Signed Response Spectrum

Question

When I am using the 'Signed Response Spectrum' command, next to the "Mode No." there is a check box with option "Signed". Can someone please explain the use of this checkbox ie when to use it and how it affects my final analysis result?

Biswatosh · Accepted Answer

This option results in the creation of signed values for all results. The sum of squares of positive values from the modes are compared to sum of squares of negative values from the modes. If the negative values are larger, the result is given a negative sign.
For example, in any end node of a member, if the shear force is for the first mode is +x1, for second mode is -x2, third mode is + x3, fourth one is -x4 and so on.....then Staad would separate negative term and positive term and then squares of the positive and negative terms are determined separately..like P=( x1^2+x3^2......) and Q=( x^2+x4^2......). Now, the sign corresponds to the larger of P and Q is taken and assigned to the final combined response quantity , in this case the shear force at the said end node. If the P is larger then positive sign would be assigned and if Q is larger then the negative sign would be assigned.

DSANJU · Answer

Please refer to the following section of Technical reference: 
 5.32.10.1.2 Response Spectrum Specification in Conjunction with the Indian IS: 1893 (Part 1)-2002

You should not use DOMINAT and SIGN option together. SIGN option is not applicable if ABS method of modal combination method is used.

DSANJU · Answer

In case of building like structures, use of sign from dominant mode of responses has been proved to be very useful. You can refer to the following papers: 
 
 communities.bentley.com/.../269668 
 
 link.springer.com/.../978-81-322-2193-7_65 
 
You can use STAAD editor to use signed option: 
 
SPECTRUM CQC 1893 X 0.04 ACC SCALE 1 DAMP 0.05 SIGN 
SOIL TYPE 1

Biswatosh · Answer

If you invoke the In the Dominant mode option, all the results will have the same sign as mode number that the user has entered. Defaults is mode 1 if no value entered. If you enter "0" value , then the mode with the greatest % mass participation in the excitation direction will be used. Now, if you define the SIGN option then the sign will be determined based on the greater of the sum of squares of positive values from the modes and sum of squares of negative values from the modes. So, determination of sign by both the methods are different and independent and hence not supposed to be used together.

Sye · Answer

Prantik : Your observation is correct. One should not have to check the Dominant to check the Sign option. There is a glitch in the software which is causing it to behave that way. These options are not supposed to be used together. We will get this addressed in the software. Till then, please use the editor to add the SIGN option as shown in the post by DSANJU

DSANJU · Answer

I would like to mention that the use of sign & dominant options should not be used together as they are meant for different way of having the sign in the final result. You need to use either DOMINANT or SIGN option. As sited earlier in case studies , DOMINANT 0 command has been proved to be very useful in design.

Sye · Answer

Also just for the records, the defect has been logged with id# 330066 in our bug tracking system.