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Seismic codes need more performance detail

Institution news

SEISMIC DESIGN codes do not provide enough detail to protect against unsafe buildings, ICE members were told last month.

Berkeley professor Vitelmo Bertero of the University of California voiced his concerns in a farewell address to a combined ICE Society for Earthquake & Civil Engineering Dynamics (SECED) and Wessex Institute of Technology (WIT) seminar.

Engineers are now under pressure from clients, he said, not only to protect lives when an earthquake strikes, but also to build structures that can still operate and fulfil their purpose.

'In California's 1989 quake few died and few buildings collapsed, but the financial and social impact - especially in Silicon Valley - was colossal, ' said Bertero.

Only 15% of buildings in San Francisco have been designed with seismic consideration. Bertero said:

'Retrofitting structures is therefore of prime importance for structural engineers and the evaluation of its cost benefits is the most important problem we face.'

He called for a change to codes to include an explicit definition of a building performance: ie is saving lives enough? Codes consider strength and base shear, which should ensure that tenants survive. But Bertero stressed that most damage is caused by deflection and plastic rotation and he urged engineers to 'provide the maximum ductility in the structure as is economically possible'.

He then underlined the need to design for waves of earthquakes rather than for just a single event. Although buildings may survive an earthquake and be passed safe to inhabit, reinforcement may have yielded or connections become elastic.

'In seismic design, there is no such thing as a single event.

Damage can accumulate so that when the big one hits - buildings do not respond as they are designed to.'

In codes, the standard load is modelled as a large pulse. This, Bertero argued, is a fallacy, as a real earthquake has a cyclical load. Such loading is equal to that of a single pulse of twice that magnitude, so many buildings, he fears, are underdesigned.

Current trends for modular construction with prestressed elements were also questioned as this dormant energy can add to that applied to the structure during a seismic event. Energy is considered to dissipate elastically through materials, which again Bertero considers an over simplification.

Base isolation techniques and the inclusion of damping units are too often inaccessible for maintenance and replacement - with potentially catastrophic consequences, Bertero warned.

In urban areas, designers are ignoring the risk posed by adjacent tall buildings, Bertero believes. Falling buildings impart a large force on those around, as was evident with the collapse of the World Trade Center.

'With over 70 years of development in seismic design the state of knowledge is largely advanced. But in practice we still use the same equations with the same assumed values, ' he added. 'There is no point in all this research if we do not transfer this knowledge into the profession.'

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