Description
This standard prescribes procedures for the seismic analysis and design of liquid-containing concrete structures. These procedures address the loading side of seismic design and are intended to complement ACI 350-06, Section 1.1.8 and Chapter 21.
INTRODUCTION
The following paragraphs highlight the development of this standard and its evolution to the present format:
From the time it embarked on the task of developing an ACI 318-dependent code, ACI Committee 350 decided to expand on and supplement Chapter 21, “Special Provisions for Seismic Design,” to provide a set of thorough and comprehensive procedures for the seismic analysis and design of all types of liquid-containing environmental concrete structures. The committee’s decision was influenced by the recognition that liquid-containing structures are unique structures whose seismic design is not adequately covered by the leading national codes and standards. A seismic design subcommittee was appointed with the charge to implement the committee’s decision.
The seismic subcommittee’s work was guided by two main objectives:
1. To produce a self-contained set of procedures that would enable a practicing engineer to perform a full seismic analysis and design of a liquid-containing structure. This meant that these procedures should cover both aspects of seismic design: the “loading side” (namely the determination of the seismic loads based on the mapped maximum considered earthquake spectral response accelerations at short periods (Ss) and 1 second (S1) obtained from the Seismic Ground Motion maps [Fig. 22-1 through 22-14 of ASCE 7-05, Chapter 22] and the geometry of the structure); and the “resistance side” (the detailed design of the structure in accordance with the provisions of ACI 350, so as to resist those loads safely); and
2. To establish the scope of the new procedures consistent with the overall scope of ACI 350. This required the inclusion of all types of tanks—rectangular, as well as circular; and reinforced concrete, as well as prestressed. (Note: While there are currently at least two national standards that provide detailed procedures for the seismic analysis and design of liquid-containing structures (ANSI/AWWA 1995a,b), these are limited to circular, prestressed concrete tanks only).
As the loading side of seismic design is outside the scope of ACI 318, Chapter 21, it was decided to maintain this practice in ACI 350 as well. Accordingly, the basic scope, format, and mandatory language of Chapter 21 of ACI 318 were retained with only enough revisions to adapt the chapter to environmental engineering structures. Provisions similar to Section 1.1.8 of ACI 318 are included in ACI 350. This approach offers at least two advantages:
1. It allows ACI 350 to maintain ACI 318’s practice of limiting its seismic design provisions to the resistance side only; and
2. It makes it easier to update these seismic provisions so as to keep up with the frequent changes and improvements in the field of seismic hazard analysis and evaluation.
The seismic force levels and R-factors included in this standard provide results at strength levels, such as those included for seismic design in the 2003 International Building Code (IBC), particularly the applicable connection provisions of 2003 IBC,
as referenced in ASCE 7-02. When comparing these provisions with other documents defining seismic forces at allowable stress levels (for example, the 1994 Uniform Building Code [UBC] or ACI 350.3-01), the seismic forces in this standard should be reduced by the applicable factors to derive comparable
forces at allowable stress levels.
The user should note the following general design methods used in this standard, which represent some of the key differences relative to traditional methodologies, such as those described in ASCE (1984):
1. Instead of assuming a rigid tank for which the acceleration is equal to the ground acceleration at all locations, this standard assumes amplification of response due to natural frequency of the tank;
2. This standard includes the response modification factor;
3. Rather than combining impulsive and convective modes by algebraic sum, this standard combines these modes by square-root-sum-of-the-squares;
4. This standard includes the effects of vertical acceleration; and
5. This standard includes an effective mass coefficient, applicable to the mass of the walls.
Keywords: circular tanks; concrete tanks; convective component; earthquake
resistance; environmental concrete structures; impulsive component;
liquid-containing structures; rectangular tanks; seismic resistance;
sloshing; storage tanks.
Table of Contents
CHAPTER 1—GENERAL REQUIREMENTS
1.1—Scope
1.2—Notation
CHAPTER 2—TYPES OF LIQUID-CONTAINING STRUCTURES
2.1—Ground-supported structures
2.2—Pedestal-mounted structures
CHAPTER 3—GENERAL CRITERIA FOR ANALYSIS AND DESIGN
3.1—Dynamic characteristics
3.2—Design loads
3.3—Design requirements
CHAPTER 4—EARTHQUAKE DESIGN LOADS
4.1—Earthquake pressures above base
4.2—Application of site-specific response spectra
CHAPTER 5—EARTHQUAKE LOAD DISTRIBUTION
5.1—General
5.2—Shear transfer
5.3—Dynamic force distribution above base
CHAPTER 6—STRESSES
6.1—Rectangular tanks
6.2—Circular tanks
CHAPTER 7—FREEBOARD
7.1—Wave oscillation
CHAPTER 8—EARTHQUAKE-INDUCED EARTH PRESSURES
8.1—General
8.2—Limitations
8.3—Alternative methods
CHAPTER 9—DYNAMIC MODEL
9.1—General
9.2—Rectangular tanks (Type 1)
9.3—Circular tanks (Type 2)
9.4—Seismic response coefficients Ci , Cc, and Ct
9.5—Site-specific seismic response coefficients Ci , Cc, and Ct
9.6—Effective mass coefficient ε
9.7—Pedestal-mounted tanks
CHAPTER 10—COMMENTARY REFERENCES
APPENDIX A—DESIGN METHOD
A.1—General outline of design method
APPENDIX B—ALTERNATIVE METHOD OF ANALYSIS
BASED ON 1997 Uniform Building Code
B.1—Introduction
B.2—Notation (not included in Section 1.2 of this standard)
B.3—Loading side, general methodology
B.4—Site-specific spectra (Section 1631.2(2))
B.5—Resistance side
B.6—Freeboard