The principle of pre-stressed concrete

Размещено на http://www.allbest.ru/

ABCTRACT

Prestressed concrete - a building material, concrete is designed to overcome the inability to resist significant stretching stresses. Construction of the prestressed concrete as compared to unstressed have a significantly smaller deflections and increased fracture toughness, having the same strength that allows you to cover large spans with equal cross-section element. In the manufacture of concrete laid armature of steel with high tensile strength, steel is then stretched with a special device and the concrete mixture is placed. After setting the pre-tensioning force of liberated steel wire or rope is passed to the surrounding concrete, so that it is compressed. This allows the creation of compressive stress is partially or completely eliminate the tensile stress on the operational load.

CONTENTS

INTRODUCTION

CHAPTER 1. PRESTRESSED CONCRETE

CHAPTER 2. OPERATION AND APPLICATION OF THE PRINCIPLE OF PRE-STRESSED CONCRETE

CONCLUSION

REFERENCES

APPENDIX А

INTRODUCTION

Prestressed concrete is a method for overcoming concrete's natural weakness in tension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. It is often used in commercial and residential construction as a foundation slab. Prestressing tendons (generally of high tensile strength steel cable or rods) are used to provide a clamping load which produces a compressive stress that balances the tensile stress that the concrete compression member would otherwise experience due to a bending load. Traditional reinforced concrete is based on the use of steel reinforcement bars, rebars, inside poured concrete. Prestressing can be accomplished in three ways: pre-tensioned concrete, and bonded or unbonded post-tensioned concrete.

Although prestressed concrete was patented by a San Francisco engineer in 1886, it did not emerge as an accepted building material until a half-century later. The shortage of steel in Europe after World War II coupled with technological advancements in high-strength concrete and steel made prestressed concrete the building material of choice during European post-war reconstruction. North America's first prestressed concrete structure, the Walnut Lane Memorial Bridge in Philadelphia, Pennsylvania, however, was not completed until 1951.

prestressed concrete construction voltage

The use of pre-stressed concrete today is not very widely used, due to the small study in this area. As for the construct of buildings with such designs require high quality of construction works, which in turn requires a specialized highly qualified personnel.

This essay describes the principles and the variety of prestressed structures, especially the application and a brief historical background on the use of them in practice.

CHARTER 1. PRESTRESSED STRUCTURE

Prestressed structure is the one whose overall integrity, stability and security depend, primarily, on a prestressing. Prestressing means the intentional creation of permanent stresses in a structure for the purpose of improving its performance under various service conditions.

There are the following basic types of prestressing:

· Precompression (mostly, with the own weight of a structure)

· Pretensioning with high-strength embedded tendons

· Post-tensioning with high-strength bonded or unbonded tendons

Today, the concept of prestressed structure is widely employed in design of buildings, underground structures, TV towers, power stations, floating storage and offshore facilities, nuclear reactor vessels, and numerous kinds of bridge systems. The idea of prestressing was, apparently, familiar to the ancient Rome architects; the tall attic wall of the Colosseum works as a stabilizing device for the wall piers beneath.

In conventional reinforced concrete, the high tensile strength of steel is combined with concrete's great compressive strength to form a structural material that is strong in both compression and tension. The principle behind prestressed concrete is that compressive stresses induced by high-strength steel tendons in a concrete member before loads are applied will balance the tensile stresses imposed in the member during service.

Prestressing removes a number of design limitations conventional concrete places on span and load and permits the building of roofs, floors, bridges, and walls with longer unsupported spans. This allows architects and engineers to design and build lighter and shallower concrete structures without sacrificing strength.

Pre-tensioned concrete is cast around steel tendons--cables or bars--while they are under tension. The concrete bonds to the tendons as it cures, and when the tension is released it is transferred to the concrete as compression by static friction. Tension subsequently imposed on the concrete is transferred directly to the tendons.

Pre-tensioning requires strong, stable anchoring points between which the tendons are to be stretched. Thus, most pre-tensioned concrete elements are prefabricated and transported to the construction site, which may limit their size. Pre-tensioned elements may be incorporated into beams, balconies, lintels, floor slabs or piles. An innovative bridge design pre-stressing is the stressed ribbon bridge.

