Severe Ground Motions Research Articles

Seismic stability of cracked concrete dams using rigid block models

Several gravity dams subjected to severe ground motions are likely to experience cracking and sliding in the upper section where dynamic amplification is important. A high acceleration spike realistically applies large inertia forces computed from mass times the acceleration. However, these impulsive inertia forces might not be applied in the same direction for a sufficient long period of time to induce significant rotational or sliding displacements detrimental to the seismic or post-seismic structural stability of the cracked components. When it is of interest to estimate residual sliding displacements, a convenient and simple tool is to perform transient rigid body “sliding block” analysis of the “cracked” component. However, this requires the definition of proper seismic input motions at the base of the block with due consideration of dynamic amplification. The possibility to compute in-structure response spectra (ISRS) at the base of the block to define suitable spectra compatible accelerograms is presented in this paper. An important conclusion is that it is not conservative to use accelerograms compatible with the linear (uncracked) dam ISRS to perform transient rigid body sliding response analyses. Dam base and upper joint cracking affects its dynamic properties such that there are modifications of the intensities and frequency content of seismic motions as they propagate over the dam’s height. An envelope of nonlinear ISRS computed from cracked beam models of the dam is recommended to obtain compatible accelerograms and to provide a conservative estimate of upper block residual sliding displacements.

Improved probabilistic quantification of drift demands for seismic evaluation

This study deals with the evaluation and formulation of methods to improve the probabilistic quantification of maximum story drift demands for structural systems exposed to severe ground motions. The focus is on flexible structures that are prone to dynamic instability. The quantification of drift demands is based on the probability distribution of the maximum story drift over the height, for a given ground motion hazard level, which in this study is represented by the spectral acceleration at the first mode period of the building. Improvements in the probabilistic estimation of maximum story drift demands are illustrated with a nine-story, moment-resisting frame building exposed to a set of 40 ground motions. It is concluded that (1) the three-parameter LN distribution more rationally describes maximum story drifts at higher values of spectral acceleration; (2) statistically adding values to replace truncated data points provides better goodness-of-fit in maximum drift demand prediction when the structure is close to the onset of dynamic instability, and (3) the least-squares fitting of the LN distribution yields parameters that provide an improved fit compared to that from maximum likelihood estimation and the method of moments.

Performance of Transportation Systems during the 2004 Niigata Ken Chuetsu, Japan, Earthquake

With the possible exception of the Shinkansen railway, transportation structures for the most part performed well in the Niigata Ken Chuetsu earthquake. There were no collapses. The damage appeared limited and repairable, indicating generally good performance despite the severe ground motions in the epicentral region. The reconnaissance team visited every bridge structure crossing the Uono and Shinano rivers in the epicentral region. All but two of the highway bridges inspected were open for at least limited traffic, and those two appeared to be open for emergency vehicles. Significant damage was observed on the Shinkansen high-speed railway, as well as on some local railway lines, but all appeared repairable. A historic first was the earthquake's derailment of a high-speed Shinkansen train.

The 2003 Bam Urban Earthquake: A Predictable Seismotectonic Pattern along the Western Margin of the Rigid Lut Block, Southeast Iran

“…King Ardeshir Babakan Sassanid [r. AD 224-241], by conquering Kerman and Bam, killed the ‘Kerm-e-Haftvad’ [the Haftvad Silk Worm] at the Bam Citadel. The gigantic worm burst with a big bang noise, which rocked the area, completely destroyed the Bam Citadel, and killed most of the inhabitants of the Citadel. King Ardeshir put an end to the rule of governor Haftvad, built the new village of Kolalan/Kojaran [Kurzan; the old Deh Shotor quarter in west Bam], and brought the ‘seven sacred fires of Bahram’ to the new village…”[Book of Deeds of Ardashir Pabagan 1878 (English Tr., original version ca. AD 272); Tabari 915; Ferdowsi Tusi 1010; & Mostaufi Qazvini 1340. The entire episode rests on the rationalization of historical events of unknown nature, and perhaps the legendary element could be a possible, mixed metaphoric reference to a ‘destructive earthquake’ or even a ‘conquering battle’ against the ancient city of Bam and its Parthian governor, Haftvad!]The impact of the Bam urban earthquake of 26 December 2003 (Mw6.6) was far more devastating than that which would be expected from a moderate-magnitude earthquake. The event followed a predictable geological/seismological pattern of a specially clustered sequence of medium- to large- magnitude earthquakes on tectonically related active faults in a region with historic slip deficits along the western margin of the rigid Lut block. The earthquake was accompanied by the coseismic rupture of sub-parallel strike-slip faults in a zone revealing a pattern of temporal clustering of seismicity, loading of adjacent faults, and a southwards progressing trend of earthquakes from the Kuh Banan to the Gowk and the Bam fault systems. As with the Agadir, Morocco, earthquake of 1960, and the great Tangshan, China, earthquake of 1976, the Bam urban earthquake painfully demonstrated the growing vulnerability of a city built on or adjacent to a seismic fault, unprepared to be tested by the severe ground motion triggered by a medium magnitude earthquake. The absence of historical seismic records regarding the occurrence of earthquakes in the region or the lengthy time spans between such disasters has been erroneously interpreted as a lack of any potential threat for the last 2,500 years in the city of Bam.

