6.3 SRSS Acceleration Criteria
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Statutory Documents - IMO Publications and Documents - Circulars - Maritime Safety Committee - MSC/Circular.616 – Evaluation of Free-Fall Lifeboat Launch Performance – (22 June 1993) - Annex – Evaluation of Free-Fall Lifeboat Launch Performance - Section 6 – Human Tolerance To Acceleration Forces - 6.3 SRSS Acceleration Criteria

6.3 SRSS Acceleration Criteria

  6.3.1 The square-root-sum-of-the-squares (SRSS) acceleration criteria is based upon the assumption that the domain of safe acceleration forces can be defined by an ellipsoidal envelope bounded in each co-ordinate direction by some value. Such an envelope for acceleration forces in the x-z plane is shown in Figure 6.3. Injury should not occur as long as the acceleration forces are within the shaded region of the envelope. The limiting values incorporated into the revise recommendation for testing lifeboats by IMO for each axis of the envelope are 15 g's in the ± x axis and 7 g's in the other co-ordinate axes. These are the values indicated on figure 6.3. The SRSS criteria was cast as an interaction equation of the form:

Figure 6.3 Ellipsoidal Safety Envelope for Acceleration Forces in the x-z plane

  6.3.2 The combined acceleration response (CAR) is a measure of the potential for the acceleration field to cause human injury. It varies with time and is computed from acceleration force time-histories measured in the axes of the seat at the seat support. Before computing the CAR time-history, the acceleration force time-histories are filtered with a 20 hertz lowpass filter because higher frequency acceleration forces generally are not injurious. The peak value of the CAR time-history is called the CAR Index occur if the CAR Index is less than unity. Injury should not occur if the CAR Index is less than utility.

  6.3.3 Although application of the SRSS criteria is very straight forward, this method for evaluating acceleration forces has a weakness in that it considers only the magnitude of the acceleration force. The duration of the force is not considered. As will be seen, the potential for an acceleration force to cause injury is dependent upon its magnitude as well as its duration. The limiting values in the SRSS procedure were selected so that injury should not occur regardless of the duration. As such, the SRSS criteria tends to overestimate the injury potential of an acceleration field. The dynamic response criteria and the Hybrid III manikin are more rational methods to evaluate injury potential because both methods provide a measure of body response to the magnitude and duration of the acceleration force.

Parent topic: Section 6 – Human Tolerance To Acceleration Forces
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