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RE: PRECAST / PRESTRESSED CONCRETE

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Mr Yango


Remember that the actual seismic displacements will be significantly larger
than those calculated using the code level forces.  Thus when the
earthquake occurs there will be extensive cracking in the concrete.

The problem withprestressed structures is that when the concrete cracks
extensively on the compression face the tensioning will not be very
effective and the section will deteriorate rapidly.

Historicaly the problem with precast concrete is that the connections are
not as good as monolythic concrete.  As a result these systems have a
tendency to fall apart.

Recently several systems have been developed to overcome these problems. 
What they have done is to design the connections as fusable links that can
absorb energy and tie the members together while protecting the rest of the
member from excessive loads.

I still believe that properly designed steel structures will probably
perform better.   having said that I would probably consider using one of
the new systems if the circumstances were favorable and the client
appreciated the risks.

I believe that the 1997 UBC has a number of new provisions dealing with
prestressed and precast members in seismic areas.



Mark Gilligan