Journal of Structural Engineering

A suggested method for design of supplemental dampers in multistory structures is presented. Optimal control theory using a linear quadratic regulator (LQR) is adapted to design linear passive viscous or viscoelastic devices dependent on their deformation and velocity (best represented by Kelvin model). The design is aimed at minimizing a performance cost function that produces a most suitable minimal configuration of devices while maximizing their effect. The method is fully effective using full-state static feedback. Since the active feedback action requires a linear combination of all states and it cannot be supplied by passive devices, the paper introduces a methodology to eliminate the off-diagonal interactions between states using various engineering ways. This paper shows the development for velocity feedback only, for the sake of simplicity. However, the full-state formulation can be manipulated similarly to obtain a combined position-velocity feedback design. This paper shows a numerical implementation of the design methodology for a structural model prepared for further experimental considerations.

This paper presents a methodology for economic evaluation of design codes. The optimal design level of a product—a component, a system, or a facility—is determined as the one that involves the highest net life-cycle benefit, i.e., benefits less costs to society. The costs may include the production cost, the operating and maintenance expenses, and the cost of malfunctioning to the product, its users, other parties that may be affected, and to the system within which it is employed. The particular application of the methodology involves seismic design of buildings. The total life-cycle cost in the case of seismic design is composed of the initial cost of the building designed for a selected level of peak ground acceleration and the expected cost of failure due to an excessive earthquake. The latter includes the cost of fatalities, of injuries, the cost of damage to buildings, and the cost of inactivity due to damage. The optimal level of the design acceleration results in a minimum total life-cycle cost. The methodology for calculation of each component of the life-cycle cost is explained, and an application example is presented.

An experimental program was conducted on a four-story full-scale reinforced concrete building designed as a ductility class “high” structure in accordance with Eurocodes 2 and 8. The activity was defined as part of a cooperative research program by the European Association of Structural Mechanics Laboratories. The pseudodynamic tests were conducted for a serviceability-level and high-level earthquake—1.5 times the reference accelerogram. The results of the pseudodynamic tests are presented. General conclusions about the performance of the structure are drawn, with attention paid to the effects of the adoption of the new high-strength B500 Tempcore steel. The results make it possible to discuss the influence of the modeling parameters to be used in predicting the behavior of this class of structures. Effects such as the pullout and the slippage of the bars in the joints, or the participation of the slab to the global stiffness were found to play a fundamental role.

Modal reduction methods are a useful alternative to fully discrete matrix models for the efficient simulation of large dynamic response problems in frame analysis. The primary advantage of modal methods is computational efficiency, since they require less memory and fewer floating-point operations relative to conventional dynamic analyses of frames. The most important limitation of modal schemes is the difficulty in capturing strong nonlinear effects while retaining the simplicity of standard modal analysis algorithms. Modal methods obtained from inverse Lanczos iteration constitute a particularly elegant protocol for obtaining approximate time histories of response for nonlinear analyses of large frames and similar flexible structures. Examples underlining the strengths and weaknesses of Lanczos approximations are presented, and conclusions as to the utility of such modal reduction schemes for nonlinear dynamic analysis are drawn.

A numerical model for the analysis of the geometric and material nonlinearities of plate and thin shell steel structures, based on the finite element method, is presented together with its application to some practical examples. To account for the nonlinear behavior of steel, a generalized biaxial constitutive model, based on a kinematic yielding surface, is adopted. The geometric nonlinear effects are included by considering equilibrium on the deformed structure. Among the possible applications, a special concern is the simulation of the existing response beyond the conventional critical design or limiting loads in order to evaluate the actual ultimate capacity of those structures once the secondary resisting mechanisms and strength reserves of the materials are considered. The performance and reliability of the model are shown through the discussion of some numerical examples for which reference theoretical and experimental results are available.

Proposing a new warping function, the basic equations for thin-walled beams with arbitrary open or closed cross sections are developed. Compared to the classical theory of thin-walled structures, the present approach is more general. The only restriction is that the cross section of the member should remain undeformed in its plane during deformation. The principle of virtual displacements has been used to give the conditions of equilibrium. The finite element method and numerical results are presented in a companion paper.

In the companion paper, using more general assumptions concerning the warping, the theory for thin-walled beams with arbitrary open or closed cross section is developed. In this paper, on the basis of the new warping function, the finite element model is presented. An updated Lagrangian formulation is used to give a linear and geometric stiffness matrix. To test the accuracy of the theory as well as the performance of the associated finite element, some numerical examples are given.

