"Wobble"
I've looked at the recent posts and informative sites regarding vibrations and what can result. I think Bear gives some good information regarding support structures and what may be going on in these newer engines.
First, if you think about the fan and all related rotating airfoils attached to one end of a cylinder, and the LPT rotor attached to the other end, 7 feet away, you have a dumb bell in a static sense. If you pick up the dumb bell at one point near the center, both ends will sag downward, the LPT end more so as it will generally be heavier. So then, to prevent this from happening in reality, a series of static structures are designed and placed to provide the backbone of the engine. These structures must maintain alignment between rotors and stators and their design is dictated by the need for stiffness, more so than strength. The major sources of load for an engine translates into forces on the structures such as maneuver loads (turning), CAT (clear air turbulence), acceleration/deceleration, gyroscopic forces, moments through the bearings into the frame hubs that support the bearings, unbalance in the rotors, internal variable pressures in the engine flowpath and lastly, thermal differences induced by the airfoils compressing the air and then the hot gasses flowing from the combustor through the turbine components. There is also the dynamic behavior of the engine and interactions with the airframe to which it is affixed. All of these features must be combined with design features and analysis to enable the static structure components to handle different types of loads.
For the designer, there are all kinds of analysis programs to assist in determining the adequacy of the structure design, redundant structural model, finite analysis model and a complete engine structural model and a first engine bending model. In addition,designs are verified by static load testing that can include built-in defects to test design robustness. Extreme engine testing can test for structural capability under extreme vibration. Low cycle and high cycle fatigue capabilities must be examined. Based on all of this, limit conditions can be determined and set so that a well designed structure will not show any sign of deformation or contribute to a loss of performance even if the limit is exceeded in a one time event. So as you can see, the engine structures are key to having successful engine capable of meeting the mission advertised to the customers.
Good structures must be capable of absorbing vibrations and not amplifying them and there are many techniques that can be incorporated to accomplish this. I used the word "robust". Care must be taken to not reduce weight of structures that could compromise long term structural integrity although this is often looked at during weight reduction/saving campaigns to improve performance.
Vibrations and rotor unbalances are always present in an engine and they change as the total engine cycles grow over time. So the engine backbone must be capable of handling all of these variables as they occur.