Effect of Column Base Flexibility on the Hysteretic Response of Wide Flange Steel Columns

Recent experiments on wide flange steel columns subjected to multi-axial cyclic loading underscore the influence of the member end boundary conditions on strength and stiffness deterioration due to coupling of local and lateral torsional buckling. Field observations complemented by recent tests indicate a similar sensitivity of response modes to the base flexibility. This paper investigates the effect of the column base flexibility on the hysteretic behavior of first story steel columns in moment resisting frames (MRFs). This is accomplished through rigorous finite element (FE) simulations, validated by full-scale column member and column base tests. Recent studies on embedded steel column base connections are used to infer the base flexibilities used in the FE simulations; these are representative of those commonly observed in first story interior columns of mid- to high-rise steel MRFs. The FE simulation findings indicate that column base flexibility delays the onset of local buckling, and subsequently reduces the axial shortening by about 50% compared to the fixed-base case. In contrast, the column base strength itself is insensitive to the column base flexibility.

Influence of embedded steel column base strength on earthquake-induced residual deformations

Hiroyuki Inamasu, Dimitrios Lignos

Recent experiments on steel columns under multi-axis cyclic loading suggest that a steel column may experience appreciable axial shortening due to local buckling. Reconnaissance reports from recent earthquakes indicate that a number of embedded column base (ECB) connections in steel frame buildings were found to be lightly damaged although they are expected to remain elastic. As a result, the amount of inelastic damage concentrated in the respective steel columns was much less than it was expected to be. This paper investigates the influence of the ECB strength on the residual deformations of first story interior steel columns in steel moment resisting frames (MRFs) under multi-axial cyclic loading. The evaluation is conducted with rigorous finite element (FE) simulations. The developed FE model is able to simulate the pinching hysteretic behavior of typical ECBs. The parametric study involves two main cases. In the first one the ECB remains in elastic. In the second case the inelastic behavior is spread to the ECB and the steel column itself. The simulation results suggest that the resultant plastic deformation within the steel column is much smaller in the latter. The onset of column local buckling is delayed; therefore, the residual column axial shortening becomes fairly minimal. In particular, it is reduced by 50% at lateral drift demands associated with low-probability of occurrence earthquakes.

2018

Seismic Stability of Wide-Flange Steel Columns Interacting with Embedded Column Base Connections

Hiroyuki Inamasu, Dimitrios Lignos

The rotational flexibility of column base connections is commonly assumed to be detrimental to the seismic response of steel moment-resisting frames (MRFs) because it increases story drift demands and promotes soft-story formation by lowering the point of inflection, thereby exacerbating the moment demand at the first-story column top end. However, recent experimental research indicates that wide-flange steel columns with flexible boundary conditions may have enhanced response because base connection flexibility delays the plastic hinge formation and local buckling progression within the column cross section near the base. Local buckling greatly reduces the lateral and torsional restraint at the column ends such that the delay in local buckling also delays member instabilities, which often control the deformation capacity of wide-flange steel columns. This effect was examined parametrically through 2,160 continuum finite-element (CFE) simulations validated against physical experiments. Parameters examined include the applied axial load ratio, the column length, and the lateral loading protocol for the column sections, along with five levels of column base connection flexibility. The results indicate that base connection flexibility has a significant (and positive) influence on key aspects of column response, including (1) lateral deformation capacity, which is on average 40%-50% greater for a realistic value of flexibility, relative to a fixed-base condition; and (2) residual column axial shortening, which is important from the standpoint of retrofit and reparability (a 30%-60% decrease). These benefits are more pronounced in cyclic (versus monotonic) loading histories and for members with cross sections more prone to out-of-plane instability. The implications of these findings are outlined for existing buildings as well as prospective design methodologies. Limitations are discussed.

2019

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

Ahmed Mohamed Ahmed Elkady, Dimitrios Lignos

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. (C) 2019 American Society of Civil Engineers.

2020

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

Ahmed Mohamed Ahmed Elkady, Dimitrios Lignos

2020

Seismic Stability of Wide-Flange Steel Columns Interacting with Embedded Column Base Connections

Hiroyuki Inamasu, Dimitrios Lignos

2019