A comparison among different spectrum compatible earthquake simulation methods

The need for the simulation of spectrum compatible earthquake time histories has existed since earthquake engineering for complicated structures began. More than the safety of the main structure, the analysis of the equipment (piping, racks, etc.) can only be assessed on the basis of the time history of the floor in which they are contained. This paper presents several methods for calculating simulated spectrum compatible earthquakes as well as a comparison between them. As a result of this comparison, the use of the phase content in real earthquakes as proposed by Ohsaki appears as an effective alternative to the classical methods. With this method, it is possible to establish an approach without the arbitrary modulation commonly used in other methods. Different procedures are described as is the influence of the different parameters which appear in the analysis. Several numerical examples are also presented, and the effectiveness of Ohsaki's method is confirmed.

Damage assessment on building structures subjected to the recent near-fault earthquake in Lorca (Spain)

The city of Lorca (Spain) was hit on May 11th 2011 by two consecutive earthquakes with 4.6 and 5.2 Mw respectively, causing casualties and important damage in buildings. Lorca is located in the south-east region of Spain and settled on the trace of the Murcia-Totana-Lorca fault. Although the magnitudes of these ground motions were not severe, the damage observed was considerable over a great amount of buildings. More than 300 of them have been demolished and many others are being retrofitted. This paper reports a field study on the damage caused by these earthquakes. The observed damage is related with the structural typology. Further, prototypes of the damaged buildings are idealized with nonlinear numerical models and their seismic behavior and proneness to damage concentration is further investigated through dynamic response analyses.

Performance of masonry buildings during the 2011 Lorca earthquake

On Wednesday 11th May 2011 at 6:47 pm (local time) a magnitude 5.1 Mw earthquake occurred 6 km northeast of Lorca with a depth of around 2 km. As a consequence of the shallow depth and the small epicentral distance, important damage was produced in several masonry constructions and even led to the collapse of some of them. Pieces of the facades of several buildings fell down onto the sidewalk, being one of the reasons for the killing of a total of 9 people. The objective of this paper is to describe and analyze the failure patterns observed in unreinforced masonry buildings ranging from 3 to 8 floors in height. First, a brief description of the local building practices of masonry buildings is given. Then, the most important failure types of masonry buildings are described and discussed. After that, a more detailed analysis of one particular building is presented.

Performance of buildings with masonry infill walls during the 2011 Lorca earthquake

On Wednesday 11th May 2011 at 6:47 pm (local time) a magnitude 5.1 Mw earthquake occurred 6 km northeast of Lorca with a depth of around 5 km. As a consequence of the shallow depth and the small epicentral distance, important damage was produced in several masonry constructions and even led to the collapse of one of them. Pieces of the facades of several buildings fell down onto the sidewalk, being one of the reasons for the killing of a total of 9 people. The objective of this paper is to describe and analyze the failure patterns observed in reinforced concrete frame buildings with masonry infill walls ranging from 3 to 8 floors in height. Structural as well as non-structural masonry walls suffered important damage that led to redistributions of forces causing in some cases the failure of columns. The importance of the interaction between the structural frames and the infill panels is analyzed by means of non-linear Finite Element Models. The resulting load levels are compared with the member capacities and the changes of the mechanical properties during the seismic event are described and discussed. In the light of the results obtained the observed failure patterns are explained. Some comments are stated concerning the adequacy of the numerical models that are usually used during the design phase for the seismic analysis.

Foreword special issue Lorca’s earthquake

(This is an excerpt from the content) On May 11 2011 at 1705 hours, a small 4.5 Mw. magnitude earthquake struck the town of Lorca in south-eastern Spain. Other than alarmed citizens, only minor damage to buildings occurred due to this quake. Unfortunately at 1847 hours, a second shock registering a magnitude of 5.1 Mw. and very shallow (just around 2 km under the city) produced the largest seismic catastrophe registered in Spain in the last 120 years. This second shock is commonly referred to as “Lorca’s earthquake” and the following papers describe the context, circumstances and consequences of the event. Spain is a country of moderate seismic hazard in a global context. Before the Lorca earthquake, the most destructive earthquake in modern times was the so-called “Andalusian earthquake” (25th December 1884) that resulted in 750 fatalities and more than 1,500 injuries, reaching X in Mercalli’s intensity scale. Despite the lack of catastrophic events in the last 120 years, Spain has always had a scientific interest in seismic ...

