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FULL SCALE EXPERIMENTS OF A COMPOSITE STEEL MOMENT RESISTING FRAME: BEHAVIORAL INSIGHTS AND IMPLICATIONS ON SEISMIC DESIGN
Abstract
This paper discusses the cyclic performance of a heavily instrumented 2-bay full-scale composite-steel moment resisting frame (CMRF) subsystem from the onset of structural damage up until incipient collapse. The CMRF featured stiffened end plate bolted connections and was subjected to a series of lateral load histories that mimicked the asymmetric hysteretic response (i.e., ratcheting) of structures prior to structural collapse. It is shown that the primary deteriorating mechanism of the CMRF subsystem was local buckling near the steel beam ends followed by crack initiation and propagation due to ultra-low cycle fatigue. While two of the four steel beams attained nearly zero flexural strength at a lateral drift demand of about 15% rad, the reserved capacity of the CMRF was at least 30% of its peak story shear resistance due to the slab restraint and framing action. Both mechanisms, which are not captured in traditional beam-to-column subassemblies with idealized boundary conditions, caused straightening of local buckles within the dissipative zones of the steel beams. The experimental data is considered unique for ongoing efforts on the further development of the new seismic design provisions of Eurocode 8 Part 1-2. 2. INTRODUCTION Past studies regarding the behaviour of composite-steel beam-to-column connections has been mostly conducted with cruciform subassemblies such as that of Figure 1a. These feature overlay simplified boundary conditions ([1]-[4]). In this case, the assumed locations of the inflection points remain constant throughout loading, thereby neglecting redistributions of forces after the onset of structural damage within the anticipated dissipative zone(s).
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This paper proposes a macro-model for simulating the hysteretic behaviorof composite-steel beams as part of fully restrained beam-to-column connec-tions in composite-steel moment-resisting frames (MRFs). Comparisons withexperimental data suggest that the proposed model captures the asymmetrichysteretic response of composite-steel beams including the cyclic deteriorationin strength and stiffness. Moreover, the proposed model captures the primaryslab-column force transfer mechanisms and predicts the slip demands inbeam-slab connections under inelastic cyclic loading. The modeling approachis employed in a system-level study to benchmark the seismic collapse risk ofcomposite-steel MRF buildings across Europe. Moreover, the beam-slab slipdemands are quantified through the development of beam-slab slip hazardcurves. The simulation studies suggest that the examined composite-steel MRFsexhibit a system overstrength of about 4. This is attributed to the drift require-ments in the current European seismic provisions.1The annualized probabilityof collapse of the prototype buildings is well below 1% over a 50-year buildinglife expectancy regardless of the design site and the degree of composite action.Beam-slab connections with a partial degree of composite action experienceminimal damage for frequently occurring seismic events (i.e., 50% probabilityof exceedance over 50 years); and light cracking in the slab for a design basisearthquake. The above are important from a seismic repairability standpoint.Accordingly, it is recommended that the 25% reduction in the shear resistanceof stud connectors is not imperative for seismic designs that feature steel beamswith depths less than 500 mm.
This paper discusses the development of a publicly available database of composite steel beam-to-column connections under cyclic loading. The database is utilized to develop recommendations for the seismic design and nonlinear performance assessment of steel and composite-steel moment-resisting frames (MRFs). In particular, the sagging/hogging plastic flexural resistance as well as the effective slab width are assessed through a comparison of the European, American and Japanese design provisions. The database is also used to quantify the plastic rotation capacity of composite steel beams under sagging/hogging bending. It is found that the Eurocode 8-Part 3 provisions overestimate the plastic rotation capacities of composite beams by 50% regardless of their web slenderness ratio. Empirical relationships are developed to predict the plastic rotation capacity of composite steel beams as a function of their geometric and material properties. These relationships can facilitate the seismic performance assessment of new and existing steel and composite-steel MRFs through nonlinear static analysis. The collected data underscores that the beam-to-column web panel zone in composite steel beam-to-column connections experience higher shear demands than their non-composite counterparts. A relative panel zone-to-beam resistance ratio is proposed that allows for controlled panel zone inelastic deformation of up to 10 times the panel zone’s shear yield distortion angle. Notably, when this criterion was imposed, there was no fracture in all the examined beam-to-column connections.
The report presents research activities on the seismic behaviour of composite steel concrete moment frames co-ordinated in Europe within the ICONS project. Research has been focused on problems of ductility and resistance in the case where the connections of the steel parts are full strength "rigid".
Tests have been realised on subassemblages, on elements, on a plane frame and on a 3-D frame. Some tests were run quasi statically, others on shaking tables. Numerical modelling was done in parallel. Summary reports of these activities are presented.
