Modern seismic codes allow the use of different methods of analysis (from fundamental-mode-method up to a non-linear time history analysis), but too less information is provided for the „average"-user of the codes, which method should be used in a certain case.

It is necessary to carry out case studies for certain structures using different methods of analysis and several variants of the structural model having different degrees of sophistication. The results should be compared and discussed in order to elaborate criteria for the selection of the most adequate model and method for a certain case.

A research project funded by JFÖNB (Austria) was carried out during the last two years. Five existing buildings have been selected for modelling, 3 of them having a rather irregular storey-cross-section. For two of them also insitu - measured eigenfrequencies were available. For each of the buildings first a detailed 3D FE-model was elaborated. Then simplified 2D- and beam models were used and fitted to the results of the detailed 3D model. For one of the regular buildings also a simplified beam model could be found, which was used for the analysis in the non-linear range. For the 3 irregular buildings no satisfying simplified models could be found up to now, since their response is governed by bending plus torsion.

This paper focuses on the advantages and the precision of various methods to model torsion effects. The FEM code ANSYS was used to model the civil engineering faculty building of Tecnical University Bratislava. The high rise building has structural walls and a height of 74 m. First, two variants of the quasi static method are applied. In the classical variant the quasi static forces are applied eccentrically to a beam model with bending and torsion stiffness. For getting realistic displacements the ground stiffness was considered approximately. In the second one the quasi static forces are applied eccentrically to a 3D model having about 11000 DOF´s. These results are compared with the results of response spectrum- and time history analysis obtained from the 3D model.

Time history analysis was carried out using ten artificially generated accelerograms. In addition one recorded set of accelerograms (Friuli, 1978) was adopted to the local conditions and used for the calculations. The time histories are fully compatible with response spectrum EC8 - soil C. In total, the structure was excited by 12 selected pairs of accelerations with duration of 12 s.

For the beam model higher stresses were obtained. Differences in comparison with time history analysis are about 60 % and are caused by the idealisation that the wall elements react independent. The positive effect of cooperating wall elements is neglected. On the other hand the maximum displacements are lower (-14%) because the simplified model does not take into account fully appropriate the soil-structure interaction.

The quasi static method applied on a 3D model gives more accurate results The results are closer to the values of time history analysis (+21% stresses, +11% displacements). The results are even better than the results from response spectrum analysis (+37% stresses, +29% displacements), but this approach makes only sense, if no dynamic module is available for the FE package and the 3D model is needed for static analysis. But this method can be used also for a simple check of the seismic design results.

Simplified methods give satisfying results if the models represent quite well the actual stiffness and the dynamic characteristics. To avoid unnecessary work in the design offices, the codes allow the use of the most simple model, which reflects in an adequate manner the real vibrational behaviour of the structure, but much more research (case studies) will be necessary to elaborate guidelines for the selection of the most adequate procedure.

An efficient transportation system plays a vital role in the development of a modern society, mainly due to the inter-reliance of various industries and the increased trend for out-sourcing of various necessary ingredients within a single activity. Hence, transportation networks are referred to as life-lines, the integrity of which has to be protected alongside water supply, electricity and gas networks. Whilst roads are a most important component of transportation networks, bridges are both more important and sensitive to damage from natural disasters, since roads are more easily repairable and may be also readily by-passed. The closure of a bridge that represents the only or most important link between two areas separated by water or some geological feature (eg. gorges) would potentially cause very severe consequences on industry, commerce and society as a whole. Recent examples abound as to the effects of earthquake damage to bridges, as discussed in subsequent sections. Two examples are quoted herein of the consequences of the closure of the Oakland Bay Bridge on traffic between San Francisco and Oakland (Loma Prieta, 1989) and the closure of several of the crossings between Kobe and Port Island (Hyogo-ken Nanbu, 1995), amongst several others. Not only did such closures affect the communities in the immediate vicinity of the bridge, but it had also knock-on effects on many other communities due to loss of business and delays in delivery of essential goods.

