He was a civil engineer, inventor, university professor, member of the Serbian Academy of Arts and Sciences, one of the founders and managers of the Institute for testing of materials and the winner of the highest domestic and international awards. Apart from lecturing at home, he gave lectures at several foreign universities. He got his education in Serbia and worked here, but in the period after the Second World War, he became one of the world pioneers in the implementation of technology of building using the reinforced concrete. The system of building that he invented and protected with over 40 patents in the period from 1950 to 1994, was known under the name IMS – Žeželj, and he applied it on over 60 built objects, including 21 bridge and 20 halls. His name is most of all connected with the patented skeletal system of industrial construction, which has been implemented on over one thousand residential buildings made of reinforced concrete in the country and in the world. He left behind him also 108 scientific and expert studies and he participated at the great number of national and international congresses. The ex-Yugoslavia was among the highly developed countries in the field of civil engineering, due to the implementation of his patents.
The most famous work of Žeželj is the Great Exhibition Hall of the Belgrade Fair, which, since 2009, has acquired the status of the monument of culture. One of the most important bridges, applying the Žeželj technology of building is the railway and highway bridge over the river Danube at Novi Sad, where due to the implementation of exceptional novelties in the construction of bridges, he was awarded in 1960 in Rome at the Congress of the World Association for Reinforcement.