IWSD M3_3 -Introduction to the Design of Structures

Objective: The objective is to provide an introduction to the philosophy and process of design for welded structures.

Module 3.3: Intoduction to the design of structures

1

Scope: Design goals and constraints, Safety, economy, durability and serviceability, Proportioning of members, Analysis and evaluation, Partial safety factors, Sources of variation in design loads and strength of structural components, Ultimate and serviceability limit states

Expected result: Clarify the set of design objectives that are placed on a structure. Illustrate the design process first concept to detail design and final evaluation. Introduce the concepts of optimization with respect to performance, manufacturability, life cycle cost, etc. Demonstrate the set of limit states that are placed on a structural system. Illustrate the possible influences of manufacturing tolerances on the final design of a structure. Explain the basic aspects of limit state design. Illustrate the use of partial safety factors in calculations. Compute loading combinations required in limit state design method. Explain sources of load variation for welded structures.

IWSD M3.3

2 IWSD M3.3

Design of welded structures

Structural design may be defined as a mixture of art and

science, combining the experienced engineer’s intuitive feeling

for the behaviour of a structure with a sound knowledge of the

principals of statics, dynamics, mechanics of materials and

structural analysis, to produce a safe economical structure that

will serve its intended purpose

Salmon and Johnson

3 IWSD M3.3

Basic loadings

P

P

P

P

P

Mv

Tensile loading

Compressive loading

Shear loading

Bending

Torsional loading

Mv

Multiaxial loading

P

Different types of loading

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Structural materials

Real behavior of material, e.g. S355 Idealized for design

Stress-strain relation in structural steel

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I-Beam subjected to bending – elastic plastic deformation

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Plate with hole – stress concentration

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Materials have properties but not shapes – when we assemble the material into structural members with shape

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Common welded shapes

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Design of structures

Classification of structural members – based on function

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Behaviour of a structure depends on how loads are transfered between members

truss structures beam structures plate structures frame structures

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Type of structural steel sections

Hot rolled sections

W (a) Wide-flange Shape

S (b) American Standard

Beam

C (c) American Standard Channel

L (d) Angle

WT or ST (e) Structural

Tee

(f) Pipe Section

(g) Structural Tubing

(h) Bars (i) Plates

a – Wide-flange : W 18 97

b – Standard (I) : S 12 35

c – Channel : C 9 20

d – Angles : L 6 4

e – Structural Tee : WT, MT or ST e.g. ST 8 76

f & g – Hollow Structural Sections HSS : 9 or 8 8

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Cold formed sections

(a) Channels (b) Zees (c) I-shaped double channels

(d) Angles (e) Hat sections

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Build up sections

Built-up (W) shapes.

Built-up (C) Channels.

Built-up (L) Angles.

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Tension members

(a) Round and rectangular bars, including eye bars and upset bars.

(b) Cables composed of many small wires.

(c) Single and double angles.

(d) Rolled W – and S – sections.

(e) Structural tee.

(f) Build-up box sections.

Perforated plates

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Compression members

(a) Rolled W-and S- sections.

(c) Structural tee.

(b) Double angles.

(e) Pipe section

(d) Structural tubing

(f) Built-up section

16 IWSD M3.3


Bending members

(a) Rolled W-and other I-shaped sections.

(c) open web joist. (b) Build-up Sections.

(f) Built-up members (d) Angle (e) Channel (g) Composite steel-Concrete

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The design process – simplified

System Desired

result Primary effect

Undesired result Secondary effect

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Structural design

It is a mixture of art and science to produce a safe and

economical structure that serves its intended purpose

Design is an optimization process

• Min. Weight.

• Min. Cost.

• Min Construction Time.

• Min. Labor Force.

• Min. Operational Cost.

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Design process – Iterative cycle

1. Planning, Function Design.

2. Preliminary Structural Configuration.

3. Establish Load Cases & Load Combinations.

4. Preliminary Member Selection.

5. Structural Analysis.

6. Evaluation of all members to meet strength and

serviceability Criteria.

7: Redesign by going to step “3” above. 8: Final Design thus optimum design is achieved.

No

Yes

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Sufficient strength Sufficient shape stability Corrossion resistance Seviceability friendly Estetic appealing Sheapest manufacturing processes Low energy consumption Recycleable produkt Welding also requires special aspects aslo:

Strength of joints Material knowledge Manufacturing Quality and Cost

Requirements and goals

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Strength of materials

General for welded structures- things needed to be checked

Static loads / Max loads – A welded joint have very good resistance against static loads Fatigue / dynamic loads – Welded joints reduces the fatigue strength considerably!

