Seismic risk assessment of buildings

Authors: Francesca De Vito, ...

Introduction

Seismic activity poses a significant threat to communities, infrastructure, and the environment in seismically active regions worldwide. Understanding and mitigating this risk requires a comprehensive assessment of the potential impacts of earthquakes on various elements at risk. This seismic risk assessment report aims to provide a thorough evaluation of the seismic hazard, vulnerability, and exposure factors, with the objective of informing decision-makers, stakeholders, and the public about the potential risks posed by seismic events.

Seismic risk assessment

Definition of risk

The definition of risk has been the center of discussion for quite a long time, nowadays all the experts agree on defining risk as the probability of suffering damage in a given period, where damage is intended as function of hazard, exposure and vulnerability. This leads to define risk as:

,

where:

H is the hazard,

V is the vulnerability,

E is the exposure.

In particular, seismic risk refers to the potential negative consequences or harm caused by earthquakes to people, structures, infrastructure, and the environment within a specific geographical area. It encompasses the likelihood of earthquakes occurring, as well as the vulnerability of the built environment and the population's exposure to seismic hazards.

Risk assessment evaluation

In order to perform a risk assessment, an analysis of its components must be made. Hence, hereafter the analysis of hazard, exposure and vulnerability is reported; this data can be aggregated following different methodologies. In the following, the process leading to a complete risk assessment is reported.

Hazard assessment

Hazard assessment is the procedure to characterize and mapping the location, magnitude, intensity, geometry, and frequency or probability of occurrence, and other characteristics of a given event, phenomenon, process, situation, or activity that may potentially be harmful to the affected population and damaging the society and the environment.

An exhaustive seismic hazard assessment should reply to the following questions:

"How strong/severe?": the parameters to be considered should be the peak ground acceleration and the time history of the maximum recorded/simulated earthquake.
"When?": two possible analysis can be performed: a prediction, intended as the possibility to assess the probability of the events in a given area, or a forecast, that is the possibility to forecast the next event within margins of error/uncertainty. Speaking about earthquakes, these events cannot be forecasted, since models oscillate between false and missed alerts; hence, they can only be predicted using historic time series or simulations, such as the Monte Carlo Simulations (MCS).
"How frequent?": in scientific terms we refer to the return period to define how frequent an event is. The return period is the calculated estimated time of natural disasters being experienced in a selected area. It is calculated most commonly from historical data, and it is used for risk assessment in the area where the data was collected or in areas with very similar factors to the original area. The return period is used to ensure the safety of the people, environment, and the buildings. When planning a structure, local government selects a proper return period for the area, which the engineers have to comply with when designing the building. Although the return period is not perfect, since the results are only chances, which means that, for example, an event with a 1% chance of being experienced in 10 years may be experienced once, twice, more, or not at all in the span of ten years, but it's still the best tool available to us to predict earthquakes.
"Where?": the two main methods to identify areas prone to earthquakes are: assessing seismogenetic zones and creating a historic earthquakes catalogue.

"How long can it last?": a main subdivision is between short time, consisting of a main shock and decreasing in the subsequent period, and long duration, consisting of repeated shocks over years.

Different methods have been implemented with the goal to obtain an hazard assessment:

Probabilistic Seismic Hazard Assessment (PSHA): PSHA estimates the likelihood of different levels of ground shaking occurring over a specified period in a given area. It considers the seismicity, fault distribution, and ground motion characteristics to calculate the probability of exceedance for different levels of ground shaking.

Deterministic Seismic Hazard Assessment (DSHA): DSHA evaluates the effects of specific seismic events on structures and infrastructure. It focuses on the maximum expected ground motion from a particular earthquake scenario, considering factors such as fault characteristics, magnitude, distance from the epicenter, and local soil conditions.
Seismic Micro zonation: This method divides an area into zones with distinct seismic characteristics, such as soil type, topography, and geology. It provides detailed information on local amplification effects and ground motion variability, helping to identify high-risk areas within a region.

