Panel Zone Deformation Research Articles

Seismic fragility analysis of steel intermediate moment frames with column-tree connections

Steel intermediate moment frames (SIMFs) with column-tree connections have an advantage in that they can ensure quality control and decrease construction cost and time by the factory welding of stub beams to columns and field bolting at the beam splice. Furthermore, the amount of steel can be reduced by distributing the seismic lateral load to the entire frame, for which beam-to-column joints are fully restrained. Despite such merits, concerns about slip at the beam splice and concentrated deformation in the weak panel zone also exist. Therefore, the influence of beam splice slip and weak panel zone deformation on the seismic performance of the SIMF with column-tree connections was examined numerically by seismic fragility assessment. For comparison, the seismic fragility of a SIMF with conventional field-welded connections was also examined. The result showed that the beams and panel zones share energy absorption in the SIMF with column-tree connections, even though the panel zones are weaker than the beams, because the beam splice slip can reduce the concentration of deformation on the weak panel zone. The collapse probability of a SIMF with column-tree connections is expected to be reduced to a value as low as a SIMF with conventional connections if the quality of test data and modeling are improved.

Parametric investigation on the design of RBS moment connections with jumbo beams and columns

Recent experiments demonstrated that reduced beam section moment connections with some of the largest hot-rolled beam and column shapes (jumbo shapes) may not have sufficient ductility for high seismic applications if designed according to current design procedures. A parametric computational study was performed to evaluate potential changes to the design procedures to enhance the ductility of these connections with jumbo shapes. First, three-dimensional finite element models were developed and calibrated to match cyclic experimental results including load vs displacement response, panel zone deformations, and deformed shape. The models were then used to conduct parametric studies with inelastic monotonic loading to examine the effect of design parameters including beam flange reinforcing fillet weld size, panel zone strength and doubler plate location on fracture potential. The results showed that the size of the reinforcing fillet weld on the underside of the bottom flange has a significant impact on the local stresses and strains at the location of fracture initiation in the tests and that larger size fillet welds should be used with jumbo beams to mitigate the concentration of inelastic strains. The results also showed that joints with higher panel zone strengths lead to lower fracture potential and that moving the column web doubler plate outwards to the edge of the flange is another effective strategy to reduce local strain concentrations. The absence of local buckling in jumbo sections eliminates the strength degradation that limits demands on the welds and needs to be taken into consideration in their design.

Seismic performance of low-rise intermediate steel moment frame buildings in South Korea

The seismic performance of low-rise intermediate steel moment frame buildings following design practices of South Korea was investigated and its benefit was presented in comparison with those of customary design practice in the United States. The major features in the design and construction practices in South Korea are that the entire frames of the building are designed to resist the lateral load and beam-to-column connections are fabricated in factories. In contrast, in the United States, only exterior frames resist the lateral load and the entire beam-to-column connections are welded at the construction site. To investigate the effects of distinctive design practices on seismic performance, the example buildings were designed and evaluated with the procedure in FEMA 355F. The effect of panel zone deformation in addition to story drift was considered in performance evaluation. The evaluation results showed that the seismic performance of the buildings designed in accordance with the practice in South Korea were not significantly different from those in the United States. Therefore, design and construction practices in South Korea are a competitive way for intermediate steel moment frame buildings.

FE parametric study of RWS/WUF-B moment connections with elliptically-based beam web openings under monotonic and cyclic loading

This paper provides numerical results investigating the behaviour of steel web-perforated beams with different shaped single openings located close to beam-to-column connections under monotonic and cyclic loading. In particular, the beams considered feature circular and patented elliptically-based perforations. Non-standard elliptically-based perforations have been proposed previously and are optimally designed to maximise resistance against Vierendeel moments and web-post buckling under static loads at the ultimate limit state. Comprehensive parametric nonlinear finite element analyses using the commercial FE package ANSYS were conducted. Initially, a FE model of the beam-to-column WUF-B moment connection was developed and calibrated against pertinent experimental results found in the literature. Next, parametric analyses were undertaken to assess the RWS/WUF-B connection regarding strength (moment), deformation (rotation) and column web shear panel zone deformation for different shapes of beam web perforations, hole sizes, and their locations. The study concludes that larger web openings are capable of moving the plastic hinge away from the column face and the CJP weld. Also, interstory drifts can be controlled with the wise use of the beam web opening size, shape, and distance from the face of the column, as suggested in the paper. Following, a step-by-step design process for RWS/WUF-B connection is presented.

