Seismic Behaviour and Design of Irregular and Complex Civil Structures III

Parametric analysis of the effect of soil compliance on the response of a slender tower to combined horizontal and rotational (rocking) of induced ground motion excitations is carried out. In contrast to earlier analyses which use theoretical decomposition of seismic wave field to obtain rotations, this study is using 6 degree of freedom ground motion records to carry on time history integration leading to structural response accounting for “true” phase interactions between horizontal and rocking excitations. The analysis leads to conclusion that the more flexible the sub-soil the smaller the response, however this effect is better pronounced for rotational than horizontal excitations. It was also demonstrated that regardless of the soil compliance, rotational component can have either increasing or decreasing influence on the structural internal forces. In the analysed cases of two sets of excitations such different results were obtained. Clearly more credible rotational-horizontal records of seismic strong ground motion are needed.

The present study was designed to demonstrate the importance of base-slab averaging and embedment effects on the foundation-level input motions due to earthquake excitations. Evaluation of foundation-level input motions based on the most commonly adopted kinematic interaction models are presented. In order to conduct this investigation, original records of horizontal accelerations for two case-study buildings were utilized. Computed foundation-level input motions, in both NS and EW directions, were compared to the actual acceleration-time histories recorded at the foundation levels. The results clearly indicate that incorporating base-slab averaging and embedment effects in seismic analyses can modify the dynamic excitation imposed at the foundation level, and, as a consequence, lead to more accurate structural response due to earthquake ground motions.

Soil-steel bridges and culverts, typically ranging from 3 to 25 m, can be used as an effective alternative for short-span bridges. They can meet the design and safety requirements of traditional bridges but at lower costs and with shorter erection time. For these reasons, soil-steel bridges are more and more often used in road and railway projects in many parts of the world. The purpose of present analysis is more advanced. Respective FEM models of a large soil steel bridge were prepared and eigen problem solved applying SAP 2000 and DIANA programs. Next the dynamic response was computed using time history response analysis with El Centro 1940 record. Two basic cases of seismic loads were analysed, i.e. “XZ” and “YZ” (seismic excitations were induced simultaneously at directions: transversal (X) and vertical (Z), and longitudinal (Y) and vertical (Z)). The totally different way of response of the large soil-steel bridges along and perpendicular to them generates key irregularity effect to study in their seismic response. Soil-steel interaction is also considered using special interface elements. Displacements and bending moments of the bridge are analysed in detail.

This chapter presents a methodology for designing irregular 3D RC frame buildings by minimizing the total steel volume while satisfying inter-story drifts and material strains limits. Indirectly, this process leads to reduction on the base shear and over-turning moments. The methodology relies only on analysis tools, without any need for knowledge or tools of structural optimization. Although iterative, the methodology requires only a few iterations for convergence. This makes the methodology very attractive for practical use.

The design or evaluation of high complexity structures requires non-traditional methods to estimate their seismic performance. Although structural engineering still relies on nonlinear static analysis for estimating seismic demands, the nonlinear response history analysis (RHA) is being now increasingly used for design-check and performance evaluation. In this approach, the engineering demand parameters are determined by performing a series of nonlinear RHAs using an ensemble of ground motion records that represent the site’s seismic hazard conditions. This type of analysis is computationally demanding and time consuming when applied to three-dimensional computer models subjected to multi-axial excitations. A procedure is presented to reduce the processing time of nonlinear RHA by minimizing the impact on the response of the structure while maximizing the time savings; it has three steps: (i) trimming a segment of the ground-motion record at the beginning; (ii) trimming a segment at the end; (iii) identifying an appropriate time-step to reduce the number of steps required to properly characterize the signal. This procedure has been tested through a parametric study by considering multiple structural periods and response modification factors for multi-story irregular buildings. Results show that the processing time can be reduced without compromising accuracy in estimates of peak EDPs.

