Journal of Engineering Mechanics

Safety assurance and detection of potential damage for space truss structures have been challenging topics. The two most critical problems are considered in this paper. One is to develop an effective damage detection method based on strain data under ambient excitation, and the other is then to optimize the installment of strain sensors owing to numerous structural members in the space truss structures. A method of damage detection for space truss structures, called the environmental excitation incomplete strain mode (EEISM) method, is proposed. Four steps are taken in the EEISM method. First, strain mode parameter identification is carried out based on the cross-correlation function of the strain responses through a combination of the empirical mode decomposition method and the peak amplitude series method. Second, the strain sensors are located optimally in the space truss structures through sensitive analysis of the strain mode perturbation matrix, which are obtained by perturbation theory. Third, the modal assurance criterion (MAC) value is applied to locate the damages; that is, the members with the larger MAC values are defined as the damaged members. Finally, a damage index obtained by solving the perturbation equation is used for damage quantification. Numerical analysis of a long-span space truss structure including damage location and quantification for single-member and multimember damages, detection of the various severities of damage, and the effect of the number of sensors is performed to verify the effectiveness of the proposed EEISM method. It is shown from the analysis results that the EEISM method is effective in the location and quantification of damages for single-member and multimember damages. The quantity of the strain sensors has an effect on the damage location and has no remarkable effect on the damage quantification for the determined damage members.

The linked two-dimensional hydrodynamic and sediment-scour models have been enhanced to simulate tsunami wave run-up on a sloping beach to determine its effects on sediment scour around a cylinder pier located in the wave breaking and run-up zone. To resolve the steep wavefront of the tsunami bore, the kinetic flux vector splitting scheme was adopted to solve the hydrodynamic model equations in the unstructured triangular mesh. The models have been validated by comparing model simulations with experimental data. The results indicate that the model predictions of water surface elevations and velocity match well with the measured data. The error for peak flood elevation ranges from 0.01 to 0.11 m, and the maximum error for the peak velocity is 6%. The model simulations adequately characterize the tsunami wave propagations and transformations as the wave approaches the beach from offshore, especially for the sharp tsunami front before it breaks and the tsunami bore runs up in the beach slope. The model simulations also reasonably describe the dynamics of the sediment scour around a cylinder pier, showing the sediment scour during wave run-up and sediment deposition during wave rundown. The Model predictions of the final scour depths after the wave impact at three measurement stations reasonably matched the experimental measurements.

This paper presents a framework of the digital generation of non-Gaussian spiky excitations. This study is focused on the random spikiness, featuring large excursions with considerable energy and monotonic (nonstochastic) variations in a local time history. A first-order non-Gaussian stochastic time series model and its spectral representation are employed for the local spiky features. The stochastic model generates not only autocorrelation properties but also a unique shape of peaks formed with random spikes and monotonic variations between spikes. The Fourier representation of the stochastic model enables an effective control of the peaks and provides a filtering operation for the local feature generation in the frame of stationary stochastic process. Several spectral models with stochastic or ensemble-averaged amplitudes and four added phase functions have been developed. Thus, the phase is different from the uncorrelated uniform phases in a conventional spectral method. The essential feature of the method is to utilize correlations in the structured phase that are responsible for the spikiness. A four-parameter system is developed that is capable of generating spiky features while simulating specified power spectra and higher-order moments. A simple procedure for the selection of phase parameter values by a graphical method is described with illustrations of surface pressure simulation.

Dam safety and flood risk management programs are dependent on damage predictions that are difficult to validate and subject to considerable uncertainty. The 1963 Baldwin Hills dam-break flood caused high-velocity flows exceeding 5 m/s and structural failure of 41 wood-framed residences built in the mid-1940s, 16 of which were completely washed out. The flood is revisited here to examine the predictive skill and variability of established structural damage models when coupled with a hydraulic flood model that predicts parcel-scale depths and velocities. Two-way coupling is introduced so that predictions of structural failure affect localized flood predictions, which in turn affects damage predictions, in contrast to one-way coupling where structural failure has no impact on flood predictions. Two damage states defined by structural failure (Level 2) and washout (Level 3) are considered, along with 10 different structural damage models. One damage model considers flood depth alone, while the remaining nine consider a combination of depth and velocity defined by a constant discharge, energy, or force. Two-way coupling is shown to yield predictions with $\sim$30% higher skill and 10% higher false alarms than one-way coupled models. Hence, there is a tradeoff between skill and false alarms that favors two-way coupling. Predictive skill is also shown to be sensitive to the structural damage classification and the damage model. Depth-based damage predictions yield low predictive skill as expected; however, across the nine velocity-based damage models, skill varies from 50 to 78% for structural failure and from 79 to 95% for washout. These results reveal a similar level of predictive uncertainty from the hydraulic model implementation, damage model, and damage classification. Results also point to flow force as a good predictor of both moderate (Level 2) and severe (Level 3) levels of damage. Through calibration, force thresholds of 0.75 and 9.5 m³/s² are found to maximize model skill at 85 and 95% for Level 2 and 3 damage, respectively, and these values compare well with previously published thresholds for similar building types. Finally, the results reveal a model bias toward a relatively wide damage zone compared with the observations; therefore, the high skill predictions are accompanied by a high rate of false alarms.

