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