In this paper, the seismic performance of a reinforced concrete dual-system building with vertical irregularities, built in the 60s in Lisbon, is addressed. The seismic behaviour and the torsional effects of the building are investigated by means of nonlinear static (pushover) analyses and extended N2 method. Then the results are compared with the nonlinear dynamic Time-History analysis, the latter considered as the reference solution. A three-dimensional numerical model of the case-study building is developed to account for torsion in the building. The evaluation of the seismic vulnerability of structure is based on performance-based assessment procedures and on the structural safety requirements proposed in Part 3 of Eurocode 8. The main targets of this study are (i) to detect and quantify the main deficiency of this typology of old existing RC buildings, which were not designed to resist the forces induced by torsional vibrations; (ii) to propose suitable retrofitting intervention for this category of buildings.

Nonlinear static analysis is currently the most popular method of analysis for the prediction of the seismic response of buildings. However, when dealing with in-plan irregular buildings, the nonlinear static analysis does not allow a proper assessment of the torsional response. In particular, the torsional response modifies the displacement demand of the two sides of the deck with respect to that of the corresponding torsionally balanced system: the displacement demand of the flexible side increases, while that of the stiff side decreases or increases depending on the features of the building. In this paper, the ratio of the maximum displacements of the asymmetric system to the maximum displacement of the corresponding planar system is determined for a large set of single storey systems. The results of this investigation are used to populate a database for the assisted assessment of buildings. This database can be interrogated by means of parameters characterizing the building to be assessed and provides the user with the amplification/deamplification of displacement demand of the two sides of the building with respect to that of the corresponding planar system. The database is conceived in such a way to include new cases without modifying its structure. This allows the database to be easily expanded when new parameters will be explored and new results will be available.

In this paper, the Mode-Adaptive Bi-directional Pushover Analysis (MABPA) previously presented by the author is modified by considering the higher (third) mode response. The modifications proposed in this paper are (a) the peak response of the third mode is estimated from the independent single-degree-of freedom (SDOF) model representing the third mode, and (b) the peak response of each frame is predicted by combining the results from the original MABPA and the third mode response using the square root of the sum of square (SRSS) rule. The modified MABPA is applied to two six-story asymmetric building models with bidirectional setback. The predicted results are compared with time-history analysis results and other simplified procedures; modal pushover analysis (MPA), improved modal pushover analysis (IMPA), a variant of MPA considering bidirectional excitation by (Manoukas G, Athanatopoulou A, Avramidis I, Soil Dyn Earthq Eng 38:88–96, 2012) and (Manoukas G, Avramidis I, Bull Earthq Eng 12(6):2607–2632, 2014), and the original MABPA. The results show that the predicted peak response at “flexible-edge” frame by each simplified procedure agree well with the time-history analysis results. On the contrary, the predicted peak response at the “stiff-edge” frame according to each simplified procedure is different: some of the predicted results, including the results via original MABPA, underestimate the time-history analysis results, while the results via modified MABPA are conservative compare to the time-history analysis results.

Masonry infill panels are commonly used in low and mid-rise RC buildings. Under earthquake loadings however, the frame/panel interaction may have significant effects on the structural behaviour of a building. In particular, strong asymmetries due to the presence of masonry infills can lead to a reduction of the building strength under combined vertical and horizontal loading. A four stories frame structure with reinforced concrete members and masonry infills is analysed for horizontal earthquake loading using numerical models. Nonlinear pushover analyses are carried out for a range of alternative configurations. The first configuration refers to the bare frame structure, while the others involve symmetrical and asymmetrical positions for masonry enclosure infill walls interacting with the frames. The infills are modelled as pin joined diagonal frames. Two situations are considered: full infill panels and panels with window openings. The seismic performance of different configurations is analysed. Conclusions are drawn about the influence of structural asymmetries on the capacity of the building. Results show that asymmetric (in plane and/or elevation) masonry enclosure walls can lead to a significant reduction of a building’s capacity to withstand earthquake loading. Soft-story configurations are particularly dangerous.

