Assessing the life-safety risk for the proposed technical specification (TS) 1170.5

A better seismic design for building and other structures is the most effective way to reduce seismic risk and avoid earthquake disaster. Adoption and implementation of new seismic safety regulations and design standards have caused serious problems in many communities in the New Madrid region, including western Kentucky, however. The main reasons for these problems are (1) misunderstanding of the national seismic hazard maps and (2) confusion between seismic hazard and seismic risk. Both are caused by probabilistic seismic hazard analysis (PSHA). PSHA is a mathematical formulation derived from a probability analysis on the distribution of earthquake magnitudes, locations, and ground-motion attenuation. Some assumptions and distributions associated with PSHA have been found to be invalid in earth science, however. In addition, PSHA contains a mathematical error: equating a dimensionless quantity (the annual probability of exceedance – exceedance probability in one year) to a dimensional quantity (the annual frequency of exceedance with the unit of per year [1/yr]). Thus, PSHA is scientifically flawed and the resulting seismic hazard and seismic risk estimates are artifacts. The national seismic hazard curves and maps are artifacts because they were produced from PSHA, even though the inputs are scientifically sound. Although seismic hazard and seismic risk have often been used interchangeably, they are two fundamentally different concepts. Seismic hazard describes the natural phenomenon or property of an earthquake, whereas seismic risk describes the probability of loss or damage that could be caused by a seismic hazard. Seismic hazard and seismic risk play different roles in engineering design and other policy considerations. Furthermore, measures for seismic hazard mitigation are different from those for seismic risk reduction. The difficulties in the development of design ground motion for NEHRP provisions are caused by the use of the national seismic hazard maps which are neither seismic hazard nor seismic risk. The resulting design ground motions for building codes and other policy considerations are therefore problematic. California’s experience proves that deterministic/scenario seismic hazard analysis is an appropriate approach for seismic hazard assessment, seismic risk assessment, as well as *Kentucky Geological Survey, University of Kentucky, 504 Rose Street, Lexington, KY 40506; phone (859) 323-0564; fax (859)257-1147; e-mail: zmwang@uky.edu. 112 Seismic Hazard Design Issues in the Central United States engineering design and other policy considerations. Deterministic/scenario seismic hazard analysis is also appropriate for engineering design and other policy considerations in the New Madrid region, as well as other regions.

The Technical Specification (TS) 1170.5 has been developed to incorporate the output of the 2022 New Zealand National Seismic Hazard Model revision (NSHM2022) [1] and update Clause B1 Verification Method 1 (B1/VM1) of the New Zealand Building Code. In this paper, we discuss the proposed site classification methodology based on Vs(30) (i.e., the time-averaged shear-wave velocity from the ground surface to 30 m depth) which is used to incorporate site effects in the TS 1170.5 design spectra. The reasoning for the use of Vs(30) for site classification, a significant departure from New Zealand Standards NZS 1170.5 [2], is first elaborated. Based on detailed scrutiny of uniform hazard spectra obtained from NSHM2022, seven site classes are proposed, with associated design spectra for six of the site classes. Multiple objectives were considered in the definition of TS 1170.5 site classes, with the principal goal being to represent relevant site conditions in a robust yet practical manner, appropriate for engineering design practice. As Vs(30) is the principal parameter in the site classification scheme, the establishment of the Vs profile at the site is a critical step. Several methods for obtaining a Vs profile, measured or inferred, and subsequent calculation of Vs(30) are recommended. Each method is associated with a different uncertainty factor that affects both site classification and consequent design spectra. In this context, a multiple site class definition must be adopted with an envelope design spectrum in cases where the range of Vs(30) values span several site classes. Importantly, the variation in design spectra due to uncertainty in the site class is relatively small compared to the uncertainty in the uniform hazard spectra themselves (due to uncertainties in NSHM2022 and PSHA). For sites with ground conditions not well-represented within the PSHA performed for NSHM2022, site-specific (special) studies are recommended.

