iwsd m3_5 -codes and standards

Objective: The student will be introduced to the use of simple and more sophisticated analytical tools in the design of structures.

Module 3.5: Design guidance documents, codes and standards

1

Scope: Dimensioning, Partial safety factors, Design load, Design strength, Limit state design / allowable stress design Expected result: Understand the procedures recommended in one design guidance document for fabricated structures. Compute design strength of a simple structural component using procedures outlined in one design guidance document. Compute the needed design load for a structure considering both static and variable loads. Identify from the design guidance document other related codes and standards related to materials, manufacturing, and calculation methods, etc. Explain the basic features of the design guidance document being used. Explain possible sources of exceptional / accidental loads on the structure being considered. Understand the relationship between design strength and design load for a structure and failure probability.

IWSD M3.5

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So what is ….

Design codes, Standards, Design Recommendations ?

Do they mean the same thing ?

Do they give the same results ?

What are the major differences ?

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Design standards and codes

BS 7608: 1993 ( British standard for steel structures)

SS-EN 1990 – 1999 (Eurocodes)

SS-EN 13445-3 (Pressure vessles)

SS-EN 13480-3 (Pipes)

SS-EN 13001(Crane standard)

DNV Offshore Standards/Practice (Offshore structures)

ASME (Pressure vessles, nuclear)

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Eurocodes

European standard within infra structure engineering

Is valid since 2007 as the swedish standards within the building and infra-structure industry

Common work within EU since 1980. The purpose is to harmonize partner countries building codes.

Advantages: Easier to wirk within Europe

Disadvantages: Many conditions give ”unprecise” standard. Difficult to grasp. Expensive.

Pretty complex and extensive

No history

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SS-EN 13445-3

Pressure vessles (not exposed to fire) –

Part 3: Design

Also: Part 1: General

Part 2: Material

Part 3: Manufacturing

Part 5: Inspection and NDT

Part 6: Design and manufacturing requirements for pressure vessles in ductile iron

The standard is intended for pressure vessels and structures subjected to high / low operating temperatures

Lots of proposed design solutions for pressure parts

Works with "allowable stresses" and not partial safety factors

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SS-EN 13480-3

Metallic industrial piping materials

Part 3: Design

Also: Part 1: General

Part 2: Material

Part 4: Manufacturing

Part 5: Inspection and NDT

Part 6: Additional requirements buried pipes

The standard is intended for pipes subjected to high / low operating temperatures / pressure

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SS-EN 13001

Standard for cranes, hoists, elevators, lifting equipment, etc.

”Crane standard”

Replaces old IKH 4.30.01-03, which does not apply anymore

Partial safety factors are applied

Limit state design similar to Eurocode is applied

SS-EN 13001-01: General principals and requirements

SS-EN 13001-02: Load estimations

SS-EN 13001-03: Allowed values for steel structures, respectively. continuous lines

One can find examples of dynamic magnification factors for different elevator / travels

Basis for wind loads on lattice structures, etc...

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DNV Offshore standards

Standard for the offshore industry

Issued by the Det Norske Veritas

Looks like the structure of Eurocodes

Partial safety factors is applied

Limit state design similar as in Eurocode

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ASME

American standard

Issued by American Society of Mechanical Engineering

Primarily for the nuclear power industry, but also applied in the pressure vessel industry

Very extensive

ASME 1

ASME 2

ASME 3 …

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IIW recommendations

”Recommendations for fatigue design of welded structures and components”

Considers fatigue of welded structures

Independent of branch / industry

Includes recommendations for both Steel and Aluminum

Note! Not a standard, only recommendations for design

IIW Doc. XIII-2151-07

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Eurocodes

EN 1990 - Basis of Structural Design

EN 1991 (EC1) – Actions on structures

EN 1992 (EC2) – Design of concrete structures

EN 1993 (EC3) – Design of steel structures

EN 1994 (EC4) - Design of composite steel and concrete structures

EN 1995 (EC5) – Design of timber structures

EN 1996 (EC6) – Design of masonry structures

EN 1997 (EC7) – Geotechnical Design

EN 1998 (EC8) - Design of structures for earthquake resistance

EN 1999 (EC9) - Design of aluminium structures

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Eurocodes – the principal

Within each code up to 12 sub-codes, national annex Each EC about 500 pages!

