[BTinSF]

Originally Posted by Kelvin

I can't recall if this point was noted previously in the thread, but probably the single largest drawback to RC construction (from a seismic point of view) is its mass. Force = Mass x Acceleration. The more mass a building has, the more force it develops under seismic ground motion (and hence acceleration). However, they are generally stiffer too, so in the end displacements and rotations are smaller and the level of stress/strain discussed above is not achieved.

I'm sure you are familiar with "performance-based design" addressing this issue and perhaps even the current controversy over it in San Francisco and, I assume, other places. If not, see http://sanfrancisco.bizjournals.com/...26/focus2.html .

Money quote:

"Codes produce a really heavy, inefficient building," said Andy Ball, CEO of Webcor Builders. "And the codes become even worse the higher you get."

A performance-designed building, Ball said, uses materials more efficiently: "It costs more to plan it out, and the first few floors are more expensive, but by the end of the building, it's cheaper."

But not everyone is ready to race to the performance-design approach. The San Francisco Department of Building Inspection -- DBI -- last fall put a moratorium on what it calls "alternative" designs that don't conform to the building code while it comes up with guidelines to handle such projects.

"Anything using alternate analysis to take exception to the code is not being approved at the moment," said Raymond Lui, manager for the structural safety and emergency division at DBI. "Code-prescriptive projects will be approved . . . ."

The difference between the two styles of design can be seen in the Millennium condo project vs. the Infinity and One Rincon, Heller said. The Millennium, he said, is slow to be built, heavy and blocky-looking, whereas the Infinity and Rincon are "light, airy and just as safe . . . .

Future projects of more than 800 feet, like the proposed Transbay towers and Renzo Piano building at First and Mission streets, hang in the balance.

"It's not really possible to design those 1,000-foot buildings safely and confidently for seismic demands without using the performance-based approach," . . . .