The historical development of limit state design in geotechnical engineering is reviewed. Total and partial factors of safety used for the design of land–based and offshore structures are compared. It is found that the factors of safety in different codes for the ultimate and serviceability limit states design of earthworks, earth retaining structures, and land-based and offshore foundations are very similar. Partial factors in the ultimate limit state design are linked to the variability of the loads and soil parameters, the design approximations, and construction tolerances. They influence the nominal probability of failure of the type of structure considered and the seriousness of failure, which differ for land-based and offshore structures. These probabilities are compared with human fatality risks of common experiences. The serviceability limit states are governed by structural and operational constraints and the intended service life of the land-based or offshore structure. The corresponding partial factors are generally taken as unity. Key words : codes, earth structures, foundations, human risks, limit states design, probability of failures, factors of safety.

This paper presents a methodology to evaluate life safety risk of coastal communities vulnerable to seismic and tsunami hazards. The work explicitly incorporates two important aspects in tsunami evacuation modeling: (1) the effect of earthquake-induced damage to buildings on building egress time, (2) the effect of earthquake-induced debris on horizontal evacuation time. The city of Seaside, Oregon, is selected as a testbed community. The hazard is based on a megathrust earthquake and tsunami from the Cascadia Subduction Zone that was defined in a previous study. The built environment consists of buildings and the transportation network for the city. Fragility analysis is used to estimate the seismic damage to buildings and resulting debris that covers portions of the road network. The horizontal evacuation time is determined based on the shortest path to shelters, including the increased travel time due to the earthquake-generated debris. The effects of different mitigation strategies are quantified. Results indicate the fatality and life safety risk of a near-field tsunami increases by 4.2–8.3 times when the effects of building egress and earthquake-induced debris are considered. The choice of population layer affects the life safety risk and thus the maximum risk is obtained when daytime populations are considered. Use of mitigation strategies result in a significant decrease in the number of fatalities. For hazards with recurrence intervals larger than 500- to 1000-years, the seismic retrofit is comparable to vertical evacuation and an effective strategy in reducing fatalities and associated risks. Implementing all mitigation strategies reduces the life safety risk by 90%.

The recent release of the 2022 national seismic hazard model has highlighted significant changes in the quantified seismic hazard for much of New Zealand that has prompted the development of draft changes to the NZS 1170.5 seismic design provisions. One proposed change is to the shape of the design spectrum, which was previously provided by a spectral shape factor, Ch(T), that is a function of site class only. However, research has shown that spectral shape is strongly affected by several additional factors including earthquake magnitude and shaking intensity. Moreover, the use of fixed spectral shapes that vary only by site class results in significant variability between the functional form of the elastic design response spectrum, C(T), and the direct results from the national seismic hazard model. International loading standards typically include a dependency on intensity and site class in the spectral shape equations and these form the basis for the approach recommended here. The functional form of the design response spectrum is also updated to better represent spectral displacement demands on longer period structures. The proposed new spectral shape equations are compared to the 2022 national seismic hazard model output and the equations used in the previous New Zealand loading standard. Results show that the proposed approach provides a significantly better approximation of the national seismic hazard model results than the current spectral shape across a range of periods, site classes, annual probabilities of exceedance, and locations.

