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附录A英文文献

Bridges

Bridges are great symbols of mankind’s conquest of space.The sight of the crimson tracery of the Golden Gate Bridge against a setting sun in the Pacific Ocean, or the atch of the Garabit Biaduct soaring triumphantly above the deep gorge. Fills one’s heart with wonder and admiration for the art of their builders[11]. They are the enduring expressions of mankind’s determination to remove all barriers in its pursuit of a better and freer world. Their design and building schemes are conceived in dream-like bisions. But vision and determination are not enough. All the physical forces of nature and gravity must be understood with mathematical precision and such forces have to be resisted by manipulating the right materials in the right pattern. This requires both the inspiration of an artist and the skill of an artisan.

Scientific knowledge about materials and structural behavior has expanded tremendously, and computing techniques are now widely available to manipulate complex theories in innumerable ways very quickly. Engineers have virtually revolutionized bridge design and construction methods in the past decade. The advances apply to short-medium and long-span bridges.

For permanent bridge，the most commonly used materials are steel and concrete. Bridge of many different type are built with these materials, used singly or in combination. Timber may be used for temporary above-water construction, for the elements of a structure that lie below the waterline (particularly timber pile s), or for short-span bridges located on secondary roads. A few short-span aluminum bridges have been built in the United States on an experimental basis.

The principal portions of a bridge may be said to be the “substructure” and the “superstructure.” This division is used here simply for convenience, since in many bridges there is no clear dividing lint between the two.

Common elements of the substructure are abutments (usually at the bridge ends) and piers (between the abutments).Piers and abutments often rest on separately constructed foundations such as concrete spread footings or groups of bearing piles;

these foundations are part of the substructure. Occasionally a bridge substructure comprises a series of pile bents in which the piles extend above the waterline and are topped by a pile cap that, in turn, supports the major structural elements of the superstructure. Such bents often are used in arepetitive fashion as part of along, low, over-water crossing.

In recent years, the dividing lines between short-medium and long-span bridge have blurred somewhat. Currently, spans of 20 to 100 ft (6.1 to 30.5m) are regarded as short by many designers, who have developed many standardized designs to handle these spans economically. Medium spans range up to, per-haps, 400ft (121.9m) in modern bridge practice, depending on the organization involved and the materials used. Long spans range up to 4000ft (1219.2m) or more, but a clear span above 1000ft (304.8m)is comparatively rare.

In the United States, highway bridges generally must meet loading, design, and construction requirements of the AASHTO Specification. The design and construction of railway bridges are governed by provisions of the AREA Manual for Railway Engineering. Design requirements for pedestrian crossings and bridges serving other purposes may be established by local or regional codes and specifications. ACI Code provisions are often incorporated by reference, and in most cases serve as model provisions for other governing documents.

Bridge spans to about 100 ft often consist of pre-cast integral-deck units. These units offer low initial cost, minimum maintenance, and fast easy constrction, with traffic interruption. Such girders are generally pretensioned, the units are placed side by side, and are often post-tensioned laterally at intermediate diaphragm lacations, After which shear keys between adjacent units are filled with non-shrinking mortar. For highway spans, an asphalt wearing surface may be applied directly to the top of the pre-cast concrete. In some cases, a cast-in-place slab is placed to provide composite action.

For medium-span highway bridges, to about 120 ft, AASHTO standard I beams are generally used. he are intended for use with a composite cast-in-place roadway slab. Such girders often combine pre-tensioning of the pre-cast member with

post-tensioning of the composite beam after the deck is placed.

Pre-cast girders may not be used for spans much in excess of 120 ft because of the problems of transporting and erecting large, heavy units. On the other hand, there is a clear trend toward the use of longer spans for bridges. Highway safety is improved by eliminating central piers and moving outer piers away from the edge of divided highways. For elevated urban expressways, long spans facilitate access and minimize obstruction to activities below. Concern for environmental damage has led to the choice of long spans for continuous viaducts. For river crossing, intermediate piers may be impossible because of requirements of navigational clearance.

Such requirements have led to the development in Europe, and more recently in the western hemisphere, of long span segmental pre-stressed concrete box girder bridges. In typical construction of this type, piers are cast-in-place, often using the slip-forming technique. A “hammerhead” section of box girder is then cast at the top of the pier, and construction proceeds in each direction by the balanced cantilever method. The construction is advanced using either cast-in-place or pre-cast segments, each post-tensioned to the previously completed construction. Finally, after the closing cast-in-place joint is made at mid-span, the structure is further post-tensioned for full continuity.

Bridge may also be classed as “deck” or “through” types. In the deck type of bridge, the roadway is above the supporting structure; that is, the load-carrying elements of the superstructure are below the roadway. In the through type of bridge, the roadway passes between the elements of the super-structure, as in a through steel-truss bridge. Deck structures predominate: they have a clean appearance, provide the motorist with a better view of the surrounding area, and are easier to widen if future traffic requires it.

Examples of short-span concrete bridges include cast-in-lace, reinforced concrete beam (and slab);simple-span, pre-stressed (this type incorporates pre-cast, pre-stressed I-girders or box girders topped by a cast-in-place deck);and cast-in place box girder.

