Redundant Structures

In an effort to reduce the impact of partial failures in a system, redundancy is typically added where feasible. Redundant structures are duplicate components or even independent additions to a design that are not necessary for standard operation but can allow for continued operation in the event of a failure. Redundant structures can be operational or standby.[1]

  • Operational: A redundant feature operates in tandem with the rest of the system and does not require any action to activate during a failure as it is already in operation. An example of an operationally redundant feature would be extra load bearing members in a bridge. Then in the event of failure of one of the members, the extra members can still support the load and the bridge can still safely hold load until the bridge can be closed and evacuated.
  • Standby: A redundant feature does not activate until a failure occurs and it is switched in. An example of a standby feature is a backup generator that is only turned on to provide electricity in the event of a power outage.

Redundant structures can also be either similar or dissimilar[1] to the feature of the system that they are "backing up"

  • Similar: The redundant feature is identical to the feature that it is supporting and performs the same function. An example of a similarly redundant feature is a full-size spare tire. In these cases, when one tire fails, the spare tire replacing it is identical and serves the same purpose.
  • Dissimilar: The redundant feature is not identical to the feature that it is supporting but, has the same function. An example of a dissimilarly redundant feature is having a secondary flight control computer on an airplane, using different hardware and software from the main computer. This way, if a failure is caused by hardware or software, it is unlikely that both computers will fail at the same time.

While redundant systems are beneficial and necessary, they should not be used as an excuse for disregarding routine maintenance or putting off repairs, especially in safety-critical systems. Overreliance on redundant structures in this manner could increase the likelihood for failure of both the initial and redundant structures.

NDE plays an important role in the case of operational redundant structures, such as extra load bearing members in buildings. Due to the increase number of members, these structures are overly constrained, or statically indeterminant. This essentially means that it is not possible to analytically solve for the specific stresses running through each member. This can make modeling difficult. Many structures that are analyzed with NDE are statically indeterminant, and NDE techniques make it possible to access the health of the structure.

References and Resources

  1. 431-REF-000370, Flight Assurance Procedure: Performing a Failure Mode and Effects Analysis, NASA/GSFC (08-10-1996)