Open thin-walled structures – Quite often used in welded structures, very sensitive to torsion and buckling

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Strength of materials

General for welded structures- things needed to be checked

Instability (column and plate buckling) – Due to welding large compressive residual stresses could be present in the structure Analysis methods

Load conditions Handbook calculations FE Analysis Fracture mechanics

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Design principals

Categoty of welded joints based on thier functionality

• Unloaded welded joints

• Static loaded welded joints

• Fatigue loaded welded joints

”Rule of thumb”

If the stress range is < 25 MPa unloaded weld

If number of cycles < 103 static loaded weld

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Quality and Cost

What can we gain by choosing a large penetration depth instead of a large outer throat thickness (a-measure) in a fillet weld?

How much is it worth to design welds so that the number of weld beads can be kept down?

Post weld improvement (Grinding, TIG remelting, UIT, etc..) – When does it pay of?

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Quality and Cost

When does it pay of by increasing the strength of the base material? What does a higher weld quality cost ? What effect does it have on my structure ? How large deformations due to welding is acceptable? Tolerances?

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What is a high quality weld ?

Dshs

Standard welds

Fatigue life

100

High quality welds

120

Geometry improvement

Mechanical improvement

150 140

Fatigue controlled by

•weld angle

•weld toe radius

•size of undercut/cold lap

•throat thickness

•penetration

a

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Quality and Cost

Cost for a fillet weld ~ (throat thickness)2

Momentary increase in cost when a weld line is not sufficient

In manual welding, this happens when a = 4-5 mm

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Quality and Cost

Rule of thumb: 1 mm more penetration corresponds to 2 mm less throat thickness (a-measure), for a fatigue loaded fillet welds

Lines of constant life in weldroot

of a load carrying cruciform joint

[ compare old rule 2*a worth 1*i ]

0

1

2

3

4

5

0 1 2 3 4 5 6 7 8 9 10

THROAT SIZE, a (mm)

PE

NE

TR

AT

ION

(m

m)

t=10

t=15

t=20

i = 2a

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Quality and Cost

Post weld improvement techniques increases the fatigue strength of welded joints

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Quality and Cost

Post weld improvement techniques Future investigation

Current situation for

improved joints in

design codes

Improved welded

high strength steels

As welded No effect of UTS

on fatigue strength

of as-welded joints

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Quality and Cost

New Volvo weld quality standard for fatigue loaded and statically loaded welded structures

3 types of quality requirements

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Quality and Cost

One step higher weld quality 2 times longer fatigue life OR

1One step higher weld quality 25% higher allowable stress

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Quality and Cost

Methods to ensure the welding quality of STD Volvo

Improvement techniques to ensure higher weld quality

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Material selection

What does the choice of a high strength material? Profit? What qualities besides strength is important for material selection?

Impact toughness Elasticity modulus Strength at high temperatures Thermal expansion Heat capacity

Tensile strength in the thickness direction Z-steel? Base material fatigue strength

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Material selection

Weight saving with high-strength materials

Z-steel to avoid lamellar tearing

For cruciform, T and corner fillet welds with full penetration of the weld

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Manufacturing aspects

Where is the product manufactured? Own production? Special requirements? (ex Puhra)

What equipment is available? Bending Machines, Cutting / cutting machines, welding equipment, etc. .. Special requirements in robotic welding. Space requirements, seam tracking, location, tolerances, deformation stability, etc. ..

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Manufacturing aspects

Smallest possible variation of the dimensions. Standardization. Warehousing Optimizing throat thickness of fillet welds. A too large throat thickness costs much.

Do not confuse the material qualities of the same manufacturer. Be aware of which material is ordered!

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Basic requirements of structures (Eurocode)

A structure shall be sized and designed to provide adequate Load resistance Serviceability

When a fire resistance should be sufficient for the time required

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Basic requirements of structures (Eurocode)

A grid system shall be sized and designed so that it is not damaged by accidents such as

Explosion Collision The effect of human error

In a scope not proportional to cause

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Requirements for Robustness

A robust structure or system shall be designed so that its sensitivity to accidental actions minimized Avoid, eliminate and reduce the risk of accidents. Example:

Put up barriers to protect the impacting vehicle Do not explosive details (eg, fuel tanks, gas lines) inside the bearing elements Set up quality measurement systems - QMS

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Choose type of structure which is less sensitive to the current risks of accident Example: Follow the guidelines in Eurocodes EN 1992 – EN 1996 Continuous structural members (vertically and horizontally) Proportion of structural detais (eg continues plate instead of dubble fillet welds in tensile loaded crossing plates) Are there alternatives paths for the forces if load-bearing structural elements are removed?