The ultimate limit state of structural tension members with stress concentrations due to geometrical (non-welding related) stress raisers is investigated. Examples of such members are pad eyes, brackets etc. The influence of the application of high strength steels (up to S690) is taken into account. The focus lies on members with a predominant static loading regime. Such members frequently occur in the marine environment as parts of lifting appliances and handling systems or as a structural detail of equipment foundations, located outside the fatigue-prone regions of the hull girder. Typically, design stresses at the stress concentration approach the yield limit of the material. Common yield criteria cannot be applied to such peak stresses, due to the small margin between design and yield. Usually, the strength integrity is based on the nominal stresses in the critical cross section. Goal of the study is to determine the ductile failure limit with a method suited for design purposes. This would enable an ultimate limit state design approach and improve the structural safety philosophy. Main question is how the post yield behavior up to failure of a notched section is influenced by the stress gradient and the properties of the high strength materials. For this purpose, the applicability of two damage models based on the work of Rice & Tracey [8] (void growth model) and Bonora [1] (damage mechanics) is studied. In combination with elastoplastic finite element analysis these models enable the prediction of local ductile crack initiation. Calculations are performed on slender tensile members with a geometrical stress raiser, assuming a range of structural steel qualities and using a static loading regime. The results are verified using small scale laboratory tests. It is shown that isolated (non-redundant) tensile members with stress raisers feature a static ductile failure mode similar to that of uniform tensile specimen. Their failure loads can be determined as the product of the material’s tensile strength and the net section area, in the same way as for uniformly stressed members. These findings are valid up to S690 materials and clear the path to a safe and sound application of such materials based on an ultimate limit state approach. It was found that the ultimate limit state is governing design for higher strength steel members with a relatively low stress concentration. A severe stress raiser may be beneficial for efficient design of high strength members, since it allows a design stress in the notch up to yield without compromising the safety up to failure. Damage calculations were found superfluous for isolated member ultimate limit state design. Damage results, however, compare well with the failure mode observed. This is useful for the design of highly stressed notches in details which are surrounded by a large main structure, providing a huge reserve strength capacity. For these so-called embedded stress raisers an ultimate load approach is not possible due to the absence of a critical cross section. Damage mechanics can then be applied to determine a failure point in terms of stress and strain, allowing an ultimate limit state design for these stress peaks as well.

This paper examines the advantages and limitations of employing ultimate limit state methods for the design of braced excavations. Braced excavation design requires both skill and careful evaluation of many factors that can affect performance. Traditionally in the US, braced excavations are designed with a serviceability approach where soil parameters are conservatively estimated and the performed analysis yields the service displacements, moments, and forces. Design forces are then calculated by applying a global safety factor on the service design results, while the wall embedment is determined by calculating limit equilibrium safety factors against wall rotation and passive resistance that range from 1.2 to 1.5. In Europe, in contrast to the US, an ultimate limit state design approach has been adopted in geotechnical design including the design of braced excavations. In this design philosophy both wall and supports are designed based on an ultimate limit condition. The ultimate design forces are typically determined by reducing the characteristic soil strength parameters or by multiplying the effects of actions and dividing the effects of resistances by various safety factors. At the end, a safety factor of one or greater is required for all structures and other types of safety factors. Back in the US, there is an increasing trend of promoting ultimate limit state design in geotechnical design, including braced excavations. In the author’s experience the ultimate limit state method works reasonably well for most limit equilibrium methods but can produce very inconsistent results in many cases when numerical analyses are employed. Hence, the advantages and limitations of the ultimate limit state design should be carefully weighted by practitioners and academia in the US before, and if, the ultimate limit state philosophy is incorporated in a legally binding building code.

Effects of earthquake damage to highway components (e.g., bridges, tunnels, roadways, etc.) can go well beyond life-safety risks and costs to repair the damaged components. Such damage can also disrupt traffic flows which, in turn, can impact the region's economic recovery and emergency response. These impacts will depend not only on the seismic performance of the components, but also on the characteristics of the overall highway system such as its network configuration and roadway-link characteristics (e.g., link locations, redundancies, and traffic capacities). Unfortunately, such traffic impacts are usually not considered in seismic risk reduction activities at state transportation departments. One reason for this has been the lack of a technically-sound and practical tool for estimating these impacts. Therefore, since the mid-1990s, the FHWA has sponsored multi-year seismic-research projects at MCEER that have included development and programming of such a tool with geospatial technologies. This has led to new software named REDARS (Risks from Earthquake DAmage to Roadway Systems) that was released for public use in March 2006.REDARS is a multi-disciplinary tool for seismic risk analysis (SRA) of highway systems nationwide based on geospatial technologies. For any given earthquake, REDARS uses state-of-knowledge models to estimate: (a) the seismic hazards (ground motions, liquefaction, and surface fault rupture) throughout the system; (b) the resulting damage states (damage extent, type, and location) for each component in the system; and (c) how each component's damage will be repaired, including its repair costs, downtimes, and time-dependent traffic states (i.e., its ability to carry traffic as the repairs proceed over time after the earthquake). REDARS incorporates these traffic states into a highway-network link-node model, in order to form a set of system-states that reflect the extent and spatial distribution of roadway closures at various times after the earthquake. Then, REDARS applies network analysis procedures to each system-state, in order to estimate how these closures affect system-wide travel times and traffic flows. Finally, REDARS estimates corresponding economic losses and increases in travel times to/from key locations or along key lifeline routes. These steps can be applied for single earthquakes and no uncertainties (deterministic analysis) or for multiple earthquakes and simulations in which uncertainties in earthquake occurrence and in estimates of seismic hazards and component damage are considered (probabilistic analysis). This presentation will provide the overview of the FHWA seismic risk analysis program, REDARS.

