Gravity Framing System Research Articles

Influence of gravity connections and damper activation forces on the seismic behavior of steel CBF buildings with dissipative floor connectors

AbstractThis paper investigates the influence of typical gravity connections on the seismic demands of steel concentrically braced frame (CBF) buildings with friction dampers as dissipative floor connectors. The investigated connections include (i) typical shear tab, (ii) clip angle, (iii) flush end‐plate and (iv) shear tab connections with bottom T‐stub. It is shown that the seismic behavior of the gravity framing system (GFS) is practically insensitive to the gravity connection type because the supplemental damping provided by the dissipative floor connectors dominates the seismic response of the GFS. While flush end‐plate and shear tab connections with bottom T‐stub generally exhibit smaller rotational demands than more flexible gravity connections, the resultant lateral drift demands in the GFS are more‐or‐less identical in all cases due to the elastic contribution of the gravity columns. A simplified method is proposed to evaluate the influence of the damper activation forces on the seismic response of multi‐story CBF buildings equipped with floor sliding friction dampers. First, the building is transformed into an equivalent single‐degree‐of‐freedom system. Subsequently, dual graphics called P‐spectra are generated through nonlinear response history analysis. These graphics allow to estimate the peak floor absolute acceleration demands and peak/residual roof displacements of the GFS as a function of the activation forces of the sliding friction dampers. An application example demonstrates that the proposed method can be reliably employed in a pre‐design phase to identify the range of damper activation forces through which an optimal performance of the steel CBF building may be anticipated.

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.

Seismic design and performance of steel concentrically braced frame buildings with dissipative floor connectors

AbstractThis paper explores the use of sliding friction dampers (SFDs) as dissipative floor connectors to mitigate higher mode effects and earthquake‐induced absolute acceleration demands on steel concentrically braced frame (CBF) buildings. The dampers connect each floor of the steel CBF system to the diaphragms of the gravity framing system (GFS) and they allow for a relative in‐plane movement between the two systems. For this purpose, a design methodology is first proposed to define the activation forces in the SFDs so as to ensure damage‐free seismic performance in the steel CBF and the diaphragms of the GFS. The efficiency of the design methodology is demonstrated through nonlinear response history analyses on a low‐ and high‐ductility six‐story steel CBF building. The simulation results suggest that (a) the determined activation forces of the SFDs are effective in mitigating higher mode effects and in preventing story drift concentrations regardless if capacity design is employed for the CBF system; (b) the absolute acceleration demands are reduced by approximately 50% relative to those in the rigid diaphragm counterpart. Similar reductions are achieved in the lateral drift demands of the GFS at seismic intensities with return periods of 475 and 2475 years. The reduction in the variability of seismic response, both in terms of absolute floor acceleration demands and story drift ratios (SDRs) in the CBF system, is noteworthy. Limitations as well as suggestions for future work are discussed.

Cyclic engagement of hysteretic steel dampers in braced buildings: a parametric investigation

Hysteretic steel dampers have been effectively used to improve the seismic performance of framed buildings by confining the dissipation of seismic energy into sacrifical, replaceable devices which are not part of the gravity framing system. The number of cycles sustained by the dampers during the earthquake is a primary design parameter, since it can be associated to low-cycle fatigue, with ensuing degradation of the mechanical properties and potential failure of the system. Current standards, like e.g. the European code EN 15129, indeed prescribe, for the initial qualification and the production control of hysteretic steel dampers, cyclic tests in which the devices are assessed over ten cycles with amplitude equal to the seismic design displacement dbd. This paper presents a parametric study focused on the number of effective cycles of the damper during a design earthquake in order to assess the reliability of the testing procedure proposed by the standards. The study considers typical applications of hysteretic steel dampers in low and medium-rise steel and reinforced concrete framed buildings and different ductility requirements. The results point out that the cyclic engagement of the damper is primarily affected by the fundamental period of the braced building and the design spectrum, and that, depending on these parameters, the actual number of cycles can be substantially smaller or larger that recommended by the standards. A more refined criterion for establishing the number of cycles to be implemented in testing protocols is eventually formulated.

