Use of Structural Steel in Bridges.

Introduction
Use of structural steel in bridges exploits its advantageous properties of economically carrying heavy loads over long spans with the minimum dead weight. Steel is however suitable for all span ranges, categorized in Table 4.1.

Span ranges of bridges

For long spans steel has been the natural solution since 1890 when the Firth of  Forth cantilever railway bridge, the world’s first major steel bridge, was completed.

For short and medium spans concrete bridges held a monopoly from 1950 to 1980 because of the introduction of prestressing and precasting. Developments in steel during this period such as higher tensile strength and improved welding techniques were applied mainly to long spans.However, improvements in construction methods from 1980 have enabled steel to improve its market share within Europe and other continents to more than 50% for short and medium spans. Contributing factors to this trend are shown in Table 4.2.

Factors contributing to the improved market share for steel in short and medium spans from 1980
Table 4.2 Factors contributing to the improved market share for steel in short and medium spans
from 1980

Where traffic disruption during construction must be minimized then steel is always suitable. Most bridgework is now carried out under these  conditions so  that structural steel will continue as a primary choice. Steel has an advantage where speed of construction is vital; it is no coincidence that this usually results in cost economies. If rapidly erected steelwork is used as a skeleton from which the slab and finishes can be carried out without need for falsework then  the advantages (summarized in Table 4.3) are fully realized.

Advantages of steel bridges
Table 4.3 Advantages of steel bridges

For short and medium span highway bridges composite deck construction is economic because the slab contributes to the capacity of the primary members.

For continuous spans it uses the attributes of steel and concrete to best advantage. In cases where construction depth is restricted, for example in developed areas, then half-through or through construction is convenient; this is common for railway and pedestrian bridges.

For long spans, including suspension or cable-stayed bridges, all-steel orthotropic plate floors are used. Although the intrinsic costs of a steel orthotropic plate are higher (often up to about four times more) than an equivalent concrete slab, the advantage in dead weight reduction (approximately 1 : 3 ratio) may more than offset this when the overall economy is considered. Steel decks are also employed when erection must be completed in limited occupations, such as for railway bridges on existing routes. Movable bridges of swing, lift, rolling-lift (‘bascule’) or retractable type usually employ steel floors to rationalize the amount of counterweighting and

the mechanical equipment.Costs of this equipment usually exceed that of the bridge superstructure. For long spans of suspension or cabled-stayed form then special  considerations affect the design including aerodynamic behaviour, the feasibility  of deep foundations in estuarial conditions, cables with anchorages, non-linear structural behaviour and the absolute necessity to include the effects of the erection procedure in the design process.

This chapter on bridges gives emphasis to initial design, an important stage of the  process, because the basic decisions as to member proportions, spacings and splice positions vitally affect economy of the structure. It is essential that the detailed analysis is based upon optimized sizes which are as accurate as possible. If this is
not achieved then the detailed design will be inefficient because time consuming repetitive work will have been expended, adversely affecting the economy of the design and the construction costs.Guidance is given on the initial design of highway bridges using composite deck construction, which is a significant proportion of the number of steel bridges built, although since 1990 a significant number of railway bridges have been constructed for new-build schemes such as the Channel Tunnel

Rail Link and in replacement of life expired structures.

Steel is particularly suitable for the strengthening and repair of existing bridges arising from increases in highway traffic loadings and the incidence of accidental impact damage from road vehicles. Steel is suitable for such work using welded or bolted strengthening.

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