In computing the total stress on the extreme fibre, it must be noted that the stresses due to weight and eccentric loading do not stress the same extreme fibres as the stress due to wind, the former stressing the extreme fibres on the top and bottom of the post, while the latter stresses those on the inner and outer sides. The total direct unit-stress is 410 500 = 8 310 pounds per square inch; and 40.42 this, added to the 8 250 pounds per square inch due to the wind, gives a total of 16.560 pounds per square inch on the extreme fibre only.
The allowable unit-stress is 16 000 — 70 X 36.7 X 12 = 12 200 8.11 pounds per square inch when wind is not taken into account, and (23) is 1 X 12 200 = 15 250 pounds per square inch when the wind is taken into account. The difference between this and the actual stress is 16 560 — 15 250 = 1 310 pounds per square inch, which shows that the section is not strong enough. The section can be in creased by widening the eover-plate or by making the plates thicker; but as this excess is due to wind only, the section being amply suffi cient under the other stresses, and is fixed to some extent by the floor-beam connection, no change will be made.
The pin at each end of the end-post will he the same—namely, 61 inches in diameter—and therefore the pin-plates will be the same at each end. The total stress in the post is 410 500 pounds, which makes a required bearing area of 410 50017.2 square inches for 24 000 both sides, or 8.6 square inches for one side, and the total required thickness of 8'6 6.25 = 1.375 square inches for one side. Since the thickness of the web plates is inch, this leaves a remainder of 1.375 — 0.5 = 0:875 inch for the thickness of the pin-plates. One plate inch thick and one plate i inch thick will be used.
The proportion of the total stress which is taken by the -R-inch rivets required to transfer the stress from the *-inch plate to the l-inch plate is = S shop rivets in single shear; and the number of rivets required to transfer the stress from both pin-plates to the web is 56 000 74 600 = 18 shop rivets in single shear. As in the case of 7 2 20 the top chord, one pin-plate should extend over the angle, and the number of rivets required in that pin-plate should go through the pin plate and the angles (see Fig.
188). The *-in. hinge plate is used for erection purposes, and is not considered as a pin-plate. It is omitted at Since this section is the same as that of the top chord, the tie plates and the lattice bars must be the same size.
88. The Pins. The design of the pins requires a simple but quite lengthy computation. Sim ple Pratt railroad trusses for single-track bridges usually have the same arrangement of tension and compression members; that is, the same tension members occupy relatively the same positions with respect to the compression mem bers. Also,while theoretically a different sized pin will be required at every joint, it is not customary to make them so. In practice the pins at the joints U, and are made of the same diameter, and those at the remainder of the joints are also made in diameter equal to each other but different from those at U, and the pins at U; and usually being larger in diameter. On account of the above conditions and facts, it is unnecessary to design the pins in spans under 200 feet, since usually they are the same for any given span and loading. Table XXVI gives the diameters of pins for spans of 100 up to 200 feet for loading E 50.
Pins for Single-Track Bridges For E 40 loading, decrease the above values by I inch; for E 30 loading, decrease them by inch. The diameter of pins for spans not given in the table can be interpolated from the given values. No pin should be less than 31 inches in diameter.
The span of this bridge is 147 (say 150) feet, and the diameter of the pins at U, and is 61- — 4 = 61 inches; and the diameter of the pins at the other panel points is 5 — i = 5 inches. It should be noted that no pin is required at point L„ as the two mem bers which join here are built-up members and are riveted together.
The above table is for single-track bridges only. The diameters of pins for double-track bridges are given in Table XXVII. These values are for E 50 loading; and for E 40 and E 30 loading, deduc tions must be made as required in the case of Table XXVI.
Pins for Double-Track Bridges Pins for highway bridges are usually much less in diameter than those for railway bridges, except in the case of first-class trusses for heavy interurban traffic or for city bridges carrying paved streets, where they should be taken equal to those given for E 30 Table XXVIII gives the diameters of pins for different length spans of simple highway bridges designed for 16-ton road-rollers or farm wagons and 100 pounds per square foot of roadway.