Buildings designed to resist earthquakes. Earthquake resistant buildings should include strategies to ensure the health, safety, and security of building occupants and assets.
Successful seismic design is in essence three-fold:
1) Design team must take a multi-hazard approach which accounts for the potential impacts of seismic forces as well as all the major hazards to which the area is vulnerable;
2) Performance-based requirements which may exceed the minimum life safety requirements of current seismic codes must be established to respond appropriately to threats and risks posed by natural hazards on the building's mission and occupants;
3) It is essential that the design team work collaboratively and have a common understanding of the terms and methods used in the seismic design process.
In addition, as a general rule, buildings designed to resist earthquakes should also resist blast (terrorism) or wind, suffering less damage.
The US Geological Survey (USGS) estimates that several million earthquakes occur in the world each year. Many earthquakes go undetected because they hit remote areas or have very small magnitudes. Historical observations show that high intensity destructive earthquakes are confined to certain areas around the world. Many heavily populated areas are located in zones of high seismic risk. Proper analysis and design should be performed when designing and detailing such buildings. Structural response to earthquake excitation depends on the characteristics of the structure and the soil nature at the site of the structure. Even in moderate seismic risk areas there may be need for design and detail the structure for seismic response depending on those characteristics. In addition, due to damage to natural gas lines, earthquakes can subsequently give rise to major fires which may be the cause of equally heavy losses and tax the already stretched emergency response force. This was the case during 1906 San Francisco earthquake and 1923 in Tokyo. Experience has shown that reinforced concrete structures have great advantage in fire resistance in those situations.
The successful performance of a large number of reinforced concrete buildings in earthquake zones in U.S. and around the world has proved that it is possible to design structures with the resilience to withstand earthquakes of relatively high magnitude and still provide resistance to the subsequent fires. The past five decades have seen a rapid development in both theory and practice of earthquake resistant design and construction. Observation of actual behavior of reinforced concrete buildings after major earthquakes, experimental research and analytical studies contributed to the advancement in seismic design approaches and code requirements. The building codes requirements for seismic design have evolved from crude treatment of the subject to sophisticated methodology addressing all factors affecting the behavior of the structure under earthquake excitation.
Prior to the year 2000, provisions for seismic design were covered in the three major model buildings codes (Uniform Building Code UBC, the Building Officials and Code Administrators International BOCA and the Standard Building Code). Each one of these codes treated the seismic provisions and requirement differently. Different states and Municipalities adopted different Codes. In 1994 the three organizations that produced and published the three model codes formed the International Code Council (ICC). The purpose of the ICC is to develop and maintain a single building code. The ICC published the first edition of the International Building Code IBC in 2000 followed by other editions in 2003, 2006 and 2009. Another standard that includes provisions for seismic analysis and design is; Minimum Design Loads for Buildings and Other Structures (ASCE 7). The seismic design provisions of the 2009 IBC are the same as ASCE 7-05 with some modifications. The ASCE 7-05 provisions are based on 2003 NEHRP Provisions (National Earthquake Hazards Reduction Program - Recommended Provisions for Seismic Regulations for New Buildings and Other Structures).
Before the publication of the first edition of the International Building Code in 2000, seismic risk and subsequently seismic design criteria in building codes depended only on the level of the earthquake ground motion. The concept of seismic zone was used. According to the UBC Code the U.S. was divided into five seismic zones 0 through 4. Zone 0 was where the earthquake ground motion the weakest and zone 4 the strongest. The method of analysis, height limits and level of detailing depended on the seismic zone in which a structure was located. Recognizing that structure performance during an earthquake depends not only on the level of the earthquake ground motion, but also on the nature of the soil on which the structure is founded, the International Building Code (IBC) established the Seismic Design Categories (SDC) as a measure for the seismic risk for a certain structure. The SDC is a function of the level of the earthquake ground motion, the soil nature at the site and the use of the structure. The IBC and ASCE 7 contain procedures to determine the Seismic Design Category. The IBC requires that a SDC be assigned to each structure. The SDC is used to determine the allowed method of analysis, height limits and level of detailing.
It is uneconomical to design a structure to respond in the elastic range to the inertial forces caused by the maximum considered earthquake. Accordingly, the design seismic lateral forces prescribed in the 2009 IBC and ASCE 7-05 are less than the elastic response inertial forces caused by the intended design earthquake. The 2009 IBC includes requirements for proportioning and detailing structural elements for different Seismic Design Categories (SDC). These requirements are the same requirements of Chapter 21 of ACI 318-08 (Building Code Requirements for Structural Concrete and Commentary). The purpose of these detailing and proportioning requirements is to avoid all forms of brittle failure and insures that the structure will have sufficient inelastic deformability. This is to enable the structure to survive without collapse when subjected to several cycles of loading within the inelastic range.
In addition to the IBC and ASCE 7 other resources are available to help understand the application of the Code and in design. In 1961 the Portland Cement Association (PCA) pioneered the work on seismic design by publishing the land mark publication, Design of Multistory Reinforced Concrete Building for Earthquake Motion by Blume, Newmark and Corning. This publication gave earthquake-resistant design of multistory reinforced concrete buildings more of a scientific basis than it ever had before. And today, it still assists engineers and researchers understand the basis of seismic design concepts. The authors wrote in their preface, "...earthquake-resistant design is not yet capable of complete and rigorous execution solely by means of mathematical analysis, design codes, specifications, or rules of procedure. It is an art as well as a science ...." These words are as true today as they were sixty years ago.
Publications
PCA Notes on ACI 318-08 Building Code Requirements for Structural Concrete with Design Applications, EB708
The tenth edition of this classic PCA resource has been updated to reflect code changes introduced in the latest version of Building Code Requirements for Structural Concrete, ACI 318-08. These notes will help users apply code provisions related to the design and construction of concrete structures. Each chapter of the manual starts with a description of the latest code changes. Emphasis is placed on “how-to-use” the code. Numerous design examples illustrate application of the code provisions.
Design of Liquid-Containing Concrete Structures for Earthquake Forces, EB219
This publication contains comprehensive information on the design and detailing requirements for concrete tanks subjected to earthquake forces. The earthquake forces are computed in accordance with a variety of codes, including the 2000 International Building Code.