Great care is necessary at the outset to ascertain the true spacing of the cross-girders, to ensure that the bolt-holes in the bottom flange of the centre girder shall come where desired. The fixing of the cross-girder brackets also needs close attention to avoid after trouble, the bolt-holes in the brackets being preferably drilled on the site after fixing. It will, for masonry abutments, be necessary to fix bedstones to receive the new centre girder, which, being carried out quite possibly under adverse traffic conditions, will perhaps leave the stones liable to settle slightly when the full load is carried. To eliminate the bad effect of this upon the ultimate adjustment, and to take up any initial set of the new girder work, which would be prejudicial in the same way, it is desirable, the centre girder being in place, to screw up the bolts temporarily and leave the work for a week or two. To ensure regularity in the screwing up process, it is convenient to prepare, for use at the bridge, a diagram somewhat similar to [Fig. 73], giving the amount by which the new and old work are to be brought together at each cross-girder, with the number of turns for each nut to effect this. With a man at each side of the girder, the whole length is traversed, giving a half-turn to each nut; this is repeated as often as necessary, and so managed as to bring all up proportionately to the final requirement, keeping tally with chalk marks over each cross-girder as a check. The preliminary screwing up should be conducted with little less care than that adopted for the later adjustment, to avoid damage to the old work. This later adjustment having in due course been effected, it is then necessary to measure for packings to fill the spaces remaining between the old cross-girders and the new centre girder. These spaces should be callipered at each of the four corners, care being taken to avoid after-confusion. The measurements ascertained will, however, be too great for the finished packings, as an allowance of not less than 1⁄10 inch (total), will commonly be wanted to cover irregularities in the surfaces. The packings, having been prepared and checked, may be slipped into place after slacking all the bolts a small amount to permit this to be done, finally screwing up tight and securing the nuts by split-pins, through holes drilled as the last operation.
As a check upon the calculations and adjustment, the “lift” of the outer girders and cross-girders, and the “drop” of the centre girder may be observed by levelling. For this purpose the author has used a staff of inches divided into tenths, with which, and a good level, very accurate readings may be taken for short distances.
No reference has been made to the effect of skew in a bridge on the above methods, the explanation given applying rather to bridges square on plan. The influence of skew on the load distribution will largely be a matter of detailed calculation. The flexure of the girders may also be sensibly affected, but may be arrived at with sufficient accuracy without any great trouble. The chief effect of skew is to modify the amount of screwing up during adjustment, which may be better understood by reference to [Fig. 74], and comparing it with [Fig. 73], the adjustment diagram for a square bridge.
To illustrate how these methods of strengthening work out, and compare as to weights of centre girders required, the case has been assumed of a wrought iron bridge of 60-feet span, having outer girders 5 feet deep, of 39 square inches gross flange area; and cross-girders, at 8-feet centres, 27-feet span, 1 foot 9 inches deep, with a gross flange area of twenty square inches. The dead load and live load on either road are each 1·75 tons per foot run.
The stress in the outer girders previous to the alteration being 6 tons per square inch gross, it is desired to relieve this to the extent of 33 per cent. by a steel centre girder. In the table here given the quantities given in italics are fixed as primary conditions:—
Centre Strengthening Girders for 60-ft. Span.
| — | Centre Girder, Stress Unknown. | Centre Girder, Depth Unknown. | Adjust- ments Unknown. | |||
|---|---|---|---|---|---|---|
| Outer Girder. | ||||||
| Deflection under modified live load | ·42 | in. | ·42 | in. | ·42 | in. |
| Lift of adjustment | nil | nil | ·153 | „ | ||
| Cross Girders. | ||||||
| Depression under live load—modified conditions of support | ·13 | in. | ·13 | in. | ·13 | „ |
| Extreme depression (m) | ·55 | „ | ·55 | „ | ·55 | „ |
| Lift of adjustment (cross-girder only) | nil | nil | ·095 | „ | ||
| Total lift of adjustment (lt) | nil | nil | ·248 | „ | ||
| Centre Girder. | ||||||
| Depth | 3·5 ft. | 8·2 ft. | 3·5 ft. | |||
| Unit stress on gross section (ex girder’s weight) | 2·14 tons | 5·0 tons | 5·0 tons | |||
| Total deflection (ex girder’s weight) | ·55 in. | ·55 in. | 1·28 | in. | ||
| Deflection excess (s) | nil | nil | ·73 | „ | ||
| Depression, or “drop” of adjustment (dt) | nil | nil | ·482 | „ | ||
| Gross area of flange | 105 sq. in. | 19·2 sq. in. | 44·5 sq. in. | |||
| Weight | 20 tons | 10·4 tons | 11·4 tons | |||
| Net flange stress (including girder’s weight) | 3·19 tons | 6·87 tons | 6·94 tons | |||
Girders subject to distributed load are treated as having uniform stress, but where this is not strictly the case, as in some light girders, it will be necessary to take the fact into account. For centre girders of wrought iron, and a unit stress on the gross section of 4 instead of 5 tons, the girder weights are between 9 and 10 per cent. greater.
