Static Pushover Research Articles

Detection of Fracture in Steel Members of Building Structures by Microstrain Measurement

This paper discusses a possibility of detecting structural damage caused by an earthquake, by measuring the time history of the strain of beams and columns before and/or after the earthquake. An index called “local stiffness” is defined as the ratio of section force amplitude to representative displacement amplitude, and this ratio can be physically interpreted as stiffness. By calculating section force amplitude at a section or node from the measured strain amplitude under a microtremor or small aftershocks and comparing it with the results of a static pushover analysis, it becomes possible to detect any structural damage, such as fractures. This methodology was applied and the microstrain data of a steel moment frame were measured in a large-scale shaking table test; beam-end fractures were observed after some excitation tests. After the beam-end fracture formed, the measured local stiffness dropped significantly below the analysis value, indicating the possibility of employing this value to detect fractures using the analysis value as a threshold value.

Assessment of Seismic Behavior Factor of Code-Designed Steel–Concrete Composite Buildings

In this study, the seismic behavior factor of steel–concrete composite buildings was examined. For this purpose, 5-, 10-, 15- and 20-story composite moment-resisting frame (CMRF) buildings having concrete-filled steel tube columns and composite beams were designed at medium ductility (DCM) and high ductility (DCH) levels as per the regulations of Eurocode 8. Examined DCM structures were designed for a PGA of 0.2 g while DCH structures were designed for a PGA of 0.4 g. SeismoStruct software was utilized in the design and performance assessment of the case study structures. Nonlinear static pushover and incremental dynamic analyses were used. In the pushover analysis, the uniform and triangular load distributions were adopted while 12 earthquake ground motions were employed in the dynamic analysis. The effect of ductility levels and number of stories on the seismic behavior of CMRFs was studied using the results of the nonlinear analysis. To this, the variation in the lateral response, global yielding value, code-based behavior factor and component, and dynamic behavior factor were presented for CMRF structures with DCM and DCH. It was observed that the behavior factor of all CMRFs exhibited well above the design assumptions, especially DCM class CMRFs.

Determination of LRFD environmental load factor in the java sea for unbraced monopod structures using reliability analysis

Case studies were carried out for unbraced monopod structures located in the Java Sea by reviewing the value of the reliability index. This sea has a depth of 33 meters and a 100-yr wave height of 6.29 meters. The structures are designed optimally with a unity check ratio of 1.00 using API RP 2A WSD and API RP 2A LRFD standards under storm condition, assumed to be unmanned. For LRFD criteria, the value of the environmental load factor is varied, namely 1.35, 1.20, 1.10, and 1.00. Collapse analysis using static pushover analysis of the model produces a minimum RSR value meeting the minimum criteria for un-manned structures of 1.6. The reliability analysis process uses the First Order Reliability Method II (FORM II). The structural reliability index value has met the minimum criteria issued by DNV of 3.71 for the structure that is designed using the environmental load factor of 1.35. It can be concluded for unbraced monopod structures designed in the Java Sea, the environmental load factor value of 1.35 is following the minimum reliability index value proposed by DNV.

Understanding the cyclic performance of composite steel-concrete connections on steel bridges

This paper describes a numerical study using the finite element method to evaluate the seismic performance of composite steel-concrete connections designed to protect critical welded interface areas between the steel pipe pile and cap beam bridge piers. Previous studies have indicated that welding the steel piles directly to a steel cap beam, regardless of weld configuration, does not avoid the undesirable brittle cracking failure mode in the welded region. Strengthening of the connection can a solution to shift the plastic hinge position from the connection to another point of the structure. Through static pushover and time-history analysis, the connection has been shown to adequately extend plastic hinging in the form of local buckling in the pipe pile wall, while avoiding unfavorable failure modes. The results of this study demonstrate that the use of composite connections can increase the shear capacity of a steel bridge; thus, improving its performance. Moreover, evaluation of the results shows that an increase in the length of the composite area will improve the overall ductility of the steel frame. Finally, a new composite connection has been proposed to improve the performance of the steel bridge; this will augment current efforts in strengthening bridges in earthquake-prone areas.

