Legislation calling for seismic retrofitting of many wood-frame soft-story San Francisco buildings was introduced at the San Francisco Board of Supervisors meeting Tuesday.

The proposed legislation would require that buildings built before 1978 that have a first floor that is not made of fully supported walls, such as a restaurant, a storefront or other openings and other "soft" features are retrofitted within the next seven years or less.

All wood-frame construction buildings that have five or more residential units and more than three stories would be required to become seismically safe, and all homes regardless of having a soft story would need to be evaluated to determine their vulnerability.

"We've known since Loma Prieta this is a top priority," Supervisor Scott Wiener said at this afternoon's board meeting at City Hall.

Board President David Chiu noted that there are as many as 3,000 vulnerable soft-story buildings scattered throughout the city, affecting even more residents.

Supervisors Wiener and Chiu along with Mayor Ed Lee put together the proposed ordinance. Supervisors Norman Yee, Mark Farrell, London Breed and Eric Mar have co-sponsored the legislation.

During Loma Prieta, a 6.9 magnitude earthquake that struck the region on Oct. 17, 1989, soft-story buildings fared poorly, and often collapsed or caught fire.

The retrofit ordinance is part of the city's Earthquake Safety Implementation Program, which is an effort of the Community Action Plan for Seismic Safety, which made a series of recommendations for policy change and retrofitting at the beginning of this year after working with earthquake, housing and building experts.

The seismic program has received some funding through several voter-approved bonds, according to the mayor's office.

The burden of paying for soft-story upgrades by 2020 would fall on building owners, who may look to the city to help with financing what earthquake officials consider mid-range retrofit costs, according to city officials.