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Designing and Analyzing Weldments

VIEWS: 4 PAGES: 50

									   Designing and
Analyzing Weldments
 Original Presentation by:
 Stephen Endersby (Solidworks)
 Edited by:
 Richard Wand (MJ Engineering)
Introduction

 Presentation Objectives
   – Basic weldment
     construction techniques
   – Beam analysis is a good
     way to get started in FEA
   – How to setup and run a
     truss/beam analysis
   – Understanding the effect of
     modelling method on the
     analysis model
Let’s get started

 Weldment Fundamentals
  – How to sketch
      o   Multiple 2D sketchs
      o   Single 3D sketch
   – Inserting Structural
     Members
      o   Rotating Profiles
      o   Locating Profiles
      o   Corner Treatments
   – Trim/Extend
      o   Can also cut members
   – Gussets
   – Custom Profiles
First Weldment




         Using a 3D sketch, don’t forget to rotate the profile
Corner Treatment




    Change the corner treatments to ensure long members are full length
First Leg
First Support
Top Trim




           Careful use of trim/extend command
Side Trim




            Careful use of trim/extend command
Mirror Leg & Support




After mirror – check total
number of bodies…
It is very easy to get
double bodies and not
realize
Brace member and trim
Hide 3D Sketch




         Hide the 3D sketch and your finished…
Analyzing Weldments
 Creating the weldment is one thing – but your boss wants to
  know it will work and isn’t over/under designed
 The objective of this part of the presentation is to understand
  the underlying assumptions in the analysis of weldment parts.
Analysis of Weldments
 In COSMOS we have three types of element Solid, Shell
  and Beam.
 What is best for the analysis of weldments?
How COSMOS Works With Weldment Features

 Analyze frame with solid elements or shell elements
   – both result in an excessive number of elements
   – mesh along with the corresponding contact conditions may
     take some time to generate and run
 Beams are the natural choice to model structural members. For
  accurate results, the length of a beam should be large compared to the
  dimensions of its cross section (10-20 times or more).
 Beams are structural elements
   – cross sectional characteristics are accounted for
     mathematically rather than geometrically.
   – As a beneficial consequence, these cross sectional
     characteristics do not need to be reflected in the finite element
     mesh

   So what does all this “stuff” mean…..?
How COSMOS Works With Weldment Features

 We can start with a weldment like this…




 Notice all the interferences – and straight away…we can
  mesh/run/solve
How COSMOS Works With Weldment Features

 Assumptions
   – In general, the beam element has two nodes with six degrees of
     freedom in each node.
   – Each beam element is defined by a straight line connecting two
     joints at its ends. A curved structural member is modelled with a
     number of straight beams.
   – The cross section of a beam is constant throughout its length.
     Internally, the programs meshes each beam by creating a number of
     beam elements. When viewing the mesh and results, beam
     elements are represented by cylinders regardless of the actual cross
     section.
How COSMOS Works With Weldment Features

 The top image shows the structural member.
   The middle image shows the joints when the
   structural member is defined as a beam. The
   lower image shows the mesh, where the
   beam is subdivided into a number of beam
   elements represented by cylinders.


 The joints coincide with the pierce point of the
   weldment profile. It is recommended that you
   locate the pierce point at the center of gravity of
   the weldment profile to avoid unintended results.
How COSMOS Works With Weldment Features

 The beam mesh resulting mesh is made out of lines connected by
  joints. While the detection of the joints is fully automated in
  COSMOSWorks, the relative position of some joints may be too close
  and we may wish to merge them, i.e. merge two segments into one. In
  the figure above, the joints 1 and 2 are relatively close and can be
  merged.
How COSMOS Works With Weldment Features
Study 2 – Discussion of Results


 Results for each element are presented in its local
  directions. There is no averaging of stresses for truss and
  beam elements. You can view
   –   uniform axial stresses
   –   torsional
   –   bending stresses in two orthogonal directions (dir 1 and dir 2)
   –   the worst stresses on extreme fibers generated by combining
       axial and bending stresses.
Study 2 – Discussion of Results

 Beam Diagrams
  – Create shear, moment, torque, or axial force diagrams
    for beams or axial force diagrams for trusses. Beam
    diagrams are generated in the local directions for each
    beam.
Analysis 1

 Objective
  – Investigate a COSMOS
    centric modelling process
    on the table we designed
    earlier
  – Where do we start??
Analysis 1: Re-Work SW model

   Now take that pretty model and mess
    it up – big time
   Remove corner treatments and trims
    – Can present issues
   Mirror
Analysis 2

 Objective
  – To investigate the
    inclusion of gussets on the
    analysis process.
  – Replace corner beams
    with gussets
Create Gusset Configuration
Delete Support Bodies
Define A Gusset
Mirror the Gusset Twice
Analysis 2 (Cont)




 Use a mixed mesh rather than beam mesh
Analysis 2 - Create beams




Note; missing beams
Analysis 2 - Create shells for the gussets
Analysis 2 - Create missing beams
Analysis 2 - Restraints
Analysis 2 - Loads
Analysis 2 - Mesh
Analysis 2 – Results
Analysis 3 – The Mixed Mesh Approach
Analysis 4

 In architecture and             In COSMOS a truss is
  structural engineering, a        special beam element that
  truss is a structure             can resist axial deformation
  comprising one or more           only..
  triangular units constructed
  with straight slender
  members whose ends are
  connected at joints.
Analysis 4

 Objective
  – To understand how to
    validate truss structures.
Analysis 4
Analysis 4

 .
Analysis 4

 .
Analysis 5

 Many large portable buildings are simply a framework
  with a non load bearing skin.
 Geodesic or geometric domes are common
  structures.
 In this lesson we will look at how COSMOS handles
  these large frames.
Analysis 5
Analysis of Welds

 In the real world the beams
  and or trusses are
  connected together by;
   – Welds
   – Bolts
 Generally the analysis of
  these features is best
  analysed by idealisation
  rather than geometry
Welded Solids

 Common Misconception: Stress calculated on welds can be
  compared to base material allowables to determine
  acceptability
   – Weld geometry in SolidWorks probably doesn’t match real
     weld
   – Failure properties in weld probably don’t match base material
   – Stress calculated in weld feature is usually singular (=
     incorrect)
 For these reasons & more, use solid weld geometry for load
  path, mass, and visualization
 Load transfer thru welds may be valid
Weld Evaluation Methods…

 Static Failure – Throat Shear Method
   – Failures Typically Occur in Weld Throat
   – Failure Criteria Convention Compares Throat Shear Stresses to
     Allowable Shear Stress
   – Most Frequently Used Code is AWS D1.1
 Fatigue Failure – Hot Spot Method
   – Failures Typically Occur at Weld Toe, However; Criteria Does Exist
     to Evaluate Failure in Weld Throat
   – Failure Criteria Compares Hotspot Stress (also referred to as
     Geometric or Structural Stress) to Appropriate Weld Classification S-
     N Curve.
   – Governing Codes: BS7608, IIW-Hobbacher, ASME, etc.



                                                       51
Summary

 Design validation gives the most benefits when carried out
  early in the design process.
 Beam and truss analysis works best before trims and corner
  treatments
 To understand material loadings look at the worst case
  stress results from a beam analysis and the axial stress in
  truss analysis.
 Gussets should be ignored where ever possible.
 If you cannot ignore the gusset or plate work create a mixed
  solid beam study.
    Thank you

sendersby@solidworks.com
rwand@mjengineering.com

								
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