Primary Stress Allocation, Loads and Design Approach

PRIMARY STRESS ALLOCATION, LOADS AND DESIGN APPROACH Loads Primary – hull girder stresses σ= My I Secondary/Tertiary Other Primary – Hull Girder Hogging/Sagging σDSM σDHM distance from neutral axis 1 above neutral axis + stress hogging 0 1 1 0.5 0 0.5 (<= compressive) stress (tension =>) 1 below neutral axis + stress sagging hogging moment sagging moment Treat “corners” of the plot i.e. σ at deck (D), hogging(H), maximum(M); σDHM σ at deck (D), sagging(S), maximum(M); σDSM and at keel (K); σKHM , σKSM For first approximation treat internal and external and external structure differently: 1 Internal – linear through 0, 0 External – Design Philosophy governs at least to start distance from neutral axis 1 0 1 1 0.5 0 0.5 (<= compressive) stress (tension =>) 1 hogging moment sagging moment hogging external sagging external sagging external hogging external first set tension at neutral axis at half the maximum in tension and compression ; 1 2  σ DHM    σ KSM  1 2  σ DSM    σ KHM  σ TNA ≡ max  σ CNA ≡ max  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 not And applies margin of 1 TSI combatant 0.5 TSI auxiliary and patrol craft stress allocated to primary distance from neutraal axis 1 0 1 0 0.5 1 => total allowable stress 1 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 used for a first estimate. One such approximation derived from a curve fit of 13 destroyer and frigate hulls (used by Asset) is as follows: MbH = -0.000457 * L2.5*B longtons*feet MbS = 0.000381 * L2.5*B longtons*feet where: L = length between perpendiculars B = maximum beam at design waterline When process starts may not be sufficiently confident to calculate I yy so we cannot estimate σ Set σ DHM = σ KSM = σ allow primary max Use linear relations for interior and exterior – with 3 yNA = yD/2 then when first estimate of scantlings are complete, calculate neutral axis, moment of inertia and bending moment and repeat the process Review handout material properties plate catalog stiffener catalog acronym PSF Maestro description Loads table Loads sketch SNAME Material properties Note 1: allowable working stress Steel σy σy  1  σu +  ex MS = 1.25 2  2.15 1.25  σ AWS =  actual close for MS Note 2: maximum allowable working stress; 13.122 will use γ S = 1.25 γ C = 1.5 for serviceability for collapse Plate/stiffeners – use for future problem sets Partial Safety Factors Tabular form – separate by origin and serviceability vs collapse note yield ≠ collapse Next page defines words Outlines where we’re going 4 not σy factored by Design Practice EXTERNAL stress allocated to primary INTERNAL stress allocated to primary 1 distance from neutral axis distance from neutraal axis 1 0 0 1 1 1 0.5 0 0.5 1 => total allowable stress 1 1 1 2  σ DHM    σ KSM  σTNA = 0 1 2  σ DSM   (use – sign for compression) σ  KHM  σCNA = 0 σ TNA ≡ max  σ CNA ≡ max  1st Iteration yNA = yD/2 ref. keel σ DHM = σ KSM = σ DSM = σ KHM = − Max Primary Stress (+ => tension) Max Primary Stress (- => compression) Relationship (y) is linear Not necessary but typically expressed such that y (ref keel) to mid ht of panel treat above and below NA separately 5 0.5 0 0.5 (<= compressive) stress (tension =>) not 1 Below Neutral axis EXTERNAL: σT(y) = σTNA + (σKSM -σTNA)*(yNA-y)/yNA σC(y) = σCNA + (σKHM -σCNA)*(yNA-y)/yNA Above Neutral axis EXTERNAL: σT(y) = σTNA + (σDHM -σTNA)*(y-yNA)/yNA σC(y) = σCNA + (σDSM -σCNA)*(y-yNA)/yNA INTERNAL: Then: above with σTNA = σCNA = 0 Maximum Stress = σMAX(y) = MAX{σT(y), -σC(y)} Have talked about mechanism for hull girder shear and bending wt – buoyancy distribution in still water or wave induced Will now consider relationships as they relate to other than primary bending effects Secondary loads: Many ways to classify, we will use Sea & Weather and Individual wave * live green sea dead heel * damage (* heel) slap We will ignore: e.g. pitch * blast missile on deck > acceleration * underwater on hull _ pressure slamming 6 not ice, snow, wind equipment weight * * Maestro includes explicitly others are input as pressures on strake 13.122 Design Loads (all expressed in ht of sea water pressure (feet)) Weather (choose largest – where applicable) Wave Hwv = yDWL + 0.55√L - y Notes: only + Hwv ignoring phase ignoring adjustment of yDWL due to dynamic effects “Smith” effect wave dynamics > exponential pressure decay with depth Maestro includes to a degree Heel HH = (yDWL – y + z*tan(α) ) * cos(α) Set α = 30°for design z*tan(α α y α yDWL-y z HH = (yDWL – y +z*tan(α)) * cos(α) 7 not Green seas (applicable to Weather Deck WD) 4 8-12 0 x FP A ≡ 8 to 12 submerged at FP linearly decreasing to constant 4’ over weather deck => 13.122 use 8 ft H GS  yo − y + 4ft    = max  yo + 8   L  2    x −  − y 2    L   Wave Slap Design value 500 psf > converted to height in feet => 500/64 lbs/ft^3 = 7.82 feet Completes weather and sea H SW = max [ HWV , H H , HWS ] Independent Live Load varies from 75 psf for living space Mezzanine Deck and up 100 psf living space below Mezzanine Deck 150 psf offices and control spaces below Mezzanine Deck to 300 psf for storerooms/magazine Use 150 psf => HLL = 150/64 lbs/ft^3 = 2.37 feet Damage (Internal structure horizontal and vertical) Flooding occurs to margin line might be worsened with heel Design approach (decks only) Compare flooded pressure with heel = 30° Margin line at deck 8 not to that panel being flooded without heel Margin line at deck (yD – y) as with heel yDWL replaced by yD Set α = 30° HDAM = MAX{(yD+z*tan(α) – y) * cos(α), (yD – y)} Dead load Weight of fixed structure 1” thick 1 ft2 plate weighs ~ 40 lbs Design: Use approximately 2.5 times plate thickness HDL = 40 * 2.5 /64 *t = 1.72 * t in feet Where t = plate thickness in inches One other criteria: maximum stiffener spacing maximum b = B (breadth of plate) N +1 23 ≤ b ≤ 28 b = stiffener spacing N = number of stiffeners Review Look at Handout Sea/Weather check applicability use largest Independent apply as appropriate 9 not