Assessment of expected damage on buildings subjected to Lorca earthquake through an energy-based seismic index method and nonlinear dynamic response analyses

The city of Lorca (Spain) was hit on May 11th, 2011, by two consecutive earth-quakes of magnitudes 4.6 and 5.2 Mw, causing casualties and important damage in buildings. Many of the damaged structures were reinforced concrete frames with wide beams. This study quantifies the expected level of damage on this structural type in the case of the Lorca earth-quake by means of a seismic index Iv that compares the energy input by the earthquake with the energy absorption/dissipation capacity of the structure. The prototype frames investigated represent structures designed in two time periods (1994–2002 and 2003–2008), in which the applicable codes were different. The influence of the masonry infill walls and the proneness of the frames to concentrate damage in a given story were further investigated through nonlinear dynamic response analyses. It is found that (1) the seismic index method predicts levels of damage that range from moderate/severe to complete collapse; this prediction is consistent with the observed damage; (2) the presence of masonry infill walls makes the structure very prone to damage concentration and reduces the overall seismic capacity of the building; and (3) a proper hierarchy of strength between beams and columns that guarantees the formation of a strong column-weak beam mechanism (as prescribed by seismic codes), as well as the adoption of counter-measures to avoid the negative interaction between non-structural infill walls and the main frame, would have reduced the level of damage from Iv=1 (collapse) to about Iv=0.5 (moderate/severe damage)

Moldels for reproducing the damage scenario of the Lorca earthquake

A damage scenario modelling is developed and compared with the damage distribution observed after the 2011 Lorca earthquake. The strong ground motion models considered include five modern ground motion prediction equations (GMPEs) amply used worldwide. Capacity and fragility curves from the Risk-UE project are utilized to model building vulnerability and expected damage. Damage estimates resulting from different combinations of GMPE and capacity/fragility curves are compared with the actual damage scenario, establishing the combination that best explains the observed damage distribution. In addition, some recommendations are proposed, including correction factors in fragility curves in order to reproduce in a better way the observed damage in masonry and reinforce concrete buildings. The lessons learned would contribute to improve the simulation of expected damages due to future earthquakes in Lorca or other regions in Spain with similar characteristics regarding attenuation and vulnerability.

Using damage from 2010 Haiti earthquake for vulnerability estimation of typical structures in Port-au-Prince (Haiti).

After the 2010 Haiti earthquake, that hits the city of Port-au-Prince, capital city of Haiti, a multidisciplinary working group of specialists (seismologist, geologists, engineers and architects) from different Spanish Universities and also from Haiti, joined effort under the SISMO-HAITI project (financed by the Universidad Politecnica de Madrid), with an objective: Evaluation of seismic hazard and risk in Haiti and its application to the seismic design, urban planning, emergency and resource management. In this paper, as a first step for a structural damage estimation of future earthquakes in the country, a calibration of damage functions has been carried out by means of a two-stage procedure. After compiling a database with observed damage in the city after the earthquake, the exposure model (building stock) has been classified and through an iteratively two-step calibration process, a specific set of damage functions for the country has been proposed. Additionally, Next Generation Attenuation Models (NGA) and Vs30 models have been analysed to choose the most appropriate for the seismic risk estimation in the city. Finally in a next paper, these functions will be used to estimate a seismic risk scenario for a future earthquake.

Study of seismic vulnerability and the damage seen in the town of Lorca after the earthquake of 2011.

Análisis de los factores de vulnerabilidad que mas influencia han tenido en el daño del terremoto de Lorca.

Impact And Explosive Loads On Concrete Buildings Using Shell And Beam Type Elements

