Steel Special Moment Resisting Frames Research Articles

Investigating seismic performance of SMRF buildings under subshear and supershear rupture conditions: Assessment of storey-level internal damage

The source effect of earthquake ruptures causes spatial-temporal variability in ground motion characteristics in the vicinity of a fault. This research employs a dynamic rupture-based method with SPECFEM3D to simulate two rupture types: subshear and supershear. While the former depends on the rupture length of the fault, the latter depends on the rupture velocity. Synthetic earthquake ground motion data are generated across the entire domain, encompassing both near-fault and far-fault locations. Ground motions exhibit multidimensionality, with subshear effects evident in the fault normal component and supershear effects distinctly reflected in the fault parallel component. These ground motions have been employed to excite low (S1L), mid (S1M), and high-rise (S1H) steel special moment resisting frame (SMRF) buildings modelled using the modified Ibarra-Medina-Krawinkler (IMK) analogy. Understanding the far-field effects of supershear earthquakes and the near-field effects of directivity in subshear earthquakes on the structural response of SMRF buildings has been aided by several engineering demand parameters (EDPs), including floor acceleration, interstorey-drift ratio, and roof drift ratio.

Development of Near-Field Earthquake Loading Protocols for the Steel Moment Connections in Iran

Steel moment frame buildings are widely used in Iran and around the world. The observations from major earthquakes underscored the importance of the beam-to-column connections in seismic force-resisting systems and their influence on such building’s performance. Since the most catastrophic earthquakes that occurred in Iran contained the characteristics of near-field earthquakes, hence understanding the behavior of these connections under such earthquakes is needed. This study proposed loading protocols for special steel moment-resisting frame systems under near-field earthquakes in Iran. Each of them had 23 damage half-cycles with a maximum inter-story drift of 0.065 radians. The proposed loading protocols were compared numerically to each other. The most influential one was chosen as the main loading protocol since it has the most destructive effect on the connection. The main loading protocol showed a faster rate of increase in the plastic strain and imposed the highest amount of dissipated energy at a former loading stage compared to other near-field earthquake loading protocols related to other countries such as SAC and Fang et al.

An improved 2DOF system for predicting the seismic performance of isolated steel moment frames

This paper proposes an improved two-degree-of-freedom (i2DOF) system with an incorporated effective base-level mass (EBLM) for evaluating the P-Delta and cyclic deterioration effects of nonlinear isolated superstructures. This study verifies the dynamic response and collapse accuracy of the i2DOF system for actual isolated buildings. The case study buildings are the National Institute of Standards and Technology’s (NIST) steel special moment-resisting frames (MRFs) of 4, 8, and 12-stories base-isolated by a friction pendulum bearing (FPB) system. Nonlinear time-history analyses of 22 coupled horizontal–vertical ground motions are performed to verify the i2DOF maximum response when subjected to uplift and pounding effects. The collapse fragility curves of the i2DOF system are compared to those of each corresponding isolated MRF using incremental dynamic analyses (IDA). The results indicate that MRF corner uplifts occur rapidly and do not significantly differ between the i2DOF system and maximum MRF results. The i2DOF system obtains median collapse intensities slightlyhigher than isolated MRFs, indicating that the i2DOF system somewhat overestimates the accurate collapse results. All types of symmetrical and regular low-to-medium-rise isolated steel MRFs with FPBs are designed using a simplified design procedure based on the proposed i2DOF system with a reasonable seismic response precision.

Effect of interior gravity framing system and composite floor slab action on the lifetime seismic risk assessment of steel frame buildings subjected to mainshock–aftershock sequence

The structural analysis practices of steel moment-resisting frame buildings often neglect the participation of the interior gravity frames and composite floor slab action in their lateral strength and stiffness. In addition, the effects of aftershocks are frequently omitted in steel frame buildings’ seismic performance evaluations. This study investigates the effects of structural modeling assumptions and the incorporation of aftershocks on the seismic vulnerability and time-dependent lifetime seismic risk for a group of steel moment-resisting frame buildings. Three ductile steel moment-resisting frame buildings of four, eight, and twelve stories are examined in this study. The results indicate that the seismic performance of steel special moment-resisting frame buildings is substantially underestimated when based on structural models that ignore additional sources of lateral stiffness and strength owing to the interior gravity frame system and the composite floor slab action. In addition, when ignoring aftershocks from the seismic performance assessment, the possibility of catastrophic failure could be underestimated by a factor of 11 at the end of an assumed 50-year lifetime for the steel frame buildings, underscoring the need to account for aftershock effects in evaluating the collapse safety of the selected buildings.

