Steel Moment-resisting Frames Research Articles

Direct displacement-based design approach for isolated structures equipped with supplemental fluid viscous damper

In this paper, direct displacement-based design (DDBD) approach is extended for the isolated structures equipped with supplemental fluid viscous damper (FVD) while this method can be also implemented for the isolated structures without supplemental FVD. The design process has been developed based on a non-iterative manner whereas a complexity from preliminary design to iterative design process can be found in previous DDBD approaches. To validate the proposed method, three steel moment-resisting frames with four, eight and twelve stories are designed using this method. Nonlinear time-history analysis has been performed under a far-field record set including twenty-two earthquakes and a near-field record set including fourteen earthquakes. The results show that the isolated frames without supplemental FVD have effective performance under far-field earthquakes. Therefore, the proposed design method can be employed as effective design approach for isolated structures. However, their performance is not very reliable under near-field earthquakes because base isolation system originally does not have effective performance under near-field earthquakes. Seismic performance of isolated frames equipped with FVD shows that their peak response is less than target drift under both far-field and near-field earthquakes and these frames have effectively achieved the desired performance levels related to both superstructure and base isolation levels. Therefore, the developed DDBD approach can be introduced as an effective design method for isolated structures equipped with supplemental FVD.

Efficient seismic risk assessment of irregular steel‐framed buildings through endurance time analysis of consistent fish‐bone model

SummaryThe seismic risk framework of building structures has been presented to reduce earthquake‐induced adverse consequences. In this context, probabilistic analysis of engineering demand parameters (EDPs) is always associated with many uncertainties and high computational demand for use in practical applications. This study has been presented to efficiently estimate the distribution of EDPs in probabilistic seismic risk assessment of irregular steel moment‐resisting frames (SMRFs). For this purpose, the incorporation of the consistent fish‐bone (CFB) model and the endurance time (ET) analysis method has been used. The proposed method (i.e., CFB‐ET) has a substantial impact on reducing the complexities of the new generation of performance‐based earthquake engineering (PBEE) by significantly reducing the degrees of freedom (DOFs) of the original building and the number of required nonlinear time history analyses (NLTHAs). The efficiency of the proposed method has been evaluated using three 3‐, 9‐, and 20‐story SMRFs irregular in bay lengths and three other SMRFs with the same number of stories but irregular in height. In this evaluation, the maximum structural response at all seismic intensity levels up to the collapse stage and also the structural response profile along the building height at service‐level earthquake (SLE), design‐based earthquake (DBE), and maximum credible earthquake (MCE) seismic intensity levels have been investigated. Next, the efficiency of the proposed method in seismic risk assessment is investigated by using seismic fragility and vulnerability curves. The results of these evaluations have been compared with the full incremental dynamic analysis (IDA) of the full DOF model of the building (i.e., SMRF‐IDA) under 44 far‐field ground motions of FEMA P‐695 in terms of computational accuracy and effort. Furthermore, a new approach to estimating the dispersion of the structural response in the ET method has been proposed, whose efficiency has been investigated in the above‐mentioned evaluations. The results show that the proposed method allows for using the new generation of PBEE in engineering applications by significantly reducing computational demand and having sufficient accuracy in the seismic assessment process.

Seismic performance of steel columns interacting with embedded column bases while exhibiting inelastic deformations

Observations from past earthquakes indicate that embedded column bases (ECBs) in steel moment resisting frames (MRFs) exhibited inelastic deformations while they were designed to be non-dissipative. Consequently, the amount of inelastic damage concentrated in the respective fixed-end steel MRF columns was somewhat eased. This nonlinear interaction is examined by means of mixed-dimension continuum finite element (CFE) parametric simulations validated to available full-scale experimental data. The examined parameters are the ECB’s peak flexural strength, its plastic deformation capacity, and its elastic stiffness. The influence of the loading conditions along with the steel column geometric characteristics are also investigated. The simulation results indicate that, when ECBs exhibit partial inelastic deformation through controlled cracking, they feature enhanced plastic deformation capacities, which are, on average, 30–50% greater than those in commonly used non-dissipative bases. Moreover, residual axial shortening of first story steel MRF columns is reduced by up to 50% at a reference lateral drift of 4% rads. These benefits are more pronounced for steel columns with slender, but seismically compact cross sections (i.e., h/tw>20), which are prone to local and/or member instabilities at modest lateral drift demands (e.g., 2% rads). The extent of the above effects is controlled by the peak flexural strength ratio between the steel column and the ECB, the plastic rotation of the ECB and the loading history (cyclic versus monotonic). Limitations are discussed.