Based on that research and on literature data, for the structural types and elements other than those of typical European composite moment frames, design rules have been developed. These rules, which are now under implementation within the 2001 EN version of Eurocode 8 are presented with comments and reference to background.
A series of six full-scale subassemblages were tested to investigate dogbone and haunch retrofits for pre-Northridge steel moment connections. Tests included matched pairs of specimens, one bare steel and one including composite slab. Data were collected to evaluate the influence and behavior of the concrete slab. Results that emphasize the influence of the composite slab on connection behavior and specific comments on the slab response are presented. The presence of a composite slab corresponded to higher-achieved overall plastic rotations and higher-peak attained moments. Existing estimates of composite beam capacity overestimated the specimen strengths in positive moment. Shear stud failures were observed, raising concerns about the capacity of shear studs under severe reversed cyclic loading.
This paper concerns the analysis and testing of 10 cantilever composite beams incorporating ribbed metal deck, representing the positive moment beam–column connections in an unbraced steel frame with composite floor beams. The positive moment beam–column connections arise from lateral forces on the unbraced frame. The effective widths of the slabs for strength and stiffness calculations have been determined from analysis. Agreement between the calculated strain distributions across the concrete slab width and the corresponding measured strain distributions was attained. Use of the calculated effective widths of the slab for strength together with a concrete strength of gave good agreement with the measured positive ultimate moment capacities of the cantilever composite beams subjected to upward end test loads.
This paper examines the influence of framing action and slab continuity on the hysteretic behavior of composite steel moment-resisting frames (MRFs) by means of high-fidelity continuum finite-element (CFE) analyses of two-bay subsystems and typical cruciform subassemblies. The CFE model, which is made publicly available, was thoroughly validated with available full-scale experiments and considers variations in the beam depth and the imposed loading history. The simulation results suggest that beams in subsystems may experience up to 25% less flexural strength degradation than those in typical subassemblies. This is because of local buckling straightening from the slab continuity and framing action evident in subsystems. For the same reason, beam axial shortening attributable to local buckling progression is up to five times lower in subsystems than in subassemblies, which is consistent with field observations. While the hysteretic behavior of interior panel zone joints is symmetric, exterior joint panel zones in subsystems experience large asymmetric shear distortions regardless of the employed lateral loading history. From a design standpoint, it is found that the probable maximum moment in deep and slender beams (d b ≥ 700 mm) may be up to 25% higher than that predicted by current design provisions with direct implications to capacity design of steel MRFs. The 25% reduction in the shear stud capacity as proposed by current seismic provisions is not imperative for MRFs comprising intermediate to shallow beams and/or featuring a high degree of composite action (η > 80%) as long as ductile shear connectors are employed.
In 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 lowprobability
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 deformationand
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.
A series of seismic tests were conducted on a 1½-bay by 1½-story special steel moment-resisting frame subassembly from the onset of damage through incipient collapse. These tests were conducted using hybrid simulation with substructuring as a mean to demonstrate efficient testing methods for system-level collapse assessment of large-scale structural subassemblies. The ½-scale specimen was designed to capture the behavior and interactions of beams, columns, panel zones, and composite floor slab. The experimental test setup permitted the application of lateral as well as varying vertical forces on the test specimen while maintaining realistic boundary conditions on the subassembly. With the overarching objective to advance knowledge on the collapse assessment of frame structures under earthquake loading, this paper focuses on the seismic performance of a steel moment-resisting frame through collapse. The failure mechanisms of the test frame are described and compared with numerical simulations based on state-of-the-art modeling approaches.
While advances in servo-hydraulic technology and computer simulation are making real-time pseudo-dynamic and shake table testing more feasible for research in earthquake engineering, quasi-static testing will remain the most prevalent form of testing for the foreseeable future. This paper describes the evolution of quasi-static testing, discusses by means of an example the main issues involved in developing a quasi-static testing program, and describes some of the advantages and limitations of this mode of testing.
This paper presents key parameters that affect numerical modeling of steel frame structures for reliable collapse simulations. The collapse assessment is based on experimental data obtained from a full-scale shaking table collapse test of a 4-story steel moment frame and a blind numerical analysis contest that was organized in parallel with the collapse test. It is shown that (1) there is no clear advantage between three-dimensional (3D) and 2D analyses in the prediction of a sidesway collapse mechanism for buildings with a regular plan view as in the case of study; (2) the assumption of Rayleigh damping leads to better predictions of structural response compared with stiffness proportional damping; and (3) accurate prediction of collapse necessitates that P-D effects always be considered in the analysis. It is also proven that accurate simulation of steel component deterioration is a key factor for reliable prediction of collapse behavior. On the basis of a synthesis of experimental and analytical studies, a few collapse mitigation alternatives are investigated. In particular, the effects of the strong-column/weak-beam ratio and exposed base plates on the collapse capacity are assessed. It is notable that a combination of bending strength increase and delay of local buckling in first-story columns is most effective for the enhancement of seismic performance against collapse.
