Bridges and buildings are often supported on deep foundations. These foundations consist of groups of piles coupled together by concrete pile caps. These pile caps, which are often massive and deeply buried, would be expected to provide significant resistance to lateral loads. However, practical procedures for computing the resistance of pile caps to lateral loads have not been developed, and, for this reason, cap resistance is usually ignored. Neglecting cap resistance results in estimates of pile group deflections and bending moments under load that may exceed the actual deflections and bending moments by 100 % or more. Advances could be realized in the design of economical pile-supported foundations, and their behavior more accurately predicted, if the cap resistance can be accurately assessed. This research provides a means of assessing and quantifying many important aspects of pile group and pile cap behavior under lateral loads. The program of work performed in this study includes developing a full-scale field test facility, conducting approximately 30 lateral load tests on pile groups and pile caps, performing laboratory geotechnical tests on natural soils obtained from the site and on imported backfill materials, and performing analytical studies. A detailed literature review was also conducted to assess the current state of practice in the area of laterally loaded pile groups. A method called the "group-equivalent pile" approach (abbreviated GEP) was developed for creating analytical models of pile groups and pile caps that are compatible with established approaches for analyzing single laterally loaded piles. A method for calculating pile cap resistance-deflection curves (p-y curves) was developed during this study, and has been programmed in the spreadsheet called PYCAP. A practical, rational, and systematic procedure was developed for assessing and quantifying the lateral resistance that pile caps provide to pile groups. Comparisons between measured and calculated load-deflection responses indicate that the analytical approach developed in this study is conservative, reasonably accurate, and suitable for use in design. The results of this research are expected to improve the current state of knowledge and practice regarding pile group and pile cap behavior. / Ph. D.

In north Mississippi, I evaluated fuel loads in the late dormant season and after leaf fall in unburned areas and after a dormant season burn. Fuel loads beneath winged elm (Ulmus alata) were assessed to determine if this shade-tolerant species impacts fine fuel loads in the area immediately near its bole. I found leaf litter fuel loads are higher after leaf fall suggesting that burns conducted closer to leaf fall may have increased burn intensity. Burning reduced leaf litter fuels and exposed mineral soil which returned to pre-burn levels following leaf fall. Declines in duff layer fuels were not evident until after leaf fall. Leaf litter fuels underneath winged elms had higher mass and percentage of winged elm litter beneath them compared to areas away from them. These results will help determine appropriate time for restoring prescribed fire and the implications of encroachment by non-oak species into upland oak forests.

No description available.

Unsteady large-scale vortices, formed by the roll-up of free shear layers separating along sharp edges, are the dominant flow characteristics of the turbulent flow over buildings. These vortical structures interact with each other and with the building surface resulting in secondary separation and severe pressure fluctuations. Moreover, the interaction of the large-scale vortices with the multiplicity of turbulence scales in the incoming wind exacerbates their unsteady motion and hence significantly affects the pressure fluctuations spectra experienced by the building. Large-eddy simulations are conducted to study the interaction of homogeneous turbulence in the incident flow with a surface-mounted prism. A compact fifth-order upwind difference scheme is used to effectively and accurately perform the simulations. Three cases of incident flow are considered. In one case, the prism is placed in a smooth uniform flow. In the second case, homogeneous isotropic turbulence with von Karman spectrum is superimposed on the uniform flow at the inflow boundary. The integral length scale is one-half the prism height. In the third case, the integral length scale is equal to the prism height. The numerical results are compared with experimental measurements reported by Tieleman et al. (2002). The results show that the highest negative mean value of the pressure coefficient on the roof and the sides is about 30% larger in case two of turbulent inflow and takes place closer to the windward edge of the prism. Moreover, the pressure coefficients on the roof and sides of the prism in the case of turbulent inflow show a higher level of variations in comparison with the case of smooth inflow conditions. The predicted mean characteristics of the pressure coefficients in the turbulent case match the experimental values in terms of both magnitude and location on the roof of the prism reported in Tieleman et al. (1998) and Tieleman et al. (2002). As for the peak value, the peak value of -2 obtained in the turbulent inflow case two is about 20% smaller than the values measured experimentally by Tieleman et al. (2002). On the other hand, it is stressed that the peak value in the simulations would increase as the duration of the simulation is increased to match that of the experimental measurement. The results also show that the turbulent case yields a non-exceedence probability for the peak pressure coefficient that is closer to the one obtained from the measured data than the smooth case data. Also, spectral and cross-spectral analysis are carried out using complex Morlet wavelet transform to investigate pressure-velocity relation. The study shows that the nonlinearity in the relationship of velocity-pressure is detected using wavelet bicoherence. / Ph. D.

