Structural Engineering Applications in Earthquake – Resilient Building Construction Part-I

Structural Engineering Applications in Earthquake – Resilient Building Construction Part-I

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The major role of structural engineering in the specific context of earthquake engineering in a traditional trend has been realization and enhancement of seismic safety. The research and technology developments have since been focused on structural and geotechnical issues to make engineered structures earthquake-resilient through appropriate design and construction engineering. Such definitive efforts have lasted for more than three quarters of this century now with varied experiences and end results. The developments in earthquake engineering research during the last 2-3 decades provide, within acceptable and permissible limits, reliable design of new structures against earthquake loads to bear the pre and post-earthquake short-time and long-term impacts. In the more developed part of the world, such methods are presently implemented in the best practice design of new structures. However, the existing older building stock (especially the high-rise buildings of mega cities) still represents a significant risk in regard to the safety of people as well as to the economical assets of the society. In less developed countries, both the new building stock and the existing building stock exhibit a significant vulnerability, mainly due to inadequate design, missing supervision during construction, insufficient quality of construction materials and inadequate ground investigation/interpretation. The paper provides the various interesting facets of development that are now taking place in different parts of the world in the specific field of structural engineering applications in building earthquake-resilient urban infrastructure.The major role of structural engineering in the specific context of earthquake engineering in a traditional trend has been realization and enhancement of seismic safety. The research and technology developments have since been focused on structural and geotechnical issues to make engineered structures earthquake-resilient through appropriate design and construction engineering. Such definitive efforts have lasted for more than three quarters of this century now with varied experiences and end results. The developments in earthquake engineering research during the last 2-3 decades provide, within acceptable and permissible limits, reliable design of new structures against earthquake loads to bear the pre and post-earthquake short-time and long-term impacts. In the more developed part of the world, such methods are presently implemented in the best practice design of new structures. However, the existing older building stock (especially the high-rise buildings of mega cities) still represents a significant risk in regard to the safety of people as well as to the economical assets of the society. In less developed countries, both the new building stock and the existing building stock exhibit a significant vulnerability, mainly due to inadequate design, missing supervision during construction, insufficient quality of construction materials and inadequate ground investigation/interpretation. The paper provides the various interesting facets of development that are now taking place in different parts of the world in the specific field of structural engineering applications in building earthquake-resilient urban infrastructure.

1.0. Seismic Resilience: Concept and Definition

1.1. Defining Resilience

Crawford Stanley Holling was the first one to introduce the concept of “resilience” in research, in his studies from 1973, concerning ecology (Holling, 1973). Since then, this concept started to be used in many other fields, like social sciences, economy and engineering. Although analyzing the resilience of a system is fundamental in order to improve it, the interest in this concept has increased only in the last decade. Resilience has emerged in the last decade as a concept for better understanding the performance of infrastructures, especially their behaviour during and after the occurrence of disturbances, e.g. natural hazards or technical failures. Recently, resilience has grown as a proactive approach to enhance the ability of infrastructures to prevent damage before disturbance events, mitigate losses during the events and improve the recovery capability after the events, beyond the concept of pure prevention and hardening (Woods, 2015).

The concept of resilience is still evolving and has been developing in various fields (Hosseini, Barker, & Ramirez-Marquez, 2016). The first definition described resilience as “a measure of the persistence of systems and of their ability to absorb change and disturbance and still maintain the same relationships between populations or state variables” (Holling, 1973). Several domain-specific resilience definitions have been proposed (Ouyang, Dueñas-Osorio, & Min, 2012) (Adger, 2000) (Pant, Barker, & Zobel, 2014) (Francis & Bekera, 2014). Further developments of this concept should include endogenous and exogenous events and recovery efforts. To include these factors, resilience is broadly defined as “the ability of a system to resist the effects of disruptive forces and to reduce performance deviations” (Nan, Sansavini, & Kröger, 2016).

The definition of resilience is highly variable depending on the subject area, which it is applied to. Essentially, the general requisites that bring together the different literature definitions of resilience pertain to the capacity of a system to absorb, adapt and recover from an external stress, while limiting disruptions to its normal functioning. Hence, depending on the subject area, different aspects contributing to resilience can be considered, i.e., infrastructure systems, safety management systems, organizational systems, social-ecological systems, economic systems and social systems (Timmerman, Vulnerability, 1981). Table 1 shows the different definitions of resilience with reference to diverse systems.

