Journal of Engineering Mechanics

This paper describes a continued study on three-dimensional temporal characteristics of earthquake ground motions at a single point. Based on an instantaneous tangential and normal acceleration decomposition of ground acceleration trajectory, a ground motion can be partitioned into a finite sequence of staggered time intervals of acceleration and deceleration. A formulation is developed to estimate speed and angular changes over a partitioned interval in terms of rates of positive and negative tangential and normal acceleration. Based on these concepts, general ground motion properties, peak ground acceleration, peak ground velocity, and peak ground displacement are examined. Several Northridge earthquake records are studied in detail. It is found that the highest peak of ground acceleration in these records corresponds to a high peak of deceleration, and a velocity maximum often precedes the peak of acceleration.

A number of optical devices are commercially available now for measurement. Some of them, such as laser devices and still and video cameras with high resolution, may be used effectively and efficiently for measuring structural displacement. This paper presents an approach to this type of application using a charge-coupled-device camera. It can acquire digitized images for low cost, to be used to identify structural displacement via digital signal processing. It is shown that this approach’s resolution for point measurement is comparable with traditional sensors such as dial gages. Furthermore, it offers a new capability of displacement measurement for a large number of points on a structure, and it can provide spatially intensive displacement data. This kind of data may be used for structural damage detection and health monitoring, as suggested and demonstrated herein.

An improved structural damage quantification algorithm is presented based on the elemental modal strain energy change before and after the occurrence of damage in a structure. The algorithm includes the analytical stiffness and mass matrices of the system in the damage quantification. It reduces significantly the modal truncation error and the finite-element modeling error from higher analytical modes in the computation, and it improves the convergence properties of the existing algorithm by Shi et al. (2000). “Structural damage detection from elemental modal strain energy change.” J. Eng. Mech., 126(12), 1216–1223]. The effectiveness of the proposed algorithm is demonstrated via a numerical example and experimental results from a two-storey steel portal frame, and it is demonstrated to be an efficient and robust method for damage quantification.

Current procedures for estimating the peaks of the stochastic response of tall buildings to wind are based on the assumption that the response is Gaussian. Those procedures are therefore inapplicable to low-rise buildings, in which time histories of wind-induced internal forces are generally non-Gaussian. In this paper, an automated procedure is developed for obtaining from such time histories sample statistics of internal force peaks for low-rise building design and codification. The procedure is designed for use in software for calculating internal force time series by the database-assisted design approach. A preliminary step in the development of the procedure is the identification of the appropriate marginal probability distribution of the time series using the probability plot correlation coefficient method. The result obtained is that the gamma distribution and a normal distribution are appropriate for estimating the peaks corresponding, respectively, to the longer and shorter tail of the time series’ histograms. The distribution of the peaks is then estimated by using the standard translation processes approach. It is found that the peak distribution can be represented by the Extreme Value Type I (Gumbel) distribution. Because estimates obtained from this approach are based on the entire information contained in the time series, they are more stable than estimates based on observed peaks. The procedure can be used to establish minimum acceptable requirements with respect to the duration and sampling rate of the time series of interest, so that the software used for database-assisted design will be both efficient and accurate.

One of the most successful means of protecting structures against severe seismic events is base isolation. However, optimal design of base isolation systems depends on the magnitude of the design level earthquake that is considered. The features of an isolation system designed for an El Centro-type earthquake typically will not be optimal for a Northridge-type earthquake and vice versa. To be effective during a wide range of seismic events, an isolation system must be adaptable. To demonstrate the efficacy of recently proposed “smart” base isolation paradigms, this paper presents the results of an experimental study of a particular adaptable, or smart, base isolation system that employs magnetorheological (MR) dampers. The experimental structure, constructed and tested at the Structural Dynamics and Control/Earthquake Engineering Laboratory at the Univ. of Notre Dame, is a base-isolated two-degree-of-freedom building model subjected to simulated ground motion. A sponge-type MR damper is installed between the base and the ground to provide controllable damping for the system. The effectiveness of the proposed smart base isolation system is demonstrated for both far-field and near-field earthquake excitations.

