The Forth Bridge/The Members forming the Cantilevers

by Wilhelm Westhofen

The Members forming the Cantilever.

The bottom members from their points of junction with the skewbacks are not curved but are carried in a straight line to the first bottom junctions. At the centre of these junctions the angle alters and the members continue straight to the next junction, and so on till the end of the cantilever is reached. At each change the angle becomes more obtuse, until in the middle of the sixth or last bay the members are all but level. Looked at on plan the bottom members are 120 ft. apart, centre to

centre at skewbacks, and 32 ft. 2 in. at the end

THE FIFE AND QUEENSFERRY PIERS

Fig. 107. Skewback on fife pier.

Fig. 108. Queensferry pier from the river.

DETAILS OF JUNCTIONS AT TOP OF TOWERS.

posts—the centre lines being straight between these two points (see the plan in Plate III.) The diameter of these members at the skewback is 12 ft., and the form circular, and the same form continues till the end of bay 4, where the diameter is 6 ft. 6 in.—a gradual taper being maintained throughout. Between the end of bay 4 and the centre of bay 5, the form of these members is rectangular with a curved outside, and after this a plain rectangle on all four sides of varying height and width, until at the end of the cantilever the form is a square of 3 ft. each way. The thickness of the plates in the bottom members is 1+¹⁄₄ in. at the skewbacks, gradually decreasing to ³⁄₄ in. at the ends. The same mode of construction with longitudinal beams and transverse diaphragms of circular or other shape is followed to the end. In all the bottom junctions the main strength is concentrated in strong webplates into which the circular tube is gradually carried over, curved plates being placed on the outside of these webs to maintain the circular appearance on the outside of the junctions. (See Fig. 126.) To the main webplates the hornplates of the ties pointing backward, and these of the struts pointing for ward, are attached. Except that the winding of the outer shell is perpendicular instead of horizontal, the construction of those junctions in principle is similar to that of the top junctions at the head of the vertical columns, but turned upside down.

The changes in the form of section in the bottom members are indicated in the general elevation, plan, and side elevations of the cantilever in Plate III.

The top members (see Fig. 116) or principal tension members are lattice girders of rectangular section, consisting of four main booms, which are braced on four sides. The webplates are carried through the whole length of the girders unbroken, only changing in depth and thickness, or in the number of thicknesses. The vertical side bracings—angles alternately on one side and other side of the webs—are double-crossed throughout. The horizontal bracing is not attached to the top and bottom flanges, but to a special plate attached to the web by angles on each side, and called the horizontal web. The horizontal bracing is also of angle bars, but for the most part in single or zig-zag fashion. The top flanges run right through to end of bay 5, where they disappear, the two web angles at top and bottom supplying a sufficient amount of section. The bottom flanges run up to the top junctions only on the inside, but right through on the outside, the loss of section being made up here in other ways. Like the junctions in the bottom members, the webplates in the top junctions are stiffened or doubled, and receive the hornplates of struts and ties, with which they form a very strong framework, stiffened by bulkheads or diaphragms.

The inclined struts (see Fig. 117) are flattened on the sides to facilitate their intersection with the ties and their attachments to bottom junctions and top junctions. They are, however, changed in form altogether in their extremities—being made rectangular the same as the diagonal struts in the central towers. A number of diaphragms are placed internally where these changes occur, to keep the struts in shape. The thicknesses of plate vary between ¹⁄₂ in. and ³⁄₈ in. according to position. Their construction is on the same principle as the other tubular members—lap joints on the circumference—butt joints at ends—all plates being 10 ft. long, breaking joint every 8 ft. with a diaphragm at each joint. Struts 1 and 2 are made up of eight plates; struts 3 and 4 of six plates; and strut 5 of four plates on the circumference.

As seen in Plate III. all struts in the cantilevers are braced and stiffened by diagonal wind-bracing girders of box shape, consisting of four corner angles with double or single cross-bracing on all four sides. These are attached to the struts by plate gussets and reverse angles. One girder is generally carried right through, the other is in two halves and is carried across by strong reverse angles and stiffening plates. At the heads of every pair of struts a horizontal lattice girder is placed between top junctions.

The tension members are all of the lattice girder type with four main booms of T shape braced on all four sides. The section of the main booms is made up of one or several webplates, and one or more flange plates, with angles to connect both together. At the point of crossing with the struts the inner flanges are cut away and the section thus lost is made up in some parts, by a deepening of the web and by a doubling and strengthening of outside angles on either side in the same manner as is done in the case of the flanges of the top members when passing the heads of struts and ties in the top junctions.

At the crossing of struts and ties large and strong gusset plates are rivetted, into which are attached the so-called vertical ties. This is clearly seen in Plate III. These are also rectangular lattice girders consisting of four booms, or in the lighter ones of four angles cross-braced on all four sides, and to the lower extremities of these are attached the bottom members by means of bent angles and gusset plates. These vertical ties, although not attached exactly midway between two of both members' junctions, serve to prevent undue deflection in these members, and in the first three bays of cantilevers serve to carry the weight of the internal viaduct by means of a plate girder across, stiffened by diagonal wind bracing.

In the tension members two top and two bottom booms break joint alternately, and all webs and flanges are double covered. The top and bottom cross-bracings are rivetted to the flanges, the side bracings to the webs. Diaphragms consisting of angles on four sides and angle cross in centre are placed at suitable distances apart.

The internal viaduct in the cantilevers is the same as described for the portion in the central towers. Figs. 119 and 120 show a portion of it in elevation and cross-section at the point where the viaduct is supported in the centre of bay 1 (see Plate III.). The two figures are not drawn on the same scale, but otherwise refer to the same point.

DETAILS OF CANTILEVERS.

As the spans in the cantilevers become shorter the viaduct diminishes in strength of sections, and from the centre of bay 2 to the end of bay 2 in depth also. At the end of bay 4 the girders altogether disappear, and the four troughs are strengthened and carry the permanent way, being supported both on the cross girders and on the diagonal wind bracings between the cantilevers.

All the various supports of the internal viaduct in the cantilevers are shown so clearly in Plate III that it is not necessary to enter into further details.

The diagonal wind bracings between the bottom members are also of rectangular box construction, consisting of four main booms made up in the longer girders by webs, flanges, and angles, and in the shorter girders of simple corner angles. They are double or single-braced on all four sides, and are joined by heavy gusset plates at the points of intersection. They are joined to the bottom members by large gusset plates top and bottom, clearly shown in Plate III., plan of bottom members.