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CROSS Safety Report

Understanding the difference between analysis and design

Report ID: 372 Published: 1 April 2015 Region: CROSS-UK

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A reporter is becoming increasingly concerned about design practice and correct understanding of the difference between analysis and design and the linking of the two.

Key Learning Outcomes

For structural design engineers:

  • It is good practice to check and validate computer output against approximate hand calculations 

  • Checkers of design models should ensure the model and its input data are appropriate, and that the output makes sense. The checker should consider if anything has been omitted or overlooked.

  • The importance of validating software is noted in the Institution of Civil Engineers Civil Engineering Journal August 2013 - The importance of understanding computer analysis in civil engineering.

Full Report

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The Full Report below has been submitted to CROSS and describes the reporter’s experience. The text has been edited for clarity and to ensure anonymity and confidentiality by removing any identifiable details. If you would like to know more about our secure reporting process or submit a report yourself, please visit the reporting to CROSS-UK page.


A reporter regularly sees output from computer models which give the sway frame moments necessary for overall stability. However, they also find quite often that in the design the relevant members are not able to perform as in the model. 

There is a prevalence of composite design, which they use themselves, but often members are designed as composite when the model, and the frame stability, requires end moments. 

Concrete and tension do not go together too well and they are sure that in almost all situations these members are designed as simply supported. What is going to happen in service?

Expert Panel Comments

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Any analysis model is just that: a model. If the structure does not match the assumptions made for the model then the whole process is invalid. Some engineers do not ensure that the designed (and constructed) structure or element accords with the computer model from which the forces have been derived. 

A reviewer has found that this is particularly the case with member end and longitudinal restraint conditions. The classic view is that engineers should be able to check computer output against approximate hand calculations and they should certainly understand what they are doing. 

When large and unusual structures are involved the task becomes particularly important. Education and continual reminders are needed to ensure that computer aided design is carried out correctly.

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