Drift Demands Research Articles

Cyclic tests of steel tee energy absorbers for precast exterior wall panels in steel building frames

This paper focuses on a new type of exterior wall system for steel building frames. The wall system consists of precast reinforced concrete wall panels that are bolted to the beams of a steel building frame and are equipped with the inter-panel steel tee energy absorbers. The precast wall panels and the steel tees are expected to remain elastic and absorb hysteretic energy under the frame inter-story drifts caused by earthquakes, respectively. To demonstrate the perceived benefits of the exterior wall system and investigate the behavior of the steel tee energy absorbers, a group of five specimens with each consisting of a steel frame, a pair of precast wall panels and a set of six steel tee energy absorbers were designed, constructed and tested. The test parameters varied among the specimens were the geometries of the steel tee energy absorbers and the technology adopted to develop them. The steel frame and the precast wall panels were re-used in all specimens. Each specimen was tested through a two-phase loading program which imposed the inter-story drift demands with progressively increasing and constant peak amplitudes, respectively. All the specimens sustained the maximum inter-story drift angle of 0.05 rad or larger and exhibited a stable energy dissipation response. The tests demonstrated the damage free response of the precast exterior wall panels as well as the replaceability of the steel tee energy absorbers. Based on an idealized curvature distribution in the webs of the steel tee energy absorbers, the classic beam theory, and the virtual work theory, this research established a simplified model for computing the strengths of the steel tee energy absorbers as well as their contribution to the lateral strength of the frame. The model was found to be able to provide acceptable predictions for the tested specimens and it is recommended for future design.

Seismic Drift Demand Estimation for Steel Moment Frame Buildings: From Mechanics-Based to Data-Driven Models

AbstractA spectrum of simplified methods for estimating building seismic drift demands is conceptualized. On one extreme are mechanics-based approaches that are derived solely from fundamental engi...

NUMERICAL ANALYSIS OF FRP REINFORCED FRAMES UNDER CYCLIC LOADING

In this research, three models of beam-column joint have analyzed under reversed cyclic loading by finite element method using ANSYS software. The first model was designed with a steel-reinforced joint (longitudinal reinforcement and stirrups), the second model performed with (GFRP) -reinforced joint in longitudinal reinforcement only and the third model performed with (CFRP) -reinforced joint in longitudinal reinforcement only. The behaviour of the three models under reversed cyclic loading, their load-storey drift envelope relationship and energy dissipation capacity were compared. The GFRP-reinforced and CFRP-reinforced models displayed a predominantly elastic activity up to failure, although the energy dissipation for GFRP and CFRP models were low, its performance in terms of total storey drift demand was satisfactory. The GFRP reinforcement's low elasticity modulus caused a decrease in the overall stiffness of the model at the service stage, resulting in the obtaining of converging drift ratios and at the same time obtaining lower forces from its acting earthquake loading.

Evaluation of rocking motion on the seismic performance of a linked column frame system

A linked column frame (LCF) is a seismic-load-resisting system with ductile behaviour. In this system, the link beam, operating as a shear fuse, reduces or eliminates damage to other components of the structure under various hazard levels. In this study, by combining rocking motion and changes in the linked column pattern, an innovative seismic-resisting system was developed. The aim was to improve the performance of the LCF system by reducing damage to the flexural beams and columns and to concentrate damage in the link beams. For this purpose, a three-storey model based on the SAC building was designed based on storey drift demands. The model was evaluated by incremental dynamic analysis (IDA) according to Fema P-695 instructions using OpenSees. Results of changes in the linked column pattern from the IDA indicated a 20% increase in the structural capacity of the LCF model. Compared with the model without rocking motion, the model with rocking motion resulted in reduced structural capacity, shear and horizontal acceleration of the storey, and maximum drift. This novel LCF seismic system provides self-centring and damage control by using features such as rocking motion.