Initial shear modulus (G0) is an important soil parameter used in geotechnical and earthquake engineering, when modelling geotechnical structures using advanced models for the soil behaviour or when modelling the soil-structure interaction in seismic analysis. Starting from the theory of wave propagation, it is known that this parameter is obtained through in situ or laboratory tests, by measurements of the shear wave velocity of the soil. This paper presents an analysis of the initial shear modulus values obtained by seismic tests in depth, carried out during the geotechnical investigations of several projects developed by the authors in Bucharest area. The results obtained from these tests were related to the stratigraphy on site, to relate the values obtained with the geological stratums typical for the city. The further performed analysis was approaching the influence of adopting different values for the initial shear modulus, within the interval obtained from the abovementioned investigations on the behaviour of retaining walls for deep excavations in the urban area of Bucharest. The analysis of the behaviour was performed through Finite Element Method, using the hyperbolic constitutive law with account of the initial shear modulus for the soil. The results obtained in terms of efforts and deformations for different values of initial shear modulus, keeping the rest of the parameters constant, are then presented and discussed. Also, the results are compared to measurements of the deformations carried out in inclinometer casings installed in diaphragm walls and in ground extensometers installed in the ground, considering real cases.

The center pillars of the common double box tunnel of subways have been investigated and their seismic performance has been verified in practice by taking advantage of the lesson of collapsed Daikai station in 1995. However, these verified box tunnels are either horizontally-arranged or vertically-arranged box culvert, and the seismic performance of the transition between these box culverts have not yet been examined. In this study, the failure mode of these uneven tunnels and its characteristic shape are analyzed, and the seismic performance regarding the safe internal spaces and the effects of cross-section shape is discussed.

In the paper, the Pushover-based Risk Assessment (PRA) method is summarized and applied to the estimation of the “failure” probability of two reinforced concrete frame buildings. The first building is a modified version of the well-known SPEAR building, designed according to Eurocode 8. Although the building is asymmetrical, the influence of torsion is moderate, so the building can be considered as a representative of regular buildings. The second building has the same structural layout, but has infill walls included only in the upper stories, which induce irregularity along the height of the building. The comparison of the “failure” probabilities obtained for the two examples indicates a lower, although still significant, seismic risk for the regular code-conforming variant of the building (0.75% over the lifetime of the structure). A three to five times larger “failure” probability is obtained for the irregular variant of the building, for which a soft first storey effect is predicted.

Nonlinear static analysis is many times chosen, in engineering practice, to predict the seismic demands in building structures. Despite its simplicity, the nonlinear static analysis based on invariant load patterns has certain limitations caused by its inability to account for the variation of the dynamic characteristics, of the building structure, resulting from inelastic behavior and the higher modes effect. The paper presents a comparative study on three building structures, for which were made adaptive and nonadaptive nonlinear static analyses and incremental nonlinear time history analyses were performed. The analyzed buildings structures have elevation irregularity, except for the smallest which is regular. The capacity curves, resulting from nonlinear static analyses were compared with the mean capacity curve resulting from incremental nonlinear time-history analyses. The study has shown that the adaptive nonlinear static analysis, with displacements load pattern, caught, with enough accuracy, the behavior of irregular building structures of low and medium height. The nonlinear static analyses may not sufficiently predict the seismic demand for the tallest building structure, having deep elevation irregularity.

The paper presents a study on a construction, erected in the 1870–1920 period, composed of two interconnected buildings. The main A building is reserved to living, having a basement, ground floor, one story, penthouse and a loft. The building is approximately rectangular with a dead wall on the North side property limit, being connected with the B building on the South side. Located on the South, the B building is an exterior staircase with a terrace floor and a penthouse, being an unique element as conception in the beginning twentieth century architecture of Bucharest. The construction is not classified as a historical monument, but is placed in the protected area of a historical monument of class B (LMI 2015: B-II-m-B-19768). The construction has several deficiencies of which the most important is the differential settlement of about 25 cm in transversal direction, respectively 10 cm in the longitudinal direction. The irregular shape, because of the link between the two buildings, leading to major torsional effects and the presence of some structural walls, supported directly by the floors, are other deficiencies of the construction. The seismic evaluation was carried out by linear static and dynamic analyses, taking into account the second order effect. There were proposed strengthening and straightening solutions for the construction. For building strengthening seismic isolation method was adopted, reducing drastically the torsion effects and the efforts in the structural elements. Building straightening was achieved by means of compensation presses placed at the basement level.

