Monthly Archive:: September 2011

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
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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
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Basic Principle of Prestressing

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 Concrete is very strong in compression but weak in tension. In an ordinary concrete beam the tensile stress at the bottom: are taken by standard steel reinforcement: But we still get cracking, which is due to both bending and shear: In prestressed concrete,
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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.
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Highrise Building Failure

House of CardsOn 16th May, 1968, Mrs Ivy Hodge, a council tenant in a block of flats called Ronan Point in east London, wandered into the kitchen of
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