Lan-ho Railway Bridge
Note: This title is likely to change, pending further investigation into the history of the bridge.
The bridge carried the Imperial Railways of North China track for the Peking-Mukden Railway across the River Lan ho. This map shows the railway betweeen Peking (Beijing) and Mukden (Shenyang) crossing the Lan Ho about 30 miles inland from the Chihli (Bohai) Sea, a short distance north east of Lanchow. It appears that the river, 滦河 , is now known variously as the Luan Ho, Luan He, or Luan River.
It was the first of several large bridges on the line. The others, as identified in The Engineer in 1914, were: the Ta-Ling Ho bridge (26 spans of 100ft); Hsiaio-Ling Ho (12 spans of 100ft); Liao Ho (20 spans of 100ft); Liu-Goo Ho (24 spans of 100ft); Shi Ho (12 spans of 100ft).[1]
For more information about the railway's history, see Wikipedia entry for the Peking-Mukden Railway.
The following partial description is transcribed from The Engineer, 4 May 1894 [2]. Not all of the illustrations are included here.
'The single line standard gauge railway between Kaiping and Shanhai-Kwan, north-east of Tientsin, built by the Chinese Government, crosses the Lan-ho on a bridge consisting of five through spans of 206ft. and ten deck spans of 103ft. 9in. between centre of bearings, as shown in Fig. 1. In accordance with instructions of Mr. Claude W. Kinder, Engineer-in-Chief, Tientsin, and Sir Benjamin Baker, Consulting Engineer, a. competition of designs and tenders was held in autumn, 1891, in Westminster . Ten manufacturing firms, viz., five British, three French and Belgian, and two American, took part in it, and eventually the order was placed with Messrs. Andrew Handyside and Co., of Derby, Mr. Max am Ende, of Westminster, acting as their engineer. The specification of strength, drawn up by Mr. Kinder ......
'.... In the case of the Lan-ho Bridge, sketches showing the system of triangulation of the main girders, the system of the floor, &c., were given, and it was further specified that the five through spans should be pin-connected and the ten deck spans riveted bridges. ....'
'Pin-connected bridges, as they are usually made in America, are considered by engineers in Europe to be wanting in stiffness and, therefore, not so durable as riveted bridges. This is ascribed to the unsatisfactory connection of the transverse bracing to the main girders; to the suspension of the cross girders from the pins by means of hangers; and to the construction of the transverse bracing of round rods with screws and angular cleats at the ends.
'Recent practice, however, has almost done away with the hangers and has adopted riveted connections between cross girders and vertical posts, as also stiff diagonal bracing with riveted connections. It is, therefore, unnecessary here to dwell upon the shortcomings of those abandoned details, while with regard to the fixed connections between the cross girders and posts, it is sufficient to remark that it causes bending stresses in the latter, and an overstraining of the inner side of main girders, in the same way as in riveted bridges with double-webbed main girders and with overhead bracing. But present American practice has not yet abandoned the chain link or eye-bar without rivet-holes between the two pin-holes, except in the end panels of the bottom chord, where compressive stresses occur, and the eye-bars have to be braced together in order to enable them to resist those stresses. It may be admitted that no objection of importance can be raised against the use of eye-bars as described for the diagonals of the web or for the bottom chord of deck bridge girders, but when they are used for the bottom chords of through bridge girders, the absence of rivet holes appears to prevent absolutely a satisfactory connection between them and the transverse bracing. If the latter is attached to the pins, the apparently insuperable difficulty arises of directing the resultant of the stresses in the diagonals into the centre of gravity of the section the chord. On the other hand, if the transverse bracing is attached to the posts above or below the heads of the eye-bars, the posts are bent in transmitting the resultant stress to the chord, even if special flanges are added in the plane of the transverse bracing, which, moreover, are objectionable on account of the distribution of the horizontal stress between the flange and the chord being rendered uncertain. In actual practice, therefore, the flange is omitted and the transverse bracing is in the state of a girder without flanges, at least flanges lying in the plane of the web, an unsatisfactory state to anyone admitting the tantamount importance of the transverse bracing to that of other parts of the bridge in the question of durability.
'Now, the details of the Lan-ho Bridge show how this may simply be obviated by the substitution of an eye–bar with rivet holes for the usual one without them, see Fig. 8. The loss of sectional area is not great. The eye-bar is made of an ordinary flat bar, 15in. wide for pin-holes of 6in. diameter, the ends are strengthened by plates riveted on each side ; the two rivet-holes of 7/8in. diameter, by which the bar is weakened to the extent of 18.2 per cent. on the remaining sectional area, may be repeated at any point between the ends without further weakening the bar, and the transverse bracing may be attached, for example, in the middle by means of rivets, and in such a manner that the resultant of the stresses acts in the centre of the section of the chord. In place of one of the two rivet holes, a slot is made to allow the bracing-bar to pass through, see Fig. 8, but this, although best, is not absolutely necessary.
