# Design of a Through Pratt Railway-Span 70

## inches, square, bars, head, total, angles and section

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Table XXIV gives the tension members and their dead-load, live-load, impact, total, and unit stresses (l5), together with the required area, the number of bars, the approximate depth of bars, and the final sizes used.

The first seven columns in Table XXIV are self-explanatory. The number of bars to be used in any particular case is a matter of judgment. One fast rule is. that an even number of bars should always be used, except in the case of counters, where one is permis sible. This is due to the fact that the placing of one of the main members in the center of the pin would create a large moment, and therefore an ex cessively large pin would be required, and accordingly a very large head on the I-bar in pro portion to its width —all of which are very undesirable and costly. In gen eral the number of I-bars should be as small as possible, and they should be so chosen that the widths of the chord members increase from the ends to ward the center of the truss, and the widths of the diago nals decrease from the ends toward the center of the truss.

The area of one bar is obtained by dividing the total stress by the num ber of bars and also by the allowable unit-stress. Thus, for the member for example, the required area of one bar is: 295 100— 9_22 2 X 16 000 square inches. • The approximate depth of this bar is determined by taking the square root of 6 times the area as above determined. It is: As this is nearer 7 than S inches, a 7-inch bar will be chosen; and looking in the first column, Carnegie Handbook, p. 248, for an area which will be equal to or in excess of 9.22, it is found that a *-inch bar satisfies this condition, and therefore the section of this member consists of two bars 7 by 1i inches.

According to (80), the first two sections for the lower chord are to be made of built-up members. This requires that' instead of I-bars they are to be made of angles and plates, or, in case the stress is light, of channels. The depth of the section is limited by the size of the greatest I-bar head. As the diameter of the I-bar head depends upon the size of the pin, it cannot of course be determined accurately before the pin is designed. It is customary to assume the largest head, and to design the section so as to clear this. The size of the

largest head for bars of given width is given in the Carnegie Hand-. book, p. 212.

The design of the member LA will depend upon the size of the largest head of the 7-inch I-bar of the member This is 171 inches; and in order that the head may have some clearance, it will be necessary to add 1 inch to the top and the bottom, making a total of 181 inches. Since the flange angle, as in the case of plate-girders, will extend over the plate about } inch, the plate itself may be 18 inches wide and still give sufficient clearance.

The total stress is 234 200 pounds, and the allowable unit stress (15) is 16 000 pounds per square inch. The required net area, then, is: According to the Specifications, the thickness of the plate cannot be less than inch. The gross area of two 1S by -inch plates is 13.5 square inches, and the gross area of four 32 by 3k by i-inch angles, which are assumed to be sufficient, is 9.92 square inches, thus making a total gross area of 23.42 square inches. If 5 rivet-holes are assumed to be taken out of each web, and one rivet-hole taken out of each angle, this will require a certain number of square inches to be deducted from the section, and this is computed as follows: Out of webs, 2 X 5 (1 + 1) X = 3.75 sq. in. Out of angles, 4 (1 + 1) X \$ .. = 1.50 " " Total = 5.25 sq. in.

The net area of the section is now determined to be 23.42 — 5.25 = 18.17 square inches. This is somewhat greater than the required net area, but must be used, for according to (39), these are the smalles t and thinnest angles that may be used.

Figs. 173 and 174 shoe the cross-section and the general detail at L,. The width of the member cannot be determined until after the section of the end-post is computed, since it must fit inside of the end-post, the horizontal legs of the angles being cut off to allow this. The end-post, Arti cle 87, is 14i inches inside. If it is assumed that all the pin-plates on the end-post are placed on the outside, and all those at on are on the inside, then the width of back to back of plates, must be 14 — (2 X + 2 X ) = 12} inches or less, -inch clearance being allowed between the sides of the angles and the web plates of the end-post (see Fig. 173).

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