Lateral Drift Demands Research Articles

Geometric tolerances of welded connections with inelastic panel zones

AbstractPrequalified beam‐to‐column connections in steel moment resisting frames usually concentrate inelastic deformations in the steel beam ends. As such, nonlinear geometric instabilities (i.e., local buckling) may occur in these regions at modest lateral drift demands, leading to potentially high residual deformations. Previous research has shown that welded connections featuring inelastic panel zones exhibit a stable hysteretic behaviour. In this case, residual deformations in steel beams are often not visually detectable after inelastic cyclic straining. This paper contrasts the residual deformations of steel beams as part of welded connections with elastic and inelastic panel zones through continuum finite element analyses. The employed finite element model is thoroughly validated with available test data. Besides the actual hysteretic response of the connections, the primary focus of the present work is on a number of geometric tolerance indicators, such as the web concavity, flange out‐of‐square and beam axial shortening. These are strongly correlated to the potential for reuse of structural steel members. The results suggest that connection designs featuring inelastic panel zones delay beam local buckling relative to their strong panel zone counterparts, achieving acceptable residual deformations even at lateral drift demands higher than 4% when considering the geometric tolerances of international standards.

Hysteretic behaviour of welded connections with highly inelastic panel zones

AbstractThe current practice in capacity‐designed steel moment resisting frames (MRFs) worldwide allows for limited shear yielding in the column web panel zone. As such, inelastic deformations concentrate near the beam ends, thereby leading to flexural strength deterioration due to local buckling often at modest lateral drift demands. Experiments on post‐Northridge welded connections suggest that prior to a story drift angle of 5% rad, panel zone kinking does not induce fracture for panel zone shear distortions of up to 10γy. Within such a context, this paper presents an alternative design of a welded moment connection with highly inelastic panel zones. The connection features simplified weld details including a customised beveled backing bar that can be kept in place after the completion of the complete joint penetration welds at the beam flange‐to‐column flange joint. Full scale experiments suggest that the hysteretic response of the connection remains stable up until a story drift angle of 8% rad. The specimen did not exhibit visible structural damage prior to story drift angles of 4% rad, which are characteristic to a maximum considered earthquake. Modest local buckling occurred near the beam ends only after a story drift angle of 5% rad, which is a typical drift threshold in current collapse assessment methodologies. This paper summarises the primary findings of the experimental program for one of the test specimens.

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.

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.

Development and Experimental Validation of Dissipative Embedded Column Base Connections for Enhanced Seismic Performance of Steel Moment-Resisting Frames

This paper proposes a novel embedded column base (ECB) connection that defies the current paradigm in capacity-designed steel moment-resisting frames (MRFs) where column bases have been traditionally considered as nondissipative. In the proposed concept, a dissipative zone is introduced as part of the embedded portion of the steel column. This zone features a reduced cross section and is decoupled from the concrete footing with a debonding material layer. The surrounding concrete effectively restrains the embedded section against nonlinear geometric instabilities, thereby retaining simplicity in the design process. Large-scale experiments suggest that, contrary to its conventional counterpart, the proposed dissipative ECB connection exhibits a stable hysteretic response until large lateral drift demands (i.e., 7% rad). It is also demonstrated that the dissipative ECB connection is resilient to local buckling-induced axial shortening, such that it lowers the repairability needs of slender wide-flange steel columns after earthquakes. Column twisting is also minimized throughout the loading history. The results indicate that both the elastic stiffness and flexural yield strength of the proposed ECB connection can be analytically derived for potential use in engineering design. It is found that a simple nondegrading bilinear model suffices to describe the hysteretic response of the proposed ECBs for performance-based assessment of steel MRFs.

Finite element modeling and behavior of dissipative embedded column base connections under cyclic loading

This paper explores, by means of continuum finite element (CFE) analysis, a recently developed concept that promotes controlled inelastic energy dissipation within the embedded portion of column base connections rather than in steel columns of conventional embedded column bases. As such, dissipative embedded column base connections become resilient to local buckling. Several loading and geometric parameters are interrogated through parametric simulations with a high-fidelity CFE model, which is thoroughly validated with available experimental data. The simulation results suggest that steel columns in conventional embedded bases are prone to local buckling even at modest lateral drift demands (i.e., 2% rad), thereby exhibiting 10 to 30 mm of axial shortening in the presence of constant compressive axial load. On the other hand, dissipative bases are resilient to local buckling till at least 4% rad lateral drift demands regardless of the column cross-sectional characteristics and loading parameters. At larger lateral drift demands, the use of horizontal stiffeners in dissipative bases alleviates localized flange deformations attributable to bearing of the embedded column flange with the reinforced concrete foundation. Axial shortening in dissipative base connections is only attributable to inelastic cyclic straining within the dissipative zone and is up to 60% less than that of steel columns in conventional bases at lateral drift demands of 2% rad. The CFE simulations reveal that the hysteretic response of dissipative bases is insensitive to bidirectional loading.

