Metal Boat Building Structural Details

Transcription

1 Metal Boat Building Structural Details Chris Barry IBEX 2003 Session 505 9:00 AM February 7, 2003.

2 Introduction Goals God is in the Details Miles Van Der Rohe Scantling Determination / Optimization Action - Reaction - Load Path Look At Deflection Rules For Transfer of Force Vibration / Fatigue Global Loads Reducing Corrosion Reducing Labor Costs Resources 2

3 Goals Reduce/Eliminate Cracking $ Most Structural Warrantee Claims Are Caused by Poor Details, Not Inadequate Scantlings $ Very Annoying For Owners $ Very Expensive To Fix - Drydocking, Downtime, Gas Freeing, Weldors Easily Eliminated By Following Good Rules Reduce Weight Reduce Corrosion Reduce Construction Labor 3

4 Good Structural Details Are: Easy to Make - Right Meet Class / Inspection Rules Pass Load Well Resist Corrosion Don t Vibrate Reduce Weight Make Outfitting Easy Look Right 4

5 Scantling Optimization Selection of Scantling Rules ABS, ABS HS, DnV HS, ISO, NVIC 11-80, etc. Contract Issues, Liability, Applicability Basic Philosophy / Tradeoffs Shell Thickness vs. Stiffener Spacing Simple Spreadsheet Type Optimization Weight/Cost per Foot Of Midships Section More Sophisticated Efforts FEA Optimization - MAESTRO (Weight, Cost, VCG) Exotic Structural Approachs Grillages On Rulings 5

6 Efficient Structures Efficiency Ranking Tension / Compression (Without Buckling) Bending (Beams) Torsion Beams: Keep Spans Short, Maximize Depth Shells Are Generally Beams - Of Constant Depth Stiffeners Can Be Deeper Remember Shear / Local / Lateral Deflection For Torsion, Use Closed Cell Members (Tubes), Diagonals Open Members May Be 1/100 as Strong Shock Loads: Look At Energy Absorption Uniformity Is Important: Make The Whole Structure Work 6

7 Action - Reaction - Load Path For Every Action, There is an Equal and Opposite Reaction Isaac Newton How Does The Load Come In? How Does It Get Back Out? What Is The Best Path To Connect The Two? Buoyancy Forces On Shell In And Up Shell Bends And Ends Load Longs Longs Bend And Ends Load Frames Frames Pass Load Across Deck / Keel Weight Matches Buoyancy (With Acceleration) Transverse Forces Balance Blodgett, Design of Welded Structures Gordon, The New Science of Strong Materials 7

8 Look At Deflection (Bending) To Understand Stress, Understand Deflection Deflection Is Proportional To Bending Moment/Stress Shear Is Proportional To Bending Moment / Stress Make Models (Plastic / Polarized Light, Stresscoat) Stress Concentrations Are Deflection Concentrations Avoid Holes, Avoid Sharp Corners Make The Whole Structure Work / Deflect Together Watch Out For Stiff And Soft Structure Parallel In Path Soft Structure Sheds Load To Stiff Structure Stiff Structure Or Adjacent Soft Structure Fails Locally Integral Bottom Tanks - All Boundaries May See Slam 8

9 Transfer of Force Force Applied To A Member Goes Into The Members Parallel To The Force. When A Force Changes Direction, There Is A Component Force That Has To Be Picked Up Be Careful About Alignment: Make Sure That Structure Doesn t Have To Deflect To Pick Up Load 9

10 Crack - Transfer Of Force 10

11 Crack Due To Hard Spot 11

12 Crack Due To Hard Spot - FEA Stiffener Ends FS 12

13 Vibration / Fatigue Vibration Vibration Amplifies Stress (As Much As 10 X!) Vibration Produces Many Cycles Of Stress Typical Problem Near Propellers Approx Formulas / FEA Survey During Trials Fatigue Repeated Wave Impacts Watch Stress Concentrations Magnfication Ratio Magnification Factor Excitation Frequency / Natural Frequency, w / w n 5.0% 10% 15% 25% 37.5% 50% 100% 13

14 Global Loads Important In Large Ships Gross Failure Unusual In Small Craft That Doesn t Mean We Can Ignore It! Lots Of Nuisance Issues Deckhouses Details Can Crack, Can Cause Cracking Potential Major Issue In Cats Torsion, Prying/Squeezing Fast Craft In Big Waves Can See Big Global Loads Courtesy U.S. Coast Guard ELC 14

