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.

406

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

-~-- -

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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408

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.

--..---

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.

410

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.