Two types of precast concrete pier systems were developed in this research. The first system is an emulation of current cast-in-place reinforced concrete pier designs, hereafter referred to as the cast-in-place (CIP) emulation pier system. The second system uses a combination of vertical, unbonded post-tensioning tendons and bonded mild steel reinforcing bars to reinforce the pier, hereafter referred to as the hybrid pier system.
Descriptions of the two systems and their expected behavior during an earthquake are presented below.
The columns and crossbeam in a CIP emulation pier are fabricated out of precast concrete and connected in the field to facilitate rapid construction.
The proposed CIP emulation pier system is shown in Figure 1.2. The foundations and diaphragm of the pier are constructed out of cast-in-place concrete. The columns are reinforced with mild steel reinforcement. The cross beam can be pre-tensioned to reduce the congestion of the reinforcement and improve the capacity of the cross beam to withstand transportation and erection loads. The connections are facilitated by extending the reinforcing bars out of both ends of the columns. The bars extending from the bottom of the column are embedded into the top portion of the cast-in-place foundation. The reinforcing bars extending from the top of the column fit into openings in the cross beam, which are filled with grout to complete the connection. Hieber et al. (2005b) presented several potential details for the column-to-footing and column-to-cross beam connections.
The connections of the precast columns to the foundation and the columns to the cross beam are designed to be stronger than the columns. Therefore, plastic hinges are expected to form at the ends of the columns during an earthquake, as shown in Confining the inelastic deformations to these regions will result in satisfactory performance, provided that the columns are appropriately confined so that they exhibit little strength degradation at large deformation demands. Because the columns are weakest, they will yield first, and the other components of the pier will remain elastic and relatively undamaged during an earthquake. This practice is commonly referred to as capacity design.
The hybrid precast pier system is reinforced with a combination of mild steel reinforcement and unbonded post-tensioning. A schematic of the proposed hybrid system is shown in As with CIP emulation piers, the columns and cross beam of the pier are precast concrete, while the foundations and diaphragm are cast-in-place concrete. The precast components are similar to those used in the CIP emulation system, except that a duct is installed in the center of the column for the post-tensioning tendons. A corresponding opening is fabricated in the cross beam.
The post-tensioning contributes to the moment capacity of the columns, allowing the required number of mild steel reinforcing bars to be decreased. This decrease reduces congestion of the column-to-cap beam connection, making the components easier to fabricate and to erect. The anchors for the post-tensioning are located in the cast-in-place concrete of the foundations and diaphragm.
For typical column lengths, furnishing the post-tensioning tendons without requiring splices should not be a problem. Hieber et al. (2005b) presented potential details for the column-to-footing and column-to-crossbeam connections. Corrosion of the post-tensioning tendons is a concern in the design of hybrid piers. A corrosion protection system is envisioned consisting of a combination of epoxy coated strand, plastic sheathing, and/or grease. Future work would be required to finalize the corrosion protection system and develop methods for inspecting the post-tensioning. 7 Figure 1.4: Hybrid Precast Concrete Pier System The hybrid piers are expected to perform differently than CIP emulation, and cast-in-place reinforced concrete piers during an earthquake. Only a portion of the mild steel reinforcement in the precast columns of a hybrid pier extends into the footing and crossbeam.
This causes the interfaces between the column and footing and column and crossbeam to be the weakest portion of the pier. Consequently, the majority of deformation during an earthquake will be concentrated at these interfaces. The deformation is expected to be dominated by one large crack at the top and bottom of the columns, and the overall behavior of the pier is expected to be similar to rocking blocks, as little cracking is expected to occur in the precast components, and plastic hinges should not form.
The interface regions of the piers must be detailed to withstand large deformations. For example, the mild steel reinforcement is unbonded in the interface region to reduce the peak strains and prevent the bars from fracturing.
The ends of the columns are also heavily confined to reduce damage to the columns caused by high local compressive stresses. The post-tensioning in the columns is designed to remain elastic during an earthquake. After an earthquake, the post-tensioning will provide a recentering force and reduce residual displacements.
The mild steel is intended to yield and to dissipate energy, reducing the maximum deflection. The proportion of post-tensioning reinforcement to mild steel reinforcement can be adjusted to balance the maximum and residual displacements