CHAPTER 2. OPERATION AND APPLICATION OF THE PRINCIPLE OF PRE-STRESSED CONCRETE

Compressive stresses are induced in prestressed concrete either by pretensioning or post-tensioning the steel reinforcement.

In pretensioning, the steel is stretched before the concrete is placed. High-strength steel tendons are placed between two abutments and stretched to 70 to 80 percent of their ultimate strength. Concrete is poured into molds around the tendons and allowed to cure. Once the concrete reaches the required strength, the stretching forces are released. As the steel reacts to regain its original length, the tensile stresses are translated into a compressive stress in the concrete. Typical products for pretensioned concrete are roof slabs, piles, poles, bridge girders, wall panels, and railroad ties.

In post-tensioning, the steel is stretched after the concrete hardens. Concrete is cast around, but not in contact with unstretched steel. In many cases, ducts are formed in the concrete unit using thin walled steel forms. Once the concrete has hardened to the required strength, the steel tendons are inserted and stretched against the ends of the unit and anchored off externally, placing the concrete into compression. Post-tensioned concrete is used for cast-in-place concrete and for bridges, large girders, floor slabs, shells, roofs, and pavements.



Bonded post-tensioned concrete is the descriptive term for a method of applying compression after pouring concrete and during the curing process (in situ). The concrete is cast around a plastic, steel or aluminum curved duct, to follow the area where otherwise tension would occur in the concrete element.

A set of tendons are fished through the duct and the concrete is poured. Once the concrete has hardened, the tendons are tensioned by hydraulic jacks that react (push) against the concrete member itself.

When the tendons have stretched sufficiently, according to the design specifications (see Hooke's law), they are wedged in position and maintain tension after the jacks are removed, transferring pressure to the concrete. The duct is then grouted to protect the tendons from corrosion.

This method is commonly used to create monolithic slabs for house construction in locations where expansive soils (sometimes called adobe clay) create problems for the typical perimeter foundation. All stresses from seasonal expansion and contraction of the underlying soil are taken into the entire tensioned slab, which supports the building without significant flexure.

Post-tensioning is also used in the construction of various bridges, both after concrete is cured after support by falsework and by the assembly of prefabricated sections, as in the segmental bridge.

Unbonded post-tensioned concrete differs from bonded post-tensioning by providing each individual cable permanent freedom of movement relative to the concrete. To achieve this, each individual tendon is coated with a grease (generally lithium based) and covered by a plastic sheathing formed in an extrusion process. The transfer of tension to the concrete is achieved by the steel cable acting against steel anchors embedded in the perimeter of the slab. The main disadvantage over bonded post-tensioning is the fact that a cable can destress itself and burst out of the slab if damaged (such as during repair on the slab). The advantages of this system over bonded post-tensioning are:

· The ability to individually adjust cables based on poor field conditions (For example: shifting a group of 4 cables around an opening by placing 2 on each side).

· The procedure of post-stress grouting is eliminated.

· The ability to de-stress the tendons before attempting repair work.

Among the advantages of system over unbonded post-tensioning are:

· Large reduction in traditional reinforcement requirements as tendons cannot destress in accidents.

· Tendons can be easily "woven" allowing a more efficient design approach.

· Higher ultimate strength due to bond generated between the strand and concrete.

· No long term issues with maintaining the integrity of the anchor/dead end.

In prestressed concrete structures, compressive stresses in the concrete and tensile stresses in the reinforcement are usually induced. The use of high-strength steel permits significant savings (up to 70 percent) in the required amount of reinforced steel --cable, wire, or deformed bars. Prestressed structures are highly resistant to cracking. In comparison with conventional structures, they have much greater rigidity, and their ability to endure repeated loads is increased. Prestressed concrete structures are most efficient for buildings and engineering structures (for example, bridges) where the spans, loads, or working conditions are such that the use of structures with unstressed reinforcement involves significant technical difficulties or large outlays of concrete and steel. Prestressed concrete is also suitable for pressure pipes, tanks, silos, and other containers that must be impermeable.

