Question
When performing a response spectrum analysis, I noticed that the displacements or deformations of the lower and upper floors appear in opposite directions. Is this correct?
Answer
When checking deformations during a response spectrum analysis, there's a possibility of partial distortion due to the sign conversion option. To address this issue, you should turn off the sign conversion option and perform the analysis to verify deformations accurately.
Specifically, the sign conversion option is found in Load > Response Spectrum Analysis Data > Response Spectrum Load Cases > Modal Combination Control > Add signs (+, -) to the Results. There are two options: "Along the Major Mode Direction" and "Along the Absolute Maximum Value."
"Along the Major Mode Direction" option displays deformation patterns following the signs (+, -) of the major mode in each direction.
"Along the Absolute Maximum Value" option uses the sign of the mode with the largest absolute value to display deformation patterns.
It's common to assume the behavior of the entire structure follows the sign of the major mode, which reflects typical structural characteristics. However, in structures with high irregularities or when it's challenging to separate the behavior of the structure into distinct modes, there's a chance of partial distortion. In such cases, it's more appropriate to use the "Along the Absolute Maximum Value" option, as it considers the sign of the mode with the largest absolute value, which can better capture the overall behavior of the structure.
[Image-1] Variation without Sign Conversion (RY)
[Image-2] Variation with Sign Conversion as the main mode (RY)
In both cases, as shown in the above figure, the absolute values of the deformations are the same, and the difference lies in the sign of the deformations.
The sign conversion option is most appropriate when exporting the reaction results from Gen to midas SDS for mat foundation design.
Furthermore, when checking member forces in structural elements, using the sign conversion option can also be reasonable. However, when combining gravity direction loads with seismic loads in load combinations, seismic loads are applied in both the positive and negative directions, so the difference in member forces is not significant.
Nevertheless, when designing steel structures, it is recommended to use sign conversion because the design member forces are heavily influenced by Cm and Cb.
In cases where the behavior of the structure is not well separated, making it difficult to distinguish the major modes, using the method of adopting the sign of the mode with the largest absolute value among the results of each mode (Along the Absolute Maximum Value) is preferable, as there may be some distortion.