The threat of impact or explosive loads is regrettably a scenario to be taken into account in the design of lifeline or critical civilian buildings. These are often made of concrete and not specifically designed for military threats. Numerical simulation of such cases may be undertaken with the aid of state of the art explicit dynamic codes, however several difficult challenges are inherent to such models: the material modeling for the concrete anisotropic failure, consideration of reinforcement bars and important structural details, adequate modeling of pressure waves from explosions in complex geometries, and efficient solution to models of complete buildings which can realistically assess failure modes. In this work we employ LS-Dyna for calculation, with Lagrangian finite elements and explicit time integration. Reinforced concrete may be represented in a fairly accurate fashion with recent models such as CSCM model [1] and segregated rebars constrained within the continuum mesh. However, such models cannot be realistically employed for complete models of large buildings, due to limitations of time and computer resources. The use of structural beam and shell elements for this purpose would be the obvious solution, with much lower computational cost. However, this modeling requires careful calibration in order to reproduce adequately the highly nonlinear response of structural concrete members, including bending with and without compression, cracking or plastic crushing, plastic deformation of reinforcement, erosion of vanished elements etc. The main objective of this work is to provide a strategy for modeling such scenarios based on structural elements, using available material models for structural elements [2] and techniques to include the reinforcement in a realistic way. These models are calibrated against fully three-dimensional models and shown to be accurate enough. At the same time they provide the basis for realistic simulation of impact and explosion on full-scale buildings

Problemas de interacción suelo estructura en cimentaciones y muros de contención. Influencia de los fenómenos de despegue y deslizamiento

Los terremotos constituyen una de las más importantes fuentes productoras de cargas dinámicas que actúan sobre las estructuras y sus cimentaciones. Cuando se produce un terremoto la energía liberada genera movimientos del terreno en forma de ondas sísmicas que pueden provocar asientos en las cimentaciones de los edificios, empujes sobre los muros de contención, vuelco de las estructuras y el suelo puede licuar perdiendo su capacidad de soporte. Los efectos de los terremotos en estructuras constituyen unos de los aspectos que involucran por su condición de interacción sueloestructura, disciplinas diversas como el Análisis Estructural, la Mecánica de Suelo y la Ingeniería Sísmica. Uno de los aspectos que han sido poco estudiados en el cálculo de estructuras sometidas a la acciones de los terremotos son los efectos del comportamiento no lineal del suelo y de los movimientos que pueden producirse bajo la acción de cargas sísmicas, tales como posibles despegues y deslizamientos. En esta Tesis se estudian primero los empujes sísmicos y posibles deslizamientos de muros de contención y se comparan las predicciones de distintos tipos de cálculos: métodos pseudo-estáticos como el de Mononobe-Okabe (1929) con la contribución de Whitman-Liao (1985), y formulaciones analíticas como la desarrollada por Veletsos y Younan (1994). En segundo lugar se estudia el efecto del comportamiento no lineal del terreno en las rigideces de una losa de cimentación superficial y circular, como la correspondiente a la chimenea de una Central Térmica o al edificio del reactor de una Central Nuclear, considerando su variación con frecuencia y con el nivel de cargas. Finalmente se estudian los posibles deslizamientos y separación de las losas de estas dos estructuras bajo la acción de terremotos, siguiendo la formulación propuesta por Wolf (1988). Para estos estudios se han desarrollado una serie de programas específicos (MUROSIS, VELETSOS, INTESES y SEPARSE) cuyos listados y detalles se incluyen en los Apéndices. En el capítulo 6 se incluyen las conclusiones resultantes de estos estudios y recomendaciones para futuras investigaciones. ABSTRACT Earthquakes constitute one of the most important sources of dynamic loads that acting on structures and foundations. When an earthquake occurs the liberated energy generates seismic waves that can give rise to structural vibrations, settlements of the foundations of buildings, pressures on retaining walls, and possible sliding, uplifting or even overturning of structures. The soil can also liquefy losing its capacity of support The study of the effects of earthquakes on structures involve the use of diverse disciplines such as Structural Analysis, Soil Mechanics and Earthquake Engineering. Some aspects that have been the subject of limited research in relation to the behavior of structures subjected to earthquakes are the effects of nonlinear soil behavior and geometric nonlinearities such as sliding and uplifting of foundations. This Thesis starts with the study of the seismic pressures and potential displacements of retaining walls comparing the predictions of two types of formulations and assessing their range of applicability and limitations: pseudo-static methods as proposed by Mononobe-Okabe (1929), with the contribution of Whitman-Liao (1985), and analytical formulations as the one developed by Veletsos and Younan (1994) for rigid walls. The Thesis deals next with the effects of nonlinear soil behavior on the dynamic stiffness of circular mat foundations like the chimney of a Thermal Power Station or the reactor building of a Nuclear Power Plant, as a function of frequency and level of forces. Finally the seismic response of these two structures accounting for the potential sliding and uplifting of the foundation under a given earthquake are studied, following an approach suggested by Wolf (1988). In order to carry out these studies a number of special purposes computer programs were developed (MUROSIS, VELETSOS, INTESES and SEPARSE). The listing and details of these programs are included in the appendices. The conclusions derived from these studies and recommendations for future work are presented in Chapter 6.

