Design of Hybrid Precast Concrete Walls for Seismic Regions

This paper presents an ongoing research project on the behavior and design of “hybrid” precast concrete wall structures for use in seismic regions. Hybrid precast walls utilize a combination of mild (e.g., Grade 60) steel and high-strength unbonded post-tensioning (PT) steel for flexural resistance across horizontal joints. The mild steel reinforcement is designed to yield in tension and compression, providing energy dissipation. The unbonded PT steel provides self-centering capability, reducing the residual (i.e., permanent) lateral displacements of the wall from a large earthquake. Both the PT steel and the mild steel contribute to the wall lateral strength, resulting in an efficient structure. The paper introduces a seismic design approach that conforms to ACI 318 (2008) and ACI ITG-5.2 (2008) for the validation of hybrid walls as special reinforced concrete shear walls based on ACI ITG-5.1 (2008). This design approach is used to design a series of test specimens investigating the following parameters: (1) relative amounts of mild steel and PT steel; (2) wall height-to-length aspect ratio; (3) concrete confinement details; and (4) presence of openings within the wall panels. A pre-test study is conducted to evaluate the design of the test specimens based on nonlinear reversed-cyclic lateral load analyses. It is shown that the specimens satisfy all of the validation and design prerequisites set forth in ACI ITG-5.1 and ACI ITG-5.2. The subsequent testing of these specimens is expected to ultimately support the code approval of the hybrid wall system for moderate and high seismic regions. INTRODUCTION AND BACKGROUND Concrete structural walls make up a large percentage of the lateral load resisting systems in building construction. As shown in Figure 1, the hybrid precast concrete wall configuration investigated in this research is constructed by placing rectangular wall panels across horizontal joints. These structures offer high quality production, simpler construction, and excellent seismic characteristics by providing self-centering to the building (i.e., the wall returns to its undisplaced “plumb” position after a large earthquake) as well as sufficient energy dissipation to control the lateral displacements. The self-centering capability is achieved by using unbonded posttensioning (PT) steel strands to connect the wall panels, and the energy dissipation is achieved by using bonded mild steel (e.g., Grade 60) bars that cross the horizontal joint at the wall base. Gap opening at the horizontal joints (primarily the base joint) allows the wall to undergo large lateral displacements with little damage. Upon unloading, the PT steel provides a restoring force to close the gaps, reducing the permanent lateral displacements of the structure. The PT force is provided by multi-strand tendons placed inside un-grouted ducts (to prevent bond between the steel and concrete) through the wall panels and the foundation. Thus, the tendons are connected to the structure only at end anchorages. The use of unbonded PT tendons delays the yielding of the strands and reduces the tensile stresses transferred to the concrete (i.e., reduced cracking) as the tendons elongate during the lateral displacements of the structure. The mild steel bars crossing the base joint are designed to yield and provide energy dissipation through the gap opening/closing behavior. A pre-determined length of the mild steel at the wall base is unbonded (by wrapping the bars) to prevent lowcycle fatigue failure. Both the PT steel and the mild steel contribute to the lateral strength of the wall, resulting in an efficient structure. Despite the desirable characteristics of hybrid precast concrete wall structures, there are significant limitations that prevent their use in seismic regions. Most importantly, Chapter 21 of ACI 318 (ACI 2008) specifies that “a reinforced concrete structural system not satisfying the requirements of this chapter shall be permitted if it is demonstrated by experimental evidence and analysis that the proposed system will have strength and toughness equal to or exceeding those provided by a comparable monolithic reinforced concrete structure satisfying this chapter.” The hybrid wall system investigated in this paper falls into this category of “non-emulative” structures that require experimental validation prior to their use in practice. The minimum experimental evidence needed to achieve the code-validation of hybrid precast walls as “special” reinforced concrete (RC) shear walls is specified in ACI ITG-5.1 (2008), which is referenced in Section 21.10.3 of ACI 318 (ACI 2008). Among the subjects covered in ITG-T5.1 are requirements for the design of the test specimens and their configurations, as well as requirements for testing, assessing, and reporting satisfactory performance. Draft design guidelines (pending experimental validation) can be found in ACI ITG-5.2 (2008). To date, limited tests and analytical studies are available for hybrid precast walls (Rahman and Restrepo 2000; Holden et al. 2001; Kurama 2002, 2005). While these results have demonstrated the excellent behavior that can be obtained from these structures, the existing knowledge is not sufficient for the required code-validation based on ACI ITG-5.1. The current paper addresses this research need. RESEARCH OBJECTIVES AND SCOPE The most pressing U.S. market need related to hybrid walls is code approval as special RC shear walls per ACI 318. Achieving this task would lift a major road block and would advance building construction, with a broad applicability in moderate and high seismic regions. To address the current market need, the primary objective of this project is to experimentally and analytically validate hybrid wall structures for code approval according to the guidelines, prerequisites, and requirements