Prestressed concrete has experienced greatest growth in the field of commercial buildings. For buildings such as shopping centers, prestressed concrete is an ideal choice because it provides the span length necessary for flexibility and alteration of the internal structure. Prestressed concrete is also used in school auditoriums, gymnasiums, and cafeterias because of its acoustical properties and its ability to provide long, open spaces. One of the most widespread uses of prestressed concrete is parking garages.

History remembers problems with bonded post-tensioned structures.The popularity of this form of prestressing for bridge construction in Europe increased significantly around the 1950s and 60s. However, a history of problems has been encountered that has cast doubt over the long-term durability of such structures.

Due to poor workmanship or quality control during construction, sometimes the ducts containing the prestressing tendons are not fully filled, leaving voids in the grout where the steel is not protected from corrosion. The situation is exacerbated if water and chloride (from de-icing salts) from the highway are able to penetrate into these voids.

Notable events are listed below:

The Ynys-y-Gwas bridge in West Glamorgan, Wales--a segmental post-tensioned structure, particularly vulnerable to defects in the post-tensioning system--collapsed without warning in 1985.

The Melle bridge, constructed in Belgium during the 1950s, collapsed in 1992 due to failure of post-tensioned tie down members following tendon corrosion.

Following discovery of tendon corrosion in several bridges in England, the Highways Agency issued a moratorium on the construction of new internal grouted post-tensioned bridges and embarked on a 5-year program of inspections on its existing post-tensioned bridge stock.

In 2000, a large number of people were injured when a section of a footbridge at the Charlotte Motor Speedway, USA, gave way and dropped to the ground. In this case, corrosion was exacerbated by calcium chloride that had been used as a concrete admixture, rather than sodium chloride from de-icing salts.

In 2011, the Hammersmith Flyover in London, England, was subject to an emergency closure after defects in the post-tensioning system were discovered.

Some of the earliest surviving bridges date from around the second Century BC. Typically, they were stone arches, a form that dominated bridge construction until the arrival of wrought iron and steel in the early 18th Century and, 150 years later, concrete. Most bridges were built by the church and two Renaissance stone bridges can still be seen in Paris - the Pont Notre Dame (1305) and the Pont Neuf (1606). It was in the 18th Century that bridge design began to develop into a science, led by an engineering school founded in Paris. Its director, Jean Perronet, perfected the masonry arch, with its low sweeping curve and slender piers. Soon afterwards, attention switched to England where the invention of the steam locomotive called for stronger bridges. In 1794, iron was first used for the chain cables of a suspension bridge over the River Tees and 1779 saw the first all-iron bridge over the Severn at Coalbrookdale. This arch bridge, spanning 100ft, is still in service.

Just when the masonry arch bridge was reaching its peak around the beginning of the 20th Century, reinforced concrete arrived on the scene. Since then, it has become the major construction material for bridges as it has for most structural and civil engineering applications, with its intrinsic versatility, design flexibility and, above all, natural durability. Although several British engineers had been using concrete early in the 19th Century, its use in British bridges did not develop until the latter half of the 20th Century. It is estimated that at least 75% of the Highways Agency concrete bridge stock has been built since 1960.

The earliest known example of a mass concrete bridge in the UK, using lime concrete, was on the District Line, near Cromwell Road, West London, designed by Thomas Marr Johnson for Sir John Fowler and built c.1865. Other British engineers began to use plain concrete for bridge superstructures, notably Philip Brannan, who erected a three-span concrete arch, including a 50ft middle span, at Seaton in Devon in 1877. The use of reinforced concrete probably started with the Homersfield Bridge over the River Waveney on the Norfolk/Suffolk border in 1870, when iron was embedded in concrete, but it was not until the first decade of the 20th Century that reinforcement, as we know it today, was introduced. This was due almost entirely to L.G. Mouchel, the UK agents for the Hennebique system. The first project in the UK was an 18ft span bridge at Chewton Glen in Hampshire in 1902, followed two years later by a 40ft span beam and slab bridge at Sutton Drain in Hull. By 1930 there were about 2000 reinforced concrete bridges in UK and notable designers such as Sir Owen Williams emerged between the two World Wars e.g. Montrose (1930). Other major bridges of this period were the Royal Tweed Bridge, Berwick (Mouchel); Chiswick and Twickenham River Thames Bridges (Considere); King George V Bridge, Glasgow and, possibly the best of the period, Waterloo Bridge, London. The outstanding feature of concrete bridges both during and after the Second World War was the advent of prestressed concrete, used to rebuild the many bridges that had been destroyed, especially on the Continent. By 1950, bridges by Freyssinet and Magnel had been built, using precast segments joined by concrete or mortar; Finsterwalder had constructed the first in-situ box girder bridge using cantilever construction and in the 1960s, the first incrementally launched bridge was built in Germany. Reinforced concrete was still being used in the 1950s for larger bridges, especially arches, notably Lune Bridge carrying the M6, but by the end of the 1960s, prestressed concrete had largely superseded reinforced concrete with box girders being the dominant structural form. Prestressed metal components are used in bridge spans, crane beams, masts, towers, power transmission line supports, and other engineering structures.