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Choose a structural element which can sustain (redundant system) A collapse of a single structural detail or A collapse of a limited part of the structure or The onset of a limited local damage

If there are structural elements as if they would collapse would mean accident consequence is excessive, these elements are termed Key Elements

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Avoid load carrying parts that can collapse without previous warning

Attempts to design so that the structure has major visible deformation or cracks before collapse Connect the structural elements to each other

If a part collapses, adjacent parts takes the load; redundancy

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Limit state (gränstillstånd)

Distinguish between: Ultimate limit state (brottgränstillstånd)

Serviceability limit state (brukgränstillstånd)

Ultimate limit state considers

• Human safety • Structural safety

Conditions just before the structure collapse is studied

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Ultimate limit state

Following conditions should be verified: Loss of equilibrium of the structure or part of the structure Failure due to large deformation1), the structure becomes a mechanism or material failure

Failure caused by fatigue or other time-dependent effect 1) Failure due to mechanical instability

Different partial safety factors for the different conditions

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Seviceability limit state

It is considered when: human well-being Structural functionality The structural appearance1)

1) Large deflection, visable cracks

• Distinguish between Reversible and Non Reversible Serviceability

limit states

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NON REVERSIBLE

REVERSIBLE

Deformation

Time

Deformation

Time

Limit value

First pass

Limit value

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Deformations affecting • The appearance • Users' wellbeing • Installations feature and function Vibrations and oscillations which • Cause discomfort to people • Limits structural fuction and operation • Other damage that will eventually affect

• The appearance • The resistance • The structures functionality

Criteria's for verification

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Limit state design

Structure and load models shall be based on the two states Verification that no limit state is exceeded when the current design values used in these models

Loads Material properties Product features Geometrical quantities

Verification shall be performed for all relevant design situations and load cases

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Limit state design

The requirements should be met by the partial safety factor method

Different design situations and critical load cases are identified Possible deviations from the assumed loading direction and loading modes shall be considered Structure and load models can be either physical or mathematical models

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Limit state design

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Limit state design

Partial safety factor – a factor applied to characteristic load, strength, etc. to take into account the risk of exceeding the characteristic value and the consequence of exceeding that value.

Note: partial safety factors do not take in to account normal variations in load, strength, etc.

Partial safety factor for the material

Resistance factor, γm (depends on material values, tolerances, etc)

Considers random deviations in dimensions and material properties and uncertanities in the calculation procedure

If tolerances variation is small and less than 6 % of the avarage value:

γm = 1,0

Otherwise: γm = 1,1

For fatigue: γm = 1,1

Structural safety

IWSD M3.3 53

Partial safety factor for the load

Load factor, γf ( depends on how well the load is known)

Risk for the load to exceed the characteristic vaule

Uncertainities in load model

γf = 1.0 – 1.5

Total load consist of several components. Since the different loads can not be estimate with the same precsion different γf for different load types, example:

γf = 1.0 for permanent loads, Gk

γf = 1.3 for largest variable load, Qk

Design load; Ld = 1.0Gk + 1.3Qk

IWSD M3.3 54

Structural safety

Partial safety factor for the consequence of failure

Consequence of failure factor, γn (safety class)

Used to garantue safety level in the structure

Depends on the consequence of failure

IWSD M3.3

Safety class Consequence of failure γn

1 Less serious 1.0

2 Serious 1.1

3 Very serious 1.2

Consequence of failure differs a factor 10 between each class. For many structures within metal industries a safety class of 1-2 give sufficient safety.

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Structural safety

Structural safety

IWSD M3.3

Partial safety factors

Den totala safety factor becomes : Stot = γm *γf *γn

Calculation procedure for static loaded structures

Requirement

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Structural safety

IWSD M3.3

The general design condition is: Ld ≤ Bd where Ld is design load and Rd is design resistance

Partial safety factor

γf

Design load

Ld = γf*Lk

Ld ≤ Rd

Resistance

Rk

Partial safety factor

γm *γn

Design resistance

Rd = Rk / γm*γn


Load

Lk

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Structural safety

IWSD M3.3


Design value for load Ld is derived by multiplying the characteristic value of the load, Lk, with a load factor, γf

