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Cable Erection

Reeling Operation

The main cable wire is generally transported to site in coils and wound on to2 mdiameter reels capable of holding about 10-15 span lengths. The end of the wire is led round a tensioning wheel (which turns against a controlled brake) and on to the reel. Ends can be spliced together with swaged couplings.

Cable Spinning

The drums of wire are taken from the reeling area and set up on electrically powered unreeling machines (in the example shown a single reel is illustrated but often up to eight wires are  simultaneously unreeled across the span). The end of the wire is passed over a series of pulleys housed in a high tower and led via a floating sheave to a strand shoe, where a temporary connection is made. The wire is then looped by hand over the spinning wheel and hauled with the endless tramway drive rope at 200-300 m/min to a strand shoe on the anchorage at the other bank. Thus two wires are simultaneously spun, one dead (i.e. that fixed to the shoe) and the other live. The dead wire is temporarily held in position with hooks spaced at approximately150 mcentres along the catwalk, while the live wire is run through sheaves to aid control. Meanwhile the empty wheel returns to the reeling side. Adjustment of sag for each wire generally takes place while spinning is in operation. As soon as the wheel passes over a tower the dead wire or) the side span is unhooked, pulled clear of the catwalk with an electric winch, lined up with a guide wire and clamped at the top of the tower, it is then manually lifted into the strand formers located along the catwalk at about150 mcenters. The procedure is then repeated in the main span after the wheel has passed the other tower. Adjustment of the remaining side span sag takes place after the wire has been looped over the strand shoe while the wheel is stationary in ready uses for the return journey to the reeling side. During this return period, the previously live wire is released from the sheaves and adjusted in a similar manner to the dead wire.

The process is repeated until a complete strand has been made, where after the loose end is spliced to the temporary connection initially provided for the start end of the wire. The wires are subsequently shaken out, temporarily banded into strands and any final sag adjustments taken up by the strand shoe. The remaining strands are spun in a similar manner until the whole cable is formed. Spinning can usually only take place when wind speeds are less than about50 km/h. The whole process typically takes up to 6 months for a1000 mspan bridge, with an average about 6 tonnes per day of spun strand (Bianculli, 2003). (more…)

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Hangers

Hanger cables are clamped directly to the suspension cable with tightly bolted bands. The main cable wrapping only extends up to the bands as these are fixed prior to the wrapping operation.

Deck unit

Two types of deck system are used for suspension bridges, namely:

1. Trussed girders.

2. Steel box sections.

Trussed girders were predominantly used on the earlier bridges with the members erected piece-by-piece from a crane running on the previously erected deck. Alternatively the trusses were assembled into large sections and lifted into place. The high costs of fabrication, inefficient use of steel inherent in the design, the large quantities of labourintensive work and maintenance costs, however, forced designers towards cellular steel box sections. These can be prefabricated cheaply and quickly delivered to site, complete with footpaths and railing. They are easily connected together with bolts and/or welding. Once the units are joined, the concrete surfacing is laid and the deck finishes completed (Richman, 2005).

Anchorages

The anchorage is fundamental to the stability of a suspension bridge, as all the load in the cable must be transferred to a fixed anchorage (a few small bridges with self anchorages similar to cable-stayed bridges have been used). There are commonly three types:

1. Rock anchors.

2. Tunnel anchorage.

3. Gravity anchorage.

Rock anchorages simply involve drilling into the rock and grouting in large bolt type anchors to which the strands are subsequently attached. Where suitable rock is available, a U-tunnel can be constructed and the two cables joined to form a loop. However, the gravity anchorage has proved to be most popular with designers. The basic arrangement consists of looped over strand shoes attached to anchor bolts located in the concrete. The cable forces are thus resisted by a combination of overburden, dead weight and bearing friction. (more…)

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Balanced Cantilever

The occasional need to have clear uninterrupted space below the bridge, for example railway sidings, private property, etc., has forced designers and constructors to develop the balanced cantilevering technique, whereby all or at least very few props are required, as shown in figure below. Erection proceeds simultaneously each side to the tower, with the first few sections over the piers, temporarily supported on false work until the tower has been erected and the cables attached. Like the other methods, a degree of cantilevering beyond the last attached cable may be possible depending upon the capability of the section to resist bending movement, the potential for this possibility being much better for steel plate than heavy precast concrete segments.

A very necessary feature of such technique is the need to have a stiff tower and fixity between the deck and tower and its foundations, because of imbalances caused by construction plant, variation in segment dead weight, and tension in the cables. Where possible, the tower design should be selected to accommodate this requirement, otherwise substantial extra staying, tempo anchor cables or a heavy deck tower fixing clamp must be provided.

