Journal of Structural Engineering

In this paper, the mean-centered first-order second-moment (FOSM) method is employed to perform probabilistic push-over analysis (POA) of structural and/or soil-structure systems. Approximations of first and second statistical moments (FSSMs) of engineering demand parameters (EDPs) of structural and/or geotechnical systems with random material parameters are computed based on finite-element (FE) response and response sensitivity analysis (RSA) results. The FE RSA is performed accurately and efficiently by using the direct differentiation method (DDM) and is employed to evaluate the relative importance (RI) of the various modeling material parameters in influencing the variability of the EDPs. The proposed approximate methodology is illustrated through probabilistic POA results for nonlinear inelastic FE models of: (1) a three-story reinforced-concrete (RC) frame building and (2) a soil-foundation-structure interaction system consisting of a RC frame structure founded on layered soil. FSSMs of EDPs computed through the FOSM method are compared with the corresponding accurate estimates obtained via Monte Carlo simulation. Results obtained from “exact” (or “local”) and “averaged” (or “global”) response sensitivities are also compared. The RI of the material parameters describing the systems is studied in both the deterministic and probabilistic sense, and presented in the form of tornado diagrams. Effects of statistical correlation between material parameters are also considered and analyzed by the FOSM method. A simple approximation of the probability density function and cumulative distribution function of EDPs due to a single random parameter at a time (while all the other parameters are fixed to their mean values) is also proposed. Conclusions are drawn on both the appropriateness of using local RSA for simplified probabilistic POA and on the application limits of the FOSM method. It is observed that the FOSM method combined with the DDM provides accurate estimates of FSSMs of EDPs for low-to-moderate level of inelastic structural or system behavior and useful qualitative information on the RI ranking of material parameters on the structural or system response for high level of inelastic behavior.

Procedure for large displacement and stability analysis of elastic plane frames subjected to temperature changes is presented. The method of analysis is based on an Eulerian (corotational) formulation, which was developed initially for static loads, and is extended herein to include thermal effects. Local element force-deformation relationships are derived using the beam-column theory taking into consideration the effect of curvature due to temperature gradient across the element cross section. The changes in element chord lengths due to thermal axial strain, and bowing due to temperature gradient, are taken into account. This “beam-column” approach, using stability and bowing functions, requires significantly fewer elements per member (i.e., a beam or a column) for the analysis of a framed structure than the “finite-element” approach. A computational technique, using Newton-Raphson iteration, is developed to determine the nonlinear responses of structures. Numerical solutions are presented for a number of benchmark structures to demonstrate the feasibility of the proposed method of analysis.

An efficient beam element, the modified elastofiber (MEF) element, has been developed to capture the overall features of the elastic and inelastic responses of slender columns and braces under axial cyclic loading without unduly heavy discretization. It consists of three fiber segments, two at the member ends and one at midspan, with two elastic segments sandwiched in between. The segments are demarcated by two exterior nodes and four interior nodes. The fiber segments are divided into 20 fibers in the cross section that run the length of the segment. The fibers exhibit nonlinear axial stress-strain behavior akin to that observed in a standard tension test of a rod in the laboratory, with a linear elastic portion, a yield plateau, and a strain-hardening portion consisting of a segment of an ellipse. All the control points on the stress-strain law are user defined. The elastic buckling of a member is tracked by updating both exterior and interior nodal coordinates at each iteration of a time step and checking force equilibrium in the updated configuration. Inelastic postbuckling response is captured by fiber yielding, fracturing, and/or rupturing in the nonlinear segments. The key features of the element include the ability to model each member using a single element, easy incorporation of geometric imperfection, partial fixity support conditions, member susceptibility to fracture defined in a probabilistic manner, and fiber rupture leading to complete severing of the member. The element is calibrated to accurately predict the Euler critical buckling load of box and I sections with a wide range of slenderness ratios ( $L/r=40$ , 80, 120, 160, and 200) and support conditions (pinned-pinned, pinned-fixed, and fixed-fixed). Elastic postbuckling of the Koiter-Roorda L frame (tubes and I sections) with various member slenderness ratios ( $L/r=40$ , 80, 120, 160, and 200) is simulated and shown to compare well against second-order analytical approximations to the solution even when using a single-MEF element to model each leg of the frame. The inelastic behavior of struts under cyclic loading observed in the experiments of Black et al., Fell et al., and Tremblay et al. is accurately captured by single-MEF-element models. A FRAME3D model (using MEF elements for braces) of a full-scale six-story braced frame structure that was pseudodynamically tested at the Building Research Institute of Japan subjected to the 1978 Miyagi-Ken-Oki earthquake record is analyzed and shown to closely mimic the experimentally observed behavior.

