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Project Summary
This project is investigating “hybrid” precast concrete wall structures that use a combination of deformed mild steel (e.g., Grade 60) reinforcing bars and high-strength post-tensioning steel strands to provide the lateral load resistance needed in seismic regions. The walls are constructed by joining precast concrete rectangular wall panels on top of each other across horizontal joints at the floor levels. The PT steel and mild steel reinforcement cross these horizontal joints through matching ducts cast inside the wall panels to provide continuity to the wall. The desired nonlinear behavior of a hybrid precast wall under lateral loads is an axial-flexural behavior governed by the opening of gaps along the horizontal joints; with little concrete cracking and little shear deformations. The gap opening behavior at the horizontal joints (primarily the base joint) allows the wall to undergo large lateral displacements with little damage. Both the PT steel and the mild steel contribute to the lateral strength of the wall, resulting in an efficient design.

The ducts containing the post-tensioning strands are not grouted, and thus, the strands are unbonded over their entire length between anchors at the roof and the foundation. The use of unbonded PT steel significantly delays the yielding of the strands (thus maintaining the PT force under cyclic loading) and reduces the tensile stresses transferred to the concrete as the strands elongate during an earthquake (thus reducing concrete cracking). The restoring effect of the PT force provides a large self-centering capability to the wall pulling the structure back towards its original undisplaced “plumb” position after an earthquake. The PT steel is designed to remain linear elastic during a Design Basis Earthquake, thus, maintaining this self-centering capability.

Unlike the PT steel, the mild steel reinforcement is designed to yield, thus, providing energy dissipation to the structure under reversed-cyclic seismic loading. The ducts for the mild steel bars are grouted to provide adequate anchorage and development to the reinforcement. To prevent fracturing of the mild steel bars and to reduce cracking of the concrete during the deformations of the bars in tension, the bond between the bars and the grout is prevented by wrapping the bars with plastic sheathing over a predetermined length right above the foundation where the largest gap opening is expected to occur. The mild steel bars are extended a sufficient height above the wall base (with adequate development length), after which the bars that are no longer needed for lateral resistance may be terminated in a staggered manner. The bars do not need to extend all the way to the top of the wall since a significant portion of the lateral strength of the structure is provided by the PT steel.

Problem to be Addressed
Concrete shear walls make up a large percentage of the lateral load systems in building construction. Hybrid precast wall structures offer high quality production and simpler construction. In addition, the combination of unbonded PT steel for self-centering and mild steel reinforcement for energy dissipation results in excellent seismic characteristics.

For seismic regions, ACI 318-08 specifies that “a reinforced concrete system not satisfying the requirements of this chapter (Chapter 21) shall be permitted if it is demonstrated by experimental evidence and analysis that the system has strength and toughness equal to or exceeding those provided by a comparable monolithic concrete structure satisfying this chapter.” The proposed hybrid wall system falls into this category of “non-emulative” structures that require experimental validation; and thus, its use in practice is severely limited. Thus, the most pressing current market need related to the project is the code approval of the new system. Achieving this task would lift a major road-block and would provide a major advance for building construction, with a broad applicability in moderate and high seismic regions.
 
Copyright 2008. University of Notre Dame.