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AN INNOVATIVE SOLUTION FOR THE REPLACEMENT OF A BRIDGE AS AN OVERALL MODEL

Kaspar Kraemer
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With pre-cast concrete girders in composite construction, we are able to build the structure in a short window of time at consistent top quality, something which is also an important cost factor in the end

Stefan Kleffel

IB-KLEFFEL REPLACE  A ROAD BRIDGE

Commissioned by Hessen Mobil, Kassel, the Kleffel engineering office for structural design is currently planning the replacement of a road bridge on the federal highway A7 between Flensburg and Füssen. The engineering office, from Rippershausen, Thuringia, specializes in demanding tasks as a structural planner in bridge construction – whether for steel, pre-stressed concrete, concrete composite or steel composite bridges. A wide range of engineering services is on offer: The portfolio runs from feasibility studies through pre-planning, design planning and structural planning to statics testing by Mr Kleffel Dipl.-Ing., tendering, site management and health and safety coordination.

For the creation of the concept for the replacement bridge, Hessen Mobil, responsible for the construction, contracted the HIG engineering office (Hensel Ingenieur GmbH) from Kassel. Here, the official design and demanding transport concept for the new building of a  6-lane bridge, was developed. Stefan Kleffel Dipl.-Ing. was glad to take on the special challenge of implementing the designed solution within the framework of detailed planning. The company Adolf Lupp GmbH & Co. KG from Nidda was tasked with the execution of the new construction and dismantling of the old.

Peter Strauß Dipl.-Ing. from the HIG engineering office was commissioned with the statics testing by the client.

As shown in figure 1, the new construction was already defined in the official design as being an integral, pre-stressed composite concrete bridge with a girder span of 27.61 meters and a  width of 40.50 meters, to be built in 5 construction phases. The construction phases in this regard considered the need to demolish the existing rigid-frame bridge and build the new one (figure 2). It was not just the realization of the replacement bridge that took place during running operations: By requirement of the client, the traffic on the A7 was to run continuously and as far as possible unhindered during the demolition works.

The spatial model created for the bridge records the oblique-angled rigid-frame bridge as a folded plate structure on pile foundations (figure 3), representing the different construction phases with in total 13 static states of construction and 145 load states. Because of the ground conditions, the pile foundations beneath both abutments are asymmetrically arranged. The superstructure is formed of 2 x 8 pre-cast girders, manufactured by the construction company Adolf Lupp and pre-tensioned in pre-casting beds.

Here, the entire bridge is recorded in an integral overall model as 2 mutually independent partial structures with beams arranged lengthwise and an orthotropic, load-spreading carriageway running transversely. The pre-cast girders are elastically clamped in the bearing walls via the steel rod reinforcements.

The use of pre-cast girders in this task turns out not only to be technically sensible but also economic: It allows the avoidance of expensive support and protective structures. As the pre-fabrication of the superstructure and the on-site creation of the substructure could be realized in parallel, the total building time could be notably shortened. The use of pre-cast girders also ensured effective quality assurance at the factory – a solid basis for high and consistent quality in execution. It is not for no reason that only in rare cases the transport routes passing underneath needed to be blocked. “With pre-cast concrete girders in composite construction, we were able to realize the structure in a short window of time at consistent top quality, something which was also an important cost factor in the end,” explains Stefan Kleffel, Dipl.-Ing.

Sub-components of the bridge are exposed to extreme stresses that are only present for a specific period in individual construction phases. This is particularly the case for the bored piles

Stefan Kleffel

CREEPAGE AND SHRINKAGE MUST ALWAYS BE ACCOUNTED FOR

Not an easy task for Stefan Kleffel, Dipl.-Ing., and his team: “Sub-components of the bridge are exposed to extreme stresses that are only present for a specific period in individual construction phases. This is particularly the case for the bored piles,” the structural engineer explains. Through earth pressure on the abutment walls, which are partially exposed in the construction phases, individual bored pile groups receive strongly eccentric loads. For this reason, the stresses from every construction phase had to be considered. The high load in the various states of construction led, in this case, to particularly high levels of reinforcement when the bending load capacity (GZT) and connected crack formation (GZG) were taken into account.

And that is not all: “Creep and shrinkage are always to be accounted for in the pre-cast composite girders as they influence the verification of load-bearing capacity and also the useability. That is, all system and load changes throughout the construction period must be able to be represented in the FE model,” Stefan Kleffel states, explaining his calculation procedures. Through suitable subdivision of the two largely independent component structures, the modeling expenditure was able to be notably reduced and the processing of statistical calculation kept transparent and efficient. In this bridge, system creepage between the subsections played an important role. Over time differences of up to 150 days, large forces built up in the on-site concrete plates, markedly affecting the assessment of the girders. “Without taking account of these time-dependent effects, an integral construction method for this bridge type cannot be implemented,” Stefan Kleffel continues.

For the demanding calculations and relatively complex assessment of the pile foundations and pre-cast composite concrete girders, the engineers relied on the software programs PONTI and TRIMAS by RIB. The PONTIbetonverbund software system supported them here in conducting calculations for the entire system, also making possible the processing of all required detailed checks. In the above example, a replacement for a relatively simple motorway overpass is described. However, this case shows clearly which special engineering and technological considerations are needed in order to create a new element of road transport while traffic is still running, safely, efficiently and economically.

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