The designer of each medium-and long-span bridge tries to devise a structure

that is best suited to the conditions encountered at that particular location. The result is an almost bewildering variety of structures that differ either in basic design principles or in design details.

General categories of steel bridge are briefly described in the following paragraphs.

Girder bridges come in two basic varieties-plate and box girders.

Plate girders are used in the United States for medium spans. They generally are continuous structures with maximum depth of girder over the piers and minimum depth at mid-span. The plate girders generally have an I cross section; they are arranged in lines that support stringers, floor-beams, and, generally, a cast-in-lace concrete deck. The girders are shop-fabricated by welding; field connections generally are by high-strength bolts.

Welded-steel box girder structures are generally similar to plate girder spans except for the configuration of the bridge cross section.

Rigid frames are used occasionally, most often for spans in the range of 75 to 100 ft (22.9 to 30.5 m) and for grade0separation structures.

Arch bridges are used for longer spans at locations where intermediate piers cannot be used and where good rock is available to withstand the thrusts at the arch abutments.

Variations in the arch bridge are specially suited in the span range of 200 to 500m and thus provide a transition between the continuous box girder bridge and the stiffened suspension cable. The cables provided above the deck and connected to the towers would permit elimination of intermediate piers facilitating a larger width for purposes of navigation. Because of the damping effect of inclined cables, the cable-stayed decks are less prone to wind-induced oscillations than suspension bridges.

Suspension bridges are used for very long spans or for shorter spans where intermediate piers cannot be built. An example is the Verrazano Narrows Bridge which was completed in 1964.The $305 million,4260ft(1298.5m)structure spans the entrance to New York Harbor to join Staten Island and Brooklyn.

Concrete bridges come in nearly as great a variety as do steel bridges.

The bridge construction in France benefits by a strong growth in rail and highway infrastructures. For the time being the competition with other material turns to the advantage of composite bridge solutions. Before presenting any features concerning the recent trends in composite bridge design it is important to clarify, the bridge market, through the analysis of some statistical data.

In France, there is a very limited market for long span bridges. In the recent construction, the demand for bridges of span length higher than 200m is rather exceptional. The main market is for bridges of span length (or multi span length) less than 100m.

In France 800 to 1200 bridges are built every year, which represent about 300,000m to 500,000m of deck surface. However the majority of bridges being erected each year are of small span length. Less than 10% of the bridge patrimony have span. Length greater than 30m and deck surface greater than 1000 m2. Now that the market has been identified lets have an idea, in term of competitiveness, of the French market situation between several bridge types. In 1977 less than 2.5%.

Of bridges were steel or composite bridges. The steel-concrete composite construction has continued to grow steadily over the last 15 years. This trend is mainly attributable to the gain in competitiveness of composite bridges against reinforcedand prestressed concrete bridges.

For short span length the majority of steel bridges is of concrete type. Bridges composed of steel beams encased in concrete are very often used for railway bridges of small span length in order to meet stiffness requirements.

The recent statistical evaluation, performed by SETRA [1] on the bridges recently built in France between 1990 to 1993 by various owners (State, Highway concession companies, Departments and Communities, SNCF) shows that the competitive span length range for steel and concrete composite bridges is between 30 and 110 m with a very distinctive peak for the interval 60 to 80 m. In that range of spans length it is noticed that 85% of bridges being built belong to the composite category (Fig. 4).

The statistical analysis of the deck cost per square metre of surface confirms that the average price for a composite bridge is less than the price for a concrete bridge for spans length within intervals of 40 to 60 m and 60 to 80 m. The difference being of 1 500 FF/m2 over a total cost of 8 200 FF/m2 (VAT excluded) in favour of the composite bridge. It means that an 18% cost difference represents a great shift in terms of competition.

The last 15 years have seen a great simplification of composite bridges for both roadway and railway bridges, which have made them, as previously indicated, very competitive compared to prestressed and reinforced bridges. These composite bridges, that we will name them as classical, have however several features which are described hereafter. Then, from these classical features, improvements have been constantly brought to the design and execution of composite bridges, which will be depicted later on.

The traditional composite roadway bridge is composed of two longitudinal girders which are connected to the concrete slab by shear connectors (usually welded stud are mostly met; however steel angle connectors are still used). A limited number of transverse cross beams joining the two longitudinal girders, usually not connected to the slab — see half cross section (a) are welded to the vertical stiffeners. The main girders have a few numbers of horizontal stiffeners, if any which are mostly needed to resist the stress state in the girder webs occurring at the launching phase.

Plain concrete is formed form a hardened mixture of cement, water, fine aggregate, coarse aggregate (crushed stone or gravel), air, and often other admixtures. The plastic mix is placed and consolidated in the formwork, then cured to facilitate the acceleration of the chemical hydration reaction of the cement/water mix, resulting in hardened concrete. The finished product has high compressive strength, and low resistance to tension, such that its tensile strength is approximately one-tenth of its compressive strength. Consequently, tensile and shear reinforcement in the tensile regions of sections has to be provided to compensate for the weak-tension regions in the reinforced concrete element.

