This issue of Heron is dedicated to Prof. ir. Jelle Witteveen, who retired in April 1997 as director of the Netherlands School for Advanced Studies in Constructions and as part time professor at Delft University. Prior to his appointment as director of the Netherlands School for Advanced Studies in Constructions in 1990 Jelle Witteveen was the director Of TNO Building and Construction Research.

Jelle Witteveen has played an important role in the life and progress of "Heron". As director of both TNO Building and Construction Research and the Netherlands School for Advanced Studies in Constructions, he did give a strong stimulus to this Dutch journal which contributes so much to the disclosure of the building and construction related research work in The Netherlands. On top of that he was editor in chief from 1979 till 1990 and it goes without saying, he also made various active contributions as an author.

The retirement of Jelle Witteveen in 1997 gave his former colleagues an opportunity to pay homage for his magnificent input by compiling this special issue. It must be said that the writers were not only his former fellow workers. They were greatly assisted by the younger researchers which shows the interest of the present generation and promises a good future for "Heron". The issue deals with the various research subjects of Jelle Witteveen himself: fire safety, plastic analysis of steel structures and numerical models.

We wish Jelle Witteveen well and hope that he and all our readers will enjoy the articles in this Heron special.

Ton Vrouwenvelder
Editor in Chief