The objective is to validate the conclusions of a study about the nonlinear behaviour of the asymmetrical RC structural wall systems that was based on the one storey equivalent model time history response. The main instrument of verification is the time history nonlinear response of the full 3D multistorey model. The time history analysis was conducted using Perform 3D software. Three structural types are selected for this study: type I (strong torsionally restrained, Ωθ = 1.4) corresponding to the most common structural wall type structure in Romania; type VI – the central core structure with perimeter frames along one main direction; and type III the central core structure without perimeter frames (Ωθ = 0.3). The parameters which were varied during the study are: (a) stiffness to mass center eccentricity, and (b) q –behavior factor. Nine spectrum compatible accelerograms were used. The seismic characteristics were the ones corresponding to Bucharest. The results were judged in function of two response parameters: (1) R – safety factor, the minimum capacity to demand displacement corresponding to a vertical element and (2) R1 – the drift amplification of the floor extremities due to torsion relative to pure translation. The main conclusions to be validated are:1.If the substantial decrease of the q behaviour factor accounted for torsionally flexible structures has a significant beneficial effect on the safety factor.2.If the perimeter frames have a beneficial effect for the behaviour of the central core structure and for the suitability of this type of structure for seismic areas.

The design of structures subjected to torsional effects is highly complex, and becomes more difficult to assess when elevation irregularity is also present. This paper has as main objective to present the particularities of design for a plan as well as elevation irregular structure (11.0 × 15.0 m layout and 15.50 m total height), built in Bucharest (corner period of 1.6 s and high displacement demand). Architectural considerations, owners wishes and structural needs join together in a complex layout: uneven in plane distribution of vertical earthquake resistant structural elements (RC walls and frames); eccentric vertical stair case circulation (coupled RC walls); one structural wall present only at ground level; at the last level the layout shrinks to the staircase circulation perimeter. In order to meet the code requirements the design had to be based on modal analysis because of the interference of translational and torsional movements. As a praxis suitable computation tool static nonlinear analysis was performed (using the SAP software, CSI analysis reference manual, University Avenue. Computers and Structures Inc., Berkeley, 1995. RC walls are modeled as frame elements with point hinges. The structural performance is evaluated in terms of structural displacements (in the center of mass and on the floor edges). The order of plastic hinge formation and their damage degree is traced. The efficiency of the chosen structural layout with respect to the reduction of general torsion effects is pointed out and the behaviour factor option is outlined. Linear dynamic analysis is performed also, provididng an upper bound for the expected displacement amplification in the nonlinear range of behaviour.

In this work the role of the effective concrete strength on the seismic assessment of RC buildings has been investigated. The concrete strength has been described after a wide investigation on the structural elements of a case-study, i.e. an existing building located in Sansepolcro (Italy), currently used as a Hospital. Alternative representations of the concrete strength distribution have been compared, consisting respectively of: assuming a uniform strength distribution for the entire structure (as required by the International Codes), assuming a mean storey strength at each storey and assigning to each tested member the strength value found by the experimental survey.The seismic performance of the building has been found with reference to three different limit states, by representing the structural response through a nonlinear static analysis and comparing the maximum chord rotation and shear stress to the limit values corresponding to the assumed concrete strength of each member. The effects of the actual distribution of the concrete strength have been finally checked by comparing the seismic performance obtained through the three different assumptions for concrete strength distribution.