Seismic performance of steel moment frames considering the effects of column-base hysteretic behavior and gravity framing system

This paper presents a parametric study conducted on five Steel Moment Frames (SMFs) varying in height (2-, 4-, 8-, 12-, and 20- story) to assess the interactive effect of the column base hysteretic behavior, continuous gravity columns and partially restrained gravity beam-column connections in their seismic performance. The frame response is examined through sophisticated Nonlinear Time History (NTH) and Nonlinear Static Pushover (NSP) analyses conducted as per FEMAp695 methodology. For each SMF, a range of base connection strengths (and their corresponding rotational stiffnesses) accompanied by different levels of gravity columns rigidity and gravity connection strengths are assigned, resulting in a total of 80 model simulations. Two collapse/failure limit states are formulated for the assessment 1) sidesway collapse defined as per FEMAp695; and 2) column-base connection failure, defined as base rotations exceeding a 0.05 rad threshold. Results from the simulations indicate that the gravity framing system profoundly affects the behavior of the SMFs analyzed by reducing their probability of collapse. In this manner, the seismic demands for the design of column-base connections can be reduced safely, incorporating their high deformation capacity as part of the energy dissipative mechanisms. Potential design implications are discussed, and limitations, as well as lines for future research, are outlined.

Seismic performance of different allowable storey drift limits: a review

Abstract Conventional seismic design approach depends on inelastic deformation of structural members for seismic energy dissipation. This concept may provide safety and economy in seismic design, but may not prevent damage in structures after strong earthquake. Consequently, the gravity framing systems in such structures may be exposed to interstoreydrift demands several times greater than would be expected on the basis of design calculations, calling into question their perceived safety. In this study, an overview of the seismic performance of the structures with respect to different design parameters are discussed. It is concluded thatthe strength and the initial stiffness of the designed frames decrease as the allowed storey drift limit of the frame increases. The maximum storey drift and beam-strain responses of the frames under the earthquake loading increases with the increase in the allowable storey drift limits.

Experimental investigation of steel building gravity framing systems under strong earthquake shaking

Hybrid simulations were conducted to evaluate the system-level seismic response of a 4-story steel moment resisting frame building and its gravity framing system through various levels of ground motion shaking. The experimental substructure, examined in detail, corresponds to a half-scale 1½-bay by 1½-story subassembly of the steel gravity frame. Unlike typical beam-to-column connection tests, the subassembly, which represented typical gravity framing steel construction, captured the redistribution of force demands in beams, columns, panel zones, and the composite floor slab while exhibiting inelastic behavior. Horizontal as well as varying vertical forces were applied on the test specimen to mimic realistic boundary and loading conditions. This paper focuses on the experimental response of the gravity framing system and its contribution to the lateral resistance of a steel frame building. In particular, the lateral resistance and failure mechanisms of the test frame are described and compared to numerical simulations based on state-of-the-art modeling approaches. The data generated from these experiments provides valuable insight on the gravity frame behavior towards improvement and verification of frame models at the system level.

Assessment of Post-Earthquake Fire Behavior of a Steel MRF Building in a Low Seismic Region

Building-level response to post-earthquake fire hazards in steel buildings has been assessed using primarily two-dimensional analyses of the lateral force resisting system. This approach may not adequately consider potential vulnerabilities in the gravity framing system. For this reason, three-dimensional (3D) finite element models of a 10-story case study building with perimeter moment resisting frames were developed to analyze post-earthquake fire events and better understand building response. Earthquakes are simulated using ground motion time histories, while Eurocode parametric time–temperature curves are used to represent compartment fires. Incremental dynamic analysis and incremental fire analysis procedures capture a range of hazard intensities. Findings show that the structural response due to earthquake and fire hazards are somewhat decoupled from one another. Regardless of the level of plastic hinging present in the moment framing system due to a seismic event, gravity column failure is the initiating failure mode in a fire event.

