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1 �BINTRODUCTION Traditionally Bridge structures were designed as statically determinate structure to exclude all secondary ef-fects. The standard bearing arrangement in such type of bridges is Rocker (Fixed) bearing on one end and Roller (Free) bearing at the other end. The span and bearing arrangement of this type ensures that vertical as well as horizontal forces are uniformly distributed on all substructures and secondary stresses do not develop in the superstructure. Deck expansion joints are provided at every pier. Selection of this configuration leads to heavier and costlier structure than statically indeterminate configurations for longer spans. The large num-ber expansion joints create potential source of corrosion initiation besides adversely affecting riding comfort. Adoption of continuous girder configuration generally results in lighter & economic design as the moments in a continuous beam are distributed over the span.

Continuous or integral bridges, on the other hand, develop secondary forces and moments due to daily & seasonal temperature changes and may cause severe local stress concentration. Secondary forces are also gen-erated in concrete girders due to creep and shrinkage effects. In the regions with high design seismic coeffi-cient, the effect of the horizontal seismic force on superstructure as well as substructure depends on the ar-rangement of bearings. Bearings, in continuous bridges, can be arranged to transfer horizontal forces to selected piers. Bearings, Expansion Joints as well as Shock Transmission Units (STU) can be effectively con-figured to manage the passage of Seismic and Thermal forces to foundation and thereby control the structural behavior of bridge superstructure under different loads.

Bearing & Expansion Joint arrangement in two medium span bridges and one curved flyover are de-scribed, hereinafter, to illustrate effective application of articulation in indeterminate structures.

2 �BTHE THIRD KARNAPHULI BRIDGE

�B2.1 Description of the Bridge The third Karnaphuli bridge is located near Chittagong in Bangladesh. The four lane road bridge was exe-cuted by Bangladesh Roads and Highway Department (RHD). It was constructed by MBEC-ACL-COPRI JV on design and construct basis. The 930m long bridge was designed by HighPoint Rendel of UK. It consists of main extradose bridge of 830m length with 100m long Viaduct on northern end.

The 830m long main bridge has a span configuration of 115m+220m+115m. The structural form is single plane extradose prestressed concrete box girder. The single box four lane deck is 24m wide with depth vary-ing from 6.75m at pier to 4.0m at midspan. The pylons of this extradose bridge are 25.75m tall. The bridge has a gentle 3200m horizontal curvature.

IABSE-JSCE Joint Conference on Advances in Bridge Engineering-II, August 8-10, 2010, Dhaka, Bangladesh. ISBN: 978-984-33-1893-0 Amin, Okui, Bhuiyan (eds.) www.iabse-bd.org

Relevance of bearings and expansion joints – case studies for indeterminate bridges

N. Bandyopadhyay, A. Ghoshal & A. Sengupta STUP Consultants P. Limited, Kolkata, India

ABSTRACT: In girder-type bridges the deck loads are transferred to the substructure through bearings. Bear-ing layout in a bridge with continuous superstructure can be done in different ways. Disposition of bearings & expansion joints in a continuous bridge controls the manner in which forces are transferred to multiple sub-structures as well as development of secondary forces in the superstructure. It is important for the designer to select the appropriate arrangement of Bearings, Expansion Joints and Shock Transmission Units (STU) for ef-fective passage of vertical as well as seismic & thermal effects to substructure. This paper presents case stud-ies for arrangement of bearings & expansion joints in two medium span bridges and one curved flyover.

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2.2 �BBearing and STU arrangement The 830m main bridge has expansion joints at two ends. Schematic elevation is shown in Figure 1. The su-perstructure box girder along with the pylons is connected to the piers through a set of bearings. The bridge is located at a high seismic zone. Therefore, besides bearings & STUs, concrete shear keys have been provided in a central opening of the pier cap.