Failure study of a bridge subjected to pounding and sliding under severe ground motions

Under strong ground excitations, highway bridge structures may experience severe nonlinear behaviors including the yielding and plastic deformation of pier members, and sometimes the pounding between adjacent decks induced by the local failure of hinge bearings necessary for restraining the girders. A kind of hinge bearing requires the use of steel dowels and is typical in many existing bridges. The objective of this study is to investigate why such hinge bearing did not function and how it contributed to the deck-falling failure of one particular highway bridge stricken by the 1999 Chi–Chi earthquake (magnitude 7.3) in Taiwan. To achieve this, incremental time history analyses for discrete dynamic systems are conducted on a group of models that incorporate sliding and impacting elements, to address nonlinear behaviors as a result of the failed bearing. Causes for discrepancy in bridge responses to seismic motions between the designed and the observed are disclosed by means of comparative study of models based on different scenarios. The study demonstrates that details of the bearing can significantly change overall behaviors of a bridge. In order to ensure the expected performance of a bridge during severe earthquakes, both designer and constructor should exercise caution.

Seismic behavior of concrete filled steel tubular arch structures

Shaking table tests of a 1:10 scale arch model performed to investigate the seismic behavior and resistance of concrete filled steel tubular (CFT) arch structures are described in this paper. The El-Centro record and Shanghai artificial wave were adopted as the input excitation. The entire test process can be divided into three stages depending on the lateral brace configurations, i.e., fully (five) braced, two braces removed, and all braces removed. A total of 46 tests, starting from the elastic state to failure condition, have been conducted. The natural vibration frequencies, responses of acceleration, displacement and strain were measured. From the test results, it is demonstrated that the CFT arch structures are capable of resisting severe ground motions and that CFT arches offer a credible alternative to reinforced concrete arches, especially in regions of high seismic intensity.

NUMERICAL STUDY OF COLLAPSE BEHAVIOR OF STEEL BUILDINGS DUE TO EXTREMELY HIGH SEISMIC LOAD

This paper focuses on the methodology for modeling the dynamic behavior of steel structures due to severe earthquake ground motions. The development of the Improved Applied Element Method for analyzing the entire behavior of large-scale steel structures up to total failure is briefly discussed. The main features of the method are illustrated. The presented case-studies show different collapse mechanisms of moment-resistance steel frame structure under severe ground motions. The results show high capability on simulating the observed damage of many steel structures due to recent earthquakes.

SEISMIC PERFORMANCE OF SLIDING ISOLATION BEARINGS DURING VERTICAL MOVEMENT OF BRIDGE GIRDERS

The behavior of sliding isolation bearings with respect to vertical movement due to earthquake excitation is investigated through experiments and simulations. Bridge girders may undergo vertical motion during severe earthquake events, and since the friction damping provided by sliding isolation bearings is dependent on maintaining contact between the bearing and the girder, there remains an issue as to whether sliding isolation bearings are appropriate for this application. It was shown in this study that multi-layer rubber horizontal load distribution (HLD) devices with steel plate reinforcement are essential for bridge applications, as non-reinforced HLD devices are likely to fail under severe vertical tensile loads. The bearing surface, however, is found to be resilient to impact with an uplifted girder, and simulations demonstrate that contact is maintained with at least one bearing during severe ground motion due to lateral rocking of the girder. The results therefore suggest that appropriately designed sliding isolation bearings are suitable for bridge applications.