Scenario-based Risk Assessment: Scenario-based assessments simulate hypothetical earthquake scenarios to estimate potential losses to buildings, infrastructure, and populations. These scenarios consider various parameters, including earthquake magnitude, location, depth, and ground shaking intensity, to model the potential impact on different elements at risk.

Exposure assessment

Exposure refers to the quantitative assessment of the number of people and number of objects, assets, infrastructures in an area exposed to a threat to a natural hazard. Exposure analyses are usually performed in economical terms. The main categories are:

population;
buildings, subdivided in residential, commercial, public, strategical;
infrastructures;
livelihoods;
cultural heritage;
agricultural areas;
natural preservation areas;
economic activities.

One main issue to be addressed before performing an exposure analysis is the spatial scale. The spatial scale is a key concept in planning and it addresses the fact that decisions and plans can be developed at different "scales", where the scale encompasses:

the level at which one is operating (local, regional, national, global);
the administrative and political levels of government that are responsible for decisions (municipality, province, region, nation);
type of decisions and activities that can be actually carried out at different levels.

In the following, the exposure models for buildings will be addressed.

Exposure models include useful information on the location, number of occupants and replacement costs of buildings, in addition to their vulnerability class. Up-to-date exposure data is often unavailable due to the rapidly altering built environment, including aging, particularly in developing countries. Normally, the exposure models rely heavily on the national housing census. Censuses are normally taken every 10 years and are performed at administrative division resolution. The efficiency of the data collected by each country is not consistent, which makes the development of a global exposure model a challenging aspect of risk analysis.

The main goal of the exposure model is to obtain a layer of uniform resolution across the study area with spatially distributed structures that are classified according to the selected building taxonomy. The taxonomy for the characterization of the exposed building stock and the description of its damage should be compatible with the fragility/vulnerability relationships that will be considered in the risk assessment process. For estimating economic and social losses, an exposure model might need to contain additional data about the estimated replacement cost of the structures and expected number of occupants depending on the time of a day. Rapid survey procedures can be performed by utilisation of satellite imagery and from volunteered data in cases where the census data is outdated or missing vital data required for the building exposure model.

Vulnerability assessment

Vulnerability refers to the predisposition of the assets to be damaged if the hazard occurs. Vulnerability definition has been and still is reason of discussion, as every field of knowledge has developed its own meaning. However, Turner et all's definition is the most comprehensive one: "Vulnerability rests in a multifaceted coupled system with connections operating at different spatiotemporal scales and commonly involving stochastic and non-linear processes".

In particular, when talking about vulnerability a distinction can be made among:

Physical vulnerability: the predisposition to be physically damaged, which means identifying the primary factors making buildings, infrastructures, people, etc, vulnerable to hazards.
Functional vulnerability: the predisposition to loss of functionality, which is the loss of capacity of critical equipment to continue functioning.
Systemic vulnerability: the predisposition to loss of services, which is the loss of capacity of a system to provide services.

Vulnerability should be assessed for all the identified assets, however, in the following the physical vulnerability of buildings will be investigated. The steps to develop a physical vulnerability assessment are:

Identify a correlation between vulnerable factors and damages;
Select a set of vulnerable parameters, such as: period of construction, type of construction, material, significant interventions over time. Based on these parameters we can classify the buildings.
Construct fragility/vulnerable curves: fragility analysis assesses the vulnerability of structures and infrastructure to seismic hazards by quantifying the probability of damage or failure at different levels of ground shaking. It uses empirical data, structural models, and damage observations to develop fragility curves for different building types and components. Fragility curves can be used to predict the probability of exceedance of certain limit/damage states for a given intensity measure value. It is common for engineers to use simple damage states such as: slight, moderate, extensive and complete damage. Consequence models that relate the cost of loss to the rebuilding cost for a given damage state, can be used to convert a set of fragility curves into vulnerability curves to predict economic losses.

Verify correlations in other real cases: checking if the assessment has been able to forecast within a margin of error a real event.

