Self-centering Moment-resisting Frames Research Articles

Performance-based seismic design of self-centering steel moment frames with SMA-bolted connections

This paper proposes a performance-based seismic design (PBSD) method for self-centering moment-resisting frames with shape memory alloy (SMA)-bolted endplate connections. PBSD can be used to achieve different performance objectives, resulting in several benefits, such as cost-effectiveness and customization of the design. This study develops a PBSD framework by establishing a relationship between the fundamental period, ductility demand, and seismic force reduction factor and using an energy-balanced concept. The design method is facilitated using an artificial neural network (ANN)-based predictive tool and an efficient experimentally-validated phenomenological model for SMA-bolted endplate connections. Maximum drift and ductility ratios are adopted as performance objectives. Two 3- and 6-story prototype frames are designed and evaluated to illustrate the proposed PBSD method. The seismic response and collapse safety of the prototype frames at different shaking intensities are evaluated. The results show that the prototype frames designed using the proposed PBSD framework meet the prescribed design objectives and the collapse safety requirements in FEMA P695. The illustrative examples confirm that the designed moment frames possess the intended self-centering capability with negligible residual deformations as low as 0.05% at the maximum considered earthquake (MCE) hazard level. The results also confirm that the designed frames exhibit uniform drift distribution along their height.

Artificial Neural Network–Based Predictive Tool for Modeling of Self-Centering Endplate Connections with SMA Bolts

Endplate moment connections with shape memory alloy (SMA) bolts provide self-centering for the seismic resilience of structures. Predicting the self-centering response of these new beam–column connections, which have not been codified yet, requires conducting experimental tests or detailed continuum finite-element simulations. Computationally efficient predictive tools are needed to facilitate the analysis, design, and assessment of self-centering connections and moment frames. In this paper, artificial neural networks (ANNs) are used to develop a MATLAB tool for predicting the moment-rotation backbone and self-centering response of extended endplate connections with SMA bolts. As the input for the neural networks, the predictive model development employs a design database of response parameters from 72 finite-element (FE) models and experimental tests of seven beam–column connection specimens. Neural networks are trained for seven response parameters, and the trained networks are used to develop a graphical user interface (GUI). The coefficient of determination for the trained ANNs is in the range of 0.92 to 0.99, indicating acceptable prediction accuracy. Furthermore, optimization studies using a multiobjective genetic algorithm are performed, seeking the minimization of material use (steel and SMA) and improved connection-response characteristics (i.e., stiffness, strength, and ductility). A phenomenological model of SMA connections is also developed in OpenSees. The use of the ANN-based predictive tool for accurate and efficient modeling of SMA-based connections and self-centering moment-resisting frames is illustrated. The computation time for predicting the moment-rotation response of a typical SMA connection is significantly reduced from seven hours in ANSYS to only three minutes in OpenSees while providing the same level of response prediction accuracy. Furthermore, the optimization results are confirmed by performing nonlinear pushover and response history analyses.

Residual drift risk of self-centering steel MRFs with novel steel column bases in near-fault regions

This paper evaluates the potential of novel steel column bases to reduce the residual drift risk of steel buildings located at near-fault regions when installed to post-tensioned self-centering moment-resisting frames (SC-MRFs). To this end, a prototype steel building is designed that consists of either conventional moment-resisting frames (MRFs) or SC-MRFs or SC-MRFs equipped with the novel steel column base (SC-MRF-CBs). The MRFs and SC-MRFs are used as benchmark frames. The frames are modelled in OpenSees where material and geometrical non-linearities are considered along with stiffness and strength degradation. A set of 91 near-fault ground motions with different pulse periods is used to perform incremental dynamic analysis (IDA), in which each ground motion is scaled appropriately until different residual storey drift limits are exceeded. The probability of exceedance of these limits is then computed as a function of the ground motion intensity and the period of the velocity pulse. Finally, the results of IDA are combined with probabilistic seismic hazard analysis models that account for near-fault directivity to evaluate and compare the residual drift risk of the frames used in this study. Results show that the predicted residual drift performance of the frames is influenced by the pulse period of the near-fault ground motions. The use of the novel steel column base significantly reduces the residual drift risk of the frames and the SC-MRF-CB exhibits the best residual drift performance. Finally, the paper highlights the effectiveness of combining post-tensioned beam-column connections with the novel steel column base, by showing that the SC-MRF-CB improves the residual drift performance of the MRF and SC-MRF by 80% and 50%, respectively.

