Seismic design of reinforced concrete frames for minimum embodied CO2 emissions

Optimum structural design of reinforced concrete (RC) frames has been the focus of extensive research. Typically, previous studies set economic cost as the main design objective despite the fact that RC structures are major contributors of CO2 emissions. The limited number of studies examining optimum design of RC frames for minimum CO2 emissions do not address seismic design considerations. However, in many countries around the world, including most of the top-10 countries in CO2 emissions from cement production, RC structures must be designed against earthquake threat. To bridge this gap, the present study develops optimum seismic designs of RC frames for minimum cradle to gate embodied CO2 emissions and compares them with optimum designs based on construction cost. The aim is to identify efficient design practices that minimize the environmental impact of earthquake-resistant RC frames and examine the trade-offs between their cost and CO2 footprint. To serve this goal, an RC frame is optimally designed according to all ductility classes of Eurocode 8 and for various design peak ground accelerations (PGAs), concrete classes and materials embodied CO2 footprint scenarios. It is found that the minimum feasible CO2 emissions of RC frames strongly depend on the adopted ductility class in regions of high seismicity, where low ductility seismic design can generate up to 60% more CO2 emissions than designs for medium and high ductility. The differences reduce, however, as the level of seismicity decreases. Furthermore, CO2 emissions increase significantly with the design PGA. On the other hand, they are less sensitive to the applied concrete class. It is also concluded that, for medium to high values of the ratio of the unit environmental impact of reinforcing steel to the respective impact of concrete, the minimum CO2 seismic designs are very closely related to the minimum cost designs. However, for low values of the same ratio, the minimum cost design solutions can generate up to 13% more emissions than the minimum CO2 designs

Life-Cycle Cost Model and Design Optimization of Base-Isolated Building Structures

Design of economic structures adequately resistant to withstand during their service life, without catastrophic failures, all possible loading conditions and to absorb the induced seismic energy in a controlled fashion, has been the subject of intensive research so far. Modern buildings usually contain extremely sensitive and costly equipment that are vital in business, commerce, education and/or health care. The building contents frequently are more valuable than the buildings them-selves. Furthermore, hospitals, communication and emergency centres, police and fire stations must be operational when needed most: immediately after an earthquake. Conventional con-struction can cause very high floor accelerations in stiff buildings and large interstorey drifts in flexible structures. These two factors cause difficulties in insuring the safety of both building and its contents. For this reason base-isolated structures are considered as an efficient alternative design practice to the conventional fixed-base one. In this study a systematic assessment of op-timized fixed and base-isolated reinforced concrete buildings is presented in terms of their initial and total cost taking into account the life-cycle cost of the structures

Relative Performance Comparison and Loss Estimation of Seismically Isolated and Fixed-based Buildings Using PBEE Approach

Current design codes generally use an equivalent linear approach for preliminary design of a seismic isolation system. The equivalent linear approach is based on effective parameters, rather than physical parameters of the system, and may not accurately account for the nonlinearity of the isolation system. The second chapter evaluates an alternative normalized strength characterization against the equivalent linear characterization. Following considerations for evaluation are included: (1) ability to effectively account for variations in ground motion intensity, (2) ability to effectively describe the energy dissipation capacity of the isolation system, and (3) conducive to developing design equations that can be implemented within a code framework. Although current code guidelines specify different seismic performance objectives for fixed-base and isolated buildings, the future of performance-based design will allow user-selected performance objectives, motivating the need for a consistent performance comparison of the two systems. Based on response history analysis to a suite of motions, constant ductility spectra are generated for fixed-base and isolated buildings in chapter three. Both superstructure force (base shear) and deformation demands in base-isolated buildings are lower than in fixed-base buildings responding with identical deformation ductility. To compare the relative performance of many systems or to predict the best system to achieve a given performance objective, a response index is developed and used for rapid prototyping of response as a function of system characteristics. When evaluated for a life safety performance objective, the superstructure design base shear of an isolated building is competitive with that of a fixed-base building with identical ductility, and the isolated building generally has improved response. Isolated buildings can meet a moderate ductility immediate-occupancy objective at low design strengths whereas comparable ductility fixed-base buildings fail to meet the objective. In chapter four and five, the life cycle performance of code-designed conventional and base-isolated steel frame buildings is evaluated using loss estimation methodologies. The results of hazard and structural response analysis for three-story moment resisting frame buildings are presented in this paper. Three-dimensional models for both buildings are created and seismic response is assessed for three scenario earthquakes. The response history analysis results indicate that the performance of the isolated building is superior to the conventional building in the design event. However, for the Maximum Considered Earthquake, the presence of outliers in the response data reduces confidence that the isolated building provides superior performance to its conventional counterpart. The outliers observed in the response of the isolated building are disconcerting and need careful evaluation in future studies

