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, Ω s larger 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, Ω d is introduced that captures the inelastic force redistribution due to dynamic loading. Steel frame buildings with perimeter SMFs achieve a Ω d > 3 regardless if the gravity framing is considered as part of the nonlinear analytical model representation.

SUMMARY This paper investigates the effect of the composite action on the seismic performance of steel special moment frames (SMFs) through collapse. A rational approach is first proposed to model the hysteretic behavior of fully restrained composite beam‐to‐column connections, with reduced beam sections. Using the proposed modeling recommendations, a system‐level analytical study is performed on archetype steel buildings that utilize perimeter steel SMFs, with different heights, designed in the West‐Coast of the USA. It is shown that in average, the composite action may enhance the seismic performance of steel SMFs. However, bottom story collapse mechanisms may be triggered leading to rapid deterioration of the global strength of steel SMFs. Because of composite action, excessive panel zone shear distortion is also observed in interior joints of steel SMFs designed with strong‐column/weak‐beam ratios larger than 1.0. It is demonstrated that when steel SMFs are designed with strong‐column/weak‐beam ratios larger than 1.5, (i) bottom story collapse mechanisms are typically avoided; (ii) a tolerable probability of collapse is achieved in a return period of 50 years; and (iii) controlled panel zone yielding is achieved while reducing the required number of welded doubler plates in interior beam‐to‐column joints. Copyright © 2014 John Wiley & Sons, Ltd.

This paper discusses the findings from ten full-scale steel column tests subjected to 4 multi-axis cyclic loading. The columns utilize deep wide-flange cross-sections typically seen in 5 steel moment-resisting frames designed in seismic regions. The effects of boundary conditions, 6 loading sequence, local web and member slenderness ratios on the column hysteretic behavior are 7 investigated. The test data underscores the influence of boundary conditions on the damage 8 progression of steel columns. Local buckling followed by out-of-plane deformations near the 9 plastified column base are the dominant failure modes in fixed base columns with a realistic flexible 10 top end. Twisting may occur only at drifts larger than 3% even when the member slenderness is 11 fairly large. The test data suggest that bidirectional loading amplifies the out-of-plane deformations 12 but does not significantly affect the overall column performance. The loading sequence strongly 13 affects the column's plastic deformation capacity but only at story-drifts larger than 2%. After this 14 drift amplitude, column axial shortening grows exponentially and becomes a controlling failure 15 mode. Measurements of out-of-plane brace force demands at the column top exceed the lateral 16 brace design force specified in North American standards. 17

Nonlinear static and dynamic analyses are utilized by engineers for performance-based seismic risk evaluation of new and existing structures. In this context, nonlinear component modeling criteria are typically based on ASCE 41 guidelines. Experiments on wide-flange steel columns suggest that the ASCE 41-13 nonlinear component models do not adequately reflect the expected steel column behavior under cyclic loading. To help bridge the gap between state-ofthe-art research and engineering practice, this paper proposes new modeling criteria for the first-cycle envelope and monotonic backbone curves of steel columns for use in nonlinear static and dynamic frame analysis. The proposed nonlinear provisions include new parameters for concentrated hinge models to facilitate modeling of strength and stiffness deterioration of steel columns under seismic loading. The associated variability in the model parameters is also quantified to facilitate reliability analyses and development of probabilistic acceptance criteria for design. Recommendations are made to account for the influence of bidirectional lateral loading and varying axial load demands on the steel column's hysteretic behavior. Also proposed is an increase in the compression axial force limit for characterizing columns as forceversus deformation-controlled in line with the new ASCE 41 provisions. The proposed modeling parameters are validated against test data and continuum finite element analyses, and they are proposed for consideration in future updates to ASCE 41 requirements for nonlinear static and dynamic analyses of steel frame buildings with wide-flange columns.

This paper presents results from an experimental evaluation on the pre-and postbuckling 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 crosssectional 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 (PEER/ATC 2010) 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 shall 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.