In terms of theoretical support arrangement, the five span girder is pinned at P8 with roller supports at other piers. Expansion Joints are provided at P6 and A2. Expansion/ contraction of 515m bridge length takes place at A2. The modular expansion joint have been designed for 500mm movement. Normally with such arrange-ment, horizontal force developed or applied anywhere on the span will be transmitted to the fixed pier P8. As stated earlier, this bridge is located in a high seismic zone and hence the design seismic forces are very high. It would have been extremely uneconomical to design the foundation and pier at P8 to resist the horizontal seismic forces developed within superstructure weighing little more than 30,000 MT. Besides the superstruc-ture girder would also be subjected to large axial force and designed accordingly. Instead, Shock Transmis-sion Units (STU), also known as Lockup Device has been provided at P7, P8 and P9, which would lockup and therefore, share fast acting forces like seismic or vehicle breaking. By design, these devices will allow slow movement like thermal expansion. The superstructure girder would, under normal circumstances, not develop thermal stresses. Horizontal forces in transverse direction are shared by four piers P7 to P10.

Figure 2 shows the layout of bearings, STUs and shear keys to achieve the structural behavior stated above. Four POT / PTFE-POT bearings are provided on each pier cap. A central cutout is provided to house the concrete shear key. A fixed STU or Pin bearing is provided under the shear key at the fixed pier P8. This bearing can only resist horizontal forces. Vertical force at each pier is carried by the POT bearings. All four bearings at P8 at free-free type as the pin is located at that pier. At piers P7, P9 and P10 there are three free-free POT bearings that take only vertical loads along with one longitudinally guided PTFE-POT bearing. This bearing would allow free movement of the deck in longitudinal direction but would restrict movement in transverse direction and thereby transmit transverse horizontal forces to the pier. STUs are provided at P7, P9 and P10, as stated earlier to contain fast acting horizontal forces in the longitudinal direction. In high seismic zones, it is a practice to provide a second line of defense against span dislodgement under extreme seismic event, whose intensity is higher than that considered in design. Concrete shear keys on the soffit of super-structure girder are provided to cater to this requirement. These shear keys are physically enclosed in a cutout on the pier cap. The space in-between is packed with elastomer to act as buffers. Figure 3 shows a photograph of the bridge nearing completion.

115m 200m 200m 200m 115m

P6 P7 P8 P9 P10 A2

Figure 1. Longituinal Elevation of Third Karnaphuli Bridge

Vertical POT Bearing

Longitudinal Guided Bearing

Fixed STU (PIN)

S.T.U

Concrete Shear Key

P10 P9 P8 P7 P6 A2

Figure 2. Schematic Plan showing layout of Bearings, STUs & Buffers

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3 NIVEDITA SETU

�B3.1 Description of the Bridge Nivedita setu across river Hooghly is located at the northern end of Kolkata, India. It forms a part of the 6.1km Tollway to provide a critical improved connectivity between the National Highway systems on eastern and western bank of Ganges. The Tollway including the bridge has been built and being operated by Second Vivekananda Bridge Tollway Company P. Ltd since 2007 under a thirty year concession from Government of India. The 880m extradose bridge has been designed by International Bridge Technologies Inc, USA. The bridge along with all other components of the Tollway has been constructed by Larsen & Toubro Ltd.

The pylons in this 880m long extradose bridge are 14m tall. The superstructure girder is a single cell con-

crete box girder with 30m wide deck carrying 6-lane traffic. The cables are in single plane. The bridge has been constructed by precast segmental construction method by free cantilevering from the pier with individ-ual segments weighing more than 150MT. Uniform depth of the prestressed concrete box girder is 3.45m. The superstructure girder is made integral with the pier.

In terms of structural arrangement the bridge consists of four 220m units (See Figure 4) that are connected at the midspan through longitudinal expansion joints. The expansion joint has been detailed in such a manner that it permits only longitudinal movement between the units allowing thermal movement. The shear force is transferred between the units through the expansion joints. This arrangement of articulation only in longitudi-nal direction allows the superstructure to behave as a single continuous girder eliminating any ‘kink’ associ-ated with midspan connection.

Figure 3. Third Karnaphuli Bridge under construction (May 2009)

110m 110m 110m

Exp Jt Exp Jt

Figure 4. One typical 220m unit of Nivedita Bridge

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�B3.2 Arrangement of Articulation at midspan The primary elements of the expansion joint arrangement are shown in Figure 5 and Figure 6. A pair of struc-tural steel box girders is placed inside the concrete box girder and they span between the cantilever ends of the two units. They are fixed to one unit with fixed disc bearing while the other end rests on sliding bearing. Thus the longitudinal differential movement between two superstructure units freely takes place. The girders are roughly 12m long. It can be observed from Figure 5 that with the arrangement of disc bearings at top and bottom face of the steel box girders, differential vertical displacement or rotation between the two units is eliminated. Therefore longitudinally the units act like one continuous beam. A modular expansion joint is provided at the bridge deck level. Movement of vehicular traffic would not produce any undesirable deforma-tion at the expansion joint. Figure 6 shows a photograph of the joint location during construction.

