Fig. 3: Unseating of Girder Resting on Elastomeric Bearing During Bhuj Earthquake (New Surajbari Bridge)
In moderate to high seismic zones, such as IV and V in India (Ref. 4), some type of external restraint is necessary for resisting seismic forces to prevent unseating or dislodgement of superstructure. Restraints in the form of linkages such as cables are popular in some countries particularly when steel superstructures are involved. However, in India and many other countries restrainers in the form of concrete ‘reaction blocks’ protruding from the pier cap are more popular. An interesting arrangement for a Railway bridge in a high seismic zone in Algeria is shown in Fig. 4. As can be seen the restrained end of the span of the 2 girder simply supported span is restrained vertically as well as horizontally by prestressed cables. The lateral restraints at both restrained and free ends is provided by reaction blocks projecting from the pier cap. Elastomeric bearings on vertical interfaces of the reaction block and superstructure ensure that there would be no damage due to violent shaking caused by a major earthquake. Another interesting arrangement of seismic restraint when superstructure rests on elastomeric bearings is shown in Fig. 5. The end diaphragm of the box girder is extended down into a recess in the pier cap. The longitudinal restraint is provided at one end only while transverse restraint is provided at both ends. All restraints are created by means of elastomeric bearings on vertical faces of the recess.
6 Volume 43
Number 4
December 2013
Fig. 4: Seismic Attachments for Superstructure: Benisaf – Ain Temouchent Line- Algeria
Fig. 5: Seismic Attachments: Box Girder Superstructure
The Eurocode EC 8 (Ref. 6) indicates a strong preference of using elastomeric bearings for vertical loads only while the earthquake forces are taken by other structural connections. This preference can be gauged by the following provisions: • Supporting member (i.e. substructures) connected to the deck should in general remain in the elastic range. Translated into earthquake forces in Ref. 4, this would amount to assuming R = 1 in Table 8 as no plastic hinges would develop in piers. Also, the large imposed deformations would have to be catered for. • In case the elastomeric bearings have to resist both non-seismic and seismic forces, they attract special provisions and testing applicable to seismic isolation devices. This issue is discussed in art. 7.0 of this paper.
The Bridge and Structural Engineer