Cantilever ever spans over150 meach side of the tower are commonly erected, but where ever possible some propping is desirable to aid stability.

Push-out Method

In some situations access beyond the abutment may not be available or deck units cannot be transported to the tower over adjoining property. To overcome these difficulties a few bridges have used the push-out method as illustrated in the figure below. The deck is assembled at one of the abutments and simple winched out over the rollers or teflon pad bearings. (more…)

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Stability conditions

The principle aim of the structural configuration of a cable-stayed arrangement is to prevent sideways and vertical movements of the tower/pylon and deck under asymmetrical live loading. By careful selection of the foundation types and connection of cable and girder it is possible to maintain stability of the whole structure by resisting only the horizontal and vertical components of the forces generated (Mitchell, 2007).

Transverse arrangement of cables

Viewed perpendicularly to the line of the bridge, the cables are usually either arranged in a single-plane or two-plane system (as shown in the figures below). Single-plane is commonly employed with a divided deck, and requires only a narrow pylon and pier. The deck itself generally has a hollow box cross section to provide torsional resistance across the deck width. In the two-plane system the cable can either be arranged to hang vertically or slope towards the top of the tower or pylon, the connection to the deck being through the outside edges.

The Pylon (Tower)

The pylon may be fabricated from steel plate, or precast concrete elements or occasionally in situ concrete. The various configurations shown, in figure below illustrate the flexibility of design options available to produce good aesthetic effect. (more…)

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This method has a lot in common of the span-by-span method in the way that the construction starts at one end of the structure and proceeds continuously to the other end. The method derives from the balanced cantilever concept. In this method segments are placed from one end to the other in successive cantilevers. Because of the length of the cantilever (one span), a movable temporary tower and stay system is required to limit the cantilever stresses to a reasonable level.

Steel Bridge

As previously stated steel bridge construction is not a popular form of construction.

Except for a few bridges, no major bridges have been constructed in steel in the recent past. Therefore, the construction of steel bridges will only be considered very briefly.

There are three main forms of steel bridge superstructures. They are:

a) Plate girder,

b) Box girder,

c) Truss.

Plate girders have largely displaced lattice girders bridges because of the availability of wide steel plates and the ease by which fabrication can be automated. (more…)

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Span-by-Span Method

The balanced cantilever construction method was developed primarily for long spans, so that construction of the deck can be carried out without the use of extensive falsework.

The span-by-span method was developed for long viaduct structures with relatively shorter spans.

In the first stage, the end span and a section of the penultimate span up to its nearest point of contraflexure is cast. The supporting formwork is then moved forward and the section of deck up to the nearest point of contraflexure of the next span is cast. This second cycle is repeated until the final section of the end span (from the point of contraflexture to the end support) is cast. The superstructure construction therefore proceeds in one direction span-by-span. To speed up the construction it is also possible to commence construction somewhere in the middle of the viaduct and work towards the ends.

The formwork is generally supported on a moving gantry system. This in effect provides a type of factory operation transplanted on the job site. It has many of the advantages of mass production commonly associated with precasting as well as the added advantage of permitting versatile adjustments on site. The gantry may be supported from the piers or from the edge of the previously completed cantilever. The formwork gantry can be either above-deck or below-deck. For the above deck gantry, the formwork is suspended from steel rods. After concreting and post-tensioning, some forms are released and the gantry rolled forward by means of outriggers on both sides gantry’s superstructure. For a belowdeck gantry, a similar procedure is followed. (more…)

Construction Engineering Assignment and Arch Bridge term paper

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It is not a secret that the bridges, tunnels, overpasses are one of the oldest engineering inventions of mankind. In ancient times, a couple of logs across the river or any physical barrier were very birth of engineering idea about the bridge construction technology.

Nowadays it is impossible to provide the existence of mega-cities, cars and railways without the bridges. Modern technology allows the construction of bridges to build complex bridge structures quickly and accurately.

Bridge building can be traced back several thousand years. In primitive times, man used fallen trees, arches made of a rock fall and suspension bridges made of vines and creepers to cross streams and ravines. These are the three natural forms of bridging methods, namely, the beam, the arch and the suspension structure. Despite the technological advances of the recent past, these three forms, either singularly or in combination, remain the basis of all bridge construction.

The vast majority of arch bridges were built with stone masonry. A few were also built of iron. Some examples of these will be presented during the lecture. Modern arch bridges are built with reinforced concrete and therefore only die construction of reinforced concrete arch bridges will be considered in this paper.

Suspension bridges are used to form exceptionally large Spans. They are a highly specialized form of construction and far more expensive than beam bridges. Their use is therefore limited to a small quantity of applications where their ability to span large distances is paramount. (more…)