Panel zone deformation demands in steel moment resisting frames

Energy is dissipated by yielding at beam ends in steel moment resisting frames subjected to earthquakes. In addition, the column panel zone can be proportioned to participate in energy dissipation. AISC360, Eurocode 3, and FEMA-355D specifications have different philosophies for the design of panel zones which lead to different amounts of deformation demands. This paper presents a numerical study undertaken to quantify the deformation demands in panel zones proportioned according to different specifications. Pursuant to this goal, the panel zone design requirements of three specifications were compared first to identify differences and similarities. A parametric study was conducted by considering the beam depth, axial load level on columns, thickness of panel zone, and seismic hazard as the prime variables. Archetypes were analyzed by employing three-dimensional explicit nonlinear finite element time history analyses. The analysis results revealed that designs according to AISC360 and Eurocode 3 resulted in significant amounts of yielding of the panel zone while the designs according to FEMA 355D resulted in minimal amount of yielding. The panel zone deformation demands were quantified and an equation used to estimate the deformation levels was developed herein.

Panel Zone Deformation Capacity as Affected by Weld Fracture at Column Kinking Location

Three full-scale specimens were tested to evaluate the cyclic performance of rehabilitated pre-Northridge steel beam-to-column moment connections. A Kaiser bolted bracket (KBB) was used on the beam bottom flange for all specimens, but different rehabilitation schemes (another KBB, a notch-tough beam flange replacement weld, or a double-tee welded bracket) were used to strengthen the top flange. All specimens were able to sustain an interstory drift angle of 0.04 radian, with large inelastic deformations in the panel zone. Two specimens experienced fracture at the replacement complete-joint-penetration (CJP) welds, mainly due to the large shear deformation in the panel zone. Because it may not be economically feasible to mitigate weak panel zones in seismic rehabilitation, an analytical model was developed to predict the panel zone deformation capacity and the associated strength. In this model, it was postulated that the ultimate panel zone deformation capacity corresponded to that when each column flange was fully yielded and excessive kinking would cause fracture of the beam flange CJP welds. This postulation was verified by the test data of two specimens that experienced weld fracture due to excessive panel zone deformation. It was shown that the deformation capacity is a function of db/tcf (beam depth-to-column flange thickness ratio). The effect of column axial load was also studied.

Simplified Method for Estimation of Beam Plastic Rotation Demand in Special Moment-Resisting Steel-Frame Structures

It is necessary to predict the plastic deformation demand of members and the story drift ratio for seismic design. In the present study, a simple method of estimating the plastic rotation of beams in special steel moment-resisting frame structures designed by the strong column-weak beam philosophy was developed. The plastic rotation of beams was calculated as the function of the story drift ratio based on the result of elastic analysis. The effects of the design parameters, such as moment redistribution, joint dimension, plastic mechanism, panel zone deformation, and the rocking effect of lateral bracing were addressed. For verification, the proposed method was applied to 6-story special moment frames with and/or without lateral bracing. The plastic rotations of beams in the same floor significantly varied according to the design parameters, although the story drift ratio was the same. The plastic rotations of beams predicted by the proposed method correlated well with those from nonlinear static analysis.