EC3-Compatible Methods for Analysis and Design of Steel Framed Structures

The behaviour of steel structures is affected by two nonlinearities—the geometric and material nonlinearity—and by the unavoidable presence of imperfections. To evaluate the ultimate capacity of a structure, these effects should be taken into consideration during the design process, either explicitly in the analysis or implicitly through the verification checks. In this context, Eurocode 3 provides several design approaches of different complexity and accuracy. The advantages and disadvantages of these approaches are discussed. Five different methods in conformity with the Eurocode provisions are applied for the design of four moment resisting steel frames of varying slenderness. The influence of nonlinearities and imperfections in respect to the slenderness of the structure is illustrated. The examined methods are compared in terms of the predicted ultimate capacity and their efficiency is assessed against the most accurate between them, i.e., an advanced geometrically and materially nonlinear analysis. It is shown that considerable differences arise between the methods. Nevertheless, except for the commonly used 2nd order analysis followed by cross-section verifications, the remaining methods are mostly on the safe side.

Performance-based seismic design method for retrofitting steel moment-resisting frames with self-centering energy-absorbing dual rocking core system

This research intends to propose an upgraded strategy for steel moment-resisting frames (SMRFs) by implementing Self-Centering Energy-absorbing Dual Rocking Core (SEDRC) systems to achieve enhanced seismic performance, improved seismic resilience, and avoid inter-story drift concentration. A performance-based method is developed to design the SEDRC system for upgrading the existing SMRF. The maximum inter-story drift and drift concentration factor are adopted as the design performance objectives. Two prototype SMRFs with three and nine stories are introduced to validate the proposed performance-based design procedure. Nonlinear static analyses were first performed for the original SMRFs and the updated ones (denoted as SEDRC-MRFs) to investigate the efficiency of the proposed retrofit strategy. The pushover curves of the analytical structures confirm that the SEDRC systems can increase the stiffness and strength of SMRFs, and SEDRC-MRFs upgraded through the proposed design procedure show no strength deterioration before the fracture of the brace in the SEDRC system. A set of 18 ground motions were chosen to study the seismic performance of the designed SEDRC-MRFs. The analysis results indicate that SEDRC-MRFs can obtain the desired performance objectives. The SEDRC system could reduce the maximum and residual inter-story drifts of SMRFs and promote uniform inter-story drift distribution. Moreover, the designed SEDRC-MRFs can be used immediately without the need for repairing structural members after the design basis earthquakes and can be repaired economically after the maximum considered earthquakes.

Development of DDBD for steel MRFs using inelastic response-based seismic lateral force distribution

Direct displacement-based design (DDBD) method is known as an effective performance-based design approach. Although this design method is theoretically stronger than force-based design approach, some previous studies reported the inability of this method to effectively achieve the design performance level. In this study, an improvement of DDBD is proposed for steel moment-resisting frames (MRFs) while the seismic design lateral force distribution is based on the inelastic response of structures. To validate its effectiveness, the steel MRFs with different heights of 4, 8 and 12 stories are designed using this modified DDBD to achieve life safety performance level under seismic hazard having 10 % exceedance probability in 50 years. The nonlinear time history analysis of these frames has been performed under twenty-two earthquake records. The results show the inability of previous DDBD to achieve the desirable performance level whereas the modified DDBD approach has effectively achieved the design performance level. All plastic hinges have been also formed in desirable locations whereas the previous DDBD approach is not quite successful in achieving this goal. The results also show that the elimination of strength-reduction factors from design process of DDBD makes drift ratio and story shear close to target drift limit and design shear. • Inability of current DDBD to achieve design performance level in steel MRFs. • Distribution of seismic lateral force based on inelastic response to improve DDBD. • Location of plastic hinges in steel MRFs designed using DDBD. • Effect of strength-reduction factors on effectiveness of DDBD.

