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Sayı: 51 - Nisan 2017


İzmir Konak Junction Pedestrian Bridge

This bridge is designed with a orthotropic box girder cross-section and passes the span of 140 meters between two abutments on intermediate supports.

This bridge is designed with a orthotropic box girder cross-section and passes the span of 140 meters between two abutments on intermediate supports. Clear bottom span in the middle section of the road is 6.5 meters and mid-span between intermediate abutments is 56 meters.   In the subject box-girder bridge, 20mm plates are used in lower surface, 15mm plates are used at the sides or parapet plates and 10mm plates are used on the top pedestrian surface. All plates along bridge are strengthened in line with orthotropic principle and these reinforcements having 10mm thickness and 100mm height are elements with continuous welding. There exist transverse beams intersecting these reinforcing elements at each 2 meters and approximately 5 of these transverse beams form 10 meter blocks, i.e. field-bolted modules. Each block-module are con­nected to each other with bolts and the bridge is formed through 13 bolt-connect­ed block-modules in total. It is aimed to provide a clean connection and an aesthetic view by leaving the bolt connections on the inside. These connections are inspired from hidden bolt connection method with face plates originating from long boats used in rowing competitions. Access doors are provided in order to access the bolts for erec­tion. These access doors are welded with full penetration weld after torqueing of the bolts for water tightness.   Another feature of our design for long life and durability in addition to aesthetics is including the loads on the bridge due to temperature changes in the calculation in design phase. In this purpose, 10cm joints are used since lateral loads would be formed due to temperature in concrete ramp braces in bridge abutments having an arch profile in order to tolerate contraction and expansion forces. Bridge is supported with roller supports under 
these joints which were produced by Arsan Kauçuk. We also had to develop a solution for repair and cleaning works which may be required in the future and observing the working principle of the joints, therefore we provid­ed a solution with maintenance pits and we placed fences around these pits.  In order to prevent transverse sliding move­ment while enabling longitudinal sliding movement, we used short pipe members. This resulted from the requirement of re­stricting transverse movement we observed in vibration calculations in theory. One of the root causes of this requirement is the slender structure of the bridge and passing the span of 150 meters with 4-meter spans. If this solution was not applied, tip of the bridge would move transversely and result in disturbance due to excess vibration by pedestrian movement. 
Comparison of these frequencies due to vibration with the natural vibration frequency of the bridge was performed as per slender bridge vibration standards from American AASHTO and British BS 5400 highway specifications. Further, we used special slender pedestrian bridge design manuals. 
One of the challenges we faced during the project was the necessity of preparing three different models. Plates had to be bent in two dimensions in space and these models were required to execute the sharp turns and double curvature. In this respect, geometric model was prepared first. In this model, main lines where plates are placed in millimeters were determined. These main lines are the bottom point of the curve and end points of side plates, and actually these points are splines in space having different curvatures. After determining each of these unique splines, spaces between the modules are filled plates required to face these splines. In addition, longitudinal reinforcements and intermedi­ate plates are placed.  
After completion, the model is transferred to 3D dynamic analysis simulation software and solved with shell members. Thus, we observed that these box-girder structures provided more accurate results when solved with shell members. We also had an ap­proximate model with frame members but final solution was through shell members due to the positive results gained through this model. At this point, meshing which takes effort and time came in the picture. Cause of this activity was that geometri­cal analysis software and 3D modelling software finite element analysis engines did not run simultaneously. We used several plug-ins and used software technologies to the full extent.  
Following finalization of these two models, we started the manufacturing model. Man­ufacturing model was prepared with Tekla software and special elements are preferred again since plates to be bent in two direc­tions were used. Beam elements were used for plates with less curvature while macro elements were used for plates with double curvature. Software executes shell expansion and enables cutting of plates in space. Another challenge we faced aside from modelling is that a metro line was passing under the projected location of the bridge and the soil underneath was backfilling. Considering that these coastal areas of Izmir are all bad-quality backfills, we had to execute soil improvement works. Dead load and seismic loads of the bridge were less­ened since the bridge was constructed with steel, however we used raft foundations with a height of 1 meter to ensure stability of the curved and delivate geometry during earthquakes. We had to construct the raft foundations at a certain distance from metro tunnel slab and shear walls and this meant revising the form of the superstruc­ture and its layout. We had to arrange a lay­out which had to locate the abutments and foundations at a certain distance from the metro tunnel below. At the same time, we observed that the safe bearing capacity of the backfill was exceeded when these heavy raft foundations are to be constructed on the backfill. Therefore, injection works for soil improvement had to be executed. Soil improvement was also beneficial to prevent damaging tunnel walls and foundation works took serious time and effort during the project execution phase.    Despite all difficulties, another chal­lenging phase of our project which was relocated due to the nature of the project and re-modelled was the manufacturing phase. Manufacturing undertaken by Eran Mühendislik proceeded with strict checks and manufacturing supervision was performed by TUV SUD as third party supervisor. Team executed a very successful work. We faced a couple of mismatches on site due to 3D geometry of the project and its slender nature, however they were cor­rected in a swift manner. There are visible traces in a couple of locations but this is acceptable in such a successful and difficult project.    One of the special factors which makes our bridge project special from architectural perspective is the pipe abutments. Pipe abutments are composed of steel pipes of S355 grade with 40mm wall thickness, which is rarely used in our country and not standard in practice. These pipes have large curve diameters, however curving process was not easy despite the large diameter. At this point, our manufacturing engineers expedited the process by providing tempo­rary long bending sections. Although those parts were useless afterwards and scrapped, this was a necessary sacrifice. The reasons of difficulty of bending process were wall thickness, large diameter and high quali­ty of the pipes. These characteristics also made the pipes hard to find, however Eran Mühendislik successfully executed the pro­curement and bending of the pipes under third party supervision. All pipe columns in our bridge are bended and this is an architectural characteristic of our structure. Reason of this architectural preference was to provide a natural appearance close to a tree form. Thus, it was aimed to provide a natural outlook with the surrounding trees. It also enabled us to decrease the main span of the bridge from 60 meters to 55 meters from structural design perspective. These tree-like columns reach up to 4.8 meters in height while they are reduced to approxi­mately 2 meters in height in the proximity of intermediate supports. From design perspective, inclination below and frac­tured structure above brought two opposite outlooks together, enabling emphasis on the curvature of the form.  Identification Tag: 
Architect: EPA Mimarlık, Ersen Gürsel, Oya Erar Structural Design and Geometrical Model­ling: ATEKNIK Structural Design, Ahmet Topbaş, Doğan Arslan, Burhan Kaplan 
Çelik Yapılar - Sayı: 51 - Nisan 2017

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