One of the serious problems facing the earthquake engineering community in reducing the earthquake risk to bridges is that whereas a feel for vulnerable parts in buildings and frequently encountered failure modes are common knowledge, engineers in general are less familiar with bridge structures. Therefore, general guidelines for conceptual design of bridges in seismic regions, the relationship between bridges characteristics and anticipated seismic response, and an appreciation of areas of known vulnerability would go a long way towards an increased awareness amongst the civil engineering profession. This in turn is likely to lead to bridge designs that are less sensitive to earthquake damage. This is the main objectives of this lecture, which covers issues of conceptual design of RC bridges, avoiding known features of vulnerability and taking stock of recent field observations following damaging earthquakes.

A brief report is given on damage patterns observed in recent earthquakes, staring from the site condition and through to super-structure distress, giving examples from the field in each case. This is followed by a discussion on the effect of plan and elevation features on anticipated seismic response of RC bridges. The various options of foundation system, foundation-pier connection, lateral resistance system, pier-deck connection and abutment details are presented and their effect on the response of the structure is highlighted.

Seismic assessment of RC bridges requires the accurate calculation of both supply and demand for shear, flexure and axial deformation capacities as well as their interaction. Since the use of full-scale testing is rather restrictive, advanced analysis assumes a role of extreme importance. In the lecture, issues of assessment of supply are addressed, in terms of commenting on model adequacy and input motion characteristics. It is concluded that the inclusion of the foundation system in the model is very important, whilst the accurate evaluation of the soil stiffness characteristics is less important. Methods of selection and scaling of input motion are also discussed and comments on the use of natural earthquakes and their scaling to preserve a consistent energy input are offered. The significance of including the vertical component of earthquake ground motion in analysis is also discussed, in the context of axial and shear supply/demand assessment in the time domain. The models and methods presented are applied to three case studies from Northridge (Collector-Distributor 36 of the Santa Monica Freeway) and Kobe (Hanshin Expressway piers P663, P664 and P665 and the Fukae Honcho structure). It is indicated that the application of sound modelling assumptions and use of appropriate input motion components, coupled with representative response models, provides a powerful tool for unravelling failure modes of RC bridge structures. The lecture is illustrated with sample results of the inelastic dynamic analysis undertaken alongside photographs of the damage the causes of which have corroborated by the analysis.

At periods longer than about 1s, near-fault ground motions are strongly influenced by the earthquake faulting mechanism and orientation of the site. At these periods the ground motions may follow certain radiation patterns, predicted by equivalent double-couple source models, and exhibit distinct long period pulses with amplitudes depending on the orientation of the site with respect to the rupture direction. For cases where the rupture front propagates toward the site and the direction of fault slip is aligned with the site., the so-called "forward rupture directivity effect" is manifested by a long period large amplitude horizontal ground motion pulse normal to the strike of the fault. In the near-fault region of an earthquake the ground motions have distinct pulse-like characters. Empirical evidences of this phenomenon can be observed in the strong motion data from several earthquakes including 1994 Northridge and 1995 Kobe events. Deterministic theoretical predictions of the ground motion can be achieved by convolution of the Green's Functions and the slip function. Green's functions can be calculated through empirical and synthetic means. Although certain predictions can be made for the total slip and the mode of faulting, associated with the DBE, no prediction can be made regarding the rupture characteristics. This necessitates the consideration different rupture models. Such deterministic predictions cannot be extended into the frequency regions above 1Hz, since, high frequency ground motions are controlled by the heterogeneities in the fault rupture, which cannot be a-priori accounted for in a deterministic manner. This requires either the use of stochastic source models or the stochastic treatment of the high frequency components in the ground motions. Based on these considerations a hybrid procedure is developed for the assessment of the time history of the design basis earthquake ground motion, compatible with the stipulated source parameters, for important engineering structures near major faults. The procedure addresses the low and high frequency components of the ground motion separately and than combines the two motions in an intelligent way. The essential elements of the procedure can be listed as follows:

