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Originally appearing in Volume V04, Page 545 of the 1911 Encyclopedia Britannica.
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SHEWING KEELSONS SHEINING GUSSETS and must be regarded as an imperfectly continuous girder. The spans were in fact designed as independent girders, the advantage of continuity being at that time imperfectly known. The vertical sides of the girders are stiffened so that they amount to 40% of the whole weight. This was partly necessary to meet the uncertain conditions in floating when the distribution of supporting forces was unknown and there were chances of distortion. Wrought iron and, later, steel plate web girders were largely 1411 VII111'INI.!IVISOa PIINIIIJV!JIIJa!III!4HhVgIIIIIIHg1011 iHV110 l IIIHWI.1111911VIVIIIIIVIIIIIII!HWVkIIIIUIHII!§IIIV!I iHIu';JHl.6IIIVNIIIIIa11611..11, I6HI'T:iIIII:IIIIIIIIIVL~ ,IIIIH,1IIH6aIIHIVIII nIVIUI VJI V6 PJII ~II,jJ,. VI,~IIIIIIIHIIIIVUIVIiIIIHIIIVIII IHIIIHIVIIiIIIIIIC' R 7M'C4NUAl #5888., area of the cellular top flange of the large-span girders is 648 sq. in., and of the bottom 585 sq. in. As no scaffolding could be used for the centre spans, the girders were built on shore, floated out and raised by hydraulic presses. The credit for the success of the Conway and Britannia bridges must be divided between the engineers. Robert Stephenson and William Fairbairn, and used for railway bridges in England after the construction of the Conway and Menai bridges, and it was in the discussions arising during their design that the proper function of the vertical web between the top and bottom flanges of a girder first came to be understood. The proportion of depth to span in the Britannia bridge was TIT. But so far as the flanges are concerned the stress to be resisted varies inversely as the depth of the girder. It would be economical, therefore, to make the girder very deep. This, however, involves a much heavier web, and therefore for any type of girder there must be a ratio of depth to span which is most economical. In the case of the plate web there must be a considerable excess of material, partly to stiffen it against buckling and partly because an excess of thickness must be provided to reduce the effect of corrosion. It was soon found that with plate webs the ratio of depth to span could not be economically increased beyond to -,. On the other hand a framed or braced web afforded opportunity for much better arrangement of material, and it very soon became apparent that open web or lattice or braced girders were more economical of material than solid web girders, except for small spans. In America such girders were used from the first and naturally followed the general design of the earlier timber bridges. Now plate web girders are only used for spans of less than roo ft. Three types of bracing for the web very early developed the Warren type in which the bracing bars form equilateral triangles, the Whipple Murphy in which the struts are vertical and the ties inclined, and the lattice in which both struts and ties are inclined at equal angles, usually 45° with the horizontal. The earliest published theoretical investigations of the stresses in bracing bars were perhaps those in the paper by W. T. Doyne and W. B. Blood (Proc. Inst. C.E., 1851, xi. p. 1), and the paper by J. Barton, " On the economic distribution of material in the sides of wrought iron beams " (Proc. Inst. C.E., 1855, xiv. P. 443) The Boyne bridge, constructed by Barton in Ireland, in 1854-1855, was a remarkable example of the confidence with which engineers began to apply theory in design. It was a bridge for two lines of railway with lattice girders continuous over three spans. The centre span was 264 ft., and the side spans 138 ft. 8 in.; depth 22 ft. 6 in. Not only were the bracing bars designed to calculated stresses, and the continuity of the girders taken into account, but the validity of the calculations was tested by a verification on the actual bridge of the position of the points of contrary flexure of the centre span. At the cal- culated position of one of the points of contrary flexure all the rivets of the top boom were cut out, and by lowering the end of the girder over the side span one inch, the joint was opened Section of Newark Dyke Bridge. FIG. 19. — in. Then the rivets were cut out similarly at the other point of contrary flexure and the joint opened. The girder held its position with both joints severed, proving that, as should be the case, there was no stress in the boom where the bending moment changes sign. By curving the top boom of a girder to form an arch and the bottom boom to form a suspension chain, the need of web except for non-uniform loading is obviated. I. K. Brunel adopted this principle for the Saltash bridge near Plymouth, built soon after the Britannia bridge. It has two spans of 455 ft. and seventeen smaller spans, the roadway being roo ft. above high water: The top boom of each girder is