Also vulnerability assessment, as exposure analysis, must be tailored to the scale of interest:

individual building: usually performed adopting a form;

local, medium size town: sampling techniques are adopted to obtain a representative specimen of the building in that area.
regional/above regional: use of statistical data.
global: based on ranking or scoring, created by agencies such as INFORM, a collaboration of the Inter-Agency Standing Committee Reference Group on Risk, Early Warning and Preparedness and the European Commission.

Risk assessment

The information gathered so far can be aggregated following different methodologies:

Loss Estimation Models: Loss estimation models integrate seismic hazard, vulnerability, and exposure data to quantify the potential economic, social, and environmental losses resulting from earthquakes. These models consider factors such as building inventory, occupancy types, replacement costs, and casualties to estimate direct and indirect losses.
Risk Matrix and Risk Maps: Risk matrices and risk maps visually represent the spatial distribution of seismic risk by combining hazard, vulnerability, and exposure data. They classify areas into different risk levels based on the likelihood and consequences of earthquake impacts, helping decision-makers prioritize risk reduction measures and emergency response planning.

Performance-based Assessment: Performance-based assessment evaluates the seismic performance of structures and infrastructure based on predefined performance objectives, such as life safety, functionality, and repairability. It considers factors such as building code compliance, structural integrity, and resilience measures to assess the expected performance during and after earthquakes.

Mitigation measures

As consequence of an efficient risk assessment, mitigation measures should be adopted in order to reduce the risk, acting on its components: hazard, exposure or vulnerability.

Mitigation measures are actions adopted to improve capacity, response and recovery of the system. They can be divided in short term and long term measures as well as in structural, construction of physical tools to reduce the physical vulnerability of buildings (such as structural retrofitting and building codes), and non-structural measures, such as emergency plans, training exercises, zoning, restauration.

Concerning seismic risk, vulnerability is the main component addressed to reduce the risk, since the hazard itself can't be reduced or impeded as well as exposure.

In the following, two main mitigation measures are investigated:

Seismic design codes

Earthquakes are common worldwide, leading many Nations to introduce rules in the design codes to take into account seismic solicitations. These codes are based on the same approach and are, hence, very similar among them. Therefore, it's interesting to analyzed and compared them; in particular, the following seismic design codes are herein analysed:

• Eurocode 8 - EN 1998-1:2004 [1]
• American Standard - ASCE/SEI 7-16 [2]
• Brazilian Standard - NBR 15421:2006 [3]
• Canadian Code – NBCC2015 [4]
• French National Annex to Eurocode 8 – NF EN 1998-1:2005 [5]
• Greek Seismic Code - EAK 2000 [6]
• Italian Technical Standard for Structures 2018 [7]
• New Zealand Standard – NZS1170.5:2004 [8]
• Turkish Building Seismic Code 2018 [9]
• Japan Building Standard Law 1981 [10]
• Design Manual for Civil Structures in Mexico: Seismic Design 2015 [11]
• Bulgarian National Annex to Eurocode 8 – BDS EN 1998-1 (EC 8-1) [12]
• Portuguese National Annex to Eurocode 8 - NP EN 1998-1:2010 [13]

Recurrence period

Different criteria have been found in the various codes for defining the recurrence periods. The Eurocode 8 [1] recommends, for the non-collapse requirement of a structure, the consideration of a recurrence period of 475 years. This corresponds to a probability of 10% of the seismic input being exceeded in 50 years.
The Brazilian [3], French [5], Portuguese [13], Greek [5] and Bulgarian [12] standards follow the same definition of Eurocode 8 [1].
The Italian code [7] defines two seismic levels for the design of conventional buildings: a Damage Limit State level using elastic spectra with recurrence period of 30 years (mainly for checking maximum displacements and non-structural damage) and a Life Preservation Limit State level using design spectra with recurrence period of 475 years (for checking structural resistance, ductility and stability).
The American Standard ASCE/SEI 7/16 [2] and the Canadian Code [4] define a recurrence period of 2475 years, i.e., a probability of 2% of the seismic input being exceeded in 50 years, corresponding to the Maximum Considered Earthquake (MCE); however, for the design of ordinary structures, a reduction factor of 2/3 is applied to the resulting values of the seismic design forces.