Seismic response of mid-rise steel MRFs: the role of geometrical irregularity, frequency components of near-fault records, and soil-structure interaction

Near-fault (NF) ground motions may induce higher demands on structures compared to far-fault records. This is often due to the velocity pulses, called forward-directivity pulses, which are commonly observed in the fault-normal ground motion components. Hence, some researchers accept such pulses as the representative of pulse-like motions. This paper is devoted to studying the nature of NF events by evaluating their impact on the structural response. To this end, a group of ten-story steel moment-resisting frames with regular and setback configuration were subjected to NF pulse-like records. The records were decomposed to distinct frequency components, including the extracted pulses (low-frequency components) and the residual (high-frequency) parts. The effect of TP/T1 (the ratio of pulse period to the first-mode structural period) was also evaluated by classifying the earthquake records into four groups, indicating TP/T1≈0.5, 1, 2, and > 2. Furthermore, soil-structure interaction (SSI) was considered through the Beam on the Nonlinear Winkler Foundation theory. Incremental dynamic analyses were conducted under the action of the original ground motions, extracted pulses and residual parts. The results deeply question the possibility of using directivity pulses as the representative of original excitation in dynamic time history analyses, especially for setback structures. The residual component plays an important role in seismic response of such structures in most cases of TP/T1. The considerable effect of this frequency component is also observed in regular structures when TP/T1≈1. Meanwhile, SSI can increase structural drift demands, especially in setback structures, when subjected to NF pulse-like records.

Structural analyses of self-centring-beam moment-frames under near-fault and far-field earthquakes

A self-centring-beam (SCB) is a new structural member with a capacity for recentring and deformation compatibility for moment-frames (MFs). This paper examines the lateral drift responses of SCB-MFs when subjected to the far-field (FF) record set, near-field pulse (NF) record set, and near-field no pulse (NP) record set. A series of 3-, 6-, 9-, and 12-storey SCB-MFs were modelled and analysed using centralized spring models in OpenSees software. Based on the observed differences in the experiments, a parametric analysis of the effect of the three design factors was conducted. The three design factors were initial stiffness, gap-opening moment, and maximum self-centring rotation at the beam ends. The analyses indicated that all of the SCB-MFs, except for 3-storey SCB-MFs, experienced an obvious increase in the inter-storey drift demand as the excitation was changed from the FF and NP to the NF records.

Effects of ductility and connection design on seismic responses of base-isolated steel moment-resisting frames

In this article, effects of ductility level and connection type on seismic responses of fixed-base and base-isolated buildings with steel moment-resisting frames are evaluated using nonlinear time history analyses. In a comparative assessment of responses, a set of twenty-four models is seismically designed, including 3- and 9-story base-isolated and conventional buildings, with ordinary (OMF), intermediate (IMF), and special (SMF) levels of ductility. Each model presents two types of connections, including WUF-W and RBS. Three-dimensional models for all buildings are created in OpenSees software, and seismic responses are assessed for two earthquake scenarios. Seismic responses of buildings, such as peak floor acceleration, peak floor shear force, peak story drift, and residual and maximum displacement of isolators are calculated and analyzed. The results indicate that the ductility levels and connection types significantly affect seismic responses of base-isolated and fixed-based buildings. The RBS connection reduces the peak drift demands compared to the WUF-W connection, and the difference increases with the increment of building's height. The SMF superstructure decreases the peak floor acceleration and peak shear force compared to the IMF and OMF ones. Moreover, the peak floor drift ratio of the OMF superstructures is larger than the IMF and SMF in base-isolated buildings. The maximum difference is computed by about 80% between the OMF and SMF superstructures.

Effect of Inadequate Lap Splice Length on the Seismic Fragility of Ground Soft-story Reinforced Concrete Frames

In this study, numerical analysis was used to derive seismic fragility curves for 3-, 6- and 9-story reinforced concrete (RC) frame with inadequate lap splice length and ground soft-story condition. The structural models were subjected to 15 far-field natural earthquake records. The peak ground acceleration (PGA) of the selected records was scaled in the range of 0.05g to 0.50g with an increment of 0.05g. Incremental dynamic analysis was employed to determine their inter-story drift demand and capacities. The obtained results indicated that the probability of severe damage to the RC frames increased as the number of stories decreased. It was also observed that the RC frames with inadequate lap splice length exhibited significantly higher probability of collapse when compared with RC frames with adequate lap splice length.