A recurrent-neural-network-based generalized ground-motion model for the Chilean subduction seismic environment

This paper proposes a deep learning-based generalized ground motion model (GGMM) for interface and intraslab subduction earthquakes recorded in Chile. A total of ∼7000 ground-motion records from ∼1700 events are used to train the proposed GGMM. Unlike common ground-motion models (GMMs), which generally consider individual ground-motion intensity measures such as peak ground acceleration and spectral accelerations at given structural periods, the proposed GGMM is based on a data-driven framework that coherently uses recurrent neural networks (RNNs) and hierarchical mixed-effects regression to output a cross-dependent vector of 35 ground-motion intensity measures (denoted as IM). The IM vector includes geometric mean of Arias intensity, peak ground velocity, peak ground acceleration, and significant duration (denoted as Iageom, PGVgeom, PGAgeom, and D5-95geom, respectively), and RotD50 spectral accelerations at 31 periods between 0.05 and 5 s for a 5 % damped oscillator (denoted as Sa(T)). The inputs to the GGMM include six causal seismic source and site parameters, including fault slab mechanism, moment magnitude, closest rupture distance, Joyne-Boore distance, soil shear-wave velocity, and hypocentral depth. The statistical evaluation of the proposed GGMM shows high prediction power with R2 > 0.7 for most IMs while maintaining the cross-IM dependencies. Furthermore, the GGMM is carefully compared against two state-of-the-art Chilean GMMs, showing that the proposed GGMM leads to better goodness of fit for all periods of Sa(T) compared to the two considered GMMs (on average 0.2 higher R2). Finally, the GGMM is implemented to select hazard-consistent ground motions for nonlinear time history analysis of a sophisticated finite-element model of a 20-story steel special moment-resisting frame. Results of this analysis are statistically compared against those for hazard-consistent ground motions selected based on the conditional mean spectrum (CMS) approach. In general, it is observed that the drift demands computed using the two approaches cannot be considered statistically similar and the GGMM leads to higher demands.

Seismic collapse safety based optimization of steel Moment-Resisting frames

Structures must have a reasonable seismic safety margin in order to ensure life-safety as well as to prevent structural collapse caused by earthquakes. On the other hand, the cost-efficiency and economy of structures are important and integral parts of structural engineering. The main aim of the current study is to propose an efficient methodology for designing cost-efficient multi-story special steel moment-resisting frames at predefined levels of seismic collapse safety. To achieve this purpose, a seismic collapse safety assessment technique is integrated with performance-based design optimization in the present study. In order to illustrate the advantages of the proposed methodology, three design examples of 3-, 6- and 12-story special steel moment-resisting frames are presented. The numerical results show that by with a 25% increase in the optimal weight of the structures, their seismic collapse capacity increases by more than 50%.

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.

Comparison of seismic and gravity progressive collapse in dual systems with special steel moment-resisting frames and braces

Progressive collapse studies generally assess the performance of the structure under gravity and blast loads, while earthquakes may also lead to the progressive collapse of a damaged structure. In this study, the progressive collapse response of concentrically braced dual systems with steel moment-resisting frames was assessed under seismic loads through pushover analysis using triangular and uniform lateral load patterns. Two different bracing types (X and inverted V braces) were considered, and their performances were compared under different lateral load patterns using the nonlinear static alternate path method recommended in the Unified Facilities Criteria (UFC) guideline. Eventually, the seismic progressive collapse resistance of models was compared to their progressive collapse response under gravity loads. These studies showed that models under the seismic progressive collapse loads satisfied UFC acceptance criteria and limited rehabilitation objective. The structures had better performance under seismic progressive collapse than models under gravity loads because of more resistance, ductility, suitable load redistribution, and more structural elements that participated in load redistribution. Furthermore, despite studies on progressive collapse under gravity loads, the dual system with X braces showed better progressive collapse performance (more resistance, residual reserve strength ratio and ductility) under seismic loads than the model with inverted V braces.

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.

