Question
In linear dynamic analysis, the bottom shear force in the X direction is significantly higher than in the Y direction, but in Pushover analysis, the Y direction has a significantly higher shear force than the X direction. What could be the reason?
I compared the bottom shear forces after performing dynamic analysis and Pushover analysis for a structure with dimensions of 42m in the X direction and 24m in the Y direction.
Answer
Generally, in nonlinear analysis, results may exhibit a different trend compared to linear elastic analysis.
In your given structure, since the stiffness in the X direction is higher than in the Y direction, the linear elastic dynamic analysis results show that the X direction would carry more load, resulting in a higher bottom shear force in the X direction compared to the Y direction.
However, in nonlinear analysis, such as Pushover analysis, it's possible for the shear force distribution to be different. In this case, the Y direction shows higher nonlinear bottom shear forces compared to the X direction. This indicates that, in the nonlinear analysis, more force has been applied in the Y direction compared to the X direction. Essentially, this suggests that the stiffness in the Y direction is relatively smaller than in the X direction.
Especially in the case of column elements, the yield moment for My may be smaller than the yield moment for Mz, meaning that yielding in the Y direction may occur earlier than in the X direction.
Once a critical column element yields, the collapse mechanism of the entire structure rapidly progresses in the direction of the yield. Hence, external forces tend to concentrate in the direction where yielding occurs first, in this case, the Y direction.
In summary, while linear elastic analysis shows load distribution mainly according to stiffness, nonlinear analysis considers the yielding behavior of the structure or its critical elements. This difference is a key distinction between elastic and nonlinear analyses. You can examine the distribution of yielding hinges step by step for each direction in Pushover analysis to confirm this behavior.