an elliptical wrought iron, tube 17 ft. wide by 12 ft. deep. The lower boom is a pair of chains, of wrought-iron links, 14 in each chain, of q in. by r in. section, the links being connected by pins. The suspending rods and cross bracing are very light. The depth of the girder at the centre is about one-eighth of the span. In both England and America in early braced bridges calf iron, generally in the form of tubes circular or octagonal in section, was used for compression members, and wrought iron for the tension members. Fig. 19 shows the Newark Dyke bridge on the Great Northern railway over the Trent. It was a pin-jointed Warren girder bridge erected from designs by C. M. Wild in 1851-1853. The span between supports was 259 ft., the clear span 24o1 ft.; depth between joint pins 16 ft. There were four girders, two to each line of way. The top flange consisted of cast iron hollow castings butted end to end, and the struts were of cast iron. The lower flange and ties were flat wrought iron links. This bridge has now been replaced by a stronger bridge to carry the greater loads imposed by modern traffic. Fig. 20 shows a Fink truss, a characteristic early American type, with cast iron compression and wrought iron tension members. The bridge is a deck bridge, the railway being carried on top. The transfer of the loads to the ends of the bridge by Newark Dyke Bridge. _.... 1710' . BRIDGES 541 long ties is uneconomical, and this type has disappeared. The Warren type, either with two sets of bracing bars or with inter-mediate verticals, affords convenient means of supporting the floor girders. In 1869 a bridge of 390 ft. span was built on this system at Louisville. Amongst remarkable American girder bridges may be mentioned the Ohio bridge on the Cincinnati & Covington railway, which is probably the largest girder span constructed. Thegirders after erection. Fig. 22 shows girders erected in this way, the dotted lines being temporary members during erection, which are removed afterwards. The side spans are erected first on staging and anchored to the piers. From these, by the aid of the temporary members, the centre span is built out from both sides. The most important cantilever bridges so far erected or projected are as follows: (I) The Forth bridge (fig. 23). The original design was for a centre span is 550 ft. and the side spans 490 ft.— centre to centre of piers. The girders are independent polygonal girders. The centre girder has a length of 545 ft. and a depth of 84 ft. between pin centres. It is 67 ft. between parapets, and carries two lines of railway, two carriageways, and two footways. The cross girders, stringers and wind-bracing are wrought iron, the rest of mild steel. The bridge was constructed in 1888 by the Phoenix Bridge Company, and was erected on staging. The total weight of iron and steel in three spans was about 5000 tons. 1o. (e) Cantilever Bridges.—It has been stated that if in a girder bridge of three or more spans, the girders were made continuous there would be an important economy of material, but that the danger of settlement of the supports, which would seriously alter the points of contrary flexure or points where the bending moment changes sign, and therefore the magnitude and distribution of the stresses, generally prevents the adoption of continuity. If, however, hinges or joints are introduced at the points of contrary flexure, they become necessarily points where the bending moment is zero and ambiguity as to the stresses vanishes. The exceptional local conditions at the site of the Forth bridge led to the adoption there of the cantilever system, till then little considered. Now it is well understood that in many positions this system is the simplest and most economical method of bridging. It is available for spans greater than those practicable with independent girders; in fact, on this system the spans are virtually reduced to smaller spans so far as the stresses are concerned. There is another advantage which in many cases is of the highest importance. The cantilevers can stiffened suspension bridge, but after the fall of the Tay bridge in 1879 this was abandoned. The bridge, which was begun in 1882 and completed in 1889, is at the only narrowing of the Forth in a distance of 5o m., at a point where the channel, about a mile in width, is divided by the island of Inchgarvie. The length of the cantilever bridge is 5330 ft., made up thus: central tower on Inchgarvie 260 ft.; Fife and Queensferry piers each 145 ft.; two central girders between cantilevers each 350 ft.; and six cantilevers each 68o ft. The two main spans are each 1710 ft. The clear headway is 157 ft., and the extreme height of the towers above high water 361 ft. The outer ends of the shore cantilevers are loaded to balance half the weight of the central girder, the rolling load, and 200 tons in addition. An internal viaduct of lattice girders carries a double line of rails. Provision is made for longitudinal expansion due to