ACI-ITG-4.3R-2007.pdf

ACI ITG-4.3R-07 Reported by ACI Innovation Task Group 4 and Other Contributors Report on Structural Design and Detailing for High-Strength Concrete in Moderate to High Seismic Applications Report on Structural Design and Detailing for High-Strength Concrete in Moderate to High Seismic Applications First Printing September 2007 ISBN 978-0-87031-254-0 American Concrete Institute ® Advancing concrete knowledge Copyright by the American Concrete Institute, Farmington Hills, MI. All rights reserved. This material may not be reproduced or copied, in whole or part, in any printed, mechanical, electronic, film, or other distribution and storage media, without the written consent of ACI. The technical committees responsible for ACI committee reports and standards strive to avoid ambiguities, omissions, and errors in these documents. 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Reference to this document shall not be made in contract documents. If items found in this document are desired by the Architect/Engineer to be a part of the contract documents, they shall be restated in mandatory language for incorporation by the Architect/Engineer. Report on Structural Design and Detailing for High-Strength Concrete in Moderate to High Seismic Applications Reported by ACI Innovation Task Group 4 and Other Contributors ACI ITG-4.3R-07 ACI ITG-4.3R presents a literature review on seismic design using high- strength concrete. The document is organized in chapters addressing the structural design of columns, beams, beam-column joints, and structural walls made with high-strength concrete, and focuses on aspects most relevant for seismic design. Each chapter concludes with a series of recommended modifications to ACI 318-05 based on the findings of the literature review. The recommendations include proposals for the modification of the equiva- lent rectangular stress block, equations to calculate the axial strength of columns subjected to concentric loading, column confinement requirements, limits on the specified yield strength of confinement reinforcement, strut factors, and provisions for the development of straight bars and hooks. An accompanying standard, ITG-4.1, is written in mandatory language in a format that can be adopted by local jurisdictions, and will allow building officials to approve the use of high-strength concrete on projects that are being constructed under the provisions of ACI 301, “Specifications for Structural Concrete,” and ACI 318, “Building Code Requirements for Structural Concrete.” ITG 4 has also developed another nonmandatory language document: ITG-4.2R. It addresses materials and quality considerations and is the supporting document for ITG-4.1. Keywords: bond; confinement; drift; flexure; high-strength concrete; high- yield-strength reinforcement; seismic application; shear; stress block; strut- and-tie. CONTENTS Chapter 1Introduction, p. ITG-4.3R-2 1.1Background 1.2Scope Chapter 2Notation, p. ITG-4.3R-4 Chapter 3Definitions, p. ITG-4.3R-7 Chapter 4Design for flexural and axial loads using equivalent rectangular stress block, p. ITG-4.3R-7 4.1Parameters of equivalent rectangular stress block 4.2Stress intensity factor 1 4.3Stress block depth parameter 1 4.4Stress block area 1 4.5Limiting strain cu 4.6Axial strength of high-strength concrete columns 4.7Comparison of different proposals for rectangular stress block 4.8Recommendations Chapter 5Confinement requirements for beams and columns, p. ITG-4.3R-19 5.1Constitutive models for confined concrete 5.2Previous research and general observations 5.3Equations to determine amount of confinement reinforcement required in columns Joseph M. BracciD. Kirk HarmanAdolfo Matamoros Michael A. CaldaroneDaniel C. JansenAndrew W. Taylor Other contributors Dominic J. KellyAndres LepageHenry G. Russell ACI Innovation Task Group 4 S. K. Ghosh Chair ITG-4.3R-2ACI COMMITTEE REPORT 5.4Definition of limiting drift ratio on basis of expected drift demand 5.5Use of high-yield-strength reinforcement for confinement 5.6Maximum hoop spacing requirements for columns 5.7Confinement requirements for high-strength concrete beams 5.8Maximum hoop spacing requirements for high- strength concrete beams 5.9Recommendations Chapter 6Shear strength of reinforced concrete flexural members, p. ITG-4.3R-35 6.1Shear strength of flexural members without shear reinforcement 6.2Effect of compressive strength on inclined cracking load of flexural members 6.3Effect of compressive strength on flexural members with intermediate to high amounts of transverse reinforcement 6.4Shear strength of members with low shear span- depth ratios 6.5Calculation of shear strength of members subjected to seismic loading 6.6Use of high-strength transverse reinforcement 6.7Recommendations Chapter 7Development length/splices, p. ITG-4.3R-44 7.1Design equations for development length of bars in high-strength concrete 7.2Design equations for development length of hooked bars in high-strength concrete 7.3Recommendations Chapter 8Design of beam-column joints, p. ITG-4.3R-48 8.1Confinement requirements for beam-column joints 8.2Shear strength of exterior joints 8.3Shear strength of interior joints 8.4Effect of transverse reinforcement on joint shear strength 8.5Development length requirements for beam-column joints 8.6Recommendations Chapter 9Design of structural walls, p. ITG-4.3R-51 9.1Boundary element requirements 9.2Shear strength of walls with low aspect ratios 9.3Minimum tensile reinforcement requirements in walls 9.4Recommendations Chapter 10List of proposed modifications to ACI 318-05, p. ITG-4.3R-53 10.1Proposed modifications to equivalent rectangular stress block 10.2Proposed modifications related to confinement of potential plastic hinge regions 10.3Proposed modifications related to bond