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Eurocode 1: SS-EN 1990

Basis of Structural Design

Classification of loads Description of limit state design

Descrition of partial saftey factors

Design by testing Dimensionering genom provning

National annex

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Combination values for variable load

Combination value, ψ0Qk, applied for verifying ultimate limit state design which includes accident loads and reversible serviceability limit state design

Frequent value, ψ1Qk, applied for verifying ultimate limit state design which includes accident loads and reversible serviceability limit state design

Quasi permanent value, ψ2Qk, applied for verifying ultimate limit state design which includes accident loads and reversible serviceability limit state design, also for long time loads

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Example of recommended values

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Permanenta load are applied laster påförs One studies a variable load, full load impact, at a time

The other variable load multiplied with ψ0, ψ1 or ψ2, depending on which limit state design is studied

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Safety classes for buildings

Safety class 1 (low) γd= 0,83 Small risk for serious personal damage Safety class 2 (normal) γd= 0,91 Some risk for serious personal damage Safety class 3 (high) γd= 1,0 Large risk for personal damage

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Several loads at the same time

Design values against loss of equilibrium of the structure (EQU)

Permanent loads

Unfavourable: Favourable:

Ed = γd*1,1*Gk,sup Ed = 0,9*Gk,inf

One variable Main load


Ed = γd*1,5*Qk,1 0

Other variable laods

Unfavourable : Favourable:

Ed = γd*1,5* ψ0,i *Qk,i 0

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Design values of structure strength (STR)

Permanent loads



Main load


Ed = γd*1,5*Qk,1 0

Other variable loads


Ed = γd*1,5* ψ0,i *Qk,i 0

One variable load

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Design values of structure strength (STR)

Permanent loads



All interacting variable loads


Ed = γd*1,5* Qk,i 0

Interacting variable loads

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Design criteria according to Euro code 3

Designing load effect Ed is determined

Requirement Ed < Rd

Resistance Rd = fk / γM

Resistance in a cross section γM = γM0 = 1,0

Resistance in a cross section when instability γM = γM1 = 1,0

Resistance in pure tension load γM = γM2 = 1,25 (Anet, fuk)

Resistance in joints γM = γM2 = 1,25 (fuk)

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Eurocode 3 – Design of steel structures

EC3 is divided as follows :

1993-1-(1-12) General

1993-2:2006 (Bridges)

1993-3-1:2006 (Towers and masts)

1993-3-2:2006 (Chimneys)

1993-4-1:2007 (Silos)

1993-4-2:2007 (Reservoirs)

1993-4-3:2007 (Pipelines)

1993-5:2007 (Piling)

1993-6:2007 (Crane Courses/tracks)

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Eurocode 3 – Design of steel structures

Content of SS EN 1993 -1:

1993-1-1:2006 General rules

1993-1-2:2006 Fire safety design

1993-1-3:2006 Cold formed profiles

1993-1-4:2006 Stainless steel

1993-1-5:2006 Plate beams

1993-1-6:2007 Shells

1993-1-7:2007 Plane plate structures with transversal loads

1993-1-8:2005 Connections and joints

1993-1-9:2005 Fatigue

1993-1-10:2005 Toughness

1993-1-11:2006 Tensile loaded members

1993-1-12:2007 High strength materials

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Eurocode 3 – Design of steel structures, fatigue

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Eurocode 3 – Design of steel structures, fatigue

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Example: EC3 vs IIW recommendations

Eurocode 3

Detail category 125 Fatigue strength = 125 MPa

IIW recommendatios

Structural detail 321 Fatigue strength = 125 MPa

@ 2 million cycles

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Eurocode 3

IIW recommendatios

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Eurocode 3

IIW

(Fatigue strength 125 MPa)

m = 3

m = 5

m = 5 FAT160

m = 3

Dilemma or a design oppertunity??

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BS 7608 : 1993 – overview

Similar to EC3 and IIW, but the ”detail category” is given by a letter. Description according to table.