PERFORMANCE-BASED ENGINEERING FOR EVALUATION AND RETROFITTING NON-DUCTILE REINFORCED CONCRETE BUILDINGS INCORPORATING AFTERSHOCK HAZARD

Performance-based engineering (PBE) provides a probabilistic tool for assessing the seismic risk and performance of buildings. Only the mainshock hazard has been included in the current PBE framework, although the concern on aftershock hazard has been increased recently. This study develops methodologies to incorporate aftershock hazard into a PBE framework, and assesses the seismic risk and performance of non-ductile reinforced concrete (RC) frame buildings under mainshock and aftershock hazards. A seismic retrofit strategy for these buildings, base isolation, is also evaluated using the developed methodologies. A methodology for synthesizing aftershock ground motions is proposed and validated to resolve the challenge imposed by limited aftershock records. Seismic risks for two nonductile RC frame buildings representing low-rise and mid-rise buildings are examined. Results show that aftershocks can increase structural responses and seismic risk. Based on the state-of-the-art mainshock-based performance assessment methodologies, a new assessment methodology is developed with incorporation of aftershock hazard. The interactive effects between a variety of post-quake decisions and aftershocks are also considered. The proposed methodology is utilized to estimate the direct loss, downtime, and fatalities for two RC frame buildings under MS-AS sequences. Results suggest that aftershocks can cause significant additional seismic loss. The characteristics of MS-AS sequences that may be the cause of the aftershock-induced additional consequence in terms of loss, downtime, and fatalities are discussed and identified through a statistical analysis. The important sources of uncertainty of post-quake decisions are also investigated though a sensitivity study. A comparative study is also performed for a RC frame building before and after being retrofitted with base isolation to determine the risk mitigation due to base isolation. The seismic risk is found to be effectively reduced by base isolation. The effect of various sources of uncertainties in the base isolation system are investigated through a sensitive study using mainshock-aftershock (MS-AS) ground motions at a variety of intensity levels. The most important uncertainty sources are identified. Life-cycle cost-benefit analysis is also performed for the two RC frame buildings to evaluate the economical effectiveness of adopting base isolation as a seismic retrofit strategy with consideration of mainshock and aftershocks. It is revealed that the benefit from base isolation can outweigh the additional cost for buildings in regions with high seismicity, and that the benefit is more significant when aftershocks are considered. The influence of aftershocks and base isolation on the structural robustness is investigated. Limitations and future works are also presented

Utilizing Base-isolation Systems to Increase Earthquake Resiliency of Healthcare and School Buildings

In the recent large earthquakes in Chile, New Zealand, and Japan, a great number of critical facilities, including hospitals, schools, bridges, factories, airports, and utility systems, experienced extensive damage resulting in their loss of their function, and consequently substantial economic losses. Heavily affected communities were paralyzed for months following these large seismic events. The recovery process is estimated to last from several years to few decades. As a result, increased attention is being placed on strategies to design facilities that are both safe and damage resistant. It is often presumed that such an approach increases costs to an unacceptable level. The study reported herein compares the repair costs and repair times considering two designs for a typical three-story steel building: conventional fixed-base and damage resistant base-isolated moment resisting frame system. Performance-based earthquake evaluation tools are used to estimate repair costs and times for five different hazard levels considering two occupancy types critical for recovery: healthcare and school. The buildings are located in a seismic region in western North America. It is shown that using seismic isolation to enhance damage resistance results in significantly smaller repair cost, repair time, and improved resilience for the base-isolated alternative compared to a conventional fixed-base design

Alternative techniques and approaches for improving the seismic performance of masonry infills