The European seismic code 8 (Eurocode 8) classifies buildings as plan-wise regular according to four criteria which are mostly qualitative and a fifth one which is based on parameters such as stiffness, eccentricity and torsional radius that can be only approximately defined for multi-story buildings. Therefore, such plan-regularity criteria need to be improved. ASCE seismic code, according to a different criterion, considers plan irregularity when the maximum story drift, at one end of the building structure, exceeds more than 1.2 times the average of the story drifts at the two ends of the structure under building static analysis. Nevertheless, both the ASCE approach and the threshold value of 1.2 need to be supported by adequate background studies, based also on nonlinear seismic analysis. In this paper a numerical analysis is carried out, by studying the seismic response of an existing r.c. school building. Static analysis is developed by progressively shifting the centre of mass, until the ratio between the maximum lateral displacement of the floor at the level considered and the average of the horizontal displacements at extreme positions of the floor at the same level matches and even exceeds the value of 1.2. Then, nonlinear dynamic analyses are carried out to check the corresponding level of response irregularity in terms of uneven plan distribution of deformation and displacement demands and performance parameters. The above comparison leads to check the suitability of the ASCE approach and, in particular, of the threshold value of 1.2 for identifying buildings plan irregularity.

For single-storey buildings, EN 1998-1 allows determination of the centre of lateral stiffness and the torsional radius by considering the translational stiffness represented by the moments of inertia of the cross-section of the vertical elements. Additionally, stiffness of the beam or shear deflections can affect the frame lateral stiffness changing the position of centre of stiffness or torsional radius. In order to investigate the influence of these two parameters, six different single storey buildings with different degree of plan irregularity were examined. All of these buildings were analysed for four different modelling assumptions, regarding the stiffness characteristic of the structural elements. Additionally, hand calculation of these structural features, according the recommendation of EN 1998-1 was carried out. Obtained results show that certain parameters have significant influence for structures with lower degree of plan irregularity, while some other parameters are more influential for structures with higher degree of irregularity. Hand calculated values for eccentricity and torsional radius are most conservative compared to the corresponding ones obtained from numerical analysis. Applied criteria for consideration of structural regularity in plan, prescribed in EN 1998-1, are compared with the criteria given by ASCE 7-10 and NZS 1170.5-2004. In order to compare the seismic response of considered single storey structures with different degree of irregularity in plan, detailed nonlinear time history analyses for seven different acceleration histories, applied in direction perpendicular to the axes of irregularity, scaled to three different levels of seismic hazard, were performed.

The objective of the present study is to investigate the importance of soil-structure interaction effects on the seismic response of a high-rise irregular reinforced-concrete residential building. In order to conduct this research, a detailed three-dimensional structure model was subjected to various earthquake excitations, also including a strong mining tremor. Soil-foundation flexibility was represented using the spring-based solutions, incorporating foundation springs and dashpots. For each soil type analyzed in this study, the foundation stiffness was calculated using the static stiffness, embedment correction factors, and dynamic stiffness modifiers. The influence of diverse soil conditions (represented by their average effective profile velocities and shear moduli) on the dynamic characteristics (e.g. fundamental vibration period) and seismic response (e.g. peak lateral accelerations) of the structure model was investigated and discussed. The numerical analysis results clearly demonstrate that the seismic performance of the building to the strong earthquake shaking can be significantly affected by the soil-structure interaction effects.

The objective of the present paper is the determination of the optimum torsion axis of multi-storey asymmetric in plan buildings on the basis of their dynamic properties. For this purpose, a three-storey reinforced concrete diaphragm system is analyzed by means of linear time-history analysis for both uniaxial and biaxial horizontal seismic excitations. The mass centers of the diaphragms are successively transposed and the resulting floor rotation angles for each case are computed. The position of the mass center which leads to the minimization of the sum of the squares of the floor rotation angles designates the location of the optimum torsion axis. The results are verified by means of modal analysis and compared with those resulting from the relevant methodology prescribed by the Greek seismic code. All three methods produce results that do not differ significantly, so the approximate procedure suggested by the Greek seismic code can be rigorously applied in order to determine the optimum torsion axis of asymmetric buildings.

