Earthquake Spectra

This paper presents the fundamental issues and policy decisions that formed the basis of the hazard and design aspects of a comprehensive research project conducted for development of the preliminary seismic design criteria for the Kingdom of Saudi Arabia. The study represents the first regionally consistent and systematic collection and critical treatment of the limited historical and instrumental data on earthquakes and seismotectonics of the region, and development of seismic design criteria. The development of the seismic design criteria followed somewhat conservative course, rather than a strict scientific approach, in estimation of the seismic hazard. One of the significant outcome of the study was the preparation of an upto-date and refined earthquake catalog of the region. The catalog contains both, the historical data available in Arabian and English literature, and the data recorded by international and local seismic monitoring agencies. The data were critically treated and superimposed upon the available tectonic information of the Arabian peninsula to obtain a preliminary seismotectonic map of the region. The results of the probabilistic seismic hazard assessment indicate that the highest relative predicted ground-motion occurs near the northwestern border neighboring the Gulf of Aqaba and also near the southwestern border, neighboring Yemen. Based on iso-acceleration map for 10% probability of being exceeded in 50 years, for the seismic design purposes the Kingdom was delineated into four seismic zones. Following UBC format the seismic zones are assigned numbers 0, 1, 2A and 2B. Widely used codes, SEAOC and UBC were selected as the model for development of seismic design recommendations. Selection of the structural system is based on structural performance category which is determined by seismic zone number and the intended occupancy category. R _w -factors for different types of structural system were assigned lower values in consonance with the design and construction practices prevalent in the country. The significance of this study is in its implementation of a global scheme in development of seismic design criteria of a region beset with limited seismic data.

The seismic hazard at the site of a new dam in Saint Lucia, West Indies, is evaluated probabilistically for engineering design purposes. The assessment takes advantage of recent high quality regional seismological data, reappraised older instrumental catalogues and current tectonic interpretations. Saint Lucia stands at a position in the Lesser Antilles island arc where two distinct, seismically active, subduction zones appear to converge at different depths. As a result, the seismic hazard is critically dependent on site position relative to the boundaries of these major source zones, which each exhibit different activity rates. The ground motion hazard at the damsite is computed using the probabilistic logic-tree program PRISK, which accepts weighted input parameter distributions and is also able to model complex source geometries such as those needed to realistically represent a subduction zone. At an exceedance probability of 2×10⁻³ per annum, the expected peak ground acceleration on rock at the site is 0.27g, and the expected peak ground velocity is 12.9 cm/sec. At an exceedance probability of 10⁻⁴ per annum, the expected peak ground acceleration on rock at the site is 0.51g, and the expected peak ground velocity is 29.1 cm/sec. The major contributor to the seismic hazard is a large magnitude earthquake occurring in the northern segment of the Lesser Antilles subduction zone. Sensitivity tests show that the results are stable in the face of rational variations in the seismicity parameters but the hazard values are dependent on the choice of attenuation relationship which, in the absence of local information, has to be adopted from other subduction zone areas. The PGA obtained in this study is markedly higher than the value suggested in current regional code recommendations.

The seismic response of adjacent buildings in series, having similar or different dynamic characteristics, is studied using SDOF systems subjected to a base motion. The phase difference (due to travelling seismic waves) in the starting times of excitation of adjacent structures is taken into account, as well as the changes on response quantities due to pounding. A parametric study is carried out to investigate the influence of the main parameters of the problem. The analysis indicated an increased response of the end structures, as well as of the most rigid structures in the series. The critical problem of the required width of seismic joint to minimise the effect of pounding was studied and found to lie well below that recommended by some modern seismic codes.

This paper explores the development of a method that is useful for design of reinforced concrete (RC) frame structures to resist earthquakes. The substitute structure method, originally proposed in the 1970s, makes an analogy between viscously damped linear and hysteretic response for the purpose of estimating maximum displacement. The evolution of the method is retraced in order to emphasize its unique reliance on experimental results, which are needed to establish rules for assignment of substitute linear properties. Recent dynamic test results are used to extend significantly the calibration of the method, which furnishes design loads on the basis of drift and damage control.

This paper presents the new seismic design specifications for highway bridges issued by the Ministry of Construction in February 1990. Revisions of the previous specifications were based on the damage characteristics of highway bridges that were developed after the recent earthquakes. The primary revised items include the seismic lateral force, evaluation of inertia force for design of substructures considering structural response, checking the bearing capacity of reinforced concrete piers for lateral load, and dynamic response analysis. Emphasis is placed on the background of the revisions introduced in the new seismic design specifications.

Strength reduction factors which permit estimation of inelastic strength demands from elastic strength demands are evaluated. Results from various investigations of strength reduction factors carried out over the last 30 years are reviewed, and their results are presented in a common format which facilitates their comparison. The main parameters that affect the magnitude of strength reductions are discussed. The evaluation of the results indicates that strength reductions are primarily influenced by the maximum tolerable displacement ductility demand, the period of the system and the soil conditions at the site. Simplified expressions of strength reduction factors to estimate inelastic design spectra as functions of these primary-influencing parameters are presented.

Disabled people are considered a high risk group in earthquake emergencies. A simulation experiment was conducted to assess the behavior of disabled occupants in earthquakes. A living room was set-up to show the after-effects of two simulated California earthquakes (low intensity R4.0 and high intensity R6.5) to a building's contents. Seventy-two participants were selected: 24 users of manual wheelchairs, 24 users of powered wheelchairs, and 24 able-bodied persons as a comparison base-line. Participants' movements were video taped and later analyzed based on a number of task performance measures. It was found that the users of manual wheelchairs performed the instructed tasks significantly faster than the users of powered wheelchairs under both low and high intensity simulations. Using a pathway analysis, the disabled groups faced a significantly greater number of obstacles and maneuvered around the objects more than the control group only in the high intensity damage scenario. The users of powered wheelchairs were forced to seek alternate escape routes and found it very difficult to maneuver around especially to carry an emergency bag while exiting (a success rate of 23%). The feasibility and weaknesses of this type of experimental approach is elaborated.

The effects of the gradual buildup of seismic pore water pressures and associated degradation of stiffness on the ground surface motion are examined for shallow saturated liquefiable deposits. Ground surface displacements and accelerations, the Fourier Transforms of the acceleration-time histories and layer gain factors are analyzed. The analysis is based on the ground surface and sub-surface accelerations and pore water pressures recorded at sites that either fully liquefied or built up considerable pore water pressures during strong earthquakes. The analysis shows that: (i) the seismic pore water pressures within the deposit and the ground surface motion are intimately related, (ii) relatively small seismic excess pore water pressures can cause a considerable lengthening of the predominant period of the ground surface motion, and (iii) these phenomena may lead to a limited maximum ground surface acceleration of approximately 0.2g, and a considerable increase of maximum ground displacements.

The authors should be commended for their pioneering research work on structural cladding and energy-dissipating connections. This discussion has been written to help ensure that this research, which is multidisciplinary in nature, leads to practical applications. All structural engineering researchers must recognize that structural cladding design is one of the broadest building-related research topics that can be tackled. They should not focus only on topics that have roots in structural engineering alone, and thus deny the integral role of structural cladding in building design. The importance of a building systems approach must be recognized. In doing so, the role of the architect will become obvious, and a promising collaborative effort will be established.

This discussion is made from the perspective of a background in structural engineering and architectural research and practice.