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. (C) 2019 American Society of Civil Engineers.

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

Experimental Evaluation of Steel Columns under Seismic Hazard-Consistent Collapse Loading Protocols

Dimitrios Lignos

This paper presents results from 21 large-scale experiments on steel columns that examined their collapse behavior as related to steel moment resisting frames (MRFs). The test specimens included wide-flange and square hollow structural shapes (HSS) and were tested until complete loss of their lateral load-carrying capacity. The test results suggest that the collapse rotations of steel columns under collapse protocols, representing near-fault events, are two to three times larger than those of their counterparts under standard symmetric cyclic protocols, regardless of the examined cross-sectional compactness. Conversely, when ground motion duration is an important seismic hazard characteristic, the collapse behavior of steel columns is reasonably traced with a standard symmetric cyclic protocol. It is shown experimentally that steel columns have an inherent reference energy dissipation capacity regardless of the employed loading history. Axial shortening attributable to local buckling is up to 10 times larger in interior than in end columns, which leads to differential axial shortening within a steel MRF story even at modest lateral drift demands during subduction zone seismic events. The test data underscore that the formation of local buckling at a column's fixed end caps the strain demands at about 1% near complete joint penetration welds between the column and the base plate, even when the transient axial load demand becomes tensile. Simpler welds may provide adequate ductility in steel columns featuring cross sections near the compactness limits for highly ductile members according to current design standards. It is shown that the estimated rotations by current ASCE 41 acceptance criteria for collapse prevention are 5-10 times smaller than the measured collapse rotations, even for columns with moderately compact profiles. (C) 2021 American Society of Civil Engineers.

ASCE-AMER SOC CIVIL ENGINEERS2021

Analytical investigation of the cyclic behavior and plastic hinge formation in deep wide-flange steel beam-columns

Ahmed Mohamed Ahmed Elkady, Dimitrios Lignos

This paper investigates the cyclic behavior of deep wide-flange sections, used as columns in steel Special Moment Frames (SMFs), through detailed finite element (FE) analysis. A wide range of wide-flange sections is subjected to symmetric cyclic lateral loading combined with different levels of constant compressive axial load ratios representing the loading conditions of interior steel columns in SMFs. The FE simulations demonstrate that wide-flange beam-columns, with web and flange slenderness ratios near the current compactness limits of seismic design provisions (AISC 341-10), experience rapid cyclic deterioration in flexural strength under high axial load ratios. It is also found that deep wide-flange slender sections shorten axially to about 10 % of their length due to severe flange and web local buckling. Based on the FE simulations, for bottom story columns, where axial load ratios are in the range of 20–35 %, a reduction to about two thirds of the current compactness limit for highly ductile members would achieve a 4 % chord rotation while maintaining a flexural strength larger than 80 % of the expected plastic flexural strength of a steel column. The FE simulation results also suggest that the pre-capping rotation predicted by current modeling recommendations for steel components (PEER/ATC 72-1) is overestimated for sections with high web and flange slenderness ratios undergoing monotonic and/or cyclic lateral loading combined with high axial load levels. © 2014, Springer Science+Business Media Dordrecht.

Springer Verlag2015

Experimental Evaluation of Steel Columns under Seismic Hazard-Consistent Collapse Loading Protocols

Dimitrios Lignos

ASCE-AMER SOC CIVIL ENGINEERS2021