iwsd m3_4 -analysis methods for structures

8/12/2019 IWSD M3_4 -Analysis Methods for Structures

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Objective:

The student will be introduced to the use of simple and more sophisticated

analytical tools in the design of structures.

Module 3.4: Analysis methods for structures

1

Scope: Spreadsheet tools, Finite Element Analysis, Design guideline formulas

Expected result:

Demonstrate use of a spreadsheet based calculation system in the design of astructural system.

Explain the basis for selecting an appropriate analysis method for a design

situation.

Explain the advantages, disadvantages and uses of different analysis methods.

Explain the use and interpretation of finite element results for steel structures

design.Correctly interpret and understand the use of design guideline formulas for

calculation of design strength and design load.

IWSD M3.4


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2IWSD M3.4

Analysis methods

Handbook calculations

Manual calculations for guidance in standards or regulations

Calculation based on analytical equations

Spreadsheet

Internally developed or purchased from software vendors

Finita element method analysis:

Own developed, in house, and commercial


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Can, for example, be used for rough estimates of stresses and

deformations in two-dimensional problems

Recommended literature

Konstruktionshandboken fr

svetsade produkter, Claes OlssonKTH:s formelsamling i hllfasthet

Roarks formula for stress and strain

SSAB plthandbok

BSK2007


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4IWSD M3.4


Used:

In the calculation of less complex structures

If you want to get a better understanding for the structural function

Assessing if the results are reasonable

As an initial rough estimates

In general, the design code based analysis

When checking the FE analysis


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5IWSD M3.4


Advantages:

Requires no expensive / complicated software's

Can be performed anywhere

Easy to assure the quality

You always know what to doEnsures necessary knowledge of the analyst

Usually the quick method

Increases gradually the knowledge of the person performing the

analyzes


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Disadvantages:

Can only be applied to less complex structures

Can be time consuming if you are not overlooking the size of theproblem from the beginning

Risk of miscalculation if not quality assurance workLate modifications in the structure means considerable extra work

The result depends on the engineer's skill


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7IWSD M3.4


Hints:

Before you make an advanced computing make simple handcalculations

The earlier in the product development the better it is with handcalculations

Prior to FE analysis: do hand calculations

Getto knowthe design by hand calculations to start with


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8IWSD M3.4

Calculations based on design codes and standards

Used:

When a specific industry required a certain method to be used

Example:

Building industry BKR, BSK2007, Eurocode

Bridges Bridge codes, Euro codes

Cranes Crane standard SS-EN 13001

Offshore DNV, among others

Pressure vessles SS-EN 13445

Pipe connections SS-EN 13480

Nuclear industry ASME


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9IWSD M3.4

Advantages:

References in strategic choices

Knowledge based platforms

Systematic and well developed methods

Simplification of complex phenomenon

I general quite easy and straight forward to use (engineering based)



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Disadvantages:

Creativity inhibitory

May cause oversized constructions

Sometimes unrecognized safety factors in the norm

Analysis conditions can be difficult to grasp

Standards often change

Dangerous if standards are used outside their validity range



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11IWSD M3.4

Hints:

Never accept a Cookbookof a norm without understanding it

Never mix together several independent standards in the same analysis

Check regularly that the norm is still valid

Always refer to the norm in the analysis (version, etc..)

Standards tend to change frequently



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12IWSD M3.4

Structured as forms that you must fill in

Often written in a spreadsheet program such as MS Excel or MathCad

Can also be written in a conventional programming languages such asFORTRAN, C + + or Visual Basic

Spread sheets

Used:

For repeated analyzes of similar types

To save you time instead of manual calculations

For educational purposes (easy to see the impact of alternativeassumptions)

For parametric studies


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13IWSD M3.4

Spread sheets

Advantages:

Customized to solve a particular problem

Normally quite easy to work with

Easy to modify

Quick answers

Save time for repeated similar calculations

Suitable for consequence analysis


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14IWSD M3.4

Spread sheets

Disadvantages:

