The best method of determining the economical spacing is either to make a comparative design or to consult the back volumes of The Engineering Record, Engineering News, or some other good engineering periodicals. Designs of buildings which have been constructed are frequently given in these periodicals; and from these the student may, in addition to the spacing of the trusses, obtain much other valuable information regarding roof construction.

Bulletin No. 1G of the University of Illinois Experiment Station gives a systematic study of roof trusses, and shows the effect on the variation in the weights of rafters and purlins due to a variation in the length of span. This bulletin, which can be had free for the asking, should be in the hands of the student. It may be had by addressing "The Director," Engineering Experiment Station, University of Illinois, Urbana, Illinois. A most valuable book giving a systematic and extensive study of roof trusses and mill buildings, is "Steel Mill Buildings," by M. S. Ketchum, Engineering News Publishing Company, New York, N. Y.

9. Stresses in Roof Trusses, and Sizes of Members. Stresses in roof trusses of any form can be computed by the methods of "Statics" (pp. 23 to 73). On account of the ease and economy of manufacture, some form of truss is usually used in which there are many members with equal stresses. The Fink truss, or some modification of it, is almost universally used (see Fig. 1, c, d, e, /). On pages 21 and 22 are shown some forms of trusses, together with the pitches which are commonly used.

The stresses in the various members due to a vertical panel load of one pound are given. To obtain the stress in that member due to any other vertical panel load, multiply the stress here given by the vertical panel load.

For example, if the stresses in U2L2 (Fig. 24) or L0Lt (Fig. 31) due to a panel load of 3 000 pounds, were required, they would be determined as follows:

U2L2 (Fig. 24) 3 000 X - 1.73 = - 5 190 pounds. L0 L1 (Fig 31) 3 000 X + 5.00 = +15 000 pounds

These diagrams are especially useful, since it is the custom of many engineers not to compute the stresses due to wind, snow, and

The Station and Approach.

Close View of Main Structure.

PASSENGER STATION OF THE CHICAGO & NORTHWESTERN RAILWAY COMPANY, RACINE, WIS.

Frost & Granger, Architects, Chicago, 111.

Built in 1901. For Other Views, See Opposite Page.

Train Platforms and Sheds.

Interior of Statical.

PASSENGER STATION OF THE CHICAGO & NORTHWESTERN RAILWAY COMPANY, RACINE, WIS.

Frost & Granger, Architects, Chicago, 111.

Exterior Views of Main Structui'e and Approach Shown on Opposite Page.

Fig. 34.

Fig. 25.

Fig. 26.

Fig. 27.

Fig. 28.

Fig. 29.

Fig. 30.

Fig. 31.

Analysis of Stresses in Various Members of Fink Truss Due to Unit-Loads.

Fig. 32.

Fig. 33.

Fig. 34.

Fig. 35.

Fig. 36.

Fig. 37.

Fig. 38.

Analysis of Stresses in Various Members of Fink Truss Due to Unit-Loads.

dead weight of roof trusses and coverings, but to compute the stresses due to a dead panel load caused by 40 pounds per square foot of horizontal projection. The stresses resulting from this procedure are very nearly equal to those produced by considering the various loads - as snow, dead load, and wind - separately or together. Whenever differences occur, they are on the safe side, except as noted below, and in the next article, in case of the stresses produced by the use of knee-bracing.

The panel load to be used when 40 pounds per square foot of horizontal projection is considered,maybe computed from the formula:

P= 40 X a X l/n in which, a= Distance between trusses, in feet; l = Span of truss, in feet; n = Number of panels in top chord of truss.

For example, let it be required to compute the panel load P for the truss of Fig. 24 when the span is 70 feet and the distance between trusses is 16 feet. Here a = 16; l = 70; and n = 8.

P = 40x16x70/8=5 600 pounds.

The truss would then be computed for a vertical panel load of 5 600 pounds, and the members designed to withstand the stresses thus obtained.

This method is applicable to all spans up to 100 feet when the truss is set on masonry walls (or steel columns built in masonry walls) and the roof covering is of corrugated steel or any of the ordinary materials. Where clay tile or slate are used, 50 pounds should be taken; and in case of concrete slabs, 65 pounds would be about right. It is better practice to compute the stresses due to wind, snow, and dead loads when clay tile, slate, or concrete are used.