change of temperature, for distortion due to the sun acting on one side of the structure, and for the wind acting on one side of the bridge. The amount of steel used was 38,000 tons exclusive of approach viaducts. (See The Forth Bridge, by W. Westhofen; Reports of the British Association (1884 and 1885) ; Die Forth Briicke, von G. Barkhausen (Berlin, 1889) ; The Forth Bridge, by Philip Phillips (1890) : Vernon Harcourt, Proc. Inst. C.E. cxxi. p. 309.) (2) The Niagara bridge of a total length of 910 ft., for two lines of. railway. Clear span between towers 495 ft. Completed in 1883, and more recently strengthened (Proc. Inst. C.E. evil. p. 18, and cxliv. pp 331). (3) The Lansdowne bridge (completed 1889) at Sukkur, over the Indus. The clear span is 790 ft., and the suspended girder 200 ft. in length. The span to the centres of the end uprights is 82o ft.; width between centres of main uprights at bed-plate loo ft., and between centres of main members at end of centilevers 20 ft. The bridge is for a single line of railway of 5 ft. 6 in. gauge. The back guys are the most heavily strained part of the structure, the stress provided for being 1200 tons. This is due to the half weight of centre girder, the weight of the cantilever itself, the rolling load on half the bridge, and the wind pressure. The anchors are built up of steel plates and angle. bars, and are buried in a large mass of concrete. The area of each anchor plate, normal to the line of stress, is 32 ft. by 12 ft. The bridge was designed by Sir A. Rendel, the consulting engineer to the Indian government (Proc. Inst. C.E. ciii. 123). (4) The Red Rock cantilever bridge over the Colorado river, with a centre span of 66o ft. Dealre'r4ow, te.:&*:•w•-.1;l ,we''farAge be built out from the piers, member by member, without any temporary scaffolding below, so that navigation is not interrupted, the cost of scaffolding is saved, and the difficulty of building in deep water is obviated. The centre girder may be built on the cantilevers and rolled into place or lifted from the water-level. Fig. 21 shows a typical cantilever bridge of American design. In this case the shore ends of the cantilevers are anchored to the abutments. J. A. L. Waddell has shown that, in some cases, it is convenient to erect simple independent spans, by building them out as cantilevers and converting them into independent (5) The Poughkeepsie bridge over the Hudson, built 1886-1887. There are five river and two shore spans. The girders over the second and fourth spans are extended as cantilevers over the adjoining spans. The shore piers carry cantilevers projecting one way over the river openings and the other way over a shore span where it is secured to an anchorage. The girder spans are 525 ft., the cantilever spans 547 ft., and the shore spans 2o1 ft. (6) The Quebec bridge (fig. 25) over the St Lawrence, which collapsed while in course of construction in 1907. This bridge, connecting very important railway systems, was designed to carry two lines of rails, a highway and electric railway on each side, all between the main trusses. Length between abutments 3240 ft.; 542 channel span 1800 ft.; suspended span 675 ft.; shore spans 5621 ft. Total weight of metal about 32,000 tons. (7) The Jubilee bridge over the Hugli, designed by Sir Bradford Leslie, is a cantilever bridge of another type (fig. 26). The girders are of the Whipple Murphy type, but with curved top booms. Thebridges. Such a bridge was the Wearmouth bridge, designed by Rowland Burdon and erected in 1793-1796, with a span of 235 ft. Southwark bridge over the Thames, designed by John Rennie with cast iron ribs and erected in 1814-1819, has a centre bridge carries a double line of railway, between the main girders. span of 240 ft. and a rise of 24 ft. In Paris the Austerlitz (1800-The central double cantilever is 36o ft. long. The two side span girders are 420 ft. long. The cantilever rests on two river piers 120 ft. apart, centre to centre. The side girders rest on the cantilevers on 15-in. pins, in pendulum links suspended from similar pins in saddles 9 ft. high. 11. (f) Metal Arch Bridges.—The first iron bridge erected was constructed by John Wilkinson (1728-1808) and Abraham Darby of pine 3 in. thick to prevent freezing. The span was 200 ft. sod + -• 1800 eet N 1806) and Carrousel (1834-1836) bridges had cast iron arches. In 1858 an aqueduct bridge was erected at Washington by M. C. Meigs (1816-1892). This had two arched ribs formed by the cast iron pipes through which the water passed. The pipes were 4 ft. in diameter inside, 11 in. thick, and were lined with staves (1750-1791) in 1773-1779 at Coalbrookdale over the Severn (fig. Fig. 28 shows one of the wrought iron arches of a bridge over the 27). It had five cast iron arched ribs with a centre span of roo ft. This curious bridge is still in use. Sir B. Baker stated that it had required patching for ninety years, because the arch and the high side arches would not work together. Expansion and contraction broke the high arch and the connexions between the arches. When it broke they fished it. Then the bolts sheared or the ironwork broke in a new place. He advised that _. \~ -\:/1./=/=/I/I -_ .