Journal of Structural Engineering

Prediction of the behavior of partially encased composite (PEC) columns by finite-element modeling presents a challenging problem due to local buckling of the thin steel flange plates and rapid volumetric expansion of the concrete near the ultimate load. The use of a dynamic explicit formulation and a concrete damage plasticity model has permitted good predictions of the capacities of both uniaxially and eccentrically loaded PEC column tests reported in the literature. The model provides good representations of the axial deformation at the peak load, the postpeak behavior, and the failure mode observed in those tests. The finite-element model is also capable of predicting the effect of different link spacings on the behavior of the PEC columns as well as determining the individual contributions of the steel and concrete to the total load carrying capacity of these columns. In addition to a description of the methodology, a discussion of the results of finite-element studies of PEC columns from three experimental programs encompassing a wide variety of geometries and loading conditions is presented in this paper. A study is also presented on the effects of local imperfections on the capacity of these columns.

The strength of a wide-flange section column-to-beam connection with split-tee stubs is dominated by out-of-plane deformation of flange plate for the wide-flange section column, especially when transverse stiffeners in the panel zone are omitted to simplify fabrication processes of a beam-to-column connection. The main purpose of the present study is to analytically derive general yield and maximum strengths of the bolted beam-to-split-tee stub connection by yield line analysis, including the case where punching-shear deformation of column flange plate or tensile deformation of connection bolts is considered. The validity of the estimated general yield and maximum strengths of the connection is investigated by comparing with the results of previous experimental studies and it is then shown that the analytical strengths give good estimates of the connection strength. The reference load-deformation relationship curve and design yield deformation are also proposed according to the experimental results; the proposed design yield deformation is appropriate for the case where flexural deformation of plate elements in the connection is critical.

This paper presents theoretical, experimental, and finite-element analysis results for the linear elastic behavior of corrugated web steel I-girders under in-plane loads. A typical corrugated web steel I-girder consists of two steel flanges welded to a corrugated steel web. Previous research has shown that a corrugated web I-girder under primary moment and shear cannot be analyzed using conventional beam theory alone, and a flange transverse bending analysis is required. A theoretical method, the fictitious load method, is presented herein as an analytical tool for quantifying flange transverse bending in corrugated web I-girders. To validate this method, four-point bending experimental results for a large-scale corrugated web I-girder are presented. The measured flange transverse displacements and flange stresses were in good agreement with the theoretical results especially in regions of constant shear. To gain additional insight, finite- element analysis results for the test girder are presented, and compared to both the experimental and theoretical results.

Existing design specifications used in the United States and abroad do not address the limit state of bearing failure that may govern in the design of tubular truss chords at the supports. The lack of treatment of this limit state seems to represent an unsatisfactory situation given the fact that very large concentrated reaction forces are often applied transversely to the ends of chord members with slender circular cross sections; long-span overhead highway sign trusses are but one example of an affected application. The bearing failure limit state is manifest as a coupled failure mechanism, consisting of a plastic collapse of the chord end and local plastification of the chord wall. In order to better understand this limit state, two full-scale experimental tests are conducted at the University of Pittsburgh as part of the current research. The specimens considered are proportioned to be representative of the bearing region of an overhead sign structure that failed in the limit state under investigation. These tests yield important information regarding the manifestation of the bearing limit state and also afford an opportunity to verify finite element modeling techniques for use in a parametric study. The parametric study reveals that the bearing capacity is influenced by many factors including the geometry of the bearing connection, the adjacent intermediate truss member, the nature of loading, and the material properties. Using regression analysis on the parametric study results and concepts from the yield line method for analyzing collapse mechanisms, general equations for predicting the bearing capacity are developed.

A series of tests on cold-formed high-strength stainless steel square and rectangular hollow sections subjected to web crippling is presented in this paper. The types of stainless steel investigated in this study were high strength austenitic and duplex material. The measured web slenderness value of the hollow sections ranged from 16.5 to 49.7. The tests were carried out under four loading conditions considered in the American Specification and Australian/New Zealand Standard for cold-formed stainless steel structures, namely end-one-flange, interior-one-flange, end-two-flange, and interior-two-flange loading conditions. The web crippling test strengths were compared with the design strengths obtained using the American, Australian/New Zealand, and European Specifications for stainless steel structures. In addition, the North American Specification for cold-formed carbon steel structural members was also used to predict the web crippling strengths and compared with the test results. It is shown that the design strengths predicted by the specifications are either unconservative or very conservative. Hence, a unified web crippling equation with new coefficients for cold-formed high strength stainless steel square and rectangular hollow sections is proposed in this paper. It is shown that the proposed web crippling equation is safe and reliable.