Assessment of Repairability Index of Building Connections with Posttensioned Strands

AbstractRecently developed steel self-centering moment-resisting frames (SC-MRFs) have been validated analytically and experimentally as having the potential to eliminate structural damage under a ...

Seismic collapse of self-centering steel MRFs with different column base structural properties

The effect of the strength and stiffness characteristics of a previously proposed novel column base on the seismic performance and collapse capacity of steel self-centering moment-resisting frames is evaluated in this paper. This is done through three normalised parameters that represent the initial stiffness, post-yield stiffness, and strength of the column base, which can be independently adjusted. For these evaluations, a prototype steel building, which serves as a case study, is designed with sixteen different cases of a self-centering moment-resisting frame with different column base stiffness and strength characteristics (SC-MRF-CBs). A self-centering moment-resisting frame with conventional column bases and the same members and beam-column connections as those of the SC-MRF-CBs, named SC-MRF, serves as a benchmark frame. A set of 44 ground motions was used to conduct non-linear dynamic analyses and evaluate the seismic performance of the frames. Incremental dynamic analyses were also performed with the same ground motions set to evaluate the collapse capacity of the frames. Collapse capacity fragility curves and adjusted collapse margin ratios of the frames were derived and used for the comparison of the seismic risk of the frames. The results show that the new self-centering column base significantly improves the seismic performance of the SC-MRF, demonstrating the potential of the SC-MRF-CBs to be redesigned with smaller member sections. Moreover, the SC-MRF-CBs achieve significant reduction in collapse risk compared to the SC-MRF. Finally, the results show that increasing the base strength and stiffness improves the seismic performance and collapse capacity of the SC-MRF-CBs.

Seismic performance of steel frame with a self-centering beam

Self-centering moment-resisting frames are capable of returning to their original plumb position after a severe earthquake, having sustained no or only slight structural damage. In this paper, a steel frame composed of a self-centering I-beam is proposed, with its capacities for self-centering and inelastic deformation provided by post-tensioned (PT) strands and energy dissipating (ED) elements, respectively. Based on an analysis of its mechanical properties, an experiment involving this self-centering frame under cyclic loading was conducted to investigate its load transfer mechanism, bearing capacity, hysteretic behaviour, ductility, energy dissipating capacity and self-centering capacity. The results agree well with the theoretical analysis, and it can be concluded that its force-displacement hysteretic curve has a typical flag shape. The plastic deformation of the specimen is concentrated on the ED elements, which can be replaced easily after an earthquake. As the gap-opening of the anchorage plate occurs, the PT strands and ED elements provide the bending stiffness for the structure. When the story drift is loaded to 4%, the skeleton curve does not have a descending section, which means that the structure still has sufficient bearing and deformation capacities. The analysis of the residual story drift and the equivalent viscous damping coefficient shows that the frame has good energy dissipating and self-centering capabilities. A finite element model is established with OpenSees to simulate the seismic behaviour of the self-centering frame. Using the model, the seismic performance of the frame is analysed in terms of its structural parameters, including the section area of the PT strands, initial stress on the PT strands and section area of the ED elements.

Numerical Assessment of the Fire Behavior of Steel Posttensioned Moment-Resisting Frames

AbstractSteel self-centering moment-resisting frames (SC-MRFs) are a class of low-damage seismic-resistant structural systems, which use posttensioned high-strength steel bars to provide self-cente...

Performance of self-centering steel moment frame considering stress relaxation in prestressed cables

Buildings can be designed to limit the earthquake-induced damage to members that can easily be repaired. Self-centering moment-resisting frames can be used as effective structural systems for this purpose. Self-centering moment-resisting frames with prestressed cables are able to return the structure to its original position after the earthquake. The internal forces in self-centering moment-resisting frames are transferred between the beam and the column by post-tensioned cables. As a main member of self-centering connections, prestressed cables play a significant role in such systems. Cable tension decreases over time due to the effect of stress relaxation on the performance of the system. Stress relaxation is a time-dependent phenomenon causing stress reduction over time in the members prestressed at a constant strain. Therefore, the effect of stress relaxation on the performance of self-centering moment-resisting frames can be significant. In this article, after simulating and validating a moment-resisting frame with self-centering connections, stiffness and moment–rotation hysteresis diagrams were analyzed after 0, 1, 5, 10, and 20 years of cable prestressing. According to the results, two equations were presented to estimate the reduction in the connection stiffness and dissipated energy by the system based on prestressing level and the time after prestressing. The proposed equations could be used to model semi-rigid connections.