SEISMIC BEHAVIOUR OF PERIMETER AND SPATIAL STEEL FRAMES

The present study deals with the seismic performance of partial perimeter and spatial moment resisting frames (MRFs) for low-to-medium rise buildings. It seeks to establish perimeter configuration systems and hence the lack of redundancy can detrimentally affect the seismic response of framed buildings. The paper tackles this key issue by com-paring the performance of a set of perimeter and spatial MRFs, which were “consistently designed”. The starting point is the set of low-(three-storey) and medium-rise (nine-storey) perimeter frames designed within the SAC Steel Project for the Los Angeles, Seattle and Boston seismic zones. Extensive design analyses (static and multi-modal) of the perimeter frame buildings and consistent design of spatial frame systems, as an alternative to the perimeter configuration, were conducted within this analytical study. The objectives of the consistent design are two-fold, i.e. obtaining fundamental periods similar to those of the perimeter frames, i.e. same lateral stiffness under design horizon-tal loads, and supplying similar yield strength. The seismic behaviour of perimeter and spatial configuration structures was evaluated by means of push-over non-linear static analyses and inelastic dynamic analyses (non linear time histories). Comparisons be-tween analysis results were developed in a well defined framework since a clear scheme to define and evaluate relevant limit states is suggested. The failure modes, either local or global, were computed and correlated to design choices, particularly those concerning the strength requirements (column overstrength factors) and stiffness (elastic stability indexes). The inelastic response exhibited by the sample MRFs under severe ground motions was assessed in a detailed fashion. Conclusions are drawn in terms of local and global performance, namely global and inter-storey drifts, beam and column plas-tic rotations, hysteretic energy. The finding is that the seismic response of perimeter and spatial MRFs is fairly similar. Therefore, an equivalent behaviour between the two configurations can be obtained if the design is “consistent”.

Energy Balance Assessment of Base-Isolated Structures

This paper explores the use of energy concepts in the analysis of base-isolated structures subject to severe earthquake ground motions. We formulate the energy balance equations in moving- and fixed-base coordinate frames and provide new physical insight into the time-dependent behavior of individual terms. Conventional wisdom in earthquake engineering circles is that systems with base isolation devices should be economically competitive and designed to: (1) minimize input energy, and (2) maximize the percentage of input energy dissipated by damping and inelastic mechanisms. Through the nonlinear time-history analysis of a base-isolated mass-spring system subject to an ensemble of severe ground motion inputs, we demonstrate that improvements in objective (2) often need to be balanced against increases in input energy. Hence, by itself, objective (1) presents an overly simplified view of desirable behavior.

Evaluation of pounding countermeasures and serviceability of elevated bridges during seismic excitation using 3D modeling

AbstractThis paper conducts elaborate analyses to evaluate the effectiveness of pounding countermeasures and the serviceability of elevated bridges subject to severe ground motions using detailed 3‐dimensional non‐linear modeling of an entire bridge structure system. A three‐span elevated steel bridge is selected for a case study. The peak and residual magnitude of gaps between girders and the maximum shear deformations of bearings are computed and used in the serviceability evaluation. The results show that under proper configurations the mitigation devices work effectively in reducing pounding actions in both the longitudinal and rotational directions. Copyright © 2004 John Wiley & Sons, Ltd.

Seismic Design of Structures by Bundled Columns

A practical technique to protect structures against the effects of earthquakes is examined and proposed for use in design. The technique involves the bundling of slender steel members to form columns that are stiff and strong vertically, yet stable and quite flexible horizontally, leading to significant deamplifications of the main effects of the earthquake excitation. Because of its lateral deformation capacity, the proposed system can be designed to remain elastic even under severe ground motions. A number of analyses are made to identify the main properties of the proposed column system, and the use of the bundled column concept as a base isolation system is also explored. Example problems representative of realistic earthquake-excited structures are reported, elucidating the main features of the proposed technique and the implications of its implementation. The presentation focuses on the fundamentals, avoiding unnecessary modeling complications, to highlight the main factors involved.

Network assesses earthquake potential in Italy's southern Alps

On 6 May 1976, a magnitude 6.4 earthquake struck the Friuli region of northeastern Italy near the towns of Gemona and Venzone. Although it was not as large as some previous earthquakes in Italy, its severe ground motion (up to 0.36 g) affected an area with numerous historical towns, resulting in 989 fatalities and 45,000 people left homeless. At least four other destructive earthquakes with epicentral intensity greater than or equal to IX on the Mercalli‐Cancani‐Sieberg scale have occurred in the Friuli region in the last 5 centuries (1511, 1700, 1794, and 1928) [Slejko et al., 1999].To better understand the seismic hazards of this vulnerable area, a network of continuously operating Global Positioning System (GPS) receivers—the Friuli Regional Deformation Network (FReDNet)—was installed to monitor the regional distribution of crustal deformation (Figure 1).

Towards a direct collapse–load method of design for concrete frames subjected to severe ground motions

AbstractMost current methods of design for concrete structures under earthquake loads rely on highly idealized ‘equivalent’ static representations of the seismic loads and linear‐elastic methods of structural analysis. With the continuing development of non‐linear methods of dynamic analysis for the overload behaviour and collapse of complete concrete structures, a more direct and more accurate design procedure becomes possible which considers conditions at system collapse. This paper describes an evaluation procedure that uses non‐linear dynamic collapse–load analysis together with global safety coefficients. A back‐calibration procedure for evaluating the global safety coefficients is also described. The aim of this paper is to open up discussion of alternative methods of design with improved accuracy which are necessary to move towards a direct collapse–load method of design. Copyright © 2002 John Wiley & Sons, Ltd.

