Why are the modal masses contribution for a Response Spectrum calculated so differently between 2 mode shapes when using AutoPIPE?
Example:
Model description:
Single straight segment on one axis (ex. X-axis) with no bends and anchors on both ends,
Results:
a. A very small cumulative modal mass of 2.0651929E-19 in X direction (corresponding to the 6th mode).
b. The Response Spectrum resulting force values have been obtained in anchors both end anchors
( Anchor Node name: 10, FX=350.87 N and Anchor Node name: 100, FX=360.91 N)
c. While for a large cumulative modal mass of 7.9421875E+01 in the X direction (corresponding to the 17th mode)
d. The Response Spectrum resulting force values have been obtained in anchors both end anchors
( Anchor Node name: 10, FX=277.91 N and Anchor Node name: 100, FX=271.42 N)
Question: how can these results be correct?
For this modeled example, look at the modal report to review the contribution of each mode and the missing mass correction in the total response. This can be reviewed at the pipe force and moments and not for the anchor reactions which may be slightly higher than the pipe FX force due to contribution from the anchor. In addition, see the following observations:A
"Mode Shape" sub report
Bentley AutoPIPE