Site classification and estimation of surface level seismic hazard using geophysical data and probabilistic approach

Design of geosynthetic reinforced column supported embankments using an interaction diagram

SummaryThe characterisation of the seismic hazard input is a critical element of any seismic design code, not only in terms of the absolute levels of ground motion considered but also of the shape of the design spectrum. In the case of Europe, future revisions of the seismic design provisions, both at a national and a pan‐European level, may implement considerable modifications to the existing provisions in light of recent seismic hazard models, such as the 2013 European Seismic Hazard Model. Constraint of the shape of the long‐period design spectrum from seismic hazard estimates on such a scale has not been possible, however, owing to the limited spectral period range of existing ground motion models. Building upon recent developments in ground motion modelling, the 2013 European Seismic Hazard Model is adapted here with a new ground motion logic tree to provide a broadband Probabilistic Seismic Hazard Analysis for rock sites across a spectral period range from 0.05 seconds to 10.0 seconds. The resulting uniform hazard spectra (UHS) are compared against existing results for European and broadband Probabilistic Seismic Hazard Analysis and against a proposed formulation of a generalised design spectrum in which controlling parameters can be optimised to best fit the uniform hazard spectra in order to demonstrate their variability on a European scale. Significant variations in the controlling parameters of the design spectrum are seen both across and within stable and active regions. These trends can help guide recalibrations of the code spectra in future revisions to seismic design codes, particularly for the longer‐period displacement spectrum.

<p>The Emilia earthquakes of May 20, 2012 (Ml 5.9, INGV; Mw 6.11, http://www.bo.ingv.it/RCMT/) and May 29, 2012 (Ml 5.8, INGV; Mw 5.96, http://www.bo.ingv.it/RCMT/) struck an area that in the national reference seismic hazard model [MPS04; http://zonesismiche.mi.ingv.it, and Stucchi et al. 2011] is characterized by expected horizontal peak ground acceleration (PGA) with a 10% probability of exceedance in 50 years that ranges between 0.10 g and 0.15 g (Figure 1), which is a medium level of seismic hazard in Italy. The strong impact of the earthquakes on a region that is not included among the most hazardous areas of Italy, and the ground motion data recorded by accelerometric networks, have given the impression to the population and the media that the current seismic hazard map is not correct, and thus needs to be updated. Since the MPS04 seismic hazard model was adopted by the current Italian building code [Norme Tecniche per le Costruzioni 2008, hereafter termed NTC08; http://www.cslp.it/cslp/] as the basis to define seismic action (the design spectra), any modification to the seismic hazard model would also affect the building code. The aim of this paper is to briefly present the data that support the seismic hazard model in the area, and to perform some comparisons between recorded ground motion with seismic hazard estimates and design spectra. All of the comparisons presented in this study are for the horizontal components only, as the Italian hazard model did not perform any estimates for the vertical component. […]</p><br />