//I/I~II/~-~\ st,\._ .~\\~1.//111///~- -- _ Rhine at Coblenz. The bridge consists of three spans of about 315 ft. each. Of large-span bridges with steel arches, one of the most important is the St Louis bridge over the Mississippi, completed in 1874 (fig. 29). The river at St Louis is confined to a single channel, 'Goo ft. wide, and in a freshet in 187o the scour reached a depth of 51 ft. Captain J. B. Eads, the engineer, determined to establish the piers and abutments on rock at a depth for the east pier and east abutment of 136 ft. below high water. This was effected by caissons with air there was nothing unsafe; it was perfectly strong and the stress in vital parts moderate. All that needed to be done was to fish the fractured ribs of the high arches, put oval holes in the fishes, and not screw up the bolts too tight. Cast iron arches of considerable span were constructed late in the 18th and early in the loth century. The difficulty of casting heavy arch ribs led to the construction of cast iron arches of cast voussoirs, somewhat like the voussoirs of masonry chambers and air locks, a feat unprecedented in the annals of engineering. The bridge has three spans, each formed of arches of cast steel. The centre span is 520 ft. and the side spans 502 ft. in the clear. The rise of the centre arch is 471 ft., and that of the side arches 46 ft. Each span has four steel double ribs of steel tubes butted and clasped by wrought iron couplings. The vertical bracing between the upper and lower members of each rib, which are 12 ft. apart, centre to centre, consolidates them into a single arch. The arches carry a double railway track and above this a roadway 54 ft. wide. The St Louis bridge is not hinged, but later bridges have been constructed with hinges at the springings and sometimes with hinges at the crown also. The Alexander III. bridge over the Seine has fifteen steel ribs hinged at crown and springings with a span of 353 ft. between centres of hinges and 358 ft. between abutments. The rise from side to centre hinges is 20 ft. 7 in. The roadway is 651 ft. wide and footways 33 ft. (Prot. Inst. C.E. cxxx. p. 335). The largest three-hinged-arch bridge constructed is the Viaur viaduct in the south of France (fig. 30). The central span is 721 ft. 9 in. and the height of the rails above the valley 38o ft. It has a very fine appearance, especially when seen in perspective and not merely in elevation. Fig. 31 shows the Douro viaduct of a total length of 1158 ft. carrying a railway 200 ft. above the water. The span of the central opening is 525 ft. The principal rib is crescent-shaped 32.8 ft. deep at the crown. Rolling load taken at 1.2 ton per ft. Weight of centre span 727 tons. The Luiz I. bridge is another arched bridge over the Douro, also designed by T. Seyrig. This has a span of 566 ft. There are an upper and a lower roadway, 164 ft. apart vertically. The arch rests on rollers and is narrowest at the crown. The reason given for this change of form was that it more conveniently allowed the lower the lattice girders above. The total weight of ironwork was 320e tons and the cost £124,000 (Annales des travaux publiques, 1884). The Victoria Falls bridge over the Zambezi, designed by Sir Douglas Fox, and completed in 1905, is a combination of girder and arch having a total length of 65o ft. The centre arch is 500 ft. span, the rise of the crown 90 ft., and depth at crown 15 ft. The width road to pass between the springings and ensured the transmission of the wind stresses to the abutments without interrupting the cross-bracing. Wire cables were used in the erection, by which the members were lifted from barges and assembled, the operations being conducted from the side piers. The Niagara Falls and Clifton steel arch (fig. 32) replaces the older Roebling suspension bridge. The centre span is a two-hinged parabolic braced rib arch, and there are side spans of 190 and 210 ft. The bridge carries two electric-car tracks, two roadways and two footways. The main span weighed 1629 tons, the side spans 154 and 166 tons (Buck, Proc. Inst. C.E. cxliv. p. 70). Prof. Claxton Fidler, speaking of the arrangement adopted for putting initial stress on the top chord, stated that this bridge marked the furthestbetween centres of ribs of main arch is 271 ft. at crown and 53 ft. 9 in. at springings. The curve of the main arch is a parabola. The bridge has a roadway of 30 ft. for two lines of rails. Each half arch was supported by cables till joined at the centre. An electric cable-way of 900 ft. span capable of carrying to tons was used in erection. 12. (g) Movable Bridges can be closed to carry a road or railway or in some cases an aqueduct, but can be opened to give free passage to navigation. They are of several types: (I) Lifting Bridges.