Adjacent Bridge Research Articles

Magnetooptical investigations of large transport currents in Y-Ba-Cu-O-thin films

Inhomogeneities in large area YBCO-thin films can be studied in external magnetic fields with a specially designed magnetooptical apparatus. Moreover, an investigation of transport current induced magnetic fields in patterned HTS-thin films is possible as well. In the present work we studied the flux penetration of the current induced fields into YBCO-bridges of 8/spl times/0.7 cm/sup 2/ dimensions as a function of the applied currents at 77 K. For the first time transport currents up to I/sub tr//spl sim/60 A could be investigated magnetooptically in HTS-thin films. Flux penetration starts just above a threshold value of the applied current I/sub tr//I/sub c//spl ges/0.3. The influence of the bridge geometry and of inhomogeneities on flux penetration has been investigated. Flux and therefore current distributions change drastically with increasing bend-angle of the YBCO-path. A magnetic interaction between adjacent bridges with parallel and antiparallel current directions as well as a thermal interaction could be observed.

Design of Seismic Cable Hinge Restrainers for Bridges

The collapse of bridges caused by unseating at intermediate (in-span) hinges during recent earthquakes emphasizes the need to provide an adequate number of restrainers to limit the relative displacement between frames in a bridge with inadequate hinge seat width. Current restrainer design procedures do not account for the factors affecting the earthquake response of multiple-frame bridges. A parameter study indicates that maximum hinge displacement is a function of the frame period ratio, frame ductility, and the characteristics of the ground motion. A new design procedure for hinge restrainers, based on a linearized model, is developed. The procedure accounts for the dynamic characteristics out-of-phase motion of adjacent bridge frames. Inelastic behavior of frames is accounted for by using the substitute structure method. Parameter studies and case studies using over 26 different ground motion records show that the new procedure limits and the relative hinge displacement to a designer-specified value for a wide range of bridges.

Use of High-Performance Concrete for Bridge Abutments: A Case Study

As part of an FHWA program to use high-performance concrete (HPC) in bridges, the state of Ohio constructed an adjacent box girder HPC bridge. HPC was used both for the prestressed and precast girders and for the cast-in-place abutments. The abutment mix was primarily designed for durability by using a low water-to-cementitious material ratio and pozzolanic materials. Before construction, laboratory trial mixes were run by using the actual material that was to be used in the abutments. After verifying the suitability of the mix in the laboratory, a few large trial batches were made at the ready-mix concrete plant to verify the plastic and hardened properties of the mix. During construction of the bridge, several problems occurred. Just before construction began, the coarse aggregate failed the state inspection and a new source of coarse aggregate had to be found. Unfortunately, HPC is sensitive to changes in aggregate, but there was no time to create trial batches before construction began. As a result, the mix had to be adjusted during construction. Soon after the initial abutment pours were made, differential shrinkage cracking was found. This was due to problems with inadequate curing. After a few tries, the mix problems were corrected and proper curing eliminated most of the cracking. Specific recommendations for quality control are made on the basis of the experience gained.

Full-Scale Testing of Shear Keys for Adjacent Box Girder Bridges

Adjacent precast, prestressed box girder bridges are widely used in the United States, but the shear keys between the girders tend to crack and leak. Three different shear key configurations were studied, i.e., a current detail where the shear key is approximately 10 in. (250 mm) from the top of the girder and grouted with non-shrink grout, this same detail grouted with epoxy rather than non-shrink grout, and a proposed mid-depth keyway grouted with non-shrink grout. The tests were conducted on a full scale, four-beam assembly which represented part of the bridge. The results showed that the currently used shear key detail cracks due to thermal stresses generated as the beams deflect upward and downward due to daily heating and cooling. The mid-depth shear key is less susceptible to these stresses and was found to be more resistant to cracking. Loading did not appear to cause new cracking, but rather seemed to propagate existing thermal cracks.

A STUDY ON SEISMIC INTERACTION BETWEEN LONG-SPAN BRIDGE AND ADJACENT APPROACH BRIDGE

長大橋に中小橋梁が隣接するとき, その質量差や材質の相違により, 設計で考慮しなかった外力が中小橋に作用する可能性がある. 特に, 振動単位ごとに固有周期が著しく異なる場合, その連結部には大きな相対変位が生じ, 可動支承の移動制限装置の存在によって予期できない荷重の作用が想定される. 本研究では, 湾岸部の連続高架橋などでよく見られる, 軟弱地盤に支持された長大橋とそのアプローチ橋を振動単位としてモデル化し, 数値シミュレーションにより, その地震時相互作用について検討した. その結果, 大地震時に桁が大きく変位し, 可動支承が移動制限装置にあたった場合, 構造物全体に大きな荷重が作用することを確認した.