Seismic resistance design requires the estimation of futuristic seismic force to the structure in terms of spectral acceleration/velocity/displacement at the corresponding natural period of the structure. These expected seismic forces are defined based on detailed seismic hazard analysis and design spectrums from recorded earthquakes in the region. In this study, we have presented seismic design criteria in the Indian Seismic Code IS 1893 since its development, state-of-the-art procedure for the seismic hazard estimation, and the development of seismic design spectrum at the Indian Rock Site from North India and South India seismic data separately. The first Indian seismic code of IS 1893 was released in 1962 based on the studies of the Geological Survey of India on past earthquakes. IS 1893 was frequently revised soon after major earthquakes in different parts of the country and the currently available version is IS 1893 (2016). The seismic zonation map of India is based on past earthquake intensities and not on systematic futuristic seismic hazard estimation accounting for probable location and size of earthquakes. The different natural period of structural design requires respective design spectral amplitude. The previous versions of IS 1893 have given seismic coefficients for seismic zones and spectral amplitude for the different periods based on earthquakes recorded in US at an epicentral distances of 50–70 km, with multiplication factors. A recent version of IS 1893 adopted a design spectrum from the Uniform Building Code, again without considering regional data. After discussing these points, a modern smoothened, and normalized way of developing the design spectrum using regional data is explained. Further, rock site seismic records from the southern and northern parts of India were collated and used to create the design spectrum. The derived design spectra presented are applicable at the rock sites for 5% damping based on inter- and intraplate regions. Our study shows North and South Indian spectrums are different from the IS 1893 spectrum and the signature of each seismotectonic region is reflected in the proposed new spectral shape.

Damaging earthquakes in Australia and other regions characterised by low seismicity are considered low probability but high consequence events. Uncertainties in modelling earthquake occurrence rates and ground motions for damaging earthquakes in these regions pose unique challenges to forecasting seismic hazard, including the use of this information as a reliable benchmark to improve seismic safety within our communities. Key challenges for assessing seismic hazards in these regions are explored, including: the completeness and continuity of earthquake catalogues; the identification and characterisation of neotectonic faults; the difficulties in characterising earthquake ground motions; the uncertainties in earthquake source modelling, and; the use of modern earthquake hazard information to support the development of future building provisions. Geoscience Australia recently released its 2018 National Seismic Hazard Assessment (NSHA18). Results from the NSHA18 indicate significantly lower seismic hazard across almost all Australian localities at the 1/500 annual exceedance probability level relative to the factors adopted for the current Australian Standard AS1170.4–2007 (R2018). These new hazard estimates have challenged notions of seismic hazard in Australia in terms of the recurrence of damaging ground motions. This raises the question of whether current practices in probabilistic seismic hazard analysis (PSHA) deliver the outcomes required to protect communities and infrastructure assets in low-seismicity regions, such as Australia. This manuscript explores a range of measures that could be undertaken to update and modernise the Australian earthquake loading standard, in the context of these modern seismic hazard estimates, including the use of alternate ground-motion exceedance probabilities for assigning seismic demands for ordinary-use structures. The estimation of seismic hazard at any location is an uncertain science, particularly in low-seismicity regions. However, as our knowledge of the physical characteristics of earthquakes improve, our estimates of the hazard will converge more closely to the actual – but unknowable – (time independent) hazard. Understanding the uncertainties in the estimation of seismic hazard is also of key importance, and new software and approaches allow hazard modellers to better understand and quantify this uncertainty. It is therefore prudent to regularly update the estimates of the seismic demands in our building codes using the best available evidence-based methods and models.

The increasing use of the finite-element method in geotechnical design has raised the question of the compliance of this design approach with Eurocode requirements for the ultimate limit state conditions, especially when a more complex soil constitutive model has been used. Past authors have identified several important issues relating to the application of the finite-element method in ultimate limit state design, including the effects of initial stresses, effects of stress history, choice of soil model, significance of the failure of structural member and the timing when the partial factor of safety is applied during the design assessment. In this paper, an advanced ‘Brick' soil model has been used to demonstrate its application in the design of different geotechnical structures and the effects of the design assumptions used in the design of these structures. The paper also demonstrates the versatility of the Brick soil model in the derivation of the new set of input parameters when the necessary partial factor is applied to the strength of the soil as required in the Eurocode design approach.