—The bridge with its platform is suspended from girders above by chains and counterweights at the four corners (fig. 33 a). 'It is lifted vertically to the required height advance yet made in this type of construction. When such a rib is erected on centering without initial stress, the subsequent compression of the arch under its weight inflicts a bending stress and excess of compression in the upper member at the crown. But the bold expedients adopted by the engineer annulled the bending action. The Gambit viaduct carries the railway near St Flour, in the Cantal department, France, at 420 ft. above low water. The deepest part of the valley is crossed by an arch of 541 ft. span, and 213 ft. rise. The bridge is similar to that at Oporto, also designed by Seyrig. It is formed by a crescent-shaped arch, continued on one side by four, on the other side by two lattice girder spans, on iron piers. The arch is formed by two lattice ribs hinged at the abutments. Its depth at the crown is 33 ft., and its centre line N.alroraasadaArlirivIppopuaisi-sr.aPaRimtripirr pr000. oo ssssw. .2= when opened. Bridges of this type are not very numerous or important. (2) Rolling Bridges.—The girders are longer than the span and the part overhanging the abutment is counter-weighted so that the centre of gravity is over the abutment when the bridge is rolled forward (fig. 30). To fill the gap in the approaches when the bridge is rolled forward a frame carrying that part of the road is moved into place sideways. At Sunderland, the bridge is first lifted by a hydraulic press so as to clear the roadway behind, and is then rolled back. follows nearly the parabolic line of pressures. The two arch ribs are 6$1 ft. apart at the springings and 201 ft. at the crown. The roadway girders are lattice, 17 ft. deep, supported from the arch ribs at four points. The total length of the viaduct is 1715 ft. The lattice girders of the side spans were first rolled into place, so as to project some distance beyond the piers, and then the arch ribs were built out, being partly supported by wire-rope cables from (3) Draw or Bascule Bridges.—The fortress draw-bridge is the original type, in which a single leaf, or bascule, turns round a horizontal hinge at one abutment. The bridge when closed is supported on abutments at each end. It is raised by chains and counterweights. A more common type is a bridge with two leaves or bascules, one hinged at each abutment. When closed the bascules are locked at the centre (see fig. 13). In these bridges each bascule is prolonged backwards beyond the hinge so as to balance at the hinge, the prolongation sinking into the piers when the bridge is opened. (4) Swing or Turning Bridges.—The largest movable bridges revolve about a vertical axis. The bridge is carried on a circular base plate with a central pivot and a circular track for a live ring and conical rollers. A circular revolving platform rests on the pivot and rollers. A toothed arc fixed to the revolving platform or to the live ring serves to give motion to the bridge. the span. The counterweight is a depressed cantilever arm 12 ft. long, overlapped by the fixed platform which sinks into a recess in the masonry when the bridge opens. In closed position the main girders rest on a bed plate on the face of the pier 4 ft. 3 in. beyond the shaft bearings. The bridge is worked by hydraulic power, an accumulator with a load of 34 tons supplying pressure water at 630 lb per sq. in. The bridge opens in 15 seconds and closes in 25 seconds. At the opening span of the Tower bridge (fig. 13) there are four main girders in each bascule. They project too ft. beyond and 62 ft. 6 in. within the face of the piers. Transverse girders and bracings are inserted between the main girders at 12 ft. intervals. The floor is of buckled plates paved with wood blocks. The arc of rotation is 82°, is 13 ft. 3 in. inside the face of the and the axis of rotation piers, and 5 ft. 7 in. below the roadway. The weight of ballast in the short arms of the bascules is 365 tons. The weight of each leaf including ballast is about 1070 tons. The axis is of forged steel 21 in. in diameter and 48 ft. long. The axis has eight bearings, consisting of rings of live rollers 4ig in. in diameter and 22 in. long. The bascules 10' 4%1, II 4MM Ito _.lam'._.-FIG. 32.