SUMMARYShrink-swell soils can cause distresses in buildings, and every year, the economic loss associated with this problem is huge. This paper presents a comprehensive system for simulating the soil-foundation-building system and its response to daily weather conditions. Weather data include rainfall, solar radiation, air temperature, relative humidity, and wind speed, all of which are readily available from a local weather station or the Internet. These data are used to determine simulation flux boundary conditions. Different methods are proposed to simulate different boundary conditions: bare soil, trees, and vegetation. A coupled hydromechanical stress analysis is used to simulate the volume change of shrink-swell soils due to both mechanical stress and water content variations. Coupled hydro-mechanical stress-jointed elements are used to simulate the interaction between the soil and the slab, and general shell elements are used to simulate structural behavior. All the models are combined into one finite element program to predict the entire system's behavior. This paper first described the theory for the simulations. A site in Arlington, Texas, is then selected to demonstrate the application of the proposed system. Simulation results are shown, and a comparison between measured and predicted movements for four footings in Arlington, Texas, over a 2-year period is presented. Finally, a threedimensional simulation is made for a virtual residential building on shrink-swell soils to identify the influence of various factors.
High quality semipolar (1122) AlN films have been grown on (1010) m-plane sapphire substrates with the help of dual moderate-temperature-grown (MTG) AlN interlayers by using metal-organic chemical vapor deposition technology. The layer thickness of the semipolar (1122) AlN film was determined by employing relative optical transmittance spectrum measured with ultraviolet-visible spectrophotometer. The effect of the insertion of 80 nm-thick MTG AlN interlayer on structural and optical properties was investigated in detail based on the characterization results of the atomic force microscopy, high-resolution X-ray diffraction, and Raman spectroscopy. Comparing with the semipolar (1122) AlN film grown without the MTG AlN interlayer, both the surface morphology and crystalline quality of the semipolar (1122) AlN film grown with the insertion of dual 80 nm-thick MTG AlN interlayers have been improved significantly. In fact, the root mean square value of the surface roughness decreased from 3.5 to 1.4 nm, and the full width at half maximum value of X-ray rocking curve decreased from 1667 to 1174 arcsec, respectively. These facts reveal that the insertion of the dual MTG AlN interlayers is a powerful method to improve the surface morphology and crystalline quality of the semipolar (1122) AlN films owing to the formation of nanoscale patterned substrate-like structure and its blocking effect on the propagation of the dislocations.
N-polar AlGaN epi-layer was realized on AlN seeding layer grown with a novel flow-modulation method. The polarity reversion from Al(Ga)-polar to N-polar for AlGaN/AlN films was confirmed by KOH etching and subsequent observation with optical microscope as well as high-resolution X-ray diffraction (HR-XRD) measurement. In particular, the dependence of crystalline quality and defect size in the N-polar AlGaN epi-layers on the Ⅴ/Ⅲ ratio was investigated with HR-XRD and scanning electron microscopy (SEM). It was found that as the full width at half maximum (FWHM) value of the X-ray rocking curve (XRC) varied with the Ⅴ/Ⅲ ratio in a "W-shape" for the N-polar AlGaN epi-layers, and a FWHM value as small as 450 arcsec was achieved for the sample grown with a V/III ratio of 988. Moreover, it was revealed by the SEM measurement that the maximum diagonal length of hexagonal cone on the surface of the N-polar AlGaN epi-layers decreased sharply when a Ⅴ/Ⅲ ratio of 1,236 was used although the crystalline quality and the surface morphology of the N-polar AlGaN epi-layers were not improved simultaneously. The peculiar migration of the group-III atoms on the surface of the N-polar AlGaN epi-layer associated with the molar ratio of TMA/(TMA+TMG) was considered to be 2 responsible for this result.
Twelve lightning flashes are successfully triggered under the positive atmospheric electric field condition. The discharge properties of the flashes, and the propagation characteristics and mechanism of the involving upward negative leaders are investigated. When lightning flashes are triggered, the average ground atmospheric electric field is around 5 kV/m, with a maximum value exceeding 13 kV/m. Except for one special event showing a discharge polarity reversal (from positive to negative) and producing multiple negative return strokes, none of the remaining 11 triggered lightning flashes involves the subsequent return stroke process. The discharge currents of these flashes are generally of the order of several hundred amperes. The successfully triggered lightning flashes start with the initiation and the upward propagation of negative stepped leaders, of which the average two-dimensional velocity is 1.85 × 10<sup>5 </sup>m/s. For a total of 132 steps captured by the high-speed video camera, the step lengths range from 0.8 m to 8.7 m, with an average of 3.9 m. During the initial stage of the upward negative stepped leader, the current and electromagnetic field present a significant impulsive feature. The mean value of pulse interval, current peak, charge transfer, half-peak-width and current rise time <i>T</i><sub>10%–90%</sub> are 17.9 μs, 81A, 364 μC, 3.1 μs, and 0.9 μs, respectively. The equivalent linear charge density of a single step is 118.5 μC/m. The branching of the leader channel generally takes place together with the stepping process in two ways: the first way is to implement the multiple connections of clustering space stems/space leaders to the leader head within an individual step cycle, and the corresponding current waveform presents a multi-peak structure, with a peak interval of about 2–3 μs (up to 6–7 μs); the second way is to reactivate those previously extinguished space stems/space leaders and to connect them to the lateral surface of the channel.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.