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

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

Discussion of “Experimental Evaluation and Numerical Modeling of Wide-Flange Steel Columns Subjected to Constant and Variable Axial Load Coupled with Lateral Drift Demands” by Julien Cravero, Ahmed Elkady, and Dimitrios G. Lignos

Discussion of “Experimental Evaluation and Numerical Modeling of Wide-Flange Steel Columns Subjected to Constant and Variable Axial Load Coupled with Lateral Drift Demands” by Julien Cravero, Ahmed Elkady, and Dimitrios G. Lignos

Closure to “Experimental Evaluation and Numerical Modeling of Wide-Flange Steel Columns Subjected to Constant and Variable Axial Load Coupled with Lateral Drift Demands” by Julien Cravero, Ahmed Elkady, and Dimitrios G. Lignos

Closure to “Experimental Evaluation and Numerical Modeling of Wide-Flange Steel Columns Subjected to Constant and Variable Axial Load Coupled with Lateral Drift Demands” by Julien Cravero, Ahmed Elkady, and Dimitrios G. Lignos

Experimental Evaluation and Numerical Modeling of Wide-Flange Steel Columns Subjected to Constant and Variable Axial Load Coupled with Lateral Drift Demands

This paper presents results from an experimental evaluation on the pre- and post-buckling behavior of 12 steel wide-flange cantilever columns under axial load and lateral drift demands. The influence of several loading and geometric parameters, including the cross-sectional local web and flange slenderness ratios, applied axial load, and lateral and axial loading history on the performance of these columns is thoroughly examined. The test data indicate that cross-sectional local buckling is highly asymmetric in steel columns under variable axial load. A relatively high compressive axial load can significantly compromise the steel column seismic stability and ductility, but this also depends on the imposed lateral loading history. The AISC axial load–bending moment interaction equation provides accurate estimates of a steel column’s yield resistance. However, the same equation underestimates by at least 30% the column’s peak resistance, regardless of the loading scenario. Measurements of column flange deformation, axial shortening, flexural resistance, and lateral drift are combined in a single graphical format aiding the process of assessing steel column repairability after earthquakes. The test data suggest that current practice-oriented nonlinear component modeling guidelines may not provide sufficient accuracy in establishing both the monotonic and first-cycle envelope curves of steel columns. It is also shown that high-fidelity continuum finite-element models should consider geometric imperfections of proper magnitude, in addition to the steel material inelasticity, to properly simulate the inelastic buckling of wide-flange steel columns and generalize the findings of physical tests. Issues arising due to similitude are also discussed to properly limit steel column instability modes in future studies.

Development of collapse‐consistent loading protocols for experimental testing of steel columns

SummaryIn order to effectively utilize results from quasi‐static cyclic testing on structural components for the earthquake‐induced collapse risk quantification of structures, the need exists to establish collapse‐consistent loading protocols representing the asymmetric lateral drift demands of structures under low‐probability of occurrence earthquakes. This paper summarizes the development of such protocols for experimental testing of steel columns prone to inelastic local buckling. The protocols are fully defined with a deformation‐ and a force‐controlled parameter. They are generally applicable to quantify the capacity and demands of steel columns experiencing constant and variable axial load coupled with lateral drift demands. Through rigorous nonlinear earthquake collapse simulations, it is found that the building height, the column's local slenderness ratio, and ground motion type have the largest influence on the dual‐parameter loading protocol indexes. Comprehensive comparisons with measured data from full‐scale shake table collapse tests suggest that unlike routinely used symmetric cyclic loading histories, the proposed loading protocol provides sufficient information for modeling strength and stiffness deterioration in steel columns at large inelastic deformations.

11.29: Updates to the ASCE‐41‐13 provisions for the nonlinear modeling of steel wide‐flange columns for performance‐based earthquake engineering

ABSTRACTPerformance‐based earthquake engineering requires numerical models that are able to reliably predict the expected component behavior of buildings under seismic excitations. The nonlinear modeling guidelines provided in ASCE‐41‐13 have historically been used in this context by practicing engineers. However, for steel wide‐flange columns, the recommendations are primarily based on experimental data that was fairly limited until recently. This paper addresses many of the limitations present in ASCE‐41‐13 for the modeling of steel wide‐flange columns. Recent experimental data from large‐ and full‐scale tests on wide‐flange steel columns, supplemented with detailed finite element studies, has been collected for a wide range of column cross‐sections and applied compressive axial loads. Multiple regression analysis is utilized to develop multivariate predictive equations for the seismic response of steel wide‐flange columns. It is found that the primary contributors that dominate the column response are the local web slenderness, member slenderness and the applied axial load ratio due to gravity loading. Guidelines for developing nonlinear component models for steel columns that can be directly used in both nonlinear static and dynamic analysis procedures are provided. The predicted responses are validated against test data that was not part of the original database. Furthermore, recommendations are proposed for modeling wide‐flange columns subjected to varying axial loads as well as bidirectional loading; and a new limit for force‐controlled elements is proposed for columns under high axial compressive loading and lateral drift demands.

