Seismic analysis
Introduction
Seismic calculation is a special case of forced vibration calculation, when the exciting effect is the ground acceleration which is time dependent and of course not periodical. The response of the structure to the ground acceleration will be a vibration like motion. The structure gets forces of inertia which is calculated ac- cording to the Newton’s law (F = m a) and it is proportional to the mass and acceleration. These equivalent forces of course cause internal forces, stresses and if they are larger than the limit value the structure may collapse.
From the above explanation we found that originally this is a dynamic phenomenon when the acceleration and so the inertia forces change in each second. The interaction of the ground and structure is complicated so in a given time the acceleration of the structure depends on several components:
- ground acceleration (the seismic magnitude and its development on time),
- the elasticity of the structure,
- the mass and mass distribution of the structure,
- the connection between the structure and ground, namely soil type.
Another complicated problem is to define exact direction of the ground motion in the seismic investigation. Generally the ground movement is assumed as an arbitrary horizontal motion but the vertical motion also may cause problem to the structure. Fundamentally the calculation process can be divided into three methods:
Time history
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Modal analysis
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Lateral force method
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National codes
2. Input data
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Dynamic calculations
Design spectrum
3. Calculations parameters and calculations steps
Calculation methods selection
Other setting possibilities
Combination rule, rotation and second order effects
Displacement calculation
4. The results of seismic calculation
5. Summation of static and seismic effects
Seismic loads in static load cases
Final results of seismic effect (Seismic max.) in load combination
Final results of seismic effect (Seismic max.) in load groups
6. Useful tips, which method to use?