This paper presents a theoretical investigation on the energy dissipation in a nearly saturated poroviscoelastic soil column under quasi-static compressional excitations. Different components of the energy dissipation are evaluated and compared. The magnitude of fluid- induced energy dissipation is primarily a function of a normalized excitation frequency $\Omega$. For small values of $\Omega$, a drained soil column is fully relaxed and essentially behaves as a dry column with negligible pore pressure. For such a soil column, fluid-induced energy dissipation is negligible, and the total damping ratio of the column is essentially the same as that of the solid skeleton. For very high values of $\Omega$, a drained soil column is fully loaded and the excitation-generated pore pressure decreases as the fluid becomes more compressible. For such a soil column, the fluid pressure gradient only exists in a thin boundary layer near the drainage boundary, where drainage occurs and fluid induces energy dissipation, whereas the rest of the column is essentially undrained. Significant fluid-induced energy dissipation occurs for moderate values of $\Omega$ because of a combination of moderate fluid pressure, pressure gradient, and fluid relative motion throughout the soil column. The effects of the boundary drainage condition, saturation, porosity, and skeleton damping ratio on fluid-induced energy dissipation are discussed.

Pedestrian loads depend on the regularity and frequency of the footfall process. Traditionally, pedestrian loads have been represented by one or more specific harmonic components with a well-defined frequency, and light footbridges have been investigated for resonance vibration generated by the harmonic components. Measurements indicate that the footfall frequency of a group of pedestrians has a coefficient of variation of the order 0.05–0.1. This is considerably larger than the response bandwidth of a typical lightly damped structure, and at resonance this has a significant influence on the magnitude of the resulting response. A frequency representation of vertical pedestrian load is developed, and a compact explicit formula is developed for the magnitude of the resulting response, in terms of the damping ratio of the structure, the bandwidth of the pedestrian load, and the mean footfall frequency. The accuracy of the formula is verified by a statistical moment analysis using the Lyapunov equations.

Numerous experiments have shown that contractile cells like fibroblasts adapt their internal structure to their microenvironment by generating and orienting a network of stress fibers (SFs). This phenomenon has been modeled in previous studies with stability analysis through calculation of the fiber’s potential or strain energy, where SFs are assigned a constant elasticity. Recent experiments have shown that the elasticity in SFs is rate dependent, resulting in a different stress fiber organization under constant or cyclic stretching. Here, a thermodynamical model that describes the anisotropic polymerization of the contractile units into SFs via the calculation of the mechanochemical potential of the two constituents is proposed. The stretch-dependent part of the SF potential is made of two terms that describe the passive and active behavior of the SF. In this paper, it is shown that the contributions of these two terms vary widely under constant or cyclic stretching as the SFs exhibit a rate-dependent elasticity and lead to two very different anisotropic SF organizations. It is further demonstrated that the substrate stiffness as well as its Poisson’s ratio and anisotropy play a crucial role in the formation and organization of the SFs, consistent with what has been observed in various experiments.

A new formulation to investigate the seismic response of symmetric and asymmetric rectangular liquid-containing structures is developed and validated in this paper. The proposed method is based on a substructuring approach, where the flexible liquid-containing structure is modeled using finite elements, while the impulsive effects of the fluid domain are modeled analytically through interaction forces at the fluid-structure interfaces. The technique takes account of geometrical or material asymmetry of the liquid-containing structure, fluid compressibility, and energy dissipation through reservoir-bottom absorption. The formulation is presented in such a way that it can be easily coded into a practical and computationally efficient program and is applied to illustrative examples highlighting the effects of geometrical and material asymmetry on the dynamic responses of liquid-containing structures. The obtained frequency- and time-domain results are successfully validated against advanced finite-element analyses.