Improved method for deflection control of reinforced concrete structures

[Н. Пецић]

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Грађевинарство и геодезија-Бетонске конструкције / Civil engineering-Concrete structures

Напредак у развоју алата за пројектовање грађевонских конструкција није пратила одговарајуће подршка у области провере деформација армирано-бетонских елемената...

Development of design tools for reinforced concrete is not followed with convenient procedures for the deflection check. Most of the software using finite element calculation does not support proper evaluation (including effects of cracking, creep and shrinkage) of the deflection of concrete structures. Use of high strength materials enables reduction of the size of structural elements. As a result, structures become more deformable and it is necessary to check deflection. Simplified and refined methods are usually presented by most of design codes or recommendations for the deflection check. The simplified methods provide faster and easier calculation. They are generally on the safe side and require enlarged dimensions of structural elements. The refined methods involve relevant properties of concrete, environmental conditions and construction schedule, allowing for optimization of the size of structural elements. Apart from not being suitable for hand calculation, they often require some additional knowledge. Eurocode 2 (EN 1992-1-1:2004) also provides two methods for the deflection check. Simplified criterion is in a form of span-to-depth ratio limit. This tool has serious limits. It is not well prepared for practical use (it is derived for an unsuitable ratio of the quasi-permanent to the ultimate load; it also does not include the compressive reinforcement other then required for ULS). The second, rigorous method is based on general approach for deflection calculation - integration of the curvatures along the element. The effective modulus method is used for calculation of long-term effects due to creep and shrinkage of concrete. Influence of concrete cracking to the stiffness is introduced by interpolation coefficient according to CEB-FIP Model code 1990. This more refined method seems to be easy applicable due to lack of the very important instruction: bending moment diagram of the statically indeterminate structures, resulting from an ordinary ULS analysis based on concrete gross sections, should be redistributed to account for effects of cracking, creep and shrinkage. This option is not usually supported by common engineering software and task becomes heavy. Evaluation of the redistributed diagram requires a time-dependant stiffness matrix and an iterative calculation following appearance of cracks...

армирани бетон, угиб, дуготрајно оптерећање, Еврокод 2, прерасподела

reinforced concrete, deflection, long-term, Eurocode 2, redistribution

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Armiranobetonske konstrukcije

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