Investigating the efficiency of DDBD approaches for RC buildings

Direct displacement-based design (DDBD) approach attempts to alleviate the shortcomings of force-based design (FBD) approach to achieve seismic resistance of structures. Many simplified procedures for DDBD proposed in the literature are conceptually similar, yet differ in their considerations for crucial parameters such as target displacement profile, equivalent viscous damping equation and base shear distribution pattern in the design process. Due to variation in the aforementioned parameters, seismic force, their distribution and element design differs, leading to significant changes in the inelastic behavior of the building. The present numerical study compares the performance of low, medium and high-rise RC frame buildings designed using six DDBD approaches for two drift levels. The seismic performance of the considered buildings has been deliberated by both nonlinear static pushover as well as nonlinear dynamic time-history methods. From analysis results, it is observed that the 84th percentile of the sample mean storey displacement of low-rise buildings designed for low storey drift exceeds the target profile, whereas, this behavior is contrary in high-rise buildings designed for higher storey drift. The analysis also highlighted that the parabolic displacement profile consideration for mid and high-rise buildings is rational than the linear displacement profile consideration. In the present case, it seems that equivalent viscous damping estimation and displacement spectra modification factors are conservative for all the DDBD approaches and needs modification to improve its rationality.

Evaluation of plastic hinges performance and the elastic stiffness factor of moment-resisting frame structures

Nowadays, to analyse and determine the maximum seismic lateral displacement for reinforced concrete (RC) structures, the most applicable procedure used by structural engineers is the nonlinear static (pushover) analysis. The nonlinear static procedure (NSP) is a common approach for analysing the seismic performance of construction structures. By directing this procedure, the weak points in each structural member can be examined, and it also determines whether the members are safe or need to rehabilitate. This process defines the level of performance and shear strength under seismic diffusion to construct each element of the structure. The displacement, the base shear, the plastic hinge model, and the effect of the different plan on seismic response of structures has been reported. When concentrating on the RC structures, it requires the ability to conduct lateral resistant force systems, which one of them is commonly known as moment-resisting frames (MRFs). In this paper, three models of RC structures considered for low-, medium-, and high-rise buildings were examined, and each model has been analysed for three different spans. These models have been analysed applying ETABS software by inputting and examining a wide range of structural parameters. A comprehensive study on the pushover curve, performance curve, among others have been performed. The aim of this study is to consider the effect of plastic hinges in various ranges of performance capacities to evaluate the elastic stiffness factor of structures

Seismic assessment of rocking timber walls with energy dissipation devices

The use of rocking walls with seismic fuses is an efficient solution for mass timber structures. This paper presents and compares mass timber wall systems with different types of seismic fuses. These devices range from conventional pinching connectors to self-centring friction connections. Pros, cons and performance measures of each system are described and discussed. Moreover, a preliminary design procedure based on the Direct Displacement Based Design (DDBD) approach is used to specify and size the damping devices. This approach was chosen to assure that the lateral displacements of the systems are appropriately controlled respecting the considered damage limit states.Numerical models are accordingly developed and subjected to non-linear static pushover and dynamic time-history analyses. The results are discussed in relation with the performance objectives considered for different systems. The results confirmed that the use of low damage friction connections as a substitute for traditional pinching connectors could considerably improve the seismic performance of mass timber structures. Nevertheless, with some of these systems, there is potential for having residual displacements that could compromise the post-earthquake performance of the system.

Gaussian process regression for seismic fragility assessment of building portfolios

Seismic fragility assessment of building portfolios is often based on the analysis of “average” building models representative of structural types (or building classes), thus neglecting building-to-building variability within a structural type. This paper proposes the use of Gaussian process (GP) regressions to develop flexible and accurate metamodels explicitly mapping building-class attributes to the seismic fragility parameters. The proposed metamodels can enable analysts to account for building-to-building variability in simulation-based seismic risk assessment of building portfolios. Unlike other commonly-used metamodels, GP regressions do not require the a-priori definition of a prediction function and they quantify the uncertainty on the predictions in a refined and explicit fashion. The proposed method is demonstrated for a portfolio of seismically-deficient reinforced concrete school buildings with construction details typical of some developing countries. Based on the available information about the building attributes (e.g. geometry, materials, detailing), building realisations are generated based on two alternative approaches, which are critically compared: design of experiment and Monte Carlo sampling. Cloud-based time-history analysis for each building realisation is performed using unscaled real ground-motion records; fragility relationships are derived for four structure-specific damage states. A GP regression is then developed for each considered fragility parameter (i.e. median and dispersion). To further increase the tractability and scalability of the methodology, alternative metamodels are defined based on numerical non-linear static pushover analyses or analytical “by-hand” pushover analyses, through the Simple Lateral Mechanism Analysis (SLaMA) method. The results show that, for the considered portfolio, the fitted GP regressions have a high predictive power in surrogating the modelled fragility, demonstrating the feasibility of the approach in practice. It is also shown that the choice of the sampling technique could be based on the input data availability, rather than on the expected computational burden. Finally, the use of simplified methods for response analysis shows acceptable error levels with respect to the full time-history analysis results. Such simplified methods can be promising alternatives to generate large training datasets for the proposed GP regressions. This increases the potential of training metamodels in practical portfolio risk assessment applications, in which a high number of building types, each characterised by a large number of attributes, is generally involved.

