accommodation to the site, girders of lengths up to 80 feet, and weights up to 20 tons may be delivered at the site in one piece. If they exceed these limits they may he sent in two or more pieces, and afterwards rivetted together in position. In such cases the joints should be so arranged that the different lengths can be conveniently joined together. When the girders are too large to be treated in this manner, or where there is a difficulty in transporting heavy pieces to the site, or of lifting them into position, then it becomes necessary to send the work away in small pieces, and all or most of the jointing and rivetting has to be done on the site by erecting the work on a stage or otherwise. In cases of this kind it is always advisable in the first instance to erect the girder complete in the bridge-builder's yard so as to insure everything fitting properly. The different bars, plates, &c., should then be carefully marked, and corresponding marks put on the erection drawing, so that each piece may find its proper place at the final erection in situ. A complete list of bolts, rivets, and other fastenings should be made, and a copy furnished to the foreman in charge. Attention to this will save a deal of trouble. It is customary to send an excess of about 10 per cent. of all rivets in order to provide for those lost or burnt.

371. Erection of Small Bridges.-In bridges of small span, where the girders are delivered on the site complete, it is only necessary to erect one or two derricks according to the size of the girders, by means of which the latter may be lifted to their place. The cross-girders are similarly treated and then the flooring laid, no scaffolding being necessary.

If the main girders are delivered at the site in two pieces, they may be jointed together on the ground, and then lifted complete in the manner explained. If too heavy or cumbersome for thus treating, a timber trestle may be erected towards the centre of the span, then each piece lifted and laid with one end on the abutment and the other on the trestle, and the two pieces rivetted together in this position. This method is very inexpensive, as little or no scaffolding is needed.

372. Erection of Large Bridges.-In bridges of large span, or where a river or ravine has to be crossed, other methods will have to be adopted. These have been very fully considered by Mr. Seyrig, and a good deal of the subsequent information on this subject is due to him.

*

The different methods of erecting large bridges may be grouped under four heads :

* Pro. Inst. of Civil Engineers, vol. lxiii.

1. Erection upon Staging;

2. Erection by Floating;

3. Erection by Protrusion, or Rolling Over of Girders;
4. Erection by Overhanging, or Building Out.

373. Erection upon Staging. This is by far the most common way of erecting bridges, though it is often expensive. The staging is usually constructed of timber, and, roughly speaking, consists of a series of timber trestles or piers upon which are laid longitudinal balks or beams. On these latter are placed a series of cross-beams which, in their turn, support the longitudinal planking, which latter forms a platform for the men to work upon. The stage, in fact, is a temporary wooden bridge.

The bottom boom of the girder is first laid on the stage resting on a series of skids and wedges, by means of which the proper camber is given to it before the web and top flange are erected. If the bridge crosses a river it will be necessary, in order to form the temporary piers, to drive piles securely into the river-bed, so that the scaffolding may not be swept away by the force of the current.

The sections of the various scantlings in the stage have to be determined specially in each case. As a rule, when the trestles are more than 30 feet apart, it will be necessary to truss the main longitudinal beams by means of wrought-iron tie-bars. The working stresses allowed to come on a temporary structure of this kind are much greater than those in permanent structures. In the former case it is not unusual for the working stress to be one-half the ultimate strength.

A Scotch crane, fixed on the stage, is very useful for lifting purposes. It is also often convenient to have a Goliath travel

ling crane running the whole length of the stage on rails placed at each side of the girder, by means of which the different members may be placed in position.

374. Erection by Floating Girders.-There are several varieties of sites which do not lend themselves to the construction of a stage. In deep or rapid rivers, or those subject to floods, the construction of a stage is troublesome and often impossible. In navigable rivers, also, it may interfere with the passing of vessels. In such cases some other method must be adopted.

The main girders may be built on the shore and then rolled on to pontoons and floated to their destination. The erection of the main tubes of the Britannia tubular bridge was done in this way. The tubes were constructed on a platform, which was erected on piles close to the shore. When the tubes were ready,

pontoons were brought underneath them through the piles at low water; when the tide rose the tubes were lifted bodily off the scaffold and floated to their position between the piers. By letting water into the pontoons, the tubes were lowered on to the masonry piers which were above water. The tubes were afterwards lifted 6 feet at a time by powerful hydraulic presses placed on the piers. After each lift the masonry was built up underneath, and then a fresh lift was made, and the operation of building the masonry continued, by which means they were eventually got to their proper level. The weight of the four large tubes, which were 470 feet long, was 1587 tons each, and they were fixed at a height of 100 feet above water-level. Six pontoons were used for floating each tube.