Ld = γf* Lk

Design value for the resistance (material/strength) Rd is derived by dividing characteristic vaule of the strength , Rk, with safety factor, γm and γn

Rd = Rk / γm*γn

58

59 IWSD M3.3

Relative and absolute design

Relativ design – Start from an existing design and proportioning the parts in relation to the properties of materials

Absolute design – A new calculation of the entire structure is made with current load conditions and control is done with permissible stresses or deformations

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Absolute design

Benefits • The ability to optimize the overall structure • Better sense of the design Disadvantages • Demanding Job effort • Often lacking reliable data on actual loads on the structure

Relative design

Benefits • Easy to perform • Experience from existing design Disadvantages • Chance to get stuck in habitual design thinking spirit • Risk to misjudge the effect of e.g. a thickness change at risk of

instability, fatigue

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Relative design

Rule of thumb The plate thickness is inversely proportional to the square root of the yield stress Valid if the plate is subjected to bending For pure axial load applies: Caution at risk of instability / fatigue

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Relative design

Change to high-strength steel in tipper basket

ReMS = 235 MPa tMS = 5 mm

ReHS = 700 MPa tHS = ?

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Relative design

eHS

eMSMSHS

R

Rtt

t = plate thickness

Re = yield stress

HS = High Strength steel

MS = Mild Strength steel

Rule of thumb

mmt

t

HS

HS

3

700/2355

tMS = 5 mm

ReMS = 235 MPa

ReHS = 700 MPa

eHS

eMSMSHS

R

Rtt

Weight reduction by 40 % and the same resistance! Control for buckling, fatigue and deformations is required!

Bending example: Lightweight design of a mobile crane

64

w

h

t Governing equations from beam theory

L

P

σmax

D

EI

LP

3

3D

I

eLP maxmax

s

deflection

bending stress

bending stiffness IE

26

2 whhtI moment of inertia

IWSD M3.3


65

w

h

t

26

2 whhtI

w x h x t 60x100x10

Weight = 1

Stiffness = 1 Deflection = 1

60x100x6

Weight = 0.61

Stiffness = 0.67 Deflection = 1.49

40x140x5

Weight = 0.61

Stiffness = 1.11 Deflection = 0.9

Reducing weight Increasing stiffness Reducing deformation

stifnessbendingIE

Lightweight Structure

IWSD M3.3


66

What do we need to design against?

ratiosslendernesplatet

h

Design against Failure

Plasticity

Yield stress (σyield)

Material dependent

Plastic collapse

Elastic instability

Buckilng stress (σcrit< σyield)

Material independent (??)

Buckling Post buckling collapse

Lightweight structure

L

P Increasing plate slenderness ratio

IWSD M3.3

67 IWSD M3.3

Challenges in Design of welded structures

o Structural performance • Elastic instability – Buckling resistance • Durability – Fatigue resistance and strength • ....

o Material selection for design

o Manufacturing and joining processes

o LCC

o Enviromental impact, etc...

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Choosing different force paths

Welding of stiffener to absorb force component P

Deformation effects

Cross section

Beam with cross section change

Stiffener in the transition

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Column and plate buckling

Relation between load P and deflection w for compressed column an simple supported plate subjected to compressive load

Ideal column

Column with imperfection

Ideal plate

Plate with initial buckles

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Bending loads

Moving material as far away from the neutral axis as possible

Bad Good

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Weakness / Stiffness in bending

Make sure that you have enough constraint to minimize the deflection – if that is one design target

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Torsion loads / Torsional Stiffness

Open recangular hollow section

Torsional stiff

Torsional weak

Closed recangular hollow section

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Torsion loads / Torsional Stiffness

• Transition from open (torsional weak) to closed (stiff) cross-section

• a) Unsuitable for torsion: Open at A and closed at B

• The transition between the weak and the stiff beam cross sections is too sudden and give rise to stress concentrations

a

b

c

Bad

Better

Good

74 IWSD M3.3

Load paths

Important to design load entries correctly; Se till att krafterna tas upp genom diaphragm action (skivverkan) and avoid introducing stress concentrations on the fatigue loaded parts.

Plate action (Plattverkan) – the forces being introduced into the head structure with a component perpendicular to the plate high bending stresses and deformations!

The ability to take up loads perpendicular to the plate

75 IWSD M3.3

Load paths

Exploit disc / diaphragm effect !

disc / diaphragm effect (Skivverkan) – loads entering the plate plane. Stresses and deformations more favorable at reduced plate thickness than in plate action

The ability to take up loads in the plate plane

76 IWSD M3.3

Load paths

Fastening of a cantilever beam to the main beam

• The various cross-sections appears as discs.

