Gravity Frames Research Articles

Near-collapse behavior of steel buildings with non-ductile concentrically braced frames

Inherent resistance to collapse has been observed in steel buildings with non-ductile concentrically braced frames (CBFs) during past major earthquakes. Understanding of the fundamental characteristics of near-collapse behavior of such buildings will help reveal seismic performance of non-ductile steel structures in existing buildings across the US, and lead to an efficient seismic retrofit of those in seismic zones. This paper presents a seismic evaluation of typical steel buildings using non-ductile CBFs as lateral load resisting structures with focus on their near-collapse behavior, based on the incremental dynamic analysis. The buildings with non-ductile CBFs were found to be fully operational up to 0.5% story drift ratio response with or without gravity frames participating in lateral-load-resisting system. However, the life safety and collapse prevention of the buildings were significantly improved by actually participating lateral-load-resisting systems including CBFs, steel gravity frames and concrete slabs. Furthermore, the post-damage response of the building was more significantly influenced by gravity frames as the damages progressed from its first brace fracture to near collapse, and the participation of the gravity frames had much more impact on the near collapse behavior of a taller building than that of a low-rise building.

Cyclic Load Tests of SFRM-Insulated Steel Gravity Frame Beam-Column Connection Assemblies

AbstractEarthquake-induced damage to structural components and spray-applied fire-resistive material (SFRM) in steel gravity frame beam-column connection regions can impact connection performance during an ensuing fire. Bolted connections are particularly susceptible to failure at elevated temperatures due to a rapid loss in bolt capacity with increasing temperature, which occurs at an accelerated rate compared to the thermal degradation of structural steel. In order to characterize structural and SFRM damage over a range of seismic drift demands, two large-scale SFRM insulated beam-column connection assemblies utilizing single-plate and unstiffened seated connection designs, with composite floor slabs, were tested under combined gravity and lateral loading. SFRM cracking, debonding, and spalling were observed during both tests, exposing critical connection elements. In addition, the single-plate connection assembly, which was developed in accordance with U.S. national guidelines but contained design feat...

Assessment of floor accelerations in special steel moment frames

The floor acceleration response of Special Steel Moment Frames subjected to earthquakes is evaluated considering different modeling options, i.e., elastic and inelastic behavior, with and without gravity frames, and considering different strength levels of the partially restrained connections in the gravity frames. The characteristics of the Special Steel Moment Frames considered in this study are based on those considered in the ATC 76-1 project. The influence of each modeling option on peak floor accelerations and on floor response spectra is then investigated. Results are also compared with the design acceleration demands on nonstructural components indicated in ASCE 7, which leads to relevant observations.

Contribution of secondary frames to the mitigation of collapse in steel buildings subjected to extreme loads

This paper examines the contribution of secondary (or gravity) frames to the mitigation of collapse in steel buildings subjected to extreme loading conditions arising from multiple hazards. By considering sidesway and vertical mechanisms as representative of most building collapse modes, this study evaluates the effects of various secondary-frame parameters on the overall building collapse capacity. Given that the beam-to-column joint response typically dominates the behaviour of secondary frames, particular emphasis is given to key connection response characteristics such as stiffness, strength and ductility. The paper starts with a brief description of the probabilistic assessment framework that serves as the basis for the study followed by the assessment of the influence of partially restrained (PR) gravity frames on the collapse capacities of steel structures by means of generalised structural sub-assemblages. Secondary frames are found to provide up to a threefold increase in the sidesway collapse resistance depending on the magnitude of second-order effects. Similarly, the vertical (progressive) collapse capacity is greatly affected by the connection depth versus beam length ratio and the secondary connection strength. A brief discussion on directions for the provision of simplified approaches for the design of secondary frames is presented and illustrated through two case studies involving steel frame buildings subjected to earthquake and blast scenarios, as well as localised fire and blast hazards. This study offers a fundamental step towards the formulation of design and assessment strategies that incorporate secondary systems into a multi-hazard structural evaluation framework.

