Prestressed Concrete Colin C Caprani-Books Pdf

Prestressed Concrete Colin C Caprani
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Civil Engineering Design 1, 1 Introduction 3, 1 1 Background 3. 1 2 Basic Principle of Prestressing 4, 1 3 Advantages of Prestressed Concrete 6. 1 4 Materials 7, 1 5 Methods of Prestressing 10, 1 6 Uses of Prestressed Concrete 15. 2 Stresses in Prestressed Members 16, 2 1 Background 16. 2 2 Basic Principle of Prestressed Concrete 19, 3 Design of PSC Members 29.
3 1 Basis 29, 3 2 Minimum Section Modulus 32, 3 3 Prestressing Force Eccentricity 37. 3 4 Eccentricity Limits and Tendon Profile 49, 4 Prestressing Losses 56. 4 1 Basis and Notation 56, 4 2 Losses in Pre Tensioned PSC 57. 4 3 Losses in Post tensioned PSC 60, 5 Ultimate Limit State Design of PSC 66. 5 1 Ultimate Moment Capacity 66, 5 2 Ultimate Shear Design 71.
2 Dr C Caprani, Civil Engineering Design 1, 1 Introduction. 1 1 Background, The idea of prestressed concrete has been around since the latter decades of the 19th. century but its use was limited by the quality of the materials at the time It took until. the 1920s and 30s for its materials development to progress to a level where. prestressed concrete could be used with confidence Freyssinet in France Magnel in. Belgium and Hoyer in Germany were the principle developers. The idea of prestressing has also been applied to many other forms such as. Wagon wheels, Barrels i e the coopers trade, In these cases heated metal is made to just fit an object When the metal cools it. contracts inducing prestress into the object, 3 Dr C Caprani. Civil Engineering Design 1, 1 2 Basic Principle of Prestressing.
Basic Example, The classic everyday example of prestressing is this a row of books can be lifted by. squeezing the ends together, The structural explanation is that the row of books has zero tensile capacity. Therefore the beam of books cannot even carry its self weight To overcome this we. provide an external initial stress the prestress which compresses the books together. Now they can only separate if the tensile stress induced by the self weight of the. books is greater than the compressive prestress introduced. Concrete is very strong in compression but weak in tension In an ordinary concrete. beam the tensile stress at the bottom, 4 Dr C Caprani. Civil Engineering Design 1, are taken by standard steel reinforcement. But we still get cracking which is due to both bending and shear. In prestressed concrete because the prestressing keeps the concrete in compression. no cracking occurs This is often preferable where durability is a concern. 5 Dr C Caprani, Civil Engineering Design 1, 1 3 Advantages of Prestressed Concrete.
The main advantages of prestressed concrete PSC are. Smaller Section Sizes, Since PSC uses the whole concrete section the second moment of area is bigger and. so the section is stiffer, Smaller Deflections, The larger second moment of area greatly reduces deflections for a given section size. Increased Spans, The smaller section size reduces self weight Hence a given section can span further. with prestressed concrete than it can with ordinary reinforced concrete. Durability, Since the entire section remains in compression no cracking of the concrete can. occur and hence there is little penetration of the cover This greatly improves the. long term durability of structures especially bridges and also means that concrete. tanks can be made as watertight as steel tanks with far greater durability. 6 Dr C Caprani, Civil Engineering Design 1, 1 4 Materials.
The main factors for concrete used in PSC are, Ordinary portland cement based concrete is used but strength usually greater. than 50 N mm2, A high early strength is required to enable quicker application of prestress. A larger elastic modulus is needed to reduce the shortening of the member. A mix that reduces creep of the concrete to minimize losses of prestress. You can see the importance creep has in PSC from this graph. 7 Dr C Caprani, Civil Engineering Design 1, The steel used for prestressing has a nominal yield strength of between 1550 to 1800. N mm2 The different forms the steel may take are, Wires individually drawn wires of 7 mm diameter. Strands a collection of wires usually 7 wound together and thus having a. diameter that is different to its area, Tendon A collection of strands encased in a duct only used in post.
tensioning, Bar a specially formed bar of high strength steel of greater than 20 mm. Prestressed concrete bridge beams typically use 15 7 mm diameter but with an area. of 150 mm2 7 wire super strand which has a breaking load of 265 kN. 8 Dr C Caprani, Civil Engineering Design 1, 9 Dr C Caprani. Civil Engineering Design 1, 1 5 Methods of Prestressing. There are two methods of prestressing, Pre tensioning Apply prestress to steel strands before casting concrete. Post tensioning Apply prestress to steel tendons after casting concrete. Pre tensioning, This is the most common form for precast sections In Stage 1 the wires or strands are.
stressed in Stage 2 the concrete is cast around the stressed wires strands and in. Stage 3 the prestressed in transferred from the external anchorages to the concrete. once it has sufficient strength, 10 Dr C Caprani, Civil Engineering Design 1. In pre tensioned members the strand is directly bonded to the concrete cast around it. Therefore at the ends of the member there is a transmission length where the strand. force is transferred to the concrete through the bond. At the ends of pre tensioned members it is sometimes necessary to debond the strand. from the concrete This is to keep the stresses within allowable limits where there is. little stress induced by self with or other loads, 11 Dr C Caprani. Civil Engineering Design 1, Post tensioned, In this method the concrete has already set but has ducts cast into it The strands or. tendons are fed through the ducts Stage 1 then tensioned Stage 2 and then. anchored to the concrete Stage 3, The anchorages to post tensioned members must distribute a large load to the. concrete and must resist bursting forces as a result A lot of ordinary reinforcement is. often necessary, A typical tendon anchorage is, 12 Dr C Caprani.
Civil Engineering Design 1, And the end of a post tensioned member has reinforcement such as. 13 Dr C Caprani, Civil Engineering Design 1, From the time the prestress is applied the prestress force gradually reduces over time. to an equilibrium level The sources of these losses depend on the method by which. prestressing is applied, In both methods, The member shortens due to the force and this relieves some of the prestress. The concrete shrinks as it further cures, The steel relaxes that is the steel stress reduces over time. The concrete creeps that is continues to strain over time. In post tensioning there are also losses due to the anchorage which can draw in an. amount and to the friction between the tendons and the duct and also initial. imperfections in the duct setting out, For now losses will just be considered as a percentage of the initial prestress.
14 Dr C Caprani, Civil Engineering Design 1, 1 6 Uses of Prestressed Concrete. There are a huge number of uses, Railway Sleepers, Communications poles. Pre tensioned precast hollowcore slabs, Pre tensioned Precast Double T units for very long spans e g 16 m span for. Pre tensioned precast inverted T beam for short span bridges. Pre tensioned precast PSC piles, Pre tensioned precast portal frame units. Post tensioned ribbed slab, In situ balanced cantilever construction post tensioned PSC.
This is glued segmental construction, Precast segments are joined by post tensioning. PSC tank precast segments post tensioned together on site Tendons around. circumference of tank, And many more, 15 Dr C Caprani. Prestressed concrete bridge beams in prestressed concrete Most of the work of PSC design involves ensuring Basic Principle of Prestressed Concrete

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