1. Bridge Analysis in GSA Neil Carstairs

2. Topics  Influence lines  Modelling  Combinations and enveloping  Bridge Analysis in GSA workshop

3. 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

4. Support Moment Influence Line

5. Structural Modelling - Definition Distilling informative, clear models from a structure to answer the questions being asked, or to show key structural behaviour.

6. Structural Modelling Manual Available on the SSN Intranet site under Reference - Guidance Notes

7. 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

8. Modelling  There is no such thing as an exact model

9. 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

10. 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?

11. 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.

12. 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

13. Checking Results  Check member orientation and relative stiffnesses  Check loading  Review modelling assumptions  Review hand analysis. Computer results may show trends not previously recognised

14. 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

15. 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

16. Envelope Options C1 C1max C1signabs

17. 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

18. 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