Aftershock seismic assessment taking into account postmainshock residual drifts

SummaryThis paper introduces and evaluates a methodology for the aftershock seismic assessment of buildings taking explicitly into account residual drift demands after the mainshock (i.e., postmainshock residual interstory drifts,RIDRo). The methodology is applied to a testbed four‐story steel moment‐resisting building designed with modern seismic design provisions when subjected to a set of near‐fault mainshock–aftershock seismic sequences that induce five levels ofRIDRo. Once the postmainshock residual drift is induced to the building model, a postmainshock incremental dynamic analysis is performed under each aftershock to obtain its collapse capacity and its capacity associated to demolition (i.e., the capacity to reach or exceed a 2% residual drift). The effect of additional sources of stiffness and strength (i.e., interior gravity frames and slab contribution) and the polarity of the aftershocks are examined in this study. Results of this investigation show that the collapse potential under aftershocks strongly depends on the modeling approach (i.e., the aftershock collapse potential is modified when additional sources of lateral stiffness and strength are included in the analytical model). Furthermore, it is demonstrated that the aftershock capacity associated to demolition (i.e., the aftershock collapse capacity associated to a residual interstory drift that leads to an imminent demolition) is lower than that of the aftershock collapse capacity, which mean that this parameter should be a better measure of the building residual capacity against aftershocks. Copyright © 2014 John Wiley & Sons, Ltd.

Comparative evaluation of innovative and traditional seismic-resisting systems using the FEMA P-58 procedure

Two new innovative structural systems are investigated using the FEMA P-58 performance assessment procedure to determine the consequences (repair times, repair costs, unsafe placards and casualties) of being subjected to seismic ground shaking. These new systems were developed to display enhanced performance across multiple ground motion intensity levels. The first new system, called a Hybrid Buckling Restrained Braced System, is similar to existing BRB systems except that the core of the brace uses multiple plates, each with a different stress–strain behavior. The second new system, called a Collapse Prevention System, is designed for use in locations where the intensity of shaking under frequent earthquakes is negligible, but life-safety under rare high-intensity shaking is still a concern. These two innovative systems illustrate that the seismic resistant design philosophy should be dependent on the nature of the seismic hazard and that Performance Based Earthquake Engineering should support hazard-dependent philosophies. This paper also discusses the influence of including the lateral resistance of the gravity framing in the structural analysis models used to predict the seismic performance of traditional Steel Special Moment Resisting Frames. This modeling assumption is investigated, as the behavior of the Collapse Prevention Systems is dependent on the behavior of the gravity framing. In this work, these new systems and modeling philosophy are shown to reduce the consequence estimations, particularly the average repair costs, across pertinent intensity levels.

Effect of gravity framing on the overstrength and collapse capacity of steel frame buildings with perimeter special moment frames

SummaryThis paper investigates the effect of the gravity framing system on the overstrength and collapse risk of steel frame buildings with perimeter special moment frames (SMFs) designed in North America. A nonlinear analytical model that simulates the pinched hysteretic response of typical shear tab connections is calibrated with past experimental data. The proposed modeling approach is implemented into nonlinear analytical models of archetype steel buildings with different heights. It is found that when the gravity framing is considered as part of the analytical model, the overall base shear strength of steel frame buildings with perimeter SMFs could be 50% larger than that of the bare SMFs. This is attributed to the gravity framing as well as the composite action provided by the concrete slab. The same analytical models (i) achieve a static overstrength factor,Ωslarger than 3.0 and (ii) pass the collapse risk evaluation criteria by FEMA P695 regardless of the assigned total system uncertainty. However, when more precise collapse metrics are considered for collapse risk assessment of steel frame buildings with perimeter SMFs, a tolerable probability of collapse is only achieved in a return period of 50 years when the perimeter SMFs of mid‐rise steel buildings are designed with a strong‐column/weak‐beam ratio larger than 1.5. The concept of the dynamic overstrength,Ωdis introduced that captures the inelastic force redistribution due to dynamic loading. Steel frame buildings with perimeter SMFs achieve aΩd > 3 regardless if the gravity framing is considered as part of the nonlinear analytical model representation. Copyright © 2014 John Wiley & Sons, Ltd.

