Ronald S. Harichandran

Estimating the spatial variation of earthquake ground motion from dense array recordings

Abstract A techniques for estimating and characterizing the spatial variation of earthquake ground motion based on recordings obtained from dense seismograph arrays is outlined. Critical steps in the estimation procedure are emphasized. Erroneous estimates may be obtained if special care is not taken in the estimation. The proposed estimation technique is validated by using it on simulated accelerograms whose spatial variability is known. Finally, the technique isused to estimate the parameters of a random field model of earthquake ground motion, for two far-field events recorded by the smart -1 seismograph array in Lotung, Taiwan.

Dynamic Time Domain Backcalculation of Layer Moduli, Damping, and Thicknesses in Flexible Pavements

A dynamic time domain method was developed to backcalculate the layer moduli, damping ratios, and thicknesses of asphalt pavements from dynamic falling weight deflectometer test data. The method uses the SAPSI program as a forward routine and a Newton-Raphson method for the backcalculation. The advantage of the time domain approach is that SAPSI can match selected features of the measured time histories directly, and inaccurate measurements at the ends of histories can be ignored. The peak deflections and the time lag between the peak of the load and the peak of the deflections at different sensors are matched. The Newton algorithm in MICHBACK is adopted to perform the backcalculation. The new algorithm is capable of backcalculating the moduli, damping ratios, and thicknesses of three-layer flexible pavements reasonably well. Backcalculations based on synthetic time histories generated using SAPSI show excellent stability and accuracy. Backcalculation using measured time histories yields less accurate results. Possible sources for the discrepancies when using field data are addressed.

Current Efficiency in Accelerated Corrosion Testing of Concrete

The suitability of accelerated corrosion testing using an impressed current, especially the accuracy of Faradays law to estimate the mass loss, was assessed in this paper. A variety of tests were performed in concrete and solution with different chloride concentrations and impressed current to assess the accuracy of using Faradays law to estimate the mass loss of corroded steel. The results indicate that Faradays law does not predict the mass loss accurately for all conditions, and the percentage error increases with decreasing chloride concentration in the system. It is hypothesized that Faradays law cannot be used to estimate the mass loss because competing reactions take place at the anode, especially at low chloride concentrations and high current. Under unfavorable conditions, only a part of the impressed current oxidizes the anode to create corrosion products while the remaining current is consumed in competing reactions.

Capacity Reduction and Fire Load Factors for Design of Steel Members Exposed to Fire

A general reliability-based methodology is proposed for developing capacity reduction and fire load factors for design of steel members exposed to fire. The effect of active fire protection systems (e.g., sprinklers, smoke and heat detectors, fire brigade, etc.) in reducing the probability of occurrence of a severe fire is included. The design parameters that significantly affect the fire design of steel members are chosen as random variables. Raw experimental data published in the literature was analyzed to obtain the statistics of parameters for which no statistical information was available in the literature. Model errors associated with the thermal analysis models are also characterized based on experimental data. It is found that uncertainty associated with the fire design parameters is significantly higher than that of room temperature design parameters. To illustrate the proposed methodology, capacity reduction and fire load factors are developed for simply supported steel beams in U.S. office buildings, and it is shown that for consistent reliability these factors should vary depending on the presence of active fire protection systems in a building.

Random vibration of laminated composite plates with material non-linearity

Structural components in space vehicles, aircraft, automobiles, etc., that are made of filamentary composite laminae are usually subjected to stochastic loads. Composite laminae have strongly anisotropic properties and display significantly non-linear behavior when loaded in shear or along directions different from those of the filaments. The method of equivalent linearization is used in conjunction with the finite element method to perform non-linear random vibration analysis of laminated composite plates. An approximate, but sufficiently accurate, series representation of the non-linear shear stress-strain law is used to facilitate the formulation. Kirchhoff plate theory is used and hence transverse shear deformation is neglected. The random vibration analysis was found to be significantly more cost-effective than deterministic simulation.

Effect of Axle Load Spectrum Characteristics on Flexible Pavement Performance

The Mechanistic–Empirical Pavement Design Guide (MEPDG) uses performance models to predict cracking and rutting in flexible pavements. A unique mechanism controls the initiation and accumulation of each distress, but each mechanism can have several causes. Axle repetitions and loads are the main causes of all load-related distress types. MEPDG incorporates axle load spectra to characterize axle loading for a site and uses them to calculate pavement response and damage accumulation. These load distributions have a bimodal shape, and a mixture of two continuous distributions can be used to model them. In this paper, closed-form solutions are developed to estimate the characteristics of a mixture of bimodal axle load distributions. The observed axle load spectra from 14 sites in different states were used to relate load distribution characteristics to predicted flexible pavement performance. The overall mean and other characteristics of a bimodal axle load distribution explained the variations in expected flexible pavement performance. Cracking, surface rutting, and ride quality are related to the fourth root of the fourth moment of axle load distributions. Rutting in the hot-mix asphalt layer is strongly associated with the overall mean, but in base and subbase layers it is related to the 95th percentile load of axle load spectra. These findings imply that cracking, rutting, and roughness growth in flexible pavements are caused mainly by axle load distributions having heavier tails with infrequent extreme loads. Heavier loads appear to cause more cracking; a higher number of load repetitions is more critical in developing additional surface rutting in flexible pavements.