Fragility functions for reinforced concrete columns incorporating recycled aggregates

Using recycled aggregates in the production of structural members is a relatively new application that has the potential to become a mainstream practice with effective green policies. Therefore, performance-based seismic assessment of structures constructed using recycled aggregates gains importance. Fragility curves, cumulative distribution functions that represent the probability of being in or exceeding a threshold for a selected engineering demand parameter, can be used in risk evaluation of these structures.In this study, fragility functions are generated by compiling the data of reinforced concrete columns incorporating recycled concrete aggregate available in the literature. The data is first evaluated through physical damage states, and three different damage states, namely Damage State 1, Damage State 2 and Damage State 3, are defined. Each damage state is then mapped to a certain repair category (cosmetic repair, structural repair or structural enhancement) so that damage can be defined in terms of economic loss, which would provide a road map in the disaster management decision making process. Drift ratio is selected as the engineering demand parameter since generally displacement demand is taken into account in the evaluation of seismic performance of structures. Based on the results of the statistical analysis, the median drift ratio values of 0.30%, 1.21%, and 3.25%, and logarithmic standard deviations of 0.95, 0.42, and 0.45 are determined for Damage State 1, Damage State 2, and Damage State 3, respectively, for columns incorporating recycled concrete aggregates. Furthermore, the effect of parameters on the relationship between drift demand and damage is investigated. In addition, seismic performances of the columns are discussed in terms of their ductility capacity. It is also shown that the ductility limit defined according to Paulay and Priestley (1992), and UBC (1997) for the reinforced concrete columns made of natural aggregate concrete is valid also for the columns made of recycled aggregate concrete.Besides, fragility functions are also generated for columns made of natural aggregate concrete in order to compare damage progression in columns made of recycled aggregate concrete with the aim of identifying the variability in drift demand. From the comparison of the fragility curves of these columns, it is found that at the same drift ratio, damage exceedance probabilities of both column types are similar for Damage State 1 and Damage State 2. However, for Damage State 3, at the same drift ratio, the damage exceedance probability of columns incorporating recycled concrete aggregates is higher than that of the columns made of natural aggregate concrete owing to relatively lower lateral load capacity of these columns.

Residual drift spectra for RC bridge columns subjected to near-fault earthquakes

Permanent displacement of a bridge column can be directly measured during the inspection after near-fault earthquakes. However, the engineer needs to estimate the expected residual drift at the design stage to determine if the bridge seismic performance is satisfactory. The most direct method to estimate the residual displacement is nonlinear response history analysis, which is time consuming and cumbersome. Alternatively, an attractive but indirect method is generating estimated residual displacement spectra that depend on displacement ductility demand, column period, site conditions, and earthquake characteristics. Given the period and the expected displacement ductility demand for the column, the residual drift response spectra curves can be utilized to estimate the residual drift demand. Residual drift spectra that are applicable to RC bridge columns in different parts of the United States were developed based on nonlinear response history analyses using a comprehensive collection of recorded and synthetic near-fault ground motions and were linked to one-second spectral acceleration (S1) of the AASHTO maps. It was also found that the residual drift ratio is below one percent when S1 is less than 0.6 g.

Rate-dependent model for simulating the hysteretic behavior of low-yield stress buckling-restrained braces under dynamic excitations

Buckling-restrained braces (BRBs) are often idealized with rate-independent simulation models. However, under dynamic loading, BRBs featuring low-yield point steel exhibit rate-dependency that may lead to appreciable amplifications of the BRB forces. This paper proposes a new rate dependent model for simulating a BRB’s response under dynamic excitations. The proposed model consists of a displacement-dependent asymmetric Menegotto-Pinto material law and a velocity-dependent bilinear oil damper model. The calibration process of the proposed model is also presented. Two approaches are demonstrated in which the proposed model can be utilized within a nonlinear frame analysis program. A comparative study based on test data from full-scale shake table tests of a five-story steel building equipped with BRBs underscores that if their rate-dependency is neglected then the BRB local force demands may be significantly underestimated. This may also lead to erroneous predictions of lateral story drift demands as well as absolute floor accelerations during earthquake shaking.

Fast Checking of Drift Demand in Multi-Storey Buildings with Asymmetry

Buildings possessing an asymmetrical arrangement of structural elements are torsionally unbalanced and can be vulnerable in a seismic event. Building codes of practices typically recommend the use of three-dimensional dynamic analysis to determine the seismic demands of a multi-storey building. Whilst most design practices are well equipped with commercial software for undertaking such analyses, designers often find it difficult to verify results. Much of the published technical articles present findings for buildings based on an idealised single-storey model. As a result of challenges in dealing with real multi-storey buildings, there has been very limited uptake of research findings in design practices. This article presents a three-tiered approach of estimating the displacement behaviour of the building in term of 3D/2D displacement ratio. The estimate can be used for verifying results reported from a computer package conveniently. The quick method provides predictions of the 3D/2D ratio and only requires the gross plan dimensions of the building to be known. The refined method requires knowledge of the torsional stiffness properties to be known, whereas the detailed method requires the eccentricity properties to be known as well. The proposed methodology is robust and reliable, as is demonstrated by case studies undertaken on six real multi-storey buildings.

Discussion of “Experimental Evaluation and Numerical Modeling of Wide-Flange Steel Columns Subjected to Constant and Variable Axial Load Coupled with Lateral Drift Demands” by Julien Cravero, Ahmed Elkady, and Dimitrios G. Lignos

Discussion of “Experimental Evaluation and Numerical Modeling of Wide-Flange Steel Columns Subjected to Constant and Variable Axial Load Coupled with Lateral Drift Demands” by Julien Cravero, Ahmed Elkady, and Dimitrios G. Lignos

Closure to “Experimental Evaluation and Numerical Modeling of Wide-Flange Steel Columns Subjected to Constant and Variable Axial Load Coupled with Lateral Drift Demands” by Julien Cravero, Ahmed Elkady, and Dimitrios G. Lignos

Closure to “Experimental Evaluation and Numerical Modeling of Wide-Flange Steel Columns Subjected to Constant and Variable Axial Load Coupled with Lateral Drift Demands” by Julien Cravero, Ahmed Elkady, and Dimitrios G. Lignos

A simplified model for inelastic seismic analysis of RC frame have shear hinge in beam-column joints

This paper presents a simplified beam-column joint modeling technique for the inelastic analysis of reinforced concrete moment-resisting frames. The joint model consists of a zero-length link element that is provided with moment-rotation lumped plasticity hinge, provided at the intersection of beam/column elements, to simulate the nonlinear shear behavior of the joint panel. The backbone of the moment-rotation constitutive law was defined using the empirical shear stress-shear strain relationship proposed by Kim and LaFave. The hysteretic response of the joint was simulated using the multi-linear hysteretic rule proposed by Sivaselvan and Reinhorn. The modeling technique was applied to two portal frames tested under quasi-static cyclic loads for predicting the cyclic force-displacement hysteretic response. The prediction was found in good agreement with the experimental response; the numerical models were efficient in simulating the shear behavior of the joints, loading/unloading stiffness and pinching behavior of the frames’ lateral force-displacement hysteretic response. The modeling technique was extended to two-story frames for nonlinear response history analysis for both design base and maximum considered earthquakes. Inter-story drift demands were obtained and critically compared to highlight the efficacy of the accurate modelling of beam-column joint using lumped plasticity shear simulation hinges. The study also confirms the beneficial role of beam-column joint detailing in resisting both design base and maximum considered earthquakes without exceeding the life safety and collapse prevention limit states respectively.

A multi-mode N2 (MN2) pushover procedure for ductility level seismic performance evaluation of jacket type offshore platforms

The static pushover analysis can be employed to check the adequate ductility of jacket type offshore platforms (JTOPs) under the abnormal level earthquake (ALE) in the design phase. The conventional pushover analysis suffers from different inherent limitations such as disregarding the higher mode effects. In this paper, it is intended to develop an advanced pushover procedure for the seismic design of JTOPs. To this end, a multi-mode N2 (MN2) pushover procedure is proposed that modifies the SRSS modal combination rule in the extended N2 procedure. The proposed procedure utilizes several modes of vibration to increase the accuracy of the method. The presented procedure is applied to two case study JTOP structures for the ductility level seismic performance evaluation. The seismic demands obtained by the MN2 procedure is compared with the average results of the benchmark nonlinear response history (NRH) analyses conducted using a suite of far-field ground motion records scaled to the ALE design response spectrum. The results of a statistical study show that there is a strong correlation between the spectral values of the records at the shorter periods of JTOPs and the responses of the foundation and the lower braced story levels of the jacket, which emphasize the important effects of the higher modes on the seismic demands of JTOPs if considering soil-pile-structure interaction. The utilized formulation for the modal combination is shown to improve the accuracy of the MN2 method at the pile foundation and at the lower braced story levels of the jacket. Moreover, the seismic drift demands predicted by the proposed MN2 method are in excellent agreement with the benchmark NRH results along the height of the platforms. Considering the simplicity and ease of application, this method can be utilized as a useful tool for checking the design ductility level earthquake.

