Question
While carrying our seismic analysis, Why does the program not take into account the weight of the lower half columns on the ground floor or an equivalent mass?
Answer
In structural analysis, particularly seismic or dynamic analysis, the mass located in the lower half of the structure especially near the ground level is generally assumed to have negligible participation in the dynamic response. This is because:
- The mass is close to the supports, which are typically fixed or heavily restrained.
- Being near the base, this portion of the structure experiences very little movement during ground shaking.
- Therefore, its effect on mode shapes and overall dynamic behavior is minimal.
This is sometimes referred to as "missing mass" mass that exists in the structure but does not participate in the vibrational modes captured during analysis.
The concept is illustrated in the figure provided for better clarity.
