Los Angeles-based technical and support services firm AECOM has merged with Hayes, Seay, Mattern & Mattern , a Roanoke, Va.-based architectural and engineering firm with more than 650 employees.Kahler Slater , Milwaukee, has acquired Performa Higher Education , Green Bay, Wis.-based consulting and design group specializing in work for small private colleges and universities.De Rito Partners Development , a Phoenix-based commercial real estate developer, has named Tom Gast president and partner of De Rito Partners West . Based out of the company's Anaheim Hills, Calif., office, Gast will be responsible for the acquisition and development of retail, mixed-use, and redevelopment opportunities in California and Nevada.Gerald T. Olson , AIA, NCARB, has been promoted to director of architecture with Durrant Group , Dubuque, Iowa.CORE Architecture + Design , Washington, D.C., has named I. Guyman Martin , AIA, principal. Martin will lead the architecture team and oversee base building projects.New York-based structural engineer Thornton Tomasetti has opened an office in London. The office will operate under the name Thornton Tomasetti Postawa de Hoog through a recent alliance with London-based structural engineer Les Postawa and Tanya de Hoog .Gray Construction , Lexington, Ky., has promoted Phil Seale to VP of the Manufacturing Market. Seale will be responsible for developing and implementing market strategies, plans, and goals for the company's work in the manufacturing sector, including projects for Mitsubishi, Kaneka, Nestlé, and P&G.The Treeline Cos. , a New York-based owner and manager of office properties, has named John M. Szubski, AIA, senior architect. Szubski will oversee all aspects of the firm's architecture department, including construction oversight, design of capital improvement projects, and coordination of tenant buildouts. For more: www.BDCnetwork.com .