An experimental study was performed to investigate the cyclic behavior of framed steel walls with thin infill plates. Five specimens with a single bay and three stories were tested. Test parameters for the specimens were the plate thickness and the strength and compactness of the column. The test results showed that unlike conventional reinforced concrete walls and braced frames, well-designed steel plate walls exhibited large ductility and energy dissipation capacity as well as high strength. The steel plate walls with relatively thin plates showed shear-dominated behavior by the moment–frame action. On the other hand, steel plate walls with thick plates showed flexure-dominated behavior by the cantilever action. The shear-dominated walls showed better ductility. To achieve large ductility, the boundary columns must resist the combined axial force and transverse force developed by the tension–field action of the infill plates. Also, the columns must have compact sections to prevent their early local buckling.

A posttensioned (PT) steel moment resisting frame is a self-centering earthquake resistant steel frame that uses posttensioning steel to compress the beam flanges against the column flanges at the connections. The posttensioning contributes to the moment capacity of the connections and provides an elastic restoring force that returns the frame to its pre-earthquake position. This paper describes the behavior and design of PT frames and PT frame systems, where a PT frame system is a PT frame with the collector elements that connect it to the floor system. The interaction of the floor system with the PT frame produces axial forces in the beams that add to those from the posttensioning. This paper outlines a performance-based seismic design approach for PT steel frame systems. Seismic performance levels, seismic input levels, structural limit states and capacities, and structural demands for PT frame systems are defined. The design objectives are outlined, design criteria are given, and a step-by-step design procedure is given. The design approach is evaluated via comparisons with time-history analysis results.

A smeared cracking approach is developed in this paper to simulate the crack propagation in plain concrete caused by coupled heat transfer and moisture transport processes. The cracking state of each concrete element is described using an original concept—local relative crack density (LRCD). Furthermore, a numerical relationship between LRCD and the moisture transport property is proposed to consider the accelerating influence of crack propagation on moisture diffusion. Finally, a global relative crack density is suggested to denote the cracking state of the entire concrete structure, which may serve as an appropriate index to evaluate the overall deterioration level of the structure. A finite element-based computational methodology is developed to simulate the coupled thermohydrochemomechanical deterioration processes, and is expanded further to implement the simulation of crack propagation. The proposed method establishes a reasonable analytical basis for quantitative long-term durability assessment of concrete structures.

Presented is a method which can predict accurately the crack width in reinforced concrete beams under repeated loading. It is known that the crack widths increase with an increase of the number of load cycles. To evaluate the crack widths realistically, the bond stress-slip relation and the variation of transfer length under repeated loading have been modeled and incorporated in the analysis model for crack width. The present study indicates that the transfer length decreases according to the increase of load cycles, which is due to the increase of residual slip. The calculated crack widths from proposed method agree very well with test data available in the literature, while the existing method shows large deviation from the test data. The proposed method also allows the calculation of the residual crack width after repeated loading. The present study provides a realistic base for the prediction of crack width, which is important for the serviceability of reinforced concrete structures subjected to repeated loads.

The behavior of fiber-reinforced cement composite (FRCC) flexural members under large displacement reversals was experimentally evaluated. Emphasis was placed on estimating the displacement capacity and shear strength of members constructed with strain-hardening FRCC materials. Two types of fibers were used: Ultrahigh molecular weight polyethylene fibers and steel hooked fibers in volume fractions ranging between 1.0 and 2.0%. The primary experimental variables were: (1) fiber type and volume fraction; (2) type of cement-based matrix (concrete or mortar); (3) average shear stress demand at flexural yielding; and (4) shear resistance provided through hoops versus total shear demand. All specimens constructed with a strain-hardening FRCC, with or without web reinforcement, exhibited drift capacities of at least 4.0%. A shear stress level of $0.30\surd f_c^{\prime }\left[\mathrm{MPa}\right]$ represented a lower bound for which no shear failure occurred in the strain-hardening FRCC test specimens, regardless of the member inelastic rotation demand. In addition, buckling of longitudinal reinforcement in the strain-hardening FRCC members without web reinforcement was not observed up to plastic hinge rotations of 4.0%.

