Àssociation for Ukrainian
|Problems of seismic design
of extended and asymmetric structures Earthquakes of last decades distinctly demonstrated, that buildings of the same type do not have sufficient earthquake resistance (e.g., buildings with walls made of insufficiently strong materials, frame buildings and others), but those of another type have rather high earthquake resistance (large-panel, monolithic and others). Calculations based on existing codes fail to explain this fact.
Main assumptions in existing codes
a) a floor is considered to be an absolutely hard disc;
b) an impact upon building is assumed to be constant lengthwise the building;
c) two most disadvantageous directions of a seismic wave are across and lengthwise the building;
d) unified generalized abstract dynamic coefficient * is accepted for any type of earthquakes.
Acceptance of these assumptions allowed to present basic calculation model of a building as a cantilever bar for all constructions without any exclusion irrespective of their dimensions. However, analysis of after-effects of destructive earthquakes of the last decades puts in front of scientists the task of perfection of models mentioned above by bringing new factors into calculations, which affect strength of a building.
The role of floors in rise of earthquake resistance had been underestimated so far. However, damages of buildings distinctly point out very important role of floors in distribution of seismic impact between bearing vertical elements. Analysis of typical damages
Typical damages of constructions
of buildings may be conditionally divided into several groups.
Big damages of the
internal walls and the middle part of floors. Oscillations of extended
buildings with inhomogeneous structure have peculiarities.When rigidities concentrate at
the butt-ends torsional oscillations are practically absent (their frequencies go beyond
scopes of spectrum taken into account in calculations), but dominance of those
three-dimensional characteristics of external effects of soil increases, which cause
deformations of floors in their plane at big distances between the butt-end diaphragms. At
distances of 0-60 m the butt-end diaphragms are almost immovable supports for floors, and
the mode of floor oscillations is determined by one half-wave. This mode and the
corresponding frequency are the most important in estimation of earthquake resistance of a
Fig. 2. Damage of the internal walls and the middle part of floors: a) maximum one of the middle part at flexural oscillations; b) the flexural oscillation mode with maximum displacements in the middle part of a building.
An earthquake resistance of frame
buildings may be enlarged by setting up stiff diaphragms. Influence of cealings as discs
being deformed in its planes in this case rises considerably, what may be seen from mode
of spatial oscillations (Fig. 2) and from dependence of seismic loadings on building
length and its calculation cheme [4,13].
Fig. 3. Shear of cross walls in vertical plane about each other and destruction of floors from their planes (earthquake in Turkey on the 27th of June in 1998)
An explanation of the fact that for different types
of buildings the character of destruction is proportional to the different modes of
oscillations is in a complex spectral structure of the seismic impact. Proximity of
prevalent periods of the seismic impact to periods of oscillations of buildings causes
deformations corresponding to these oscillations.