Difference Between Center of Gravity and Center of Stiffness

If the location of the center of gravity and the stiffness center differ within a structural system, it can result in various implications affecting the structural behavior and dynamic response to applied loads, especially under dynamic or seismic conditions. Here’s an overview of potential consequences:

1. Torsional Effects: Misalignment between the mass center and stiffness center can induce torsional effects, causing the structural system to twist when subjected to dynamic loads. This occurs because the load application point (mass center) and the point of resistance (stiffness center) are not in alignment. Torsional effects may lead to additional stresses and deformations in structural components.
2. Uneven Load Distribution: The discrepancy in spacing between the mass center and stiffness center may cause an uneven distribution of loads among structural elements. Consequently, certain elements might bear a disproportionate share of the load, potentially resulting in overstressed or underutilized components. This imbalance can affect the structural efficiency and safety.
3. Increased Deflections and Drifts: Torsional effects and uneven load distribution can contribute to heightened deflections and story drifts. These elevated deflections may lead to serviceability issues, structural damage, or failure under extreme loading conditions. Moreover, larger deflections could cause discomfort to occupants, while increased story drifts might damage non-structural elements like partitions or glazing.
4. Dynamic Response: The disparity between the mass center and stiffness center can introduce a phenomenon known as “structure-soil-structure interaction” (SSSI) during dynamic analysis. SSSI can significantly alter the dynamic response of the structure, including changes in natural frequencies, mode shapes, and damping ratios.
5. Analysis and Design Challenges: Non-coinciding mass and stiffness centers make the structural analysis and design process more intricate. Engineers must consider torsional effects, uneven load distribution, increased deflections, and potential SSSI in their calculations. This complexity may necessitate the utilization of advanced analysis techniques and greater computational resources.