This paper presents the method and analysis of a study whose objective was to numerically examine the effect that nonstructural finishes and nonstructural partition walls would have on the performance-based seismic design of a wood frame structure at the system level. The incremental dynamic analysis (IDA) approach was modified slightly, termed incremental mass analysis (IMA), and used as an investigative tool in order to determine what seismic mass numerically produces a prescribed performance level for a wood frame structure or subassembly. An illustrative example for a single wood shearwall using the IMA method that utilized an existing suite of 20 earthquakes at three different seismic hazard levels produced 60 curves. These curves for weight per unit length versus resulting transient drift from nonlinear dynamic time history analysis were analyzed using basic statistics to produce 16th, 50th, and 84th percentile IMA curves for the wood shearwalls. A one-story wood frame house was designed based on standard wood frame design concepts including windows and doors. Then, using the IMA approach presented for the wood shearwall, but with a single earthquake record, the weight at roof level versus transient drift curve was developed for three different seismic hazard levels. In order to investigate the effect of nonstructural elements numerically, analyses were performed for four versions of the wood frame structure: (1) the exterior wood shearwalls with only oriented strand board (OSB) and no finishes; (2) the exterior wood shearwalls with OSB and gypsum wall board (GWB); (3) the exterior wood shearwalls with OSB and GWB and a nonstructural partition wall; and (4) the finished structure with stucco included. Based on the analysis presented in this study, it is concluded that the allowable seismic mass can be significantly increased when non-structural elements are included in the design, in some cases more than 70 percent.

The combined approximations approach, developed originally for linear static reanalysis, is used for dynamic reanalysis of structures. The approach is based on the integration of several concepts and methods, including series expansion, reduced basis, matrix factorization, and Gram-Schmidt orthogonalizations. The advantage is that efficient local approximations and accurate global approximations are combined to achieve an effective solution procedure. Considering modal analysis, the computational effort involved in reanalysis is significantly reduced. However, the accuracy of the higher mode shapes might be insufficient. A procedure intended to improve the accuracy of the results is developed. A reduced eigenproblem is introduced, and a method for determining the basis vectors is presented. Improved basis vectors, using the concepts of shifts and Gram-Schmidt orthogonalizations, are developed, and eigenproblem reanalysis by combined approximations using inverse iteration with shifts is introduced. Numerical examples demonstrate the quality of the results. It is shown that high accuracy is achieved efficiently for various types of changes in the structure.

A frequency response function-based damage identification method is presented that accurately identifies both the location and severity of damage in structural systems using a limited amount of measurement information. Damage is identified by minimizing the error between measured and analytically computed frequency response functions obtained through finite element model updating. The impact that the type of genetic algorithm representation has on performance is evaluated for a fixed representation and an implicit redundant representation, which simplifies the search by exploiting the unstructured nature of damage identification. The performance of the proposed damage identification method is evaluated for beam and frame structures that consider different damage scenarios and measurement layouts. The impact of measurement noise on performance is also investigated. The damage identification method developed using the implicit redundant genetic algorithm provides greater accuracy in identifying the location and severity of damage in all case studies even in the presence of noise. For larger frame structures, the implicit redundant genetic algorithm performed well, while no valid results were obtained using the fixed genetic algorithm representation.

For performance evaluation of structural systems under seismic loads, accurate modeling of structural component and system response behavior in the nonlinear range is required. In this study, the differential-equation-based hysteresis (Bouc-Wen) model is incorporated into ABAQUS to describe the inelastic connection behaviors of moment frames, taking into account capacity uncertainties of connections documented in the FEMA/SAC projects. A three-dimensional (3D) finite-element model is developed to examine the effects of bi-axial motions including those due to torsion oscillation. Effects of gravity frames, panel zones, and inelastic column deformation are also considered. Numerical examples are carried out to evaluate displacement demand. System capacity against incipient collapse is determined by the incremental dynamic analysis (IDA). Both inherent variability and modeling error are considered. Results indicate that torsional effects due to asymmetric member failures are important. The conventional lumped-plasticity model limits the development of plasticity of columns, thus is not appropriate in IDA for system capacity analysis. As expected, fracture failures of the pre-Northridge connections have a severe impact on the building performance. The methods of modeling and response analysis developed will be used in the reliability and redundancy study in the companion paper.

In NEHRP 2003, a new mechanism-based reliability/redundancy factor, $\rho$ , is proposed to replace the current $\rho$ factor in building codes. A multiplier of $\rho =1.3$ for design lateral force is recommended for structures deemed being lack of redundancy. The effects of detailed structural configurations, nonlinear structural behaviors, and uncertainty in demand and capacity, however, have not been considered in this factor. To examine the adequacy of this new factor, a systematic investigation on the above effects is carried out. It focuses on buildings with $5\times 5$ bays and nine different configurations of special moment resisting frames. The nonlinear structural response behaviors and uncertainty in demand and capacity based on a three-dimensional (3D) incremental dynamic analysis are properly considered. Structural reliability and redundancy are determined. The biaxial spectral acceleration is found to be a more effective predictor of system demand than drift and therefore used as the response measure in the analysis of reliability and redundancy. A uniform-risk redundancy factor, $R_R$ , for design lateral force is proposed to provide equal reliability for buildings of different degrees of redundancy. This $R_R$ factor is compared with the $\rho$ factor and areas for improvement of the latter are pointed out.