Cited By (since 1996):7, Export Date: 21 January 2014, Source: Scopus, CODEN: JSEND, doi: 10.1061/(ASCE)0733-9445(2008)134:2(279), Language of Original Document: English, Correspondence Address: Ricles, J.M.; ATLSS Center, Dept. of Civil and Environmental Engineering, Lehigh Univ., 117 ATLSS Dr., Bethlehem, PA 18015, United States; email: jricles@lehigh.edu, References: (1999) Building Code Requirements for Structural Concrete, , American Concrete Institute (ACI).. . ACI 318-99 and Commentary. ACI 318R-99, Farmington Hills, Mich;
This paper describes an experimental study on radius cut reduced beam section (RBS) moment connections for use in seismic resistant steel moment frames. The effects of panel zone strength, composite behavior with a concrete slab, and the beam web-to-column flange connection were specifically addressed in these tests. In total, eight double-sided specimens were designed, fabricated, and tested in this study, providing data for sixteen individual RBS connections. Each specimen was subjected to a standard quasi-static cyclic load pattern. Overall, the specimens performed well with seven of the eight achieving total (elastic plus plastic) story drift ratios of at least 0.04 radians in magnitude before experiencing 20% strength degradation. The other test was stopped due to out-of-plane instability after being loaded to 0.03 radians of total story drift. Comparison of the response of specimens with strong panel zones, balanced panel zones, and weak panel zones relative to beam strength led to the conclusion that weak panel zones allow for the most stable hysteretic response at large drift levels. Inclusion of a composite slab in these tests appeared to stabilize the beams against lateral torsional buckling with no consistently detectable increase in the strains in the bottom beam flange. Welding the beam web to the column flange seemed to decrease the likelihood of weld fracture in these specimens.
The results of an experimental study of the seismic performance of improved, welded unreinforced beam-to-column moment connections are presented. The study involved the inelastic cyclic testing of I I full-scale connection specimens to evaluate the effects of weld access hole geometry, beam web attachment detail, panel zone strength, continuity plates, and composite slab on connection performance. With a high toughness weld metal and modified detailing, it is demonstrated that a welded unreinforced flange moment connection can reliably achieve an inelastic rotation of 0.03 rad or more prior to failure. The modified details include the use of a weld access hole with a modified geometry and a welded beam web. The test results indicate that a strong panel zone enhances inelastic connection performance. Based on the results of the study recommendations are given for the seismic-resistant design of improved welded unreinforced connections for steel moment-resisting frames.
A research program is summarized in which collapse of a steel frame structure is predicted numerically and the accuracy of prediction is validated experimentally through earthquake simulator tests of two 1:8 scale models of a 4-story code-compliant prototype moment-resisting frame. We demonstrate that (1) sidesway collapse can occur for realistic combinations of structural framing and earthquake ground motion; (2) P−Δeffects and component deterioration dominate behavior of the frame near collapse; (3) prediction of collapse is feasible using relatively simple analytical models provided that component deterioration is adequately represented in the analytical model; and (4) response of the framing system near collapse is sensitive to the history that every important component of the frames experiences, implying that symmetric cyclic loading histories that are routinely used to test components provide insufficient information for modeling deterioration near collapse. Copyright © 2010 John Wiley & Sons, Ltd.
Submitted to the Department of Civil and Environmental Engineering. Copyright by the author. Thesis (Ph. D.)--Stanford University, 2005.
Submitted to the Department of Civil and Environmental Engineering. Copyright by the author. Thesis (Ph. D.)--Stanford University, 2004.
This paper presents an experimental study on a steel moment frame with reinforced concrete (RC) floor slab that was subjected to horizontal cyclic loading leading to very large deformations. The primary objective of this test was to examine the interaction (composite action) between the steel beam and the RC floor slab. The steel beam moment capacity increased about 1.5 times in positive bending with the presence of the RC floor slab. During small beam rotations of 0.002-0.003 rad, the beam strength increased 1.2-1.4 times in negative loading, but composite action did not affect the negative beam moment capacity for larger rotations. The effective width estimated from the slab compressive force approximately equals the column width. Fracture at the bottom flange was notable due to the presence of the slab in the composite beam. Complete separation of the beam-to-column connections was not achieved even at a beam rotation of 0.13 rad, under which the composite connections still possessed bending resistance equal to about 10% of the maximum bending capacity when sustaining positive bending.
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