15 Reducing Corrosion Water Should Not Lie On Metal Good Drainage, Large Limbers Slope Horizontals To Limbers Access For Inspection, Maintenance, Painting Inerting Spaces: Foam? - Flocoat? Seal Faying Surfaces Full Welds Around Lapped Joints No Hemmed Seams Fully Bed Insulation, Etc. And Seal Edges Be Careful About Edges - Coatings Pull Thin Rounded Edges, Stripe Coats, Special Paints Weld-Through Primers 15

16 Reducing Labor Costs Accessible Details Easy To Make - Easy To Make Right How Does The Welder Get To It? Look At Assembly Order - Modular Construction Standardize Details Re-Invent Processes Developable Surfaces Other Structural Details Think About Structural Details And Outfitting We Always Make Money on Steel, But Outfit Eats Our Lunch Shipyard President 16

17 Work Assembly Drawings Courtesy Bender Shipbuilding 17

18 Good Detailing For CNC Brackets Integral With Frames Longs Cut To Hard Way Welded Flanges On Curved Edges Bent Flanges On Straight Edges Nice Slots For Longs Self-Jigging Courtesy Victoria Ship Yards 18

19 Standardize Details Reinforcing Ring Buy Special Bender? Re-Engineering Process Standardize With Group Technology Make Or Buy? Integrate With Outfit Process Courtesy U.S. Coast Guard YARD 19

20 Example Re-Invented Process Precut Parts - (CAL/NCC) Subdivide Into Panel Blocks Make Up Plate On Ruling Jig Install Longs, Web Frames Install Foundations, Brackets Blast, Prime, Finish Paint, Mount Outfit Join Bottom Panels, Outfit Some More Add Deck, Outfit, Add House, Etc. Oetter, Barry, Duffty, & Welter Block Construction Journal of Ship Production SNAME, May 2002 Angle, Heel Up Trailer Hitch Ball 20

21 Developable Surfaces Review Developable Surfaces (Article) Computers Can t Do It Alone May Find Surface You Don t Want May Be Developable, But Not Fair An Open Ended Hull Forces Straight Rulings Use Old Hand Techniques (Easy In 3D CAD!) Rabl s Conic and Multi-Conic Methods Kilgore s Technique (Using UCS In ACAD) Split Surfaces On Desired Rulings Use Software Controls 21

22 Structural Details And Outfitting Reuse Structure For Outfit If Possible Incorporate Vents, Drains Into Stiffeners Integral Tanks (Watch Out About Potable Water!) Design Structure To Act As Foundations Design Structure To Act As Shelves, Etc. Think About Outfitter s Access IWO Structure Coordinate Framing (Especially Pillars) With Arrgt Interior Designer & Structural Designer Should Work Together Don t Use Structure For Outfit / Finish Details If Not Necessary Welded On Pipes / Rods To Finish Structure, Etc. Heavy, Costly, Produces Distortion Use Wood, Plastic, Etc. Remember That Beams Can Be Above Deck 22

23 Some Ideas For Structural Details Longs Cut or Formed To Hard Way Curve Build Up Bulb Tees For Z, Buckling Concerns Bent Flanges - Special Machine Transverses With Natural Joints, Outfit Access Laps to Avoid Fitup Lapstrake Hulls? Penetrations Created By Assembly Corrugated Bulkheads Alignment Clips - Poka Yoke CNC Cut Into Parts Use Mushroom Brackets To Align/Control Angles, Special Sections RHT, Custom Extrusions Oetter, Barry, Duffty, & Welter, Ibid 23

24 Resources Marine Technology Journal Of Ship Production Ship Production Symposium T&R Panels Ship Structures Committee ASTM / ISO / IACS Standards NSRP: AWS: ShipConstructor: MAESTRO: Books, Videos, Magazines, Awards: Welding Design & Fabrication: FMA: AISC: Yahoo Group: Tinboats ([email protected]) 24

25 Review Goals God is in the Details Miles Van Der Rohe Scantling Determination / Optimization Action - Reaction - Load Path Look At Deflection Rules For Transfer of Force Vibration / Fatigue Global Loads Reducing Corrosion Reducing Labor Costs Resources 25