Experimental Investigation of Self-Centering Beams for Moment-Resisting Frames

AbstractPreviously proposed self-centering moment-resisting frames relied on gap opening and closing in the beam–column interface to achieve recentering capability and energy dissipation capacity. ...

Progressive collapse resistance of steel self-centering MRFs including the effects of the composite floor

This paper presents progressive collapse simulations to assess the robustness of a seismic-resistant building using self-centering moment resisting frames (SC-MRFs) under a sudden column loss scenario. The first floor of the building, including the composite floor, was modelled in ABAQUS using a mixture of finite element types and simulation methods to balance computational cost and accuracy. First, key components of the numerical model, including the composite beams, the fin-plate beam-column connections, and the perimeter SC-MRFs, were validated against available experimental results to ensure a reliable simulation. The validated model was then used to study the robustness of the building under a sudden column loss event. Both nonlinear static and dynamic analyses were employed. The simulations allowed for the identification of all possible failure modes and the quantification of the contribution of the composite floor to the robustness of the frame. The results show that the building can withstand the code-prescribed load with a safety factor of 2 and that the structural limit state that triggers progressive collapse is the buckling of the gravity columns. The Dynamic Increase Factor (DIF) was also identified by comparing the static and dynamic responses.

Structural evaluation of steel self-centering moment-resisting frames under far-field and near-field earthquakes

The design formulations, structural performance, and energy-based results of steel buildings of self-centering moment-resisting frames (SC-MRFs) with post-tensioned (PT) connections of three different energy dissipation devices including bolted top and seat angles, beam bottom flange friction devices, and bolted web friction devices are investigated under far-field and near-field earthquakes. The results of nonlinear time-history analysis using DRAIN-2DX program show that the simplified analysis used to estimate the inertial forces distributed through collector beams is not sufficiently precise since it leads to the underestimation of design beam axial forces and connection moments in exterior bays. The overestimation of design displacement demands of SC-MRFs with bottom flange friction devices results in bigger sections of beams and columns than are necessary. In addition, this SC-MRF system is not as effective as other two systems to dissipate the input energy. It is recommended to symmetrically use beam flange friction devices to achieve higher level of energy dissipation and lower fabrication costs. On the other hand, SC-MRFs with web friction devices is identified as the most efficient system to minimize the recoverable energy and SC-MRFs with top and seat angles provides medium level of energy dissipation. Different from far-field earthquakes, the input energies of SC-MRF structures under near-field ground motions are distinguishable by hikes of maximum values at the early stages of earthquakes. Furthermore, absolute energy components are substantially higher than relative energy components under near-field earthquakes. These phenomena should be considered in the formulations of energy factor and energy-based design procedures of SC-MRF systems.

Seismic performance of a self-centering steel moment frame building: From component-level modeling to economic loss assessment

The seismic performance and economic seismic losses of a 6-story self-centering moment resisting frame (SC-MRF) building using post-tensioned (PT) connections with top-and-seat angles is evaluated. A phenomenological model that captures the lateral load response of PT connections is developed and verified using previous experiments. A 2D model of the 6-story SC-MRF is constructed in OpenSees using the newly developed phenomenological model. Using the same member sizes as the SC-MRF, a model is also created for a welded moment resisting frame (WMRF) with reduced beam section (RBS) connections. Nonlinear static and incremental dynamic analyses are performed on the SC-MRF and WMRF models. The lateral load carrying capacity of the SC-MRF is found to be 40% lower than that of the WMRF. The dynamic analysis results show that the WMRF has higher collapse resistance, whereas the SC-MRF undergoes smaller residual drifts. Finally, the earthquake-induced economic impact to the two buildings is assessed using the FEMA P58 methodology, where the expected annual loss for the SC-MRF is computed to be 21% higher than that for the WMRF building. More specifically, the SC-MRF building (with PT connections and top-and-seat angles) has a lower expected loss associated with excessive residual drifts but higher collapse losses.

