ENGINEERING ANALYSIS OF A TYPICAL PRE-CODE URM BUILDING IN ZAGREB

Full-scale Experimental Pushover Testing of an Existing URM Building

Evaluation of Load Distributions in Pushover Analysis Nonlinear pushover and time history analyses are common used methods to determine the nonlinear behavior of structures under earthquake loading. The aim of the present study is to evaluate the push over analysis, which is commonly used to determine the performance of R/C structures, comparing with time history analysis. For this purpose, 3, 5, 8 and 15-story R/C frame structures with various natural periods are analyzed by performing nonlinear pushover and dynamic time history analyses. The load distributions for pushover analyses are chosen as rectangular, triangular (IBC, k=1) and parabolic (IBC, k=2). To evaluate the results from the pushover analysis, nonlinear dynamic time history analysis is performed for 50 different stations’ data during various earthquakes. Finally, the best load distribution is determined for pushover analysis and the results are evaluated regarding with story height of the selected structures.

This paper evaluates the seismic behavior of four methods for retrofitting typical reinforced concrete school building. The original school (SO) suffers from limited ductility and lateral resistance. The four seismic upgrading methods are using carbon fiber reinforced polymers (SUFRP), steel jacket (SUSJ), bracing (SUBR), or concrete jacket (SUCJ). Nonlinear pushover and time history analyses are made for the five schools. Three code-scaled natural earthquakes are utilized for the inelastic response history analysis. The pushover analysis revealed that providing the school with bracing (SUBR) results in reducing the lateral displacements. However, the three other types of seismic retrofit (SUFRP, SUSJ, and SUCJ) develop larger lateral displacements when compared to the original school (SO). The translation ductility and energy dissipation for school SUBR is poor while they are good for schools SUFRP, SUSJ, and SUCJ. The base shear is increased for the four upgrading methods (SUFRP, SUSJ, SUBR, and SUCJ) compared to school SO. From the nonlinear time-history analysis, the maximum interstorey drift was approximately similar for schools SO, SUFRP, and SUCJ. Lesser values for the maximum interstorey drift are obtained for schools SUSJ and SUBR. The nonlinear time-history analysis also gives similar base shear for all the upgrading methods compared to school SO with the exception of school SUCJ which showed increase in the base shear.

Existing unreinforced masonry (URM) buildings represent a significant part of the constructed facilities. Unfortunately, in case of seismic actions, their structural behavior is negatively affected by the low capacity of masonry components to withstand lateral forces. For this reason, in the past decades, URM buildings have been responsible for fatalities and large economic losses even in the case of moderate earthquakes. This article presents the seismic loss assessment of an old masonry building damaged during the 2014 South Napa earthquake using the framework of the Pacific Earthquake Engineering Research Center’s Performance-Based Earthquake Engineering. For this purpose, the performance is expressed in terms of expected monetary loss curves for different hazard scenarios. Structural and non-structural losses are considered in the analysis using a practical, yet accurate, structural idealization of the URM building, which is validated by the observed damage from the 2014 South Napa earthquake.

Groups of contiguous unreinforced stone masonry buildings are a common type of housing seen in old European downtowns. However, assessing their response to earthquakes poses several challenges to the analysts, especially when the housing units are laid out in compact configurations. In fact, in those circumstances a modeling technique that allows for the dynamic interaction of the units is required. The numerical study carried out in this paper makes use of a rigid block modeling approach implemented into in-house software tools to simulate the static behavior and dynamic response of an aggregate stone masonry building. Said approach is used to reproduce the results of bi-axial shake-table tests that were performed on a building prototype as part of the activities organized within the Adjacent Interacting Masonry Structures project, sponsored by the Seismology and Earthquake Engineering Research Infrastructure Alliance for Europe. The experimental mock-up consisted of two adjacent interacting units with matching layout but different height. Two rigid block models are used to investigate the seismic response of the mock-up: a 3D model allowing for the limit analysis of the building on one hand, and a 2D model allowing for the non-linear static pushover and time-history analysis on the other. The 3D model was built for the blind prediction of the test results, as part of a competition organized to test different modeling approaches that are nowadays available to the analysts. The 2D model was implemented once the experimental data were made available, to deepen the investigation by non-linear static pushover and time-history analysis. In both models, the stonework is idealized into an assemblage of rigid blocks interacting via no-tension frictional interfaces, and mathematical programming is utilized to solve the optimization problems associated to the different types of analysis. Differences between numerical and experimental failure mechanisms, base shears, peak ground accelerations, and displacement histories are discussed. Potentialities and limitations of the adopted rigid block models for limit, pushover and time-history analyses are pointed out on the basis of their comparisons with the experimental results.