This doctoral dissertation aims to report on the research work carried out in the field of alternative techniques and approaches for improving the seismic performance of masonry infills. An aspect often overlooked in the design and/or verification of Reinforced Concrete (RC) frame buildings it is the one related to the so-called "non-structural" elements, that are elements without a main structural function, but capable of causing damage to things and people during the seismic action. A typical example of non-structural elements are the external infills of RC frame buildings, which often have masses and stiffnesses able to significantly modify the behavior and response of the structure during the seismic action. Typically, the UnReinforced Masonry (URM) infill walls are made of single or double facing hollow bricks placed inside the meshes of RC frames. The main damage mechanisms observed in URM infill walls during seismic events include in the plane (IP) or out of the plane (OOP) damage mechanisms, both characterized by degradation of strength, stiffness and low energy dissipation. In this doctoral dissertation, the analysis of the influence of the in-plane and out-of-plane behavior of UnReinforced Masonry infill walls on global seismic performances of different RC frame buildings is presented. The research mainly focuses on: i) numerical investigation of the in- plane / out-of-plane interaction in order to evaluate its entity and severity pointing out the correct description of the damage scenarios; ii) identification of alternative intervention solutions, aimed at mitigating the phenomenon of overturning of the infill panels; iii) estimate of the expected economic losses. Rough cost-benefit analyses have been carried out in order to compare the sustainability of alternative seismic rehabilitation techniques, thus providing a rational base and objective criteria that can be used in the design and/or preliminary screening phase by insurance companies, to reduce the seismic risk and the impact of earthquakes on a community

Financial Feasibility of High Performance Low Rise Steel Buildings

Comparative performance evaluation including life cycle cost is currently being conducted on a series of conventional and base-isolated case study buildings. Alternative design approaches and their influence in cost were to be evaluated . This investigation is intended to contribute in the development of isolated structures by allowing engineers to communicate the cost of higher performance systems to their clients. The reported effort is part of a larger cost-benefit study for isolated steel buildings, and the purpose of this thesis is to compare initial investment of 3-story conventional and isolated steel buildings and determine how isolation affects the cost of the structure. The relative cost of seismic isolation, as a percentage of the total cost, may be higher in this study than for typical U.S. isolation applications because the relative premium is greater for a short building than a tall building. The cost of isolation layer for this building is in the order of 11.7% to 12.4% of the total cost. Such a large cost premium may be a huge restraint for most owners; therefore, strategies to reduce the isolation premium cost need to be investigated in detail

Quantification of resilience improvements for critical facilities through advanced technologies

A relevant number of critical facilities, such as hospitals, schools and city halls, experienced extensive damage that resulted in loss of their functionality, and consequently huge economic losses and slow restoration processes after earthquakes. In some communities not well organized, the recovery process can last several years and the community or the system is never brought back to the initial functionality. The awareness that damage can not be avoided has increased the attention on designing buildings that are both safe and resilient, however it is often assumed that such design can increase costs to unacceptable levels. The paper compares life-cycle costs and resilience of respectively a hospital and a school which have been retrofitted using both a moment resisting frame system and a base isolated system. Performance-based earthquake evaluation tools are used to estimate the total cost of ownership, including expenses associated with initial construction, damage repair, loss of functionality and resilience. Numerical analyses have shown that resilience of both schools and hospitals can be improved by using a seismic isolation system that will also reduce life-cycle costs and downtime with respect to a conventional fixed-based design

Risk, Resilience, and Sustainability-Informed Assessment and Management of Aging Structural Systems

