In 1961, a partnership was formed between Rietveld and Van Dillen, who had joined the firm in 1958, and Van Tricht, who joined in 1959. Rietveld functioned in this partnership primarily as the designer. Van Dillen and Van Tricht were the ones who took care of the organization of the growing practice and contributed their architectural knowledge when carrying out extensive assignments. They also produce their own designs, although to a lesser extent than Rietveld. The number of employees at the beginning of the 1960s is 22. In July 1964, Rietveld died at the age of 76. Van Dillen and Van Tricht continue work on the remaining commissions, some of which were already given before the association. As of January 1, 1965, the name of the partnership is changed to Van Dillen and Van Tricht. In March 1966, Van Dillen became incurably ill. He dies in December of that year. Van Tricht then leads the firm alone. In September 1968, he associates with Timmler, who joined the firm in 1961. The bureau's name now becomes Van Tricht and Timmler. Van Tricht's duties consist of overall bureau management, completing and commissioning unfinished designs by Rietveld, and creating designs for new commissions. Timmler's task is to complete designs by Van Dillen and to monitor architectural quality. The firm moves to Sterkenburgerlaan in Doorn at the end of 1971. There the partnership breaks up into two offices; in 1973 the partnership is dissolved. Van Tricht continues the office in a smaller size. In 1975 Van Tricht moved his office to the Otselaan in Werkhoven. He transfers the office in 1986 to the architectural and engineering firm Ovink, Simonetti and Thijssen, based in Doetinchem and Werkhoven.