For example: Class ”S” = welded bolts for transfering of shear forces Class ”T” = welds in steel pipes where the stress range is calculated with hot spot stress method Class ”W” = crack trough the weld

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BS 7608 : 1993 – overview

For the slope of the SN-curves

• At constant amplitude, fatigue limit is at 107 cycles

• At variable amplitude, slope is

changed after 107 cycles with m´=m+2

• No fatigue limit for variable

amplitude

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Eurocode 3 – overview

• EC3 defines ”detail category” the same the IIW ”FAT-class” is defined

• The same way as IIW defines special SN-curves for shear stresses (FAT 80 and 100)

• Some additional joint types are find in EC3. If the specific joint type you are analysing is not found in IIW, DNV, BS then check EC3

• Special detail categories for hot spot stresses

• Weld root failure is designed against normal stresses perpendicular to the weld, σw, and shear stresses parallell to the weld, τw

• EC3 do not consider parallell normal stress in the evaluation

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DNV-RP-C203 – overview

The FAT values is represented with a detail category of class with a letter

For example: Class ”B” = Base material Class ”T” = welds in steel pipes where the stress range is calculated with hot spot stress method Class ”W” = crack trough the weld

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DNV-RP-C203 – overview The SN slopes

• For costant amplitude loading, fatigue limit at 107 cycles • For variable amplitude loading, m2 = 5 after 107 cycles • No fatigue limit for variable amplitude loading

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Calculation of fatigue crack through the throat thickness of fillet weld

• Should be based on the following expression

• σ|| is not included in the calculations

• Stresses σ and in a double sided fillet weld is calculated according

• Where σpl is the stress in the main plate and t is the thickness

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Cumulative damage calculations according to Palmgren-Minor

• Requirement D 1 • Safety against failure is regulated with ”Design Fatigue Factor”, DFF • DFF = 1 2.3 % failure probability • DFF is connected to the cumulative damge by: D 1 / DFF

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Summary – fatigue design codes (IIW, EC, DNV, BS)

• Most design codes have chosen to define the S-N curve slope with m = 3. All the codes give a change in the slope (m = 3.5 - 5) for the un welded base material. Except BSK07.

• Characteristic fatigue strength could vary considerably for the same structural detail. Especially for high FAT-values, e.g. base material

• DNV and BS assumes no fatigue limit if the loading is variable amplitude (load spectrum)

• IIW give generally one step higher fatigue class than BSK07, although the same failure probability. However, difference in considering the thickness factor.

• Only BSK07 make difference / consider the weld quality according to ISO5817

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Summary – fatigue design codes (IIW, EC3, DNV, BS)

• IIW and EC3 gives specific SN-curves for shear stresses

• Only BSK consider σ|| in the calculation through the throat thickness (a-mått)

• Most of the codes use the maximum principal stress at the weld toe for stress evaluation. However, not BSK07, where the stress components are evaluated in relation to the welds direction

• Multiaxial stress state is evaluated in a very different ways in the codes

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Multiaxial stress state

• Effective stresses are not suitable for evaulation of welded joint. Welds are notch sensitive in the direction of loading. However, effective stresses are used frequently due to lack of other parameters

• Principal stress ranges in multiaxial stress state

• Some codes uses interaction formulas for stress components or damage

• In some FAT values multiaxiality is considered

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Multiaxial stress state • Proportional loading: Stresses are varied simultaneously and in phase

• Non-proportional loading: Stresses are varied out of phase and independent

of each other. Principal stress directions are varying during the load cycle

• IIW, EC3 considers both variants. Is also considered in DNV by summation of the damages in the different directions

• BS – No specific consideration (Requirement that the direction of the principal stress should not differ more than 45°)

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Example 3.5.1 Design standards and codes comparision

• Butt weld in a X-groove • How many load cycle can the weld resist before failure of the joint

a) Δσ = 200 MPa b) Δσ = 100 MPa

• Weld quality corresponds to WB according to BSK07, failure probability 2.3 %.