Seismic analysis of CFST frames considering the effect of the floor slab

This paper describes the refined 3-D finite element (FE) modeling of composite frames composed of concrete-filled steel tubular (CFST) columns and steel-concrete composite beams based on the test to get a better understanding of the seismic behavior of the steel-concrete composite frames. A number of material nonlinearities and contact nonlinearities, as well as geometry nonlinearities, were taken into account. The elastoplastic behavior, as well as fracture and post-fracture behavior, of the FE models were in good agreement with those of the specimens. Besides, the beam and panel zone deformation of the analysis models fitted well with the corresponding deformation of the specimens. Parametric studies were conducted based on the refined finite elememt (FE) model. The analyzed parameters include slab width, slab thickness, shear connection degree and axial force ratio. The influences of these parameters, together with the presence of transverse beam, on the seismic behavior of the composite frame were studied. And some advices for the corresponding seismic design provisions of composite structures were proposed.

Evaluation of seismic behaviour of steel special moment frame buildings with vertical irregularities

AbstractThis paper concentrates on investigating the seismic behaviours of vertically irregular steel special moment frame (SMF) buildings by comparison with the regular counterpart. All buildings in this study were assumed to be located in Los Angeles, and subjected to 20 earthquake ground motions with a seismic hazard level of 2% probability of exceedance in 50 years. The beam–column connections of the buildings were modelled to consider the panel zone deformation. In addition, a ductile connection model accompanied by strength degradation was incorporated to the analysis programme in an effort to obtain more accurate response results. Three types of irregularities (mass, stiffness and strength irregularity) specified as vertical irregularities in the IBC 2000 provisions were imposed to the original building. Nonlinear static and dynamic analyses were performed, and the confidence levels of which the performance objective will be satisfied were calculated as well. The effects of different irregularity types and levels on the seismic behaviours of the buildings were investigated and discussed in terms of the height‐wise distribution of storey drifts, maximum storey drift demands, global collapse storey drift capacities and confidence levels. Copyright © 2010 John Wiley & Sons, Ltd.

Finite Element Analysis on Load-Bearing Behavior of Concrete Filled Steel Tubular Composite Frames

This paper presents a nonlinear finite element (FE) analysis on the mechanic behavior of concrete filled steel tubular (CFST) composite frames. The main purpose of the FE analysis was to investigate the seismic behavior of composite frames. Three kinds of nonlinearity, namely material nonlinearity, contact nonlinearity and geometry nonlinearity, were taken into account in the FE model using MSC.Marc. The element type, connection between element, material constitutive law and boundary condition was described in detail. The elasto-plastic behavior, as well as fracture and post-fracture behavior, of the FE analysis models fitted well with those of the test specimens. The beam and panel zone deformation of the analysis models is also in good agreement with that of the test specimen. It is concluded that FE model of CFST composite frame is reliable and could be regarded as a helpful tool to expand the information on seismic behavior of CFST composite frame.

Nonlinear Analysis of CFT Composite Frame with Floor Slab

Nonlinear finite element (FE) analysis models of CFT composite frames with floor slab were established by Msc.Marc to investigate the seismic behavior of composite frames. A number of material nonlinearities and contact nonlinearities, as well as geometry nonlinearities, were taken into account, including elastoplastic properties of steel and concrete, concrete cracking and tension stiffening, steel fracture, interface slip between concrete slabs and steel beams, P-D effects etc. The elasto-plastic behavior, as well as fracture and post-fracture behavior, of the FE analysis models agreed well with those of the test specimens. The beam and panel zone deformation of the analysis models is also in good agreement with that of the test specimen. It is concluded that FE analysis is useful not only for monotonic load analysis but also for cyclic load analysis. It is a helpful tool to expand the information on seismic behavior of composite frame.

Analytical studies of a full-scale steel building shaken to collapse

A full-scale four-story welded steel moment-frame building was shaken to collapse on the E-Defense shaking table in Japan in September 2007. This building was used as a case study to investigate modeling techniques for nonlinear structural and collapse simulations. This paper presents various kinds of numerical models for beams, columns, and panel zones. The analytical studies suggest that the fiber-beam frame model considering composite beam effects simulates the responses of the four-story building specimen better than the hinge-beam frame model. Considering panel zone deformations in the numerical model can improve the accuracy of computing the displacement-related responses. Moreover, considering the effects of the composite beam action on the panel zone stiffness and strength can further enhance the accuracy of the response simulations. The column model incorporating fiber-beam–column element using cyclic buckling fibers proposed in this study can satisfactorily simulate the degrading force versus deformation relationships of the three-dimensional shell column model due to the local buckling. A frame model incorporating the same type of fibered column element could predict the collapse time of the test building. Based on these nonlinear response analyses, it is illustrated that the responses of the building can be estimated satisfactorily by using the modeling techniques presented in this study.

