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notes_20_design_appr

# notes_20_design_appr - Overall Design Approach(top level...

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Overall Design Approach (top level overview) Ship Structure Design is a stochastic and time dependent process. What parameters are uncertain? Loads waves, sea state, speed, direction, etc… Load effects Assumptions in analysis Variation in application Modeling assumptions, e.g. shear lag Materials Dimensions Properties Fabrication Loads Live load variation (hotel balcony, whale watching) Equipment Sea state and response Note which affect limit Limit & load effect Want load effect < limit Have distributions for pdf(R), and pdf(Q): 0 20 40 60 80 0 0.01 0.02 0.03 0.04 0.05 0 Q L x ( ) Q x ( ) f x ( ) k 100 0 x 100 1 notes_20_design_appr.doc

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Recognizing stochastic nature of situation Risk = = prob ( ) failure prob L Q Q Safety = prob ( ) ( ) 1 L f Q Q P < = What has been the traditional approach? 1. Philosophy Safe Life Fail Safe Safe Life - rule out any damage or failure throughout life of ship Fail Safe - accept some risk of damage as long as life or ship survivability not at risk Tradition => Safe Life with few exceptions involving unlikely catastrophic events, e.g. plating at missile magazine boundary load blast. 2. Establish strength criteria and/or allowable strength limits to prevent a. yielding b. elastic instability or buckling - Example yielding apply safety factor to MS 1.25 = allowable (working) stress = 1.25 Y σ apply SF to other materials based on MS but dependent on ultimate F e.g. Max Stress = 1 2 Y Y F F factor factor µ µ + - Buckling/instability buckling – define maximum column strength as function (end restraints, slenderness ratio C F radius gyration L ρ ) beams (including plate/stiffener) – combination stresses calculate C F column strength apply factor e.g., allowable stress < 60% ... etc.
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