The Anatomy of Bridgework

by William Henry Thorpe

All the foregoing instances relate to wrought-iron bridges. Two cases of steel construction are here added, the first of these furnishing an example of high girder stress somewhat remarkable. This was found in a trough girder of a strange pattern, of which a section is here given ([Fig. 46]). The bridge to which it belonged carried a siding, over which engines of less than the heaviest class sometimes passed at a crawling pace. The larger of the two girders carrying the rails was 15 feet 8 inches effective span. The sides of the trough consisted each of two vertical plates, originally ¹⁄₂ inch thick, but wasted to an aggregate thickness of ⁵⁄₈ inch. These plates 6 inches deep, were connected at their lower edges to angle bars, 3 inches by 3 inches by ¹⁄₂ inch, which again were riveted to a bottom plate 16 inches wide, originally ¹⁄₂ inch thick, wasted to ³⁄₈ inch. Lying in the bottom of the trough, and riveted through the inner angle flanges, was a bridge-rail. Assuming that the metal retained its elastic properties from top to bottom of the section, at whatever stress, this works out at 32 tons per square inch at the extreme top fibre, and 15 tons at the bottom, on the net section. As puddled steel, of which the girders were made, may have a tenacity of 45 to 55 tons per square inch, the assumption is probably correct. The author has no record of the deflection, but it may be remarked it was such that to stand under the girder, with a tank engine passing over, required some determination.

A point of additional interest in this little bridge is that, though made of steel, it dates as far back as 1861, having been in use thirty-two years when removed. The particular variety of steel used was known as Firth’s puddled. The evidence of this consists in correspondence showing that permission had been asked of the controlling authority, by the only users of the siding, to apply this material, with no evidence of any refusal. At about the same time this steel was also used upon the railway concerned in the top flanges of some girders of considerable span. The appearance of the trough girders to which the foregoing particulars apply was distinctly different to that which might be expected in ordinary wrought iron. The top edges of the vertical plates were wasted away, smooth, and rounded in a manner strongly suggestive of a steely character. Finally, the way in which the girders held up to their work for so long is, by itself, conclusive on the point. The bridge-rail appeared to be of wrought iron, the different modulus of elasticity of which has been included in the calculation upon which the preceding results are based. That these girders stood so well is, perhaps, largely due to the fact that the load carried by them was, though varying within wide limits, practically free from impact, which, had the load passed over quickly, would, with girders so small, shallow and flexible, have been very sensible.

The second instance of steel construction in which somewhat high stress is manifest is that of some steel troughing of the Lindsay pattern, used in a bridge built in 1885. The troughs ran parallel to the rails, having an effective span of 18 feet 8 inches. The depth of the section (which is shown in [Fig. 47]), was 8¹⁄₂ inches, making a ratio of depth to span of ¹⁄₂₈. The road was of ballast, sleepers, chairs, and 85-lb. rails.

Fig. 47.

Assuming this to be carried on six troughs, which corresponds to 11 feet 3 inches of width, the extreme fibre stress works out at 7·5 tons per square inch, under usual engine-loads. The bridge when examined after fourteen years’ use was in good condition, and at that time but little rusted; but the end seam rivets were, as is not uncommon with such troughing, loose. The traffic over the bridge was considerable, but not at great speed.

On the opposite page are set out the results which have been given, in tabulated form, as was done for rivet stresses, to enable ready comparison to be made.

Examples of High Stress.

—			Span in Feet.				Part Stressed.	Stress per Square Inch.		Tension or Compression.	Condition.
								Webs Included.	Webs not Included.
Wrought-iron	main girders,	plate	60·0				Flange	..	6·3	Tension				Good.
„	„	„	64·0				„	..	6·8	„				Good.
„	„	„	76·0				„	..	7·5	„				Fair.
„	„	„	29·5	-			„	7·4	9·7	„			-	Good.
							„	6·3	8·3	Compression
„	„	lattice	63·0				„	7·1		Tension				Fair.
„	„	plate	47·0				Flange edge	10·0	..	Compression				Fair.
Wrought-iron	cross-girders,	plate	26·0				Flange	..	6·7	Tension				Fair.
„	„	„	11.0				„	6·3	6·5	„				Bad; loose rivets.
„	„	„	19·0				„	8·4	9·1	„				Good, but rusted.
„	„	„	22·0				„	10·5	11·1	„				Good, but rusted.
Steel trough girder			15.7	-			„	15·0		„			-	Fair, but rusted.
							Top edge	32·0		Compression
Steel troughing			18·7				Flanges	7·5	..	Tension and Compression				Fair, but rusted.