Industrial stacks are built in order to reduce atmospheric pollution and release gases to the higher level of the atmosphere. Since they are high and slender structures, they are significantly affected by lateral forces, in particular, induced by winds and earthquakes. This paper aims to summarize the design principles for steel stacks given in the international standards, codes and specifications considering the evaluation of the results from the investigation on nonlinear dynamic response of a typical steel stack subjected to the earthquake. For this purpose, a typical self-supporting steel stack has been designed and its performance under earthquake and wind forces has been investigated in terms of the applied design principles, such as the displacement limit and the potential plastic deformations. The study is accomplished by using finite element models employing nonlinear time history and pushover analyses. The numerical results are presented particularly by considering the design principles used for the model. The results show that wind loads considered are more pronounced in the design of industrial steel stacks than earthquake especially in terms of limiting top displacement. The flue openings which reduce strength and lateral stiffness of the stack web is significantly effective in the lateral behavior of the stack. Moreover, the principles considered are found applicable for the design of such steel stacks. Based on the results from nonlinear time history analyses, no yielding and plastic deformation are detected along the stack.

Due to the poor seismic performance of unreinforced masonry (URM) buildings there is a need for a cost effective minimally-invasive seismic retrofit technique. Most research considering out-of-plane seismic retrofit of URM walls has been conducted using laboratory-based studies with well defined but artificial boundary conditions, using constituent construction materials that attempt to simulate the material properties of masonry found in historic URM buildings. Thus, in-situ testing of retrofitted URM walls is required to provide data with which to validate the accuracy of laboratory-based studies. An experimental campaign was executed to investigate the performance of a near-surface mounted (NSM) carbon fibre reinforced polymer (CFRP) seismic retrofit solution. This campaign involved testing of five masonry walls loaded out-of-plane in four different buildings located in New Zealand. Testing confirmed that the CFRP retrofit technique is an excellent minimally-invasive and cost effective option for seismic strengthening of URM buildings. Details of the history of the buildings, and the methods used to undertake the field tests are reported, and experimental results are presented.

The paper examines unreinforced masonry (URM) buildings, which are common in small historic centres around the world. These buildings are often constructed in aggregate, a configuration that not only results in significant structural interaction but also interaction with and through the foundation soil. The seismic performance of an aggregate can be influenced by foundation-soil-foundation interaction (FSFI), in addition to standard soil-foundation-structure interaction (SFSI) and site effects (SE). While reliable and time-efficient approaches are available in the literature to address all these issues for standalone buildings, buildings in aggregate are frequently modelled as isolated and fixed at their base, particularly when developing fragility curves. This paper investigates the effects of SFSI and FSFI on the period and damping ratio estimates of typical URM buildings. Specifically, it examines the impact of SE, SFSI, and FSFI on the fragility curves of two aggregated URM buildings. These latter are representative of Visso, a town heavily affected by the 2016–2017 Central Italy earthquake, known for site amplification phenomena due to soft soils. Fragility curves were developed through nonlinear dynamic analyses of equivalent 3D frame models of the two archetypes, analysed under both fixed and compliant base conditions. In the latter scenario, the structural model is equipped with springs at its base, with stiffness first calibrated to account for SFSI effects and subsequently adjusted to include the additional FSFI contribution. The results indicate a higher fragility in the fixed-base model. Specifically, the ratio of the median values of the fragility curves for the compliant base model to the fixed-base model ranges from 20 to 60%. Finally, the results from the cross-interacting models exhibited slightly higher values than those considering only SFSI, suggesting a moderate impact from the additional contribution of footing-footing interaction, at least for the case studies examined.

