# Design of a Through Pratt Railway-Span 70

## inches, square, plates, rivets, width and pin

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176 shows a cross-section of this member as above determined.

This member will be connected to the upper chord and end-post by means of a pin which is 61 inches in diameter, the diameter of the pin being determined later. The total stress is 141 600 pounds, and this will be taken by two plates, one on either side of the member. The net section of the member is 9.08 square inches, and the section through the pin (26) must be 25 per cent in excess of this, making a total of 11.35 square inches, or 5.68 square inches for each plate. The total width of these plates will be taken as 12 inches, and this (see Fig. 177) will make the required thickness: 5. GS5.68 —0.99 inch.

2c 12.00 — 6.25 Fig. 177 shows the details of these pin-plates. Since the above thickness is too great to be punched in one single piece, the above thickness will be made up of two plates, each 1 inch thick. The area at section A—A must be equal to that of the body of the bar. It is 12.00 X 1 = 6.00 square inches for one side, or 12.00 square inches for both sides. As this is greater than the 8.85 square inches as above computed, the area at A—A is sufficient, as is also the width of the plate, which was assumed as 12 inches.

One of the plates will be riveted directly to the member, and the other will be riveted to it as a pin-plate. The section back of the pin (26) must be equal to the net section in the body of the member. The net section is 4.54 square inches for one side, and the total thickness of the pin-plates is 7 .125 inches, making the distance from the end of the member to the pin _ 48 inches, and the distance to i the center of the pin 41 + 74 inches.

The joint between the plates and the main member will be weak in shear, the rivets tending to shear off between the 1-inch angles and the plate, and also between the two plates themselves. As each side takes one-half of the above stress, the number of rivets required to connect the plates to the main member will be 141 600 ÷ 2 = 10 shop rivets, 7 220 and the number of rivets required to connect the inner 1-inch plate to the outer one which is connected to the member itself will be: 141 600 _ 4 — 5 shop rivets.

7 220 The distance from the center of the pin to the top of the main part must be greater than one-half the diameter of the largest I-bar head—that is, 171 ± 2 = say, 9 inches.

At the lower end, this member is connected to the bottom chord by means of a couple of clip angles and four or five rivets, Only sufficient rivets are required to prevent the sagging of the bottom chord, since the floor-beam is connected to the hip vertical above the lower chord, and hence no stress corns on the joint at the lower end (see Fig. 178).

The width of the plate has been assumed as 8 inches. This width is liable to be changed after the design of the intermediate posts has been made, since it will be economical to have all the inter mediate floor-beams of the same length; and therefore the width of this plate will be changed so as to make the width of the hip vertical the same as the width of the intermediate posts.

85. The Intermediate Posts. The post must be designed to stand a total stress of 163 600 pounds. Where possible, it is economical to make the intermediate posts out of channels, as this saves a large amount of riveting. As seen by the stress sheet, the length of these posts is 30.1 feet center to center of end pins. It is usually required that l must not be more than 100, and this con y dition requires that the least radius of gyration cannot be less than 30.1X 12 = 3.62. 100 From Carnegie Handbook, P. 101, it is seen that a 12-inch 30 pound channel has a radius of gyration of 4.28, and will fulfil the conditions. The area of two of these channels is 17.64 square inches. The unit allowable stress (16) is: P = 16 000 — 70 X 30.1X 12 10 090 pounds per square inch.

. 4.28 163 600_ The required area is then determined to be 10 090 = 16.2 square inches; and as this coincides very closely with the area given, these channels are efficient and will be used.

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