Design of prestressed, jointed columns for enhanced seismic performance

Abstract

Jointed, prestressed columns have been shown to enable accelerated construction and to offer superior seismic performance, as compared with cast-in-place columns without prestressing. During an earthquake, such columns deform primarily through concentrated rotations at the joints, and special detailing at the ends of the columns prevents crushing and spalling. Unbonded prestressing steel, running vertically through the columns, provides a restoring moment that returns the columns to plumb after the ground motion stops. The use of prefabricated substructure elements speeds up the bridge assembly by eliminating building formwork, fixing steel, casting and curing concrete on-site. A variety of detailing strategies have been developed for jointed, prestressed columns, but their fundamental behavior is similar. More importantly, this behavior differs from that of conventional reinforced concrete columns for which current seismic code provisions were developed. This paper develops a displacement-based procedure for designing jointed, prestressed columns that is based on the framework provided by the AASHTO Guide Specifications for LRFD Seismic Bridge Design. To account for the unique characteristics of these systems, the design procedure introduces new performance criteria; it also provides recommendations for proportioning the prestressed and non-prestressed steel, calculating the effective column stiffness, and estimating the column displacement capacity. The focus of the paper is on columns with pretensioned strands and internal energy dissipaters, although the procedure would work for post-tensioned systems and those with external dissipaters as well. This design approach is critically evaluated through comparison to the results of both cyclic tests of cantilever columns and shaking table tests of a two-span bridge system.

Introduction

Many precast concrete connection concepts have been developed to facilitate the rapid construction of seismically resilient bridges. Such connections can either be designed to emulate the behavior and performance of cast-in-place concrete connections, e.g., [1], [2], [3], [4], [5], or they can be designed to leverage the jointed nature of precast construction to increase the seismic performance of the system, e.g., [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. These jointed systems are typically prestressed to provide stability and to recenter the structure.

As an example, Fig. 1 shows schematically one bent in a prestressed, jointed system, used here with a spread footing. It consists of:

• Precast columns, which are prestressed with strands or bars that are deliberately debonded through the height of the column;

• Foundations, to which the columns are connected on-site in a manner that both anchors the prestressing steel and deformed bar reinforcement from the columns and creates joints between the elements, where concentrated rotation, or “rocking”, is intended to occur; and

• A precast cap beam that is placed on top of the columns, which similarly anchors the reinforcement from the columns and produces joints where rotations will occur.

The use of prefabricated components speeds the construction process. The precast column and cap can be placed and grouted, then the girders can be set. This sequence eliminates several steps of building formwork, fixing steel, and casting and curing concrete, thereby reducing the on-site construction time.

As the bridge superstructure displaces horizontally, the columns deform primarily through concentrated rotations at the joints located directly below the cap beam and above the foundation. These rotations cause the prestressing steel to elongate but, because the prestressing steel is deliberately debonded over the whole column height and the elongations are spread over that length, the strain increases are small enough for the prestressing steel to remain essentially elastic for most earthquakes. The prestressing therefore provides an elastic restoring moment, with a relatively low tangent stiffness, which returns the columns to vertical after the ground motion stops. Residual drifts are essentially eliminated. The deformed bar reinforcement yields alternately in tension and compression, adding strength and dissipating energy. It may be debonded locally at the joints, over a length chosen by the designer, to allow larger rotations before bar fracture. The regions of the column adjacent to the joints are protected against premature crushing by armoring the ends of the columns with steel or fiber-reinforced polymer jackets or through the use of high-strength, fiber-reinforced concrete.

Jointed, prestressed bridge systems build on earlier conceptual developments. This approach was originally developed for precast building frames, e.g., [6], [7], [8], [9], extended to precast walls, e.g., [10], [11], [12], and later adapted to bridges by vertically post-tensioning segmental, precast columns, e.g., [13], [14], [15]. Because unbonded prestressing steel alone dissipates very little energy, e.g., [16], [17], energy dissipaters have often been added at the connections to reduce the peak displacements during an earthquake. Supplemental energy dissipation has been achieved either by including deformed bars within the body of the column, e.g., [17], [18], [19], [20], by affixing replaceable devices on the outside of the column, e.g., [20], [21], [22], [23], [24], or through friction within the joints between precast segments, e.g., [25]. External dissipaters are easier to inspect and replace after a seismic event, but they could also be more susceptible to vandalism and environmental degradation.

In jointed, prestressed systems, large contact stresses develop at the interfaces between prefabricated components, which can lead to excessive damage to the concrete unless the ends of the precast elements are protected. Researchers have protected these regions by using fiber-reinforced concrete in the plastic hinge region [14], [26], [27], [28], [29], [30], [31], by wrapping the precast column in a fiber-reinforced polymer [24], [27], [32], by encasing the end of the column in a steel jacket [13], [21], [23], by attaching steel end plates or angles to the base of the columns [18], [20], or a combination of these strategies [33], [34], [35].

While the details of these jointed column systems differ, many aspects of their behavior are similar. More importantly, their behavior is fundamentally different from that of conventional reinforced concrete columns, for which the current code provisions were developed. This paper builds on the framework of AASHTO displacement-based design procedure [36] to develop a procedure to design prestressed, jointed columns for enhanced seismic performance. The focus of the paper is for precast columns with pretensioned strands and internal energy dissipaters, although the procedure would, in principle, work for jointed cast-in-place construction, post-tensioned systems, or columns with external hysteretic dissipaters, with minimal additional information. The proposed procedure accounts for the unique behavior of the jointed columns and is straightforward to implement in current design practice. This design approach is evaluated through comparison to the results of both cyclic subassembly tests of precast cantilever columns and shaking table tests of a two-span bridge system. A summary of these experiments is found in Table 1.