you want to place a brace at an antinode.
determining which bending mode will come down to the frequency at which the component is vibrating.
If you have determined there is 3 bending modes on a pump discharge pipe. Say the first is at 1700cpm, the second occurs at 2250cpm and the third at 3890cpm. The motor driving the pump is on a VFD and has an operating range from 1500 to 1785RPM. Measured vibration on the pipe is highest when machine shaft speed approaches 1700RPM.
In this case the 1st bending mode is becoming resonant. Place the brace at the point of peak displacement.
Bracing the pipe will increase stiffness which will in turn may shift other natural frequencies higher in the spectra. Additional testing should be performed after bracing to ensure other natural frequencies have not aligned with different forcing frequencies.