-Niagara Falls and Clifton Bridge. The main girders rest on the revolving platform, and the ends are rotated by pinions driven by hydraulic engines working in steel of the bridge are circular arcs fitting the fixed roadway. Three sectors 42 ft. radius (Prot. Inst. C.E. cxxvii. p. 35). As an example of a swing bridge, that between Duluth and arrangements are found: (a) the axis of rotation is on a pier at Superior at the head of Lake Superior over the St Louis river may be the centre of the river and the bridge is equal armed (fig. 33 c), so described. The centre opening is 500 ft., spanned by a turning bridge, that two navigation passages are opened simultaneously. (b) The 58 ft. wide. The girders weighing 2000 tons carry a double track for axis of rotation is on one trains between the girders and on each side on cantilevers a trolley abutment, and the bridge roadway and footway. The bride can be opened in 2 minutes, and is operated by two large electric motors. These have a is then usually unequal speed reduction from armature shaft to bridge column of t oo~to 1, armed(flg. 33d), the shorter through four intermediate spur gears and a worm gear. The end arm being over the land, lifts which transfer the weight of the bridge to the piers when the c In some over the land. span is closed consist of massive eccentrics having a throw of 4 in. The clearance is 2 in., so that the ends are lifted 2 in. This gives a the shorter arm is vertical load of 50 tons per eccentric. One motor is placed at each end of and the bridge turns on a the span to operate the eccentrics and also to release the latches kind of vertical crane post and raise the rails of the steam track. at the abutment At Riga there is a floating pontoon bridge over the Duna. It (fig. 33e). consists of fourteen rafts, 105 It. in length, each supported by two (5) Floating Bridges, the pontoons placed 64 ft. apart. The pairs of rafts are joined by three roadway being carried on baulks ?5 ft. long laid in parallel grooves in the framing. Two spans pontoons moored in the are arranged for opening easily. The total length is 1720 ft. and the width 46 ft. The pontoons are of iron, 851 ft. in length, and their stream. section is elliptical, 10; ft. horizontal and 12 ft. vertical. The dis- The movable bridge in placement of each pontoon is 18o tons and its weight 22 tons. The its closed position must be mooring chains, weighing 22 lb per ft., are taken from the upstream bportioned like a fixed end of each pontoon to a downstream screw pile mooring and from / ridge, but it has also other the downstream end to an upstream screw pile. WA conditions about to a fulfil. If it re-/ vertical axis its centre of gravity must always lie in that axis; if it rolls the centre of gravity must always lie over 'the abutment. It must have strength to support safely its own overhanging weight when moving. At Konigsberg there is a road bridge of two fixed spans of 39 ft., and a central span of 6o ft. between bearings, or 41 ft. clear, with balanced bascules over the centre span. Each bascule consists of two main girders with cross girders and stringers. The main girders are hung at each side on a horizontal shaft 8$ in. in diameter, and are 6 ft. deep at the hinge, diminishing tot ft. 7 in. at the centre of -*- t e 13. Transporter Bridges.—This new type of bridge consists of a high level bridge from which is suspended a car at a low level. The car receives the traffic and conveys it across the river, being caused to travel by electric machinery on the high level bridge. Bridges of this type have been erected at Portugalete, Bizerta, Rouen, Rochefort and more recently across the Mersey between the towns of Widnes and Runcorn. The Runcorn bridge crosses the Manchester Ship Canal and the Mersey in one span of 1000 ft., and four approach spans of 551 ft. on one side and one span on the other. The low-level approach roadways are 35 ft. wide with footpaths 6 ft. wide on each side. The supporting structure is a cable suspension bridge with stiffening girders A car is suspended from the bridge, carried by a trolley running on the underside of the stiffening girders, the car being propelled electrically from one side to the other. The underside of the stiffening girder is 82 ft. above the river. The car is 55 ft. long by 241 ft. wide. The electric motors are under the control of the driver in a cabin on the car. The trolley is an articulated frame 77 ft. long in five sections coupled together with pins. To this are fixed the bearings of the running wheels, fourteen on each side. There are two steel-clad series-wound motors of 36 B.H.P. For a test load of 120 tons the tractive force is 70 lb per ton, which is sufficient for acceleration, and maintaining speed against wind pressure. The brakes are magnetic, with auxiliary handbrakes. Electricity is obtained by two gas engines (one spare) each of 75 B.H.P. by dredging, or some form of mechanical excavator, until the formation is reached which is to support the pier; the concrete is then shot into the enclosed space from a height of about to ft., and rammed down in layers about r ft. thick; it soon consolidates into a permanent artificial stone. Piles are used as foundations in compressible or loose soil. The heads of the piles are sawn off, and a platform of timber or concrete rests on them. Cast iron and concrete reinforced piles are now used. Screw piles are cast iron piles which are screwed rim,~~~u5_N.~i_~E~.~-7.~~~~~r*ri ,=. ..:..u. - .eri~ril~~~1~~-~-2Go moo 00 +' :,•,... ... .. .. . +. ... ]o iw J590 -'~x -.x7470• 1000.0' •-•--_Ai--•---• '
End of Article: SHEWING

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