Probabilistic Seismic Assessment of Moment Resisting Steel Buildings Considering Soft-story and Torsional Irregularities

In this study, the fragility curves were developed for three-, five-, and eight-story moment resisting steel frame structures with considering soft story and torsional irregularities during the earthquake mainshock to assess the probabilistic effects of irregularities in plan and height of steel structures. These models were designed according to Iranian seismic codes. 3D analytical models of steel structures were created in the OpenSees software platform and Incremental Dynamic Analysis (IDA) was conducted to plot the IDA curves. The maximum value of inter-story drift was selected as the demand parameter and the capacity is determined according to the HAZUS-MH limit states; and finally, the corresponding fragility curves were developed. The results of the 3D nonlinear dynamic analysis indicated that the damage state of the structure due to soft story irregularity was decreased with increasing stories. On the other hand, the damage caused by torsional irregularity in plan was increased by increasing the height of the structure. For example, in the 3-story structure, soft-story effect on damage probability was more dominant than torsional irregularity.

Effects of Steel Braces on Robustness of Steel Frames against Progressive Collapse

External installation of steel braces is an effective approach to increase the lateral load resistance of steel moment-resisting frames. However, the effects of existence of steel braces on the robustness of steel moment-resisting frames to resist progressive collapse is still not clear because little study has been carried out. To fill this gap, in this paper, six multistory steel moment-resisting subframes (three bare frames and three braced frames) were fabricated and tested. Test results indicated that the specimen with reduced beam section in the connection zone performed best among three types of connections due to the guaranteed formation of plastic hinges at the location of reduced section and avoiding brittle fracture of weld at the connection. Experimental results proved that steel braces could increase the load-resisting capacity by 45.1% and 83.9% of the frame with weld connection and end plate connection, respectively. The gusset plate restricted the rotation of the plastic hinges in the second story of the braced frames with V-shaped bracing, which decreased its deformation capacity and degraded its catenary action capacity. Actually, the ultimate load of the braced frames with V-shaped bracing is only 87.5% of that of the counterpart without any braces. Because the compressive braces were severely buckled before the displacement reached 0.4% of the beam span, it had little effect on yield load but increased the initial stiffness of the bare frames. Thus, a majority of the benefits of the bracing system were attributed to the tensile braces. Moreover, the analytical results evaluated the differences in load resistance and development of load-resisting mechanisms in different stories. Furthermore, the contribution of compressive and tensile braces was decomposed individually by analytical analysis.

Multi-objective optimal design of steel MRF buildings based on life-cycle cost using a swift algorithm

This paper proposed a swift multi-objective optimization algorithm for the performance-based design of steel moment-resisting frame (MRF) buildings considering life-cycle cost (LCC). Two components of the life-cycle cost, including the initial construction cost and the lifetime seismic damage cost of the building, are considered as two separate objectives of the optimization problem and are minimized simultaneously using the proposed algorithm. The proposed algorithm called Multi-objective Uniform Damage Optimization (MUDO) is an adaptive method that utilizes the concept of uniform damage (deformation) theory. MUDO leads to design alternatives for the building that are optimal (minimum) for both mentioned objectives. These alternatives, called Pareto optimal designs, allow decision-makers to choose the optimal investment option according to the project goals and by the trade-off between optimization objectives. The proposed algorithm is used to optimize three steel MRF buildings with 3, 6, and 9 stories. The lifetime seismic damage cost is calculated by assessing seismic losses at several ground motion intensities and using a probabilistic approach presented in HAZUS. A Monte Carlo analysis is used in this research to account for inherent uncertainties in structural response. The results demonstrate that MUDO can find competitive Pareto optimal designs in a much shorter time compared to a well-known metaheuristic algorithm called non-dominated sorting genetic algorithm II (NSGA-II).