Prestressed structures are designed by the limiting state method, taking into account the actual physical and mechanical properties of the concrete and steel. It is assumed in the design that the induced stresses do not remain constant until service loads are applied. Losses of prestress may be caused by technological factors, such as heat treatment of products and structural members, or by such physical and mechanical properties of the concrete and steel as shrinkage and creep in the concrete and relaxation of stresses in the steel. Prestress may also be lost as a result of design characteristics of the prestressed structures and of the equipment used for tensioning the reinforcement. Such engineering factors may lead to loss of prestress caused by such factors as deformation of the anchors and friction between the tendons and the surface of the concrete in the channels and grooves.

The Sydney Opera House was built with the use of pre-stressed concrete. The shells of the competition entry were originally of undefined geometry, but, early in the design process, the "shells" were perceived as a series of parabolas supported by precast concrete ribs. However, engineers Ove Arup and Partners were unable to find an acceptable solution to constructing them. The formwork for using in-situ concrete would have been prohibitively expensive, and, because there was no repetition in any of the roof forms, the construction of precast concrete for each individual section would possibly have been even more expensive.



From 1957 to 1963, the design team went through at least 12 iterations of the form of the shells trying to find an economically acceptable form (including schemes with parabolas, circular ribs and ellipsoids) before a workable solution was completed. The design work on the shells involved one of the earliest uses of computers in structural analysis, to understand the complex forces to which the shells would be subjected. The computer system was also used in the assembly of the arches. The pins in the arches were surveyed at the end of each day, and the information was entered into the computer so the next arch could be properly placed the following day. In mid-1961, the design team found a solution to the problem: the shells all being created as sections from a sphere. This solution allows arches of varying length to be cast in a common mould, and a number of arch segments of common length to be placed adjacent to one another, to form a spherical section. With whom exactly this solution originated has been the subject of some controversy. It was originally credited to Utzon. Ove Arup's letter to Ashworth, a member of the Sydney Opera House Executive Committee, states: "Utzon came up with an idea of making all the shells of uniform curvature throughout in both directions."Peter Jones, the author of Ove Arup's biography, states that "the architect and his supporters alike claimed to recall the precise eureka moment ... ; the engineers and some of their associates, with equal conviction, recall discussion in both central London and at Ove's house."

He goes on to claim that "the existing evidence shows that Arup's canvassed several possibilities for the geometry of the shells, from parabolas to ellipsoids and spheres." Yuzo Mikami, a member of the design team, presents an opposite view in his book on the project, Utzon's Sphere. It is unlikely that the truth will ever be categorically known, but there is a clear consensus that the design team worked very well indeed for the first part of the project and that Utzon, Arup, and Ronald Jenkins (partner of Ove Arup and Partners responsible for the Opera House project) all played a very significant part in the design development.

The design of the roof was tested on scale models in wind tunnels at Southampton University and later NPL in order to establish the wind-pressure distribution around the roof shape in very high winds, which helped in the design of the roof tiles and their fixtures.

