In the chapter on Flange Stress we mentioned the fact of the existence of of certain forces causing stresses which we have no exact way of determining. To these may be added those due to the changes of temperature, to the longitudinal thrust of the moving load, to vibration, and to the applica-. tion of the brakes on the wheels over the girder. The amount of some of these forces can be easily determined, and their effects calculated. Thus the stress due to the changes of temperature is simply the amount of friction at the sliding end of the girder, and is either tension or compression, according as the girder is contracting or expanding. The amount of force due to the application of brakes on wheels is simply that of friction existing between the rail and those wheels.

But it is doubtful if even once in the life of a bridge the combined action of all these forces will take place. Even if it did, we are quite certain that the large factor of safety we have assumed at the beginning will cover every possible condition of loading.

Weight

The actual weight of the girder can now be determined. It is to be remembered that a bar of iron of 1 square inch cross-section, and one yard long, weighs very nearly 10 lbs. Some steel weighs about 2 per cent, more than iron. The total weight of rivets in a girder is about 6 per cent, of the total weight of the girder, but in determining the actual or shipping weight of a girder, only their heads are to be taken, as their bodies merely fill up the holes. A pair of rivet heads, as usually made, weighs:

Weight Of 50 Feet Span Girder

8 Flange angles 3«"Ã--5"Ã--«" (40 lbs. p. y.)

Ã--18 - 0' weigh, 1,920 lbs. 8 Flange angles 3«Ã--5Ã--« (40 lbs. p. y.) Ã--33'

- 3¬ weigh, 3,549 lbs.

24 Stiffener angles 3Ã--4Ã--« (32 lbs. p. y.) Ã--4'

- 6 ' weigh, 1,152 lbs. 32 Stiffener angles 3Ã--4Ã--⅜ (24 lbs. P. y.) Ã--4'

Ã--6" weigh, 1,152 lbs. 6 Lateral brace angles 3Ã--3Ã--⅜ (21 lbs. p. y.)

Ã--6 - 8" weigh, 280 lbs. 2 Lateral brace angles 3Ã--3Ã--« (28 lbs. p. y.)

Ã--6 - 8" weigh, 123 lbs. 2 Lateral brace angles 3Ã--3Ã--⅝ (38 lbs. p. y.)

Ã--7 - 5" weigh, 195 lbs. 8 Cross brace angles 3Ã--3Ã--⅜ (21 lbs. p. y.)

x 7 - 2" weigh, 403 lbs. 8 Cross brace angles 3Ã--3Ã--⅜ (21 lbs. p. y.)

Ã--5 - 0" weigh, 280 lbs. 4 End cross brace angles 3Ã--3Ã--⅝ (38 lbs. p. y.)

Ã--6 - 10" weigh 348 lbs. 4 End cross brace angles 3Ã--3Ã--⅝ (38 lbs. p. y.)

Ã--5 - 0" weigh, 253 lbs. 4 Cover angles 3Ã--4Ã--⅝Ã--4 - 0" weigh, 215 lbs. 2,530 pairs of rivet heads weigh, ],265 lbs. 4 anchor bolts 1⅛" diam. 9" long, nuts, etc., weigh, 11 lbs.

Making the total weight=28,404 ".

Estimate Of Cost

The cost of a girder depends upon the market price of the material of which it is made, the distance of transportation, and the nature of the site of the crossing. Under ordinary circumstances, with the following prices of material, our 50 foot girder, not including transportation, will cost about as estimated below :

General Remarks

Girder much larger than 50 feet have usually to be shipped in more than one piece. Since the riveting done in the field is more or less imperfect, as compared with that done in the shop, some allowance should be made in the number of those rivets in the design. The use of bolts instead of held rivets should be avoided, unless they are turned and made conical, and holes drilled to fit them, thus insuring perfect contact.

In shop work machine riveting is to he preferred to hand work. The use of cast iron in any part of the work is to he avoided, especially for railroad work, as its strength is unreliable where it is subjected to shocks.

In our 50 foot girder we have riveted connection plates to the girder, but when the distance of transportation is very great and reloading necessary, they may become bent or broken ; consequently, under such circumstances, they should be shipped separately.

Appendix

Weights Of Iron In Plate Girders On Lines Of Hokkaido Railway. Deck 3pans

Through Spans

Floor beams, spaced about 10.5 ft. apart. Stringers of 12-inch I beams.

These girders have been designed according to following specifications: