Journal of Structural Engineering

A method for identifying the element properties of a truss is developed in this paper. The measured natural frequencies and modal shapes of the truss are used in the identification process. The finite-element method is used to formulate the eigenequation of the truss. The identification problem is then formulated as an optimization program in which the error norm of the eigenequation is minimized. It is shown that given sufficient modal data, the element properties can be directly attained without iterations. Further, the solution is unique and global minimal. The identifiability of the inverse problem is then studied in depth. The perturbation method is adopted to investigate the influence of measurement errors on the identification results. A numerical example is presented to demonstrate the effectiveness of the proposed method in property identification and damage detection.

A large twist rotation model for the nonlinear inelastic analysis of the nonuniform torsion of I-section beams is presented in this paper. A finite-element procedure has been developed based on the model. The nonlinear equilibrium equations of beams in nonuniform torsion including the effect of the large twist rotation have been derived. The elastic-plastic behavior of beams in nonuniform torsion is studied using the finite-element procedure and the results are compared with tests. It is found that I-section beams have much larger torsional capacities than can be predicted by linear plastic collapse analysis, and that torsional failure occurs not by the formation of a mechanism but by the tensile rupture of the flanges. A method is proposed for calculating the full plastic nonuniform torque for practical design purposes.

The viscoelastic analysis of composite steel-concrete continuous beams, with prestressed slab and flexible shear connectors, is presented. Two prestressing systems, involving the application of prestressing forces before or after the connection of the concrete slab to the steel beam, are considered. The mathematical formulation of the problem involves internal compatibility equations between beam and slab and between slab and prestressing cables. Such conditions are formulated taking into account the integral-type creep law for the concrete slab. With the model proposed, some relevant examples were investigated and discussed. Partial longitudinal prestressing of the slab, performed using cable with length less than the whole structure, is also analyzed. Particular attention is paid to examining the “primary” and the “secondary” effects of the prestressing, and the shear force distribution at beam-slab interface. A parametric study is then performed by varying the stiffness of shear connectors and the combining time between slab and beam.

This paper describes a new method of analysis and design for steel frames with concrete or masonry infilling walls subjected to in-plane forces. The method is based on data generated from previous experiments as well as results from a series of nonlinear finite-element (NLFE) analyses. The method accounts for elastic and plastic behavior of infilled frames considering the limited ductility of infill materials. The proposed method predicts the strength and stiffness of infilled frames as well as the infill diagonal cracking load. The method also allows for the major practical imperfections, such as lack of fit and shrinkage of the infill. Variations, such as infill aspect ratio and beams having different strength and stiffness from the columns, are accounted for and it is concluded that the behavior of frames with pin or semirigid joints can also be predicted. The method is further developed to model multistory infilled frames as braced frames, replacing the infills by equivalent diagonal struts.

The effects of wind and wave loads on the response of tension leg platforms have been studied extensively over the past decade; however, this study represents the first attempt to compare the response predictions to a model study of a tension leg platform performed in a properly scaled wind/wave environment. Experimental results and numerical simulations were in agreement. The paper discusses some of the difficulties encountered in combined wind/wave testing. The numerical models include first- and second-order wave loads and the simultaneous action of dynamic wind loads. The effect of wind on the surge response of the platform was strongly dependent on the existing waves. The wind-induced response tends to dominate the overall surge response in cases where the overwater fetch is small (i.e., small waves) and the wind is strong; however, for large waves, the reverse is true. The inclusion of wind forces significantly increased the heave response of the tension leg platform.

This paper presents a general numerical method to calculate the nonlinear behavior of load-bearing members under elevated temperature conditions. The method, although similar in concept to earlier approaches, differs with respect to both solution procedure and capability. The governing equations are formulated in a succinct format and an innovative iteration procedure is used to obtain the solution. The method can take into account the combined actions of axial force and biaxial bending, external restraints, temperature variation over the cross section and along the member, material nonlinearity, geometric nonlinearity, unloading and reloading, residual or initial stresses, and initial “out-of-straightness” of the member. It applies to members of any cross-sectional shape and with any elevated temperature stress-strain relationship for the material, including one with zero or negative slope. The application of the method is illustrated using a practical design example in which the fire resistance of an unprotected steel column of large cross-sectional area was evaluated.

A general numerical procedure to analyze the behavior of load-bearing members under elevated temperature conditions has been developed by the writers. In this paper, the results obtained by using the numerical procedure are compared with the data from 18 steel column tests conducted at the Norwegian Institute of Technology. The tests comprised one column tested at room temperature and the remaining 17 at elevated temperatures, with different heating rates, axial loads, load eccentricities and end-support conditions (unrestrained, rotationally restrained, and axially restrained). For the column tested at room temperature, the numerical procedure closely reproduced both the ascending and descending branches of the load-deflection curve. The accuracy of the numerical results for the columns tested at elevated temperatures is largely dependent on the elevated temperature material model used. Although elevated temperature stress-strain curves were not determined for the steel columns, a reasonable prediction of the behavior of the columns was obtained when the Eurocode 3 ( Eurocode 1990) stress-strain relationship were used.

