Bridge+Use+and+Application

= = = What are some bridge applications? = Where a bridge is, what climate it's in, and the material used can affect the structure and durability of it.

__** Building Materials **__ The traditional building materials for bridges are stone, timber, steel, and (more recently) reinforced and prestressed concrete.

__ Natural Stone __ Sound weather-resisting stone must be chosen, and fundamental rock like granite, gneiss, porphyry, diabas or crystallized limestone are especially suitable. With stone, one can build bridges which are both beautiful, durable and of a large span, up to 150m. This also means that at certain times, stone can not be used because the span may not be long enough to reach where you want it to.

__ Artificial Stone __ Clinker and brick. Clinker and hard-burned brick are used in bridges both as liners and for bearing vaults.

__ Steel __ • Amongst bridge materials, steel has the highest and most favourable strength qualities, and it is therefore suitable for the most daring bridges with the longest spans. • Normal building steel has compressive and tensile strengths of 370 N/mm2, about ten times the compressive strength of a medium concrete, and a hundred times its tensile strength. • A special merit of steel is its ductility, due to which it deforms considerably before it breaks; it begins to yield at a certain stress level. • This yield strength is used as the first term in standard quality terms. For bridges, high strength steel is often preferred. • The higher the strength, the smaller the proportional difference between the yield strength and the tensile strength.

__ Reinforced and Prestressed Concrete __ • Its tensile strength is low, and the use of concrete alone is therefore limited to structures which are only subject to compressive stresses. • Tensile stresses also occur in abutments and piers due to earth pressure, wind, breaking forces, and to internal temperature gradients. • To resist these tensile forces, steel bars must be embedded in the concrete, the so-called reinforcing bars, and this has lead to the development of reinforced concrete. • The steel bars only really come into play after the concrete cracks under tensile stresses. • If the reinforcing bars are correctly designed and placed, then these cracks remain as fine "hair cracks" and are harmless. A second method of resisting tensile forces in concrete structures is by prestressing. • The zones of concrete girders, which are under tensile stress due to loads or other actions, are first put under compression - are pre-compressed - so that the tensile forces must first reduce these compressive stresses before actual tensile stresses come into being. • This pre-compression is obtained by tensioning high strength steel bars or wire bundles, which are in ducts inside the concrete girder. • Tensioning elongates the steel bars and they are anchored in this state at the ends of the girder, transferring this tensioning force as a compressive force onto the girder. • These girders, prestressed with "active steel" (prestressing steel) are, in addition, reinforced with "passive steel" (non-stressed steel bars) for various reasons. • Prestressed concrete revolutionized the design and construction of bridges in the fifties. • With prestressed concrete, beams could be made more slender and span considerably greater distances than with reinforced concrete. • Prestressed concrete - if correctly designed - also has a high fatigue strength under the heaviest traffic loads. • Prestressed concrete bridges soon became much cheaper than steel bridges, and they need almost no maintenance - again, assuming that they are well-designed and constructed, and not exposed to de-icing salt. • So as from the fifties, prestressed concrete came well to the fore in the design of bridges. • There is one great disadvantage to concrete as it emerges from the forms: the inexpressive, dull grey color of the cement skin. • The surfaces frequently show stains, irregular streaks from placing the concrete in varying layers, and pores or even cavities from deficient compaction, which ire then patched more or less successfully. • These deficiencies have lead to a widespread aversion to concrete, as well as to efforts for improvement. • Some of the methods used to achieve a good concrete finish in buildings, like profiles and patterns on the formwork, ribs or accentuated timber veins, etc., are not generally suitable, and this means that high strength steels are not as ductile as those with normal strength. • The high strengths of steel allow small cross-sections of beams or girders, and therefore a low dead load of the structure. • It was thus possible to develop the light-weight "orthotropic plate" steel decks for roadways, which have now become common with an asphalt wearing course, 60 to 80mm thick.

__ Timber __ • Timber has favorable qualities of strength for resisting compression, tension and bending. • Rough tree trunks or sawn timber beams have been used since primitive times for beam bridges; raking frames and arches soon allowed larger spans. • Timber should be protected against rain and therefore, covered bridges with a roof and sidewalls with windows evolved, and many of these are rightly preserved in the Alpine countries, testifying to the high standard of their craftsmanship. • Many now only serve pedestrians. Recently, timber bridges have been given a new impetus by glue technology which allows larger cross-sections and larger lengths of beams to be made, rather than grow naturally. • Moreover, timber can now be better protected against weather and insect attack. • So, new possibilities have arisen for the choice of structure, for its shaping and for the size. • Large timber trusses and even folded space trusses have been built using steel gusset plates for jointing the members. • Timber bridges, however, have limits of span and carrying capacity, confining them mainly to bridges for pedestrians or for secondary roads.

__ Geographic Conditions of Bridges and their relevance! __

Depending on whether they’re built over wide or narrow canyons, on marshes or bays, or on areas that are prone to earthquakes. Some higher bridges may be built over ocean liners, rather than river over barges. Some may rest on bedrock towards the surface, or they could rest on bedrock under many, many feet of soft mud. The bridge type used could depend on the cost or building method used, which could vary, depending on loation whether country or state. Different types of materials could possibly also be used depending on how sturdy or not the surrounding ground area is. For example, for soft quicksand or quicksand like soils, caisson- or large, bottomless boxes or cylinders- may be used. Then, once the caisson is placed and the soil removed, the caisson is filled with concrete.

__ Climate Conditions on Bridges __ In warm parts of the world, suspension bridges were built using ropes of vines and grasses. In the Himalayas, suspension bridges were also used, but they were made from rocks thrown across them and thin ropes hung across them to carry the road. The cantilever bridge originated in India because that was the right climate for that bridge at the time. The Romans were great bridge builders because they knew what climate to build them in.

__ Tacoma Narrows Bridge __ The Tacoma Narrows Bridge opened on July 1, 1940. There was a vertical movement of the deck during windy conditions and that's how it got it's nickname. The bridge collapsed on November 7, 1940 due to high wind conditions. After it collapsed there was a replacement bridge that they built. The replacement bridge opened on October 14, 1950. This bridge stills stands today. At first the wind made the bridge sway back and forth while cars were on it. The wind started to get worse and made the bridge sway even harder until it finally collapsed. The builders who built this bridge must not have paid attention to the climate conditions while building it. When building a bridge it is very important to pay attention to things like climate or else the bridge might collapse like this one did.