Seismic fragility assessment of steel frame structures equipped with steel slit shear walls

The steel slit shear wall (SSSW) dissipates seismic energy through the developed plasticity at steel plate links separated by cutting slits. Its shear hysteretic behavior is influenced by parameters such as link configuration, amplitude of out-of-plane deformation and steel property. To investigate the contributing factors for different types of shear hysteretic behavior, four SSSW specimens with design parameters of link configuration (flat or twisted) and steel property (common carbon steel or low yield steel with significant strain hardening) are designed and tested under quasi-static cyclic loading. Four types of shear hysteretic behaviors are obtained: pinched, pinched without cyclic degradation, plump and combined (plump first and pinched later). Though pinching also occurs in the SSSW with twisted links, there is no strength and stiffness degradation in the hysteresis. Thanks to the strain hardening of low yield steel, the low yield SSSW presents plump hysteresis. These different shear hysteretic behaviors are successfully simulated using macro models firstly, including user-defined models for the pinched type without cyclic degradation and the combined type. Then moment resisting steel frame (MRSF) models equipped with these SSSWs are built respectively to study their influence on the overall structural performance. Results of fragility analysis show that four types of SSSWs enhance the seismic performance of MRSF structures at both small and moderate damage states nearly in the same degree. At extensive and collapse damage states, the SSSW with twisted links performs the best, followed by the low yield SSSW.

Seismic performance assessment of steel moment-resisting frames with self-centering viscous-hysteretic devices

This study evaluates the seismic performance of the steel frame with self-centering viscous-hysteretic devices (SC-VHD frame) and investigates the effect of design parameters of the SC-VHD, preload, loading stiffness, unloading stiffness and viscous damping ratio, on seismic performance through conducting nonlinear static pushover analysis, fragility analysis and risk assessment on 4-, 8-, and 12- story SC-VHD frames with different design parameters. Results indicate that the maximum normalized base shear and the ultimate roof drift ratios of the SC-VHD frames increase by more than 13.3% and 42.9%, respectively, compared with the conventional frames. The SC-VHD frames have acceptable low probabilities of collapse and probabilities exceeding considered residual drift under the maximum considered earthquake (MCE) and during a 50-year period. Accordingly, the most sensitive parameter regarding the probabilities of collapse is loading stiffness, followed sequentially by unloading stiffness, viscous damping ratio and preload. While for the probability exceeding considered residual drift under the MCE, the parameters of ring springs, preload, loading stiffness and unloading stiffness, have a significant effect on the 4- and 8-story SC-VHD frames, and the viscous damping ratio of the viscous damper has the most significant effect on the 12-story SC-VHD frame. For the probability exceeding considered residual drift during a 50-year period, the most sensitive parameter is the viscous damping ratio.

Lifetime risk-based seismic performance assessment of buildings equipped with supplemental damping and base isolation systems under probable mainshock-aftershock scenarios

This research attempts to investigate the efficacy of different passive vibration control systems, from both technical and economic points of view, to mitigate the lifetime seismic risk of typical buildings under probable mainshock-aftershocks sequences via a novel probabilistic framework. In this regard, three separate common building models are considered, including 5-, 10-, and 15-story intermediate steel moment-resisting frame buildings. These models are designed optimally with/without passive vibration control systems, including hysteretic damping, linear viscous damping, and base isolation systems as independent alternatives. Next, for the hazard modeling, all active faults located in the greater Tehran area are taken into account to simulate the earthquake scenario. Afterward, probable earthquake scenarios during the 50 years of buildings’ life-spans are generated randomly, including the events, as well as their corresponding synthetic stochastic accelerograms. In the next step, responses of buildings and their cumulative losses are calculated under the whole scenarios considering various damage types such as structural, non-structural, and content-related. In the end, by using the Monte-Carlo simulation method, the seismic risk of different building alternatives is estimated. Interestingly, the application of each of the considered vibration control devices leads to a significant reduction in lifetime losses (both physical and economic). It is revealed that the viscous damper has the highest efficacy among different systems. Importantly, by neglecting aftershocks' effects, the estimated lifetime risk will be underestimated by about 27 to 57% below the expected value. Finally, among the active faults in the greater area of Tehran, the North Tehran, Kahrizak, and Mosha faults have the highest contribution in the estimated lifetime risk of buildings.