Stress-strain relationship for a steel bar embedded in concrete differs somewhat to that of a bare steel bar because the surrounding concrete is bonded to the bar. This behavior is known as tension stiffening. This paper presents the results of an analytical and experimental study on the performance of reinforced concrete beams subjected to pure torsion. In particular, the effect of the tension stiffening was discussed and included in the analytical study. Nine RC beams having different torsional reinforcements were tested. Although the torsional strength of RC beams according to the existing design codes (ACI 318-05, EC2, and JSCE-02) depends on neither the average yield stress of steel bars nor the tension stiffening effect, the test results indicated that the steel stress of the beams at peak load increased as the total percentage of reinforcement decreased due to the tension stiffening effect. A new equation including tension stiffening effect was proposed to predict the torsional moment capacities of RC beams. Comparisons between tested and calculated torsional moments of the seventy-one beams showed reasonable agreement.

Extensive experimental research aimed at defining the in-plane cyclic behavior of three types of load-bearing masonry walls, assembled with perforated clay units, and various types of head and bed joints was carried out. Experimental behavior was modeled with four types of nonlinear finite-element models. Both macromodeling and micromodeling strategies, implementing either isotropic or orthotropic material laws, were adopted. Two simplified criteria were proposed for calibrating the models, one for defining orthotropic properties starting from perforated unit geometry and the other for defining expanded unit and interface element properties in micromodels. The procedures adopted for model calibration established the reliability of various modeling strategies. Results allow some conclusions to be drawn about the reliability of diagonal compression tests for large unit masonry, the stress distribution and different behaviors of masonry made with different head and bed joints, and the influence of unit strength on the in-plane behavior of masonry.

While the database of tests on shear in reinforced concrete members without stirrups is extensive, the pool of test data for fiber-reinforced specimens is limited. Fewer still are tests undertaken on high-performance fiber-reinforced concrete members with the fiber concrete designed to carry the full shear capacity. This paper reports the results of a testing program on ultrahigh-performance steel fiber-reinforced concrete beams. Eight prestressed concrete beams were tested in shear with the test variables being the shear span-to-depth ratio and the quantity and type of steel fibers. The results of the tests, together with additional tests reported in the literature, are compared to the values derived from the plastic shear variable engagement predictive model for the determination of shear strength of steel fiber-reinforced concrete beams. A good correlation is observed with a mean model to experimental strength ratio of 0.92 and coefficient of variation of 0.12.

Most prior experimental research on the composite behavior of concrete-filled tubes (CFTs) in flexure has been performed using minimodels. The conclusions that have been reached based on the prior research stipulate that the interaction between the concrete and the steel is maximized by steel tube walls confining the concrete, rendering the addition of shear connectors unnecessary. The goal of the current study is to examine the composite flexural behavior of full-scale CFTs, which could be used in actual construction. Four, full-scale, 20-ft- (6-m)-long simple-supported CFT beams were tested under four-point loads. Two were rectangular with a width of 12 in. (305 mm) and the other two beams were circular with a diameter of 18 in. (457 mm). Load, deflection, steel strain, and concrete movement were digitally recorded, and the concrete cracks in each section were mapped and photographed. Test results showed that composite action was improved by shear connectors particularly for circular CFT beams and that the AISC nominal moment capacity prediction was not conservative for circular CFT beams without shear connectors.

The strength and ductility of thin-walled steel columns under cyclic loads are considerably upgraded by filling concrete into hollow spaces surrounded by steel tube and diaphragms. The above thin-walled steel columns filled with concrete are referred here to as thin-walled CFT columns. Up to the present, no sufficient and precise research has been conducted on the versatile finite-element model (FE model) analysis that can take into account the upgrading mechanism of thin-walled CFT columns in a direct manner. Herein, an accurate FE model is investigated in order to fully include the important factors such as cyclic local buckling of steel tube, nonlinear behavior of confined concrete, and interface action between steel tube and in-filled concrete. The validity of the proposed models is examined by comparing with the results of cyclic loading experiments on CFT columns. With the proposed model, the effect of in-filled concrete on the upgrading mechanism of thin-walled CFT columns is discussed in detail.