Assessing and engineering systems resilience is emerging as a fundamental concern in risk research (Woods & Hollnagel, 2006) (Haimes, 2009) (McCarthy, et al., 2007) (McDaniels, Chang, Cole, Mikawoz, & Longstaff, 2008). Resilience adds a dynamical and proactive perspective into risk governance by focusing (i) on the evolution of system performance during undesired system conditions, and (ii) on surprises (“known unknowns” or “unknown unknowns”), i.e. disruptive events and operating regimes which were not considered likely design conditions. Resilience encompasses the concept of vulnerability (Johansson & Hassel, 2010; Kröger & Zio, 2011) as a strategy to strengthen the system response and foster graceful degradation against a wide spectrum of known and unknown hazards.

 

 

1.1. Seismic Resilience: A Theoretical Overview

Derived from ecology, the concept of resilience is chiefly defined as “the measure of the persistence of systems and of their ability to absorb change and disturbance and still maintain the same relationships between populations or state variables” (Holling, 1973). The concept of resilience has a rich history (Folke, 2006), sometimes with a considerable stretch from its original meaning (Gallopin, 2006). Thus, it is possible to identify a sequence of resilience concepts in ecology, from narrow to broad: engineering resilience, ecosystem resilience, social-ecological resilience (Folke, 2006). Indeed, the most important development over the past thirty years is the increasing recognition across the disciplines that human and ecological systems are interlinked and that their resilience relates to the functioning and interaction of the systems rather than to the stability of their components or the ability to maintain or return to some equilibrium state (Klein et al., 2003). Henceforth, the concept is used by various scientific disciplines and there is a tension between the original descriptive concept of resilience first defined in ecological science and a more recent, vague, and malleable notion of resilience used as an approach or boundary object by different scientific disciplines (Brand and Jax, 2007). Bruneau and Reinborn (2006) discussed the earthquake resilience of building structures and infrastructures. They defined “the resilient structures” as (1) those with small collapse probability, (2) those with reduced consequences from failures in terms of lives lost, damage, and negative economic and social consequences, (3) reduced time to recovery.

The polysemy of the resilience concept is not a problem in itself; it is even productive in terms of heuristic and methodological issues (Folke, 2006). The difficulties arise when the polysemy seems to legitimize a semantic blur that creates theoretical and operational dead ends. In view of occasional contrary injunctions, the concept is “inoperative” and reduced to some sort of unattainable discursive utopia, to the point where some researchers have considered the concept too vague to be used in order to prevent disaster (Manyena, 2006). Nevertheless, resilience was Immediately seen as an opportunity to enrich or even renew the management systems such as the policies contributing to reduce hazards and disasters. It must be emphasized that hazard research can hardly be independent from this passage from theory to practice.

The increasing interest in resilience requires methodological frameworks to be developed to assess it. Measuring disaster resilience might help understanding and improving the capability of urban systems to withstand risks, and implement effective strategies to recover. To this end, different studies have been developed, which propose operational frameworks to quantify disaster resilience and other properties related to it. In addition, disaster resilience is directly related to the management of urban environments that is to the field of civil engineering. In fact, urban systems are mainly constituted by physical subsystems, which are built and managed according to civil engineering concepts and methodologies. Figure 1 shows the correlation between resilience and civil engineering.

Civil engineering plays a crucial role in the recovery of a complex system, made of physical subsystems, as is an urban environment. Consequently, whether techniques and technologies used in civil engineering for the recovery of an urban system are effective, it is restored quickly and efficiently. Hence, the urban system is resilient. Essentially, resilience is assessed according to two main approaches: qualitative and quantitative

1.2. A New Comprehensive Framework for Seismic Resilience Understanding

The conceived framework for understanding resilience provides a structured and comprehensive overview of historical and multi-disciplinary dimensions (Figure 2). A detailed literature review of the historical and multi-disciplinary origins of resilience can be found by Verruci (2009) and Verrucci and Twigg (2012).Five topical macro-areas have been identified:

