DESIGN CONSIDERATIONS

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• DESIGN CONSIDERATIONS

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General - Stiffener/web connection
Size of welding gun compared to typical stiffener
height
Continuous welding difficult Start and stop
defects
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General - Stiffener/web connection
Difficult to weld Small shear capacity
Bracket or flat bar added Not too difficult to
weld
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General - Stiffener/web connection
Cover plate will improve shear capacity and
improve force distribution
If possible avoid scallops, snipe corner of
collar plate to allow space for longitudinal weld
and then weld continuously around collar plate
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General - Brackets
Edge Stiffening Requirements
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General - Tipping Brackets
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General - Soft Toe Brackets
10 - 15mm
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General - Termination of Sniped Stiffeners
30 degrees

• Sniping allowed when
• buckling stiffeners
• dynamic loads/vibrations of minor importance

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General - Pillar Connections to Deck
Should not be applied if tension through pillar
may occur
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General - Pillar Connections/Cross Ties connection
Applicable to pillars/cross ties that may be
under compression and tension
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General - Alignment in Way of Section Joints
Easier to get acceptable alignment when a
continuous member is penetrating the bulkhead or
deck structure
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General - Longitudinals through Web Frames
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Longitudinals at Watertight Bulkhead
Longitudinal continuous through bulkhead
Longitudinal terminated at bulkhead with
soft-nose brackets
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Edge Support of Cut-Outs
Edge support when length of free edge exceed 50t
or D?50t Cut-outs to be kept well clear of end of
brackets and locations where shear stresses are
high
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Hard Chine / Knuckle

• Problem Cracking due to unfavourable detail
  design.
• Possible Cause The knuckle represents an elastic
  element in the plate, the effective flange will
  be reduced and the stresses will increase locally
  in way of the web / bulkhead, in addition large
  dynamic pressures act on the planing strip /
  spray rail.

Cracks in web and shell plating
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Hard Chine / Knuckle
Solution
CHINE SUPPORT
KNUCKLE SUPPORT
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Pillar Connections/Cross Ties connection

• Problem Cracking and buckling of frame webs and
  ties.
• Possible Cause Unfavourable detail design.

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Pillar Connections/Cross Ties connection
Solution Additional tripping brackets should be
fitted to support cross tie or pillar. Soft toe
brackets to be fitted
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Transverse Frames - Buckling

• ProblemBuckling and tripping of frame in way of
  sharp radius.

Position

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Transverse Frames Buckling

• Solution as illustrated below, or various
  combinations to increase buckling capability

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SKEGS FOR DIRECTIONAL STABILITY

• Problem Cracks in various locations starting
  from weld to bottom plating, edges and slot welds
• Possible Cause
• Unfavourable detail design.
• Insufficient stiffening
• Unfavourable weld method

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SKEGS FOR DIRECTIONAL STABILITY

• Solution
• Introduce additional stiffening.
• Improve weld connections by using double sided
  fillet welds or full penetration with backing bar
  where access is difficult.
• Avoid scallop welds, replace with continuous
  welds against permanent backing

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SCALLOPS AND CHAIN WELDS

• Problem
• Cracks originating from end of chain weld
  propagating into stiffener. Cracks originating
  from scallops propagating into web or shell
  plating, along HAZ of weld
• Possible Cause
• Vibrations from machinery or high dynamic
  sealoads. Unfavourable detail design for area.
  Unfavourable weld execution at starts and stops
  of chain welds and at ends/terminations of
  scallops

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SCALLOPS AND CHAIN WELDS

• Solution
• Longitudinal stiffeners in area should be welded
  with double continuous welds. Scallops should be
  avoided, necessary drain holes may be cut in web
  above weld to bottom plating

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Thrust Bearing, Shaft Bearing and Engine
Foundations

• Problem
• Cracks in various parts of foundations
  originating from weld toes at ends of brackets,
  stiffeners and scallops, ends of sniped flanges,
  or at ends of buttwelds in flanges in way of
  thickness transitions.
• Possible Cause
• Unfavourable detail design, weld execution at
  ends, in way of scallops and at buttwelds of
  thickness transitions. Insufficient thickness of
  critical parts.

