The Union Trust Building in Downtown Pittsburgh has secured a major office tenant at last.

Siemens Power Generation Inc. will consolidate about 500 local employees on three floors at the nearly 600,000-square-foot, 11-story structure — formerly known as Two Mellon Center. It has been mostly vacant since Mellon Financial Corp. pulled its employees out of the property in May 2006.

The 185,000-square-foot lease, which puts a significant dent in one of Pittsburgh’s largest office vacancies, will run for 10 years at the 501 Grant St. building, according to the Pittsburgh office of CB Richard Ellis Inc., which represented both the building’s owner and Siemens in the deal.

Orlando, Fla.-based Siemens Power Generation’s local operations, known collectively as Siemens Environmental Systems and Services, have employees at three Pittsburgh sites: 441 Smithfield St., Downtown; One Oxford Centre, at 301 Grant St., also Downtown; and 40 24th St., in the Strip District.

Earlier this year the state pledged $2.95 million in grants and tax credits to Downtown Pittsburgh-based Wheelabrator Air Pollution Control Inc., the division of Siemens Power Generation based at 441 Smithfield St., to assist with a $5 million expansion. The company agreed to hire at least 550 employees over three years as part of the expansion.

The company will begin moving into the Union Trust Building later this year with full occupancy set for the fourth quarter of 2008.

A Siemens spokesperson was not immediately available for comment Monday.

Simultaneously, the building’s new owners — California investors led by Los Angeles-based Mika Realty — are conducting an extensive renovation of the property with plans for LEED certification, a new lobby and installation of a lower level parking facility.

Serving as an operations center for Mellon Bank since the mid-1980s, the Union Trust Building was originally constructed between 1915 and 1917 by steel magnate Henry Clay Frick as a shopping arcade.