  • Built-in resilience: deriving from the technocratic approach (International Strategy for Disaster Reduction., 2004) and from the application of resilience in structural engineering (Bosher et al., 2007), it relates to aspects of resilience that can be addressed with appropriate design or with strengthening;
  • Planning and Land-use: relates to the geographers point of view (Burton et al., 1978) and to the ecological tradition for which resilience is achieved by shaping the spatial location of future development based on hazard awareness (Mileti, 1999, Burby et al., 2000). Development choices are directly linked to the physical exposure of people and built environment to the hazard;
  • Continued functioning or Redundancy of Critical service and Infrastructures: stemming from the technocratic approach and from the application of resilience in systems engineering (Bruneau et al., 2003), it is based on the observation that damage to key services and infrastructures is cause of great economic losses and hinders recovery;
  • Distribution of resources: derives from the socio-vulnerability perspective (O’Keefe et al., 1976, Blaikie et al., 2003) and from the ecological perspective applied to communities (Folke, 2006), it notes that availability of resources, including monetary resources, is essential for fast and appropriate recovery;
  • Social cohesion: developed from disaster studies (Quarantelli and Dynes, 1977) and from the ecological perspective applied to communities, it emphasises the role of citizens as first responders to disasters. The first three topical macro-areas have received more attention in the past as they relate to systems which performance can be more easily measured in quantitative ways. Social aspects, like the distribution of resources and social cohesion in particular, are less commonly covered by the literature.

1.3. Engineering System (Infrastructure)-Centered Resilience

Research on people perceptions of different dimensions determining resilience (em-BRACE, 2013) revealed that inhabitants of disaster-prone areas identify disaster-resistant buildings and infrastructures as the most important indicators of resilience. Therefore, infrastructure resilience is investigated not only by researchers of the geophysics-seismic engineering, safety, contingencies and infrastructures disciplines (Boin and McConnell, 2007; Bruneau et al, 2003; Fritzon et al, 2007; Hellström, 2007), but also in some extend intervenes in psychological resilience research area. As T. O’Rourke (2007) states, a resilient engineering system is one that manifests itself as diminishing failure probabilities; reducing consequences from failures (in terms of lives lost, damage, and negative economic and social consequences); shortening time for recovery. Within safety and security disciplines, the most important assets, systems and networks (physical or virtual) to be preserved against natural and manmade disasters are those, that deemed so vital to the country that their “their incapacitation or destruction would have a debilitating effect on security, national economic security, public health and/or safety” (US Department of Homeland Security, 2010). They are defined as critical infrastructures. This term encompass different sectors, for example: energy, water, transportation systems, food and agriculture, dams, emergency services, healthcare and public health, government facilities, chemical and critical manufacturing, national monuments and icons, commercial facilities, communications and information technology etc. When research comes to the question of what are components of resilient infrastructure, scholars and practitioners agree on those: robustness, recovery, resourcefulness. Resilience notion plays key role in this methodology, as it is determined by preparation, response, and recovery, which characteristics represent overall efficiency and effectiveness of risk management. The key integrated indicators of Resilience Index are those mentioned above (robustness, recovery and resourcefulness) and they represent first level of resilience assessment (Figure 3).

Figure 3 demonstrates the logic of Resilience Index development and a fragment of all three level indicators. Risk index, which includes resiliency, vulnerability and criticality indexes, helps to understand better the relationship between components of critical infrastructures, between critical infrastructures and environment, therefore, allows better preparation to react in case of disaster or any kind of crisis.

