# How to determine the sign of the response spectrum results?

Hi support,

I know there is an option to consider plus/minus sign for the results of the response spectrum analysis in midas. I went through the online help but I don't get it how exactly it is determined. Could you please explain it with examples? Thanks.

Regards,

Answer:

Hi,

Let me explain with a simple model.

Total number of modes = 2

Modal Participation Masses

From the table above, we know that the major mode for RX load is mode 1.

The following two images show bending moment diagram (My) with sign due to RX for mode 1 and mode 2 respectively.

The bending moment diagram was obtained from the following input data.

You can see that the sign is different between two modes for the beams highlighted with the yellow circle.

And the absolute maximum moment for mode 1 and mode 2 is 1.4 and 7.9 respectively. The value is larger in the mode 2

From the online help:

- Along the Major Mode Direction: Restore the signs according to the signs (+, -) of the principal mode for every loading direction.

- Along the Absolute Maximum Value: Restore the signs according to the signs of the absolute maximum values of the modal results.

So, if the 'Along the Major Mode Direction' option is selected, the sign of bending moments will follow that of mode 1 which is the major mode for X .direction (RX)

If the 'Along the Absolute Maximum Value' option is selected, the sign of bending moments will follow that of mode 2 in which the bending moment is larger.

Here are the results for each option. In this case, two modes are selected.

As expected, the sign of beam highlighted in the yellow circle follows mode 1 for the Major Mode option.

And it follows mode 2 for the Absolute Maximum Value option.

For the rest of elements, the sign will be determined as follows:

For the Major Mode option, the sign will be determined based on the sign of the major mode.

For the Absolute Maximum Value option, the sign will be determined based on the sign and absolute value of each element.

Regards,

DK Lee

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