Progressive Collapse-Resisting Capacity of Steel Moment Frames considering Panel Zone Deformation

Panel zone deformation is known to be a major parameter affecting the nonlinear behaviour of moment resisting steel frames subjected to earthquake loads, especially in weak column-strong beam conditions. In this study, the reinforcing effect of the panel zone on the progressive collapse and the capacity of moment resisting steel frames was investigated with the aid of nonlinear dynamic analysis. Both gravity load-resisting system structures with weak panel zones and lateral load-resisting system structures with strong panel zones were studied. The analysis results show that the panel zone deformation is highly dependent on the location of removed columns. The panel zone deformation turns out to be very significant when a column adjacent to an exterior panel zone is removed. On the other hand, the panel zone effect is not significant if a column adjacent to an interior panel zone is removed. As the number of stories increases, the effect of a weak panel zone on the overall behaviour decreases regardless of the panel zone strength and the location of any column removed.

Computation of story drifts considering panel zone deformations for multistory steel moment frames with welded flange plate connections

AbstractThe 2003 International Building Code (IBC) requires that the contribution of panel zone deformations to overall story drifts be included in the mathematical model of the physical steel moment‐resisting frame systems. A finite‐element model can accurately take the panel zone deformations into consideration for the computation of story drifts, yet the preparation of the model is usually very time consuming. A line‐element model significantly simplifies the frame analysis, yet it cannot address the issue of panel zone deformations. These two types of models are statistically evaluated in this paper for steel moment frames with welded flange plate (WFP) connections. A total of 16 beam–column combinations are evaluated for each type of model. The elastic story drift of each beam–column combination is then obtained from each type of model. Finally, adjustment factors are developed to correct the story drifts that are computed using the line‐element models for the frames with WFP connections. Copyright © 2007 John Wiley & Sons, Ltd.

Seismic analysis of inelastic moment‐resisting frames Part II: Energy dissipation in deformable panel zones

AbstractThe simple analytical method developed in the companion paper is applied to study the dynamic behavior and energy dissipation of inelastic structures with panel zone deformation using rotational springs. Both shear deformation and inelastic behaviors of the panel zone are modeled using rotational springs to capture the actual vibration characteristics of the overall structural responses. The modified force analogy method with static condensation proposed in the companion paper is employed to study the effects of panel zone deformation on the dynamic behavior of inelastic structures, while rigid end offsets are also included to give a more accurate structural model. Based on this proposed analytical method, numerical simulation is first performed to study the responses of a one‐story one‐bay moment‐resisting frame. Results show that rigidity of panel zones plays a major role in the overall structural responses during earthquake excitations. Numerical simulations are then performed on a multi‐degree of freedom structure to demonstrate the use of the proposed method. Comparison of results between modeling structures with various panel zone rigidities and yield moments in the rotational springs shows that significant differences in both structural responses and energy dissipations exist between the models, and therefore the effects of panel zone deformation should be considered in dynamic analysis. Copyright © 2007 John Wiley & Sons, Ltd.

Seismic analysis of inelastic moment‐resisting frames Part I: Modified force analogy method for end offsets

AbstractAn analytical method of performing inelastic structural dynamic analysis based on the force analogy method to study real moment‐resisting frames with plastic hinge offsets from member ends and rigid panel zones is proposed. New stiffness matrices used in the force analogy method for members with two ends being rigid are theoretically derived using the unit displacement method. Through static condensation, each term in the stiffness matrices is found to be modified simply through multiplying by a scale factor. This is an enhancement to the original force analogy method or other conventional methods, where the condensed form reduces the size of the structural problem by eliminating the unnecessary additional degrees of freedom that must be placed at the plastic hinges. But more importantly, this is a significant improvement on other conventional methods where the present inelastic analysis procedure is more of a theoretical nature rather than numerical. Numerical simulation for seismic analysis is performed on a single degree of freedom system, and the results show that plastic hinge end offsets have significant effects on the seismic response and therefore they should be carefully considered in dynamic analysis. The companion paper further discusses the influence of panel zone deformation on the dynamic response based on the method proposed in this part of the research. Copyright © 2007 John Wiley & Sons, Ltd.

