Earthquake engineering
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It has been suggested that Seismic performance, Vibration control, Earthquake Protector, Earthquake Performance Evaluation Tool, Earthquake simulation and Earthquake construction be merged into this article or section. (Discuss) |
Earthquake engineering is a subset of structural engineering which covers behavior of buildings and structures subjected to earthquake loading including but not limited to horizontal and vertical shaking, supporting ground failure, tsunami [1].
The main objectives of earthquake engineering are:
- Understand interaction of buildings and civil infrastructure with the shaking ground during an earthquake.
- Anticipate consequences of possible strong tremors.
- Conceive, design and construct those structures to perform effectively at the anticipated earthquake exposure up to the expectations and in compliance with the applicable building code.
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[edit] Seismic performance
Earthquake or seismic performance is an execution of a building's or structure's ability to sustain their due functions, such as its safety and serviceability, at and after a particular earthquake exposure. A structure is, normally, considered safe if it does not endanger the lives and wellbeing of those in or around it by partially or completely collapsing. A structure may be considered serviceable if it is able to fulfil its operational functions for which it was designed.
Basic concepts of the earthquake engineering, implemented in the major building codes, assume that a building should survive The Big One (the most powerful anticipated earthquake) though with partial destruction. Drawing an analogy with a human body, it will have dislocated joints, fractured ribs, traumatized spine and knocked out teeth but be alive and, therefore, quite O.K. according to the presciptive building codes [2]. This situation is a major barrier to implementation of any structural innovations in the earthquake engineering technologies employing the seismic vibration control and, particularly, the most effective brands of base isolation.
However, alternative earthquake performance-based design approaches already exist. Some of them, for assessment or comparison of the anticipated earthquake performance, use the Story Performance Rating R as a major criterion [3] while the Seismic Performance Ratio (SPR) is used for a rather accurate prediction of earthquake performance of a building up to the point of its state of “severe damage” [4].
[edit] Seismic performance evaluation
Performance evaluation of building structures at their earthquake exposure is one of the hotest topics of earthquake engineering [5]. Engineers need to know the quantified level of an actual or anticipated seismic performance associated with the direct damage to an individual building subject to a specified ground shaking.
The best way to do it is to put the structure on a shake-table that simulates the earth shaking and watch what may happen next (see the picture below [6]).
Another way is to evaluate the earthquake performance analytically. Thus, a publicly accessible research software called Earthquake Performance Evaluation Tool (EPET) enables concurrent virtual experiments on building models with and without vibration control using a kind of seismic base isolation called Earthquake Protector[7]. On demand, all virtual EPET experiments on two identical building models can be animated [8]. Any building or its model is treated by EPET as an essentially nonlinear system.
Major building seismic performance evaluation parameters are the following: Ground Acceleration Mitigation Factor, when only some maximum accelerations on a structure are available, and Story Performance Rating, when the story drifts are also known. The parameter called Seismic Performance Ratio is chosen as the primary parameter which would control anticipated losses due to a particular seismic exposure of the building under consideration.
NEEScentral portal hosts valuable data on experimental validation of EPET, including some movie clips on the comparative shake-table testing of 6- and 12-story building models [9]. EPET can use both real time-historis and earthquake simulations and predict seismic performance of a building up to the point of its virtual state of “severe damage”.
[edit] Seismic analysis
Seismic analysis or earthquake performance analysis is the process of breaking a complex topic of earthquake engineering into smaller parts to gain a better understanding of it. The technique as a formal concept is a relatively recent development.
In genral, seismic analysis is based on the methods of structural dynamics. For decades, the most prominant instrument of seismic analysis has been the earthquake response spectrum method which, also, contributed to the proposed building code's concept of today [10].
However, those spectra are good, mostly, for single-degree-of-freedom systems. Numerical step-by-step integration, applied with the charts of seismic performance [11], proved to be an effective method of analysis for multi-degree-of-freedom structural systems with severe non-linearity.
[edit] Research for earthquake engineering
Research for earthquake engineering incorporates investigation or experimentation aimed at the discovery and interpretation of facts, revision of accepted theories or laws in the light of new facts, or practical application of such new or revised theories or laws.
The National Science Foundation (NSF) is the main United States government agency that supports fundamental research and education in all fields of earthquake engineering. In particular, it focuses on experimental, analytical, and computational research on design and performance enhancement of structural systems.
The NSF programs support research on new technologies for improving the behavior and response of structural systems subject to earthquake hazards; fundamental research on safety and reliability of constructed systems; innovative developments in analysis and model based simulation of structural behavior and response including soil-structure interaction; design concepts that improve structure performance and flexibility; and application of new control techniques for structural systems [12].