Seismic response estimation of steel buildings with deep columns and PMRF

The responses of steel buildings with perimeter moment resisting frames (PMRF) with medium size columns (W14) are estimated and compared with those of buildings with deep columns (W27), which are selected according to two criteria: equivalent resistance and equivalent weight. It is shown that buildings with W27 columns have no problems of lateral torsional, local or shear buckling in panel zone. Whether the response is larger for W14 or W27 columns, depends on the level of deformation, the response parameter and the structural modeling under consideration. Modeling buildings as two-dimensional structures result in an overestimation of the response. For multiple response parameters, the W14 columns produce larger responses for elastic behavior. The axial load on columns may be significantly larger for the buildings with W14 columns. The interstory displacements are always larger for W14 columns, particularly for equivalent weight and plane models, implying that using deep columns helps to reduce interstory displacements. This is particularly important for tall buildings where the design is usually controlled by the drift limit state. The interstory shears in interior gravity frames (GF) are significantly reduced when deep columns are used. This helps to counteract the no conservative effect that results in design practice, when lateral seismic loads are not considered in GF of steel buildings with PMRF. Thus, the behavior of steel buildings with deep columns, in general, may be superior to that of buildings with medium columns, using less weight and representing, therefore, a lower cost.

Seismic response estimation of steel buildings with deep columns and PMRF

The responses of steel buildings with perimeter moment resisting frames (PMRF) with medium size columns (W14) are estimated and compared with those of buildings with deep columns (W27), which are selected according to two criteria: equivalent resistance and equivalent weight. It is shown that buildings with W27 columns have no problems of lateral torsional, local or shear buckling in panel zone. Whether the response is larger for W14 or W27 columns, depends on the level of deformation, the response parameter and the structural modeling under consideration. Modeling buildings as two-dimensional structures result in an overestimation of the response. For multiple response parameters, the W14 columns produce larger responses for elastic behavior. The axial load on columns may be significantly larger for the buildings with W14 columns. The interstory displacements are always larger for W14 columns, particularly for equivalent weight and plane models, implying that using deep columns helps to reduce interstory displacements. This is particularly important for tall buildings where the design is usually controlled by the drift limit state. The interstory shears in interior gravity frames (GF) are significantly reduced when deep columns are used. This helps to counteract the no conservative effect that results in design practice, when lateral seismic loads are not considered in GF of steel buildings with PMRF. Thus, the behavior of steel buildings with deep columns, in general, may be superior to that of buildings with medium columns, using less weight and representing, therefore, a lower cost.

Disformal transformation of cosmological perturbations

We investigate the gauge-invariant cosmological perturbations in the gravity and matter frames in the general scalar–tensor theory where two frames are related by the disformal transformation. The gravity and matter frames are the extensions of the Einstein and Jordan frames in the scalar–tensor theory where two frames are related by the conformal transformation, respectively. First, it is shown that the curvature perturbation in the comoving gauge to the scalar field is disformally invariant as well as conformally invariant, which gives the predictions from the cosmological model where the scalar field is responsible both for inflation and cosmological perturbations. Second, in case that the disformally coupled matter sector also contributes to curvature perturbations, we derive the evolution equations of the curvature perturbation in the uniform matter energy density gauge from the energy (non)conservation in the matter sector, which are independent of the choice of the gravity sector. While in the matter frame the curvature perturbation in the uniform matter energy density gauge is conserved on superhorizon scales for the vanishing nonadiabatic pressure, in the gravity frame it is not conserved even if the nonadiabatic pressure vanishes. The formula relating two frames gives the amplitude of the curvature perturbation in the matter frame, once it is evaluated in the gravity frame.

Behavior of Steel Shear Connections under Column-Removal Demands

An understanding of the behavior of shear connections in steel gravity frames under the unique combinations of moment, shear, and axial force relevant to extreme loading conditions is necessary to assess the vulnerability of a structure to disproportionate collapse. However, such an understanding is currently limited by a deficiency of physical test data. To investigate the inherent robustness of commonly used steel shear connections, an experimental program consisting of 35 full-scale physical tests was completed. Specimens included shear-tab, welded-bolted single-angle, bolted-bolted single-angle, and bolted-bolted double-angle connections. A testing procedure was developed that imposed on a connection the force and deformation demands that would be expected in the bays immediately adjacent to a compromised column in a symmetrical multibay frame. Various geometric arrangements of each connection type were tested, and each arrangement was subjected to a range of loading histories representing different column-removal scenarios. The test results characterize the load-development history, deformation mechanisms, and expected failure modes following column removal for each type of connection. Connection strength and ductility limits under the effects of this unique type of combined loading are quantified.