Evaluating seismic performance is essential for structures in earthquake-prone regions, particularly reinforced concrete (RC) frames commonly used in countries like Indonesia. This study compares two prominent assessment methods: nonlinear time history analysis (THA), known for its high accuracy but computational demand, and the capacity spectrum method (CSM), which offers greater efficiency through pushover analysis but requires validation. A six-story RC frame with vertical irregularities, such as setbacks at intermediate levels, was modeled and analyzed under seismic loading based on the 1940 El Centro ground motion. Numerical results from both THA and CSM were benchmarked against experimental data from a previously conducted shake table test documented in the literature. The comparison focused on inter-story drift responses, which serve as key indicators of seismic performance. The results show that both methods produced slightly higher drift values compared to the experimental data, though the differences remained within acceptable engineering tolerances. The findings demonstrate that while THA remains a benchmark for detailed analysis, CSM provides a viable and faster alternative for the seismic assessment of irregular RC structures, especially during preliminary design or evaluation stages. This reinforces the potential of CSM to support performance-based seismic design in a more efficient manner without significant loss of accuracy.

In regions with low to moderate seismicity, unreinforced masonry (URM) is commonly used for the construction of low to mid-rise buildings. When these structures are subjected to seismic loading, the stiff URM walls attract considerable part of the lateral forces and need therefore to be considered in seismic design and assessment. Nevertheless, the response of URM walls subjected to lateral in-plane loading is not yet fully understood and given estimates for some of the crucial design parameters are unsatisfactory (the displacement capacity and the effective stiffness). Two series of URM walls tested under lateral in-plane loading are presented. First series was built at full-scale using a typical Swiss hollow clay brick and a commercially available standard cement-based mortar. Second series represented the same walls at half-scale. During all tests, the walls were subjected to quasi-static cycles of increasing drift demands, while controlling the boundary conditions (axial load and moment restraint at the top of the walls) such to simulate the typical loading of a ground floor wall in an URM building. From the experiments several new insights on the kinematics of URM walls are drawn. For instance, it is identified that once several diagonal cracks develop in the walls, the deformation capacity of the walls is governed by the separation of the rectangular wall into two triangles. From comparison between both test series at different scales, new recommendations for the correct scaling of hollow core masonry are derived and proposed. In order to compare our own results to other existing wall tests, an existing dataset of URM walls is extended and reanalyzed. Finally, a dataset of 64 quasi-static tests on modern URM walls of different heights and masonry types is presented. The evaluation of the dataset confirms the influence of the boundary conditions on the drift capacity. Moreover, it reveals the presence of a strong size effect and an empirical equation is proposed to estimate the ultimate drift capacity while accounting for these factors. Throughout the wall tests, an optical measurement system is used to track the displacement field of the walls. This measurement was synchronized with the force measurement such that global and local engineering demand parameters of the walls could be linked. This point was crucial for the following contributions of this thesis: (i) proposition of two sets of limit states (LSs) that link local damage states to characteristic points of the global force-displacement curve of URM walls; (ii) the study of different deformation parameters for the validation of mechanical and numerical models at local level. A new mechanical model is proposed which describes the nonlinear force-displacement response of flexural dominated URM walls. For this, first, an analytical part is derived based on the plane section hypothesis and a non-tension material with a linear-elastic constitutive material law in compression. It assumes that only the compressed part of the wall contributes to the wall resistance and accounts for a softening due to the reduction of the effective area. In a second step, new criteria are developed which predict the occurrence of the previous proposed local LSs, which are then incorporated in the analytical model. The new complete mechanical model is validated through the comparison with experimental evidence yielding to good estimates for the effective stiffness and the displacement capacity.

