To nonresidents of San Francisco, it may be surprising that the design of a new, more earthquake-resistant Bay Bridge is only now nearing completion nine years after the 1989 Loma Prieta earthquake. But for residents of this city encumbered by politics and special interest groups, replacement of the 58-year-old span cannot be over-studied, over-discussed or over-designed.

Currently on the boards (or on the screens) are four alternative designs for the two-mile long, $1.2 billion east span of the San Francisco-Oakland Bay Bridge, slated to begin construction in the year 2000 and open in 2003-2004. Under the aegis of the California Department of Transportation and T.Y. Linn International, the San Francisco architectural firm Donald MacDonald Architects is developing two of the bridge designs using ArchiCAD.

"You get 3D results instantly with ArchiCAD, which is crucial in the early design stage,² MacDonald said.

With the help of ArchiCAD, MacDonald¹s team modeled and presented six suspension bridge schemes to the Bay Bridge Design Task Force and Engineering and Design Advisory Panel in only four weeks.

In addition to the obvious requirement of earthquake resistance ‹ the two mile span must withstand an earthquake of 8.3 on the Richter scale ‹ MacDonald had to include a bike lane, study views for both cyclists and motorists, consider the effects of added pollution to the Bay and adjacent Oakland wetlands, and accommodate the constraints of navigation channels and setting foundations on the Bay¹s muddy floor.

MacDonald notes that the bridge design should work in the context of the man-made environment. "The designs are based not only on how to carry the stresses, but also the context in which they are placed. We decided to keep the necklace of suspension bridges across the Bay intact to keep the continuity of design. It is more modern but similar in scale,² MacDonald said.

Bridge design is not new to MacDonald. A recently completed project to retrofit the Golden Gate Bridge afforded him the opportunity to further understand not only the engineering and architectural principles which drive design, but the social awareness surrounding a national landmark as well. Since opening his practice in 1965, MacDonald has received numerous awards for such diverse projects as low-to middle-income housing, bridge design, urban infill and commercial projects.

The Bay Bridge Design Task Force commissioned two teams to develop designs for self-anchored suspension and cable-stayed bridges. MacDonald¹s team developed six suspension designs while another team within T.Y. Linn¹s office developed the cable stay schemes. After an initial review in early March, two suspension ‹ a twin tower and a single tower ‹ and two cable stay designs were selected for further development. If chosen, the single-tower, self-anchored suspension bridge (shown at left) would be the first in the world. The task force is scheduled to select among the four designs in June of 1998.

Having used ArchiCAD for many projects in the past two years, MacDonald appreciates that he can keep his overhead low thanks to the software¹s efficiency. "I¹m able to keep my office lean and productive now,² MacDonald said. "You can get started so quickly because the learning curve is so much shorter than other CAD programs.²