Product(s):	Sacs
Version(s):	16.1
Area:	Fatigue

Community Question:

Hello Bentley, I'm working on a fatigue analysis in which I could see changes in transfer functions when using the flexibility option, this affected a few connections but caused me curiosity about the reason for this change, next, you can see a little example of this issue, this is s  bug in the fatigue module?

Answer 

The Joint flexibility's main effect is to "soften" the structure by introducing springs between the member and the node which represents the flexibility of the connection. This in effect will reduce the nominal stresses in the brace and chord elements and increase the fatigue life. Joint flexibility is just a way to increase the fatigue life of the structure, the behavior show in the upper image is not a bug in the module, for example, the discrepancy in the results between w/ and w/o joint flexibility (JF) is due to K2NE - 0163 member internal load variation as phase angle changes at fatigue load case 10. 

the discrepancy in the results between w/ and w/o joint flexibility (JF) is due to K2NE - 0163 member internal load variation as phase angle changes at fatigue load case 10. As an example, let's look at HSSR/Height values at waves 67 and 68 for the model w/ JF activated. In the wave 67 case, the member's internal axial load is positive (in tension) across all 18 phase angles which yields a significantly small HSSR/Height ratio (3.37 MPA/m). However, in the wave 68 case, the member's internal axial load is positive for the first 9 phase angles (in tension), and then becomes negative for 10-18 phase angles (in compression) which results in increasing the HSSR value and subsequent HSSR/Height ratio significantly. To be more specific, the JF option activation results in changing the internal axial load state of the K2NE-0163 member to switch from tension to compression as the phase angle increases at a given wave number