The general levels of the work in the vicinity of the viaduct did not permit of any greater headway above high-water mark and the underside of the main or cross-girders than that shown in Fig. 34.

Fig. 41. Scale 1 inch = 1 foot. Scale 1 inch = 1 foot.

Fig. 40. Fig. 41. Scale 1 inch = 1 foot. Scale 1 inch = 1 foot.

In such cases, not infrequent in jetty work, it becomes desirable to counteract a possible uplifting force from beneath caused by the displacement of floating craft, such as barges, which may by mischance have been caught underneath the girders on a rising tide, and tending to displace the girderwork above them.

This is effected by the holding-down bolts 2 inches in diameter, shown in Fig. 39, passing through the bottom flange of the main girders and the bedstones, and carried down a sufficient distance into the concrete filling of the cylinder, and having at their lower ends the cast-iron ribbed washer-plates shown in Fig. 42 and Fig. 43.

These bolts are carefully fixed in position by templets, and with their cast-iron washers embedded in the concrete as the filling progresses, the girder-beds being slipped over them. To allow for possible errors in the levels of setting, the screwed ends are kept well above the nuts as shown. This is a wise precaution wherever foundation bolts are liable to displacement by sinking during the progress of the work, and allows a margin of error in construction. The excess of length can, if thought desirable, be cut off after the work is completed.

FIG. 42. Scale 1 inch = 1 foot.

FIG. 42. Scale 1 inch = 1 foot.

Girderwork As Applied To Bridge Construction 43

Fig. 43.

The seating of the ends of the main girders with their holding-down bolts is also shown in elevation in Fig. 44, and in sectional plan in Fig. 45.

The entire cylinder was filled with Portland cement concrete, after the sinking was completed and the bottom ascertained to be satisfactory. The cylinder was then loaded at the top with a dead weight of pig-iron.

The transverse resistance of cylinders, in such a situation as the present, when exposed to the shock of a bump from floating craft or other force tending to shift the cylinders laterally, must always be carefully considered, and is usually met by the adoption of a system of bracing, which will vary in detail in accordance with the circumstances of the case.

Scale  inch = 1 foot.

Fig. 44. Scale inch = 1 foot.

Scale  inch = 1 foot.

Fig. 45. Scale inch = 1 foot.

The efficiency of such a system of bracing will largely depend upon its depth, but in many situations it may be advisable to avoid underwater connections, which can only be made by diver's work, and are not easily accessible for examination or repair. Such a course is frequently not open to the designer of marine work, but in the present instance the method of bracing shown in Fig. 34 was adopted as affording a practically efficient depth of brace, while the connections with the cylinders could be made without much difficulty without diver's work.

Scale  inch = 1 foot.

Fig. 46. Scale inch = 1 foot.

The bracing consisted of a deep lattice girder as shown in Fig. 34 The-exact length of this girder between the cylinders was obtained after the latter were sunk, to ensure accuracy of fitting. Any inaccuracy in the distance apart of the centres of the cylinders could be adjusted within limits, as far as the main and cross-girders were concerned, by an adjustment of the girder-beds and bolts within the cylinders themselves.

The detail of connection of the lattice girder with the cylinder is shown in elevation in Fig. 46, while Fig. 47 shows the detail of the bolting up in section on the line B in Fig. 39.

The centre bay of the braced girder is shown in elevation in Fig. 48, and vertical cross-sections of the girder are shown at the end and at the centre respectively in Figs. 49 and 50.

The main girders over the 60-feet openings were of the single-web type, designed to carry the heaviest loads which could arise either from ordinary road traffic, wheeled and passenger, or from the heaviest rolling load on the rails which could be anticipated to arise.

Scale 1 inch = 1 foot.

Fig. 47. Scale 1 inch = 1 foot.

The section of these girders at the centre is shown in Fig. 51, which also shows the attachment of the cross-girders supporting the roadway, and of the plate girder cantilevers supporting a footway for passengers on both sides of the viaduct.

The mode of giving rigidity to the fixed end of the cantilever is indicated in the figure, this end being securely clamped between the bottom flange of the main girder and the plate stiffener which occurs at every cross-girder and cantilever, 5 feet apart, and securely riveted to both, as well as the web - a form of construction sufficiently rigid for the maximum load which can come upon it in this case.

Scale  inch = 1 foot.

Fig. 48. Scale inch = 1 foot.

A timber bolster is bolted on the top flange of the cantilever, to which is spiked the 2 -inch timber flooring forming the footway.

The outer ends of the cantilevers are connected with a continuous angle-steel carrying a moulded timber fascia, upon and through which are bolted the cast-iron handrail standards with gas-tube handrails, all as shown in the figure.

Scale  inch = 1 foot.

Fig. 49. Scale inch = 1 foot.

Scale  inch = 1 foot.

Fig. 50. Scale inch = 1 foot.