NSF supported George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) [13] advances knowledge discovery and innovation for earthquake and tsunami loss reduction of the nation's civil infrastructure, and new experimental simulation techniques and instrumentation. NEES comprises a network of 15 earthquake engineering experimental equipment sites available for experimentation on-site or in the field and through telepresence. NEES equipment sites include shake-tables, geotechnical centrifuges, a tsunami wave basin, unique large-scale testing laboratory facilities, and mobile and permanently installed field equipment.
NEES Cyberinfrastructure Center (NEESit) connects, via Internet2, the equipment sites as well as provides telepresence, a curated central data repository, simulation tools, and collaborative tools for facilitating on-line planning, execution, and post-processing of experiments.
[edit] Earthquake simulation
Dynamic experiments on building and non-building structures may be physical, like shake-table testing, or virtual ones. In both cases, to verify a structure's expected seismic performance, some researchers prefer to deal with so called “real time-histories” though the last cannot be “real” for a hypothetical earthquake specified by either a building code or by some particular research requirements. Therefore, there is a strong incentive to engage an earthquake simulation which is the seismic input that possesses only essential features of a real event, like, e.g., the earthquake simulating displacement time-history Cone [14] presented above.
Earthquake simulations have been widely used in the research of Earthquake Protector and development of the EPET which was supported by The George E. Brown Network for Earthquake Engineering Simulation (NEES) [15].
Sometimes, earthquake simulation is understood as a re-creation of local effects of a strong earth shaking [16].
[edit] Seismic vibration control
Seismic vibration control is a set of technical means aimed to mitigate seismic impacts in building and non-building structures [17]. All seismic vibration control devices may be classified as passive, active or hybrid [18] where:
- passive control devices have no feedback capability between them, structural elements and the ground;
- active control devices are force delivery devices integrated with real-time processing sensors and evaluators/controllers within the structure and on the ground;
- hybrid control devices have combined features of active and passive control systems.
When ground seismic waves reach up and start to penetrate a base of a building, their energy flow density, due to reflections, reduces dramatically: usually, up to 90%. However, the remaining portions of the incident waves during a major earthquake still bear a huge devastating potential.
After the seismic waves enter a superstructure, there is a number of ways to control them in order to sooth their damaging effect and improve the building's seismic performance, for instance:
• to dissipate the wave energy inside a superstructure with properly engineered dampers;
• to disperse the wave energy between a wider range of frequencies by adequately configuring a building elevation [19];
• to absorb the resonant portions of the whole wave frequencies band with the help of so called mass dampers [20].
Devices of the last kind, abbreviated correspondingly as TMD for the tuned (passive), as AMD for the active, and as HMD for the hybrid mass dampers, have been studied and installed in high-rise buildings, predominantly in Japan, for a quarter of a century [21].
To increase the shielded range of forcing frequencies, the concept of Multi-Frequency Quieting Building System (MFQBS) was developed in U.S. [22].
However, there is quite another approach: partial suppression of the seismic energy flow into the superstructure known as seismic or base isolation.
For this, some pads are inserted into or under all major load-carrying elements in the base of the building which should substantially decouple a superstructure from its substructure resting on a shaking ground.
The first evidence of earthquake protection by using the principle of base isolation was discovered in Pasargadae, a city in ancient Persia, now Iran: it goes back to VI century BC.
[edit] Earthquake protector
Heavy damping mechanism sometimes incorporated in vibration control technologies and, particularly, in base isolation devices may be considered a valuable source of suppressing vibrations thus enhancing a building's seismic performance. However, for the very pliant systems such as base isolated structures, with a relatively low bearing stiffness but with an high damping, the so-called "damping force" may turn out the main pushing force at a strong earthquake[23]. This finding created a theoretical ground for the new damping-disengaged base isolation technology called Earthquake Protector [24].
A video clip of concurrent shake-table experiments with two identical and kinematically equivalent to their 12-story prototype building models is presented at [25]. The right model is resting on Earthquake Protectors, while the left one, caught at the time of its collapse, is fixed to the shake-table platen [26]. At this experiment, the fundamental natural period of the building model superstructure is about 1.2s, the isolated period of the Earthquake Protector is 5.0s, the range of the earthquake simulation periods is 0.02 - 2.00s, and the maximum ground acceleration is about 1.0g [27].
Analytical software called Earthquake Performance Evaluation Tool (EPET) enables concurrent virtual experiments on the same building models with and without Earthquake Protectors.
[edit] Seismic design
The term seismic design is usually understood as the authorized engineering procedures, principles and criteria in order to design structures to be subjected to earthquake exposure. The price of poor seismic design may be enormous.
However, seismic design has always been a trial and error process no matter it was based upon physical laws or empirical knowledge of the structural performance of different shapes and materials.
