30 November 1999
Waiting in the wood panelled office for Fritz Leonhardt to arrive takes me back to my school days. It's a bit like waiting for the headmaster when you weren't quite sure whether you had been summoned for punishment or congratulations. The thrill I felt when he first agreed to be interviewed has been replaced by a mixture of excitement and fear and I have run through the interview so many times in my mind that I am word perfect.
Although he now visits his practice only occasionally, Leonhardt's office is kept exactly as he likes it. From outside in the corridor, the room even has the forbidding appearance of a headmaster's office; on the door is a large wooden frame with one carefully written word - LEONHARDT - inside it. The wood panelled room is hung with numerous pictures of his bridges and television masts, and portraits of some of his lecturers.
Various messages on the desk await his response, and the latest bridge books are ready for perusal. Leonhardt's involvement in bridge design stretches back sixty-five years over a well-documented career in which he has been at the forefront of numerous technological developments - not just in bridges but also in other sectors of structural engineering - written many highly regarded books, including his seminal 'Bridges: Aesthetics and Design', and founded and led his internationally respected design office Leonhardt, Andr' und Partner.
Having celebrated his 90th birthday this year, Leonhardt has slowed down considerably, although even up to a year ago he was a regular visitor to the office, which is based in three converted buildings in a residential area of Stuttgart. But despite having difficulty moving around, his mind is still sharp and he takes a firm interest in current projects and events.
As part of his birthday celebrations he set up a new structural engineering award - the Fritz Leonhardt prize - and at a major presentation earlier this year the first award was given to Michel Virlogeux. In the bridge world, Leonhardt attracts particular respect for his devotion to the subject of aesthetics, the central theme of his famous book on bridge design.
He has spent his whole career attempting to both understand the rules governing the aesthetics of bridges, and apply these successfully to all of his designs. It is perhaps not surprising to learn, therefore, that at an early age he had to make a decision between architecture and engineering. "My father was an architect," he explains, "but my professional choice was civil engineering because it covers a much wider range than architecture - bridges, towers, roads, dams - and gives greater scope."
But despite the options and freedom that the profession offered, Leonhardt very rapidly demonstrated his interest in bridges. He studied civil engineering in his home town - at Stuttgart University - and after four years studying for his diploma, won an exchange scholarship to the USA to continue his graduate studies. The scholarship was to Purdue University in West-Lafayette, Indiana. While he was there he spent some time working with his uncle on the design of a river bridge in Detroit, but more significantly, when he first arrived in New York, he immediately arranged meetings with David Steinman and Othmar Ammann.
Talking to the two greatest living bridge designers in the USA only strengthened his interest in bridges, and this experience was to stand him in good stead when he graduated. Not only did his visit to the USA bring him into contact with engineers who had extensive knowledge of suspension bridge design, but events in his own country during his absence made him aware of his total ignorance of politics.
"Before I went to the USA I had had absolutely no political education," Leonhardt recalls, "and I was completely surprised at what was happening in my own country." When Adolf Hitler came to power he remembers being staggered by the interest from his fellow students. "I was overwhelmed by questions from the other students," he says, "and I couldn't answer any of these questions." In fact Leonhardt ended up working as an engineer in the government's Autobahn department.
He considered himself very lucky to land the job in 1934 as there was very little work for civil engineers in Germany at the time. Of his 40 fellow graduates, only two found jobs in the country - most of them had to emigrate. In fact if the job in the highways department had not been available for him, Leonhardt believes he would have stayed in the USA.
It was in the early days of his career with the roads department that he was commissioned to design a major suspension bridge, the first in Germany. Until this time the country had not needed long span bridge structures for its relatively modest crossings, but the industrial development of the Rhine valley and the expansion of the motorway system made it necessary to adopt this structural type for new bridges.
"One of my first tasks was to design a bridge to go over the river in Koln," says Leonhardt. "I was only 25 years old and was put in charge because the chief of highways and bridges in Berlin knew that I had studied these bridges during my time in the USA," he says. He was understandably shocked that he had been asked to do this, but saw it as a challenge and resolved to meet it.
The bridge was the Rodenkirchen Bridge over the Rhone which had a main span of 378m. Despite the knowledge he had gained during his meetings in New York, Leonhardt chose not to imitate the heavy truss girder common to USA bridges, but instead designed his own version - a slim deck stiffened with plates. Ironically the design was copied later by Ammann for the Bronx-Whitestone Bridge and the ill-fated Tacoma Narrows Bridge - unsuccessfully in the latter due to the aerodynamic flutter effect which was caused by the deck being high above the water.
Leonhardt's first major bridge did not survive the destruction of the war and was rebuilt between 1951 and 1954 and later still, widened under the design of the same office. Rodenkirchen was not the only casualty of the war, and once hostilities were over there was a major programme of rebuilding which kept Leonhardt busy for many years. He had already founded an office in Munich in 1939, and in 1945 the practice moved to Stuttgart and a partnership was set up with Wolfhart Andra and Willi Bauer.
The company has been solely a structural engineering practice ever since - now with four offices and some 170 staff working on bridge design and structural engineering projects around the world. Also among Leonhardt's early projects was the Koln-Deutz bridge over the Rhine, completed in 1948, which was the first really slender large-span hollow box beam. It has a span of 185m and a slenderness ratio l/d of 56. When the bridge was widened in the late seventies, Leonhardt demonstrated his skill with prestressed concrete technology as well as his commitment to aesthetics, by designing a twin structure of a lightweight prestressed concrete box girder with exactly the same shape in elevation as the original bridge.