The shells were constructed by Hornibrook Group Pty Ltd, who were also responsible for construction in Stage III. Hornibrook manufactured the 2400 precast ribs and 4000 roof panels in an on-site factory and also developed the construction processes. The achievement of this solution avoided the need for expensive formwork construction by allowing the use of precast units (it also allowed the roof tiles to be prefabricated in sheets on the ground, instead of being stuck on individually at height). Ove Arup and Partners' site engineer supervised the construction of the shells, which used an innovative adjustable steel-trussed "erection arch" to support the different roofs before completion. On 6 April 1962, it was estimated that the Opera House would be completed between August 1964 and March 1965

CONCLUSION

Prestressing has been used throughout history, in conjunction with most construction materials. Prestressing may be used to achieve functionality, improve structural performance, simplify connections and to preclude reversals of stress. It is often useful for structural intervention, to stabilize and strengthen systems. In the context of space trusses, the conceptualization of new tensegric topologies remains challenging. The choice between hypostatic and hyperstatic prestressed forms and reliance on geometrically non-linear behaviour are producing innovative designs. Conceptualization of practical erection and prestressing procedures that give desired prestress states is essential and often difficult. Verifying the static stability, stiffness, strength and reliability of prestressed systems can require advanced engineering modelling and analyses. And in particular, the dynamic behaviour of geometrically non-linear systems is poorly understood.

REFERENCES

1. Nilson, Arthur H. Design of Prestressed Concrete./ John Wiley & Sons // New York 1987.

2. Nawy, Edward G. Prestressed Concrete. / Prentice Hal // 1989.

3. Crom, J, Prestressed reinforcement for domed concrete tanks / Engineering News-Record // 1936 .

4. Rosov, I, Prestressed reinforced concrete and its possibilities for bridge construction / ASCE Transactions // New York 1938.

5. N. Krishna Raju , Prestressed concrete / Tata McGraw-Hill Education // 1986.

APPENDIX А

1. Concrete - Бетон

2. Post-and-lintel construction - балочно-стоечная конструкция; балочно-стоечный каркас.

3. Prestressed concrete - преднапряженный бетон

4. reinforcement - армаматура , усиление

5. fittings - арматура

6. structure - здание, конструкция

7. mortar-раствор

8. raw materials-сырье

9. concrete mixer- бетономешалка10. arch-арка

10. dismantling демонтаж

11. drain дренаж

12. bow; arc - дуга

13. greace - смазка

14. Precompression - предварительное сжатие

15. poor mixture тощая смесь

16. overload-перегрузка

17. porous - пористый

18. framework-основа,каркас,рама

19. wrought iron сварное железо

20. cast steel-литая сталь

21. reinforcement metal- железная арматура

22. roofing steel-кровельная сталь

23. roof - кровля

24. steel - сталь

25. adjacent ~ пристройка

26. cost -смета

27. cantilever ~ консоль

28. armature frameworks - каркас арматурные

29. hollow brick - пустотелый кирпич

30. Full brick - полнотелый кирпич

31. plinth wall -- цоколь

32. vault -- свод

33. dome - купол

34. veranda(h) -- веранда

35. vestibule -- вестибюль, притвор, тамбур

36. support -- опора

37. tectonics -- тектоника

38. string, stringer -- косоур

39. string-board -- тетива

40. roof -- крыша

41. roofing -- кровля, покрытие

42. plllar -- колонна, пилон

43. grill, grillage -- ростверк

44. front -- фасад

45. column -- колонна, столп

46. asbestos cement - асбоцемент

47. Atrium- атриум

48. Superintendent - прораб

49. bonded - связанный

50. unbonded - несвязанный

51. high-strength concrete - высокопрочный бетон

52. Pretensioning - предварительное натяжение

53. Precompression - Предварительное сжатие

54. Post-tensioning - постнатяженный

55. compressive stresses - сжимающие напряжения

56. steel tendons- Стальные сухожилия(тросы)

57. ribbon bridge- ленточный мост

58. abutment- упор

59. concrete hardens- твердение бетона

60. compression- сжатие

61. monolithic - монолитный

62. underlying soil - нижележащие слои почвы

63. steel anchors - стальные анкеры

64. maintaining - поддержание

65. internal structure - Внутренняя структура

66. open spaces - открытые пространства

67. poor workmanship - недостаточная квалификация

68. quality control during construction- контроль качества в процессе производства

69. tendon corrosion - коррозия арматуры (сухожилий)

70. precast segments - сборные сегменты

71. basement- фундамент, основание, подвал

72. wall - стена

73. dead load - критическая нагрузка

Размещено на Allbest.ru

...