1.4. Need for Resilience in Critical Infrastructure

Resilience calls for developing a strategy rather than performing an assessment. If on the one hand it is important to quantify and measure resilience in the context of risk management, it is even more important that the quantification effort enables the engineering of resilience into critical infrastructures. Especially for emerging, not-well-understood hazards and “surprises” (Pate-Cornell, 2012), resilience integrates very smoothly into risk management, and expediently focuses the perspective on the ex-ante system design process. Following this perspective, risk thinking becomes increasingly embedded into the system design process. The application of resilience-building strategies look particularly promising for critical interdependent infrastructures, also called systems-of-systems, because of its dynamical perspective in which the system responds to the shock event, adapting and self-healing, and eventually recovers to a suitable level of performance. Such perspective well suits the characteristics of these complex systems, i.e. i) the coexistence of multiple time scales, from infrastructure evolution to real-time contingencies; ii) multiple levels of interdependencies and lack of fixed boundaries, i.e. they are made of multiple layers (management, information & control, energy, physical infrastructure); iii) broad spectrum of hazards and threats; iv) different types of physical flows, i.e. mass, information, power, vehicles; v) presence of organizational and human factors, which play a major role in severe accidents, highlighting the importance of assessing the performance of the social system together with the technical systems. As a key system of interdependent infrastructures, the energy infrastructure is well suited to resilience engineering. In the context of security of supply and security of the operations, resilience encompasses the concept of flexibility in energy systems. Flexibility providers, i.e. hydro and gas-fired plants, cross-border exchanges, storage technologies, demand management, decentralized generation, ensure enough coping capacity, redundancy and diversity during supply shortages, uncertain fluctuating operating conditions and unforeseen contingencies (Roege, Collier, Mancillas, McDonagh, & Linkov, 2014; Skea, et al., 2011).

1.5. Building Resilience in Critical Infrastructures

In the context of critical infrastructures, resilience can be developed by focusing on the different phases of the transient performance following a disturbance (also called resilience curve), and devising strategies and improvements which strengthen the system response. Focusing mainly on the technical aspects, these strategies can be summarized as:

  • Planning ahead during the design phase: robust or stochastic optimization against uncertain future scenarios i.e. attacks or uncertain future demand in the energy infrastructure can be used in the system planning or expansion process; uncertain scenarios provide the basis to design resilient systems.
  • Self-healing, adaptation and control, i.e. graceful degradation: the system cannot be design with respect to every uncertain scenario; therefore a resilient design should consider how to prevent the disturbance from spreading across the whole system, creating systemic contagion and system-wide collapse. In this respect, cascading failures analysis, and engineering network systems to be robust against outbreak of outages and propagations of cascading failures across their elements are key strategies. Control engineering can provide strategies to create robust feedback loops capable of enabling infrastructures to absorb shocks and avoid instabilities. Designing structures and topologies, which prevent failure propagation, and devising flexible topologies by switching elements which allow graceful degradation of system performances after disruptions, are also valuable resilience-enhancing techniques.
  • Recovering quickly from the minimum performance level: robust or stochastic optimization of the recovery and restoration process in the face of uncertainties in the repair process or in the disruption scenarios.
  • Effective system restoration: through the combination of restoration strategies, e.g. repairing the failed elements and building new elements, the infrastructure can achieve a higher performance with respect to the pre-disruption conditions.
  • Exploiting interdependencies among infrastructures: interdependencies and couplings in systems operations can foster the propagations of failure across coupled system; on the other hands, interdependencies might also provide additional flexibility in disrupted conditions and additional resources that can facilitate achieving stable conditions of the coupled system.

Resilience can be quantified through computational experiments in which disruptions are triggered; the system performance is analyzed (Figure 4), and integrated resilience metrics are computed (Nan, Sansavini, & Kröger, 2016). By repeating this process, different system design solutions can be ranked with respect to resilience. By the same token, resilience against various disruptions can be assessed, and resilience-improving strategies compared. During the last decade, researchers have proposed different methods for quantifying resilience. In 2003, the first conceptual framework was proposed to measure the seismic resilience of a community (Bruneau, et al., 2003), by introducing the concept of Resilience Loss, later also referred to as “resilience triangle”.

1.6. What does ‘Resilient Design’ entail when it comes to Building? 

To design a building with resiliency means to start the design process by thinking carefully about the typical use scenarios of the building, common points of stress due to normal use, as well as the most likely disaster situations in the environment that could challenge the integrity of the building and/or endanger its occupants. The local environment always plays a critical role in determining the factors that make a building resilient or not, and so resilient design is always locally specific (Figure 5).

For example, New York City has a wet climate, and water is a part of its environmental challenges throughout the year. In New York City, the most common and likely natural disaster scenarios involve water: hurricanes, flooding, storm surges, and blizzards. Resilient building in New York City needs to plan for all of these types of events, as well as the day-to-day stress that comes from significant precipitation year round, high-humidity, and the alternation of humidity (in the summer), with extremely dry interior air (Figure 6). Of course, builders in New York City also need to design to withstand seismic activity, high heat loads in the summer, power outages, manmade disasters like terrorism, as well as the normal damage that comes with thousands of people moving through spaces in rapid succession. On the West Coast of the United States, seismic considerations are obviously much more of a concern, as well as fire danger. Thinking through every potential problem and possible disaster situation can be overwhelming for designers, which is why a sensible approach starts by examining the most likely problem situations and pulling from local wisdom, knowledge and experience.