Seismic zonation and design seismic ground motion values

Eurocode 8 transfers the responsibility for defining the seismic zonation to each of the National Authorities, Introduced in the respective National Annexes. In this standard, the parameters that define the local seismicity are the ZPA (“Zero Period Acceleration”), value of the reference peak ground acceleration on rock (a_g) and the magnitude that prevails in the seismic risk of the analysed site, that defines two different spectral types to be used in the design. The definition by only a single parameter (“Zero Period Acceleration”) is found in all other codes with the exception of Canadian Code [4] and ASCE/SEI 7/16 [2].
In ASCE/SEI 7/16 [2], the seismic input is defined through three basic parameters, i.e., the peak ground accelerations at spectral periods 0.2 s and 1.0 s and the period TD that defines the displacement governed region of the spectrum. These parameters are defined in the standard through very detailed maps.
In Canadian code [4] it is possible to draw equal probability design spectra for a specific location from information given in the sites of the National Research Council of Canada (NRCC).
In New Zealand code [8], the return period depends on the Importance Level (IL) assigned to a building. For the Life-Safety LS, the following return periods are defined:
• IL1 (e.g. farm buildings): 100 years
• IL2 (standard commercial or residential building): 500 years
• IL3 (high occupancy building, school, airport): 1000 years
• IL4 (hospital, fire station, post-disaster facility): 2500 years
In Turkish code, the seismic input levels depend on the building type and target performance level:
• DD-1: 2475 years return period;
• DD-2: 475 years return period;
• DD-3: 72 years return period;
• DD-4: 43 years return period.
The seismic design input in Japan [10] corresponds to a return period of approximately 500 years.
Mexico code [11] defines an optimum design criterion, based on an additive function of the initial cost of the buildings and the expected value of losses. This approach conducts to optimum return periods in zones of high seismicity close to 200 years and in zones of low seismicity, to more than 2000 years.

The elastic response spectrum as defined in Eurocode 8, is presented in Fig 1.

In this spectrum, as well as in the elastic spectra of all the other analyzed standards, the pseudo-accelerations (S_e) are given as a function of the structural periods (T). The spectra vary proportionally to the peak ground acceleration (a_g), times a soil coefficient S, related to the soil amplification and considers the parameter η, correction factor for damping values different from 5%. All other analyzed standards, excepting the Italian standard, consider, for the definition of the spectra, the nominal structural damping of 5%.

The region between reference periods T_B and T_C is controlled by acceleration (constant acceleration); the region between periods T_C and T_D is controlled by velocity; the region for periods superior to T_D is governed by displacement. The region between 0 and T_B is the transition region between the peak ground acceleration and the maximum spectral accelerations.

For Eurocode 8 [1], the values of S, T_B, T_C and T_D are defined as a function of the type of subsoil in the two spectral types defined in the code, Types 1 or 2, related respectively to higher and lower seismicity regions, respectively.

The ASCE/SEI 7/16 [2] defines this region showing the period T_D through maps.

Numerical example:

This item presents the application of some points discussed previously in the seismic analysis of a prototype reinforced concrete building. The item reproduces partially the work developed by Santos et al. [15], where some of the presently analyzed codes were considered.

Allowed procedures for the seismic analysis

For regular and simple structures, all the standards allow for a lateral force (static equivalent) method of analysis, in the cases that the contribution of the fundamental mode in each horizontal direction is predominant in the dynamic response. All the standards provide also formulas for the approximate evaluation of the fundamental periods of a structure.
All the standards allow the use of the modal response spectrum analysis. The standards allow also linear time-history analysis, using recorded or artificial time-histories matching the design response spectra. Some codes (as the Eurocode 8 [1]) admit non-linear analyses in the time domain, but as long as substantiated with respect to more conventional methods. Some codes (as the Eurocode 8 [1]) allow also for non-linear static (pushover) analyses.

Discussion

The analysis of the different seismic standards indicates a general agreement regarding the desired characteristics of a seismic resistant structure. Structures should be designed and detailed to provide enough ductility for the dissipation of energy in the non-linear range. On the other hand, differences in the shapes of the design spectra lead to big differences in results.All analyzed standards show the seismicity in their respective countries through maps defining accelerations to be considered in the design.