and develop- ment of reinforcement 10.4Proposed modifications related to strut-and-tie models Acknowledgments, p. ITG-4.3R-56 Chapter 11Cited references, p. ITG-4.3R-56 CHAPTER 1INTRODUCTION 1.1Background The origin of ACI Innovation Task Group (ITG) 4, High- Strength Concrete for Seismic Applications, can be traced back to the International Conference of Building Officials (ICBO) (now International Code Council ICC) Evaluation Report ER-5536, “Seismic Design Utilizing High-Strength Concrete” (ICBO 2001). Evaluation Reports (ER) are issued by Evaluation Service subsidiaries of model code groups. An ER essentially states that although a particular method, process, or product is not specifically addressed by a particular edition of a certain model code, it is in compliance with the requirements of that particular edition of that model code. ER-5536 (ICBO 2001), first issued in April 2001, was generated by Englekirk Systems Development Inc. for the seismic design of moment-resisting frame elements using high-strength concrete. High-strength concrete was defined as “normalweight concrete with a design compressive strength greater than 6000 psi (41 MPa) and up to a maximum of 12,000 psi (83 MPa).” It was based on research carried out at the University of Southern California and the University of California at San Diego to support building construction in Southern California using concrete with compressive strengths greater than 6000 psi (41 MPa). The Portland Cement Association performed a review* of ER-5536 and brought up several concerns that focused on inconsistencies between the evaluation report and existing industry documents in two primary areas: material and structural. Despite those concerns, it was evident that the evaluation report had been created because quality assurance and design provisions were needed by local jurisdictions, such as the City of Los Angeles, to allow the use of high-strength concrete without undue restrictions. ACI has assumed a proactive role in the development of such provisions with the goal of creating a document that can be adopted nationwide. ACI considered its own Committee 363, High Strength Concrete, to be the best choice to address the materials and quality aspects of the document, while ACI Subcommittee 318-H, Structural Concrete Building CodeSeismic Provisions, was considered the best choice to address the seismic detailing aspects. Because 318-H is a subcommittee of a code-writing body, the development of a technical document of this kind is not part of its intended mission. In addition, producing a document through a technical committee can be a lengthy process. Based on these limitations, a request was made to form an ITG that would have the advantage of following a shorter timeline to completion. In *Unpublished report available from PCA, Skokie, Ill., Aug. 2001. STRUCTURAL DESIGN AND DETAILING FOR HIGH-STRENGTH CONCRETE IN SEISMIC APPLICATIONSITG-4.3R-3 response to the request, the Technical Activities Committee (TAC) of ACI approved the formation of ITG 4 and estab- lished its mission. The mission was to develop an ACI document that addressed the application of high-strength concrete in structures located in areas of moderate and high seismicity. The document was intended to cover structural design, material properties, construction procedures, and quality-control measures. It was to contain language in a format that allowed building officials to approve the use of high- strength concrete in projects being constructed under the provisions of ACI 301-05, “Specifications for Structural Concrete,” and ACI 318, “Building Code Requirements for Structural Concrete.” The concept of “moderate to high seismic applications,” stated in the mission of the document, dates back to when U.S. seismic codes divided the country into seismic zones. These seismic zones were defined as regions in which seismic ground motion on rock, corresponding to a certain probability of occurrence, remained within certain ranges. Present-day seismic codes (ASCE/SEI 2006) follow a different approach to characterizing a seismic hazard. Given that public safety is a primary code objective, and that not all buildings in a given seismic zone are equally crucial to public safety, a new mechanism for triggering seismic design requirements and restrictions, called the seismic performance category (SPC), was developed. The SPC classification includes not only the seismicity at the site, but also the occupancy of the structure. Recognizing that building performance during a seismic event depends not only on the severity of bedrock acceleration, but also on the type of soil that a structure is founded on, seismic design criteria in more recent seismic codes are based on seismic design categories (SDC). The SDC is a function of location, building occupancy, and soil type. The TAC Technology Transfer Committee (TTTC)-estab- lished mission of ITG 4 was interpreted to mean that the Task Group was to address the application of high-strength concrete in structures that are: Located in Seismic Zones 2, 3, or 4 of the “Uniform Building Code” (ICBO 1997); or Assigned to SDC C, D, or E of “The BOCA National Building Code” (BOCA 1993 and subsequent editions) or the “Standard Building Code” (SBCCI 1994); or SDC C, D, E, or F of the “International Building Code” (IBC 2003) or the National Fire Protection Association (NFPA) NFPA 5000 “Building Construction and Safety Code” (2003). SPC or SDC C is also referred to as the “intermediate” category. Similarly, SPC D and E or SDC D, E, and F are referred to as “high” categories. The terminology “moderate to high seismic applications,” however, is used throughout this document. 1.2Scope This document addresses the material and design consider- ations when using normalweight concrete