The performance-based seismic response of special steel MRF: Effects of pulse-like ground motion and foundation safety factor

This study aims to investigate the effect of near-fault pulse-like earthquake ground motions as well as the foundation safety factor, on the performance-based seismic response (PBSR) of special steel moment-resisting frame (MRF). The Beam on the Nonlinear Winkler Foundation (BNWF) approach was employed for modeling the soil-foundation system. In this regard, values of 2, 3, and 5 were considered for the foundation vertical safety factor (FSv). The PBSR of the MRF has been assessed under 32 far-fault (FF) ground motions as well as 55 near-fault (NF) pulse-like records, through Incremental Dynamic Analyses (IDAs). The effects of extracted pulses and residual components of near-fault records were investigated as well. Results show that regardless of the earthquake record type, FSv has no considerable effect on the performance-based drift demand in the structure. However, larger settlement and rotation of the foundation are expected when a small FSv is accounted for. This leads to difficulties in providing the Immediate Occupancy (IO) performance objective for structures with operational performance levels. The effect of the input record type on the median foundation settlement is also inconsiderable. As well, a similarity was observed between the seismic performance of the structure under the residual components and the far-fault records. What’s more, neither the extracted pulses nor the residual components of near-fault earthquake records can represent the original near-fault pulse-like records. Also, the residual component plays an important role in the structural responses under low-intensity earthquake events.

Resilient seismic design of steel frames with hysteretic fuses in a code-oriented format

Modern earthquake engineering has to evolve toward resilience-based design, where structures are expected to survive major earthquakes with reduced or null damage that could be easily reparable in a reasonable time. With such a design strategy, three important goals are warranted: a) life safety of people, b) preserve of building property within a reasonable investment to repair light damage, and c) minimize economic losses because of building's usage interruption. One way to achieve such goal is by using hysteretic energy dissipation devices as structural fuses. Although any global strategy could be used for the successful resilient design of buildings with structural fuses, the authors have work extensively to define global design parameters in order to introduce resilient-based design in the current force-oriented format of most seismic building codes worldwide. Then, global design parameters obtained from very comprehensive parametric studies for special moment-resisting steel frames with hysteretic fuses are presented and discussed. Also, it is shown that the proposed code-oriented methodology is successful to achieve resilient seismic design when subjected to strong earthquake records that may even surpass those considered in their design spectra.

Strength resistance factors for seismic design of exposed based plate connections in special steel moment resisting frames

This study presents a critical assessment of the reliability of current base plate connections in steel special moment resisting frames (SMRFs). Using a probabilistic outlook, this research evaluates the reliability of exposed column base (ECB) connections in SMRFs designed based on the current seismic design provisions; it suggests (and implements) a statistical approach to compute resistance factors for three modes of failure (concrete bearing, base plate yielding at tensile interface, and anchor bolt fracture) of ECB connections to achieve a target reliability index, β, of 4.5. Since ECB connections are limited to short buildings, therefore, this study is conducted on two-story and four-story SMRFs which are analyzed using a suite of 120 ground motions originating from strike-slip and reverse faults. ECB connections for the two-story building are designed to simulate pinned connection, while the bases of the four-story building represent moment connections. Detailed methodology for calculating the β of ECB connections is presented considering the three limit states in a moment–axial load interaction curve. Results indicate that the implementation of current seismic provisions results in β ~ 3.3 for non-moment resisting ECB connections for all tried combinations of resistance factors. For moment resisting ECB connections, however, only the designs based on a resistance factor for concrete bearing failure mode less than the current 0.65 result in an acceptable reliability factor of β > 4.5.

Performance of intensity measures for seismic collapse assessment of structures with vertical mass irregularity

Intensity measures (IMs) are typically utilized to make connection between the prediction of engineering demand parameters and the results obtained from seismic hazard analysis. An optimal IM has four desired features including efficiency, sufficiency, scaling robustness and predictability. Most of the studies conducted on the performance of IMs (i.e., efficiency and sufficiency) are related to seismic response assessment of regular structures. In this study, the performance of some advanced and well-known scalar and vector IMs are investigated for seismic collapse assessment of steel special moment resisting frames (SMRFs) with vertical mass irregularity. The results show that SaavgM as an optimal scalar IM, and (Sa(T1), Sa(T1)/DSI) as an optimal vector IM satisfy the desired features of an optimal IM to predict the collapse capacity of low- and mid-rise steel SMRFs with vertical mass irregularity, respectively. Therefore, the fragility functions obtained using these optimal IMs for this type of structures are more reliable.