Regarding the definition of the spectral shapes, for most of the analyzed standards, the shape is governed by a single parameter, the peak ground acceleration. Eurocode 8 defines two spectra, associated with the type of source that prevails in the seismic risk of the analyzed site. In standard ASCE/SEI 7/16 [2], the spectral shape is defined with three basic parameters, i.e., the peak ground accelerations for the spectral periods of 0.2s and 1.0s and the period T_D that defines the displacement governed region of the spectrum.
Another issue is the definition of the recurrence period. The ASCE/SEI 7-16 [2] and NBCC [4] 2015 already redefined this parameter from 475 to 2475 years. This led to an important increase in the design seismic forces, implying in the level of reliability that our constructions will possess.

Seismic Retrofitting

This involves modifying existing structures to make them more resistant to seismic events. Techniques include adding bracing, strengthening walls, or reinforcing foundations.

Base Isolation Systems:

Base isolation systems are widely used as an effective and practical solution to protect the structure and non-structural elements from seismic hazards. Base isolation systems reduce structural excitation by physically decoupling the structure from the ground. This type of solution requires that the entire structure be cut loose and separated from the foundation system and isolation pads inserted in between the two.

The growth in seismic isolation technology has led to the development of innovative base isolation systems which exhibit adaptive behavior. The behavior is denoted adaptive when the properties of the device change substantially depending on the loading level. Thus, the response can be tailored to the hazard level based on the softening and subsequent stiffening response and/or changing damping ratio as displacement increases. Recently, the concept of adaptive behavior has gained significant attention within the research community. Some types of adaptive devices have the remarkable ability to dissipate the input energy at severe events, others can effectively reduce the responses at low to moderate earthquake ground motions while limiting the displacement at extreme events.

Although active control means can effectively reduce the response, due to the high external power requirement, lack of system reliability and robustness, high budget requirement, and some other challenges, implementation of these devices is not yet widely accepted. Among the well-developed isolation bearings, systems with adaptive behavior continue to be an active area of research.

Rubber isolator:The Seismic Isolating Rubber Bearing consists of alternating laminations of thin rubber layers and steel plates (shims), bonded together to provide vertical rigidity and horizontal flexibility. Vertical rigidity assures the isolator will support the weight of the structure, while horizontal flexibility converts destructive horizontal shaking into gentle movement.

Friction isolator: FPS (friction pendulum system) is a widely used bearing based on the principle of sliding system and with a pendulum type isolator to provide a damping function using friction. The FPS isolator has an articulated slider moving on a spherical friction surface. The surface of the articulated slider in contact with the spherical friction surface is all coated with a self-lubricating composite material. The other side of the slider is attached to the stainless steel concave, spherical surface and also covered with low-friction composite material. When the slider moves over the spherical surface, the supported mass will be lifted and the movement will provide the restoring force to the system. Under extreme loads such as earthquakes, the slider moves along the concave surface, causing the supported structure to move in small arcs like a pendulum. The isolators reduce transmission of the earthquake forces to the structure by deflection (the pendulum motion) and by friction (damping) on the sliders. The radius of the curvature of the concave surface will dominate the effective stiffness and the system period.

Spring isolator: These are heavy-duty isolators used for building systems and industry. Sometimes they serve as mounts for a concrete block, which provides further isolation.

Damping Systems:

Damping devices, such as tuned mass dampers or viscous dampers, absorb and dissipate seismic energy, reducing the magnitude of vibrations experienced by the structure. The energy generated by floor vibration and building displacement is absorbed by the dampers and dissipated though heat energy. Seismic dampers are used in both new construction and retrofit applications. In new construction, they are typically installed between the floors or walls of a building. In retrofit applications, they are often installed on the roof or in the basement. Seismic dampers can also be used in combination with other earthquake-resistant measures, such as base isolation and shock-absorbing systems.

Benefits

There are a number of benefits that seismic dampers offer for civil engineers.

