable to determine the stresses due to maximum snow load, the values given in Table IX. may be considered as the maximum possible loads for different portions of the country.

When the wind pressure is treated as a vertical load, the values given in Table X. should be used as representing the maximum load due to wind and snow combined.

109. Wind Pressure

For roofs having a pitch of 4 ins. or more to the foot, the effect of the wind must be taken into

Table IX. - Allowance For Snow In Pounds Per Square Foot Of Roof Surface

Location.

Pitch of Roof.

1/2

1/3

1/4

1/5

1/6 or less.

* †

* †

* †

Southern States and Pacific Slope..

0 - 0

0 - 5

0 - 5

5

5

Central States.............

0 - 5

7 - 10

15 - 20

22

30

Rocky Mountain States.............

0 - 10

10 - 15

20 - 25

27

35

New England States..........

0 - 10

10 - 15

20 - 25

35

40

Northwest States...........

0 - 12

12 - 18

25 - 30

37

45

Columns headed by an asterisk (*) are for slate, tile, or metal; those headed by a dagger ( †) are for shingle roof. When Snow guards are to be placed on the roof, the same allowance should be made for a half pitch as for one-third pitch, and the larger figures should be used.

account in determining the stresses. Two ways of doing this are in vogue. The more common method for wooden trusses and for steel trusses of the Fink or Fan types is to include the wind pressure with the vertical loads and to make a single allowance for both wind and snow.

This is not a correct assumption, as the wind acts on a roof in a direction normal (at right angles) to its surface and the stresses produced by such a force are quite different from those produced by a vertical force of the same or even greater intensity.

As a matter of practical experience, however, it is found that for ordinary types of wooden trusses, having an inclination not exceeding 450, and for steel trusses of the Fink or triangular type, this method is sufficiently accurate.

Mr. Bryan, the designing engineer of the Edgemoor Bridge Works, states that: "In the Fink trusses a partial load due to wind or snow never causes any maximum stresses, so that it is customary to calculate these trusses for a uniform load over the entire truss, the wind and snow loads combined being usually assumed at 30 pounds per square foot of area covered," i. c., horizontal surface. "It is not generally assumed that the maximum wind pressure and the snow load can act on the same half of the roof at the same time."

To be absolutely safe, however, the author recommends that the allowance for wind and snow combined be not less than indicated in Table X.

Table X. - Allowance For Wind And Snow Combined In Pounds Per Square Foot Of Roof Surface

Location.

Pitch of Roof.

60°

45°

1/3

1/4

1/5

1/6

Northwest States..............

30

30

25

30

37

45

New England States...............

30

30

25

25

35

40

Rocky Mountain States..............

30

30

25

25

27

35

Central States.................

30

30

25

25

22

30

Southern and Pacific States.............

30

30

25

25

22

20

110. The other and more exact method is to find the stresses for all of the different loadings to which the truss may be subjected, separately and then combine them so as to obtain the greatest possible stress that may occur in each member under any possible combination of loads.

This method should be followed in determining the stresses for all trusses having an inclination of more than 450 or a span of more than 100 ft. (except for flat roofs), also for all trusses in which a partial load may produce maximum stresses or call for counter bracing, as is the case in quadrilateral trusses and trusses with curved chords.