Sensitivity analysis of the influence of ground motion intensity levels on the seismic behavior of steel frames in assessment of the target displacement considering near-fault effects

Nonlinear time-history analysis conducted as part of a performance-based seismic design approach often require that the ground motion records are selected and then scaled to a specified level of seismic intensity. In such analyses, besides an adequate structural model, a set of acceleration time-series is needed as the most realistic representation of the seismic action. In this paper, the effects of scaling procedure on seismic demands of steel frames are investigated. To this, two special steel moment-resisting frames with considerable higher mode effects, and two sets of ground motions, including near-fault and far-fault motions are considered. Moreover, three scaling procedures are introduced for performing nonlinear dynamic time-history analysis of structures. Among different demands, lateral roof displacement and interstory drift are selected as seismic demands. The height-wise distribution of demands shows that the inelastic seismic demands of the near-fault pulse-like ground motions differ considerably from those of far-fault ones. These results show that the story drifts are mostly larger for far-fault motions in the upper story levels in comparison to near-fault records and in the lower floors, the reverse is true. Thus, the scaling procedures directly affect the results of seismic demands and choosing different methods would result in varying responses. Moreover, a low-cost and fairly effective procedure is proposed to estimate the target displacement demands of buildings from response-spectrum analyses, considering near-fault effects. The precision of this method is verified by nonlinear time-history analysis results, as the benchmark solution, and acceptable improvements have been achieved.

New vector-valued intensity measure for predicting the collapse capacity of steel moment resisting frames with viscous dampers

Viscous Dampers (VDs), as a passive energy dissipation system, are increasingly applied for the seismic design and retrofit of structures. In general, probabilistic seismic assessment of structures with VDs is performed by traditional scalar Intensity Measures (IMs) such as pseudo-spectral acceleration at the first mode period of the structure, Sa(T1). In this paper, the seismic collapse of steel Special Moment Resisting Frames (SMRFs) with VDs is assessed by 18 vector-valued IMs. The first component of the vector-valued IMs is Sa(T1) and their second components are un-scalable. Incremental Dynamic Analyses (IDAs) are performed to compute the seismic collapse capacity, Sacol, values of steel SMRFs with VDs. Then, the efficiency and sufficiency of the IMs for the prediction of the collapse capacity of the structures are studied, and (Sa(T1), SaRatioM-D) is proposed as an advanced vector-valued IM that includes the spectral shape and Ground Motion (GM) duration effects. The results indicate that (Sa(T1), SaRatioM-D) possesses high efficiency and sufficiency, and can reliably predict the collapse capacity of steel SMRFs with VDs.

Seismic Fragility Assessment of Steel SMRF Structures under Various Types of Near and Far Fault Ground Motions

In this paper, the seismic collapse probability of special steel moment-resisting frame (SSMRF) structures, designed to 4th edition of Iranian seismic design code, under near fault pulse-like and far fault ordinary ground motions is evaluated through fragility analysis. For this purpose, five sample frames with 3 to 15 stories are designed and imposed to the ground motion excitations with different characteristics. Fragility curves are derived for the sample frames using the results of incremental dynamic analyses. Three sets of near fault ground motion records with different range of pulse period and one set of far fault ordinary records are used in dynamic analyses. Each record set involves ten acceleration time histories on soil type III. Based on the obtained results, it was found that pulse-like motions with medium- and long-period pulses are significantly more destructive than other types of ground motions. Fragility analysis reveals that the average collapse probability for the case study frames under the far and near fault ground motions at the intensity of 0.35g equals to 4.3% and 10.3%, respectively. These values are 15.9%and 38.6%, for PGA of 0.53g. It is also found that the increase in the height, leads to increase in higher modes effect to transfer drift demands toward upper stories.

Multihazard Design and Cost-Benefit Analysis of Buildings with Special Moment–Resisting Steel Frames

This study evaluates the economic viability of allowing for controlled inelastic deformation of special moment–resisting buildings during wind storms. This is achieved through a set of case studies involving steel buildings of varying heights made of special moment–resisting frames located in three different cities in the United States, with different intensity levels of wind and seismic hazards. The evaluation procedure utilizes the latest developments in the field of performance-based earthquake engineering but in a multihazard context. By comparing the capital costs of the buildings with the expected life-cycle losses, it is shown that controlled inelastic deformation of buildings under wind load may be economically advantageous, depending upon the properties of the structures and characteristics of both hazards. It is also shown that, irrespective of whether a building is designed following the current code provisions or allowing for controlled inelastic deformation under wind excitations in seismically active regions, neglecting one hazard during performance-based cost-benefit analysis of the building can result in very large errors in the predicted life-cycle losses.