2.0. Sustainable Design Considerations in Earthquake-Resilient  Building Construction

It has been noted that the design of isolated buildings and bridges can be problematic due to the apparent over conservativeness of some design provisions, but also because of challenges encountered when designing isolated systems to resist intense near-field ground motions. Various efforts to make design procedures more transparent, based on performance-base engineering concepts, are underway worldwide. Several approaches are being pursued at Berkeley, including novel combinations of elastomeric isolators and nonlinear viscous dampers. Another promising approach is the newly developed Triple Pendulum Slider (Earthquake Protection Systems, 2007). This device has three independent pendulum mechanisms (Figure 5). By strategically selecting friction coefficients for each mechanism, hysteretic characteristics can be optimized for occasional, rare and very rare events. Tests (Figure 5) and analyses demonstrate that the devices can be designed to achieve about the same isolator displacements for a large event, but with smaller drifts and accelerations in the superstructure and with a far greater degree of isolation during smaller events (Figure 7) (Morgan and Mahin, 2007).

2.1. Rocking Foundations

Most structures are designed to have a fixed base. For example, conventional bridge structures residing on competent soil are typically designed with rectangular spread footings proportioned to allow for a fixed base Figure 5. Triple pendulum slider and test specimen response. This generally leads to inelastic behavior at or near the column to footing interface during design level earthquakes. This mode of behavior dissipates input energy, but results in damage to the column and potential permanent lateral displacements. By permitting the bridge piers to rock or uplift on the supporting soil introduces other modes of nonlinearity (rocking) and energy dissipation (soil inelasticity). Explicit consideration of rocking as an acceptable mode of response can reduce the required footing sizes. Also, the simultaneous rocking of a properly designed foundation and flexural deformation of the supported column appears to eliminate, or substantially reduce, damage in the column and residual displacements in a bridge with moderate to long fundamental periods following a major earthquake. Of the three approaches discussed in this paper, rocking of bridge foundation relies the most on conventional design and construction methodologies. The work at UC Berkeley focuses on development of design procedures, and validating these via more refined structural analyses and earthquake shaking table tests of moderate-scale models of reinforced concrete bridge columns under multidirectional earthquake excitations. The work at UC Davis focuses on validating this work through a series of geotechnical centrifuge tests. Consideration is limited to good soil conditions where the factor of safety under gravity loads alone exceeds three. Some to the models used in the tests and analyses are shown in Figure 8.

Simplified analytic model, UC Berkeley shaking table tests and UC Davis centrifuge tests are used to study rocking foundations (Figure 9)

For the shaking table model, a series of tests examine the effect of one, two and three components of excitation for a simple 1:4.5-scale column on a spread foundation supported on a 50 mm thick neoprene (Duro-60) pad. This pad highly idealizes the soil beneath the footing. The column has a diameter of 410 mm, a longitudinal reinforcement ratio of 1.2%, and spiral reinforcement. Experimental and analytical investigations indicate that this rocking mechanism provides a viable means of resisting earthquake effects. Except for short period structures, column displacements were similar to or smaller than would be expected for comparable elastic or yielding pier with a fixed foundation, and the column showed no signs of damage and re-centered following the end of the ground shaking. For cases where column yielding occurred, the damage and permanent displacements are greatly reduced. It is recognized that rocking of spread footings may not be possible in many important situations (e.g., poor soil conditions). In such cases, development of special foundation details that permit rocking on pile-supported foundations may be feasible. Extension of these concepts to braced frame buildings is currently under investigation.