Numerical Analysis Of Progressive Collapse Of Steel Frame Building

In this study, the progressive collapse resistance of a two-story steel gravity frame was investigated numerically after the sudden removal of a perimeter column in the first floor. A comparison of the numerical analyses results and experimental results was available in the literature [1]. After the model validation, the effect of infill steel panels and bracing on enhancing the progressive collapse resisting capacity of gravity frame were evaluated. The progressive collapse models were simulated by arbitrarily removing a perimeter column and carrying out nonlinear static analyses using the finite element analysis code ABAQUS.

Influence of the gravity framing system on the collapse performance of special steel moment frames

This paper investigates the influence of the gravity framing system on the seismic performance of special steel moment frames (SMFs). The buildings used in this study were taken from one of the examples that form part of the ATC-76-1 project, which used the FEMA P-695 methodology to assess the collapse probability of SMF systems. Two, four and eight story SMFs were analyzed with and without the gravity frame to quantify their collapse performance. Aspects of the gravity frame that were investigated include the contribution of stiffness and strength of beam to column connections, and the location of splices in the gravity columns. Moreover, this research investigates the potential for the development of inelastic deformations in the gravity columns, and the effect of such deformations on structural response. The results show that gravity connections and gravity column continuity profoundly affect the computed response and collapse probability. The inelastic behavior in gravity columns has a less important effect but should be included in the analysis.

Large‐scale real‐time hybrid simulation of a three‐story steel frame building with magneto‐rheological dampers

SUMMARYA series of large‐scale real‐time hybrid simulations (RTHSs) are conducted on a 0.6‐scale 3‐story steel frame building with magneto‐rheological (MR) dampers. The lateral force resisting system of the prototype building for the study consists of moment resisting frames and damped brace frames (DBFs). The experimental substructure for the RTHS is the DBF with the MR dampers, whereas the remaining structural components of the building including the moment resisting frame and gravity frames are modeled via a nonlinear analytical substructure. Performing RTHS with an experimental substructure that consists of the complete DBF enables the effects of member and connection component deformations on system and damper performance to be accurately accounted for. Data from these tests enable numerical simulation models to be calibrated, provide an understanding and validation of the in‐situ performance of MR dampers, and a means of experimentally validating performance‐based seismic design procedures for real structures. The details of the RTHS procedure are given, including the test setup, the integration algorithm, and actuator control. The results from a series of RTHS are presented that includes actuator control, damper behavior, and the structural response for different MR control laws. The use of the MR dampers is experimentally demonstrated to reduce the response of the structure to strong ground motions. Comparisons of the RTHS results are made with numerical simulations. Based on the results of the study, it is concluded that RTHS can be conducted on realistic structural systems with dampers to enable advancements in resilient earthquake resistant design to be achieved. Copyright © 2014 John Wiley & Sons, Ltd.

Computational study of self‐centering buckling‐restrained braced frame seismic performance

SUMMARYRecent research developed and experimentally validated a self‐centering buckling‐restrained brace (SC‐BRB) that employs a restoring mechanism created using concentric tubes held flush with pretensioned shape memory alloy rods, in conjunction with a buckling‐restrained brace (BRB) that dissipates seismic energy. The present computational study investigated how the SC‐BRB can be implemented in real buildings to improve seismic performance. First, a computational brace model was developed and calibrated against experimental data, including the definition of a new cyclic material model for superelastic NiTi shape memory alloy. A parametric study were then conducted to explore the design space for SC‐BRBs. Finally, a set of prototype buildings was designed and computationally subjected to a suite of ground motions. The effect of the lateral resistance of gravity framing on self‐centering was also examined.From the component study, the SC‐BRB was found to dissipate sufficient energy even with large self‐centering ratios (as large as 4) based on criteria found in the literature for limiting peak drifts. From the prototype building study, a SC‐BRB self‐centering ratio of 0.5 was capable of reliably limiting residual drifts to negligible values, which is consistent with a dynamic form of self‐centering discussed in the literature. Because large self‐centering ratios can create significant overstrength, the most efficient SC‐BRB frame designs had a self‐centering ratio in the range of 0.5–1.5. Ambient building resistance (e.g., gravity framing) was found to reduce peak drifts, but had a negligible effect on residual drifts. Copyright © 2014 John Wiley & Sons, Ltd.

