Learn the requirements, limitations, advantages, and disadvantages of the various analysis methods typically used in seismic design of structures. Learning Outcomes Upon completion of this course you will be able to: Describe the primary motivating factor used in earthquake design Match the scientific principles of earthquake engineering and how they relate to dynamic effects, quantification of ground motions, inclusion of inelastic effects, and prediction of response using structural analysis.
Structural Engineering Research Activities
Apply lessons learned from previous earthquakes to development of good practices in building codes and building construction. Quantify the effects of ground shaking and mitigate the ground shaking hazards that result from earthquakes. Discuss how geological processes generate earthquakes, and investigate the best ways to mitigate the various hazards that results from earthquakes. Match a building made of different materials to the damping they would likely exhibit.
Identify the advantages and disadvantages of the various structural systems that are allowed in seismic design and methods of structural analysis that are provided by ASCE 7. Recognize ways in which configuration irregularities, excessive torsional response, and lack of redundancy can have severe consequences on the seismic performance of building structures. The addition of sufficient monitoring to identify the ground motion characteristics that trigger liquefaction, lateral spreading, and landslides. This will also allow the risk to be calculated in a manner consistent with similar calculations for the design of structures.
This is important because the mitigation of geologic hazards is often carried out from a deterministic perspective, without regard for the probability of occurrence.
Substantial savings in foundation costs, as well as expanded opportunities for building on otherwise questionable sites, will result. The addition of sufficient urban monitoring i. This does not require an instrument in every building—rather, there should be an instrument sufficiently close to record the ground motion that was experi-. This equates to at least one instrument in every zip code and one instrument on every active geologic structure fault or fold within that zip code.
The resulting records would provide the minimum amount of consistent information needed to understand the earthquake motion and provide the opportunity to develop the statistical data necessary both to calibrate assessment techniques and to develop appropriate performance indicators. The urban monitoring instruments proposed for the ANSS will partially accomplish this goal. The addition of sufficient structural monitoring of enough buildings nationwide to fully document the performance of all common building types during an event in terms of the lateral forces resisted, displacements experienced, the location and demand on elements developing ductility, and foundation-soil interaction.
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Instrumentation must allow for a complete determination of the demand on all structural and nonstructural elements. The resulting records will, in time, provide the data needed for the development of new analysis techniques that fully capture the linear and nonlinear performance of the structure. Current techniques are unable to estimate the deterioration of structural elements under strong shaking and therefore often overstate the significance of damage. In order to minimize the cost of seismic design and rehabilitation, more accurate techniques for estimating damage are needed.
Special emphasis should be placed on instrumenting publicly owned buildings, especially federal buildings, to ensure continuity of maintenance of the instruments, open access to information about structural design and construction history, timely access to the monitoring records and to the buildings themselves so that recorded shaking levels can be correlated to actual building damage, and avoidance of liability issues that may concern private building owners.
The development and deployment of new methods for monitoring buildings to directly record inter-story drift 2 demand at critical locations from both structural and nonstructural perspectives.
Current building instrumentation packages record acceleration, and integrate the waveforms to determine velocity and displacement. There is considerable controversy surrounding the accuracy of the calculated displacements, especially when they are used to calculate inter-story drift. Directly measured inter-story drift is expected to provide the most reliable ability to assess damage potential.
Inter-story drift is the amount of horizontal movement that occurs between floors during earthquake shaking. For example, if the tenth floor of a building deflects 20 inches and the ninth floor deflects 18 inches at the same time, the inter-story drift between the ninth and tenth floors is 2 inches.
Lifeline systems include transportation, water, wastewater, electric power, telecommunications, and gas and liquid fuel systems. They must perform successfully as complete systems to ensure uninterrupted operation of essential services. The addition of sufficient monitoring of lifeline systems to fully capture the interdependence of the related structures e. This will allow a full understanding of the source and impact of element failures in the system that will lead to more robust designs.
When assessing the value of improved seismic monitoring as it relates to performance-based engineering, three parameters must be considered. These include the value of the built environment within the United States, the rate of construction, and the annual expected loss from earthquakes.
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Other critical assumptions about the built environment relate to the cost of seismic design and the cost of seismic rehabilitation. Only broad-brush estimates are needed to encompass the various design styles and performance-based engineering techniques. Recent experience in various design practices has suggested anecdotally that the cost of including seismic design can range from 1 to 10 percent of a project. The difference relates mostly to the performance level selected, the sophistication of the design team, and its willingness to incorporate seismic design in the conceptual framework of the project and work to minimize its impact.
Similarly, design office experience suggests that the cost of seismic rehabilitation can range from 10 to percent of the replacement cost of the structure, depending on the structure, its condition, the seismic performance objective selected, and whether the structure will be occupied during reconstruction. A good generalized average cost is 20 percent of the replacement value. Another important assumption relates to the number of existing buildings that need seismic strengthening. Of the 50 states and the District of Columbia, 42 have some degree of earthquake potential, and 18 are considered to have high or very high seismicity.
Based on a variety of building inventories and extensive seismic rehabilitation experience, it is reasonable to assume that about 10 percent of existing buildings within the earthquake-prone areas of the United States need seismic strengthening. Finally, since the majority of buildings remain in use until destroyed by natural disasters or neglect, all cost savings were calculated under the assumption that all buildings would eventually experience a design-level earthquake. The total value was then translated to an annualized cost by multiplying the total cost by 0.