'The above surplus of 18·2 per cent. of metal refers to the bottom chord, with the exception of the end panels and to the main diagonals, in all to 86 tons in a total of 156 tons for each span . This percentage is, however, reduced by taking into account the stresses from the weight of the bars themselves. The width of an ordinary eye-bar on the 6in. pins would not be more than 8in., with a length of 25ft. 9in., and is often less in proportion to the length. This corresponds within the limit of elasticity to an additional stress of 1.1 ton, whereas the 15in. bar has only an additional stress of 0.59 ton. The difference amounts to about 7 per cent., so that the surplus required is about 6.2 per cent. of 36 tons, or 2¼ ton in 156 tons. It can hardly be denied that this small additional quantity attending the substitution of riveted eye-bars is amply compensated by the satisfactory state of the transverse bracing, and by the fact that these eye-bars can be manufactured without the employment of expensive forging apparatus and without reheating the steel. It is, therefore, unlikely that British manufacturers will ever be induced to set up such apparatus, even if pin connected girder bridges should come into fashion in Europe. Chain suspension bridges have almost been abandoned, and in those rare cases where eye-bars without rivet-holes are required, it will be more profitable to cut them out of plain flat bars. The raking struts in the through spans of the Lan-ho Bridge are made, contrary to common practice, to turn freely upon the end pins, because, as one of the reasons why pins are used is to a void the bending stresses from fixed connections, it appeared inexpedient to introduce pins and at the same time to retain the bending stresses, except in the horizontal part of the top chord , see Figs. 6 and 7. The diagonals of the lateral bracing are constructed so as to be capable of resisting thrust, and, as already mentioned , are made to act in the central axis of the chord -see Figs. 8 and 9. The shearing stress is finally brought to the end pin in the direction of its axis by means of a transverse strut, as shown in Fig 11. The head of the pin transfers the stress to the cast steel bedplate , to which also the shearing stress from the overhead bracing, see Fig. 8, is conducted through the raking strut by the 1¾ in. turned bolt, see Figs. 11 and 12, while the flange stress is brought to the bed-plate through the large pin. The inclined end frame s, which usually have only a transverse bracing on the top, have in the Lan – Ho bridge a similar one also at the bottom , the latter consisting of a plate girder 8ft. deep and two angle bars struts in the plane of the frame ; the required open profile for the passage of the train is therefore accurately enclosed by the framework - see Fig. 12. In this way the lateral movement of the upper part of the bridge relatively to the lower, from forces or vibrations, is reduced to a minimum. The ends of the rail bearers have their support direct on the masonry, and are here fixed to the strut which takes the stress from the lower transverse bracing to the end pins as described, see Figs. 4, 11, and 12. The inclined plate girder passes through openings in the rail bearers, so that the two structures are independent of each other. If this were not the case, the inclined plate girder, being prevented from bending downwards, and being very stiff horizontally, would be subjected to very great bending stresses when the chords of the main girders extend under the load, while the rail bearers would only to some extent take part in this extension. Stresses from this cause, increasing f rom the middle to the abutments, occur in all bridges in the cross girders and the diagonals of the transverse bracing, if the latter lies in or near the level of the chords, unless the rail bearers and the diagonals are furnished with expansion joints : but this is not usually done, and with the diagonals meeting, as here , at the chords in the middle of the panel, the stresses are less intense. The vertical posts of the main girders are not made, as usually, of a tubular section with braced sides, but of an H section with a. solid web, the reason being that they can be better painted , and that the pin connections at their ends are more accessible. At half height all posts and diagonals are connected together by two horizontal bars, see Fig. 2. The other details of the large span of the Lan-ho Bridge, see Figs. 13, 14, and 15, are without particular interest. The weight of steel in one span is as follows:- [Table omitted] ....
'.... Few remarks will suffice to characterise the construction of the ten small spans, see Figs. 16 to 19. The heavy connections of the diagonals with the flanges, consequent upon adherence to the given outline of the main girder, induced the majority of competitors to propose box-shaped flanges, so as to avoid excessive length of the groups of rivets, although this construction 1s unusual in bridges of about 100ft. span, because the box shape is needlessly complicated, and the girder difficult to paint properly. In the present case, where the flanges are T-shaped, the excessive length of the connections is avoided by making the web thick enough to carry a rivet in double shear. This thickness is 5/8in., the rivets being 7/8in. diameter. The diagonals, being in pairs, are riveted on each side of the web - see Fig. 19; the number of rivets is therefore half of what it would be in a box-shaped girder. The 5/8in. space between the bars is closed by 2in. x 5/8in. strips at the edges. The main girders are 7 ft . apart, and the sleepers rest direct on the top flange, which is strengthened for the purpose of sustaining the bending stress from the load. There is a horizontal transverse bracing between the top flanges and a vertical transverse bracing between the uprights, as also a plate girder over the bearings. The weight of metal in one span is 84½ tons. The diagram of stresses for the through span is given in Fig. 5, and that for the deck span in Fig. 18.'
See Also
Sources of Information
- ↑ [1] The Engineer, 24 July 1914
- ↑ The Engineer, 4 May 1894