• The bending moment caused by the force P is taken up in the form of a force couple in the flanges, and the lateral force taken up by shear forces in the webs

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Load paths

Plate action d1 < d

Less local bending

Disc action is exploited good!

Suitable is the beam is subjected to fatigue loads

Exemple of load path with reduced local bending of the beam flange

78 IWSD M3.3

Load paths

Options to reduce stress concentrations

Good when the beam is statically loaded

Good when the beam is subjected to fatigue loads

Bad Better

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Load paths – welding of a lifting lug

The load is fed directly into the web, most appropriate section to take up forces

Plate across the beam, the flange is deformed

Weld in line with the web section will take a heavy load

Also the web is affected

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Improper placement of the lug

Bending stresses in the web

The forces should instead be directed to the flanges Placing the lug to avoid bending

stresses in web

Risk for Buckling

Use disc action

81 IWSD M3.3


Improper placement of the lug

Bending stresses in the web

The forces should instead be directed to the flanges

Use disc action

Stiffeners for leading in forces on the web section Prevents flanges deform

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Reduce th risk for buckling

Introduce stiffeners - easiest way to distribute the load

Sandwich structure

83 IWSD M3.3

Reduce th risk for buckling

Example: Stiffners to reduce the risk for buckling. Laserwelded plate to main structure within civil aircraft industry.

84 IWSD M3.3

Design to avoid corrosion

Bad Good Best

Drainage hole

85 IWSD M3.3

Design to adopt for manufacturing

Save material, details and costs

Bad Good

Reduce tolerance dependence Bad Good

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Avoid accumulation of welds in one point Bad Good

Reduce shrinkage stress/STRAIN

Bad Good

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Accessibility welding

Bad Good

Unambiguous positioning of the parts for welding

Bad Good

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Redesign to self fixturing so that they directly in assembly may well defined positions

Dåligt Bra

BEFORE AFTER

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Design the welds as simple as possible

Welded joints – examples of weld joint design

NOT a rivet joint (nitförband)

Good rivet joints / bolted joints = bad welded joints

Wrong! Correct!

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Be sure of the accessibility

Welding pistol

91 IWSD M3.3


Avoid tensile stresses perpendicular to the sheet thickness

Stress direction

Non-load carrying weld

Failure in weld toe

Stress direction

Load carrying weld

Failure in weld toe and/or root

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If the the joint is unymmetric turned in the right direction in relation to the load

weld root side critical and often not accesable avoid tensile stresses

Wrong! Correct!

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Design the welded details

Place the welds right with respect to the direction of forces

• Avoid tensile stresses at weld root

• Root side in a symmetric welds is almost always the joints weakest point

94 IWSD M3.3

Welds – example of proper design

Butt welds are better than fillet welds in fatigue loaded structures

Choosing the right joint geometry

Bad Good

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Welds – example of proper design

Transition between different sheet thicknesses leads to increased loading

Avoid high stress concentrations at fatigue loading

Acceptable for static loadind

Good from fatigue point of view

Better from fatigue point of view

Slope min 1:2

Recommended 1:4

96 IWSD M3.3

Design for welding

Be careful when welding thick material

• Large temperature gradients uneven and rapid cooling

• Large welding residual stresses small possibilities to be reduced by deformations

Avoid welding of thin materials to thick

• Thick material cools rapidly cracks in the weld

• Gradually decreasing thicknesses

97 IWSD M3.3

Design for welding

Design section transitions so that the notch effect is as small as possible

Low notch effects

High notch effect

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Design for welding

In fatigue loaded structures - avoid the stress concentrations in highly stressed areas (welds, holes, etc.)

Examples of placement of stress concentrations in low stressed areas

Better

Bad

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Reduced weld filler material Accessability

100 IWSD M3.3

Adjust for mechanised welding

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Tolerance requirements for weld tourch angle

Butt weld

Bevel butt weld

Fillet weld

102 IWSD M3.3

Welded Rectangular Hollow Sections (RHS)

Welding of fillet welds facilitates mechanization

Backing plate for butt-welding

Few continuous, intermittent welds reduces the risk of deformations

Welds symmetrically to the neutral axis

103 IWSD M3.3

Welded RHS

Butt Welded corners with and without stiffener

Frame corners

The corner is reinforced with a RHS

104 IWSD M3.3

Example: Weld design of a mobile crane

105 IWSD M3.3


Previous design

Only one plate Only one weld No weld preparation, groove

Weld is not in the neutral axis of the beam Give rise to high bending stresses in the weld