Response Reduction Factor for Mansory Buildings

Most of the seismic codes used today incorporate the nonlinear response of a structure by providing an appropriate response reduction factor so that a linear elastic force-based approach can be used in designs. This study focuses on evaluating the response reduction factor for masonry buildings with different mechanical properties, which are used in modern codes to scale down the elastic response of the structure. Using a similar frame-approach, a nonlinear static pushover analysis is carried out on the analytical models of masonry building in finite element analysis software SAP2000v20.0.0. The response reduction factor components, flexibility, and over strength were computed from the results obtained from the nonlinear static pushover analysis. Finally, the response reduction factor is evaluated for different masonry buildings. It is concluded that the R-value given in IS: 1893-2016 for unreinforced masonry is not recommended for random rubble stonemasonry buildings in mud mortar.

The effects of changing story number and structural footprint area on building performance in reinforced-concrete buildings

Structural footprint area and number of stories occupy an important place among the factors affecting the behavior of buildings under earthquake effects. In this study, footprint area and number of stories are considered as two different variables. A sample reinforced-concrete building with all values such as structural system elements, dimensions, materials, material models and loading status is selected same in all different structural models. The structural analyzes were made for a single direction since the structure was chosen symmetrically. In each building model, the axle clearance has been increased by one meter in both directions. As the second variable, four different story numbers were chosen as 5, 6, 7 and 8. Eigenvalue and static pushover analyzes were performed for the each structural model. Target displacement for damage estimation, period, stiffness and base shear force values were obtained for all models, respectively. As the building footprint area and number of story increase, period, displacement and target displacement increase, while stiffness decreases in this study.

Optimal seismic retrofitting of reinforced concrete buildings by steel-jacketing using a genetic algorithm-based framework

Retrofitting of existing reinforced concrete (RC) frame structures by steel angles and battens (steel-jacketing) is a commonly employed technique used to retrofit beams and columns against gravity and seismic loads. Steel-jacketing (SJ) effectively provides additional deformation and strength capacity to RC members but its application is associated with noticeable downtime of the building and non-negligible costs, depending on the amount of structural and non-structural manufacturing and materials. This paper presents an optimization framework aimed at the minimization of seismic retrofitting-related costs by an optimal placement (topological optimization) and amount of steel-jacketing reinforcement. In the proposed framework a 3D RC frame fiber-section model implemented in OpenSees is handled by a genetic algorithm routine that iterates reinforcement configurations to match the optimal solution. The feasibility of each solution is controlled by the outcomes of a static pushover analysis in the framework of N2 method. Results will provide optimized location and amount of steel-jacketing reinforcement, showing how effective and sustainable reduction of retrofitting costs is achievable maintaining a specified safety level.

Seismic design and assessment of steel-concrete frame structures with welded dissipative fuses

This research presents the design and numerical assessment of composite steel-concrete frame structures with welded dissipative fuses. The assessment has been carried out based on linear response spectrum, nonlinear static pushover and time history procedures. The analytical expressions which define the envelope of the nonlinear response of the dissipative fuses are first presented and calibrated against experimental results available in literature. The assessment is then carried out according to a design methodology proposed herein. Outcomes of the numerical assessment indicate that the use of welded dissipative fuses successfully limited damage within the replaceable parts. Furthermore, although structures with dissipative fuses present lower strength and, generally, lower displacement capacity, their displacement ductility and global dissipative performance are generally higher than conventional structures, especially when the structure with dissipative fuses presents a dissipative configuration adjusted to the bending moment distribution diagram calculated for the applied seismic action.

Betonarme Binaların Güçlendirilmesinde Uygun Merkezi Çelik Çapraz Yerleşiminin Belirlenmesi

Repair or strengthening of building may be required due to the change of the design codes, adding new floors to the building, changing the purpose of the building etc. There are many methods to retrofit buildings. One of them is to strengthen using steel braces. In this strengthening method one of the important questions that should be answered at the initial stage is what kind of steel brace is to be used and how the steel braces are to be arranged. In this study first, the performance analysis of a three-span (all spans are 4.5m), nine-storey (story height is 3m on each floor) reinforced concrete frame building analysis was performed. As a result of the performance analysis, it was determined that the structure needed to be strengthened. Using centric X braces 5 different reinforcing models were created. In this way, the effect of steel braces on the seismic behavior of the building was determined. In the comparison of the models, the results of the performance evaluation, with the Nonlinear Static Pushover Analysis, were used. As a result of the comparison, the optimum results were obtained from the model where central steel braces were used in each story from the first floor without vertical discontinuity.

