# notes_21_primary_stress_loads - PRIMARY STRESS ALLOCATION,...

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PRIMARY STRESS ALLOCATION, LOADS AND DESIGN APPROACH Loads Primary – hull girder stresses y M I σ = Secondary/Tertiary Other Primary – Hull Girder Hogging/Sagging σ DSM σ DHM 1 0.5 0 0.5 1 1 0 1 hogging moment sagging moment (<= compressive) stress (tension =>) distance from neutral axis above neutral axis + stress hogging below neutral axis + stress sagging Treat “corners” of the plot i.e. σ at deck (D), hogging(H), maximum(M); σ DHM σ at deck (D), sagging(S), maximum(M); σ DSM and at keel (K); σ KHM , σ KSM For first approximation treat internal and external and external structure differently: 1 notes_21_primary_stress_loads.doc

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Internal – linear through 0, 0 External – Design Philosophy governs at least to start 1 0.5 0 0.5 1 1 0 1 hogging moment sagging moment hogging external sagging external sagging external hogging external (<= compressive) stress (tension =>) distance from neutral axis first set tension at neutral axis at half the maximum in tension and compression ; 1 max 2 DHM TNA KSM σ     1 max 2 DSM CNA KHM Then linear Design Philosophy further allocates a fraction of allowable stress to primary MS 8.5 TSI 19.04 KSI HTS 9.5 TSI 21.28 KSI HY-80 10.5 TSI 23.52 KSI HY-100 11.5 TSI 25.76 KSI 2 notes_21_primary_stress_loads.doc
And applies margin of 1 TSI combatant 0.5 TSI auxiliary and patrol craft 0 0.5 1 1 0 1 stress allocated to primary 1 => total allowable stress distance from neutraal axis If required, bending moments could be estimated as follows: With sufficient knowledge of the design, a bending moment can be calculated (static or stochastic etc.) Frequently to get started on the design spiral an initial estimate of the structural weight and scantlings is desired. Estimates can be

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