The primary purpose of this dissertation is to investigate the stability of reticulated domes under multiple static and dynamic loads. Two elastic geometrically nonlinear structural models of a reticulated dome with 21 and 39 degrees of freedom are considered. The nonlinear response of the system to static loads is obtained using nonlinear programming and discrete perturbation techniques. The nonlinear programming technique is used to obtain a starting solution for the discrete perturbation technique and to optimize the choice of the perturbation parameter. Convergence criteria and error estimates to limit errors in a perturbation scheme are developed. A method for selecting a "suitable" perturbation parameter for imperfection sensitive systems is proposed. The investigation of stability of equilibrium of the system subjected to finite disturbances is based on the concept of "degree of stability" and the associated sufficient stability condition. The stability condition is derived from a theorem on extent of asymptotic stability of Liapunov's direct method of the theory of stability of motion. Its application requires the determination of the nonlinear fundamental path and the "nearest" unstable post-buckling path. This is obtained via static analysis. The perturbed motion of the system under a given set of perturbations is obtained by numerically integrating the nonlinear equations of motion. The dynamic stability tests confirm the sufficiency of the dynamic stability condition. However, they also indicate that there is a dynamic disturbance with a specific spatial distribution for which the sufficient condition of stability is also a necessary condition for each equilibrium state tested. Since in practice, the spatial distribution of the disturbances cannot be controlled, the sufficient dynamic stability condition employed is practical for the design of reticulated domes. The stability boundaries corresponding to two independent loads on the models are presented. Limit points lie on a boundary which is convex towards the region of stability. Bifurcation points lie on a continuous but piecewise differentiable boundary. Each piece of the boundary containing bifurcation points appears to be convex towards the region of stability. / Ph. D.

A three-dimensional finite element code to analyze coupled thermomechanical deformations of composites has been developed. It incorporates geometric nonlinearities, delamination between adjoining layers, and damage due to fiber breakage, fiber/matrix debonding, and matrix cracking. The three damage modes are modeled using the theory of internal variables and the delamination by postulating a failure envelope in terms of the transverse stresses; the damage degrades elastic moduli. The delamination of adjoining layers is simulated by the nodal release technique. Coupled nonlinear partial differential equations governing deformations of a composite, and the pertinent initial and boundary conditions are first reduced to coupled ordinary differential equations (ODEs) by the Galerkin method. These are integrated with respect to time with the Livermore solver for ODEs. After each time step, the damage in an element is computed, and material properties modified. The code has been used to analyze several static and transient problems; computed results have been found to compare well with the corresponding test results. The effect of various factors such as the fiber orientation, ply stacking sequence, and laminate thickness on composite's resistance to shock loads induced by underwater explosions has been delineated. / Ph. D.

An analytical modal response tool was developed to investigate the characteristics of and to estimate static and dynamic launch vehicle responses to ground wind loads (GWL). The motivation of this study was to estimate the magnitude of response of the Ares I-X launch vehicle to ground winds and wind-induced oscillation (WIO) during roll-out and on the pad. This method can be extended to other launch vehicle designs or structures that possess a nearly cylindrical cross-section. Presented in this thesis is an overview of the theory used, a comparison of the theory with wind tunnel data, further investigation of the data to support the assumptions used within the analysis, and a prediction of the full-scale Ares I-X response. Additionally, an analytical investigation is presented that estimates the effect of atmospheric turbulence on WIO response. Most of the wind tunnel data presented in this report is taken from the GWL Checkout Model tested in the NASA Langley Transonic Dynamics Tunnel (TDT) in April 2007. The objective of the GWL Checkout Model was to reestablish and evaluate the capability of the facility to conduct GWL testing and to operate the associated equipment. This wind tunnel test was not necessarily intended to predict the full scale Ares vehicle response to GWL; however, it can be used to help validate the newly developed analytical method described in this thesis. A detailed GWL test incorporating updated vehicle designs and launch pad configurations of the Ares I-X flight test vehicle was also conducted in the TDT during the fall of 2008. This test provides more accurate predictions of the second bending mode response of the Ares I-X, and it models effects of the nearby tower and support structures. The proposed analytical method is also compared to select data from the Ares I-X GWL test; however, it is presented as normalized values to protect the sensitivity of the data. Results of the proposed analytical method show reasonable correlation to wind tunnel data. Also, this method was the first to determine that second bending mode WIO response was not only possible for the Ares I-X, but will also produce the most critical loads. Finally, an explanation is offered in this thesis regarding discrepancies between wind tunnel and full-scale WIO response data. / Master of Science