A hysteresis model for thin infill steel plates was developed to evaluate the nonlinear cyclic behavior of steel plate shear walls. Nonlinear finite-element analysis was performed for thin steel plates with a rigid boundary frame. Based on the analysis results, the hysteretic behavior of the infill steel plate was simplified as an equivalent uniaxial stress-strain relationship in the direction of tension-field action. The proposed hysteresis model was implemented in macroscopic analysis models for infill steel plates, i.e., the tension strip model and equivalent tension brace model. The proposed method was applied to existing test specimens with various design parameters and loading conditions. The prediction results were compared with the test results.

A novel damped outrigger system has been recently proposed for tall buildings, and is quite promising. To gain insight into the conceptual design of such systems, a simple beam-damper system model for a building with such dampers installed is developed and studied. A partial differential equation governing the motion is derived assuming a Bernoulli-Euler beam. A closed-form analytical solution is developed for vibration of the beam by analyzing the regions above and below the damper separately using separation of variables. By applying appropriate boundary conditions at the ends, a transcendental characteristic equation is obtained that governs the system’s complex natural frequencies. An explicit form for the complex mode shape is determined for dynamic analysis. A numerical iteration scheme is adopted to solve the characteristic equation for the complex eigenvalues (i.e., the system modal frequencies and damping ratios). This solution was used to determine design curves for optimal damper position and size. For engineering convenience, empirical equations were provided by fitting numerical results. These equations include one for determining the optimal location of the damper for each mode, and two for determining the optimal damping coefficient of the damper, and for calculating the maximum modal damping ratio of the system while the beam vibrates in its first mode. Furthermore, relatively accurate approximations of the pseudoundamped natural frequency and damping ratio of the first mode were obtained using a Taylor expansion of the characteristic equation. All of the results obtained are nondimensionalized for convenience of analysis and application.

This paper presents an iterative automated method for optimizing structures with multiple deflection criteria and load cases. The method is based on the principle of virtual work. Discrete sections are selected for structures with fixed geometries. An optimal structure is one which meets all strength and deflection criteria using minimal material. Four case studies are considered in this paper. A simple portal frame is presented to show how the method works. A 60-story frame is optimized to demonstrate the effectiveness of the method for large structures. A warehouse designed by professional engineers is presented to show how the method can be used for structures subjected to complex loading conditions and deflection criteria. The automated method’s solution is 4.5% lighter than the engineers’. Finally, a stepped cantilever is optimized and compared to results in literature. Material savings of up to 14.4% are realized.

High-rise construction projects commonly use a tower crane attached to the building by braced connections along its height. The presence of the tower crane modifies the aerodynamic cross section of the building and can alter the expected wind loading characteristics. Wind tunnel tests were carried out to examine the impact of the solidity ratio, and location of the tower crane, on the wind loading for the overall building-crane system. Tower cranes of two solidity ratios, 20 and 100%, were tested. A tower crane having one of the two solidity ratios tested was placed at one of three locations along the building face in order to define a quantitative range of increased wind loads. Increases in mean and fluctuating loading were observed for particular wind directions, and were more pronounced for the greater solidity ratio. The increase was the most significant for the torsion base moment. The impact of the tower crane was reduced as its location neared the center of the building face. Recommendations are made for the choice of tower crane location.

Recent advances in multistory timber building design have led to new structural systems that allow open floor plans with large spans between frames and/or walls. Timber-concrete composite (TCC) flooring can achieve the spans required but has the potential to be flexible under diaphragm actions, which can significantly alter the seismic response of a building. In-plane experimental tests on a 3 m by 3 m one-third scale TCC floor were performed using quasi-static earthquake loading simulation. The experimental results indicate that the deformation between the floor and lateral load resisting systems (LLRS) is much greater than the in-plane deformation of the floor diaphragm. Hence, a floor system with similar aspect ratio can be modeled as a single-degree-of-freedom for future structural analyses. Different connections were considered between the floor unit and lateral restraints, which simulate the LLRS. The connection was either timber-to-timber or concrete-to-timber and incorporated screws or nails acting as dowels or inclined at 45°. Each connection type performed differently in terms of stiffness, strength, ductility capacity, and induced damage. Screws that were oriented at 45° to the connection interface were significantly stiffer than fasteners aligned orthogonal to the interface. There was little difference in the initial stiffness for the concrete-to-timber connection compared to the timber-to-timber connection. The testing indicated that a timber-to-timber interface is more desirable because of construction ease and reparability. The in-plane response of the floor system is modeled using finite elements and compared to experimental results. Design recommendations are provided for the cyclic strength of inclined wood fasteners.