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Earthquake design philosophy outdated and crude, says expert

ICE news

THE THINKING behind most earthquake engineering is out of date, resulting in inappropriate designs, a leading authority warned last month.

'Current design philosophy and practice are much too crude when compared with the sophisticated analytical tools commonly utilised in the design office, ' said New Zealand-based academic and consulting engineer, professor Nigel Priestly.

Speaking at the Society of Civil Engineering & Earthquake Dynamics' (SECED) ninth Mallet Milne Lecture, Priestly revisited the controversial themes he presented 10 years ago in a lecture entitled 'Myths and fallacies in earthquake design'.

Then, Priestly contended that there were a number of fundamental problems with the forcebased design criteria used in structural codes.

Seismic design is based on structural strength and elastic structural characteristics. These may be modified by the ductile capacity of materials and behavioural factors. This approach leads designers in directions that are not always rational, said Priestly.

He offered an alternative design philosophy - direct displacement based design (DDBD).

Current design practice calculates a performance limit state for a structure or member under a given seismic intensity. The DDBD approach looks to estimate a structure's performance at a peak displacement.

'The design approach attempts to design a structure which would achieve, rather than be bounded by, a given performance limit state under a given seismic intensity, ' said Priestly.

Over the last 10 years this method has been developed in a more complete form and applied to a wider category of structures.

'It is simpler to apply, and better suited to incorporation into design codes, ' said Priestly.

Seismic design typically ignores duration effects and condenses response into 'snapshots' of behaviour under maximum conditions, he claimed. It also assumes that maximum transient response is more relevant than the final at-rest condition of the structure. Both approaches are inadequate, Priestly believes.

He also argued that stiffness estimates used in seismic design of concrete members generally refer to uncracked stiffness. As critical elements like beams usually undergo some cracking under gravity loading this assumption would be invalid.

This means the stiffness of members will be lower than that specified, leading to an inappropriate estimate of seismic forces.

Since his first lecture Priestly said that general awareness of DDBD has increased within the research community but that the impact on designers was minor.

He feels that there is still a real need for improvements in basic modelling of elements.

Designers would also benefit from simplified design methods, with computer analysis used as a tool to check validity of the final designs, he argued. 'Simplicity in seismic design is a virtue and this can easily be achieved.'

Details of Priestly's proposed changes can be found in Myths and fallacies in earthquake engineering revisited. For copies contact Pauline Arundel (020) 7665 2236.

INFOPLUS www. seced. org. uk/mm2003a. htm

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