First, they can help to reduce the amount of damage that a building sustains during an earthquake.
Second, they can help to prolong the life of a building by reducing the amount of stress that is placed on the structure during an earthquake.
Third, they can help to improve the safety of a building by reducing the risk of collapse or other failure.

Limitations

One of the limitations of seismic dampers is that they can only protect the building from a certain amount of shaking. Once the damper has reached its maximum capacity, the building will be vulnerable to further damage. Another limitation is that seismic dampers are only effective if they are properly designed and installed. If they are not, they can actually increase the risk of damage to the building.

Classification

There are several solutions to limit the base displacement, including adding active control systems, using supplementary semi-active or passive dampers such as fluid viscous dampers and hysteretic dampers.

Viscous dampers: they are used to reduce the shock waves that pass through the ground during earthquakes. Dampers are usually constructed of a heavy fluid contained in cylinders or tanks and are placed along the length of an oil or gas pipeline. The fluid inside each cylinder is undisturbed until it is subjected to a force. When an earthquake strikes, the fluid moves from one end of the cylinder to another and absorbs energy from the seismic wave. The more viscous the liquid, the slower this energy will be absorbed. Viscous Seismic Dampers have been used for over 30 years.

Viscoelastic dampers: it is a device that absorbs shock by converting mechanical energy into another form of energy. The dampers are made from polyurethane and are placed between the floor and the upholstery. The material is able to absorb repetitive impacts through its structure, rather than just at the surface, which is how conventional suspension works. Viscoelastic dampers are used to eliminate the vibrations caused by any structure or machine. Vibration is a problem for many structures because it can cause damage over time. These damped structures have rubber elements which absorb some of the vibrations in order to ensure that they don’t cause any problems. There are two main types of viscoelastic dampers.
Friction dampers: Friction dampers are commonly used in industrial machinery and material handling equipment to reduce the amount of friction generated between two moving objects. These friction dampers are made from a variety of materials ranging from steel to urethane as well as metals such as aluminum, stainless steel, brass and more. The kinetic energy of moving parts is converted into thermal energy by friction dampers, which lessen abrupt stops or prevent excessively high vibration amplitudes. These types of seismic dampers in buildings more cost effective when compared some of other methods.

Tuned mass dampers: A tuned mass damper is a device that’s installed in buildings to protect them against vibrations. Tuned mass dampers are essentially giant pendulums that slow down swaying as a building vibrates. A seismic damper or harmonic absorber are other names for a tuned mass damper. It is a mechanical vibration-dampening device that is fixed to buildings and consists of a mass mounted on one or more damped springs.
Yielding dampers: Yield dampers, commonly referred to as metallic dampers, are typically constructed of steel. In order to absorb the energy when a building vibrates during an earthquake, they are made to deform excessively. After a seismic incident, yield dampers typically cannot revert to their previous configuration.

Seismic zoning:

Implementing regulations and guidelines to restrict development in high-risk seismic zones or to ensure that new constructions adhere to seismic-resistant building codes.

The seismic hazard is described in terms of a single parameter, i.e. the value of the reference peak ground acceleration, thus national territories are subdivided by the National Authorities into seismic zones, in the interior of which the hazard is assumed to be constant. For each seismic zone the reference peak ground acceleration corresponds to the reference probability of exceedance in 50 years, PNCR, of the seismic action for the no-collapse requirement. Further, depending upon the value of the design ground acceleration and the ground type, it is possible in the National Annex to define cases of low or very low seismicity, where reduced/simplified or no seismic design procedures for certain types or categories of structures may be followed.

Conclusion

Seismic risk assessments are essential for enhancing building resilience and ensuring occupant safety during earthquakes. These assessments rely on data-driven analysis to identify vulnerable buildings and prioritize mitigation efforts efficiently. Engaging integrating seismic risk assessments with broader resilience strategies are crucial steps in fostering community awareness and collaborative action. Continuous improvement, policy development, capacity building, and global collaboration play significant roles in advancing seismic risk assessment practices and promoting equitable resilience across diverse communities.