Deck modeling for seismic analysis of skewed slab-girder bridges
Shervin Maleki ∗
Department of Civil Engineering & Construction, Bradley University, Peoria, IL 61625, USA Received 5 November 2001; received in revised form 3 March 2002; accepted 7 May 2002
Abstract Seismic analysis of slab-girder single-spanskewed bridges is greatly simpliﬁed if one can assume the concrete deck to be rigid in its own plane. In effect, the deck can be replaced with a rigid bar and this will eliminate many degrees of freedom associated with the superstructure. In this study, the importance of this modeling assumption, in conjunction with AASHTO’s seismic analysis methods, is highlighted. Furthermore, to investigate theeffect of this assumption on analysis, a parametric study is performed on bridges with skews ranging from 0 to 60 degrees and with spans up to 30 m. It is assumed that the bridges are elastically supported with elastomeric or pinned bearings in the longitudinal direction, and cross-frames in the skew direction at each end. Linear ﬁnite element response spectrum dynamic analysis is performed onbridges with decks modeled as rigid and ﬂexible shell elements. The effects of deck stiffness on the translational and torsional periods of vibration are noted. Stresses for ﬂexible decks are evaluated and shown to be negligible. Seismic demand on supporting elements with rigid and non-rigid decks are compared. It is shown that the rigid deck assumption simpliﬁes the analysis and is valid for practicalranges of slab-girder bridges. 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Skewed Bridge; Bridge deck; Slab-girder; Seismic analysis; Cross-frame; Elastomeric bearing
1. Introduction Slab-girder bridges are the most common type of bridge construction used for short to medium range spans, Figs. 1 and 2. They consist of a concrete deck spanning over concrete or steel longitudinalgirders. For longer spans, shear connectors are provided at top of beams to ensure composite action with the concrete deck for gravity loads. In modeling slab-girder bridges for ﬁnite element analysis, the designer is often faced with several choices of varied complexity. The most complicated choice is to model the deck with all of its components, using shell and beam elements. In this method,the interface of the deck and the beam requires special consideration to account for the beam’s center of gravity position [1–3]. Another option is to replace the superstructure with a grillage . This model eliminates the shell elements and uses beam elements in two directions. The above models can be used for gravity and seismic loading.
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However, if only seismic lateral loading is of concern, simpler models have been proposed. A popular choice for seismic loading is the equivalent beam model. In this method, the deck is modeled with a beam element that has the same mass and stiffness as the entire superstructure . All these modeling techniques try to capture the stiffness of thesuperstructure. For seismic analysis, a much simpler model can be constructed if the deck is assumed to be rigid in its own plane [6,7]. In effect, this assumption eliminates all the degrees of freedom and stiffness associated with superstructure components. The only concern is the stiffness of the supporting elements of the superstructure. Knowing that rigidity is a relative term, deck stiffness shouldbe viewed along with other lateral load resisting elements that contribute to the stiffness 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. In addition, the type of superstructure, such as slab-girder, box girder, plate girder, etc., plays an important role...