Structural behaviour and design criteria of under-deck cable-stayed bridges subjected to seismic action

Under-deck cable-stayed bridges are very effective structural systems for which the strong contribution of the stay cables under live loading allows for the design of very slender decks for persistent and transient loading scenarios. Their behaviour when subjected to seismic excitation is investigated herein and a set of design criteria are presented that relate to the type and arrangement of bearings, the number and configuration of struts, and the transverse distribution of stay cables. The nonlinear behaviour of these bridges when subject to both near-field and far-field accelerograms has been thoroughly investigated through the use of incremental dynamic analyses. An intensity measure that reflects the pertinent contributions to response when several vibration modes are activated was proposed and is shown to be effective for the analysis of this structural type. The under-deck cable-stay system contributes in a very positive manner to reducing the response when the bridges are subject to very strong seismic excitation. For such scenarios, the reduction in the stiffness of the deck because of crack formation, when prestressed concrete decks are used, mobilises the cable system and enhances the overall performance of the system. Sets of natural accelerograms that are compliant with the prescriptions of Eurocode 8 were also applied to propose a set of design criteria for this bridge type in areas prone to earthquakes. Particular attention is given to outlining the optimal strategies for the deployment of bearings

The direct boundary element method: 2D site effects assessment on laterally varying layered media (methodology)

The Direct Boundary Element Method (DBEM) is presented to solve the elastodynamic field equations in 2D, and a complete comprehensive implementation is given. The DBEM is a useful approach to obtain reliable numerical estimates of site effects on seismic ground motion due to irregular geological configurations, both of layering and topography. The method is based on the discretization of the classical Somigliana's elastodynamic representation equation which stems from the reciprocity theorem. This equation is given in terms of the Green's function which is the full-space harmonic steady-state fundamental solution. The formulation permits the treatment of viscoelastic media, therefore site models with intrinsic attenuation can be examined. By means of this approach, the calculation of 2D scattering of seismic waves, due to the incidence of P and SV waves on irregular topographical profiles is performed. Sites such as, canyons, mountains and valleys in irregular multilayered media are computed to test the technique. The obtained transfer functions show excellent agreement with already published results.

Pounding force assessment in performance-based design of bridges

Bridges with deck supported on either sliding or elastomeric bearings are very common in mid-seismicity regions. Their main seismic vulnerabilities are related to the pounding of the deck against abutments or between the different deck elements. A simplified model of the longitudinal behavior of those bridges will allow to characterize the reaction forces developed during pounding using the Pacific Earthquake Engineering Research Center framework formula. In order to ensure the general applicability of the results obtained, a large number of system parameter combinations will be considered. The heart of the formula is the identification of suitable intermediate variables. First, the pseudo acceleration spectral value for the fundamental period of the system (Sa(Ts)) will be used as an intensity measure (IM). This IM will result in a very large non-explained variability of the engineering demand parameter. A portion of this variability will be proved to be related to the relative content of high-frequency energy in the input motion. Two vector-valued IMs including a second parameter taking this energy content into account will then be considered. For both of them, a suitable form for the conditional intensity dependence of the response will be obtained. The question of which one to choose will also be analyzed. Finally, additional issues related to the IM will be studied: its applicability to pulse-type records, the validity of scaling records and the sufficiency of the IM.

Simplified computational method for non-linear seismic analysis of bridges

After the experience gained during the past years it seems clear that nonlinear analysis of bridges are very important to compute ductility demands and to localize potential hinges. This is specially true for irregular bridges in which it is not clear weather or not it is possible to use a linear computation followed by a correction using a behaviour factor. To simplify the numerical effort several approximate methods have been proposed. Among them, the so-called Dynamic Plastic Hinge Method in which an evolutionary shape function is used to reduce the structure to a single degree of freedom system seems to mantein a good balance between accuracy and simplicity. This paper presents results obtained in a parametric study conducted under the auspicies of PREC-8 european research program.