375. Erection by Protrusion, or Rolling Over of Girders.-This method is principally applicable to continuous girders extending over several spans, and dispenses with the necessity of staging. It is important when this plan of erection is to be carried out that the girders be designed so as to make it practicable. When a girder projects in the form of a cantilever, severe stresses are incurred of a nature and intensity to which it will not be exposed when fixed in its final position. In order to provide against these temporary stresses, it is sometimes necessary to make the girder stronger in certain parts than is needed in the permanent structure. The extra strength and stiffness are sometimes provided for by means of temporary bracings and stays, which are removed after the girder is finally fixed in position.

There are several advantages attending this system of erection. The bridge may be put together on the bank, and the operation of rolling it over is not, as a rule, attended with much risk or expense if properly carried out It is equally applicable to lofty viaducts and bridges crossing rivers.

Fig. 254 explains how a girder A B, continuous over three spans, may be rolled across into its final position. The girder rests on rollers which should be grooved so as not to interfere with the rivet-heads in the bottom flange, or the rollers may revolve in a frame fixed to the girder and run on a rail laid on the ground. A crab, C, is fixed on the opposite bank, and a chain from it fixed to the extremity B of the girder.

If the girders are deep and narrow, and rolled over separately, they should be fixed in a cradle to prevent their falling over sideways.

In order to prevent an undue side-stress on the piers, a tie might be taken from the top of the pier and fastened to the bank from which the girder is rolled.

Fig. 255 explains a method applicable when there are no piers. A crane, C, is fixed on the opposite bank to that on which the girder rests. This supports the end B while the girder is being rolled across. A counterbalance weight, A, is often used in order to relieve the stress on the crane. By applying levers or jacks to the end A, the girder may be pushed over. Instead of the bridge, the staging may be rolled across; in some cases this will be found preferable, but should only be had recourse to when other methods are not applicable, on account of the expense

involved. The stage in such case may be constructed of timber lattice-trusses, connected together with cross-framing.

In bridges crossing rivers and canals, a method of erection might in some cases be employed with advantage, which is a combination of the methods of rolling and floating. The girder is first thrust forward to nearly one-half its length. A boat is then placed under the projecting end, which can be raised by pumping water out of the vessel; the girder can then be drawn over and landed on the opposite abutment.

Fig. 255.

376. Erection by Overhanging, or Building Out. By this system no scaffolding whatever is required, the structure itself being made use of for its own erection. This plan is adopted in situations where it is impossible to erect staging, and where the design of the structure lends itself to the method, such as braced arches or bridges of the double cantilever form. Among notable examples, where this method has been successfully adopted, may be mentioned the Forth and Douro Bridges.

The bridges are built out, starting at the abutments or piers.

In the case of a bridge with a single span, the erection is started at each abutment, and built out panel by panel until the two portions meet at the centre. The top members of each portion must be tied back to the abutment, while the lower members may for a certain distance be supported by inclined struts.

In bridges with more than one span, by starting at a pier and building out on either side simultaneously, two cantilevers are formed which balance each other.

377. Cost of Erection. The cost of erection of bridges varies a great deal, and may be roughly stated to be from £1 to £10 per ton. When a great deal of rivetting has to be done, it is advisable to use portable rivetting machines driven either by hydraulic or pneumatic power; the latter being preferable in cold climates, as it is not interfered with by the frost.

After a bridge has been erected and before it is opened to traffic it should be tested and the deflection noted. In railway bridges it is usual to send a string of locomotives coupled together over it at different speeds.

378. Erection of Iron Roofs and Buildings.-If a roof rests on columns, the first thing to be done is to fix these in position. The foundation of a column may simply consist of a stone bedded in the ground, it is best, however, to have concrete or brickwork underneath the stone to insure the stability of the foundation, and the tops of the stones should be dressed off smooth and level.

The columns may be fixed to the foundations by Lewis' bolts, which latter are fixed to the stone by running them with lead, sulphur, or other suitable substance. When lead is used it is poured into the dovetail space between the bolt and the stone, and, to make a good job, it should afterwards be caulked as the lead contracts in cooling. Sulphur does not require caulking as it expands in cooling. In the case of long columns, which require extra anchorage, long bolts should be used which pass down through the concrete or brick foundation, and are secured thereto by anchor plates. It is usual to have four holding-down bolts for each column.

For good work the bases of the columns should be faced; when this is not done one or more layers of felt or a layer of sheet lead should be placed between the column and the stone. These packings will yield wherever the pressure is greatest, so that it becomes distributed approximately over the base. Another plan for packing the bases of columns and getting them truly vertical is to put iron wedges at the different corners; then by driving one or other of these wedges, the column may be made quite plumb. When all the columns are thus set true, and their tops