Seismic fragility assessment of existing reinforced concrete buildings in Patna, India

Assessment of seismic fragility of reinforced concrete (RC) buildings requires appropriate consideration of uncertainty in aleatory and epistemic sources. Aleatory uncertainty is typically attributed to randomness in ground motion records, while epistemic is due to the uncertainties in structural modelling. The present work focuses on obtaining the variability functions and fragility curves by considering those uncertainties for low, mid and high rise RC buildings in Patna, India, which is a region of high seismicity.Modelling uncertainties requires consideration of material, geometrical, and design as random parameters. These parameters mainly comprise material strength, storey height, plan, typology, design provisions, and various irregularities. In this study, the selection of the random parameters is purely based on the investigation of buildings in the Patna region. Considering the multiple combinations of random parameters, the buildings are modelled and analysed using nonlinear static (NLS) pushover analysis. The monotonic load in a triangular pattern is applied for NLS analysis to get pushover curves. The pushover curves are subsequently converted into capacity curves considering the dynamic characteristic of the first mode of vibration. The obtained capacity curves are utilised to derive the variability in the capacity spectrum for different categories of RC buildings. Subsequently, variability due to randomness in ground motion records is determined by generating several artificial (comprising near field and far field) ground motions considering the seismotectonic map of the region.Thence, by accounting for both the uncertainties, lognormal variability functions are determined, and seismic fragility assessment is performed for different building categories in Patna, India. Furthermore, the seismic vulnerability of existing buildings can be determined based on the obtained fragility curve.

Seismic Assessment of Asymmetric Single-storey R/C Buildings by Two New Methodologies: Enforced Displacement-Based and Forced-Based Pushover Procedures

?wo new documented non-linear static (pushover) procedures on asymmetric single-storey R/C buildings are presented in detail herein, aiming directly at the Near Collapse state. Both procedures apply relative to the “Capable Near Collapse Principal reference system” of the single-storey building. The main objective of the two proposed procedures is to fully consider the coupling between torsional and translational vibrations of the floor-diaphragm under translational seismic excitation of the building’s base. The first pushover procedure, which is a Direct Displacement-Based one, uses floor enforced-displacements as action. In the second pushover procedure, which is a Force-Based one, the floor lateral static forces are applied eccentrically to centre of mass using suitable inelastic design eccentricities (dynamic plus accidental ones). The floor enforced-translations/rotation and the appropriate inelastic dynamic eccentricities used in the two proposed procedures derive from extensive parametric non-linear response history analysis and are given by figures or equations. In order to clarify in detail and evaluate the new pushover procedures, a torsionally-flexible, double-asymmetric, single-storey R/C building is seismically assessed. The validation of both procedures relative to the results of non-linear response history analysis shows that both predict with safety the in-plan displacements of the building.

Yığma Duvarların Mikro Seviye Modellemesinde Tuğlanın Basınç Dayanımı ve Kayma Gerilmesi İletim Katsayısının Nümerik Çözümler Üzerindeki Etkisinin İncelenmesi

In this study, the effectiveness of 3-dimensional fixed smeared crack model in the micro-model analysis of masonry walls is investigated. The three parameters concrete model which special case of William Warnke model, is used to calculate the nonlinear behavior of mortar and brick regions of this wall. For numerical analysis, Solid65 concrete element in Ansys finite element program is selected. For the comparison of the numerical results, the base shear force-top displacement curve and the cracking zones obtained from the experimental results of the Eindhoven walls JD4, JD6 and JD7 are used. An equation for compressive strength of the brick, and the values for shear stress transfer coefficients are recommended with purpose of using in micro level static push-over analyzes of masonry walls.