Can only follow a predetermined calculation procedure

Requires a lot of discipline at the documentation of results

Problems with the quality assurance

Creativity Inhibiting


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15IWSD M3.4

Spread sheets

Literature:

MS Excels manuals

General programmering manuals

Engineering with Excel, 2:nd edition, R.W. Larsen, Montana StateUniversity, Pearson education Inc. New York 2005, ISBN 0-13-147511-8

Engineering with the Spreadsheet. Craig, Christy, ASCE Press , 2005ISBN0-7844-0827-0


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16IWSD M3.4

Spread sheets own developed

Hint:

The spread sheet should be self-explanatory

Fill up with "help buttons" to provide the necessary information

Use a lot of graphics, facilitating understanding and ability totroubleshoot

Documenting how the calculations are carried out - preferably in thespread sheet

Facilitate documentation and traceability (revision number)

Enter references, so that you can perform manual checks

Lock the fields that it is useless for users to write in

Example:Analysis and assessment of fatigue data from tests, MSExcel, Zuheir Barsoum, KTH


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Spread sheets commercial

Exempel :SAFE (general intelligence in strength calculations)

CUMDAM (culmutive damage calculations) TechStrat, 2007,www.techstrat.se

Maskinskruv (Analysis of screws VDI 2230), HighTech Engineering
http://www.techstrat.se/http://www.techstrat.se/http://www.techstrat.se/http://www.techstrat.se/http://www.techstrat.se/http://www.techstrat.se/


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21IWSD M3.4

Spread sheets commercial

Hint:

Perform in-depth inspection of the program before putting it into use

Do either manual calculations and FEM analysis to verify thespreadsheet program

If the calculations must be approved by a court, so make sure that they

accepts spreadsheet programYou need to understand how the program works. There may never be ablack box

Never accept a result, you do not understand

Document all analyzes closely with project number, name, date andversion of the spreadsheet program

Remember that you can blame the responsibility on to the softwaresupplier, although there are obvious errors in the program


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22IWSD M3.4

Spread sheets open source

Analysis for windows; FEA application for analysis of 2D and 3D frames and

trussesFree ware, can be downloaded at:

http://www.freebyte.com/cad/fea.htm#fea
http://www.freebyte.com/cad/fea.htmhttp://www.freebyte.com/cad/fea.htmhttp://www.freebyte.com/cad/fea.htmhttp://www.freebyte.com/cad/fea.htmhttp://www.freebyte.com/cad/fea.htmhttp://www.freebyte.com/cad/fea.htmhttp://www.freebyte.com/cad/fea.htmhttp://www.freebyte.com/cad/fea.htmhttp://www.freebyte.com/cad/fea.htmhttp://www.freebyte.com/cad/fea.htm


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23IWSD M3.4

Spread sheets open source

For structural analysis and beam theory: Altas, BeamBoy, Hamlet, etc..

http://www.craftsmanspace.com/free-software/mechanical-calculators.html

Atlas BeamBoy
http://www.craftsmanspace.com/free-software/mechanical-calculators.htmlhttp://www.craftsmanspace.com/free-software/mechanical-calculators.htmlhttp://www.craftsmanspace.com/free-software/mechanical-calculators.htmlhttp://www.craftsmanspace.com/free-software/mechanical-calculators.htmlhttp://www.craftsmanspace.com/free-software/mechanical-calculators.htmlhttp://www.craftsmanspace.com/free-software/mechanical-calculators.htmlhttp://www.craftsmanspace.com/free-software/mechanical-calculators.htmlhttp://www.craftsmanspace.com/free-software/mechanical-calculators.htmlhttp://www.craftsmanspace.com/free-software/mechanical-calculators.htmlhttp://www.craftsmanspace.com/free-software/mechanical-calculators.htmlhttp://www.craftsmanspace.com/free-software/mechanical-calculators.htmlhttp://www.craftsmanspace.com/free-software/mechanical-calculators.htmlhttp://www.craftsmanspace.com/free-software/mechanical-calculators.htmlhttp://www.craftsmanspace.com/free-software/mechanical-calculators.htmlhttp://www.craftsmanspace.com/free-software/mechanical-calculators.htmlhttp://www.craftsmanspace.com/free-software/mechanical-calculators.html