Seismic performance of steel MRF building with nonlinear viscous dampers

This paper presents an experimental study of the seismic response of a 0.6-scale three-story seismicresistant building structure consisting of a moment resisting frame (MRF) with reduced beam sections (RBS), and a frame with nonlinear viscous dampers and associated bracing (called the DBF). The emphasis is on assessing the seismic performance for the design basis earthquake (DBE) and maximum considered earthquake (MCE). Three MRF designs were studied, with the MRF designed for 100%, 75%, and 60%, respectively, of the required base shear design strength determined according to ASCE 7-10. The DBF with nonlinear viscous dampers was designed to control the lateral drift demands. Earthquake simulations using ensembles of DBE and MCE ground motions were conducted using the real-time hybrid simulation method. The results show the drift demand and damage that occurs in the MRF under seismic loading. Overall, the results show that a high level of seismic performance can be achieved under DBE and MCE ground motions, even for a building structure designed for as little as 60% of the base shear design strength required by ASCE 7-10 for a structure without dampers.

Seismic Response and Performance of a Steel MRF Building with Nonlinear Viscous Dampers under DBE and MCE

AbstractThis paper presents an experimental study of the seismic response of a 0.6-scale three-story building with nonlinear viscous dampers under the design basis earthquake (DBE) and the maximum considered earthquake (MCE). The test structure consists of a moment-resisting frame (MRF) with reduced beam sections (RBS), a frame with nonlinear viscous dampers and associated bracing (called the DBF), and a gravity load system. The paper focuses on quantifying and assessing the seismic response of the test structure under the DBE and MCE. Three MRF designs were studied for 100, 75, and 60%, respectively, of the required base shear design strength according to ASCE 7-10. The DBF with nonlinear viscous dampers was designed to control the lateral drift demands. Earthquake simulations using ensembles of ground motions were conducted using the real-time hybrid simulation method. Experimental results show that a high level of seismic performance can be achieved under DBE and MCE ground motions, even for a building...

Damage Assessment of RC Frame Structures under Multi-Earthquake Sequences

This paper presents the results of a numerical investigation aimed at evaluation of damage accumulation in structures under different seismic sequences. For this purpose, a multi-story RC frame model representing the typical buildings was built and subjected to four sets of different repeating seismic sequences with different magnitude. The results indicate that the aftershock sequences will increase the damage accumulation in structures, and the damage index based on the Park and Ang model can qualitatively and quantitatively ascertain the damage state of the structure instead of the single index based on the lateral drift demands. Additionally, different sequences with the identical energy input will cause the same damage state in structures.

Damage Assessment of RC Frame Structures under Multi-Earthquake Sequences

This paper presents the results of a numerical investigation aimed at evaluation of damage accumulation in structures under different seismic sequences. For this purpose, a multi-story RC frame model representing the typical buildings was built and subjected to four sets of different repeating seismic sequences with different magnitude. The results indicate that the aftershock sequences will increase the damage accumulation in structures, and the damage index based on the Park and Ang model can qualitatively and quantitatively ascertain the damage state of the structure instead of the single index based on the lateral drift demands. Additionally, different sequences with the identical energy input will cause the same damage state in structures.

Methods for the evaluation of the damage limit state and stiffness regularity in elevation of frame systems in seismic areas

Recent earthquake resistant design codes (e.g. Eurocode 8, last Italian seismic code) underline the importance of stiffness regularity in elevation for frame systems located in seismic areas. Large deviations from a continuous variation with height of stiffness are likely to induce poor and often dangerous structural response (Paulay and Priestley). New codes emphasize moreover the importance of limiting non-structural damage under a seismic action having a large probability of occurrence during the economic life of a building. Actually nonstructural components play a significant role in performance-based earthquake engineering. Damage limitation requirements consist in limiting interstorey drifts. Therefore, to evaluate and eventually redefine a new building’s preliminary design, it seems very useful to have a method to quickly calculate lateral stiffness of each storey so as to quantify its variation along the height of the building as well as a simple procedure to estimate the maximum lateral drift demands. Two approximate methods will be proposed for achieving these purposes.

Approximate Lateral Drift Demands in Multistory Buildings with Nonuniform Stiffness

An approximate method is presented to estimate the maximum lateral drift demands in multistory buildings with nonuniform lateral stiffness responding primarily in the fundamental mode when subjected to earthquake ground motions. The method is aimed at the estimation of the maximum roof displacement and of the maximum interstory drift ratio for a given response spectrum. A simplified model of the multistory building is used based on an equivalent continuum structure with nonuniform lateral stiffness distribution consisting of a combination of a flexural cantilever beam and a shear cantilever beam. The effect of the type and amount of reduction in lateral stiffness along the height of the building and of the ratio of overall flexural and shear deformations on the ratio of the spectral displacement to the roof displacement and on the ratio of the maximum interstory drift ratio to the roof drift ratio is investigated. It is shown that reductions in lateral stiffness along the height have a negligible effect on the ratio of spectral displacement to maximum roof displacement and only a small effect on the ratio of maximum interstory drift ratio to roof drift ratio. The use of the method is exemplified with a ten-story steel moment-resisting frame building with nonuniform lateral stiffness. Results computed with the simplified method are compared with those computed using linear and nonlinear time history analyses. It is shown that the method provides good approximations that are adequate for the preliminary design of new buildings or for a rapid evaluation of existing buildings.