During their service life, structural systems (e.g., civil and marine structures) may be subjected to aggressive deteriorations such as corrosion and fatigue and/or extreme events such as floods, collisions, earthquakes, and fires. These deteriorations may start from the day the structures enter in service and, if not effectively managed, can cause a significant reduction in structural functionality and safety. Maintaining performance and functionality of structural systems under these adverse effects is gaining increased attention. This highlights the necessity of effective assessment and management of civil and marine structures in a life-cycle context.The main objective of this study is to develop a risk, sustainability and resilience-informed approach for the life-cycle management of structural systems with emphasis on highway bridges, bridge networks, buildings, interdependent structural systems, and ship structures. Risk - based performance indicators combining the probability of structural failure with the consequences associated with a particular failure event are investigated in this study. Furthermore, a wide range of performance measures is covered under “sustainability” to reflect three aspects: economic, social, and environmental. Sustainability is described as “meeting the needs of present without altering the needs of future generations” (Adams 2006). Sustainability can serve as a useful tool in decision making and risk mitigation associated with civil and marine structures. In addition to risk and sustainability, resilience is another indicator that accounts for structural functionality and recovery patterns after extreme events. Presidential Policy Directive (PPD 2013) defines resilience as “a structure’s ability to prepare for and adapt to changing conditions while simultaneously being able to withstand and recover rapidly from functionality disruptions”. Overall, risk, sustainability, and resilience assessment considering aging and multi-hazard effects are of vital importance to ensure structural safety and functionality of structural systems during their service life.Risk is assessed for highway bridges under the effects of climate change and multiple hazards, including aging effects, flood-induced scour, and earthquake, whereas the adverse effects associated with aging and earthquake are investigated for bridge networks. The sustainability of highway bridges and bridge networks is assessed considering social, economic, and environmental metrics. The seismic resilience of highway bridges under mainshock (MS) only and mainshock-aftershock (MSAS) sequences is investigated to account for structural performance and recovery patterns under extreme events. Additionally, the seismic performance of buildings and interdependent healthcare - bridge network systems is investigated considering correlation effects and uncertainties. Furthermore, a probabilistic methodology to establish optimum pre-earthquake retrofit plans of bridge networks based on risk and sustainability is developed. For ship structures, a decision support system considering structural deteriorations (i.e., corrosion and fatigue) and extreme events (e.g., collision) is established. Specifically, the probabilistic ship collision risk and sustainability are investigated incorporating the attitude of a decision maker. A novel approach is developed to evaluate the time-variant risk of ship structures under corrosion and fatigue during the investigated time interval. Furthermore, a multi-objective optimization problem, which accounts for structural deteriorations and various uncertainties, is formulated to determine optimum inspection planning that reduces the extent of adverse consequence associated with ship failure while simultaneously minimizing the expected total maintenance cost. Additionally, a probabilistic approach for reliability and risk updating of both inspected and uninspected fatigue-sensitive details at both component and system levels is developed considering uncertainties and correlation effects. Overall, this study provides methodologies for the risk, sustainability, and resilience-informed assessment and management of structural systems under structural deteriorations and extreme events in a life-cycle context. Based on the inspection information, the reliability and risk could be updated for the near real-time decision making of deteriorating structures. The proposed probabilistic frameworks are illustrated on highway bridges, bridge networks, buildings, interdependent structural systems, and ship structures. The proposed methodology can be used to assist decision making regarding risk mitigation activities and, ultimately, improve the sustainability of structural systems in a life-cycle context

GIS procedure to evaluate the relationship between the period of construction and the outcomes of compliance with building safety standards. The case of the earthquake in L’Aquila (2009)

The earthquake (Ml=5.8; Mw=6.3) that shook L’Aquila (Abruzzo region, Italy) on 6 April 2009 and caused huge widespread damage in the other 56 municipalities of the seismic crater has also provided important input to reflect proactively on the need to avoid the repetition of similar tragedies, learning from the calamities that have occurred. In fact, L’Aquila and the other municipalities hit by the earthquake represent an open-air analysis laboratory to reveal and directly see the weak points of the different buildings on the field which did not adequately resist the shocks. In order to provide important data for social utility, in this paper we illustrate the steps which constitute a GIS procedure that we have thought in order to evaluate the relationship between the period of construction and the outcomes of compliance with building safety standards. Through sequential activities which have enabled us to also produce three-dimensional scenarios – of immediate communicative impact and able to show details for interdisciplinary analysis and strategical planning – we have portrayed the urban evolution of L’Aquila per period of construction and mapped the level of damage to the buildings. The relational analysis and quantitative data have permitted us to show that in the case of L’Aquila the major percentages of “unusable buildings”, and also these together with “condemned buildings due to external risks” concern the structures erected until 1955 and then in the 1956- 1975 period, followed by the ones constructed in the periods of 1976-1988 and 1989-1994. Similar results, in conjunction with other specific information, can offer the possibility to define and apply the consolidation measures necessary to tackle future earthquakes in an appropriate way, without a passive sense of resignation and with a deeper awareness of seismic risk