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Summary

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• Load carrying fillet weld • The lifting ear lug ( 300 mm long) is loaded with a load range of Fr =

200 kN. The weld throat thickness is 6 mm. How many load cycles can the weld take ? Failure probability 2.3 %

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• First a general comparision, IIW, BS, EC3, DNV

For a four sided fillet weld, cruciform joint

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IIW – Structural detail nr 414 gives FAT = 45

Number of cycles to failure is calculated according

where Number of cycles to failure

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BS7608:1993 – Type number 8.5

We gets class “W” with the following properties

Since we only have the stresses perpendicular to the weld

If Sr is substituted by SP

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EC3 – Detail category 36

Number of cycles to failure according to EC3:

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DNV-RP-C203 – Welded joint accoding to table A.8.2

We get class “W3” with the following properties:

Since we only have the normal stresses perpendicular to the weld:

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Summary

The different codes consider “continues fillet welds”

Why is there a difference ??

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• I – beam bent in its stiff direction • The beam is loaded with a bending moment in one of the track beam

legs

• Weld quality B according ISO5817 is required from customer

Fmin = 10 kN Fmax = 60 kN

+

-

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• The longitudinal weld to the flange is exposed to a multiaxial stress state

• In the example the fatigue lifes are compared according to IIW, BS and DNV

• The calculated lifes are compared with fatigue testing results of the beam, e.g. 50 % failure probability

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• The stresses are calculated with Finite element analysis

Four critical points:

- two in the middle of the beam

- two, 120 mm from the middle

[MPa] Mid point 120 mm from mid point

σr║ σr┴ τr║ σr║ σr┴ τr║

WEB 83 73 0 78 73 -

FLANGE 117 123 0 118 131 4

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• According to IIW

FAT║ = 90 FAT┴ = 80

Shear stress range is below 15% of normal stress range shear stress neglected

In phase loading; no interaction

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• According to IIW

Fatigue life for the different critical locations

Mid points (flange)

σr║= 117 MPa N = 2 000 000 cycles

σr┴ = 123 MPa N = 1 209 000 cycles

120 mm from mid points (flange)

σr║= 118 MPa N = 1 950 000 cycles

σr┴ = 131 MPa N = 1 001 000 cycles (failure!)

φ = 1.3 ( 50 % failure probability)

FAT║ = 90

FAT┴ = 80

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• According to British Standard

Weld class parallel D Weld class perpendicular F

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• According to British Standard

• Shear stress range is below 15% of normal stress range shear stress neglected

• The stress components are considered separetly. Principal stress ranges are valid for weld toe failure. Here, the principal stresses (1 and 2) are in the direction of the parallel and perpendicular stresses

• Fatigue life is calculated according:

Mid points (flange)

N = 2 490 000 cycles

N = 928 000 cycles


N = 2 427 000 cycles

N = 768 000 cycles (failure!)

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• According to DNV RP-C203

Detail category parallel C2 Detail category perpendicular E • t < 25 mm detail category E

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• The stress components are considered separetly. Principal stress ranges are valid for weld toe failure. Here, the principal stresses (1 and 2) are in the direction of the parallel and perpendicular stresses

• The fatigue life is calculate according:

• The characteristic fatigue strength is divided by 1.3 to get the 50 % failure probability

• If fatigue without corrosion;

Detail category

Detail category

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Mid points (flange)

N = 2 740 000 cycles

N = 1 210 000 cycles


N = 2 670 000 cycles

N = 1 000 000 cycles (failure!)

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• Fatigue testing – 2 beams where tested

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• Fatigue testing 1st test 2nd test

Nf = 1 920 000 cycles

Crack started in mid section; crack growed perpendicular to weld

web

flange

Nf = 1 193 000 cycles

Crack started close to mid section, in the weld to and propagated parallell to the weld

Mean value from testing: N = 1 556 500 cycles

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• Summary

• The welds did not fulfill the requirement of weld quality B according to ISO 5817 in the inspection after testing

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Design standards and codes - comparision

• Further reading....

• Comparsion of BSK 99, EC3, IIW, BS, etc...

• Weld Evaluation using FEM, chapter 10, Åsa Eriksson mfl, Industrilitteratur, ISBN – 91-7548-665-2

• Have been distributed to you in advance !

iwsd m3_5 -codes and standards

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