Construction materials’ properties: Investigation of unreinforced masonry buildings in the cities of Tirana, Durres and Elbasan

An accurate optimal pushover analysis is developed to predict the higher-mode influence in multi degree of freedom (DOF) buildings. Performance levels are determined and seismic demands are calculated using two different ground acceleration records. The methodology is based on elastic structural dynamics theory and retains the simplicity of standard pushover procedures that rigorous time history analyses do not provide. A variant inertial load pattern is determined by updating the building’s vibration properties at each stage of yielding. Each load pattern is derived using one mode shape at every yielding stage thereby capturing the higher-mode effects (HME). Using the energy under the building’s capacity curve then enables a single optimal mode shape to be determined; this defines a representative single-DOF system (R-SDOF). Towards this end, an optimization algorithm is written to minimize the target displacement error between the pushover and nonlinear time-history analyses of a few buildings for a certain ground record. Optimal capacity and demand parameters are then determined and used to combine the individual mode shapes from each stage of yielding into the single optimal mode shape. This approach enables HME and phenomena such as HME and P-∆ effects to be captured through the optimal parameter identification. These same parameters are then applied in a pushover analysis of other buildings. The results are shown to compare favorably to a separate nonlinear time-history analysis.

The article presents seismic resistance evaluation study of unreinforced brick masonry buildings. The study was carried out as part of the Ph.D. research work of the first author. As part of the study, in addition to the standard laboratory tests, a dynamic field test was carried out on single-story, single-room unreinforced masonry structure. The model structure was tested in actual ground conditions against simulated earthquake vibrations produced through controlled explosions, especially designed for this purpose. Based on masonry properties accrued from lab and field tests, finite element models of the brickwork system were also studied. Finally, the software named, “Shear Damage Index (SDI),” developed as part of this study, was used to plot contours of shear demand (shear stress) to shear capacity (shear strength) ratio on the numerical model and hence to identify potential weak zones in the model for possible strengthening of those locations.

The seismic behavior of the steel connection equipped with the lead extrusion damper (LED) is examined through the experimental and analytical studies. The LED implemented has spherical type bulge and does not consist any lubricant layer. The experimental study is performed in Structural and Earthquake Engineering Laboratory of Istanbul Technical University (STEELAB). In the experimental part, force-displacement relations of the connections with and without LED are achieved for the reversal displacement cycles. The measures of the dissipated energy, the equivalent damping ratio and the effective stiffness are obtained from the experimental study and compared for the connections. The total energy dissipated by the connection with the LED is approximately 175% greater than the total energy dissipated by the bare connection. In the analytical part of the study, nonlinear dynamic time history analyses are performed for the selected earthquakes and mass condition using the analytical models for the connections with and without LED. The maximum top displacement decreases approximately 50% for the connection equipped with the LED respect to the bare connection for the considered earthquakes. The maximum value of restoring force for the connection equipped with LED is approximately 25% greater than the bare connection for the accomplished analyses. Finally, the LED responses obtained in the nonlinear time history analyses are compared with the isolated LED test results.

Nowadays, seismic isolation system has been widely applied in the world to mitigate damage risk of structures. Although maximum displacement demand can be obtained through nonlinear time history (NLTH) analysis, many approximate methods are frequently recommended in structural specifications to reduce the required computational time. One of the best-known methods is the equivalent linear (EL) method, in which the nonlinear response of isolator can be adequately modeled using a fictitious viscously damped elastic structure. In this paper, a comparison between existing expressions supplying the state of research is carried out and then, an improved expression is presented for equivalent linearization of structures supported on lead rubber bearings (LRB). Based on the concept of secant stiffness, the optimal damping ratios, which minimize the errors of maximum displacement between EL analysis and NLTH analysis, are calculated and averaged over 12 ground motions. Then, a rational model to estimate equivalent damping ratio is derived through statistic analysis of the optimal damping ratios. To examine the prediction accuracy of the proposed model, mean ratios of approximate to exact maximum displacement and root mean square error for different isolated period are calculated as evaluation indicator. Compared with other EL models, the newly proposed model predicts a displacement that is in better agreement with the one obtained through NLTH analysis.