Two kinds of errors are frequently encountered during the construction of cable-stayed bridges: one is force-application error in the jacking cable forces, and the other is geometry error in controlling elevations of the girders. These errors are normally kept within the prescribed tolerances by adjusting cable forces. However, the influence of creep in prestressed concrete cable-stayed bridges cannot be ignored. If this influence is not considered, it is possible to exceed the allowable stay-force range at the time when creep ends. This paper presents an optimization application in which the influence of creep can be taken into account. Some optimizing criteria are proposed, and the optimum adjusting cable forces are obtained by minimizing the amount of work due to these forces. Residual stresses due to force-application errors can also be minimized. The procedures are illustrated by several examples.

Because the cost of steel bridges in Japan is generally estimated using the price per unit weight, bridge designers and fabricators are required to decrease the steel weight of the bridge. This results in increasing the number of cross-section variations and the number of weld lines. However, because of the shortage of technicians and increase in labor cost, it is becoming necessary to consider the possibilities of simplifying and accelerating the fabrication process to get a more rational and a near optimal design. Knowledge-based systems, when combined with actual design data of available bridges, can support the decision-making process for improving both design and fabrication. This paper discusses the creation of a knowledge base that includes data on previously designed plate-girder bridges, and its application in considering the effects of simplifying the design on the cost of the bridge. First, the steps used in the creation and processing of the knowledge base are explained. Then, the knowledge base is used to investigate several design alternatives by selecting and redesigning some bridges. Several suggestions for a more rational design are given, based on previous design information.

The effects of uncertain material properties on the elastic stability of structural members and frames are analyzed using a stochastic finite-element method. The uncertain material properties are modeled as continuous random fields by a series expansion involving orthogonal functions. The expanded random fields are incorporated into the finite-element formulation leading to an eigenvalue problem involving random parameters. A mean-centered second-order perturbation technique is used to find the probabilistic characteristics of the buckling load. Illustrations involving a simply supported beam, a simply supported beam on an elastic foundation, and a frame are presented. These illustrations demonstrate the impact on instability of characteristics of the random fields, including correlation length and coefficients of variation of random flexural rigidity and/or random elastic foundation modulus, finite-element mesh selection, and the interrelation between statistical and finite-element modeling. A comparison of the perturbation solutions with the results from Monte Carlo simulation serves to validate the solutions and identify some of their limitations.

The phenomena that occur during the early age of concrete curing are complex, as the coupling between chemical-thermal-hydration and mechanical effects must be considered. Among the main attributes that govern the nonlinear behavior of early-age concrete are stiffness evolution, the development of thermal strains, creep, and cracking. To properly accommodate these effects in a finite-element analysis of a large structure such as a concrete dam, efficient computational strategies are needed. In this paper, a finite-element numerical algorithm is developed for analyzing thermomechanical damage in young concrete. Thermal analysis procedures include consideration of heat generation (hydration) and dissipation within mass concrete, including the effects of ambient temperature. The stress-deformation-damage analysis procedures include temperature-induced, creep, and autogenous deformations. Development of concrete properties, including Young's modulus, tensile strength, and limiting tensile strain is described by experimentally obtained functions. Damage in concrete is considered to be the result of crushing, cracking, and a mixed failure mode. Failure criteria for each failure mode, along with constitutive relationships for prefailure and postfailure states are developed for both loading and unloading conditions. The developed finite-element program is used to simulate the construction of the first four layers of a concrete dam. The cracking damage predicted by the finite-element analysis is shown to qualitatively match with field observations.

Addition of discrete steel fibers to concrete enhances its properties, especially in the areas of serviceability and toughness. With the increasing use of shallow sections made of high-strength fiber concrete and capable of meeting the strength requirements, deflection behavior becomes an important factor that can control the design. This paper presents an analytical method that predicts the moment-curvature and load-deflection relationships for beams made of fiber concrete and containing conventional reinforcement. The proposed method evaluates the immediate deformation as well as the long-term deformation as affected by creep and shrinkage. The tension stiffening effect is incorporated to obtain a better prediction of the curvature and deflection. The analytical algorithm proposed to generate the complete moment-curvature and load-deflection curves provides a good correlation between predicted values and experimental test data reported in the literature. A sensitivity analysis is conducted to evaluate the effect of the material parameters on the predicted beam behavior.

