| Bridge partners
on winning streak |
Prof. F.Leonhardt |
German engineers have
thrived on intense competition and none more so than the enduring
design practice of Leonhardt & Andrä whose founders celebrate
30 years of partnership this year.
"I was the first consulting engineer to make compatitive design
submissions for large bridges" says Professor Fritz Leonhardt
recalling a Germany of the 1940s when he was establishing his
association with Dr. Wolfhart Andrä. |
Leonhardt and Andrä
have accrued considerable steel design experience for their
practice and they claim responsibility for a series of significant
innovations, including streamlined box girder decks and unusual
ideas for cable stayed bridges. Though these claims are disputed
in Germany and overseas, many engineers would agree that the
firm´s two senior partners have helped push forward design boundaries
through their joint aggressive pursuit of work. |
Leonhardt is a "great innovator"
says one old colleague, and he
is considered to have been the salesman of the team.
His dominating interest in bridges was sparked in the early
1930s when he organised and won Germany´s first student
exchange scheme with the United States, and he hitchhiked around
North America between design offices and from one construction
site to another.
The more reclusive Andrä is reckoned to have provided analytical
backbone for the steady stream of alternative design schemes.
|
The two men met in pre- Second
world War Germany when Leonhardt was just beginning to work
independently though he was still an employee of the German
Autobahn Authority.
He was in his late twenties in 1938 and threatening to leave
for greater responsibility in the private sector when he was
offered charge of the design and the construction of Germany´s
first long span suspension bridge stretching 378 m over the
river Rhine at Cologne-Rodenkirchen. He stayed on in the public
service.
"This job gave me a chance to look out for good young |
engineers and to set up my own team
of reliable collaborators" he said nearly 40 years later accepting
the British Institution of Structural Engineers´Gold Medal for
original contributions to structural design.
Andrä had joined the suspension bridge site as design and structural
engineer in 1939 after five years´ university training. There
he forged a link with his later partner though subsequent work
together was postponed until after the world war during which
both men were forced to abandon engineering temporarily. |
Leonhardt had opened
an office in Munich in 1939 but after the war he was expelled
as a non- Bavarian to his birthplace of Stuttgart. He took a
job with the Cologne City Authority even though the occasional
300 km journey home then took "two nights and three days".
Andrä recalls working as a farm labourer in 1946 when Leonhardt
arrived on a bicycle and invited collaboration for a new, streamlined
steel box girder bridge at Cologne-Deutz. |
"There was little raw
material after the war and we were looking for the most economical
solution" says Andrä, explaining how the design team arrived
at West Germany´s first box girder bridge with a 184 m long
main span of "extreme slenderness" completed in 1948.
"It was far outside German code regulations" says Leonhardt,
noting that the bridge´s design deflection was nearly three
times greater than the officially allowed limit. |
Dr. W. Andrä |
Results from later
wind tunnel tests on slender decks were used to design a streamlined
suspended crossing of the river Rhine at Emmerich. But German
authorities this time felt the design too advanced says Leonhardt,
and a deck design using more conventional trusses was built.
"It could have been the first modern suspension bridge" he says.
"Instead its dismissal led to Freeman Fox & Partners leading
the field with the Severn bridge." |
The rejected Emmerich
design would have also introduced the monocable suspension system,
earlier advanced by Leonhardt
for Portugal´s Tagus bridge in Lisbon, where deck suspenders
are hung from a single catenary instead of two.
After the Cologne-Deutz crossing was finished the Leonhardt
practice of six employees including Andrä began developing prestressed
concrete techniques for West Germany. |
"We translated Freyssinet
works into German" says Andrä "and recognised a future." Prestressing
techniques were refined and specialist equipment invented. One
of the early inno- vations which became known as the Leoba system,
dispensed with individual wire wedges in favour of a single
wedge for the entire prestressing cable, and is still widely
used in Germany (NCE October).
In 1953, Andrä became a partner in the growing practice as overseas
work started coming L&A´s way. |
The firm had not sought work abroad
says Leonhardt, but it was approached by a Brazilian contractor
with German connections which was looking for prestressed concrete
expertise. "There were over 5km of bridges to be built" remembers
Andrä and L&A was kept busy in Brazil until the end of the decade.
Meanwhile the practice had been devising and promoting what
has now become a symbol of the German construction industry,
telecommunications towers in concrete. |
Leonhardt is a "stubborn, intuitive
engineer" says one colleague.
He was convinced that a television tower in concrete was a better
idea than yet another one in steel, both economically and aesthetically,
and he harangued Stuttgart´s broadcasting authorities until
they thought so too.
Construction of the 217 m high tower was completed in 1955 and
triggered a new demand in Germany which saw more than one concrete
tower built every year until the late 1970s. |
Further developments in bridge building
were pursued by L&A, most notably the evolution of incremental
launching: "It just appeared simpler where there was plenty
of space behind the abutment" says Dr.Andrä, though the idea
had been sparked rather more obscurely. "I used to read `Reader´s
Digest´ to get to sleep" recalls Leonhardt, when one night he
was prompted by an article on the domestic benefits of non-
stick Teflon to write to manufacturer DuPont about the engineering
possibilities for the new material. |
| "They listened and Teflon sliding
bearings were developed very quickly and accepted all over the
world." The earliest launched bridge using bearings drived from
frying pan surfaces was the first crossing of Venezuela´s Caroni
river in 1961 says Andrä. The entire 480 km long super-structure
was cast before launching. Such monolithic launching eased construction
but proved too costly for general application, and so the partnership
looked at a system of casting successive deck units and incrementally
launching the bridge. |
The method was first applied in
its modern form in 1968 at Kufstein in Austria for the Inn river
crossing.
Development is much slower these days concedes Andrä, but he
stresses that innovations will always be avidly sought while
economies in construction are chased. "Old systems must be improved"
he says, adding that "you can sometimes only learn through mistakes."
Now L&A as partner of Gruppo Lambertini claims to be in a position
to win the proposed design contract for the Messina Straits
crossing between Italy |
and Sicily with a spectacular cable
stayed bridge.
The proposed 3.5 km long crossing has a 1800 ,, long main span,
nearly four times the length of the greatest cable stayed span
built so far.
"We´ve been convinced in the past that our calculations have
indicated that their ideas were not feasible" says one L&A employee.
He continued:"But they´ve said `your assumptions must be wrong,
try again´, and they´ve been proved correct." |