Self-centering steel column base with metallic energy dissipation devices

Column bases of seismic-resistant steel frames are typically designed as full-strength to ensure that plastic hinges develop in the bottom end of the first-storey columns. Alternatively, column bases may be designed as partial-strength and dissipate energy through inelastic deformations in their main components (i.e., base plate, steel anchor rods). Both design philosophies result in difficult-to-repair damage and residual drifts. Moreover, the second design philosophy results in complex hysteretic behaviour with strength and stiffness deterioration. This paper proposes a partial-strength low-damage self-centering steel column base. The column base provides flexibility in the design as its rotational stiffness and moment resistance can be independently tuned. The paper presents an analytical model that predicts the stiffness, strength, and hysteretic behaviour of the column base. In addition, a design procedure and detailed finite element models are presented. The paper evaluates the effectiveness of the column base by carrying out nonlinear dynamic analyses on a prototype steel building designed as post-tensioned self-centering moment-resisting frame. The results demonstrate the potential of the column base to reduce the residual first-storey drifts and protect the first-storey columns from yielding.

Robustness assessment of a steel self-centering moment-resisting frame under column loss

The robustness of a seismically-designed steel self-centering moment-resisting frame (SC-MRF) under a column loss scenario is numerically assessed. The prototype SC-MRF is equipped with post-tensioned bars and optimised stainless steel energy dissipation devices. The SC-MRF was modelled in full detail using solid finite elements. The numerical model was calibrated using results from previous tests on post-tensioned beam-column connections and isolated component tests on the energy dissipation devices. Quasi-static analyses were carried out to identify the failure modes of the SC-MRF under a column loss scenario. Nonlinear dynamic analyses were also performed to evaluate the dynamic response of the frame and to assess the acceptance criteria against progressive collapse according to current formalised procedures. The results show that the SC-MRF has superior robustness and it can guarantee a high level of safety under a sudden column loss scenario due to the high fracture capacity of the stainless steel energy dissipation devices and the tie force resistance provided by the post-tensioned bars.

Collapse risk and residual drift performance of steel buildings using post-tensioned MRFs and viscous dampers in near-fault regions

The potential of post-tensioned self-centering moment-resisting frames (SC-MRFs) and viscous dampers to reduce the collapse risk and improve the residual drift performance of steel buildings in near-fault regions is evaluated. For this purpose, a prototype steel building is designed using different seismic-resistant frames, i.e.: moment-resisting frames (MRFs); MRFs with viscous dampers; SC-MRFs; and SC-MRFs with viscous dampers. The frames are modeled in OpenSees where material and geometrical nonlinearities are taken into account as well as stiffness and strength deterioration. A database of 91 near-fault, pulse-like ground motions with varying pulse periods is used to conduct incremental dynamic analysis (IDA), in which each ground motion is scaled until collapse occurs. The probability of collapse and the probability of exceeding different residual story drift threshold values are calculated as a function of the ground motion intensity and the period of the velocity pulse. The results of IDA are then combined with probabilistic seismic hazard analysis models that account for near-fault directivity to assess and compare the collapse risk and the residual drift performance of the frames. The paper highlights the benefit of combining the post-tensioning and supplemental viscous damping technologies in the near-source. In particular, the SC-MRF with viscous dampers is found to achieve significant reductions in collapse risk and probability of exceedance of residual story drift threshold values compared to the MRF.

Probabilistic economic seismic loss estimation in steel buildings using post‐tensioned moment‐resisting frames and viscous dampers

SummaryThe potential of post‐tensioned self‐centering moment‐resisting frames (SC‐MRFs) and viscous dampers to reduce the economic seismic losses in steel buildings is evaluated. The evaluation is based on a prototype steel building designed using four different seismic‐resistant frames: (i) conventional moment resisting frames (MRFs); (ii) MRFs with viscous dampers; (iii) SC‐MRFs; or (iv) SC‐MRFs with viscous dampers. All frames are designed according to Eurocode 8 and have the same column/beam cross sections and similar periods of vibration. Viscous dampers are designed to reduce the peak story drift under the design basis earthquake (DBE) from 1.8% to 1.2%. Losses are estimated by developing vulnerability functions according to the FEMA P‐58 methodology, which considers uncertainties in earthquake ground motion, structural response, and repair costs. Both the probability of collapse and the probability of demolition because of excessive residual story drifts are taken into account. Incremental dynamic analyses are conducted using models capable to simulate all limit states up to collapse. A parametric study on the effect of the residual story drift threshold beyond which is less expensive to rebuild a structure than to repair is also conducted. It is shown that viscous dampers are more effective than post‐tensioning for seismic intensities equal or lower than the maximum considered earthquake (MCE). Post‐tensioning is effective in reducing repair costs only for seismic intensities higher than the DBE. The paper also highlights the effectiveness of combining post‐tensioning and supplemental viscous damping by showing that the SC‐MRF with viscous dampers achieves significant repair cost reductions compared to the conventional MRF. Copyright © 2016 John Wiley & Sons, Ltd.

