Fig. 7. Theoretical Determination of Normal Component.

CLYBOURNE STATION OF THE CHICAGO & NORTHWESTERN RAILWAY COMPANY.

Frost & Granger, Architects, Chicago, 111. Built in 1900. For Interior, See Opposite Page.

INTERIOR OF CLYBOURNE STATION OF THE CHICAGO & NORTHWESTERN RAILWAY COMPANY.

Frost & Granger, Architects, Chicago, 111. Exterior View Shown on Opposite Page.

 Pitch Normal Wind Pressure 1/3.... ....34 pounds per square foot. 30°.... ....32 " " " " ¼.... ...... 30 " " " " 1/5 ....26 " " " " 1/6 .... ....22 " " " "

If the normal pressure on a roof making any other angle with the horizontal is desired, see "Statics," p. 24.

The determination of these values is based for the most part on data obtained by experiment. In the computations relative to the design of buildings, the wind is usually assumed to exert a pressure on the walls of 30 pounds per square foot.

The snowfall varies with the locality. The heaviest snow loads which come upon a roof are not always in the locality of the heaviest snowfall, since a comparatively light snowfall may occur, and if this is followed by wind and sleet, the result will be a load greatly in excess of the snowfall itself. The snow load per square foot of roof surface varies with the pitch of the roof, and will be greater the smaller the pitch. The ice and sleet will be comparatively constant. Fig. 8* gives values of snow and sleet loads which are recommended for use. It is customary to figure the snow load by taking it as so much per square foot of horizontal projection.

## Examples For Practice

1. Compute the wind panel load on a roof whose pitch is ¼, and whose panel length is 15 feet, the distance between trusses being 16 feet.

2. Compute the snow panel load for the truss of Problem 1, above.

*Ketchura's "Steel Mill Buildings," p. 11.

4. Weights of Roof Trusses. The weight of a roof truss varies with the material of which it is constructed, the span, the distance between trusses, the pitch, and the capacity of the truss. The actual weight, of course, cannot be determined until after the truss is designed; but an approximate weight may be obtained from any of the empirical formulae which are now in use. Table II gives the most common and best of the empirical formulae, together with the names of their authors.

## Table II Formulsae Giving Weights Of Roof Trusses

 Formula Author w = ¾ al ( 1+ l/10) Mansfield Merriman w = al ( 1 + l/25 ) E. R. Maurer, (p. 23, "Statics") W = al2 ( 1/25 + l/ 6 000 ). Wooden trusses. N. C. Ricker W = Pal/45 ( 1 + l / Milo S. Ketchum* W = 2a (4 + l / 25 ) C. W. Bryan W = al (0.06 l + 0.6) for heavy loads C. E. Fowler W = al (0.04 l + 0.4) " light "

In the above formulae,

W = Weight of steel in truss, in pounds;

P = Capacity of truss in pounds per square foot of horizontal projection of roof; r = Rise of peak, in feet; a = Distance center to center of trusses, in feet;

I = Span of truss, in feet.