Journal of Structural Engineering

In order to investigate the seismic performance and to provide experimental evidence for developing guidelines for seismic design of concrete and clay tile roofs for typical low-rise residential or commercial buildings, 24 full-scale tile roof models have been tested under simulated earthquake loading using shaking tables. The roof models were constructed following the installation requirements of the 1994 Uniform Building Code. Experimental parameters include tile type, roof slope, input direction of horizontal vibration, and peak input acceleration at the roof level. Test results indicate that tile roofs can sustain horizontal accelerations much higher than 0.8g, which is considered the design level acceleration for tile roofs of ordinary buildings in high seismic regions. Ultimate performance criteria for tile roofs have been identified based on the test results.

New formulations are developed to account for the effect of uncalculated high frequency rigid modes in the seismic analysis of coupled primary–secondary systems by modal synthesis approach. The effect of uncalculated rigid modes is included in terms of residual modal vectors for both the uncoupled primary and the multiply supported secondary systems. The reduced eigenvalue problem, obtained by transforming the original equation of motion in which the displacements at the secondary system degrees of freedom (DOF) are expressed relative to the base of the primary system, gives incorrect results when all the modes of the secondary system are not included and the effect of high frequency modes is included through residual modal vectors. An alternate formulation is presented in which the original equation of motion is transformed such that the displacements at the secondary system DOF are expressed relative to the primary system connecting DOF. The reduced eigenvalue problem is solved using the transformed equation of motion for the nonrigid and the residual modal vectors of the two uncoupled systems.

This is the second paper of two papers that describe the procedures to include the effect of uncalculated high frequency modes in coupled modal synthesis. Because the residual modal vectors of a multiply supported secondary system cannot be included directly in a coupled modal synthesis, formulations are developed to calculate the residual response due to “missing mass” contained in the uncalculated high frequency rigid modes and combine it with the response due to nonrigid modes. A criterion is developed to determine a rigid (cut-off) frequency for the uncoupled secondary system modes and estimate the upper bound of error in the secondary system residual response.

This paper proposes a simplified theory to predict and compare the seismic performance of viscoelastical (VE)- and elastoplastical (EP)-damped passive control systems, and to demonstrate their ability to protect structures during a major seismic event. Closed-form expressions for “equivalent period” and “equivalent damping” of these systems are proposed by idealizing them as linear single-degree-of-freedom (SDOF) systems. The expressions are used to clarify and compare the structural parameters, seismic drift, and force for the systems, based on a common high-damping linear spectrum. By extending the SDOF theory, a seismically deficient 14-story steel moment resisting frame (MRF) is upgraded by inserting VE or EP dampers. Extensive three-dimensional multi-degree-of-freedom nonlinear dynamic analyses are performed for the MRF, VE system, and EP system using earthquakes of various intensities. The 14-story VE and EP systems responded as predicted by the SDOF theory. They show much smaller drifts than the original MRF, keep frame members elastic, and protect nonstructural components, even against major earthquakes.

Identical mathematical expressions for viscoelastic and viscous damper-brace components are derived as the function of two fundamental nondimensional parameters. These two parameters are used to investigate the properties of viscoelastic (VE) and viscous (VS) systems by conducting parameter analysis. Usually both VS and VE damper-brace components provide added stiffness and damping to the whole system. The magnitude of the added stiffness and damping depends not only on the damper, but also on the interaction of the damper with other members of the frame. Under conditions of low frequency and stiff brace, the added stiffness provided by a VS damper-brace component is negligible. Based on harmonic theory, closed-form solutions for seismic response prediction are presented and are used to determine the optimal design. As an example, the seismic performance of a 10-story steel frame incorporated with VE and VS dampers is studied.

Structures subjected to strong earthquake ground shaking may be left in a displaced condition. The amount of this residual or permanent displacement is determined by structural characteristics, including mass and hysteretic behavior, as well as ground motion. Residual displacement is undesirable because it presents problems during repair and reconstruction after an earthquake. This paper describes analyses of many single-degree-of-freedom bilinear oscillators, with different natural periods, damping ratios, ductility factors, bilinear factors, and input ground motions. These analyses were carried out in order to obtain a method of estimating the likely residual displacements of real single-degree-of-freedom structures. The residual displacement was normalized by the maximum possible residual displacement. Although there was scatter in the normalized residual displacement for different earthquake records, trends in behavior were dominated by the slope of post-yielding branch of bilinear loop. A residual displacement response spectrum is presented for design of a single-degree-of-freedom structures and a design application is also provided.

The seismic damage analysis of steel frames based on the use of global evaluation indices is addressed. A general procedure is proposed aimed at estimating the degree of damage induced by severe earthquakes in structures not supplied with monitoring systems. Special reference is made to the Park-Ang, McCabe-Hall, and plastic fatigue indices. The procedure is characterized by a form of automatic calibration of the free coefficients contained in the expressions of these indices. This allows one to constrain the arbitrariness in the coefficient choices, representing the greatest source of uncertainties for practical damage field assessment. Story stiffness degradation and permanent interstory displacements are assumed as the a posteriori measurable quantities upon which the reliability of the computational predictions supporting damage estimation are checked. The suitability of this procedure as well as of the considered indices for seismic damage characterization are discussed with reference to two case studies, represented by one shear-type and one concentrically braced, two-story plane steel frame, experimentally tested to failure.

