Bridge Analysis in GSA

Neil Carstairs

Topics

Influence lines

Modelling

Combinations and enveloping

Bridge Analysis in GSA workshop

Influence Lines

Gives effect of moving load at a point

Influence line only gives results for the point chosen

Be selective, use engineering judgement

Don’t look at every point on a beam

Support Moment Influence Line

P inP in P in P in

xy

z

Part is excluded by volumeScale: 1:433.209Influence Lines: 0.250000 /pic.cmBeam Influence Effect 2: suppmomCase: A4 :"Influence"

Structural Modelling - Definition

Distilling

informative,

clear models

from a structure

to answer the questions being asked,

or to show key structural behaviour.

Structural Modelling Manual

Available on the SSN Intranet site under Reference -

Guidance Notes

Modelling

We do not analyse the structure but a mathematical model

The properties and behaviour of the model are chosen by the engineer

Approximations and assumptions must be made to create the model

Modelling

There is no such thing as an exact model

Modelling

The results of computer analysis are ‘precise’ but not necessarily ‘accurate’

If the model is not carefully chosen the results could be dangerously wrong

Before Starting

What needs to be demonstrated?

How simple a model will do this?

What is ignored by this model?

Are these effects significant?

What results will be produced?

Can the results be used?

What happens to the reactions?

Preliminary Hand Analysis

Engineer must be able to visualise how he expects the structure to work.

He must have an idea of the magnitude and form of answers before any analysis is done.

Sketch the expected deflected form and do simple hand calculations.

Checking Results

Always confirm no warning messages

Look at deflected forms for each basic loadcase and check magnitude

Check sum of reactions against loads

Check restraints and reactions

Compare results with predictions

In case of discrepancy check data again

Checking Results

Check member orientation and relative stiffnesses

Check loading

Review modelling assumptions

Review hand analysis. Computer results may show trends not previously recognised

Checking Results

If computer results and hand calculations cannot be reconciled seek advice

Never accept the results of a computer

analysis without an engineering explanation for the behaviour predicted by the computer

Combinations and Enveloping

Keep an eye on permutations

if C1 has 5 permutations and C2 has 6, C1+C2 has 30 (record 500 million).

Enveloping operators: C1max + C2max has one permutation, but loses coexistent effects.

Analysis envelopes retain coexistent effects

Watch it with graphics on envelopes

Envelope Options

C1

xy

z

Element list: P1 Part is excluded by volumeScale: 1:595.847Moment, Myy: 5000.00 kN m/pic.cmCase: C7 :"EnvULS(Max)"

xy

z

Element list: P1 Part is excluded by volumeScale: 1:595.847Moment, Myy: 5000.00 kN m/pic.cmCase: C10 :"envmax"

C1max

xy

z

Element list: P1 Part is excluded by volumeScale: 1:595.847Moment, Myy: 5000.00 kN m/pic.cmCase: C11 :"envsgnabs"

C1signabs

GSA Bridge Analysis

See Step-by-Step Guide in Help 3 methods: Full optimisation for UK/HK highway

loading Lane by lane optimisation for other loads Direct placing of vehicles on the model

Analysis stages – construction sequence

Influence Line Analysis

Influence lines on centreline of each path

Beam forces and displacements

Node displacements and reactions

Combined effects possible

Graphical and tabular output

Influence line lobes tabulated