This study evaluates the dynamic interaction between rail vehicles and the track on which they travel. The measurements were analyzed in the time and frequency domain. The effects of external parameters such as the vehicle design, subgrade stiffness, and degree of curvature were analyzed and compared for both the vehicle response and the track geometry data. This study was conducted in close cooperation with the Transportation Technology Center, Inc. (TTCI) in Pueblo, Colorado. The track and vehicle response data was collected on the High Tonnage Loop at TTCI over a span of approximately ten years. For the purpose of this study, TTCI provided this data to the Advanced Vehicle Dynamics Laboratories (AVDL) at Virginia Tech. The analysis of the data was first completed for the vehicle response and track geometry measurements separately. In this manner, the effects of the subgrade stiffness, the degree of curvature, and the vehicle design could be evaluated for each of the measurements. The analysis of the track and rail vehicle response was successfully completed, and the results from this analysis are presented in the following chapters. The analysis showed that the subgrade stiffness had a significant effect on the vertical wheel loads, the track crosslevel, and the track gauge. For an increase in the degree of curvature, the variation of the crosslevel increased. The dynamic response of the vertical and lateral wheel loads showed a significant dependence on the alignment of the rails. Improvements for future studies of the interaction between the track and rail vehicles have been recommended. / Master of Science

Energy is lost when a rope transfers from a slack state to a taut state. This transfer is called a snap load and can be very violent. It is proposed to use synthetic fiber ropes as a type of passive control device in new or existing structures to mitigate seismic response. Experimental static and snap load (dynamic) tests were conducted on various synthetic fiber ropes. An eleven-foot-tall drop tower was built in the Virginia Tech Structures and Materials Laboratory in order to conduct these tests. Force and acceleration of the drop plate, which slides vertically within the drop tower, were measured with respect to time for all dynamic tests. Acceleration data was integrated using the trapezoidal or midpoint rule to obtain velocity and displacement values. Plots were made for each test in order to give a better representation of the results. These plots include representations of force and acceleration vs. time, force vs. absolute displacement, force vs. velocity, and force, acceleration, velocity, and displacement vs. time (during the initial taut phase only). Test results show that energy was dissipated in all of the dynamic drop tests, which was expected. Also, the displacement of each rope did not return to zero at the same time that the force returned to zero after the initial snap load. This proves that the ropes undergo some permanent elongation under load. The stiffness of each rope increased with continuous testing. As more tests are conducted on each rope, the strands are pulled tighter into the braided configuration, which causes the rope to become stiffer. / Master of Science

The purpose of this study is to develop a simple one-dimensional model to analyze axially loaded beam-plates containing cracks which extend through the thickness of the beam-plates. Although the material analyzed is isotropic, these cracks will be referred to as delaminations. Buckling, postbuckling, and growth of delaminations in these beam-plates will be analyzed. A finite element method in which all of the terms of the stiffness matrices are obtained by exact integration is employed to determine the linear buckling load and postbuckling solution. The energy release rate is then determined using the postbuckling solution. Curves are provided to show the effect of delamination length and location on buckling loads, energy release rates, and strengths of the beam-plates. The problem of buckling and postbuckling of beams with multiple delaminations is also considered. A method of calculating the energy release rate for beams with multiple delaminations using numerical differentiation is introduced. / Master of Science