Newcastle-based Ryder has merged with the fifth biggest practice in the US, HKS, to form the 10th largest international office in the world.
The new firm - which will have 700 staff and joint head offices in Newcastle and Dallas - will be called RyderHKS and have a turnover of £100 million-plus.
The merger was, according to outgoing RIBA president and Ryder chairman Paul Hyett, instigated by a joint healthcare collaboration between the two.
Hyett, the new chairman of RyderHKS, said the move reflected problems he has highlighted during his presidency, both in the profession and the wider industry. 'The construction industry is dysfunctional and often underperforms for clients, ' he said. 'The RyderHKS goal is to redefine the way the industry works in order to respond to clients' real needs.'