For the synthesis of the low frequency ground motion (DC-1Hz), a discrete wave number approach is used. The wave radiation from the source is decoupled into P-SV and SH motions and treated separately in 3-D wave plane propagation using a propagator-based approach. Analytical expressions, calculated for the displacements due to unidirectional unit impulses, are used to compute the solutions for double couples. These solutions are integrated over the fault area to yield the seismic ground motion. For complementing this motion with frequency components above 1Hz a stochastic simulation procedure is utilised. The stochastic simulation procedure is based on the conformity with theoretical Fourier Amplitude Spectrum and the coda shapes. The basic tenets of the simulation procedure are chosen to:

Examples are provided to illustrate the applications of this procedure are provided.

Three phenomena contribute to the spatial variability of seismic excitations:

These effects appear as loss of coherency and phase delays between signals measured at stations A and B on the surface of the ground. Recent studies (e.g. Der Kiureghian 1996) show that a unified treatment of all the spatial effects is possible through complex coherency terms describing each of the above mentioned phenomenon.

Since mid eighties when the results of SMART-1 array are available research in the field of spatial seismic effects on structures is particularly intensive. Particularly affected by these effects are multi-support structures like bridges or lifelines.

The purpose of the lecture presented during the Seminar was to summarise results of the recent research of the response of multi-support structures to spatial seismic excitations (Der Kiureghian 1996, Zembaty and Krenk 1993, Zembaty 1996, 1997, Zembaty and Rutenberg 1997):

REFERENCES

For the purpose of seismic microzonation, factors controlling structural response during earthquakes may be considered as; earthquake source characteristics and local geotechnical conditions. In the first phase of a microzonation study, earthquake source characteristics representing effects of geology and tectonic formations of the region need to be evaluated. However, effects of these factors are on more macro level and may not be sufficient to explain variations that may exist within relatively short distances. In addition observations during the past earthquakes have shown that due to regional differences each earthquake could possess unique properties representing the local tectonic formations and earthquake source mechanisms.

The second phase of a microzonation study involves investigations to determine the properties of local geotechnical conditions that may be very different due to thickness and properties of soil layers, depth of bedrock and water table. Soil layers as well as modifying properties of earthquake excitations as they pass through, would also be affected by them and could experience significant reductions in stress-strain and shear strength characteristics.

The first alternative to estimate effects of soil layers during a possible earthquake is to utilise empirical relationships developed based on insitu measured soil parameters and site amplification. A more comprehensive alternative is to use numerical models developed for site response analysis to estimate earthquake characteristics on the ground surface. In this situation the results obtained will be directly dependent on the characteristics of the input earthquake motion. Therefore the selection of an appropriate and realistic design earthquake is important in site response studies.

An attempt was made to demonstrate the importance and the variability in these two basic factors controlling microzonation studies based on a case study conducted after 1992 Erzincan Earthquake.

During 1992 Erzincan Earthquake, three main factors (source characteristics, local soil conditions and structural aspects) controlling structural damage were unfavourable for the city of Erzincan. The epicentre of the earthquake was very close to the city and ground accelerations were relatively high. The deep alluvial deposit has led to significant amplifications and most of the reinforced concrete structures were not designed and constructed properly. As a result, large number of the reinforced concrete frame structures collapsed completely or were heavily damaged.

The paper is devoted to problems of applied approaches of seismic analysis and design in relation to simplified or more general structural and seismic input models. The wave theory is presented with reference to non-linear phenomena and complex shear moduli of soils. The attention is concentrated to the effect of multi-component seismic inputs including rotation components. Structural models of symmetrical and asymmetrical structures respond to different excitation. It is pointed out that stress distribution indicates places and points in the structural system where the failure can start. Theoretical analysis is explained on examples where the seismic response is calculated for cases when only translate input components are used in calculations and those with included rotation input components. The response can be obtained in time domain and also using response spectra method. In the simple approach of the response spectra method the rotation response spectra are presented. Comparative case study presents the seismic response of symmetrical and asymmetrical models that were tested in laboratory on shaking table. The calculated and measured seismic responses illustrate the effects that should be expected and considered in the seismic design. The capacity design method according to Eurocode 8 is discussed. Briefly are noted the measures that could increase the seismic resistance.