Methodology for Regularization of Force-Based Elements to Model Reinforced Concrete Columns with Short Lap Splices

In reinforced concrete structures built before the 1970s, it was common for columns to be constructed with short lap splices and widely spaced transverse bars at the column base, leading to increased likelihoods of pull-out failures and structural collapse during seismic events. In the modeling and analysis of reinforced concrete columns with short lap splices, several challenges arise, particularly with softening behavior that leads to strain localization and scaling and convergence issues in analyses. This research presents a methodology to regularize force-based beam-column elements with softening lap-splice material response. In the methodology, a constant energy release criterion is imposed with the constant postpeak energy of the lap-splice region determined from relevant experimental data. With the proposed regularization, the numerical model shows objective results that are independent of the length of the element and number of integration points used. Whereas the accuracy in estimating, for example, the displacement at 20% strength drop using existing nonregularized models changes with the number of integration points, the accuracy using the proposed model remains constant across numbers of integration points used with a mean accuracy of 94% compared with experimental tests. Results from static pushover and cyclic analyses show an order of magnitude decrease in standard deviation of the response using the regularized model. Increased accuracy and increased convergence for the method across the number of integration points used is shown in static and dynamic analyses. The proposed regularization approach is able to alleviate strain localization issues and facilitates the scaling of analyses from small-scale to full-scale structures.

Experimental and numerical investigations of replaceable moment-resisting viscoelastic damper for steel frames

An innovative type of replaceable moment-resisting viscoelastic damper (RMVD) was developed to increase the level of inherent damping of steel moment frame buildings to control wind-induced and/or earthquake-induced dynamic vibrations. The RMVDs are installed in lieu of existing steel moment connections, which occupies no additional architectural space. The system level damping of the steel moment frames could be increased from the viscoelastic segment of the RMVDs. Besides, the proposed damper exhibits passively adaptive performance. The energy dissipation mechanism of the RMVD could be easily shifted between the viscoelastic segment and fuse segment under different levels of inter-story drifts. The sacrificial fuse segment of the damper, which could be easily replaced after an earthquake event, provides ductile and stable performance when the inter-story drifts exceed a predefined threshold. The paper first presents a systemic investigation of the performance of viscoelastic material. Then, a series of dynamic experimental studies were carried out on the viscoelastic segment when subjected to input motions that feature different frequencies and magnitudes, providing a comprehensive overview of the damper performance. Furthermore, the numerical simulation of the damper is provided at the component level. Finally, a steel portal frame is simulated as a case study to present the structural performance with the dampers under static pushover loads. The result of the study shows that the energy dissipation capacity of steel moment frames could be improved throughout the implementation of such devices.

Assessing the seismic behavior of steel moment frames equipped by elliptical brace through incremental dynamic analysis (IDA)

Incremental dynamic analysis and nonlinear static pushover analysis are carried out on a performance-based design to determine the seismic demands and capacities of an elliptic braced moment resisting frame (ELBRF). The objective is to assess ductility, overstrength and response modification factors in a modern steel-braced structural system based on incremental dynamic analysis. This integrated system is connected to a beam and column with an appropriate length while providing enough architectural space to allow for an opening without having the common problems associated with architectural spaces in braced systems. Several different classes of buildings are considered on soil type II. Linear dynamic analysis, nonlinear static pushover analysis and incremental nonlinear dynamic analysis related to 12 records from past earthquakes are carried out using OpenSees software. The factors of ductility, overstrength and response modification are calculated for this system. The values of 9.5 and 6.5 are found and suggested only for the response modification factor for ELBRF systems in allowable stress and ultimate limit state methods, respectively. The fragility curves are plotted for the first time for this type of bracing, which contributes to the assessment of building seismic damage.

Sensitivity analysis of double layer Diamatic dome space structure collapse behavior

Collapse behavior of double layer dome space structures depends on different factors such as load pattern, material mechanical properties, and member’s physical characteristics, which have a random nature that makes it difficult to predict accurate behavior of this type of structure. In this paper, in order to comprehensively assess the double-layer dome space structure behavior, different uncertain parameters such as the mechanical characteristics of steel material, mass and applied gravity load, and also structure’s and members’ geometric imperfection are taken into account in nonlinear static pushover analysis. Different models with rise to span (R/S) ratios of 0.1, 0.2, and 0.3 under incremental symmetric and asymmetric snow loads are investigated. In order to determine the importance of these uncertain parameters and rank them according to their effects on the structural response parameters, two different methods of Tornado Diagram Analysis (TDA) and First-Order-Second-Moment (FOSM) are utilized. Using these methods enables the sensitivity analysis of the collapse behavior of the structures to the considered random variables. The results of the analyses reveal that there is a close agreement between the TDA and FOSM analyses results in ranking the random variables according to their importance. In addition, these methods highlight the high sensitivity of the structural response parameters to the various sources of uncertainty, and indicate the necessity of applying these uncertainties in the analysis and design of this type of structure.