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