Hence stiffeners must be introduced in order to shorten the elementary columns into which the web is assumed to be divided. Let the stiffeners be spaced every 32 inches horizontally, at the attachment of cross-girders and midway between them, in which case the horizontal distance between the centres 30 of rivets in stiffeners is 30 inches; and therefore H = = 48, and the safe intensity of stress by Cooper's rule is 6787 lbs., or about 3 tons.

Rankine's rule for the buckling stress gives

16

1

1+ (68) 2

3000

= 6.3 tons per square inch nearly; hence the factor of safety against buckling is—

Hence the web may be adopted at the ends. Assume that the web is inch thick in the the end, then the intensity of shearing stress is

second bay from

the pressure on the bearing area of the rivets is square inch.

7.5 tons per

Cooper's rule gives for the web 5454 lbs., as the safe intensity of shearing stress per square inch; hence inch may be adopted.

In a similar manner it may be shown that the other thickness shown on Plate I. may be adopted.

The equation of bending moments is

We may tabulate the moments and stresses as follows:

:

T

The moment of resistance of the angle irons in tension is

5.25 × 4 × 5.5 115.5 foot-tons

The moment of resistance of each plate in tension is(24-2) × 4 x 55 242 foot-tons

The details of the bridge are sufficiently illustrated in Plate I. The design of the group joints in the booms, and angle wrappers, have been fully considered in Chapter XIII.

The weight of the girder may now be found.

Total weight of two main girders 34'4 tons. The approximate weight assumed in the calculations was 35.5 tons, so that no recalculation is necessary.

The weight of the flanges in one girder is 26,191 pounds, and of the web 10,497 pounds; so that more than of the total weight is in the flanges.

The main girders may be riveted up in two lengths, leaving the rivets in the central covers and web joint to be put in on the site. There would be no difficulty in introducing two more joints in the angle wrappers similar to the central joint, in which case the girder could be riveted up in four lengths, leaving the rivets in the three group joints, in the covers, and in the three web joints to be put in on the site.

CHAPTER XVIII.

EXAMPLE OF THE DESIGN OF A STEEL AMERICAN TRUSS BRIDGE, WITH PIN CONNECTIONS, FOR A SINGLE LINE OF RAILWAY.

THE order of procedure which should be followed in the design of a bridge of this class is to first consider the deck and the floor beams in the manner explained in Chapter XIV. The deck and floor beams designed in Chapter XIV. will be used in this bridge. The panel loads due to the weight of the structure should next be determined in the manner explained in Chapter XV., and a skeleton diagram prepared for calculating the stresses due to the dead load of the structure, and those due to the live load. In this bridge the concentrated-load system explained in Chapter XV. will be used to determine the stresses due to the live load. The total maximum stresses should be then determined and written on the skeleton diagram of the truss, from which the sectional areas of the various members and the details of construction may be provisionally considered, to be afterwards reconsidered in regard to the additional stresses due to wind.

It will probably be most convenient in this example to describe briefly the bridge as actually designed before considering the stresses, in order that the functions of the various members may be more clearly understood.

Fig. 339 shows a skeleton diagram of half a span of the bridge in isometric projection, with the names of the various members written on the diagram; and Plate II. shows the details of the truss, with the more important joints and connections.

It will be seen, by referring to Plate II., that the two trusses forming one span are 180 feet between centres of bearings on the supports or piers, and are spaced 16 feet apart from centre