PRESTRESSED CONCRETE.pdf

CE -515: Design of Steel Structures

CE-505: Prestressed Concrete Design M. Engg. (Civil), Fall 2016 2016 Lecture 01-02: Introduction Introduction

CE -505: Prestressed Concrete Design

Fall 2016 CE -515: Design of Steel Structures

Course Outline Basic concepts of prestressed concrete, Systems of prestressing, prestressing, materials. Partial prestressing, prestressing, prestress prestress losses. Use of high strength concret concrete. e. Structural behaviour behaviour of Beams for Elastic and Ultimate ranges for Bending and Shear. Moment curvature relationship, Camber and deflections. Detailed Detailed design of simple and continuous beams for Service and Ultimate loads. Design of End Anchorages. Determination Determination of Cable layout. Construction techniques. Precast and in-situ preprestressed concrete concrete members. Applications to special structures.

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Recommended Books •

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Nilson, A. H. (1987). Design of Prestressed Concrete. John Wiley & Sons Nawy, E. G. (2010). Prestressed Concrete: A Fundamental Approach, Fifth Edition. Prentice Hall Naaman, A. E. (2012). Prestressed Concrete Analysis and Design: Fundamentals , Third Edition. Techno Press 3000



Evaluation & Assessment •

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Sessional evaluation –

Assignments (10)

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Mid-term test (10)

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Final test (10)

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Presentation (10)

Final exam

(40 marks)

(60 marks)

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Introduction •

Concrete is strong in compression, but weak in tension.

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Flexural cracks develop at early stages of loading.

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In order to reduce or prevent such cracks from developing, a concentric or eccentric force is applied in the longitudinal direction of the structural element. This force prevents the cracks by eliminating or considerably reducing tensile stresses at critical sections thus raising the capacities. Such an imposed longitudinal force is called prestressing force , a compressive force that prestresses the sections along the span of the structural element prior to the application of the transverse gravity dead and live loads.



Introduction Linear Prestressing Since the prestressing force is applied longitudinally along or parallel to the axis of the member, the prestressing principle involved is commonly known as linear prestressing.

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Introduction Circular Prestressing Used in liquid containment tanks, pipes, and pressure reactor vessels, essentially follows the same basic principles as linear prestressing. The circumferential hoop stress on the cylindrical or spherical structure, neutralizes the tensile stresses at the outer fibres of the curvilinear surface caused by the internal pressure.



Comparison with Reinforced Concrete •

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In prestressed concrete, permanent stresses are created in the structural member before the application of full dead and live loads. The objective is to reduce or eliminate the net tensile stressed caused by these loads. In reinforced concrete, net tensile strength is neglected. Tensile forces resulting from bending are resisted by bond created in reinforcement process. Cracking and deflection are irrecoverable in RC once the member has reached its limit state at service load.

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Comparison with Reinforced Concrete...

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Reinforcement in RC member does not exert any force of its own on the member, contrary to the action of prestressing steel. The steel required to produce the prestressing force in the member actively preloads the member, permitting a relatively high controlled recovery of cracking and deflection. Once the flexural tensile strength of the concrete is exceeded, the prestressed member starts to act like a RC element.



Economics of Prestressed Concrete •

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Prestressed members are shallower in depth than their RC counterparts for the same span and loading conditions. In general, the depth of a prestressed concrete member is usually about 65 to 80 % of the depth of the equivalent RC member. PC member requires less concrete, and about 20 to 35 % of the amount of reinforcement. This saving in material weight is balanced by the higher cost of materials needed in prestressing. Also regardless of the system used, prestressing operations themselves result in added cost. Formwork is more complex, since the geometry of PC sections is usually composed of flanged sections with thin webs.

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Economics of Prestressed Concrete •

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In spite of these additional costs, if a large enough number of precast units are manufactured, the difference between at least the initial costs of PC and RC systems is usually not very large. The indirect long-term savings are quite substantial, because: –

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less maintenance is needed, Longer working life is possible due to better quality control of concrete, Lighter foundations are achieved due to the smaller cumulative weight of the superstructure.

Once the beam span of RC exceeds 70 to 90 ft, the dead load becomes excessive, resulting in heavier members and, consequently, greater long-term deflection and cracking. For longer spans, PC becomes mandatory. Very large spans such as segmental bridges or cable-stayed bridges can only be constructed through the use of prestressing.



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Historical Development •

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Prestressing concept dates back to 1872, when P.H. Jackson, an engineer from California, patented a prestressing system that used a tie rod to construct beams or arches from individual blocks. In 1888, C.W. Doehring of Germany obtained a patent for prestressing slabs with metal wires. These early attempts were not successful because of loss of prestress with time. J. Lund of Norway and G.R. Steiner of the US tried early in the twentieth century to solve this problem, but did not succeed.



Historical Development… •

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Unavailability of high-strength steel to overcome prestress loss was the main hurdle. R.E. Dill of Nebraska, recognized the effect of shrinkage and creep of concrete on the loss of prestress. He subsequently developed the idea that successive posttensioning of unbonded rods would compensate for the time-dependent loss of stress in the rods due to decrease in the length of the member because of creep and shrinkage. In 1920s, W.H. Hewett of Minneapolis developed the principles of circular prestressing.

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Linear prestressing continued to develop in Europe and in France. Eugene Freyssinet proposed in 1926 through 1928 methods to overcome prestress losses through the use of high-strength and high-ductility steels. In 1940, he introduced the Freyssinet system comprising the conical wedge anchor for 12-wire tendons. During and after the World War II, it became necessary to reconstruct bridges destroyed by war. G. Magnel of Belgium and Y. Guyon of Paris extensively developed and used the concept of prestressing for the design and construction of bridges in Europe.




