Seismic Design Of Building Structures: A Professional's Introduction To Earthquake Forces And Design
Earthquakes can virtually happen anywhere in the U.S., but the high-risk areas include California, Oregon, Washington, Alaska, Missouri, Arkansas, Tennessee, Kentucky, South Carolina, and New England. These areas are held to higher, stricter building standards as published by the NEHRP Recommended Seismic Provisions. The buildings may have to endure radical movement and foundation shifts in order to minimize damage and protect the people inside and around them. If they fracture or collapse, no emergency plan can protect the people from harm. Earthquake-resistant building designs consider the following characteristics that influence their structural integrity: stiffness and strength, regularity, redundancy, foundations, and load paths.
Seismic Design of Building Structures: A Professional's Introduction to Earthquake Forces and Design
Essential facilities such as hospitals, police and fire stations, power plants, or water treatment facilities are examples of higher level occupancy categories (III or IV), which can require a higher level of analysis, design, and detailing than a lower occupancy category building in the same region of the country. Essential facilities like these require immediate occupancy or continued use after an earthquake, which can require continued function of MEP components after an earthquake as well. Life safety systems such as fire sprinkler systems and essential electrical systems require seismic bracing to stay in service.
To determine the level of analysis, design, and detailing that will be required for the structural, architectural, and MEP components, the structural engineer will need to calculate the seismic design category. This calculation takes into account the location of the building near a fault, the occupancy category of the building (as previously mentioned), and the soil characteristics of the site. Seismic design categories A, B, or C are deemed low to moderate, whereas categories D, E, or F are deemed high to severe. Structures located in California, for example, will typically fall into a high to severe category, while structures located in Wisconsin will fall into low to moderate. Once the seismic design category has been determined, the analysis and design begins. The design of MEP supports and anchorages is covered in ASCE sections 13.3, 13.4, and 13.6.
Earthquakes are major physical occurrences that can have devastating effects on both existing infrastructure and human life. Due to advancement through research and better code development, structural engineers are better able to design buildings to withstand earthquakes and significantly reduce loss of life and destruction of property. Unfortunately, MEP components are not always taken into account when earthquakes are considered, but properly attaching the components to the structure can be just as important as properly attaching beams or columns. With careful communication and coordination with the structural engineer of record as well as a firm grasp of existing building codes, an MEP engineer can design a system that functions properly before, during, and after an earthquake.
Seismic design is a vital process of structural analysis while designing a building, which is subjected to Earthquake ground motions, such that the facility continues to function and serve its purpose even after an Earthquake.
Seismic resistance of a building can be increased by adopting different types of structural systems viz., seismic isolation system, energy dissipation system and active control systems etc., which enhances the seismic behavior of a building by dissipating the lateral forces without damaging the structural elements. The development of new structural systems and devices will proliferate the non-traditional civil engineering materials and techniques. By adopting such advanced system approaches based on dynamic analysis gives better representation of the behavior when simulated for seismic design conditions.
This resource page provides an introduction to the concepts and principles of seismic design, including strategies for designing earthquake-resistant buildings to ensure the health, safety, and security of building occupants and assets.
The essence of successful seismic design is three-fold. First, the design team must take a multi-hazard approach towards design that accounts for the potential impacts of seismic forces as well as all the major hazards to which an area is vulnerable. Second, performance-based requirements, which may exceed the minimum life safety requirements of current seismic codes, must be established to respond appropriately to the threats and risks posed by natural hazards on the building's mission and occupants. Third, and as important as the others, because earthquake forces are dynamic and each building responds according to its own design complexity, 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.
Intensity is the amount of damage the earthquake causes locally, which can be characterized by the 12 level Modified Mercalli Scale (MM) where each level designates a certain amount of destruction correlated to ground acceleration. Earthquake damage will vary depending on distance from origin (or epicenter), local soil conditions, and the type of construction.
The aforementioned seismic measures are used to calculate forces that earthquakes impose on buildings. Ground shaking (pushing back and forth, sideways, up and down) generates internal forces within buildings called the Inertial Force (FInertial), which in turn causes most seismic damage.
The greater the mass (weight of the building), the greater the internal inertial forces generated. Lightweight construction with less mass is typically an advantage in seismic design. Greater mass generates greater lateral forces, thereby increasing the possibility of columns being displaced, out of plumb, and/or buckling under vertical load (P delta Effect).
Earthquakes generate waves that may be slow and long, or short and abrupt. The length of a full cycle in seconds is the Period of the wave and is the inverse of the Frequency. All objects, including buildings, have a natural or fundamental period at which they vibrate if jolted by a shock. The natural period is a primary consideration for seismic design, although other aspects of the building design may also contribute to a lesser degree to the mitigation measures. If the period of the shock wave and the natural period of the building coincide, then the building will "resonate" and its vibration will increase or "amplify" several times.
The soil also has a period varying between 0.4 and 1.5 sec., very soft soil being 2.0 sec. Soft soils generally have a tendency to increase shaking as much as 2 to 6 times as compared to rock. Also, the period of the soil coinciding with the natural period of the building can greatly amplify acceleration of the building and is therefore a design consideration.
Torsion: Objects and buildings have a center of mass, a point by which the object (building) can be balanced without rotation occurring. If the mass is uniformly distributed then the geometric center of the floor and the center of mass may coincide. Uneven mass distribution will position the center of mass outside of the geometric center causing "torsion" generating stress concentrations. A certain amount of torsion is unavoidable in every building design. Symmetrical arrangement of masses, however, will result in balanced stiffness against either direction and keep torsion within a manageable range.
Building Configuration: This term defines a building's size and shape, and structural and nonstructural elements. Building configuration determines the way seismic forces are distributed within the structure, their relative magnitude, and problematic design concerns.
Knowledge of the building's period, torsion, damping, ductility, strength, stiffness, and configuration can help one determine the most appropriate seismic design devices and mitigation strategies to employ.
Base Isolation: This seismic design strategy involves separating the building from the foundation and acts to absorb shock. As the ground moves, the building moves at a slower pace because the isolators dissipate a large part of the shock. The building must be designed to act as a unit, or "rigid box", of appropriate height (to avoid overturning) and have flexible utility connections to accommodate movement at its base. Base Isolation is easiest to incorporate in the design of new construction. Existing buildings may require alterations to be made more rigid to move as a unit with foundations separated from the superstructure to insert the Base Isolators. Additional space (a "moat") must be provided for horizontal displacement (the whole building will move back and forth a whole foot or more). Base Isolation retrofit is a costly operation that is most commonly appropriate in high asset value facilities and may require partial or the full removal of building occupants during installation.
A performance-based approach to establishing seismic design objectives is recommended. This determines a level of predictable building behavior by responding to the maximum considered earthquake. A threat/vulnerability assessment and risk analysis can be used to define the level of performance desired for the building project. Some suggested seismic design performance goals are:
Seismic design objectives can greatly influence the selection of the most appropriate structural system and related building systems for the project. Some construction type options, and corresponding seismic properties, are:
Many building codes and governmental standards exist pertaining to design and construction for seismic hazard mitigation. As previously mentioned, building code requirements are primarily prescriptive and define seismic zones and minimum safety factors to "design to." Codes pertaining to seismic requirements may be local, state, or regional building codes or amendments and should be researched thoroughly by the design professional.