Discrete ground structure topology optimization design methods have to date received considerable attention in structural engineering. An alternative class of structural topology optimization methods, which have not yet received much attention in structural engineering, but which have undergone considerable development in the past decade, are the so-called continuum formulations. In this work, a continuum structural topology optimization formulation is presented and applied to the concept design optimization of structural bracing systems that are needed to stiffen tall structures against sidesway under lateral-wind and seismic-type loading. Although demonstrated here in the context of these specific design examples, continuum structural topology optimization methods are believed to hold potential as a design tool for a wide range of civil engineering type structures. A variety of continuum topology design formulations, including static compliance minimization and eigenvalue optimization, are explored, and solution parameters are varied to show that a number of design possibilities can be realized as solutions.

A design procedure incorporating a simple genetic algorithm (GA) is developed for discrete optimization of two-dimensional structures. The objective function considered is the total weight (or cost) of the structure. The objective function is minimized subjected to serviceability and strength requirements. The GA-based design procedure FEAPGEN is developed as a module in the Finite Element Analysis Program (FEAP). Special features of FEAPGEN include discrete design variables, an open format for prescribing constraints, design checking using the American Institute of Steel Construction Allowable Stress Design (AISC-ASD) specifications, multiple loading conditions, and a comprehensive AISC database of available structural steel members. Several strategies for reproduction and crossover are investigated. In particular, a group selection scheme for reproduction that does not require fitness scaling is applied. Various fitness and penalty functions are investigated for their appropriateness to the ASD design of two-dimensional structures. A comparison is presented between FEAPGEN genetic search design procedure and a classical continuous optimization method based on the optimality criterion.

Previous test results of two-way fully restrained reinforced concrete slabs are used to optimize the model parameter values involved in a finite element program. The parameters whose sensitivities control the simulation process are identified through extensive parametric studies using both the displacement and load control strategies, and values are identified for them. Use of such values has been shown to guarantee reliable predictions of both the peak load and the corresponding deflection of arbitrary fully restrained rectangular slabs. A series of numerical predictions of over 850 “computer model” slabs are undertaken, varying the geometric and material properties, as well as the reinforcement ratios. The result is used for developing charts that may be used for the strength and displacement determination of arbitrary fully restrained slabs. The charts are used for the predictions of 36 previously tested slabs, in which the slab strength is predicted to within a mean accuracy of 2%, and the deflection is predicted to within a mean accuracy of 4%. Other practical applications of reliable numerical predictions are discussed.

Construction of concrete structures involves at least three different materials: concrete, steel, and formwork. As such, design of these structures should be based on cost rather than weight minimization. A review of papers on cost optimization of concrete structures published in archival journals is presented in this paper. These structures include beams, slabs, columns, frame structures, bridges, water tanks, folded plates, shear walls, pipes, and tensile members. Interesting and important results and conclusions are summarized. A review of reliability-based cost optimization is also included in the paper. It is concluded that there is a need to perform research on cost optimization of realistic three-dimensional structures, especially large structures with hundreds of members where optimization can result in substantial savings. The results of such research efforts will be of great value to practicing engineers. Additional research needs to be done on life-cycle cost optimization of structures where the life-cycle cost of the structure over its lifetime is minimized instead of its initial cost of construction only.

The design or assessment of wall panels subject to blast loading requires the formulation of a dynamic flexural resistance-deflection function for the element. With the exception of situations in which the explosion occurs at very close proximity to the panel, the resistance to blast loading may be based upon a quasi-static resistance function that subsequently must be modified for dynamic effects. This paper describes a proposed method for the derivation of quasi-static elastic/plastic resistance functions for reinforced concrete wall panels with door and window openings based upon finite element analysis and yield-line theory. This approach is compared with the results of tests on model wall panels. It is shown to produce a conservative value of ultimate load but may underestimate the deflection at the limit of elastic behavior.

The analysis and design of composite beams prestressed by external tendons involves difficulties mainly related to the cable slip at the saddle points and to the construction sequences. The paper presents a model for analyzing such structural systems under general conditions. Generic nonlinear constitutive laws are considered. No restrictions are assumed for tendon profiles and static scheme. Usual construction sequences can be considered. Kinematics and the compatibility equations refer to a beam model preserving the cross section planarity, and the tendon is free to slip at the saddle points. The characteristics of the problem make it convenient to obtain the balance condition from the virtual work principle. The numerical solution was performed by variational approximating methods and the nonlinear equations were solved by the Newton-Raphson procedure. The paper presents a tool for the analysis of externally prestressed composite beams up to failure in order to investigate the influence of typical design parameters on the ultimate capacity. The effectiveness of the presented model is shown in a numerical example.