Seismic Loss Estimation in Pre-1970 Residential RC Buildings: The Role of Infills and Services in Low–Mid-Rise Case Studies

The lessons learned after recent earthquakes have highlighted the key role played by infills and services in damage and loss of Reinforced Concrete (RC) buildings. Their influence in seismic performance and loss estimation of selected RC building case studies is thoroughly analyzed here. The case study selection aims to be representative of existing buildings built in Italy before 1970, and covers a different number of stories and design typologies. The seismic responses of the case-study buildings are numerically analyzed by means of non-linear static pushover analysis (PO) considering a lumped plasticity approach with a quadri-linear flexural response for beam/column elements (properly calibrated for RC elements reinforced with plain bars) and a tri-linear compressive-only axial response with diagonal concentric struts for infill panels (empirically derived from experimental data on hollow clay masonry walls). Economic loss estimation is carried out via a component-based methodology that relies on the main repairing activities and resultant costs required for the refurbishment of infills and services for different damage levels. Accordingly, a damage analysis is performed herein, given the intensity measure, based on a comparison between Interstory drift demand from PO analysis and drift-based fragility functions specific for masonry infills. Loss curves, relating the total building repair cost to peak ground acceleration (PGA), are presented and compared for the analyzed case study buildings to show their trends and quantify the incidence of infills and services with respect to the reconstruction cost. A comparison between these outcomes and those recently found in the literature emphasizes the robustness of the considered approach and the reliability of the hypotheses about damage and loss assessment.

Effect of Soil-Structure Interaction on the Seismic Response of Existing Low and Mid-Rise RC Buildings

Reconnaissance studies performed after destructive earthquakes have shown that seismic performance of existing buildings, especially constructed on weak soils, is significantly low. This situation implies the negative effects of soil-structure interaction on the seismic performance of buildings. In order to investigate these effects, 40 existing buildings from Turkey were selected and nonlinear models were constructed by considering fixed-base and stiff, moderate and soft soil conditions. Buildings designed before and after Turkish Earthquake code of 1998 were grouped as old and new buildings, respectively. Different soil conditions classified according to shear wave velocities were reflected by using substructure method. Inelastic deformation demands were obtained by using nonlinear time history analysis and 20 real acceleration records selected from major earthquakes were used. The results have shown that soil-structure interaction, especially in soft soil cases, significantly affects the seismic response of old buildings. The most significant increase in drift demands occurred in first stories and the results corresponding to fixed-base, stiff and moderate cases are closer to each other with respect to soft soil cases. Distribution of results has indicated that effect of soil-structure interaction on the seismic performance of new buildings is limited with respect to old buildings.

A database of seismic designs, nonlinear models, and seismic responses for steel moment-resisting frame buildings

A number of simplified methodologies have been developed and used to estimate seismic drift demands in buildings. However, none of them have been systematically tested against a large number of buildings subjected to a diverse set of ground motions. This is partly attributed to the lack of existing databases of building designs, nonlinear structural models, and simulated seismic responses. This article introduces the development of a comprehensive database, which includes 621 special steel moment-resisting frames designed in accordance with modern codes and standards and their corresponding nonlinear structural models and seismic responses (i.e. peak story drifts, peak floor accelerations, and residual story drifts). The seismic responses for a subgroup of 100 steel moment-resisting frames subjected to three groups of site-specific ground motions (with 40 records each), at the service-level, design-based, and maximum considered earthquakes, are also included. The database has been utilized by the authors (in a separate study) to evaluate the performance of existing methods and develop data-driven and hybrid (combination of mechanics-based and data-driven) models for estimating seismic structural drift demands. The database can also be utilized in the development and implementation of a performance-based analytics-driven seismic design methodology.

Seismic upgrade of deficient RC frames using different configurations of eccentric steel braces

Deficient reinforced concrete frames in many developing countries need upgrade and effective retrofitting techniques to enhance the strength, stiffness and energy dissipation capacity of the system. This paper presents eccentric steel braces (ESB) retrofitting technique for deficient reinforced concrete moment-resisting frames. A hollow box steel brace was connected to the beam at different lengths from the beam-column joint that is at Lbeam/8, Lbeam/6 and Lbeam/4. A nonlinear static analysis was carried out on three-story deficient and ESB retrofitted frames and the force–deformation hysteretic response was noted. The ESB retrofitted frames considerably changed the lateral forces load transfer path and as a result increased the strength, stiffness and hysteretic damping of the frames as opposed to the deficient bare frame. Furthermore, a dynamic time history analysis was performed on five and seven-story ESB retrofitted frames and story drift demands were identified. ESB retrofitted frames significantly reduced story drift demands as compared to bare frames.