Expansion Joint

Steel Girder Fixed Bearing

Sliding Bearings

Figure 5. Longitudinal View of Expansion Joint

Steel Girder

Figure 6. Cross section at Expansion Joint

Figure 7. Nivedita Setu : Midpsan Expansion Joint

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4 CIRCULAR RAMP NEAR K.L. CENTRAL

�B4.1 Description of the Structure Number of grade separated interchanges was planned around the KL Sentral Steysan and Jalan Tun Samban-than in Kualam Lumpur (Malaysia) to improve the traffic movement. One such ramp with complicated ge-ometry would be constructed over Sungai (river) Klang at the loop of Seputih junction and Jalan Syed Putra.

The geometry of the ramp formed a trumpet junction connecting the existing elevated roadway over Sungai Klang at Seputih to the perpendicular lower level roadway, Jalan Syed Putra (road parallel to the Klang river). The road link had to be constructed over the sungai Klang only with two circular curves connected by a small straight stretch at the middle. The radii of the two circular curves are 37 m and 35 m respectively having su-per elevation of 6%. The new ramp at start needs to be merged with the existing ramp at Seputih.

The superstructure consists of four continuous spans of 41 m each with the starting span of simply sup-ported T-beam of 12 m required for merging with existing bridge. The superstructure is of elegant box shape, constructed using precast segmental method. Prestressing was carried out at the blisters at the junction of web and deck or at the junction of web and soffit. The substructure is of circular pier matching with the super-structure profile. The superstructure is placed over the bearing to transfer the load to the substructure. Figure 8 shows the plan of the ramp.

4.2 �BBearing arrangement in curved continuous spans Proper layout of bearings on a curved span along with estimation of movement is a critical and important ac-tivity for the durability of the structure. Due to continuity in superstructure the longitudinal forces due to braking, tractive effect, temperature, creep & shrinkage movement will be transferred to the bearings, not only depending on the shear rating of the bearings but also on the stiffness of the pier and foundation. In case of straight continuous span the distribution of horizontal force can be carried out effectively with relative

simple computation. Whereas for curved continuous span the transverse stiffness of deck due to curvature

Figure 8. Plan of Ramp-4

Direction of traffic

Sliding/ guided Fixed

Free FreeFree

Pier Abutment Abutment

Figure 9. Bearing Layout for Straight Continuos Span Bridge

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along with the location and direction of movement of bearing becomes an important factor. The bearing lay-out for a straight continuous span is shown in Figure 9. The bearing layout as shown in the first sketch is very effective for transferring all the loads including the horizontal load due to braking, tractive or force resulting from movement due to temperature, shrinkage and creep. All stresses in the bearing and as well on the super-structure will be of normal nature and the system can be effectively designed for all loading conditions. The

span length and the loading pattern will determine the size of bearing.

�B4.3 Arrangement of Bearings The arrangement of bearings on the four span ramp was made with the above principle. Analysis was car-

ried out for different load conditions like Dead Load, Vehicular Load, Temperature, Creep & Shrinkage and Hyperstatic effects before sizing the bearings. It was observed that secondary effects contributed significantly towards the loads on the guided bearings. Figure 11 shows the schematic arrangement of bearings on Four Span Ramp 4.

5 CONCLUSION

The internal forces as well the support reactions in a indeterminate structure depends upon the imposed boundary conditions. An indeterminate structure can have solved for different combinations of boundary con-ditions. Out of all those sets, one or two sets yield desirable design results in terms of minimum secondary forces as well as manageable support reactions. Bearings and expansion joints translate to the boundary con-ditions for stress analysis. Therefore, it is necessary to give careful consideration to bearing layout in con-tinuous bridges. The examples cited demonstrate the same.

�BREFERENCES

Ramberger, G. 2002. Structural Bearings and Expansion Joints for Bridges – IABSE SED 6

Fixed

Sliding/ guided

Figure 10. Desirable Bearing layout for Curved Span

Figure 11. Arrangement of Bearings in Four Span Continuous Bridge