Earthquake-Induced Collapse Risk and Loss Assessment of Steel Concentrically Braced Frames

This paper quantifies the collapse risk and earthquake-induced losses for a wide range of archetype buildings with special concentrically braced frames (SCBFs). The collapse risk and expected economic losses associated with repair, demolition and collapse are computed based on a performance-based earthquake engineering framework developed within the Pacific Earthquake Engineering Research Center. It is shown that the collapse risk of the steel SCBF archetypes may be significantly overestimated when the influence of the gravity framing system on the lateral frame strength and stiffness is ignored. It is also found that the building-specific earthquake loss assessment is significantly overestimated at low probability of occurrence seismic events (i.e., 2% probability of occurrence in 50 years) when the gravity framing system is not modeled explicitly as part of the nonlinear building model. For frequent and design-basis seismic events (i.e., 50 and 10% probability of exceedance over 50 years of building life expectancy), acceleration-sensitive nonstructural component repairs govern the building losses regardless of the employed nonlinear building model representation. For the same seismic events, steel brace flexural buckling contributes to structural repair losses.

SEISMIC PERFORMANCE OF STEEL MOMENT FRAME OFFICE BUILDINGS WITH RECTANGULAR CONCRETE‐FILLED STEEL TUBE GRAVITY COLUMNS

AbstractIn conventional seismic design, the lateral strength in the gravity framing systems is not utilized. However, utilizing the lateral strength of a concrete‐filled steel tube (CFT) gravity column system has the potential to increase lateral capacity and improve global behaviour, compared to buildings with wide‐flange gravity columns. In this paper, the seismic performance of 1‐story, 2‐story and 4‐story office buildings with perimeter steel moment frames and gravity columns consisting of rectangular CFTs or wide flange members is evaluated. The results predicted that the CFTs generally improve seismic performance of the buildings, with reduced repair costs and repair times, compared to buildings with wide‐flange gravity columns.

Effect of Modeling Assumptions on the Earthquake-Induced Losses and Collapse Risk of Steel-Frame Buildings with Special Concentrically Braced Frames

This paper quantifies the collapse risk and earthquake-induced economic losses of steel-frame buildings with special concentrically braced frames designed in urban California. A probabilistic building-specific loss estimation methodology that can explicitly account for the main sources of variability related to seismic hazards and structural response is used for this purpose. It is shown that, depending on the choice of the loss metric, at seismic events with low probability of occurrence (i.e., 2% probability of occurrence in 50 years), losses because of demolition and structural collapse in steel-frame buildings with special concentrically braced frames designed in highly seismic zones may be significantly overestimated when ignoring the contribution of the composite floor and gravity framing system to the analytical model building representation. For frequent and moderately frequent seismic events (i.e., 50 and 10% probability of exceedance over 50 years of building life expectancy), acceleration-sensitive nonstructural component repairs govern building losses regardless of the analytical model representation used. For the same seismic events, an appreciable contributor to total losses in steel-frame buildings with special concentrically braced frames is structural repairs because of steel brace flexural buckling. It is suggested that dual-parameter rather than drift-based steel brace fragility curves should be used in loss computations conditioned on a single seismic intensity. Otherwise, the expected annual losses should be used as a metric for building-specific loss assessment of steel-frame buildings with special concentrically braced frames.