Local site conditions generate large amplifications as well as spatial variations in the seismic motions that must be accounted for in the earthquake resistant design of structures. The present paper aims to evaluate the influence of complex site effects on the non-linear response of irregular buildings. To achieve this purpose, site dependent ground motions are produced via Boundary Element Method (BEM). An ensemble of nine earthquakes recorded at the outcropping rock are considered as an input at the seismic bed of complex geological profiles, and acceleration time histories at the ground surface are computed. Several complex geological configurations are considered, taking into account the following key parameters: (i) canyon topography, (ii) layering and (iii) material gradient effect. Two 5-storey buildings are considered: a symmetric and an asymmetric in plan building. A series of Nonlinear Time History Analyses are conducted. The results of this study demonstrate that the presence of local site conditions influence the inelastic dynamic response of buildings.

One of the most common structural systems in earthquake prone areas are Reinforced Concrete (R/C) buildings with masonry infills. The observation of post-earthquake damages has led to conclusion that the masonry infills can greatly modify the seismic performance of these buildings. In the context of the direct assessment of the buildings’ seismic vulnerability, many researches have been conducted aiming to use the capacities of artificial intelligence, such as the Artificial Neural Networks (ANNs). The present study examines the influence of the infills’ irregular distribution on the seismic damage level of R/C buildings using Multilayer Feedforward Perceptron (MFP) ANNs. More specifically, a 5-storey R/C building with a large number of different masonry infills’ distributions possessing several degrees of irregularities is analyzed by means of Nonlinear Time History Analysis for 65 actual ground motions. The optimum configured and trained networks are applied for the rapid estimation of the damage of the examined building. The results of these applications show that the best configured and trained networks are capable to adequately estimate the damage of the R/C buildings with asymmetry caused by the irregular location of masonry infills.

Α new version about a method of documented application of pushover analysis on reinforced concrete, asymmetric single-storey buildings has recently been presented. To rationally consider the coupling between torsional (about vertical axis) and translational vibrations, the equivalent lateral static floor force of the proposed pushover method is applied using suitable dynamic eccentricities which are added with the accidental ones in such a way that the final design eccentricities move the application point of the external, lateral, static, floor force away from the diaphragm Mass Centre. The appropriate dynamic eccentricities for the stiff and the flexible side of the floor plan are related to the so-called “Capable Near Collapse Principal Axes”, reference to the Near Collapse limit state, derive from extensive parametric analysis, and are calculated by graphs and suitable equations. In the present work, a numerical example of a (double) asymmetric single-storey building is presented to clarify the step by step application of the proposed pushover analysis method. It is a torsional sensitive r/c building, designed in accordance with Eurocodes EN 1992-1 and 1998-1 for ductility class high (DCH). The proposed method is evaluated relative to the results of non-linear response history analysis (RHA). The final results show that the proposed method of pushover analysis predicts with safety the displacement of the stiff side of the building as well as that of the flexible side.

A seismic design methodology is described aiming to provide an insight into the parameters which mitigate the torsional response. The proposed methodology applies to wall-frame dual concrete systems and incorporates the concept that stiffness and strength are interrelated in concrete structures. In practice two conditions should be satisfied in order to obtain a virtually translational behavior: (i) the initially elastic response should be of minimum torsion and, (ii) the strength allocations in the lateral force resisting elements (LFRE) should be based on a static analysis of the symmetric counterpart structure. Τhe first condition is satisfied when the stiffness centre (m-CR) of an equivalent single story system lies close to the mass axis of the real building. Τhe second condition requires a static analysis under a lateral loading simulating the first mode of vibration of the uncoupled structure. As a planar static analysis is finally required, this enables the designer to assess the element flexural rigidities in relation to the assigned strengths to LFRE’s. Only an estimate of the design shear is required at this stage, which may be assessed by experience or by means of Yield Point Spectra (YPS). The proposed methodology is presented in ten story dual concrete buildings, under the seismic excitation of Erzincan-NS 1992, where the base shear assigned to the frame sub-system varies from 20% to 45% of the total design shear.

