Keywords: continuous bridges, bridge deck, slab-girder, diaphragm, cross-frame, seismic analysis.

This paper is an analytical investigation of the effects of modelling assumptions for concrete decks of slab-girder continuous bridges subjected to earthquake forces. The American bridge design code, AASHTO[1], has many specific details regarding the seismic analysis and design of bridges. However, in seismic modelling of a bridge superstructure, the question of whether or not the deck slab should be assumed to be rigid in its plane, is left to the discretion of designer. It is shown that the practicality of current AASHTO seismic analysis procedures is highly dependent on the validity of this assumption. An important factor affecting this decision is the stiffness of supporting elements of a bridge. These elements can be a part of the superstructure, such as, end cross- frames, diaphragms and elastomeric bearings, or a part of the substructure, such as, piers and abutments. The type of superstructure, such as slab-beam, box girder, plate girder, etc., is another factor to be considered. Finally, the geometry and mass of a bridge will affect this decision as well. This is an attempt to study the effect of in-plane deck rigidity on the vibration of slab-beam, symmetric, continuous, bridges. It is assumed that the bridges are elastically supported by end cross-frames in the transverse direction. The effect of varying the combined cross-frame and pier stiffness at intermediate supports is included by varying the stiffness of support springs. Three-dimensional finite element analysis is performed on two- and three-span continuous slab-girder bridge models. The seismic demand on the deck and supporting elements, based on rigid and flexible deck assumptions, are compared. It is shown that the assumption of rigid deck simplifies the analysis and it is safe to use when considering longitudinal vibration. It is also safe with respect to deck stresses and displacements. For transverse vibration, the rigid deck assumption underestimates the intermediate support shears, and overestimates the end support shears. It is concluded that rigid deck assumption could be employed in analysis to simplify modelling. However, the designer should use the higher shear force at abutments for intermediate pier support design. This approach is conservative. Alternatively, a three-dimensional flexible deck model would always yield exact forces for supporting elements. Similar studies have been performed in the past. However, continuous spans and slab-girder bridges and three-dimensional finite element analysis distinguish this work from others. Reference[2] considers deck stiffness for a box girder bridge. Alfawakhiri and Bruneau[3] use a beam model to derive a relationship between the support and deck stiffness for single-span bridges. Zahrai and Bruneau[4] consider a single span beam with cross-frames at the ends without considering deck rigidity as a parameter.

1

American Assoc. of State Highway and Transportation Officials, AASHTO LRFD bridge design specification, 2nd Edition, Washington, D.C., 1998.

2

Elnashai, A.S., McClure, D.C., "Effect of Modeling Assumptions and Input Motion Characteristics on Seismic Design Parameters of RC Bridge Piers", Earthquake Engineering & Structural Dynamics, 25,435-463, 1996. doi:10.1002/(SICI)1096-9845(199605)25:5<435::AID-EQE562>3.0.CO;2-P

3

Alfawakhiri, F., Bruneau M., "Flexibility of Superstructures and Supports in the Seismic Analysis of Simple Bridges", Earthquake Engineering & Structural Dynamics, 29, 711-729, 2000. doi:10.1002/(SICI)1096-9845(200005)29:5<711::AID-EQE936>3.0.CO;2-#

4

Zahrai S.M., Bruneau M., "Impact of Diaphragms on Seismic Response of Straight Slab-on-girder Bridges", Journal of Structural Engineering, ASCE, 124(8), 938-947, 1998. doi:10.1061/(ASCE)0733-9445(1998)124:8(938)

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