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Thrust Bearing, Shaft Bearing and Engine
Foundations

• Solution
• Improve detail design, by avoiding brackets and
  stiffening with unfavourable details as indicated
  previously.
• Increase thickness in certain locations. Improve
  position and profile of transitions between thick
  and thinner parts.
• Improve structural continuity by adding brackets
  or new flange pieces replacing sniped flanges.
• Careful grinding of critical welds and rounding
  of edges of foundations, flanges and transitions
  will improve fatigue life

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Thrust Bearing, Shaft Bearing and Engine
Foundations

• Foundations supporting oscillatory equipment will
  be exposed to varying degrees of vibrations at
  high frequency.
• Fatigue failure must therefore be taken into
  consideration.
• Typical details that may give rise to high stress
  concentrations and for which it is difficult to
  obtain a good surface finish are scallops,
  cutouts, sniped flanges etc.
• It is therefore very important to maintain
  structural continuity, avoid scallops and sniped
  flanges whenever possible and improve details by
  grinding smooth surfaces at critical locations.

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FOIL AND APPENDAGE FOUNDATION

• Problem
• Cracks in various parts of foundations
  originating from weld toes at ends of brackets,
  stiffeners and scallops, or at ends of buttwelds
  in flanges in way of thickness transitions.
• Possible Causes
• Unfavourable detail design. Poor execution of
  welds. Insufficient dimensions of critical parts.
  Lack of structural continuity

Foil internal supporting structure
Cracks in various of the flange junctions and
bracket end connections
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FOIL AND APPENDAGE FOUNDATION

• Solution
• Improving detail design by ensuring soft
  transitions at cruciform joints, positioning of
  buttwelds outside areas of stress concentrations,
  avoiding scallops or ensuring proper weld
  finishing at the scallop toes, fitting of soft
  toe brackets at ends of girders.

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BULKHEAD STIFFENING ARRANGEMENT

• Problem Cracks in brackets or stiffener at ends
  or cracks in bulkhead plating in way of
  unsupported stiffener ends
• Possible Cause Unfavourable detail design.
  Fatigue cracking from sloshing of liquid in tanks
  or varying sea pressure

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BULKHEAD STIFFENING ARRANGEMENT

• Solution
• Soft toe brackets
• Provide full shear connection of webs
• Provide end support to unsupported stiffeners.

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BRACKET CONNECTIONS

• Problem
• Cracks in brackets and longitudinal stiffener,
  cracks originating from weld toes in various
  locations propagating into plating or along HAZ
  of weld.
• Possible Causes
• Vibrations from machinery or high dynamic
  sealoads. Unfavourable detail design for the
  area. Unfavourable workmanship. Insufficient
  scantlings.

Cracks
Intermittent welding
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BRACKET CONNECTIONS

• Solution
• Connect webs of longitudinals to bulkheads or
  frames with continuous welds, avoid scallops and
  weld start / stops in critical positions. Fit
  soft toe brackets. Toe height should be limited -
  refer soft toe bracket detail

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TRANSVERSE FRAMES

• Problem
• Cracking in plate buttweld in knuckle. Cracking
  in webs and bulkheads in way of knuckle.
• Possible Causes
• Unfavourable detail design. The knuckle
  represents an elastic element in the plate, the
  effective flange will be reduced and the stresses
  will increase locally in way of the web /
  bulkhead.

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TRANSVERSE FRAMES

• Solution
• Knuckles should be supported with brackets which
  are properly connected to stiffeners on bulkheads
  or flanges of web frames.
• For knuckles with small change of direction, a
  plate insert made from a bent plate may be
  introduced to move butt weld out of the knuckle
  area.

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STRUCTURAL DESIGN - POINTS TO REMEMBER

• 1. Bottom slamming will be dimensioning to all
  structures in this area. Shear area requirement
  to longitudinal stiffener may be severe.
• 2. Chines and knuckles have to be supported
• 3. Side plating (Lgt50m) may need increased
  thickness for shear buckling.
• 4. Vertical pillars to be positioned correctly
  first time.
• 5. Transverse deck girders will often have long
  spans which need appropriate section properties.
• 6. Deck plates of catamarans need to be checked
  for buckling due to transverse bending moment.