Scantlings determination for frp-structuresMarkku Hentinen 2016(Karl-Johan Furustam)
a
ARp
fPst 1
ISO 12215• An international standard for boat
structures, combined of seven(harmonised) parts:– ISO 12215-1 Materials - Thermosetting
resins, glass fibre reinforcements andreference laminate.
– ISO 12215-2 Materials - Core materials forsandwich construction, embedded materials.
– ISO 12215-3 Materials - Steel, aluminium,wood, other material.
– ISO 12215-4 Workshop and manufacturing.
– ISO 12215-5 Design pressures, allowablestresses, scantling determination.
– ISO 12215-6 Structural arrangement andDetails
– ISO 12215-8 Rudders• New parts coming:
– ISO 12215-7 Scantling determination ofmultihulls
– ISO 12215-9 Appendages (kölit ym.)– ISO 12215-10 Rig loads and attachments
Design pressures for powerboatsaccording to ISO 12215-5
LDC
C
C
WLcg m
BVB
Ln22
4,0 )50()084,010
(32,0
)1(1,0 5,0cgDC
CWL
LDCBMPBASE nk
BLmP
Design pressure P is determined according to the calculatory verticalacceleration ncg and design category:
Designcategory
A B C D
Value of kDC 1 0,8 0,6 0,4
Table 2 — Values of kDC according to design category
In any case, nCG need not be taken > 7
Allowable stresses
• Design methods usually determineallowable stresses in relation to thebreaking (ultimate) strength.
• Factors of safety vary between differentsimplified methods– shall not to be regarded as “real” factor of
safety, because the method may have beenadjusted to use certain safety factors
– FE-analyses may require different safetyfactors
Allowable deflections
• Typically 1/60 - 1/100 of span.• May be dominant criteria (as in
ABS/ORC).• May be left free for panels (as in ISO
12215).• Allowing large deflection exploits full
strength of materials, but requires toughmaterials and good detail design
Most common material values neededfor scantlings determination
• Tensile strength and stiffness• Compressive strength and stiffness• Bending strength and stiffness• If laminate theory and FEM are used, material
values for each lamina and orientation areusually needed
• In-plane shear strength• Core shear strength in sandwich structures
Methods to determine the material values
• By testing• By calculation• Using literature values
Property Test method RequirementUltimate tensile strength ISO 527-1, ISO
527-4 80 MPaTensile modulus ISO 527-1, ISO
527-4 6 350 MPaUltimate flexural strength ISO 178 135 MPaFlexural modulus ISO 178 5 200 MPaIn-plane shear ASTM D 4255 50 MPaApparent interlaminar shear strength(short-beam shear)
ISO 14130 15 MPa
a The reference laminate shall consist of glass chopped-strand mat and resinwith a glass content not exceeding 30 % by mass of the fully cured laminate.b The test data shall be achieved after a post cure schedule of max. 24 h at max.50 ºC.
Table 4 — Minimum mechanical properties of reference laminatea
Structural modelling
• Plates (panels): Shell, bulkheads, decks• Beams: Stiffeners; stringers, frames• Rods: Rig attachment, shrouds• Buckling struts: Columns, masts• Torsion parts: Rudder and propeller shafts
Plates (panels)
a
ARp
fPst 1
3 2
EfkPst AR
d
Strength criteria
Deflection criteria
Adequate panel thickness shall ensure that the stress in the material doesnot exceed the allowable stress (taking into account fatigue, manufacturingfaults etc., i.e. suitable factors of safety), and that the deflection of thepanel does not exceed the allowable deflection:
Sandwich panels
Sandwich panels
Sandwich panels
Aspect ratio effect
Assuming that lateral pressure is the dominant loading,
•Stresses larger in the shorter direction (b-b) fiber orientation 0 /90
•If a b, 45 the best orientation
Beams (frames, stringers etc)
32
12mmlsPSM
ai
Beams are dimensioned so that the stress in flange does not exceedallowable tensile or compressive stress. Additionally, it shall bechecked that the deflection is not larger than the allowable deflection.
If the height of the beam is large, the shear stress in the web shall bechecked.
Significance of structural arrangement
• Alternative A. Thick base laminate – imperfectstiffener arrangement can be allowed
• Alternative B. Thin base laminate – effectivestiffener arrangement needed
• Designing a light structure requiresunderstanding of the behaviour of the structure
• Logical structural arrangement is easier toanalyse better end result
Example: Bottom structure ofa powerboat
Frame Bulkhead
Loading areasA = loading area for panel
B = loading area for stringer
C = loading area for (web)frame/bulkhead
Transverse frame(web frame)
Long
itudi
nal
strin
ger
Hard chine
Keel
Non-rectangular or very wide panels
Real panel shapes often needinterpretations how to assessthe dimensions for theequations
Design example• Planing hull powerboat for fast cruising, design cat. B
Lh =9 mLwl = 7,5 mBch = 2,5 mmLDC = 4000 kgV = 40 knß = 18
• Bottom panels and stiffenersb = 320 mml = 2000 mms = 450 mmMixed mat, roving & multiaxial GRP, fibre content 30 %MASS
=18°
450
320
2000
kylki
laipio
palle
pohja
Design pressure
8,94000
5,240)1850()084,05,210
5,7(32,022
cgn
LDC
C
C
WLcg m
BVB
Ln22
4,0 )50()084,010
(32,0
)1(1,0 5,0cgDC
CWL
LDCBMPBASE nk
BLmP kDC = 0,8 (design cat. B)
kPaPBMPBASE 5,114)9,48,01(5,25,7
40001,0 5,0
3,0
15,01,0
D
LDCRAR A
mkk kR = 1,0 (planing craft bottom)
522,0103205,2
40001,013,062
15,0
ARk
kPakkPP LARBMPBASEBMP 8,590,1522,05,114
17,05,0LDC
cg mVn(1) or, if (1) results to ncg>3,0 : (2)
9,44000
405,0 17,0cgn
Laminate thicknesswhere• b is the short dimension of the panel in millimetres;• kc is the curvature correction factor for curved panels
given in Table 6;• P is the design pressure (bottom, side, deck, etc.) of the
panel in kilonewtons per square metre;• k2 is the panel aspect ratio factor for bending strength
given in Table 5;• d is the design stress for FRP plating, which is 0,5 uf
(half of the minimum ultimate flexural strength), N/mm².
mmkPkbtd
C 10002
Laminate thickness (2)
t = minimum thickness of the structural laminate without gelcoat or topcoat
mmmmt 3,61525,010005,08,590,1320
mmkPkbtd
C 10002
Stiffener dimensions
Stiffener dimensions (2)
bb = 130 mm, tp = 6,3 mmSuitable dimensions:H 135 mmtW/2 = 2,34·wf =0,3)
tW/2 5 mm
226 2,510635,0
20004506,365 cmcmAW
32,010200033,0
40001,013,062
15,0
ARk kPakkPP LARBMPBASE 6,360,132,05,114
3292
12910855,0
20004506,360,13,83 cmcmSM
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