the useless housekeeper
DESCRIPTION
book on home keepingTRANSCRIPT
The Use of Computer for
Engineering Calculations
Background
• Ever increasing use of computers
• Deliver results of calculation very quickly
• Less time to think and rethink
• Industry feedback – over reliance on
computers
• Incorrect modelling has led to mistakes,
even disasters.
Planning – establishing purpose
• The first step in modelling -- define clearly
the purpose of the work.
– What do we want to learn from modelling the
engineering system?
– How are the results to be used
– How accurate do they need to be
Planning – establishing purpose
Examples:
• To decide the required size of beams, etc.
• To check whether a design meets the
requirements in a code of practice
• To predict deformation and stress distribution
• To study dynamic behaviour
• To investigate the effect of structural damage
Understand the purpose of the work
• The roof of the Hartford civic stadium incorporated several
novel features.
• In fact the computer provided
only an estimate of the forces
in the members.
• The capacity of the members to sustain these loads, especially those in compression, was not checked and the original errors went unchecked.
• The roof collapsed under the weight of snow.
January 18, 1978
Planning - model planning
Model Resource
Engineering model
(Real structures) Hardware
Conceptual model Software
Computational model People
Calculate
Acceptance criteria Performance criteria
Model validation
Results verification
Review
Revise no meet requirements yes Proceed
Clearly define acceptance criteria
• An engineer was asked to produce the outline
design for a 3D space-truss structure
• Drawings produced by a CAD program on which
the member forces were listed.
• However, in order to detail the connections the
steel fabricator needed information about the
forces at the joints in specific directions
• The program had to be re-run to provide
information in this form
Modelling
• Be clear about the purpose of the model and how
the results are used
• Do not use a model that is more complex than
necessary
• Do not examine local behaviour in a model which
deals with overall behaviour
• Use hand calculation to supplement engineering
judgement and assess expected behaviour
• Start from simple and prepared for modification
Engineering Model
• A description of the physical entry to be
studied in the modelling:
• The overall geometry
• The load bearing components
• Supports
• Materials
• Loading
Conceptual Models
• Derived from an engineering model
• Introduces assumptions
• Consider material behaviour
• Structural theory
Conceptual Models
• Should the model be 2D or 3D?
• Where are the load paths and how does the structure carry the load? Bending or torsion?
• What are the potential failure mechanism?
• Is local bending/stress important?
• What are support conditions to be imposed?
• How do the joints interact with other parts?
• Does the structure interact with other bodies?
• What will the loading be?
• Will any second order effects be significant?
Appropriate Conceptual Models
• The structural framework, supporting
pulverising machinery in a cement mill,
experienced intense horizontal vibrations
which affect the entire building.
• Instead of using a full 3D model to study
the causes, the engineers mistakenly
selected a 2D model, which could represent
only vertical vibration.
Engineering Model
Conceptual Model
A Floor with Pinned Supports
Conceptual Model
A Floor with Fixed Supports
Conceptual Model
A Floor-Column Model
Computational Models
• The result of describing a conceptual model
in a specific computer analysis program.
20 kN
x
y
A
B
Fy = 20 kN
X
Y
Z
Requirements for engineering model
Identify essential features of structures and the
loading on structures
• Clear spans, frame centres, storey heights, wall
and slab sizes
• Structural components and non-structural
components
• Supports and connections
Requirements of Conceptual Model
It should adequately describe the behaviour
of the engineering model. For example
• Have the B.C. been adequately modelled?
• Do loads and load combinations cover all cases?
• Have suitable material properties been selected?
• How are the eccentricities considered?
Requirements of Computational Model
Whether it solves the conceptual model to a
satisfactory accuracy? For example
• The date input should be peer reviewed
• Warning and error messages are removed.
• Results for standard solutions are satisfactory
• Discretisation errors are removed.
Model Validation
• Demonstrate that a model is suitable for its
intended purpose.
• Validation can be fully addressed only
when the results are available.
• All assumptions and approximations
adopted should be systematically revisited
and evidence sought to support them
Model Validation
Evidence may invalidate a model, for example
• Stresses higher than yield in a linear analysis
• Higher stresses outside recognised load paths
• Large deformation in a ‘small-deflection’ model
• Boundary effect contrary to expectation or
assumptions
Checking all assumptions
• An engineer modelled the cables in a stadium
glass window frame
• Bar elements were used, which are capable to
carry both tension and compression
• The window failed during a wind storm
• The resulting compression overcame initial
tension
• Failed to check the ‘no compression’ assumption
Verifying Results
• Demonstrate that the computational model has been solved correctly.
• Do the results correspond to what was expected?
• Check input and output for obvious errors
• Check overall equilibrium
• Check support conditions have been applied
• Check for symmetry if present
• Check deflection shape and distribution of stress
• Compare results with those from other programs
Verifying Results -0.9883E-3
X
Y
Z
Estimated Bending Stress Distribution Along the Cross-Section AB
0.00
0.04
0.08
0.12
0.16
0.20
-300.0 -200.0 -100.0 0.0 100.0 200.0 300.0
Bending Stress x (N/mm2)
Dis
tan
ce f
rom
B (
m)
Lusas
Cantilever
Fixed-Fixed
77.9926E6
CONTOURS OF SE
Loadcase 1
LOAD CASE = 1
RESULTS FILE = 1
STRESS
99.2633E6
106.354E6
92.1731E6
85.0829E6
42.5414E6
63.8122E6
70.9024E6
56.7219E6
49.6317E6
21.2707E6
35.4512E6
28.361E6
14.1805E6
7.09024E6
0
Max 0.1169E+09 at Node 578
Min 0.3465E+07 at Node 279
Check data and verify output
• The roof of an aircraft hanger was designed as a reinforced concrete folded plate
• A computer aided design software was used
• The designer failed to tick the appropriate box on the input form and as a result self-weight was not taken into account
• The structure had to be strengthened, at considerable expenses, after completion
Typical Composite Structure
Checking Results
• Use a simplified or equivalent model for
hand calculation.
• Convert an unknown problem to a known
problem
• Apply the basic concepts
Modelling in Your Designs
• 2D or 3D model?
• How to model cables?
• How to model towers?
• How to model trusses?
• How to model a connection?
A temporary grandstand
Back of the Grandstand
Engineering (AutoCad) Model
Computer Model
18004 mm 25720 mm
11000 mm
X
ZY
First Vibration Mode