The development of the modern cable-stayed bridge is often cited as one of Leonhardt's great achievements, and although he was neither the first nor the only engineer involved in designing this new form, he was one of the first engineers to exploit its potential. One of Leonhardt's earliest cable-stayed designs was the Theodor Heuss Bridge in Dusseldorf, the first of four built in the city. It was built in 1957 and had a main span of 260m supported by four pylons.
But the most famous of the so-called Dusseldorf bridge family is the Knie Bridge. At the time of completion in 1969 it had a world-record main span of 320m. Leonhardt went on to design many cable-stayed bridges around the world in collaboration with other engineers in his practice. His coaching was also instrumental in forming the foundation for a number of other influential structural engineers, such as Jorg Schlaich who worked with Leonhardt until 1980.
As well as his work on cable-stayed and suspension bridges, Leonhardt was also closely involved in the development of the incremental launching method of construction. With this new system, which is still regarded as highly specialist and technically challenging even today, the process of constructing viaducts over steep valleys underwent a radical revision. Of course Leonhardt did not just make his name with his innovative bridge designs - he challenged convention with his work on cable net structures, such as the roof for the Munich Olympic Stadium, and he built a number of high rise concrete television towers.
His first design, for the 210m high television tower in Stuttgart which was opened in 1955, was the first high rise reinforced concrete tower in the world. It set a trend for construction of similar towers all over the world, but the tower in Stuttgart must class as one of the most elegant examples, with the gentle taper of the 160m high main shaft, and the inclined walls of the services pod at the top. "Every time I look at it, I know that this was the best design possible - I could not have done it any better," he says.
Designing bridges which were not only structurally efficient, but which looked good too, was high on the agenda for Leonhardt right from the start of his career. Although he had been writing books on various aspects of structural engineering since 1939, it was not until 1982 that he published his famous book Br'cken: 'sthetik und Gestaltung (Bridges: Aesthetics and Design). This extensive publication on the design and aesthetics of bridges was not only very influential at the time, but has stood the test of time and is still seen as the bible for bridge designers.
Leonhardt says that his main reason for writing the book was not just in order to educate engineers but for the benefit of everyone - planners, architects, clients and even the general public. His goal was to analyse bridge aesthetics, develop rules and set them down in clear terms so that everyone could understand them. "It was necessary to have a good book on bridges, not on the technical aspects but on the aesthetics. At the time it was regarded as something very new - no one had really thought about it before," he explains. The book took two years to research and write, and Leonhardt maintains that its contents are as relevant now as they were when it was published.
If he was to rewrite it now, he says, he would not change anything. He allows himself an indulgent smile when I tell him the story of a friend for whom the book was so inspiring that he decided to become a civil engineer. I suspect that he may have heard similar stories before, such is the influence of the publication.
Despite being the best known book, it is not by any means his only publication. Aside from some 300 reports and papers in various journals, Leonhardt has also published more than a dozen books, ranging from his technical 'red books' which are still bought every year by engineering students and which have been translated into eight languages, to a historical book on towers. Although he has had many great achievements throughout his career, it is obvious that his early work still holds great significance for him. When asked about his favourite structures, his immediate response is to name the Rodenkirchen Bridge for a very simple reason; "because it was my first big bridge".
The pride in his achievement at such a young age is still evident. His other favourite is the Neckar Valley Bridge in Weitingen - a slender viaduct which steps across the valley on tiptoe like a number of Leonhardt's other structures. Built in 1978 the bridge has tall slender central piers, with more sturdy double piers at either end.
He explains that the reason for the thin central piers was to reduce the visual impact the bridge would have on the landscape - "so we would have as little pier volume as possible in the valley". Double piers at either end were designed to resist torsion in the structure, but he recalls the difficulties he faced in getting the design accepted. "Most engineers had no feeling for the concept of torsional stiffness - in fact torsion is still a problem for many of them," he explains.
But as with so many of his new ideas, his arguments were persuasive enough to win the day. In passing he also mentions the Knie Bridge in Dusseldorf as one of the most interesting. Despite holding these up as his favourite bridges, Leonhardt says that he is happy with all of his designs - he confidently states that he does not think that he has made any mistakes in his work. And his experience is very wide ranging. "I have never counted how many bridges I have designed," he claims, "but I think it is quite a big number."
He admires the work of Jean Muller but says sadly that he believes there are not many engineers who can design really good bridges. Such skills do not come solely through education, he says, "it is a certain gift". His education at Stuttgart University was obviously inspirational though; he was strongly influenced by three of his professors, those who specialised in concrete structures, bridges and materials. But there were also outside influences, notably from architect Gerd Lohmer with whom Leonhardt worked in K'ln. "He was a very good adviser," recalls Leonhardt, "and he gave me some very valuable hints. Some of my knowledge about aesthetics definitely came from him."
This discussion leads naturally to the question of collaboration between architects and engineers. Leonhardt's response is that the two should definitely work together - a process which he is very familiar with - but he would prefer to see the two professions combined. Engineering and architecture should be brought together and taught as a single subject, he believes. Leaving aside such a radical move, he regards the engineer's job as being one of restraint.
"Engineers have to stop architects from creating nonsense," he states baldly. "In several cases I have had to stop architects from doing this," he reveals, "because some of them just wish to be sensational." In contrast he also believes that engineers can learn from architects, as long as the architect has an understanding of engineering and is not just seeking to attract attention. He is firm about the cost of building a beautiful bridge. "The cheapest design will not necessarily be the best design, and the owner must be willing to spend a little more in order to have a beautiful structure," he suggests. Leonhardt believes that owners in Germany are generally agreeable to this suggestion, but he says that this is not the case in other countries, for example in the USA.