Progressive collapse potential of different types of irregular buildings located in diverse seismic sites

This paper evaluates the effects of severity of Torsional Irregularity (TI) and In-plane Discontinuity in Vertical Lateral force-resisting element Irregularity (IDVLI) together with seismic strength of the building on the progressive collapse potential of steel Special Moment-Resisting Frames (steel SMRFs), which were designed based on common seismic codes. In order to investigate the progressive collapse potential according to GSA 2013 guidelines, an interior or exterior column is removed in 3D modeled building using nonlinear dynamic analysis. Various TIs by defining the ratio of maximum relative lateral displacement of the story to average relative lateral displacement of the story between 1 to 1.6 and IDVLIs by disconnecting one or two columns in the first and second stories are selected. Buildings are 3, 6 and 9 stories high, and Los Angeles, California andGeorgia sites with high, moderate and low levels of seismicity, respectively, are considered. All corresponding buildings have similar seismic mass and are designed for approximately equal values of earthquake base shear, so the comparison process can be possible due to the comparison of equivalent-designed buildings. Gravity and seismic loads of buildings are applied based on ASCE 7-05, and steel design is carried out based on AISC 2010. The results show that buildings designed with greater TI have greater resistance to the progressive collapse phenomenon. Furthermore, buildings in a site with higher seismicity level have less progressive collapse potential. In IDVLI, the buildings located in a site with low seismicity are always rejected against progressive failure based on GSA 2013, whereas buildings located in a site with high seismicity are always acceptable. In addition, in a system with IDVLI, the scenario of external column removal always creates more critical conditions. Results toward the combined effects of irregularity and seismicity level of sites are presented.

CONTRIBUTION of GRAVITY FRAMES TO SEISMIC PERFORMANCE OF STEEL MOMENT RESISTING FRAMES

Traditionally, lateral stiffness and strength of the gravity frames in steel buildings are neglected in structural analysis. During the past earthquakes, such as Northridge, USA, 1994 and Kobe, Japan, 1995, unexpected failures were detected at beam-to-column connections of steel moment resisting frames (MRFs). In the aftermath of these earthquakes, extensive research has been carried out to reveal the causes of these failures. Based on the detailed observations, it is likely that the reserve capacity provided by the gravity frames prevented the highly damaged steel buildings from collapsing, since majority of the moment-resisting connections failed prematurely during the Northridge earthquake (1994). Even though the influence of gravity frames (GFs) on structural behavior can be substantial, little attention is paid to evaluate its impact on structural response. With this paper, the contribution of interior GFs in seismic performance of special moment resisting steel frames (SMRFs) is evaluated. For this purpose, 4- and 9-story SMRFs were designed in accordance with the requirements of Draft Turkish Seismic Code (2016). The frames are, then, subjected to incremental dynamic analysis. To evaluate the contribution of the interior GFs on the overall seismic performance of structural system, inelastic behavior of shear tab (simple) connections at beam-to-gravity-only columns were idealized as semi-rigid joints. A general purpose structural analysis software, ETABS, is utilized for the analyses. The results of the study are presented in terms of story drifts, base shear vs. roof displacement.

EVALUACIÓN DEL DISEÑO SÍSMICO RESILIENTE CONFORME AL MÉTODO DE LAS FUERZAS DE MARCOS DÚCTILES DE ACERO CON DISIPADORES DE ENERGÍA HISTERÉTICOS

Se presentan los resultados de análisis dinámicos no lineales realizados a doce modelos de edificios que se diseñaron conforme a una metodología que emplea el método de las fuerzas y principios de diseño por capacidad y de fusible estructural, la cual permite obtener diseños sísmicos resilientes de estructuras con base en marcos contraventeados de acero estructural dúctiles con disipadores de energía histeréticos. Se demuestra que los mecanismos de diseño sísmico resiliente corroborados previamente mediante análisis pushover se cumplen para la enorme mayoría de los modelos ante la acción de un registro de aceleración compatible con el espectro elástico considerado en el diseño de los modelos, y si la excitación dinámica rebasa los parámetros de resistencia o demanda supuestos en el diseño de los modelos, se activa exclusivamente la segunda línea de defensa inelástica planeada para el sistema. Con base en los resultados presentados, se confirma que es muy factible diseñar resilientemente al sistema en estudio, incluyendo a edificios altos y esbeltos. Por lo tanto, se pueden emplear con confianza los parámetros globales de diseño sísmico y balances óptimos de rigideces derivados en estudios previos.