in ACI ITG-5.1 and ACI ITG-5.2. The specific project objectives are to develop: (1) a validated seismic design procedure for the new system; (2) validated analytical models and design tools; and (3) practical guidelines and experimental evidence demonstrating the performance of these structures under seismic loading. The key deliverable from the project will be a seismic design procedure document, which is outlined and used in this FIGURE 1 – ELEVATION, DISPLACED POSITION, AND CROSS-SECTION OF HYBRID WALL SYSTEM paper to design a series of test specimens with the following parameters: (1) relative amounts of mild steel and PT steel; (2) wall height-to-length aspect ratio; (3) concrete confinement details; and (4) presence of openings in the wall panels. As required by ACI ITG-5.1, a pre-test study is conducted to evaluate the design of the test specimens based on nonlinear lateral load analyses. The objective of this study is not only the confirmation of the specimen designs but also the pretest development of the analytical models for subsequent validation using the test data. OVERVIEW OF VALIDATION AND DESIGN REQUIREMENTS The roadmap to the required ACI Code validation for hybrid precast concrete walls is provided by several essential documents. ACI ITG-5.1 lays out the minimum experimental evidence needed to achieve ACI 318 acceptance. Specific requirements are given with regards to the tested wall roof drift, measured wall lateral strength to the predicted strength ratio, PT strand stresses and strains, amount of energy dissipation, wall strength degradation, and shear slip at the horizontal joints, among other requirements. The design is conducted at two levels of roof drift as follows: (1) the design-level roof drift, θwd, which is determined according to ASCE 7 (2006); and (2) the validation-level roof drift, which is determined from ACI ITG-5.1 as ( ) % 0 . 3 5 . 0 8 . 0 % 9 . 0 ≤ + ≤ = w w wm l h θ (1) where, hw is the height of the wall; and lw is the length of the wall. Prior to conducting the validation testing, ACI ITG-5.1 requires that a pre-test design/analysis procedure for the wall specimens be established. The design procedure used to determine the details of the test specimens in this study is presented below. A few key ACI ITG-5.1 requirements for the test specimens include: (1) a minimum of two wall panels (in order to model a representative panelto-panel joint as well as the base-panel-to-foundation joint) unless the prototype structure uses a single panel for the full height of the wall; (2) a minimum specimen scale of one-third; (3) a minimum wall height-to-length aspect ratio of 0.5; and (4) the use of similar reinforcement details and representative building materials in the test specimens as in the prototype structure. PROTOTYPE WALL DESIGN A prototype structure was designed in collaboration with the Consulting Engineers Group (CEG), Texas, following the basic guidelines in ACI ITG-5.2 (2008) and ACI 318 (2008). As shown in Figure 2(A), the prototype structure is a four-story regularly-shaped precast concrete parking garage with a footprint area of approximately 42000 ft. The first story height is 12 ft while the upper story heights are 11 ft. The structure is located in Los Angeles, California, where the seismic response coefficient for the building was calculated as Cs=0.182g. The lateral load resistance of the building in the N-S direction is provided by seven hybrid walls (see Figure 2(B) for the elevation of a typical wall). Using the equivalent lateral force procedure in ASCE 7 (2006), the design base moment demand for the exterior walls (where the lateral force demand is largest considering accidental torsion effects) can be determined as Mwd=20000 kip.ft. A response modification factor of R=6.0 for special RC shear walls is used in the design. For the selected wall dimensions of hw=45 ft and lw=20 ft (resulting in an aspect ratio of 2.25), the validation-level drift from Equation (1) is θwm=2.3% and the design-level drift from ASCE 7 is θwd=0.4%. In the calculation of θwd, a deflection amplification factor of Cd=5.0 was applied to the linear-elastic deflection (flexural plus shear deflections) of the wall determined using an assumed cracked flexural stiffness of Icr=0.60Igross and a shear area of Ash=0.67Agross. PT Steel and Mild Steel Areas Energy dissipating mild steel reinforcement near the wall centerline and self-centering PT steel placed in two bundles outside of the mild steel bars cross the base joint for lateral resistance. As shown in Figure 3(A), the PT steel and mild steel areas are determined to satisfy the wall design base moment demand, Mwd=20000 kip.ft. at the design-level drift, θwd. To determine the required steel areas, the “design mild steel moment ratio,” κd is defined as ) /( wn wp ws d M M M + = κ (2) with wd wn wp ws M M M M = + + (3) where, Mws, Mwp, and Mwn represent the contributions of the mild steel reinforcement, PT steel reinforcement, and applied (external) wall design gravity axial load (Nwd), respectively, to satisfy the design base moment demand, Mwd. In the design of the prototype structure, a gravity load combination of 100% of the design dead load plus 25% of the design live load was used (note that in practice, the governing load combinations from ASCE 7 should be followed). As described in Kurama (2005), the κd ratio is a measure of the relative amounts of the energy dissipating resistance (from the mild steel reinforcement) and the self-centering resistance (from the PT steel reinforcement and the external gravity load) in the wall. Designs using larger κd result in walls with larger amounts of mild steel reinforcement. If the mild steel contribution is too small (i.e., κd ratio is too small), then the energy dissipation of the wall may be very small. (A) (B) FIGURE 2 – PROTOTYPE STRUCTURE: (A) PLAN (COURTESY OF CEG); (B) WALL ELEVATION