Earthquake‐induced loss assessment of steel frame buildings with special moment frames designed in highly seismic regions

SummaryThis paper discusses an analytical study that quantifies the expected earthquake‐induced losses in typical office steel frame buildings designed with perimeter special moment frames in highly seismic regions. It is shown that for seismic events associated with low probabilities of occurrence, losses due to demolition and collapse may be significantly overestimated when the expected loss computations are based on analytical models that ignore the composite beam effects and the interior gravity framing system of a steel frame building. For frequently occurring seismic events building losses are dominated by non‐structural content repairs. In this case, the choice of the analytical model representation of the steel frame building becomes less important. Losses due to demolition and collapse in steel frame buildings with special moment frames designed with strong‐column/weak‐beam ratio larger than 2.0 are reduced by a factor of two compared with those in the same frames designed with a strong‐column/weak‐beam ratio larger than 1.0 as recommended in ANSI/AISC‐341‐10. The expected annual losses (EALs) of steel frame buildings with SMFs vary from 0.38% to 0.74% over the building life expectancy. The EALs are dominated by repairs of acceleration‐sensitive non‐structural content followed by repairs of drift‐sensitive non‐structural components. It is found that the effect of strong‐column/weak‐beam ratio on EALs is negligible. This is not the case when the present value of life‐cycle costs is selected as a loss‐metric. It is advisable to employ a combination of loss‐metrics to assess the earthquake‐induced losses in steel frame buildings with special moment frames depending on the seismic performance level of interest. Copyright © 2017 John Wiley & Sons, Ltd.

Integrity of Bolted Angle Connections Subjected to Simulated Column Removal.

Large-scale tests of steel gravity framing systems (SGFSs) have shown that the connections are critical to the system integrity, when a column suffers damage that compromises its ability to carry gravity loads. When supporting columns were removed, the SGFSs redistributed gravity loads through the development of an alternate load path in a sustained tensile configuration resulting from large vertical deflections. The ability of the system to sustain such an alternate load path depends on the capacity of the gravity connections to remain intact after undergoing large rotation and axial extension demands, for which they were not designed. This study experimentally evaluates the performance of steel bolted angle connections subjected to loading consistent with an interior column removal. The characteristic connection behaviors are described and the performance of multiple connection configurations are compared in terms of their peak resistances and deformation capacities.

Application of Experimental Results to Computational Evaluation of Structural Integrity of Steel Gravity Framing Systems with Composite Slabs

AbstractThe simplicity of steel gravity framing systems in their load paths and construction has led to questions regarding their structural integrity or robustness. When considering structural int...

Large-Scale Hybrid Simulation of a Steel Moment Frame Building Structure through Collapse

The implementation of two series of hybrid simulations that aim to trace the system-level seismic response of a four-story steel moment frame building structure through collapse is presented. In the first series of tests, a half-scale 1½-bay by 1½-story physical substructure of a special steel moment-resisting frame is considered, while in the second series the physical substructure corresponds to the gravity framing system with a similar-sized specimen. An objective of these tests is to demonstrate the potential of hybrid simulation with substructuring as a cost-effective alternative to earthquake simulators for large-scale system-level testing of structural frame subassemblies. The performance of a recently developed substructuring technique and time-stepping integration method for hybrid simulation are evaluated when employed with large and complex numerical substructures exhibiting large levels of nonlinear response. The substructuring technique simplifies the experimental setup by reducing the number of required actuators while adequately approximating the boundary conditions including lateral displacements and axial loads on columns. The test method was found to be reliable with capabilities to provide insight into experimental behavior of structural subassemblies under realistic seismic loading and boundary conditions.

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.

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.

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.

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.

Modeling and Analysis of Single-Plate Shear Connections under Column Loss

AbstractThis paper presents a computational assessment of the behavior of single-plate shear (shear tab) connections in gravity frames under column-loss scenarios. Two-span beam assemblies without floor slabs are considered under push-down loading of the unsupported center column. Both detailed and reduced modeling approaches are used in the computational assessment, and comparisons with experimental data are presented to establish confidence in the models. The models are used to investigate the influence of factors such as end support conditions, span length, connection strength, and postultimate connection behavior on the collapse resistance of gravity framing systems. Rotational capacities of single-plate shear connections under column-loss scenarios are, in some cases, less than half the values based on seismic test data, owing to the axial extension imposed on the connections in addition to rotation.