Kinematic loads resulting from earthquakes or human induced events can act on engineering structures. Poland is not an active seismic region. In south part of Poland there are located quarries and mines. Surface and underground exploitation of mineral resources results in regional seismic phenomena. Seismic waves originating from these phenomena are travelling towards surface and in form of surface wave can impact the structures. Mining-related surface vibrations show a lot of similarities with earthquakes but also some differences. In previous years the main load used in design procedure of buildings in mining regions in Poland were dead and live load, technology load and gusts of wind. The structures were not designed to dynamic loads resulting from mining-related vibrations of ground. Dynamic loads acting on buildings result in extra inertia forces which load those structures. This specially concerns to residential masonry objects which present group of structures frequently occurring in mining areas. In many cases, these buildings are characterized by irregularities in their construction, and this significantly reduces their dynamic resistance. The paper presents the selected problems of analyzing selected masonry building with irregularities in structure using finite element method (FEM). The analysis relates to dynamic effects caused by surface vibrations. The selected problems with assuming the kinematic excitations and soil-structure interaction are presented as well as some aspects of constitutive modelling of masonry structures. The paper also includes comparison of earthquakes and mining-related tremors. The differences in frequency domain and significant duration of intensive phase are also discussed.

Progressive collapse is a phenomenon in which a minor damage leads to total failure of the structure or the collapse of large parts of it. In this paper, influence of mass irregularity in height of the structures on the progressive collapse potential was investigated. To this end, four-, eight- and twelve-story steel moment resisting frame structures are designed and progressive collapse potential was evaluated using alternate load path method, recommended by the American General Service Administration 2003. Three dimensional buildings studied in the current paper were modeled using finite element method in ABAQUS software. Results revealed that in the cases with column removal in the first floor, increasing the number of floors, decreases progressive collapse potential. Maximum dynamic displacement under the removed column in regular 4-story building is about 1.41 times larger than that of regular 8-story building and about 2.16 times larger than that of regular 12-story building. Moreover, comparing performance of regular and irregular buildings showed that regardless location of column removal and story number, mass irregularity in height increases progressive collapse potential. Maximum dynamic displacement due to removed column in irregular buildings subjected to column removal in penultimate floor is 19, 20 and 22 percent larger than that of regular one for four-, eight- and twelve-story buildings respectively.

Past experience of earthquakes has shown the importance of asymmetry effects on the severity of the damage sustained by structures. In order to mitigate torsional response of asymmetric structures under seismic loads, employing passive energy dissipating equipment, such as viscous dampers, has been investigated by many researchers during recent years. One decisive factors contributing to the seismic behaviour of asymmetric structures is the torsional component of strong ground motion. The main purpose of this research is to compute a damping eccentricity which minimizes torsional response of asymmetric structures. To this end, several one-storey, three-dimensional steel moment frames with various stiffness and strength eccentricities are studied. Due to the limitations in recording torsional component of earthquakes, translational components are used to develop torsional counterparts. Models were analyzed using time history analysis method under 7 records of strong ground motions. Results indicate that incorporating viscous dampers could not only mitigate structural response, but it also could suppress torsional deflections due to asymmetry in structures. Results showed by increasing the asymmetry of the structure, optimal damping eccentricity should be shifted towards the flexible edge to reduce the displacement difference considerably.

Irregularity in plan can significantly increase the vulnerability of structures exposed to strong earthquakes. Application of base isolation systems is one of the possible solutions to avoid the negative effects from irregularity of structures in plan. In order to investigate the advantages of the application of these systems, a detailed evaluation of the behaviour of four five story reinforced concrete frame structures was performed. All structures are rectangular in plan and are analysed as fixed base and base isolated. The fixed base models differ between them according to the degree of irregularity. Because base isolated models have a significantly lower eccentricity between the centre of stiffness and the centre of masses, these models are regular in plan. Total obtained displacements in base isolated models are higher, but main part of them are concentrated at the level of the isolation system which results with lower interstorey drifts and small floor rotations.

