The Effect of Vertical Earthquake on the Behavior of Precast Segmental Bridge's Decks Post-Tensioned by Unbonded Cables

Precast segmental construction methods can decrease bridge construction costs by reducing construction time while maintaining quality control. In addition, the absence of scaffold can minimize traffic congestion and environmental impact. Because of these great advantages of using the precast segmental bridges, their usage is increasing in the world, but it is limited in high seismicity areas due to lack of sufficient knowledge about the dynamic response under seismic loads. According to the precast construction of these types of bridges and their post-tensioning contact on segment joints, it is expected that under earthquake excitation specialy vertical earthquake, superstructure behavior is affected by joints operation in the presence of long-term loads. This issue is very probable in non-continuous post-tensioned bridges. This study is trying to investigate the effects of vertical earthquake on the superstructure of the bridges in near-fault regions by studying a sample model and obtaining structure response including joints response and their openings, force-displacement response of the system, stress and strain in concrete and cables and the level of nonlinearity of them.

Recent research has shown that segment joints can undergo very large rotations that open up gaps in the superstructure and The primary seismic concerns regarding segmental construction are focused on the behavior of joints between segments as no mild reinforcement crosses such as joints. The lack of reinforcement across segment joints allows to an increased rate of construction, yet creates inherent regions of weakness that act as crack initiators and can result in large localized rotations. According to phase I experiment results by Megally et al. [1-3] which are concentrated in regions of high moment and low shear (i.e. near midspan), models were created using the computer software OpenSees V2.4.4 and using detailed 2D nonlinear time history analysis under a suite of ten near-field earthquake records to quantify effect of vertical motion on the joint response. The prototype bridge structure used for this study is single-cell box girder bridge that consist of a 50m span, sixteen 3m segments long with unbonded tendons constructed with the span by span construction method. The segments of the superstructure are modelled using linear elastic frame type members except for a region at the ends of each segment which is discretized into several axial non-linear zerolength springs. The springs are connected to the ends of the superstructure beam elements through rigid body links. Results indicate that the vertical components of earthquake can affect the response of these bridges and segment-to-segment joints opening are very probable particularly at the mid-span joints and superstructure can collapse under upward acceleration demand because of concentrate greater part of concrete on top flange and lack of tension material on top of joints or sliding on elastomer caused by decrease of effective weight. the joint compressive strain remained way below the concrete spalling limit state minimizing the damage and stiffness reduction of the superstructure; the cables remained in the elastic range and closed all joints after earthquake even high seismic intensity levels and the residual vertical displacements were negligible.