Conventional weld

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Current design

Laser-hybrid welds

Laser welding where no bevel groove is necessary Hybrid welding; where corner effects from bending results in variable gap along the seam welds

107 IWSD M3.3


Production-related reasons for new design

Difficult to position the plate ends for correct welding

Difficult to guarantee full penetration welds without penetrating through / backing plates

108 IWSD M3.3


Mechanical-related reasons for new design

Large global bending stresses in the weld

Great local deformations / stresses from telescopic beams plates

Tangential stresses at the beam ends

Longitudinal stresses

109 IWSD M3.3


Laser hybrid welding

Welding at Luleå Technical University, Sweden

110 IWSD M3.3

Stiffeners in the corners

In static load and low fatigue loads

At large fatigue loads

The best solution is achieved if the cut out is designed as a third arc or a prolonged arc. Both of these configurations provide two elastic tongues that provides a favorable stress flow at welding around the cut-out corner. Grinding as in figure e) gives soft transition

111 IWSD M3.3

Bad design practice

Lack of access Lack of visibility

Lack of clearance

112 IWSD M3.3

Case study: Lightweight design of bogie beam structure in a Volvo CE Hauler

Study on weld quality and good design

Bogie beam

Bogie beam

Goal decrease weight by 20 % by • Using High Strength Steel • Redesign

• Improvement techniques

• Validation by testing

• Reduce production cost

• Justify less environmental impact

113 IWSD M3.3


Old design compared with new design

Old design • t = 15 / 8 mm • 183 kg • Steel grade = 350 MPa

New design • t = 12 / 6 mm • 143 kg (- 22 %) • Steel grade; use High Strength Steel

114 IWSD M3.3


Max static load Desicision of material grades

• Bearing 350 MPa grade • Flanges 460 MPa grade • Webs 600 MPa grade

Buckling strength is approx. 4 times max load

115 IWSD M3.3


Fatigue testing and life prediction

Bogie beam (fatigue life in hours)

Target > 1000

Testing 930

Prediction (Global) 120

Prediction (Local) 700

116 IWSD M3.3


Study on weld quality and good / bad design

Test of bogie No. 1 – Failure after 120 hours

• Expected fatigue life > 1000 hours • Failure after 120 hours

• Due to lack of fusion (weld penetration requirements I = 2 mm)

117 IWSD M3.3




• To verify test No. 1 • Result: Failure after 250 hours in tack weld / fixture • Cause: lack of fusion due to production mistake in plug weld

118 IWSD M3.3




• Full penetration fillet weld • Now failure from toe side

119 IWSD M3.3



Test of bogie No. 4 – first crack at 790 hours, test stopped at 940 hours

• Full penetration fillet weld • TIG dressing at weld toe • No tack weld • Improved flange geometry

120 IWSD M3.3


Cost estimation for production

Lower weight and production cost ”Win-Win” situation for customer and producer!

121 IWSD M3.3


Environmental impact and cost saving during the life time of the vehicle

• 90 % of environmental savings are during the use phase • Weight reduction lead to less fuel consumption, increased payload... • Life time cost saving of 840 Euro /vehicle

Total CO2 savings due to weight reduction of the bogie beam

122 IWSD M3.3

Design philosophy - fatigue

Infinite-Life

Loads are well below the threshold

Safe-Life

Load spectra known, part replaced at intervals

Fail-Safe

Safety factor, multiple load paths and redundancy

Damage Tolerance

Regular inspections, fracture mechanics

Heavy structures

Optimized structures

123 IWSD M3.3

Design philosophy - general

• Allowable stress design

- allowable stress method = actual loads wiyh an arbitrary factor on strength

• Limit state design

- ultimate limit state method = (differently) factored loads and actual strength

- serviceability limit state method = actual loads and actual behavior

124 IWSD M3.3

Design philosophy - general

• Allowable stress design - allowable stress method = actual loads with an arbitrary factor on strength, used for over 100 years

• Limit state design - ultimate limit state method = (differently) factored loads and actual strength

- serviceability limit state method = actual loads and actual behavior

- A limit state means “A set of conditions at which a structure ceases to fulfill its intended function

- First introduced in 1986

IWSD M3_3 -Introduction to the Design of Structures

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Transcript of IWSD M3_3 -Introduction to the Design of Structures