EP 1110-2-12
30 Sep 95
Table 7-1
Combining Modal Responses: Square Root of the Sum of the Squares Method (SRSS) and Ten
Percent Method (TPM)
STATISTICAL METHODS consider the phasing of the modes by utilizing a "coupling coefficient" between the various modes as
expressed by the basic equation:
N
1/2
N
Ri Pij Rj
R
i 1
j 1
N = number of modes to be considered
where:
R = total modal response
Ri = maximum modal response in the ith mode
Rj = maximum modal response in the jth mode
Pij = coupling coefficient between modes i and j
There are several methods for determining the Pij values. They are given below in the order of complexity:
Method 1: Square Root of the Sum of the Squares (SRSS)
1.0 if i j
Pij
0.0 if i ≠ j
The basic equation then reduces to
N
2 1/2
R
Ri
i
1
Method 2: Ten Percent Method (TPM)
ωj ωi
≤ 0.1
1.0 if
ωi
Pij
0.0 if ωj ωi > 0.1
ωj
where
This method gives additional accounting for modes with nearly the same frequency. If none exist, TPM reduces to SRSS.
7-9
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