This paper presents guidelines for classification of ductile coupled shear walls (CSWs). It includes two parts: the evaluation of the degree of coupling (DC) between the walls and the coupling beams, and the classification of CSWs. In present practice, DC is defined as the percentage of the base overturning moment carried by the axial tension and compression forces resulting from shears in the coupling beams. The evaluation of DC requires a structural analysis of CSW, which presents serious shortcomings during the preliminary proportioning process. A simple method is proposed in this paper for the evaluation of DC without need for structural analyses. A classification method based on the dynamic properties of CSWs is also proposed. This allows the classification of CSWs into one of the following three categories: a series of linked isolated walls, CSWs, and walls with openings (pierced walls). Finally, numerical examples demonstrate the applicability of the proposed methods. This paper is thought to be very useful for the practicing engineer.

This paper presents a systematic approach for estimating fragility curves and damage probability matrices for different structural systems. Both fragility curves and damage probability matrices express the probabilities that a structure will sustain different degrees of damage at given ground motion levels. In contrast to previous approaches, this paper presents a method that is based on nonlinear dynamic analysis of the structure rather than on heuristics or on empirical data. The ground motion level for fragility curves is characterized by spectral acceleration. For damage probability matrices, modified Mercalli intensity is used as the ground motion parameter. The probabilities associated with the different damage states at a specified ground motion level are evaluated using the Monte Carlo-simulation technique. The nonstationary autoregressive moving average (ARMA) model is used for the generation of earthquake time histories. The approach presented in this paper is used to obtain fragility curves and damage probability matrices for reinforced concrete frames. Three different classes of reinforced concrete frames, based on the number of stories, are considered. Park and Ang's damage index is used to identify the different degrees of damage.

Excessive vibrations and deflections of modern flexible structures can be imposed by crowd movements. Dynamic forces generated by moving crowds of people while walking were measured using a specially designed, large, instrumented force platform consisting of six independent honeycomb plates. Experiments were conducted for individuals and groups of two and four people. A linear regression model was used to statistically characterize the footstep load-time history. Group loads were randomly simulated using the experimental parameters affecting individual footsteps and the coherency among the people in a group. The accuracy of the group load model is verified by frequency-domain comparison of simulated and measured data for groups of four people. Based on the work presented, loads are simulated for large crowds and design live loads are suggested for structural serviceability requirements under occupant walking activities. The design live loads vary both with footstep frequency and the size of the crowd.

Wind-induced fatigue loading and damage to roof claddings on gable-roof and hip-roof buildings are investigated in this study through both wind tunnel tests and computer analyses. The magnitude and distribution of wind pressures on both roofs are first measured at different wind directions. Fatigue characteristics of wind pressures are identified based on a rainflow count method for each tap at its critical wind direction in which its largest negative pressure occurs. Total fatigue loading at each tap on both roofs is then computed using the obtained fatigue characteristics and information on wind climate. Finally, fatigue damage to screw-fastened light-gauge steel roof sheetings of different profiles is estimated in terms of previously obtained S-N curves and a modified Miner's rule. The distributions of fatigue damage index over both roofs show that under the same strong winds, fatigue damage to the hip roof cladding is much less severe than that to the gable roof cladding. For each type of roof configuration, there are considerable differences in fatigue loading and fatigue damage with respect to different locations of pressure taps and different profiles of roof sheetings.

This paper presents a method to define two-dimensional (2D) and three-dimensional (3D) elastic-stiffness matrices for nonprismatic elements (tapered or haunched), based on traditional beam theory and the flexibility method. The proposed formulation includes deformations and the shape of the cross section but neglects warping deformations. Although more rigorous formulations for tapered elements have already been addressed, the proposed procedure is presented so its direct application or implementation in computer programs for structural analysis is straightforward. The procedure is compared against the design tables of the Portland Cement Association (PCA). It is demonstrated that the PCA tables are obsolete for today's state-of-the-knowledge on nonprismatic members because they can lead to significant errors. A new set of design aids for most common cross sections used in building structures have been developed to substitute the PCA handbook of frame constants. Closed-form solutions for linearly tapered elements of rectangular, square, and circular cross sections are provided. Finally, it is demonstrated that the stiffness factors for nonprismatic elements depend on the span-to-depth ratio of the element ( L /h). To the writer's knowledge, no one has proved this fact before.