Seismic design utilizes, mostly, the same relatively small number of basic structural elements (to say nothing of vibration control devices) to build up complex structural systems.
Normally, according to building codes, structures are designed to "withstand" the largest earthquake of a certain probability that is likely to occur at their location. This means the loss of life should be minimized by preventing collapse of the buildings. Seismic design is carried out by understanding the possible failure modes of a structure and providing the structure with appropriate strength, stiffness and configuration to ensure those modes cannot occur.
[edit] Failure modes
The lack of reinforcement coupled with poor mortar and inadequate roof-to-wall ties can result in substantial damage to a unreinforced masonry building. Severely cracked or leaning walls are some of the most common earthquake damage. Also hazardous is the damage that may occur between the walls and roof or floor diaphragms. Separation between the framing and the walls can jeopardize the vertical support of roof and floor systems.
Soft story effect. Absence of adequate shear walls on the garage level exacerbated damage to this structure. A close examination of the image reveals that the rough board siding, once covered by a brick veneer, has been completely dismantled from the studwall. Only the rigidity of the floor above combined with the support on the two hidden sides by continuous walls, not penetrated with large doors as on the street sides, is preventing full collapse of the structure.
Soil liquefaction. In cases where the soil consists of loose granular deposited materials with the tendency to develop excessive hydrostatic pore water pressure of sufficient magnitude and compact, liquefaction of those loose saturated deposits may result in non-uniform settlements and tilting of structures. This caused major damage to thousands of buildings in Niigata, Japan in 1964.
Landslide rock fall. A landslide is a geological phenomenon which includes a wide range of ground movement, including rock falls. Typically, the action of gravity is the primary driving force for a landslide to occur though in this case there was another contributing factor which affected the original slope stability: the landslide required an earthquake trigger before being released.
Pounding against adjacent building. This is a photograph of the collapsed five-story tower, St. Joseph's Seminary, Los Altos, California. One person working in the tower was killed. During Loma Prieta earthquake, the tower pounded against the independently vibrating adjacent building behind. A possibility of pounding depends on both buildings' lateral displacements which should be accurately estimated and accounted for.
At Northridge earthquake, the Kaiser Permanente concrete frame office building had joints completely shattered, revealing inadequate confinement steel, which resulted in the second story collapse. In the transverse direction, composite end shear walls, consisting of two wythes of brick and a layer of shotcrete that carried the lateral load, peeled apart because of inadequate through-ties and failed.
7-story reinforced concrete buildings on steep slope collapse due to the following:
- Improper construction site
- Poor detailing of the reinforcement ( lack of concrete confinement in the columns and at the beam-column joints, inadequate splice length)
- Weak/soft story (open space at the first floors)
- Long cantilevers with heavy load
Sliding off foundations effect of a relatively rigid residential building structure during 1987 Whittier Narrows earthquake. The magnitude 5.9 earthquake pounded the Garvey West Apartment building in Monterey Park, California and shifted its superstructure about 10 inches to the east on its foundation.
If a superstructure is not mounted on a base isolation system, its shifting on the basement should be prevented.
Reinforced concrete column burst at Northridge earthquake due to insufficient shear reinforcement mode which allows main reinforcement to buckle outwards. The deck unseated at the hinge and failed in shear. As a result, the La Cienega-Venice underpass section of the 10 Freeway collapsed.
Loma Prieta earthquake: side view of reinforced concrete support-columns failure and the upper deck collapse onto the lower deck of two-level Cypress viaduct of Interstate Highway 880, Oakland, CA.
Retaining wall failure at Loma Prieta earthquake in Santa Cruz Mountains area: prominent northwest-trending extensional cracks up to 12 cm (4.7 in) wide in the concrete spillway to Austrian Dam, the north abutment.
Ground shaking triggered liquefaction in a subsurface layer of sand, producing differential lateral and vertical movement in a overlying carapace of unliquified sand and silt. This mode of ground failure, termed lateral spreading, is a principal cause of liquefaction-related earthquake damage.
Severely damaged building of Agriculture Development Bank of China after 2008 Sichuan earthquake: most of the beams and pier columns are sheared. Large diagonal cracks in maisonry and veneer are due to in-plane loads while abrupt settlement of the right end of the building should be attributed to a landfill.
2004 Indian Ocean earthquake of December 26, 2004, with an epicenter off the west coast of Sumatra, Indonesia, triggered a series of devastating tsunamis, killing more than 225,000 people in eleven countries by inundating coastal communities with huge waves up to 30 meters (100 feet).
Failure modes also include:
- Shear failure of floor structure (beams with insufficient lateral restraint may fall over and fail)
- Shear failure of column heads (the lateral movement of a floor shears the floor from the supporting column)
