Chinese real estate conglomerate Oceanwide Holdings has broken ground for a mixed-use project in downtown San Francisco, which contains what is expected to be the second- tallest building in the city upon completion in 2021.

"Oceanwide Center", containing two towers, is the Beijing-based company's first mixed-use development in San Francisco, comprising office, residential, hotel and retail.

The 910-foot, 61-story building facing First Street would be surpassed by the 1,070-foot Salesforce Tower (to open in 2018) and would top the current tallest building, the 853-foot Pyramid.

The residential building will have 109 condos from the 41st to the 61st floor, and 1 million square feet of office space from the seventh to the 40th floor. The condos will be the highest residential units in San Francisco, said the company.

The other tower facing the Mission Street will be 625 foot tall and have 54 stories. It will be home to the Waldorf Astoria San Francisco and 156 high-end residences on the top floors.

Han Xiaosheng, executive board director and president of Oceanwide Holdings, said the project was an "important milestone", while addressing the ground-breaking ceremony in front of the construction site on Thursday.

"The groundbreaking ceremony is a symbolic step to represent our confidence in San Francisco," he said. "San Francisco has enjoyed great growth in recent years, and has the potential to continue attracting investment from all over the world."

Instead of talking about the skyline, San Francisco Mayor Ed Lee said he wanted to focus on "how tall the benefits are".

"This project is more than a skyscraper. It is a partnership that reflects a lot of sensitive discussion in the last a year and a half. It reflected almost $130 million of fantastic benefit to our city," said Lee.

Aside from the increase in job opportunities, he said the open space would be increased and used to build houses for the city.

"We'll make a tremendous contribution - in fact, a $44 million contribution - in housing to make sure people within a mile benefit from this actual project," said Lee. "People in Chinatown can see more of the open space preserved and enhanced."

According to the company, Oceanwide Center spans eight lots and totals approximately 60,000 square feet, and half the area will be dedicated to public open spaces. There also will be a 68-foot-tall public square, or "Urban Plaza", adjacent to First Street.

Han said the company's goal was to create high-quality mixed-use real estate downtown and establish its brand while seeking long-term partnerships with local communities, such as preserving the architectural heritage of the city.

Oceanwide Center will also include renovation and restoration of two historic buildings at 78 and 88 First Street, both more than 100 years old. Those buildings will provide additional office and retail space.

The project's design team - the London-based Foster + Partners and Heller Manus Architects - acknowledged that it's a "difficult" and "huge responsibility" to "get it right" against the "incredible skyline".

The higher tower at the First Street takes on a crystalline form to articulate the faades on the skyline. It will showcase intriguing metal framework that forms angled planes, similar to the facets of a diamond.

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