Seismic acceleration demand and fragility assessment of storage tanks installed in industrial steel moment-resisting frame structures

Recent seismic events highlighted the importance of non-structural elements in seismic assessment and performance-based earthquake engineering. When referring to industrial facilities, storage tanks represent one of the most critical components for the continuous functionality of industrial and building plants after seismic events. At the same time, the dispersion of dangerous or inflammable materials, due to their collapse, could become a safety issue in terms of both human life and environmental impact. This study focuses on the seismic performance assessment of a liquid-storage tank installed in an industrial steel moment-resisting frame building. The performance is assessed in terms of fragility curves, that are provided for the meaningful limit states that affect the liquid-storage tank and considering several intensity measures. The results related to the structural performance are also discussed in detail in terms of fragility functions and floor response. The assessment is performed through nonlinear dynamic analyses using two different modelling approaches to evaluate the structural and non-structural performance. The first modelling approach explicitly accounts for the tank-structure interaction, while in the second one the tank is only modelled considering its seismic mass. The results show the importance of the modelling assumptions to accurately estimate the seismic demand on liquid-storage tanks and corresponding fragility, which is calculated in terms of different intensity measures (IM): average ground spectral acceleration, peak floor acceleration, and floor spectral accelerations. Peak floor acceleration, drift profiles, and absolute acceleration and relative displacement floor response spectra are instead scrutinised for what concerns demand. The comparison with the code formulations available in the literature for estimating the seismic demand to non-structural elements pointed out their inaccuracy for the analysed liquid-storage tank and the need for further efforts to develop more accurate procedures. Furthermore, the outcomes indicate that the IM adopted for secondary components, i.e. peak floor acceleration, may not always be the optimal one hence additional attention should also be paid to the definition of updated fragility curves for non-structural elements, identifying and adopting more suitable IMs.

Consistent one-bay frame simplified model for efficient seismic evaluation of steel moment frame buildings with equal and unequal bay lengths

A Consistent One-Bay Frame (COBF) simplified model for the efficient estimation of Engineering Demand Parameters (EDPs) in Steel Moment Resisting Frames (SMRFs) with equal and unequal bay lengths is proposed. The geometry of the Substitute One-Bay Frame (SOBF) simplified model has been used, and the elastic and inelastic properties of the beams and columns in this model are determined using an energy consistent approach in a way that the energy balance is established between the members of the original SMRF and the SOBF simplified model. This way, the SOBF simplified model, which originally could only be used for MRFs with equal bay lengths, can now be used for MRFs with any geometric arrangement for bay lengths. Two 9- and 20-story SMRFs with equal bay lengths and two others with unequal bay lengths have been employed as structural models. The concentrated plastic hinges with modified Ibarra-Medina-Krawinkler (IMK) deterioration model have been used to apply strength and stiffness deterioration in numerical models. The accuracy of the proposed simplified model in estimating the EDPs along the height of the structure has been evaluated using Nonlinear Time History Analysis (NLTHA) under 22 pairs of FEMA P-695 earthquake records at the three seismic intensity levels of SLE, DBE, and MCE. The proposed simplified model has been compared with the Consistent Generic (CG) simplified model previously developed for SMRFs. This comparison suggests that the proposed simplified model is more efficient and reliable. Evaluating the seismic performance of the proposed simplified model using the conventional Incremental Dynamic Analysis (IDA) method and the Endurance Time Analysis (ETA) method at all of the seismic intensity levels reveals that using the COBF simplified model along with the ETA simplified method not only reduces the Degrees of Freedom (DOFs) of the original SMRF and the number of the NLTHAs but also leads to a proper estimation of the EDPs of the original SMRF from low seismic intensity level to collapse intensity level. Therefore, it is very helpful to use it for reducing the complexities of the new generation of Performance-Based Earthquake Evaluation.

Modeling flexural overstrength factor for steel beams using heuristic soft-computing methods