It is this deviation in the composition of a reinforced concrete section from the

homogeneity of standard wood or steel sections that requires a modified approach to the basic principles of structural design. The two components f the heterogeneous reinforced concrete section are to be so arranged and proportioned that optimal use is made of the materials involved. That is possible because concrete can easily be given any desired shape by placing and compacting the wet mixture of the constituent ingredients into suitable forms in which the plastic mass hardens. If the various ingredients are properly proportioned, the finished product becomes strong, durable, and, in combination with the reinforcing bars, adaptable for use as main members of any structural system.

The techniques necessary for placing concrete depend on the type of member to be cast: that is, whether it is a column, a beam, a wall, a slab, a foundation, amass concrete dam, or an extension of previously placed and hardened concrete. For beams, columns, and walls, the forms should be well oiled after cleaning them, and the reinforcement earth should be compacted and thoroughly moistened to about 6 in. in depth to avoid absorption of the moisture present in the wet concrete. Concrete should always be placed in horizontal layers which are compacted by means of high-prequency power-driven vibrators of either the immersion or external type, as the case requires, unless it is placed by pumping. It must be kept in mind, however, that over vibration can be harmful since it could cause segregation of the aggregate and bleeding of the concrete.

Hydration of the cement takes place in the presence of moisture at temperatures

50. It is necessary to maintain such a condition in order that the chemical above F

hydration reaction can take place. If drying is too rapid, surface cracking takes place. This would result in reduction of concrete strength due to cracking as well as the failure to attain full chemical hydration.

It is clear that a large number of parameters have to be dealt with in proportioning a reinforced concrete element, such as geometrical width, depth, area of reinforcement, steel strain, concrete strain, steel strees, and so on. Consequently, trial and adjustment is necessary in the choice of concrete sections, with assumptions

based on conditions at site, availability of the constituent materials, particular demands of the owners, architectural and headroom requirements, the applicable codes, and environmental conditions. Such an array of parameters has to be considered because of the fact that reinforced concrete is often a site-constructed composite, in contrast to the standard mill-fabricate beam and column sections in steel structures.

A trial section has to be chosen for each critical location in a structural system. The trial section has to be analyzed to determine if its nominal resisting strength is adequate to carry the applied factored load. Since more than one trial is often necessary to arrive at the required section, the first design input step generates into a series of trial-and-adjustment analyses.

The trial-and-adjustment procedures for the choice of a concrete section lead to the convergence of analysis and design. Hence every design is an analysis once a trial section is chosen. The availability of approach as a more efficient, compact, and speedy instructional method compared with the traditional approach of treating the analysis of reinforced concrete separately from pure design.

The rapid growth from 1945 onwards in the prestressing of concrete shows that there was a real need for this high-quality material. The quality must be high because the worst conditions of loading normally occur at the beginning of the life of the member, at the transfer of stress later, when the concrete has become stronger and the stress in the steel has decreased because of creep in the steel and the concrete, and shrinkage of the concrete. Faulty members are therefore observed and thrown out early, before they enter the structure, or at least before it becomes inconvenient and expensive to remove them.

The main advantages of prestressed concrete in comparison with reinforced concrete are:

(a) The whole concrete cross-section resists load. In reinforced concrete about half the section, the cracked area below the neutral axis, does no useful work. Working deflections are smaller.

(b) High working stresses are possible. In reinforced concrete they are not

usually possible because hey result in severe cracking which is always ugly and may be dangerous if it causes rusting of the steel.

(c) Cracking is almost completely avoided in prestressed concrete.

The main disadvantage of prestressed concrete is that much more care is needed to make it than reinforced concrete and it is therefore more expensive, but because it is of higher quality less of it needs to be used.

It can therefore happen that a solution of a structural problem may be cheaper in prestressed concrete than in reinforced concrete, and it does often happen that a solution is possible with prestressing but impossible without it.

Prestressing of the concrete means that it is placed under compression before it carries any working load. This means that the section can be designed so that it takes no tension or very little under the full design load. It therefore has theoretically no cracks and in practice the concrete in which it is embedded has hardened. After the concrete has hardened enough to take the stress from the steel, some of the stress is transferred from the steel to the concrete. In a bridge with abutments able to resist thrust, the prestress can be applied without steel in the concrete. It is applied by jacks forcing the bridge inwards from the abutments. This method has the advantage that the jacking force, or prestress, can be varied during the life of the structure as required.

In the ten years from 1950 to 1960 prestressed concrete ceased to be an experimental material and engineers won confidence in its use. With this confidence came an increase in the use of precast prestressed concrete particularly for long-span floors or the decks of motorways. Wherever the 500 m long, provided that most of the spans could be made the same and not much longer than 18 m, it became economical to use factory-precast prestressed beams, at least in industrial areas near a precasting factory. Most of these beams are heat-cured so as to free the forms quickly or reuse.

In this period also, in the United States, precast prestressed roof beams and floor beams were used in many school buildings, occasionally 32 m long or more. Such long beams over a single span could not possibly be successful in reinforced concrete unless they were cast on site because they would have to be much deeper and much