In the case of strong earthquakes, the ability of inelastic response of ductile reinforced concrete
buildings is of great importance in order to assure adequate collapse risk of structures, which are
designed with consideration of the capacity design principles and the design force, which is obtained
by taking into account post-cracking stiffness of structural elements. However, there is no uniform
criteria for consideration of effective stiffness of structural elements. In this graduation thesis, several
approaches for determination of effective shear and flexural stiffness are presented and demonstrated
by means of an example of a six-storey reinforced concrete building. Nine different models of the
structure were evaluated considering various seismic design codes, two of which reflected the fact that
flexural cracking varies along the element length. In the later cases, the effective stiffness of the
elements corresponded either to the initation of yielding of the reinforcement or to the estimated
seismic response of the structure. Therefore, an iterative procedure was required. It is shown, that the
results using uniform stiffness reduction throughout entire structure have small variation and that the
results for model, where effective inertia is calculated relative to centroid of uncracked section, are not
representative. Calculating effective stiffness relative to centroid of cracked sections reflects in higher
stiffness reduction, thus the displacement demands are larger, although design forces are often smaller.
In the case of the investigated building, the base shear was reduced by 29 % and the maximum drift
was increased by 24 % in comparison to values obtained in the case of model proposed by EC8.
Consequently, the required amount of reinforcement is also smaller. Which model of effective
stiffness is more appropriate for earthquake-resistant design could only be evaluated by estimating
seismic collapse risk for the buildings, which is beyond the scope of this thesis.