From conclusions of a paper it is necessary to point out that space seismic response differs from the plane one. Therefore, both structural model and inputs should be built in space mode. Such approach implies changes in natural frequencies and modes of vibration. Different positions of initiative failure regions should be expected in relation to used structural model and inputs. Rotation seismic inputs cause the increase of stresses in edge structural elements. Remarkable torsion effects can appear also in case of symmetrical structures. The increase of stresses is obvious in torsion-translate response of asymmetrical structures. The important tasks arise to pay larger attention to rotation seismic inputs and to spread the response analysis to combined inputs. It should be emphasised the necessity to follow the properties of soil media in time domain. The wave velocities have to be checked in relation to the dimensions of structure foundation, together with the continuous control of seismic resistance capacity in critical parts of structure. Simultaneously, the appropriate changes should be introduced into structural model depending on the degree of non-linear behaviour. The verification of stresses in critical parts gives the answer whether the design is sufficient or it should be modified.

The strengthening strategy for an existing structure has to unavoidably go through a detailed study of the existent. This analysis can be approached in different manners according to the age of the building, its history and the nature of the building, and also the basic hypotheses that preceded the original design.

The engineer who has to analyse it must begin by collecting as much information as possible concerning this history. The research work, in the form of an investigation, is very often than not a difficult task, for it is not easy to find the most important documents, beginning with the drawings, the hypotheses and the calculation notes. One must not only content oneself with the construction conditions: the way in which the building was maintained can also influence the diagnosis.

The most interesting data are those linked to the initial design of the building. The first important aspect concerns regulations and the taking into consideration or not of earthquake protection measures at the start. The second important aspect concerns the calculation methods used: one has more and more important means at hand, and the size of the calculation models increases as computers become more powerful. Nevertheless, a diagnosis should not divert from the initial design : improvement of the methods will most certainly help to have a better appreciation of the capacity of the structure to stand up to earthquakes, and it is therefore beneficial to the diagnosis, but it is important to note that different calculation methods may lead to different designs of the building.

A technique has been elaborated for the re-analysis of important industrial buildings mainly in the case of structures made of R.C. walls. This technique consists in the search at each point of the structure of a safety factor in respect to seismic action, based on a finite element analysis, using the following procedure :

- check the different combinations of static loads ;

- on a given section, form a set of combinations : G_i + l _ij E_j ;

Preliminary conclusions may be reached, based on the previous procedure :

The diagnosis concerning the resistance of a structure is not only connected to the calculation in itself, which although a necessity is not sufficient. It has been known for some time, which accounts for the construction rules in the codes, that some detailing must be respected as well as the calculation rules, and this has to be examined carefully.

To conclude, the evaluation of the resistance of an existing building is generally much more complex than the design of a new building. It is therefore advisable to retain the following points:

The evaluation of seismic risk has attracted great interest in the last few years. The lecture is divided into two parts. The first part introduces a new method for the evaluation of seismic risk of existing reinforced concrete buildings. The method takes into account tens of parameters that characterise the vulnerability of existing R.C. buildings. These have been obtained through observed behaviour during actual earthquakes, codes and theoretical research. The method has been tested using actual cases of buildings that suffered different levels of damage during past earthquakes and it has found very promising.

The second part introduces a new method to evaluate seismic risk of existing urban areas. This situation is more challenging since one is faced with different types of structural systems, materials and actual conditions. The method was applied to Giza city. Different maps showing levels of expected damages under different earthquake scenarios were prepared. Also it can be seen that different strategies for strengthening of existing buildings lead to different levels of damage and loss of lives.