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P.W. Abeles of England introduced the concept of partial prestressing between the 1930s and 1960s. F. Leonhardt of Germany, V. Mikhailov of Russia, and T.Y. Lin of the US also contributed a great deal to the art and science of the design of prestress concrete. Lin’s load-balancing method simplified the design process, particularly in continuous structures.

Today, prestressed concrete is used in buildings, underground structures, towers, floating storage and offshore structures, power stations, nuclear reactor vessels, and numerous types of bridge systems including segmental and cable-stayed bridges.

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Concept of Prestressing

Concrete fibre stress distribution in a rectangular beam with straight tendon. (a) Concentric tendon, prestress only, (b) Concentric tendon, self-weight added .



Concept of Prestressing… The compressive stress on the beam cross-section is uniform and has an intensity (1.1)

(1.2a)

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(1.2b)

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Concept of Prestressing…

Concrete fibre stress distribution in a rectangular beam with straight tendon. (c) Eccentric tendon, prestress only, (b) Eccentric tendon, self-weight added.



Concept of Prestressing… If the tendon is placed at eccentricity e from the centre of gravity of the concrete (cgc line), it creates a moment Pe. The stresses at the mid-span become (1.3a)

(1.3b) Since the support section of a simply supported beam carries no moment from the external transverse load, high tensile stresses at the top fibres are caused by the eccentric prestressing force. To limit such stresses, eccentricity of the prestressing tendon profile is reduced at the support section than at the midspan section.

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Stress Control by Prestressing

Consider the plain, unreinforced concrete beam, which carries a single concentrated load at the centre resulting in flexural stresses. At a relatively low load, the tensile stress in the bottom of the member will reach the tensile strength of the material f r ., and a crack will form. Since no restraint is provided against upward extension of the crack, the member will collapse without further increase of load.




Now consider identical beam, in which a longitudinal axial force P is introduced prior to vertical loading. The longitudinal prestressing force will produce a uniform axial compressive stress. This force can be adjusted in magnitude, so that, when the transverse load Q is applied, the superposition of stresses due to P and Q will result in zero tensile stress at the bottom of the beam.

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It would be more logical to apply the prestressing force near the bottom. The force P with same value but applied with eccentricity e = h / 6 relative to concrete centroid, will produce a longitudinal compressive stress distribution varying from zero at the top surface to a maximum of 2 f c at the bottom. The stress at bottom will be twice as compared to axial prestressing, consequently, the transverse load may now be twice.



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Stress Control by Prestressing The transverse load produced moment that varies along the span from zero at the supports to maximum at the centre. The best arrangement for prestressing would be to pro duce counter moment, acting in the opposite sense. This can be d one by using varying eccentricity. For each characteristic load arrangement, there is “best” tendon profile in the sense that it produces a prestress moment diagram that corresponds to that of applied load. If the prestressing counter moment is made exactly equal and opposite to the moment from the loads, the beam is subjected to axial compressive stresses. There will be no cracking and deflection in the member.



Equivalent loads

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Equivalent loads



Overload Behaviour and Strength in Flexure

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Partial Prestressing •

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Complete elimination of tensile stresses in members at normal service loads is called Full prestressing. In Partial prestressing , tension in concrete and some flexural cracking is permitted at full service load. Full prestressing offers possibility of complete elimination of cracks at full service load, it may produce members with large camber.



Prestressing Methods •

Pretensioning

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Prestressing Methods Pretensioning: •

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Tendon usually in the form of multiple-wire stranded cables, are stretched between abutments that are a permanent part of plant facility. With the forms in place, the concrete is cast around the stressed tendon. After sufficient strength is attained, the jacking pressure is released. The prestressed force is transferred to concrete by bond and no special anchorage is needed. Pretensioning is suited to mass production of beams.



Prestressing Methods •

Post-tensioning

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Prestressing Methods Post-tensioning: •

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In post-tensioning, hollow conduits containing the unstressed tendons are placed in the beam forms before pouring concrete. When sufficient strength is gained, the concrete beam itself is used to provide reaction for the stressing jack. With the tendon anchored by special fittings at the far end of the member, it is stretched, then anchored at the jacking end by similar fittings and the jack removed. Tendons are normally grouted in the conduits after they are stressed.



Changes in Prestress Force •

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The magnitude of prestress force does not remain constant but assumes different values during the life of the member. Some changes are: –

Instantaneous

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Time dependant

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Function of superimposed loading

All changes must be accounted for the in the design. Early attempts to prestressed concrete were not successful mainly due to neglect of time-dependant losses.

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Generally, the greatest force that the member is subjected to is the jacking force. For a post-tensioned member, this force is applied as a reaction directly upon the concrete member. With pretensioning, the jacking force reacts against external anchorages and does not act on the concrete at all. At the moment of transfer of prestress force from jack to the achorage fittings that grip the tendon, there is immediate reduction in force. There is small amount of slip at the achorages.




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There is instantaneous stress loss because of the elastic shortening of concrete as the prestressing force is transferred to it. This always occurs in pretensioning, but occurs in post-tensioning only if there are two or more tendons tensioned sequentially. In post-tensioned members, friction between steel and conduit through which it passes results in immediate loss of prestressing force. Due to these immediate losses, jacking force is reduced to a lower value, defined as initial prestress force . With the passage of time, the steel stress is further reduced. Shrinkage and creep in concrete also result in time-dependant losses. The initial prestressed force is reduced to effective prestress force .

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PRESTRESSED CONCRETE.pdf

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