When evaluating the structural performance of steel moment-resisting frames, significant consideration is given to the flexural behavior of steel beams. This analysis focuses on the primary response parameters, such as the ultimate flexural resistance and rotation capacity that dictate the behavior of the steel beam. In practice, the flexural behavior of steel beams determines the ductile design of steel structures, and a sufficient ductility of steel permits plastic hinge rotations until a full-collapse mechanism is attained. However, it is relatively impossible to guarantee the occurrence of a full-collapse mechanism if the beam’s flexural overstrength is not appropriately quantified by the application of capacity design procedures. Therefore, this study developed analytical formulations to predict the steel beam’s flexural overstrength factor (s) using the multilayer perceptron (MLP)11AMFF: adaptive multilayer feed forward; ANFIS-PSO: adaptive neuro fuzzy system with particle swarm optimization; ANOVA: analysis of variance; GA: genetic algorithm; KKT: Karush–Kuhn–Tucker; LSSVR: least-square support vector regression; M5 Tree: M5 model tree; MAE: mean-absolute errors; MARS: multivariate adaptive regression splines; MLP: multi-layer perceptron; MLR: multiple linear regression; PSO: particle swarm optimization; RBNN: radial basis-function-based neural network; RHS: rectangular hollow section; RMSE: root-mean-square errors; SHS: square hollow sections; SIS: swarm intelligence systems., radial-basis-function-based neural network (RBNN), least-squares support vector regression (LSSVR), adaptive neuro fuzzy system with particle swarm optimization (ANFIS-PSO), multivariate adaptive regression splines (MARS), M5 model tree (M5 Tree), and multiple linear regression (MLR). Numerous cross-sectional geometries were tested with square and rectangular hollow sections, and with I and H sections. The data used for the analysis were sourced from completed experimental studies available in the open literature. The data were categorized as training and testing datasets to develop the models based on the aforementioned methods. For the models, the mechanical properties of the materials, shear length of steel beams, and the geometric properties of the sections constituted the independent variables. Overall, the RBNN demonstrated the best prediction of the steel beam overstrength with a root-mean-square error of 0.099 mm, mean absolute error of 0.066 mm, and a correlation coefficient of 0.825. It was observed that all six machine learning methods provided better estimates than the MLR. The machine learning models developed in this study are expected to serve as a hand tool for engineers and steel structure constructors.

Blast Performance Evaluation of Steel Moment-Resisting Frame Equipped with Smart Bolted Connection

This paper deterministically evaluates the blast performance of steel moment-resisting frame (MRF) smart structures equipped with nickel titanium shape-memory alloy (NiTi SMA) bolted connections. The recentering capacity of the NiTi SMA bolts is exploited to limit the structural damage and reduce the residual drift after a near-field explosion event. A simplified blast over-pressure calculation approach is used to draw the blast overpressure profile. Numerically, the NiTi SMA–based connection is designed conforming to current European standards and is modeled with a solid finite element. The effect of high-strain-rate actions is considered. The response of the NiTi SMA–based connection is obtained by the nonlinear transient analysis and compared with the steel bolted connection. The NiTi SMA–based connection provided excellent recentering capacity. Later, the moment-rotation response of the NiTi SMA–based connection is utilized in the MRF structure. A simplified self-centering connection is proposed to produce smart behavior in the MRF structures. The results reveal that using NiTi SMA connections keep interstory drift relatively low and substantially provide reasonable structural lateral resistance under near-field blast loading.

The effect of pulse-like ground motion on the performance-based confidence level of setback special steel moment-resisting frames

This paper addresses the effects of near-fault (NF) earthquake records containing forward directivity pulses and the pulse-to-structure period ratio (TP/T1) on the performance-based confidence levels of regular and setback frames. For this purpose, ten steel moment-resisting frames, including a regular structure and nine setback frames, were subjected to 55 near-fault pulse-like records as well as their low- and high-frequency components called the “pulse” and “residual” portions, respectively. To study the effect of the TP/T1 ratio, the records were classified into four groups representing average values of TP/T1 ∼ 0.5, 1, 2, and >2. In addition to NF motions, 32 far-fault records were imposed on the studied structures. The results from incremental dynamic analyses revealed the higher confidence level of meeting performance levels under NF records compared to far-fault ones for both regular and setback frames (the case of TP/T1≥1 for the regular frame and the cases of TP/T1 ∼ 1 and 2 for the irregular ones). The importance of considering the TP/T1 ratio was shown by comparing the performance-based confidence levels under FF and NF ground motions as well as those under original NF records and their frequency components. It was found that ignoring such classification can lead to reversed results for both regular and irregular structures. Furthermore, directivity pulses with TP/T1>1 can represent original NF records when evaluating the confidence levels of the regular structures, especially as LS and CP limit states. However, it is highly recommended to impose original NF records on irregular structures to assess their performance-based confidence levels.

