The project involves the design of two bridges inthe Landschaftspark Mechtenberg, a countrypark. The bridges are part of a system ofpathways crossing the park to join severalresidential areas. Because of the location, theintention of the designers is to provide asculptural, as well as a functional, road crossing.The main bridge, that is described here, is 150min length and has eight separate spans, whichincrease successively in length from 10m to 30m.The full length of the bridge is required toaccommodate a gentle rise, suitable for hikers,cyclists and wheelchairs, from ground level upto the required clearance height over the road. The second, smaller, bridge spans 30m over a

stream and a footpath, set in a cutting and hastwo clear spans. The same standard dimensionsand details have been used in both bridges.Construction began earlier in the year. Thesteelwork will be prefabricated in the workshopand trial erected there. The project is due forcompletion by the end of 2002.

Concept One of the benefits of working together for manyyears is that ideas born in one project can benurtured and realised in others. In this case thelong established relationship between TedHappold and Frei Otto has allowed ideas thatwere never realised in the work on the Germanmagnetic elevated transport system to beexplored further. Ideas and schemes from theelevated train track have been used as a baseand scaled down to pedestrian size.The elegant form of the bridge has beenachieved by using only two primarycomponents for the structure: a 70mm diametersolid circular bar and a forge-pressed clampdetail that connects these together. The primarybars are arranged at various angles radiatingfrom a fair faced concrete pier. The diagonal fanmembers are tied together at deck level by ahorizontal bar to create a triangulated fanelement. These triangulated fan elements areoverlapped with those of the adjacent fan toform a continuous structural frame.In total approximately 3,500m of grade St52 barhas been used with a weight of 110tonnes,together with some 1,050 clamp details.The geometry is based on one primary rule: atdeck level the fan members are always spaced1.6m apart and this then defines all the otherangles. The 1.6m is the optimum length of thelongitudinal which, as well as tying the fanstogether, also supports the cross beams andmust act as a bending element between the fanelements. The cross beams are at 0.8m centres,offset by 0.4m from the fan members and thus sitat third points of the 1.6m grid.

Fan ElementsIn the early stages of the design two options forthe structural elements were considered, circularsolid bars and circular hollow sections (CHSs).To keep the elegance of the bridge, the diameterof the fan members should be the smallestpossible. The radius of gyration i decreasesfrom CHS to solid bars, therefore themathematical slenderness le/i increases, whichleads to a lower maximum buckling length le forsolid bars. On the other side, the area of steelincreases from CHS to solid bars which results ina higher axial capacity. The graph shows that

for a given outer diameter of the bar and a givenlength the axial bearing capacity for a solid baris higher, than for a CHS, despite the fact that thebuckling length is shorter. Despite the higheraxial capacity of the solid, the structuralperformance per weight is therefore superior forthe hollow section.However, there were also other considerations.A solid section removes the risk of internalcorrosion which can arise if tubes are notproperly sealed. Also, at the joint locations, ahigh clamping force is needed to mobilise thefriction required to transfer the load from onebar to another. The magnitude of this forcemeant that a hollow section could be at risk oflocal collapse at the clamp location. Being up tothree times heavier, the bar also added weight tothe system which was actually of benefit whenthe dynamic analysis was carried out.Consequently, the solid bar was chosen. The bars were required in lengths varying from3.5m to 22m across the fans. With a maximumsupplied length of 12m the longer bars requireda splice.The park in which the bridge is located ispredominately reclaimed land and in this areathere is between four and eight metres of poorquality fill. In order to limit settlement, boredpiles were used to transfer the load into theunderlying marl. Raking piles were used totransfer the high horizontal loads into theground.

Structural SystemEach fan can be considered as a single unit, thecentral, more vertical, elements are subjected tolow axial loads and are short in length. Thismeans that they have a high stiffness comparedto the longer outer elements of the fan. Ahorizontal bar at mid-point ties all the fanelements so that as the longer elements start tomove out of plane they transfer load to the stiffcentral elements which stop the system frombuckling. On the three larger fans where thelength of bar is between 7m and 15m it wasnecessary to clamp a second bar to the fan toincrease the out of plane stiffness.The stability across the bridge is provided bytraditional cross bracing at each of the supports,and in the longitudinal direction stability relieson the overlapping of the fans that forms a seriesof pinned arches. The deck is a 10mm steel platestiffened with 60mm ribs and forms a stiff shearplane under horizontal wind loads. Because ofthe short spans and close centres of each fanunder dynamic loads the deck level itself is stiff.Lower natural frequency result at the crown overthe road which deflects up to 12cm under deadand live loads. There is a degree of structural

Mechtenberg BridgeMartin Strewinski and Stuart Brumpton

This bridge uses a structure consisting of solid steel bars arranged in fans. They are connected togetherby clamps designed with the help of a scaffolding manufacturer. The design draws on earlier ideas fromFrei Otto and Ted Happold.

INTERNATIONAL STEEL CONSTRUCTION

redundancy in this crown which, althoughmobilised under asymmetric live loads,primarily provides a strong architectural accentover the road. After consultations with thePrüfengineer (checking engineer) and the client,it was decided not to include any dampers at thisstage, but to test the actual response of thissection of the bridge before completion.As with all long structures, the potential fortemperature induced forces became a primarydesign criteria. The initial concept was to releasethe forces in the traditional manner by placing thesupports of the bridge on rollers to allow thebridge to expand. But as the design progressedand the interaction of the elements within the fanwas more fully understood it became undesirableto allow the central bars, which provide restraint

to the outer bars, to move. Additionally, lateralmovement of the base generates large compressiveforces above the deck level which could againincrease the risk of buckling.The bridge now relies on a series of joints along itslength each providing +/- 10mm movement.Because the bridge relies on the connection ofone fan to the other for stability, the jointing isnot total. The joints are detailed so that the barscan slide relative to one another but vertical loadcan be transferred.

ClampsThe standard clamp is bolted around each barand relies on the friction between the bar and theinner face of the clamp to transfer the load ateach connection. Four M16 bolts are used each

with a pre-stress tension of 100kN to cater for atransfer of load of between 3t and 12t dependingon the angle of the fan bar. To accommodate thenumerous different connections required a singletype of clamp was used, made in two halves. Thebridge required 1,050 connections each using apair of the standard clamp components weldedback to back, at the various angles required. Inorder to achieve the required tolerances theywere welded together in the works.The development of the clamp detail was basedon the early analysis of forces and was discussedwith manufacturers of scaffolding systems toagree a suitable detail. Many variations weremodelled with timber and clay before an agreedprototype was manufactured for testing. The detail was tested at the University ofDortmund with which the design team hadpreviously worked on the testing of the papertubes used on the Japanese Pavilion at the EXPO2000 in Hanover. Various geometricarrangements of bars together withcombinations of moments and torsion have beentested. The results from these tests were fedback into the design to confirm the results of thedetailed computer analysis.

Dipl.-Ing Martin Strewinski and StuartBrumpton BSc, CEng, MIStructE, are ProjectLeaders at Happold Ingenieurbüro GmbH.

CreditsName of Project

Mechtenburg Brüken

LocationLandschaftspark Mechtenburg,Gelsenkirchen

ClientKommunalvervand Ruhrgebiet

Planning teamPlanungsgemeinschaft MechtenburgBrüken Atelier Frei Otto Warmbronn Happold Ingenieurbüro GmbHProf Dr Ing Hilbers IngenieurgesellschaftmbH

INTERNATIONAL STEEL CONSTRUCTION