19 Century Iron Lenticular Truss Bridges from the Berlin ...
Development of structural connections of steel truss bridges ... K.pdfDevelopment of structural...
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Development of structural connectionsof steel truss bridges around 1900
The development of structures and its details isdetermined by both structural requirements and themain stream of contemporary architectural art.
An interesting detai] of such a development isthe change of the constructional type of rivetedconnections of truss girder bridges in cities.
Within some decades, the form and the dimensionsof the connections using so caBed gusset pJates havechanged considerably. That can be recognised by
some bridges in BerJin starting fram a simple form,after that changing to a more ambitious type causedby discussions about architectural aspects of subway
viaducts in Berlin during erection of the first subwayline and at the end coming to a simple technical typewhich is not satisfactory in terms of architecturalquaJity, however, following the new ideas of Bauhaus
and Functionalism and aspects of structural safety.The development is presented by giving some
examples and mentioning the discussion at that time.
An engineering aspect is introduced by looking atthe structural behaviour under severe loading andexplaining «weak» structural details which had to bechanged for the design of future bridges.
Some research work performed recently atBAM, Berlin, opens the opportunity of rational
investigations and rating of those bridges forextending their service Jife considerably, sometimes
with some strengthening and additional inspection
procedures.
Klaus Brandes
Figure 1Viaduct of the line ] of the subway in Berlin-Kreuzberg,
comp1eted 1902, a fiJigran truss girder system
Proceedings of the First International Congress on Construction History, Madrid, 20th-24th January 2003, ed. S. Huerta, Madrid: I. Juan de Herrera, SEdHC, ETSAM, A. E. Benvenuto, COAM, F. Dragados, 2003.
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INTRODUCTION
An important and interesting part of architecture is«Engineering Architecture» which not necessarily is
the outcome of architectural design, rather then ofengineering thinking and insight. Neverthe]ess, this
manner of engineering design is influenced by thestream of contemporary arts. ExampJes forEngineering Architecture are numeraus, e. g. the
Eiffel Tower, Paris and the CN Tower, Toranto, andbridges ]ike the Golden Gate Bridge, San Francisco,
and the Tower Bridge, London.A less ambitious structure is the stee! viaduct of the
subway system in BerJin which had been completed
in 1902 -exact]y hundred years ago. Studying theviaduct, the combination of optimal design in terms
of economy and safety and in terms of estheticsbecomes obvious (Landsberg ]904), figure 1.
The forms of Art Nouveau (calJed Jugendstil inGermany) have influenced the design with the pleasantshape of structural elements and details. However, thevery t1!igran truss girder system -giving a feeling of
lightness- contlicts to some extend with therequirement of structura] safety. The contrast to !ater
on built bridges is obvious when ]ooking at figure 2.While most buildings wiIJ be designed only folJowing
architect's ideas about forms which do not concemquestions of statics ~. g. the Buildings by Gaudi in
Barcelona- bridges have to be safe in its first priority.And this requirement causes conflicts in many cases.
There are severa! examples of structures whichout]ine the paths offorces like the Eiffel Tower or theOlympic Roof at Munich. Their form is satisfying
also in terms of esthetics, however, sometimes it is
Figure 2Bridge of the Berlin subway built some years later on
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K. Brandes
Figure 3
A railway bridge in Berlin-Spandau built in 1997. Theheigth of the girder follows ¡he stressing of ¡he girder
only interesting fram an engineer's point of view like
the new railway bridge in BerJin-Spandau, figure 3.Truss girders were an outcome of looking for stable
bridge structures which minimise the weight. Their
overall design as well as their structural details aresubject of looking for adequate appearance.
STEEL VIADUCT OF THE SUBWAY BERUN
The «Hoch- und Untergrundbahn» (subway) Berlinopened its service in 1902. The system has been
Figure 4A node of ¡heviaduct
Development of structuraJ connections of steel truss bridges around 1900 407
extended during the following years. Some of theoriginal parts have survived war time and demolition
mania later on. Now they are protected by law asmonuments of Engineering Art (Jager und Wachter1999).