Improvements in the direct displacement-based design procedure for mid-rise steel MRFs equipped with viscous dampers

This paper aims to propose straightforward modifications in the direct displacement-based design (DDBD) procedure to reach an optimal design for mid-rise steel moment-resisting frames (MRFs) equipped with linear fluid viscous dampers (FVDs). The modifications include using an alternative equivalent damping as well as applying the ratio of pseudo-spectral velocity to spectral velocity. To compare the proposed method with respect to the conventional DDBD procedure, the seismic performance of a set of low to mid-rise structures consisting of 3-, 6-, 9- and 12-story steel MRFs is investigated. This is carried out via nonlinear time-history analyses by using a set of design spectrum compatible ground motion records at two seismic hazard levels, i.e., the design earthquake (DE) and maximum considered earthquake (MCE). Furthermore, two different patterns, i.e., uniform and non-uniform, for distribution of damping coefficients at the height of the structures are used. Results show that the proposed method can meet the performance targets defined at the DE and MCE hazard levels. Moreover, the peak roof displacement of the structures that is estimated by the proposed method is acceptably the same as captured via the nonlinear analyses. Finally, implementing the proposed method leads to a reduction in the weight of the steel used in the MRFs in comparison with the structures designed by the conventional DDBD procedure and smaller damper forces, i.e., lighter damper constants, can also be achieved.

Hysteretic Behaviour of Shear Stud Connectors in Composite Steel Moment‐Resisting Frames

AbstractIn steel moment resisting frames (MRFs) with composite floor slabs, seismic loads are transmitted from the slab into the beam through shear studs. Shear strength degradation of the studs due to cyclic loading results into the loss of the load transfer mechanism and composite action. To date, several low‐cyclic, high‐amplitude push‐out tests have been conducted to characterize the shear stud hysteretic behaviour. These isolated tests showed considerable degradation in the shear stud resistance; nevertheless, they do not simulate the actual hysteretic behaviour of shear studs within a composite steel beam. To elaborate, conventional cyclic push‐out tests do not replicate the actual stress state in the concrete slab, nor do they account for the force redistribution between the shear studs. This paper investigates the hysteretic behaviour of shear studs in steel MRFs with composite floor slabs. The load‐slip hysteretic behaviour of the shear studs is examined through continuum finite element (CFE) models of 2‐bay sub‐systems and their corresponding cruciform subassemblies. Moreover, shear force and interlayer slip envelopes are obtained along the composite steel beams. Results of the CFE models are used to propose new design recommendations for shear studs in composite‐steel MRFs.

Seismic response of a steel resilient frame equipped with self‐centering column bases with friction devices

AbstractIn the last two decades many researchers focused on the development of innovative building structures with the aim of achieving seismic resilience. Among others, steel Moment Resisting Frames (MRFs) equipped with friction devices in beam‐to‐column joints have emerged as an effective solution able to dissipate the seismic input energy while also ensuring the damage‐free behaviour of the system. However, to date, little attention has been paid to their column bases, which represent fundamental components in order to achieve resilience. In fact, column bases designed by current conventional approaches lead to significant seismic damage and residual drifts leading to difficult‐to‐repair structures. The present paper evaluates the seismic performance of steel MRFs equipped with an innovative damage‐free, self‐centring, rocking column base joints. The proposed column base consists of a rocking splice joint where the seismic behaviour is controlled by a combination of friction devices, providing energy dissipation capacity, and pre‐loaded threaded bars with disk springs, introducing restoring forces in the joint. The design procedure of the column base is presented, a numerical OpenSees model is developed to simulate the seismic response of a perimeter seismic‐resistant frame, including the hysteretic behaviour of the connection. Non‐linear dynamic analyses have been carried out on a set of ground motions records to investigate the effectiveness of the column base in protecting the first storey columns from yielding and in reducing the residual storey drifts. Incremental Dynamic Analyses are used to investigate the influence of the record‐to‐record variability and to derive fragility curves for the whole structure and for several local engineering demand parameters of the frame and of the column base connection. The results show that the damage‐free behaviour of the column bases is a key requirement when self‐centering of MRFs is a design objective.