Composite Floor Systems under Column Loss: Collapse Resistance and Tie Force Requirements

This paper presents a computational assessment of the performance of steel gravity framing systems with single-plate shear connections and composite floor slabs under column loss scenarios. The computational assessment uses a reduced modeling approach, while comparisons with detailed model results are presented to establish confidence in the reduced models. The reduced modeling approach enables large multibay systems to be analyzed much more efficiently than the detailed modeling approaches used in previous studies. Both quasi-static and sudden column loss scenarios are considered, and an energy-based approximate procedure for analysis of sudden column loss is adopted after verification through comparisons with direct dynamic analyses, further enhancing the efficiency of the reduced modeling approach. Reduced models are used to investigate the influence of factors such as span length, slab continuity, and the mode of connection failure on the collapse resistance of gravity frame systems. The adequacy of current structural integrity requirements is also assessed, and based on the computational results a new relationship is proposed between the uniform load intensity and the tie forces required for collapse prevention.

Fluctuation with dust of de Sitter ground state of scalar-tensor gravity

An exact de Sitter solution of scalar-tensor gravity is found in our recent work, in which the non-minimal coupling scalar is rolling along a non-constant potential. Based on this solution, a dust-filled FRW universe is explored in frame of scalar-tensor gravity in this article. The effective dark energy induced by the sole non-minimal scalar can be quintessence-like, phantom-like, and more significantly, can cross the phantom divide. The rich and varied properties of scalar-tensor gravity even with only one scalar is shown.

Seismic response of 3D steel buildings considering the effect of PR connections and gravity frames.

The nonlinear seismic responses of 3D steel buildings with perimeter moment resisting frames (PMRF) and interior gravity frames (IGF) are studied explicitly considering the contribution of the IGF. The effect on the structural response of the stiffness of the beam-to-column connections of the IGF, which is usually neglected, is also studied. It is commonly believed that the flexibility of shear connections is negligible and that 2D models can be used to properly represent 3D real structures. The results of the study indicate, however, that the moments developed on columns of IGF can be considerable and that modeling buildings as plane frames may result in very conservative designs. The contribution of IGF to the lateral structural resistance may be significant. The contribution increases when their connections are assumed to be partially restrained (PR). The incremented participation of IGF when the stiffness of their connections is considered helps to counteract the no conservative effect that results in practice when lateral seismic loads are not considered in IGF while designing steel buildings with PMRF. Thus, if the structural system under consideration is used, the three-dimensional model should be used in seismic analysis and the IGF and the stiffness of their connections should be considered as part of the lateral resistance system.

Mathematical Model of Hybrid Precast Gravity Frames for Smart Construction and Engineering

The structural stability, constructability, economic feasibility, environmental-friendliness, and energy efficiency of hybrid composite frame systems have been demonstrated by practical application and research. A hybrid composite frame system combines the economy of precast concrete structures with the constructability of steel frame structures, including erection speed. Novel composite frames will ultimately maximize the efficiency of structural design and facilitate construction. This paper presents hybrid precast frames, which are precast composite frames based on a simple connection between precast concrete columns and beams. The hybrid precast frames designed to resist gravity loading consist of PC columns, PC beams, and steel inserted in the precast members. Steel sections located between the precast columns were simply connected to steel inserted at each end of the precast beams. Dynamic analysis of a 15-story building designed with the proposed composite frame was performed to determine the dynamic characteristics of a building constructed of hybrid frames, including frequencies and mode shapes.

Integrity of Steel Single Plate Shear Connections Subjected to Simulated Column Removal

Steel gravity framing systems, one of the most commonly used structural systems in the United States, have an unknown resistance to collapse when a column suffers damage that compromises its ability to carry gravity loads. One potential mechanism for these flexible systems to arrest collapse is through the development of an alternate load path in a sustained tensile configuration resulting from large vertical deflections. The ability of the system to develop such an alternate load path is partly dependent on the ability of the gravity connections to remain intact after undergoing extreme local deformations. This study experimentally evaluates the resistance of steel gravity connection subassemblages to loading consistent with the removal of an interior column. Characteristic connection behaviors are identified and peak resistance values and connection demands are reported for several different connection configurations. An approach to determine the deformations of fibers, used to discretize the connections, is also proposed that can predict fiber deformations from system displacement. Here, the approach is used to determine the fiber displacements at connection failure.