Examples of Rivet Stresses, etc., in Lattice Girders.
| — | Cross- Girders as Simple Beams. | Cross- Girders as Con- tinuous Beams. | Correct Results. |
|---|---|---|---|
| Stress in Tons per Square Inch. | |||
| Centre girder, 63 ft. span (both roads loaded): | |||
| Rivets in diagonals—Shear | 13·7 | 17·2 | 14·9 |
| RivetDo.diagonals—Bearing pressure | 15·0 | 18·8 | 16·3 |
| Rivets in diagonals—Flange stress | 6·8 | 8·5 | 7·1 |
| Outer girder, 66 ft. span (near road loaded): | |||
| Rivets in diagonals—Shear | 9·6 | 8·2 | 9·0 |
| RivetDo.diagonals—Bearing pressure | 9·6 | 8·2 | 9·0 |
| Rivets in diagonals—Flange stress | 5·9 | 5·1 | 5·7 |
The material and workmanship of the bridge were good. The rivets of the centre girder end diagonals, 1 inch in diameter, were originally 7⁄8 inch, but on becoming loose were cut out, the holes reamered, and replaced by the larger size, which remained tight, and to which the stress figures apply. The rivets in the diagonals near the centre, 7⁄8 inch in diameter, which were subject to reversal of stress, occasionally worked loose, and were more than once replaced. The riveting in the outer girder diagonals, subject to smaller stresses, much more frequently developed, also gave trouble, particularly those liable to counter stresses.
Apart from looseness of rivets, the general appearance and behaviour of the bridge, which had been in existence about twenty years, was not suggestive of any weakness.
Of smaller girders, an example showing the necessity for care in discriminating, if it be possible, between looseness of rivets resulting from over-stress and that due to other influences may first be quoted. Two trough girders, of 11 feet effective span, each of the section shown in [Fig. 30], 111⁄2 inches deep at the ends, 14 inches at the middle, with 1⁄4-inch webs, and rivets 3⁄4 inch in diameter, of 41⁄2-inch pitch, showed certain defects, of which one, it may be incidentally mentioned, was a cracked web ([Fig. 31]). From the nature of the arrangement the lower web rivets, which were loose, would receive the first shock of the load coming upon the span, but there were evidences indicative of original bad work. The angle bars gaped, suggesting that these had first been riveted to the bottom plate, and left sufficiently wide to allow the web to be afterwards inserted, the rivets failing to pull the work close, and then readily working loose. Here there is considerable uncertainty as to how much of the loosening is to be attributed to bad work, and how much to stress. It may, however, be remarked that whatever bearing stress was the ultimate result of the load hammering on the lower angle flanges, loosening rivets never perhaps really tight, the stress of 7 tons per square inch bearing pressure on the upper rivets, though aggravated by considerable impactive force, was not sufficient to loosen these. The girders were taken out after being in place sixteen years.
Fig. 30. Fig. 32.
Fig. 30. and Fig. 32.
Fig. 31.