The present work deals with the numerical simulation of fiber reinforced polymers (FRP) poststrengthened scaled concrete slabs in order to predict their load carrying behavior. The used strengthening materials are FRPs which are of increasing interest in civil engineering applications such as textile reinforced concrete tubes, cables of cable-stayed bridges, or even entire bridges. The numerical results are compared with experiments which were conducted at the Univ. of California, San Diego, and are described in detail by Hörmann in 1997; Hörmann et al. in 1998; and later by Seim et al. in 2001. The slabs are modeled for the numerical simulation first in a two-dimensional design space, assuming a plane stress condition for the concrete and the fiber reinforced polymer, and second in a three-dimensional design space with multilayered shell elements in order to include the varying response across the depth of the slabs. The used material model for reinforced concrete was developed by Menrath et al. in 1998 and has been enhanced for multilayered shell elements by Haufe et al. in 1999. It is based on multisurface plasticity, consisting of two Drucker–Prager yield surfaces and a spherical cap, and exhibits fracture energy based evolution laws for the softening regime. The reinforcement is taken into account as additional stress contribution in a smeared manner using a one-dimensional constitutive law based on an elastoplastic isotropic hardening model.

To effectively and efficiently utilize fiber-reinforced plastic (FRP) laminates (plates or sheets) in strengthening civil infrastructures, a design strategy integrating the properties of FRP reinforcement and composite structural behavior needs to be adopted. The interfacial stress transfer behavior including debonding should be considered to be one of the most important effects on the composite structural behavior. In this paper, two kinds of nonlinear interfacial constitutive laws describing the pre- and postinterfacial microdebonding behavior are introduced to solve the nonlinear interfacial stress transfer and fracture propagation problems for different kinds of adhesive joints in FRP/steel-strengthened concrete or steel structures. Expressions for the maximum transferable load, interfacial shear stress distribution, and initiation and propagation of interfacial cracks (debonding) are derived analytically. In addition, numerical simulations are performed to discuss the factors influencing the interfacial behavior and the theoretical derivations are validated by finite-element analysis.

This paper presents a general formulation of the model predictive control (MPC) scheme with special reference to acceleration feedback in structural control under earthquakes. The MPC scheme is based on a prediction model of the system response to obtain the control action by minimizing an objective function. Optimization objectives include minimization of the difference between the predicted and desired response trajectories, and of the control effort subject to certain constraints. The effectiveness of MPC has been demonstrated to be equivalent to the optimal control. In this study, the prediction model is formulated using a feedback loop containing acceleration measurements from various locations in the structure. The state observer utilizes the Kalman-Bucy filter to estimate the states of the system from the acceleration feedback. Examples of single-story and three-story buildings equipped with control devices are used to demonstrate the effectiveness of the MPC scheme based on acceleration feedback. Both buildings are analyzed using an active tendon control device and an active mass damper (AMD). A two-story building with an AMD is used to experimentally validate the numerical control scheme. The results demonstrate the effectiveness of the MPC scheme using acceleration feedback. The acceleration feedback framework developed in this paper should serve as a building block for future extensions of MPC in capturing and benefiting from the attractive features of MPC, i.e., computational expediency, real-time applications, intrinsic compensation for time delays, and treatment of constraints, for implementation in civil structures.

A procedure to identify the material parameters of an elastoplastic model is presented. It aims to predict uniaxial stress–strain curves in tension or compression. The Drucker–Prager model is chosen because of its simplicity. The procedure takes into account the hardening/softening regime by varying two physical parameters related to the model: the cohesion and the friction angle.

This technical note presents an analytical method, accompanied by a numerical scheme, to evaluate the response of beams on nonuniform elastic foundations, namely, when the foundation modulus is $k=k\left(x\right)\text{.}$ The method employs a Green’s function formulation, which results in a system of nonsingular integral equations for the distributed reaction $q\left(x\right)\text{.}$ These equations can be discretized in a straightforward manner to yield a system of linear algebraic equations that can be solved by elementary numerical techniques.

A unified model considering the microstructure instability and a set of complete constitutive relations, which describes the volumetric collapse and shearing collapse simultaneously, is developed for collapsible loess in the framework of the catastrophe theory. The validity of the model is verified by comparing the results calculated using the model with the data of triaxial compression tests performed on Lanzhou loess. The following characteristics of collapsing deformation are explained and reproduced successfully using the model: (1) the existence of two turning points on each deformation curve, (2) the shear curves under various stress ratios intersect one another, (3) the ratio of the shear strain to the volumetric strain is a function of the stress ratio, and (4) the initial collapse condition is approximately a circle.