Design arches pdf

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Explore Documents. Uploaded by Patricia Guerzo Tumaliuan. Document Information click to expand document information Description: different arches.

Did you find this document useful? Is this content inappropriate? Report this Document. Description: different arches. The design and construction of arches Chanchal Kaushik. A short summary of this paper. Download Download PDF. Translate PDF. Arches which work principally in compression, are among the most beautiful of structures, effortlessly spanning great distances.

However, arches can be labour intensive to build, and consequently had somewhat fallen out of favour. More recently, the growing awareness of the concept of sustainable development has renewed interest in this form of construction that minimises the materials employed, and has stimulated the search for more economical methods of construction.

Most other types of bridge require side spans either side of a main span, which extends the costly long span technology over additional length. Large-span stone and concrete arches have been used very extensively in China, where it is reported that some 70 per cent of all highway bridges are of this form of construction [1].

Chinese engineers have developed a variety of innovative methods of construction. Any loads applied to an arch will create a line of thrust. If the arch is made exactly the same shape as the line of thrust, it can carry the loads without bending moments. The shape of the line of thrust may be visualised by applying the loads to a weightless suspended cable, Figure The shape of this loaded cable is known as a funicular diagram.

Clearly, the sag of the loaded cable, and its tension, will depend on how taut it was before application of the loads. This is an exact analogy to the line of thrust, Figure The design and construction of arches Figure If the arch has hinges in its length, the bending moment must be zero at the hinge positions, which means that the line of thrust must pass through those points. If the arch has three hinges, typically at the springings and at the crown, it is statically determinate.

In this case, the line of thrust is also determinate, and can be drawn, superimposed on the arch. The line of thrust shown in Figure The thrust at the arch springings and the compression at any point along the arch may then be calculated by simple statics. The bending moment at any point of the arch may also be calculated by taking moments about that point, or by measuring the distance between the line of thrust and the neutral axis of the arch.

If the arch has less than three hinges, and is consequently indeterminate, the line of thrust may not be drawn precisely without elastic analysis. The correct shape for an arch carrying a uniformly distributed vertical load is a parabola.

For an arch of constant thickness carrying only its self weight, the correct shape is a catenary which is very close to a parabola. For an arch carrying its self weight and external loads, the best shape is a compromise between a catenary and the funicular diagram of the loads. Where the line of thrust is within the middle Figure If the line of thrust is locally outside the middle third, one face of the arch ring at that section will be in tension, and the arch will crack. In stone or brick arches these cracks usually occur in the joints and may not be noticed.

If the line of thrust touches or crosses the intrados or extrados of the arch, a hinge will form at those points. When four hinges form, the arch will become a mechanism and fail. For masonry arches the cause of failure is almost always the mechanical instability described, not the inadequate strength of the masonry.

An unreinforced concrete arch will behave very much as a masonry arch. The arch is not threatened, but the cracks may be unsightly. Masonry arch bridges have been built with spans up to m, such as the Wuchao River Bridge in Hunan Province in China. The line of thrust for a uniform load, for instance, will be parabolic. Clearly the line of thrust will be outside the middle third over much of the length, and the structure will crack. The material above and below this incorporated arch is redundant, and could be removed, without affecting the performance of the arch.

Figure They do not depend on their thickness and shape to give them Figure However, their economy depends on their shape being close to that of the line of thrust. A two-pinned portal may be seen as a crude approximation to an arch. When subjected to a uniform load, the superimposition of the parabolic line of thrust on the portal illustrates graphically the bending moments that are proportional to the distance between its neutral axis and the line of thrust, Figure The loads are carried by a combination of arching action that are represented by the thrust at the arch feet, and bending.

At the same time the thrust T will increase as the rise of the line of thrust decreases, and as all the loads are carried in arching action. If solid abutments are available, the arch may be simply an economical replacement for a slab roof. The mm thick arch spanned 9. The arch is three-centred, that is it had a larger radius over the central section than over the Figure Over most of its length it rested on abutments cut into massive sandstone.

As the hard rock fell away, it was replaced by unreinforced concrete walls, either dressing the soft rocks of the coal measures, or for short lengths acting as cantilever retaining walls. The arch was cast in-situ on a set of steel travelling shutters, Figure Such an arch needs only light reinforcement, and the bars should if possible be of small diameter such that they will lie to the arch radius without pre-bending, preferably under their own weight.

Two alternative designs carried out by Benaim demonstrate the economy of reinforced concrete arches. Both carried the M74 motorway in Scotland on very skew crossings of twin-track railways, Figure The arch thickness for New Cowdens was mm, and mm for Maryville. The greater thickness of the former was adopted to allay concerns about the impact resistance of the structure in the event of a derailment. These alternatives consumed approximately one-third of the materials of the conforming designs they replaced, Figure The precast arches were erected in a matter of days, and once in place they protected the railway from the remaining construction activities.

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