Seismic monitoring programs in place today will continue to generate benefits from performance-based engineering to the extent that they capture and record damaging events. The proposed improvements to the monitoring program are considered in terms of incremental improvements in seismic monitoring capabilities. The second is the implementation of the initial phase of the ANSS program, and the final step would be to add sufficient seismic monitoring nationwide to ensure that every damaging earthquake that occurred would be recorded to the extent necessary to advance the engineering design standards as much as possible.
If these proposed enhancements are not done, the existing networks will continue to deteriorate due to age and obsolete technology, and eventually little seismic monitoring will exist to capture data from future. An approximate annualized value is derived by multiplying the dollar value of the capital stock by a plausible long-term interest rate, estimated as 4 percent. The current implementation of USArray should lead to a long-term improvement in the understanding of the seismic hazard nationwide, although it will not provide any additional information related to the performance of structures in damaging earthquakes or provide any immediate benefit to the hazard assessment of the nation used in structural design.
The implementation of ANSS—required to maintain what is currently available and achieve some of the enhancements stated above—should generate at least six significant benefits discussed below: two that are short-term 1 and 2 and four that are intermediate- to long-term 3 to 6. The extent and timelines of achieving these benefits will depend on significant earthquakes occurring in areas that are instrumented, as well as the timing of program funding.
Proof testing of instrumented buildings: Buildings that are instrumented and experience damaging earthquakes will provide new insights into how to better design buildings to predictable performance levels. The December 22, , San Simeon earthquake may have been close to the maximum likely earthquake for that area and provides an example of this proof-testing benefit. The U. Geological Survey USGS probabilistic seismic hazard maps define design-level events for that part of the central California coastal area in terms of peak ground acceleration, as well as for short-period and 1.
Templeton Hospital, located in the area of strongest shaking, was instrumented and recorded strong motions at about the maximum design level expected. The building experienced only slight damage and did not experience any disruption of function.
Because the building has been essentially proof-tested, and the records of this testing are available, it is likely that no seismic strengthening is needed. Currently, approximately buildings are instrumented nationwide, and this number will increase to approximately under the ANSS program.
Earthquake Engineering & Structural Dynamics: Early View
The original calculations in a prerelease draft of this section were based on instrumentation of 3, buildings nationwide by ANSS. Clarification of implementation plans provided by USGS indicates that approximately buildings will be instrumented with multiple sensors, so this figure is used in the calculations that follow. Post-earthquake repair of instrumented buildings: Structural engineers responsible for evaluating the post-earthquake condition of a building that is instrumented will have the advantage of knowing what level of ground shaking caused the observed damage, and they can determine how the shaking compares to the event for which the building was designed as well as the event that it has to be repaired to resist.
This information will generally lead to lower repair costs, because the adequacy of the existing building will be better understood along with its key vulnerabilities—the repair and rehabilitation efforts can be better focused to address actual deficiencies. Engineering experience indicates that repair cost savings—ranging from 5 to 20 percent—are expected to occur for 20 percent of the currently instrumented buildings and 30 percent of the buildings to be instrumented under ANSS. Improved seismic hazard maps: The decision to design for seismic conditions—or rehabilitate because of seismic conditions—depends first on an understanding of the hazards anticipated at a particular building site.
Although detailed site-specific seismic hazard studies can be performed, the costs of such studies are too high for most building projects. Seismic hazard maps have been available for decades to allow for a less rigorous assessment of seismic risk. Scientifically defensible maps were produced by USGS in that—for the first time—used seismic monitoring data in conjunction with engineering-based parameters. The limited distribution of the seismic monitoring data on which these maps are based has meant that they can only be used accompanied by a number of assumptions that lead to conservative assessments.
Even with these limitations, these maps have refined our understanding of the nationwide distribution of earthquake hazards. Improved seismic monitoring using adequate free-field instruments is a critical requirement for further refining these maps. In addition, there is a need to better understand the relationship between the particular source characteristics of an earthquake and the strong ground motions that are produced. Instruments have to be located to identify the effects of the geologic setting and local site conditions on ground motions, with a special focus on the seismic zonation of urban areas to identify locations at which unusual ground motions may occur.
Improved seismic monitoring of weak and strong ground shaking will ultimately lead to improvements in the hazard maps used for design. The expected improvements will have a direct impact on the cost of construction and the level of.
Earthquake engineering syllabus
Engineering experience suggests that an additional 1 percent savings in construction cost could occur with implementation of each of the incremental seismic monitoring programs—USArray, a revitalization of the USNSN, and full implementation of ANSS—as they provide improved seismic hazard information. However, the remaining waves possess significant potential for damage when they reach the superstructure.
Vibration control devices assist in the reduction of the damaging effects, and enhance the seismic performance characteristics of the building.
When the seismic waves penetrate a superstructure, these are dissipated by the use of dampers, or dispersed in a wide range of frequencies. Mass dampers are also employed to absorb the resonant wave frequencies of seismic waves, thus reducing the damaging effects. Seismic isolation techniques are sometimes used to partly suppress the flow of seismic energy into the superstructure by the insertion of pads into or beneath the load bearing elements in the base of the structure.
Thus, the structure is protected from the damaging consequences of an earthquake by decoupling the structure from the shaking ground. In order to properly understand how buildings and structures can stand up to earthquakes, extensive research has also been conducted on earthquakes.