Pounding between neighboring structures during seismic events has been revealed as one of the most commonly observed reasons for severe damage or even total collapse of the adjacent buildings. Therefore, pounding effects have recently become an issue of great interest of many numerical and experimental investigations in many earthquake-prone regions of the world. It has also been observed that the differences in dynamic characteristics is the key reason leading to interaction between colliding, insufficiently separated structures. The problem is much more complicated for complex arrangements of structures, for example, in the case of collisions between few structures in a row. A lot of different approaches have been considered to mitigate earthquake-induced structural pounding. One method is based on placing between the structures some viscoelastic elements acting as bumpers. Another one is stiff linking the structures. It allows the forces to be transmitted between buildings and thus eliminate undesired interactions. The aim of this paper is to present the results of experimental research focused on mitigation of pounding between buildings in complex arrangements by using polymer elements installed between structures. In the present study, three steel models characterized by various dynamic properties and different in-between distances were investigated. Additional masses were mounted at the top of each model in order to obtain different dynamic characteristics. The unidirectional shaking table, available at the Gdansk University of Technology (Poland), was employed to conduct this study. Experimental models were mounted to shaking table platform. The results of the study explicitly show that the approach of using polymer elements can be an effective pounding mitigation technique in the case of complex arrangement of buildings. It may partially or fully prevent from damaging collisions between adjacent buildings during seismic events. It also enhances the dynamic response leading to the reduction in lateral vibrations under different strong ground excitations.

Seismic response predictability of irregular buildings structures is a challenging task which engaged many studies. To improve the seismic performance of irregular buildings structures, the base isolation method can be used. By positioning the isolation system components, equilibrated, aside and the other of the mass centre and due to the large flexibility of the isolation system in the horizontal direction, the effect of irregularities is drastically diminished. The paper presents a nonlinear static analysis procedure, applicable to existing buildings structures, retrofitted through seismic isolation. There are performed nonlinear static analyses on a fixed base plan structure, with elevation irregularity and an isolation system is proposed. Based on the capacity curves (F-D curves), of the fixed base structure and of the proposed isolation system, there is computed the F-D curve of the isolated building structure. On the basis of the proposed non-linear static analysis procedure, there is determined the performance point of the seismically isolated building structure.

The main building of Uto City Hall was constructed in 1965 and severely damaged in the 2016 Kumamoto Earthquake. In the present article, the damage observed is described and discussed. First, observations of damage to the outside of the building are described. Based on the first damage observation, a damage surveillance mission using a mobile rescue robot was planned and carried out. From these observations, it can be concluded that most of the structural damage in the main building was concentrated on the fourth and fifth stories. Some of the columns on the fourth floor had lost the axial strength capacity to sustain vertical loads.

In this paper, the seismic capacity and torsional irregularity of the main building of Uto City Hall are evaluated by using a simple method based on the building’s structural drawing. The simplified evaluation method of seismic capacity, which was proposed by Shiga in the 1970s, is based on the wall-area index and the average shear stress in walls and columns. For evaluation of its seismic capacity, the following two cases are considered: the building is assumed to behave as a unit building, and each of the structural blocks responding independently. The evaluation of the torsional parameters, stiffness eccentricity and radius of torsional stiffness with respect to the center of stiffness are based on the sectional area of the columns and walls, which is presented in the Japanese Standard for the seismic evaluation of existing reinforced concrete (RC) buildings. The main findings of this paper are as follows. (a) The seismic capacity of the main building of Uto City Hall is insufficient to survive severe earthquakes. However, the evaluated results of both cases cannot explain the damage observed in upper stories. (b) The ratio of the stiffness eccentricity to radius of torsional stiffness evaluated in each story exceeds 0.15, while the radius ratio of the torsional stiffness with respect to center of stiffness to the gyration of the whole mass above the considered story is smaller than 1. Therefore, the main building of Uto City Hall is sensitive to torsional response: it may be classified as a “torsionally flexible building”.