Margins design safety factors

Risk Assessment Code, Item 1 3D. The required design safety factors and hydraulic pressure bleedoff rates will not permit a catastrophic result in the event of pressure loss. At most, in the event of a continuous oil pressure loss, the elevator would descend slowly to the ground floor level at a rate consistent with that of the loss. Therefore, a severity level of marginal has been assigned and a probability of occurrence level of remote has been determined. 

Designers of most structures specify material stresses and strains well within the pro-portional/elastic limit. Where required (with no or limited experience on a particular type product materialwise and/or process-wise) this practice builds in a margin of safety to accommodate the effects of improper material processing conditions and/or unforeseen loads and environmental factors. This practice also allows the designer to use design equations based on the assumptions of small deformation and purely elastic material behavior. Other properties derived from stress-strain data that are used include modulus of elasticity and tensile strength. 

Large safety factors have been built into the design of the EDS vessel and the procedures for its operation. The mechanical integrity of the vessel was evaluated by Sandia National Laboratories using a combination of small-scale failure analysis tests and computer simulations. This evaluation indicated that the EDS-1 containment vessel could withstand several thousand detonations with more than 1 pound of explosive, providing a significant margin of safety for a system with an intended life of 500 detonations (SNL, 2000). 

Besides the gain margin there is another safety factor which is used for the design of a feedback control system the phase margin. Consider again Figure 18.5. Let m be the phase lag at the frequency for which AR = 1. The phase margin is defined as follows ... 

Since for a given reliability level, the safety factor C5 obj may be specified as an objective, it is then possible to calculate the safety margin. In Equation 6 Rk and 5 are such that P (R>Rk) = k% and P S < Sk) = k%. Generally in design codes, k = 99. Thus, the safety margin can be expressed as ... 

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