This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Table IX, p. 46, gives the gauge lines for different lengths of angle legs. If S gives a value less than 2| inches, the leg of the angle against the web must be 5 inches or more, on account of practicaI limitations of manufacture.
In order to produce the maximum moment, the wheel must be placed as shown in Fig. 120. The left reaction is 12 000 (2.125 + 10.00 + 3.625) ÷ 20 = 9 450. The moment under wheel 1 is 9 450 X 7.875 X 12 = S94 000 pound-inches, which requires a section modulus of 894 000 ÷ 15 000 = 59.60. Looking in the Carnegie Handbook, pp. 97 and 98, it is seen that a 15-inch 42-pound
Fig. 120. Position for Maximum Moment for Problem 1 on Page 91.
Fig. 121. Position for Maximum Moment for Problem 2 on Page 91.
I-beam with a section modulus of 58.9 will be sufficient, since the section modulus is less than 2½ per cent under that required.
2. Design a runway girder for a 30-ton crane of 60-foot span, the wheel loads and wheel base being as given in Table XV, and the distance between trusses 20 feet.
The wheels are placed in position as shown in Fig. 121. The left reaction is 52 000(12.75 + 1.75) / 20 = 37 700 pounds; and the maximum moment, which occurs under wheel 1, is 37 700 X 7. 25 X 12 = 3 285 000 pound-inches. The maximum shear occurs when the wheels are in position as shown in Fig. 112, p. 85, and is 75 400 pounds. The required thickness of the web is 75 400 / 10 000 X 24 =0.314 inch, the depth being 20 ÷ 4 10 = 2 feet - 24 inches. The web will be made 24 inches wide and 3/8 inch thick.
The required net flange area is 3 285 000/ 15 000 X (24 - 2) = 9.97 square inches for two angles, or 4.99 square inches for one angle. An angle 6 by 6 by ½-inch gives a gross area of 5.75 square inches and a net area of 5.75 - 0.50 = 5.25 square inches, one ⅞-inch rivet-hole being taken out of the section. Since this area coincides quite closely with the required area and is larger, it will be used. A 6 by 3½ by ⅜-inch angle would have been better in regard to area, but the rivet spacing is less than 2f inches at the end, and this required a double gauge line and therefore a leg 5 inches or over.
The maximum shears at the tenth-points are now computed, and are tabulated as follows:
75 400 pounds.
The value of the shear to be used in any particular case is given above. In this case, P = 52 000 pounds; v == 6 570 pounds, ⅞-inch rivets being used; and hg is 24¼ - (2 X 2¼ + -2½ / 2)= 18½ inches. The rivet spacing for the first division or first two feet of the span is:
Fig. 122. Stress Sheet of Runway Girder of Problem 2 on Page 91.
Fig. 123. Determination of Size of Stiffener.
S = 6 570 / - 1.495, say 1½ inches.
The rivet spacing for the other divisions may be computed by the student. It is given in Fig. 122. The web of the girder should be stiffened as shown in the figure, by angles placed as there indicated The thickness of the angles should not be less than 5/16 inch, nor greater than ½ inch. The size of the angles should be such that the outstanding leg does not reach beyond the leg of the flange angle (see Fig. 123). This makes their size as shown in Fig. 122. The crane rail may be connected directly to these and the flange angles; or a channel may be placed over the flange angles and riveted to them in a manner similar to that employed in the case of I-beams, the crane rail being fastened to that. If this latter detail is employed, the area of the channel is reckoned as forming part of the upper flange; and the net area of the angle must then be equal to the required net area, less the net area of the channel. 27. Ventilators. Mill buildings may be ventilated by means of small circular ventilators such as shown in Fig. 69, p. 41, placed at certain intervals along the ridge or peak of the roof, or by means of monitors as shown in Fig. 71. The sides of these monitors may be fitted with swinging glass windows, with wooden or metal louvres, or, in case a large amount of ventilation is required, may be simply left open. Figs. 124,125, and 126 give details of monitors, and show how they are connected to the trusses.
Fig. 124. Section of Glass Louvres in Monitor.
Fig. 125. Section of Metal Louvres in Monitor.
Fig. 126. Section of Open Monitor.
Fig. 127. Gable Details for Corrugated Steel.
Fig. 128. Gable Details for Corrugated Steel.