CHAPTER LXII

ERECTION AND FALSEWORK

VARIOUS methods for erecting bridges have been developed to fit the different types of structures and the diverse conditions prevailing at the bridge sites. These methods may conveniently be grouped in two general classes, viz.:

First, erection with falsework; and second, erection without falsework. The choice between these two methods will depend on the type of structure and the conditions at the bridge site. As a help in making such a choice for any particular case, the salient features of each method will be briefly set forth. The several types of bridge spans that the erector may be called upon to build are as follows:

1. Masonry arches.

2. Concrete girders and arches, both plain and reinforced.

3. Steel girders.

4. Viaducts and elevated railroads.

5. Truss spans.

6. Movable spans.

7. Suspension bridges.

Where a span is composed of numerous members that have to be assembled in final position, such as trusses, it is usually best and most economical to employ falsework, if the conditions at the site permit. Likewise, masonry and concrete arches, which require continuous support, are constructed on falsework, or centres, as the same is frequently termed. Those conditions at site favorable to the building of falsework are a river bed that will permit the driving of piles, an interval between floods sufficient to allow of the span or spans being assembled, riveted up, and swung, freedom from interference by river navigation, and the absence of deep water, swift current, drift-wood, and ice.

For single-track truss-spans, where no passing trains have to be provided for, it is customary to use falsework consisting of four-pile bents driven at intervals to correspond with the panel points of the truss. If a traveller is to be employed in erection, these bents are made wide enough to permit the placing at each end of a pair of 8" X 16" stringers outside of the span in order to support the rails on which the traveller runs. For shorter spans, where a derrick car will handle the material satisfactorily, the bents need be wide enough to carry only the two trusses. If the piles are sufficiently long to reach to the top of the falsework, they are capped with 12" × 12" timbers and sway-braced with 4′′ x 8′′ planks. In case

the said piles are too short so to reach, they are capped, and a framed bent is erected on top. Horizontal 6′′ × 8" timbers, running longitudinally, are used to tie the bents together and to give additional stiffness, when the height of the bents exceeds twice their width. For falsework on dry land or hard bottom, framed bents resting on mud sills may be constructed. The posts are usually 12" X 12" timbers with the outside pieces battered to give additional stability. Caps, sways, and longitudinal wales correspond to those in pile bents.

Should provision have to be made for carrying trains on a single-track bridge during erection, six piles or posts are used for each bent. Adequate longitudinal bracing similar to that required for timber trestles will have to be provided, in order to resist the thrust of braked trains. For the designing of falsework, the reader is referred to Chapter XXXV on "Wooden Bridges and Trestles," where he will find all the intensities of working stresses and other necessary information

It is usually desirable to erect the floor system first, and afterward the trusses; but occasionally it is best to erect the trusses first. This question is discussed quite fully by Mr. Reichmann on page 335. All truss connections are to be riveted up as soon as possible after the truss members are erected, in order that the span may be self-supporting in case the falsework is washed out.

For spans over 250 feet in length, erection is best carried on by means of a traveller. This is essentially a frame-work in the shape of an inverted U, supported on at least four rollers or wheels that rest on rails laid along the stringers of the falsework previously mentioned. This allows of the traveller's being readily moved along as erection progresses. At the top are convenient platforms for the workmen and tools; and on each side are hung several sets of blocks and tackle for raising the members of the truss. A hoisting engine is mounted on a lower platform for operating the tackle. Frequently swinging booms are placed at the forward corners so that they can be handled like a derrick. In large cantilever bridges it is practicable to employ one or two very small, comparatively speaking, travellers or "mules" riding on the top chords and picking up the material for erection from cars on the deck below. For spans under 250 feet and for trestles and elevated railroads, the traveller may be dispensed with and a derrick car or locomotive crane used for raising the parts into place.

The falsework, or centering, for masonry and concrete arches is more complicated than that required for truss spans, because the curved form of the arch necessitates special construction, and because the loads are distributed along the span length instead of being concentrated at panel points, as in trusses. This latter calls for continuous support, so that lagging and beams are necessary to transfer the load to the columns or bearings. Furthermore, the centering must be braced in order to resist the distortions produced by partial or unsymmetrical loadings. Set

tlement of the supports is to be avoided as much as possible. Centering is sometimes built on top of temporary trusses, but in such cases provision must be made to offset the deflection of such trusses. Further provision must be made for a gradual lowering of these centres so as to bring every part of the arch into action at the same time. This is readily accomplished by using wedges under the centres, which wedges can be gradually loosened at all the supports. Sand-jacks are also frequently employed for the same purpose.

Where conditions do not admit of falsework being constructed, trussspans may be erected on barges at some distance, if need be, from the site

and then floated into place and lowered onto the piers. This lowering is accomplished by means of jacks or by taking on water ballast. This method was adopted for the spread span of the author's bridge over the Fraser River at New Westminster, B. C. In that instance a depth of water of 80 feet and a reversing current of five miles per hour were encountered. The spread span, which was about 232 feet long and 136 feet wide at the wide end, while the narrow end was of the ordinary width of 19 feet, was erected on three barges placed in triangular formation, as shown in Fig. 62a. These were then floated into proper place, water ballast was admitted, and the span was thus lowered into final position on its piers. A detailed description, setting forth some of the unique features of the work, is given in the Engineering Record, Vol. 50, pages 192 to 194 inclusive.

Where it is not practicable to build falsework nor to erect the span on barges and float it into place, the structure can be erected by the

cantilever method. In this case special provision must be made in designing the bridge to take care of the erection stresses resulting from the temporary and unusual loading. A good example, and one of the earliest of this method of erection, is that of the Kentucky River High Bridge for the Cincinnati, New Orleans, and Texas Pacific Railway, twenty-one miles south of Lexington, Ky. The semi-cantilevering method of erecting simple spans was first proposed by the author over a quarter of a century ago, and has lately been used by him on several

bridges for the Canadian Northern Pacific Railway Company over the Fraser and the Thompson rivers in British Columbia. Here the swift current and hard bottom prevented the use of falsework in the main channels of both rivers. In the Fraser River bridge the centre span, which was 290 feet long, was erected from both ends by cantilevering out from each adjacent span; but for one of the Thompson River bridges, work could proceed from only one end, so that it was necessary to erect several contiguous spans by cantilevering the full span-length of 128 feet. Fig. 25g gives a view of the Fraser River bridge during the operation of semi-cantilevering.

Trussed arches are often erected by the cantilever method. One of the early examples of this was the erection of the Niagara Arch, described in Engineering News, Vol. 37, page 252. A later example is the Crooked River Bridge for the Oregon Trunk Railway, described in Engineering News, Vol. 69, page 549.

The author's 425' arch span near Cisco over the Fraser River was erected by cantilevering out the two halves till they met at the middle,

FIG. 62c. Counterweight for Anchoring, During Erection, the South Half of the Arch Span of the Canadian Northern Pacific Railway Bridge