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...