The figures 4 and 5 give some impression about thedevelopment of the systems and the details.
Considering the viaduct at Berlin-Kreuzberg, theoptimisation of the gusset plates can be studied. Evennow, we are able to analyse the stressing of different
structural elements, also -to some extend- thestresses in the gusset plates and their response tofatigue loading -the ever and ever repeated loadingby traffic which- at the end -causes fatigue cracks
and failure (Brandes 2000).As far as we could find out by modern engineering
approaches and by many fatigue testing on completeoriginal bridges (Helmerich und Brandes 2002), theengineers who designed the viaducts have succeeded
in constructing very small gusset plates, howeverreaching a stress level at the most stressed regionsthat caused doubts about the resistance to fatigueloading for 100 years or even more.
Al'ter extensive investigations, we found, that thereal stress level permits a life time for more thanhundred years, may be two hundred years.Nevertheless, in some other cases, bridges of a similartype had to be replaced al'ter only some years beca use
of cracks in the gusset plates.
Figure 5
Development of gusset plates' form : Fullline: real system:
small dotted line: unfavourabJe in terms of appearance;
dotted line: stressing of the gusset plate becomes to large
THE MECHANICAL EVALUATlON
The investigation, evaluation and rating of existingbridges is more and more common, mainly stimulatedby the lack of financial means of the owners of bridges.
In most of the cases, hidden reserves of load carryingcapacity can be discovered. That means that the bridge
can be used for some more years or decades,sometimes with some minor repair or strengthening.
For engineers, the investigation of an existingbridge is quite different to the straight forwardprocedure of designing a new bridge according totoday standards. Al'ter many decades of service,bridge elements may have suffered from loading or
the attacks from the environment. That has to beincluded in a safety assessment, however, noapproved procedure is available how to do this in asatisfactory manner.
lt has be en a very interesting challenge to find outthe real behaviour of the viaducts by measurement ofthe torces within the structural elements under trafficloading (Herter, Fischer und Brandes 2002) and bytesting some dismantled bridge s of the same kind
which had been replaced before, in the big testing hallof BAM (Helmerich und Brandes 2002), figure 6.
During the tests in the testing hall, we took theopportunity to search for hidden cracks in the gussetplates by radiographic inspection, figure 7. This task
is very complicated because the cracks always startfrom rivet holes and are covered by some steel layers
Figure 6Fatigue test on an original truss girder of the subway Berlinin the testing haJl of BAM
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Figure 7
Radiographic inspection during tests on subway truss girders
atBAM
and the heads of the respective rivet. The result of thetests was very encouraging. We found that cracks canbe detected when they reach about 4 mm length.Then, the future propagation of cracks can be
assessed. In most cases, many years are needed tocome to a crack length which may cause danger forthe bridge, figure 8, figure 9.
While we performed the tests on a load level more
then double of the real load, in reality, the crackpropagation -if any crack wilJ be initiated- wilJ bevery slow.
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K. Brandes
Figure 8
Crack in a gusset plate after millions of loading cycles
The overall result of the investigations is that theengineers who designed the viaduct had found theoptimum of dimensioning the gusset p]ates. They are
the weakest elements of the viaducts as theinvestigation showed, however, they are strong enough
to resist the loading for a long period of time.It should be mentioned, that some years after the
construction of the viaducts in question, gusset plateshave been dimensioned much thicker and larger. The
Figure 9A detected crack on an X-ray film
Development of structural connections of stee1 truss bridges around 1900 409
reason for this change was that gusset plates shall notbe the weakest structural elements because theinspection of gusset plates is nearly impossible.However, if other structural members are weaker,they exhibit first cracks which can be detected easilyby routine inspection, and this is crucial for structural
safety.
How 1'0 PROTECT OLD STEEL BRlDGES FROM BEING
REPLACED ?