2.2. Self-Centering Reinforced Concrete Column Systems

Where the base of a bridge column is fixed by large spread footings or piles, a modification of conventional ductile reinforced concrete columns is possible that can greatly improve post-earthquake operability and reduce the extent of necessary repairs. In this case, some of the reinforced concrete column’s (RC) longitudinal mild reinforcement is replaced by unbounded post-tensioning strands (typically located in a central conduit). The seismic performance of such partially pre-stressed, reinforced concrete columns (PRC) is being investigated by the Pacific Earthquake Engineering Research Center through quasi-static and dynamic analyses and shaking table tests of single columns and simple bridge systems. The design approach used for these columns (Sakai and Mahin, 2004; Sakai et al, 2006) is to reduce the amount of longitudinal reinforcement needed for a conventional ductile design (Caltrans, 1998) by about half, but to add unbonded post-tensioning strands and adjust the amount of transverse confinement such that the envelop of stiffness and strength under loading conditions is similar to that used for a conventional ductile column. However, under unloading, the columns exhibit a characteristic hourglass or center-oriented hysteretic loop shape. This can be seen in Figure 10.The technology and design approaches provide several alternative ways to achieve durable building and transportation structures that increase post-earthquake serviceability and reduce the need for repair. By permitting structures to undergo significant inelastic deformations during seismic events, yet suffer little damage that would require post-earthquake repairs and impair operability, structural engineers can achieve designs that are durable, dependable, and economical in terms of initial construction cost and the potential losses that might occur in the event of a damaging earthquake. As such, these approaches address the basic principles articulated by sustainable design.

3.0. The Basic Principles of Earthquake- Resilient Design

Modern earthquake design has its genesis in the 1920’s and 1930’s. At that time earthquake design typically involved the application of 10% of the building weight as a lateral force on the structure, applied uniformly up the height of the building. Indeed it was not until the 1960’s that strong ground motion accelerographs became more generally available. These instruments record the ground motion generated by earthquakes. When used in conjunction with strong motion recording devices which were able to be installed at different levels within buildings themselves, it became possible to measure and understands the dynamic response of buildings when they were subjected to real earthquake induced ground motion. By using actual earthquake motion records as input to the, then, recently developed inelastic integrated time history analysis packages, it became apparent that many buildings designed to earlier codes had inadequate strength to withstand design level earthquakes without experiencing significant damage. However, observations of the in-service behaviour of buildings showed that this lack of strength did not necessarily result in building failure or even severe damage when they were subjected to severe earthquake attack.

The key to successful modern earthquake engineering design lies therefore in the detailing of the structural elements so that desirable post-elastic mechanisms are identified and promoted while the formation of undesirable response modes are precluded. Desirable mechanisms are those which are sufficiently strong to resist normal imposed actions without damage, yet are capable of accommodating substantial inelastic deformation without significant loss of strength or load carrying capacity. Such mechanisms have been found to generally involve the flexural response of reinforced concrete or steel structural elements or the flexural steel dowel response of timber connectors. Undesirable post-elastic response mechanisms within specific structural elements have brittle characteristics and include shear failure within reinforced concrete, reinforcing bar bond failures, the loss of axial load carrying capacity or buckling of compression members such as columns, and the tensile failure of brittle components such as timber or under-reinforced concrete.

Earthquake forces are generated by the dynamic response of the building to earthquake induced ground motion. This makes earthquake actions fundamentally different from any other imposed loads. Thus the earthquake forces imposed are directly influenced by the dynamic inelastic characteristics of the structure itself. While this is a complication, it provides an opportunity for the designer to heavily influence the earthquake forces imposed on the building. Through the careful selection of appropriate, well distributed lateral load resisting systems, and by ensuring the building is reasonably regular in both plan and elevation, the influence of many second order effects, such as torsional effects, can be minimised and significant simplifications can be made to model the dynamic building response (Figure 11).Most buildings can be reasonably considered as behaving as a laterally loaded vertical cantilever. The inertia generated earthquake forces are generally considered to act as lumped masses at each floor (or level). The magnitudes of these earthquake forces are usually assessed as being the product of seismic mass (dead load plus long-term live load) present at each level and the seismic acceleration generated at that level. The design process involves ensuring that the resistance provided at each level is sufficient to reliably sustain the sum of the lateral shear forces generated above that level (Figure 9).

 

Dr. A.N. Sarkar
Ex-Senior Professor (International Business) & Dean (Research), Asia-Pacific Institute of
Management, New Delhi

The Part II will be continued in the next edition

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