Finite Element Analysis of Self-centering Moment Resistant Frames with and without Steel Plate Shear Wall

The Northridge earthquake on January 1994 damaged a large number of buildings all through Los Angeles. The most recurrent pattern of structural damage was the brittle failures at beam column junction in steel moment-resisting frames (MRF's)[6] This event brought out the need for further improving the Moment Resistant Frames and its connections. A new system of Moment Resisting Frames (MRF) commonly known as Self-Centering Moment Resisting Frames (SC-MRF) was developed. These designs are capable of resisting the seismic loads without sustaining much damage. The paper studies the behaviour of Self-Centering Moment Resisting Frames (SC-MRF) with and without steel plate shear walls subjected to cyclic loading. A detailed three dimensional finite element modeling using plate elements available in ANSYS FEA code is done. Geometric and material nonlinear stress analysis of the frame and a detailed comparison of the frame with and without steel plate shear walls is done. It can be observed that the sway of the SC-MRF frame is much higher than that of a conventional frame, but by introducing a thin steel plate shear wall this deformation can be brought down to a much lower value.

EC8-based seismic design and assessment of self-centering post-tensioned steel frames with viscous dampers

This paper focuses on seismic design and assessment of steel self-centering moment-resisting frames (SC-MRFs) with viscous dampers within the framework of Eurocode 8 (EC8). Performance levels are defined with respect to drifts, residual drifts and limit states in the post-tensioned (PT) connections. A preliminary pushover analysis is conducted at the early phase of the design process to estimate rotations and axial forces in post-tensioned (PT) connections instead of using approximate formulae. Different designs of an SC-MRF with viscous dampers are considered to investigate all possible scenarios, i.e. use of dampers to achieve drifts significantly lower than the EC8 drift limit; to significantly reduce steel weight without exceeding the EC8 drift limit; or to reduce steel weight and achieve drifts lower than the EC8 drift limit. Nonlinear dynamic analyses using models capable of simulating all structural limit states up to collapse confirm the minimal-damage performance of the SC-MRFs. It is shown that the use of the preliminary pushover analysis makes the design procedure very accurate in predicting structural and non-structural limit states. Supplemental damping along with strict design criteria for the post-tensioned connections are found to significantly improve the seismic performance of the SC-MRFs. Moreover, the paper shows that SC-MRFs with viscous dampers have superior collapse resistance compared to conventional steel MRFs even when the SC-MRF is significantly lighter than the conventional MRF.

Seismic Performance of Steel Self-Centering, Moment-Resisting Frame: Hybrid Simulations under Design Basis Earthquake

AbstractThe behavior of a self-centering moment-resisting frame (SC-MRF) is characterized by the connection gap opening and closing at beam-column interfaces under earthquake loading. A SC-MRF uses high-strength posttensioning strands to precompress the beams to the columns and to close the gaps that develop under earthquake loading, returning the frame to its initial position (i.e., the frame is self-centering). In this study, a beam web friction device is included in each beam-column connection to dissipate energy under seismic loading. Unlike a special steel moment-resisting frame with welded connections (W-SMRF), a SC-MRF can be designed to survive the design basis earthquake (DBE) without structural damage, leading to the potential for immediate occupancy performance, and to suffer only modest damage under the maximum considered earthquake, leading to collapse prevention performance. A 7-bay, 4-story SC-MRF prototype building was designed for a location in the Los Angeles area. A 0.6-scale model of t...

Reduction of Maximum and Residual Drifts on Posttensioned Steel Frames with Semirigid Connections

The aim of this paper is to study the seismic performance of self-centering moment-resisting steel frames with posttensioned connections taking into account nonlinear material behavior, for better understanding of the advantages of this type of structural system. Further, the seismic performance of traditional structures with rigid connections is compared with the corresponding equivalent posttensioned structures with semirigid connections. Nonlinear time history analyses are developed for both types of structural systems to obtain the maximum and the residual interstory drifts. Thirty long-duration narrow-banded earthquake ground motions recorded on soft soil sites of Mexico City are used for the analyses. It is concluded that the structural response of steel buildings with posttensioned connections subjected to intense earthquake ground motions is reduced compared with the seismic response of traditional buildings with welded connections. Moreover, residual interstory drift demands are considerably reduced for the system with posttensioned connections, which is important to avoid the demolition of the buildings after an earthquake.