A direct displacement‐based seismic design procedure to fulfill multiple performance objectives

Damage evidence after the Chi‐Chi earthquake highlighted the im‐portance of performance‐based earthquake engineering (PBEE), which stresses the inelastic behavior of structural systems under severe earthquake ground motions. In this paper, a direct displacement‐based seismic design procedure is implemented and demonstrated through an example to fulfill multiple performance objectives in practical design.

Seismic Investigation of Steel Pile Bents: I. Evaluation of Performance

The main objective of this study is to assess the seismic vulnerability of a class of highway bridges existing in certain regions of the eastern and central states, where steel H-piles extends out of the soil to support the pier cap. During severe ground motions, the overall performance of the bridge will be governed by the local performance of the pile-to-cap beam connection. The scope of work was divided into several tasks as follows: (1) a theory was developed to predict the performance of the connection under lateral loading; (2) an initial experimental program was conducted to investigate the seismic behavior of the steel bents; (3) a retrofit strategy is proposed; (4) a second experimental study was carried out to validate the proposed retrofit method; and (5) fragility curves for such structures were developed. This paper deals with the first two tasks of the study. The other three tasks are the subject of a second companion paper (Shama 2002).

Approximate Evaluation of Deflection Amplification Factor

In seismic design provisions, the maximum story drift occurring in major earthquakes is estimated by amplifying the drifts computed from an elastic analysis at the prescribed design seismic force level with a deflection amplification factor. A survey of several seismic design provisions indicates that the deflection amplification factor, in most of the provisions, is independent of significant factors such as ductility ratio, fundamental period, and number of stories. In this paper, a statistical evaluation of deflection amplification factor is presented in order to indicate the parameters that have an effect on this factor. Elastoplastic multidegree of freedom (MDOF) models with various dynamic characteristics are first considered. Consequently, nonlinear and corresponding linear time history analyses for 21 severe earthquake ground motions are performed in order to obtain structural response data, which are then used to derive an empirical formula for the deflection amplification factor. Finally, an app...

An analytical model to predict the inelastic seismic behavior of shear-wall, reinforced concrete structures

The development of an analytical model to predict the inelastic seismic response of reinforced concrete shear-wall buildings, including both the flexural and shear failure modes is presented. The use of shear-wall buildings is quite common in a number of seismic countries as a result of their successful seismic behavior during past severe earthquakes. The objective of this study has been to develop a computer model capable of predicting the seismic behavior of shear-wall buildings. Such model would allow better estimations to be obtained of both the ultimate lateral strength of these buildings as well as their inelastic deformation demand under severe ground motions. Such information may be used in the implementation of performance-based design procedures, and to improve present code design procedures. To fulfill this objective, a shear failure mode model based on experimental results has been added to the computer program larz. This paper discusses the most relevant problems and solutions devised during the development of this model. Validation of the model proposed to predict the inelastic seismic response of shear-wall structures was carried out by comparing its results with the actual response of two real buildings during the March 3, 1985 Chilean earthquake. In spite of the fact that the model is two-dimensional and, hence, it ignores the torsional response, the results obtained are satisfactory.

Nonlinear dynamic response of concrete gravity dams: cavitation effect

A finite element method for the dynamic analysis of concrete gravity dams is presented. Displacement based formulation is used for both fluid and structural domains. During severe ground motion, the impounding fluid in the reservoir may separate from the dam and cause forming of micro bubbles. As a result, the compressibility of water is reduced. This nonlinear phenomenon of the reservoir is termed cavitation. When the direction of the ground motion is changed, the micro bubble's region of fluid collapses, and an impact will occur. By using different damping ratios in the fluid and solid domains the spurious oscillations which were caused by the impact are removed. The cavitation is confined to the upper part of the reservoir, where it has an effect of paramount importance on the tensile stresses. To illustrate the cavitation effect, the response of the non-overflow monolith of the Pine Flat dam subjected to the first 6.5 s of the May 1940 El-Centro, California earthquake, is considered. In order that the cavitation phenomenon take place more widely, maximum acceleration was scaled to give an amplitude of 1 g.

Simulation of near source ground motion and its characteristics

It is fact that the severe ground motions of shear waves have a strong effect on the dynamic behavior of buildings and civil structures. We simulate near source strong motions of a pure shear wave and synthesize small motions, using the parameters based on the recorded accelerograms at the site that is regarded as a base rock in the Osaka basin, Japan. By making use of a stochastic technique, we can easily introduce higher frequency contents in the motions and apply the technique to the synthesis of small waves regarding as a green function. We also introduce to the analysis the useful relationships among the time duration Td, the seismic moment M0, the corner frequency fc and the high cutoff frequency fmax which were regressed by a simple representation scheme. Considering two active faults that may affect severe damage on buildings and civil structures, we try to predict strong ground motions in Osaka basin and show the characteristics of them.