Evaluation of FEMA-350 Seismic Provisions for Steel Panel Zones

Steel panel zone design provisions have undergone large changes in the past four decades. The recently introduced FEMA-350 (2000) guidelines are substantially different from previous provisions, and are no longer a function of the plastic strengths of the panel zone and adjoining beams. Rather, they are based on the premise that the framing beams and the panel zone both yield simultaneously to promote controlled inelastic participation of both components. This paper evaluates and discusses the new provisions based on an examination of published test data and the results of transient analyses of buildings with four-, eight-, and sixteen-story moment resisting steel frames. The limited test results reviewed do not confirm the adequacy of the new panel zone provisions and furthermore suggest that panel zone deformation demands could be affected by connection detailing. The frame analysis results show that the FEMA-350 provisions lead to a rather low level of panel zone participation. A discussion of design implications and research needs is provided.

Ductile details for welded unreinforced moment connections subject to inelastic cyclic loading

The results of a 3-D finite element study of welded unreinforced flange beam-to-column moment connections in steel special moment resisting frames are presented. Computer models of connection subassemblies were developed using the general-purpose nonlinear finite element program abaqus. Several issues were addressed in the study: (1) geometry and size of the weld access hole; (2) benefit of a welded beam web; (3) control of inelastic panel zone deformations; and (4) effectiveness of continuity plates in reducing local demand on the connection. The analytical results provided information related to basic performance and the effects that these connection parameters have on inelastic cyclic performance, thereby furthering the current understanding of welded moment connection behavior under seismic loading conditions and leading to improved design criteria. Based on the results of the analytical study, recommendations for the seismic design of connections are given. The incorporation of the recommendations into an experimental test program showed good connection ductility in the test specimens.

Relating the Seismic Drift Demands of SMRFs to Element Deformation Demands

A procedure is outlined and evaluated for estimation of beam and panel zone deformation demands for given estimates of story drift demands for regular steel moment resisting frame (SMRF) structures. The total story drift demand is related to the story plastic drift demand by estimating the story yield drift based on the weakest element at the connection. The story plastic drift demand is related to the panel zone and beam plastic deformation demands by a function based on story geometry and member properties. The procedure is verified for a series of code compliant SMRFs. It complements a process presented in the literature that permits the estimation of seismic drift demand for frame structures from the spectral displacement demand at the first mode period of the structure. The combined process should prove useful in conceptual design, in estimating deformation demands for performance assessment, and in improving basic understanding of seismic behavior of steel frame structures.

Seismic Performance Evaluation of Pre-Northridge Steel Frame Buildings with Brittle Connections

As part of the (FEMA/SAC) Phase 2 program, this study focused on performance prediction and evaluation of existing steel SMRF buildings assumed to be built prior to the Northridge Earthquake. Several classes of buildings, which are representative of typical buildings based on year of construction and brittle pre-Northridge connections, were designed in accordance with the 1973, 1985, and 1994 UBC provisions. Then, the frame analysis models were developed including the effects of brittle connections, panel zone deformation, and interior gravity frames. Based on the drift demands and capacities calculated using each set of 20 SAC ground motions representing 2/50 and 50/50 hazard levels, a performance prediction and evaluation procedure based on the reliability framework is presented. Confidence levels that existing buildings will exceed the predefined performance level for different hazard levels are calculated. The pre-Northridge design and construction represented by the old building codes and brittle connections force the buildings to experience large seismic demand and result in a low confidence level in achieving the desired performance levels.