The modulus of rupture of concrete, which characterizes the bending strength of unreinforced beams, is known to depend on the beam size. Because there is no large stable growth of a crack before the maximum load is reached, this size effect, unlike that in many other types of failure of concrete structures, cannot be explained by energy release due to fracture. Rather, this size effect must be explained by the fact that distributed microcracking and slips with strain softening take place in the boundary layer of the beam before the maximum load is reached. The beam is considered to fail before any macroscopic cracks are formed. A simple formula describing the size effect is derived. Asymptotic analysis of the strain softening in the boundary layer shows that the excess of the modulus of rupture over the direct tensile strength is inversely proportional to the beam depth and proportional to the thickness of the boundary layer, which itself is approximately proportional to the maximum aggregate size. The proposed formula agrees with the existing experimental data quite well. The formula is further generalized to describe the effect of the gradient of normal strains near the concrete surface. Finally, it is shown that approximate analysis of the size effect by linear elastic fracture mechanics yields similar formulas. Those formulas, however, have some questionable features; for example, they indicate the size effect magnitude depends on the span-to-depth ratio of the beam, which has not been observed in experiments.

High load multirotational (HLMR) bridge bearings are used to support large loads while accommodating rotations resulting from traffic loading, construction tolerances, thermal effects, and other deformations. Pot, disk, and spherical bearings qualify as HLMR bearings; a series of experiments were performed to evaluate their behavior, and to develop design recommendations. The experiments were performed on a specially designed rig that applies a constant compressive load, a constant shear force, and a cyclic rotation. Sixteen tests were performed on pot bearings, four on spherical bearings, and two on disk bearings. The test results are summarized, and general observations on the bearing behavior and design considerations are noted in this paper. Each bearing type has advantages and potential problems. Pot bearings are able to support large compressive loads, but their elastomer might leak and their sealing rings might suffer wear or damage. Disk bearings are susceptible to uplift during rotation, which may limit their use in bearings with polytetrafluoroethylene (PTFE) sliding surfaces. Spherical bearings are able to sustain large rotations but require proper clearances, very smooth and accurate machining.

The introduction of the standing seam panel for roof systems provided the building industry with an efficient solution to watertightness problems. These panels have two major advantages over the conventional trapezoidal through-fastened panel: an elevated seam and concealed “hold-down” clips. The relatively small depth-to-width ratio of the panel guarantees that overall, the panel is laterally stable under gravity load. The capacity of laterally stable standing-seam panels may, however, be reduced due to lateral instability of the unsupported compression flange. This mode of instability has been referred to in the literature as distortional buckling—a form of instability for unsupported lengths between those for local and lateral buckling. In the present paper, an analytical expression, based on a beam on elastic foundation model, is presented for estimating the elastic distortional buckling stress in sections with laterally unsupported compression flanges. The beam is assumed to comprise the outstanding leg, and the elastic foundation is provided by the rotational restraint of the tension flange and the web. A comparison of the analytical results with the results from elastic finite-strip buckling analyses shows good agreement between the two methods of analysis.

The design of standing seam roof panels must necessarily consider two loading conditions, construction and in use (occupancy) loading, in addition to the effects of ponding (if sufficient drainage or specific drainage mechanisms are not provided to avoid accumulation of water). In the present paper, the results of tests on single panels with laterally unsupported compression flanges are presented along with two design methods for the interaction between local and distortional buckling. The design methods are based on the analytical expression presented in the first paper and are conceptually consistent with the design procedures currently used for flexural, torsional, and lateral-torsional buckling. Overall, the new design methods give better approximations of the maximum capacity of wide-flange standing-seam panels than previous design methods. More research is still needed to demonstrate the applicability of the methods presented here to full-scale panel systems in which the outstanding legs are crimped together.

A continuum model for evaluating partially composite insulated concrete sandwich panel deflections with discrete shear connectors due to differential volume changes such as those caused by differences in temperature between the wythes is evaluated for accuracy by comparison with finite-element analysis. Design equations for computing these deflections are provided. Analysis of sample panels indicates that the continuum model provides acceptable accuracy for calculating thermal bowing of wall panels. The resulting equations show that, for long panels, the amount of thermal bowing in the panels is relatively insensitive to the stiffness of the connectors. This implies that long insulated sandwich panels with low connecting-layer stiffness will experience nearly the same amount of thermal bowing as fully composite panels.

Approximate methods are useful in preliminary design, and also provide the analyst and the designer with a rapid means of rough checking “exact” solutions. The approximate analysis of building frames for vertical loads is covered in several papers and textbooks. The existing storywise summation method is complicated in computation, and the other approximate methods usually have poor accuracy. This paper presents the simplified storywise summation method, which may be summarized as follows: (1) Divide a building frame into many single-story frames whose number equals the number of stories of the building frames; (2) approximately analyze each single-story frame; and (3) sum the single-story frames. The method is almost as accurate as the existing storywise summation method, and may provide better accuracy than the other approximate methods. The intent of this paper is to develop an approximate analysis of building frames for vertical loads, which may involve less labor in computation than the existing storywise summation method.