The recently developed methods of investigating steelbridges which are about 100 years old, otfer a rationaltool to assess the safety of the bridge sufficiently.
During the last decades, many old steel bridges
have been replaced by new bridges because theknowledge about the condition of the bridges had not
been satisfactory. Only improvement of the methodsavailable for the evaluation of old structures has leadto a better situation. Just now, it should be impossibleto demolish an old structure without applying all thenew tools of minimal invasive methods for a soundrating.
Minimal invasive methods comprise all themethods which give a better understanding of thebehaviour and the state 01' condition 01' a structure:Exact visual inspection, measurement of geometricconditions and strains and deformation under loading,non-destructive testing like ultrasound andradiographic inspection, dynamic identification ofthe
structure etc.The idea 01'mini mal invasive techniques originates
from the medical sciences where exact]y the samemethods are used to obtain a comprehensive view 01'the conditions 01'a patient.
There is much more to develop to avoid thatmonuments will be demolished only because 01' lack
01' knowledge about their structural safety. However,a first important step has been done.
ANOTHER EXAMPLE
A very important bridge in terms 01' history ofengineering is the Stubenrauch-brücke in Berlin
which was built in 1908, designed by the famous
engineer Karl Bernhard (Bemhard 1908), figure 10.
About 15 years later on, the bridge has be en
Figure 10Stubenrauchbrücke in Berlin crossing river Spree
Figure 11Strengthened gusset plate of the Stubenrauchbrücke
strengthened and at a second time in the 1930ths. Thesecond strengthening concerned the gusset plateswhich had to be enlarged when following the newideas 01' engineers from that time. AIso the barsbetween the two arches were strengthened by addingbroad steel plates which changed the appearance tothe worse. In figure 11, the type 01' strengthening isclearly visible. The gusset plates had been enlargedby additional steel sheets which were welded to the
existing plates. At that time, welding 01' the oldmaterial was not favourable, as we know now.However, when investigating the bridge in 1996, we
could not find cracks near the weld seams.With our newly developed tools, presumably, an
investigation as that in the 30th had had resulted in not
doing any strengthening.
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SUMMARY
The design of engineering structures like bridgesreferred in its forms to those of contemporary arto Inperforming an actual design, the requirements of
structural safety sometimes conflicted -and contlicttoday- with ideas about forms adequate to the mainstream of architecture of the time in question. It is ofinterest for understanding of the engineering art tostudy the development of structural systems anddetailing regarding mechanical structural analysis onthe one hand and favoured architectural forms on theother hand.
The shape of gusset plates is a very special case in
that field of construction history.Remark: It should be mentioned that for the
preservation of monuments like bridge s whichunderlie safety requirements as long as they are inuse, methods of investigation as we have developed
are indispensable.
K. Brandes
REFERENCE LIST
Brandes, K]aus. 2000. Life Span Extension of O]d Steel
Bridges. Congress Report, 16'" Congress. 1nt. Ass. Bridge
and Structural Engineering. Lucerne, Sept. 2000. Paper
337.Bernhard, Karl. ] 908. Die Stubenrauchbrücke über die
Oberspree bei BerlinZeitschrift des Vereins Deutscher
1ngeniellre. Sonderdrllck 1908. 1-20.
Helmerich. R; K. Brandes. 2002. Ermüdungsversuche an
Fachwerktriigern der Berliner V.Bahn.Linie VI.Stahlhau, 71, 790-5.
Herter, J. M. Fischer und K. Brandes. 2002. 100 Jahre
V.Bahn in Deutschland -Zur Ermüdungssicherheit
eines Stah]viadukts der Linie 1 in Berlin. Stahlball,71,97-] 07.
Jiiger, Barbara; Gabrie1a Wachter. eds. 1999. Ahriss oder
Ertiichtigllng. Vice Versa Verlag, Berlin.
Landsberg. Th. 1904. BrÜckenhau. Wilh. Engelmann,Leipzig.