Effect of axle load measurement errors on pavement performance and design reliability

In traffic characterization, axle load spectra (ALS) are one of the most critical inputs in the new Mechanistic–Empirical Pavement Design Guide (MEPDG). ALS have a significant impact on the predicted pavement performance. At the design stage, it is typically assumed that ALS as measured by weigh-in-motion (WIM) systems have adequate data quality and accuracy. In fact, the quality of WIM-based data has inherent uncertainties because of inaccuracy and systematic bias. While WIM data accuracy depends on the sensor technology, calibration errors and drift over time may introduce a systematic bias. Several studies have investigated the impact of traffic data collection technologies, data coverage, accuracy, and calibration errors on pavement loading and performance prediction. However, these studies were limited to a few distress measures and did not address design reliability aspects as considered in the MEPDG. This study investigated the impact of probable WIM errors on the ALS and quantified the effects of these errors on the performance of both flexible and rigid pavements. Furthermore, the impact of uncertainties in ALS on design reliabilities is discussed in this paper. Although most findings reinforce existing concepts, the study provides a systematic overview of WIM data accuracy and calibration requirements, and the effect of associated uncertainties in the pavement design process. The results show that cracking in both flexible and rigid pavements is the distress most affected by ALS variations, while rutting in flexible pavements is moderately affected.

Improved Repair of Concrete Structures Using Polymer Concrete Patch and FRP Overlay

Concrete structures that are damaged due to cracking and spalling, associated with corrosion of reinforcing bars, are often repaired using shallow depth surface patches. Cracking and full or partial delamination of the patch repairs due to shrinkage and continued corrosion is generally unavoidable. The repair typically lasts only for a few years in corrosive environments associated with coastal regions or the use of deicing salts. To provide more durable patch repairs, the use of a fiber-reinforced polymer (FRP) fabric applied as an overlay on top of a traditional polymer concrete patching material was investigated. The FRP overlay can serve as a secondary reinforcement and act as a barrier against the diffusion of moisture and chloride ions, thereby improving the performance of the patch by reducing cracking and slowing down the corrosion process. Two-dimensional finite-element analysis was used to select a suitable FRP material and configuration for the overlay. The effectiveness of the proposed improved repair method was assessed using accelerated corrosion testing of rectangular concrete prism specimens with a polymer concrete patch and an FRP overlay. Reinforcement mass loss and expansive strains in concrete due to corrosion were measured on specimens with and without the FRP overlay. The measurements indicated reductions in concrete strains, crack propagation, and corrosion level when the FRP overlay was used. Three-dimensional finite-element analysis was conducted to understand the influence of using an FRP overlay on the concrete stress distribution and the arrest of cracking.

Impact of Systematic Axle Load Measurement Error on Pavement Design Using Mechanistic-Empirical Pavement Design Guide

In traffic characterization, axle load spectra (ALS) are one of the most critical inputs in the new Mechanistic-Empirical Pavement Design Guide (MEPDG). Axle load spectra have a significant effect on predicted pavement performance and, thus, the design life. Typically, axle load spectra as measured by weigh-in-motion (WIM) systems are assumed to have adequate data quality and accuracy. In fact, the quality of WIM-based data has inherent uncertainties attributable to inaccuracy and systematic bias. Whereas WIM data accuracy depends on the sensor technology, calibration errors and drift over time may introduce a systematic bias. This technical note investigates the effect of axle load measurement bias on pavement design for flexible and rigid pavements. The results show that negative bias in axle load measurements significantly affects cracking performance for both pavement types. The bias is more critical for rigid pavements with thinner slabs. Therefore, a measurement bias limit of less than ±5% should be...

Closed-Form Solutions for Bimodal Axle Load Spectra and Relative Pavement Damage Estimation

The mechanistic-empirical (ME) design procedures utilize axle load spectra to characterize the individual traffic loadings for a site. These loading characteristics are employed to calculate pavement response and for subsequent damage computations. Generally, these axle load distributions exhibit a bimodal shape and a combination of two continuous statistical distributions can be used to model them. In this paper, closed-form solutions are developed to estimate the parameters of the bimodal distribution from data. A combination of two normal distributions is shown to reasonably fit observed axle spectra. Since it is anticipated that the AASHTO equivalent single-axle load (ESAL) concept will continue to be used by pavement engineers even after the full adoption of ME design methods, a closed-form statistical relationship between ESALs and axle load spectra is proposed. Such a relationship will be useful in estimating a traffic level index from an axle distribution. In addition, the relationship can provide an estimate of the relative pavement damage caused by axle distributions, and be used to rank axle load spectra within a geographical region, or between regions in order to identify heavier traffic loading corridors.