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24IWSD M3.4

Spread sheets example #1

Use software Beam Boy to calculate bending moment, stresses and

deflection


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Use softwareAtlasto calculate bending moment, stresses and deflection

q = 10 kN/m F = 20 kN

5 m

2.5 m

4 m

100 mm

100 mm

t = 10 mm


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26IWSD M3.4


nom

Non-load carrying

cruciform welded

joint, sensitive to weld

toe cracking

Plate thickness = 10 mm

Throat thickness = 7 mm

N (1000)

(MPa)

100

50

30

100 300 500

Total cycles

in block =900 000

Use IIW recomm. to carry out parametric study of the weld design withspecified load history. Use CumDam spread sheet software to study the

effect of Load block Partial safety factors Improvement techniques, etc

Load spectrum block


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27IWSD M3.4

Finite element analysis

FEM is a numerical method used to solve many engineering problems,

structural mechanics, heat transfer, electromagnetics, fluid mechanics andalso linked assays (e.g. thermal-mechanical)

Finit element mesh v. Mises stress field Contact pressure between shaft and rotor

3D finite elements


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A simple description: like building Lego


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Hint:

Automated, computer-assisted manual calculations

Means that the structure is cut up into a number of "elements."

The elements defined between nodes

Different types of elements

Beam elements

Shell elements

Volume / Solid elements


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Finite element analysis - applications

Literature in swedish:

FEM i praktiken, Staffan Sunnersj, Sveriges verkstadsindustriers frlag

Dimensionering av svetsade konstruktioner, Claes Olsson, Sverigesverkstadsindustrier

Svetsutvrdering med FEM, sa Eriksson, A-M Lignell, Claes Olsson,

Hans Spennare, Sveriges verkstadsindustrie

Could be purshased by Industriliteratur AB in Stockholm


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Used:

For complex structures that can not be analyzed analytically or by handbook calculations

If it is easy to transfer data models from a CAD program


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Advantages:

General solutions, not tied to any norm or standard

Ability to solve complex problems

Ability to take into account different physical processes, loadcombinations, boundary conditions, etc..

Accurate solutions

Reuse of the FE model for different analyzes


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Disadvantages:

Could require large work effort

Requires strong quality assurance to be reliable

Easy to interpreted results wrong

Gives the impression of being more accurate than what is

Harder to get an overview and sense of the design than manual analysis

Expensive software's


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Finite element analysis basic steps

Pre processing (Phase #1):

In this phase the problem is described numerically in a discrete form

Definition of geometry, material properties, boundary conditions, loads,etc

Meshing (discretizing of the geometry by finite elements)

FE model of a bulted joint in pressure vessle;

global model, mesh, element nodes och loads


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Finite element analysis basic steps

Processor / Solution phase (Phase #2):

In this phase the analysis type is defined; static, linear/ non-linear,dynamic, buckling, etc

Other details describing the calculation strategy and solving process

Post processor /Evaluation (Phase #3):

In this phase the results are presented in different forms; graphs,animations, figures, stress plots, etc...

von Mises equivalent

stress distribution (MPa)


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Type of finite elements

1-D (line) element

(spring, rod, beam, pipe elements)

2-D (Plane) element

(Membrane, plates, shells)

3-D (Solid) element

(3D-fields; temperature, displacement, stresses,etc)


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1 D - elements

Rod and beam elements

Most of the structural analysis can be treated as linear static problems

with the following assupmtions:

Smal deformations

Elastic material behviour

Quasi-static load

Rod element Beam element


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38IWSD M3.4

1 D - elements

Beam elements

Give tge same accuracy/results as analytical handbook solutions

Problem with correct stifness at connecting points

Can take

Tension, compression, bending, shear

Torsion (St Vernant and Vlosov)

Beam element


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1 D - elements

Beam elements

If the cross sectional centre of gravity and the shear centre do notcoinside, this have to be considered in the model (example, U-beam)

Modelling w/o the connenctionpoints excentricity

Modelling with the connenctionpoints excentricity


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40IWSD M3.4

1 D - elements

Beam elements

Evaluation of a welded joint

Bending moment distribution of thediagonal beam

Detail model of the connection

points

Alternative at large models


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Example of a truss structure FEA beam elements

E-site Flexenclosure


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Shell elements

Used often in welded structures

Can handle automatic Vlasov torsion The shells system line coinside with the steel plates mid line:

Difficult to automatic mesh

Problem with thick plates

Difficult to model fillet welds

Account for singularities ?

5 Degrees of Freedom


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Shell elements

Triangular Quadratic


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Shell elements

disc elements (skivelement)

Plane elements which is loaded and deformed in its own planeCan distribute

Tension, Compressiom, Shear

Plate element (plattelement)

Can only take bending loads, no tension and compression loads in the

plane


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Shell elements

A combination of plate-and disc elements

Thin shells

Do not take into account the shear deformation in the thickness direction

(Kirchhoffs plate theory)

Thick shells

Take into account the shear deformation in the thickness direction

(Mindilins plate-theory)

Today thick shells is often used


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Shell elements

Example of applications


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Volume elements

Solid volym elements, 3D

3D elastic stress fied Stress strain relation

Strain displacement

Equilibrium

l l


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Volume elements

Typical 3D solid elements

Tetrahedron (tets) Penta

Hexahedron (brick-element)Avoid to use linear (base function), 4-noders, tetra-elements in 3D stress

analysis (not accurate, but OK in staticdeformation and/or vibration analysis)

V l l


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Volume elements

3D solid element

Hexahedron (brick-element)

V l l


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Volume elements

3D solid element

Tetrahedron (tetra-element)

V l l t


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Volume elements

3D solid element

Easier to use when evaluating welded structures than shell elements

Problem in connection between solid-shell models

Sub modelling when many welds need to be evaluated and in largemodels

Fi it l t l i


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Consider the problems physical characteristic

Warping of open thin walled profiles ?Buckling, tilting, column buckling, global buckling ?

Linear or non linear analysis ?

Residual stresses?

Contact problems ? Non linear properties ? Large deformations ?

Fatigue ? Dynamics ? Wave distribution in the material ?

etc..

Simplifications are required !

Fi it l t l i


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Critical evaluation of the results

Control of reaction forcesCarry out simple hand calculations in areas of the model which are easyto control

Present results by the element solution (not by nodal solutions; mean

value)Study the deformation distribution of the model

Never accept the FE results you do not understand or can explain !

Fi it l t l i


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Exemple of Solvers (FE program)

ADINA, ANSYS, ABAQUS, MSc MARC, NASTRAN,

Integrated systems

IDEAS, ANSYS, MEchaica, Cosmos

Exemple of PRE/POST processors

Patran, Pro-E, Hypermesh


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Singular points

i l i


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Singular points

Si l i


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Singular points

Evaluation of singular points

Never read stresses which are affected

by singular points !Evaluation of nominal stresses meanextrapolation of stresses.

Example 3.4.1


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Example 3.4.1

Calculate the forces in all the elements in the truss system

Hint: All the elements in the truss system only takes tensile or compressive

loads. Static/Force equilibrium in the joints!

Use FEA software Analysis for WindowsP = 10 kN, L = 500 mm

iwsd m3_4 -analysis methods for structures

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