Orientational anisotropy in oxygen dissociation on Rh(110)

Scanning tunneling microscopy study of ${\mathrm{O}}_{2}$ dissociation on Rh(110) at 170 K shows a low-energy dissociation coordinate for a precursor with the O-O axis aligned in the [001] azimuth, indicating a strong influence of the surface anisotropy on the potential-energy surface. This results in pairs of oxygen atoms oriented along [001], which aggregate in chains along $[11\ifmmode\bar\else\textasciimacron\fi{}0].$ The atomic distance in the pairs, 3.3 \AA{}, is shorter than the [001] lattice constant, which led to the conclusion that the oxygen sits in adjacent asymmetric short bridge sites and that the creation of ``hot'' oxygen atoms is not favored.

Reinvestigation of the substituent effects of fluorine on the1H NMR chemical shifts of the adjacent methylene group in fluorinated [2.2]paracyclophanes

Introduction of a fluorine substituent into an aromatic ring of [2.2]paracyclophane causes distinct shielding and deshielding effects at the anti- and syn-protons, respectively, of the adjacent bridge methylene group. A recent contradictory claim in the literature was disproved by 2D H,H-NOESY, long-range H,H-COSY and H,C-HMBC experiments. Copyright © 1999 John Wiley & Sons, Ltd.

Testing of High-Performance Concrete Single-Span Box Girder

As part of a multistate research program on use of high-performance concrete (HPC) in highway bridges, a bridge originally designed as a three-span adjacent box girder bridge was converted to a single-span bridge by using 70-MPa HPC and 15-mm strands. As part of the research, a test beam was constructed and tested. Instruments placed in the beam before casting were used to measure transfer length, which was found to be approximately 1.22 m, larger than the 50-bar diameters usually used in the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications but consistent with recent studies. After the beam concrete reached the required compressive strength, it was tested to destruction. The beam was able to resist the required AASHTO ultimate moment without failure. It was found that the AASHTO cracking load was conservative for this beam, mostly because the measured modulus of rupture greatly exceeded the value assumed in the AASHTO specifications. The behavior of the beam was successfully predicted using a section analysis.

Ab initio HF/DFT studies of the chemisorption of hydrogen on the cluster simulated Si(111)-(√3 × √3)R30°-Al and -Ga surfaces

The chemisorption of atomic hydrogen on the Si(111)-(√3 × √3)R30°-Al and -Ga surfaces has been studied by first-principles all-electron Hartree-Fock/density functional theory (DFT) cluster calculations. It has been found that the hydrogen chemisorption behaviour on the Ga terminated cluster modelled surface is similar to that for the Al terminated cluster. Hydrogen atoms chemisorbing near an adatom site are found to cause the breaking of SiAl (or SiGa) bonds and the formation of HSi bonds. The aluminium (or gallium) adatoms move from their original threefold sites to the adjacent bridge sites (one hydrogen atom), and then to the neighbouring on-top sites (two hydrogen atoms). Calculations of the binding energies have shown that aluminium or gallium adatoms occupying the bridge and on-top sites may be able to migrate on the real surface. This suggests the possible formation of Al (or Ga) islands, together with a local (1 × 1)H surface reconstruction. Further exposure to hydrogen does not cause any further structural transformations and supports the (√3 × √3)-(1 × 1) phase transition.

Ab initio cluster calculations of the chemisorption of hydrogen on the Si(111)√3 × √3R30°-B surface

First principles all-electron Hartree-Fock and density functional theory cluster calculations have been performed to investigate the chemisorption of atomic hydrogen on the Si(111)√3 × √3R30°-B surface. Both the equilibrium geometries corresponding to n hydrogen atoms (1 ≤ n ≤ 5) chemisorbing on the B-S 5 and B-T 4 reconstructed structures, and the desorption energies for a silicon or boron adatom bonded to x hydrogen atoms (0 ≤ x ≤ 3), have been obtained. As successively more hydrogen is chemisorbed, a silicon or boron adatom is found to move from its original threefold site to an adjacent bridge site, and then to a neighbouring on-top site. It is also found that boron will most likely occupy a subsurface substitutional S 5 site at low hydrogen coverages (≤ 0.67ML), but appear as an adatom at an on-top site directly above one of the first-layer silicon atoms for hydrogen exposures higher than 0.67 ML. This boron segregation at high hydrogen exposures prevents the formation of SiH 2 and SiH 3 complexes and leads to the prediction that only SiH and BH 2 will be observed on an Si(111):B hydrogenated surface. It also provides an explanation for the lack of Si surface etching at high hydrogen exposure. All of these results are in good agreement with the available experimental data on hydrogenated B/Si(111) surfaces.

Hartree-Fock-DFT cluster calculations of the Si(111)7 × 7:H system

Ab initio Hartree-Fock-DFT cluster calculations have been performed of the interaction of hydrogen with the Si(111)7 × 7 surface. These calculations have enabled the determination of the various equilibrium structures which result when up to five hydrogen atoms are allowed to adsorb near the adatom and rest atom sites of the Si(111)7 × 7 surface. As progressively more hydrogen atoms are chemisorbed near an adatom site, each hydrogen-bonded adatom is found to move from its original three-fold (T 4) site (one hydrogen atom) to an adjacent bridge site (two and three hydrogen atoms), and then on-top of a neighbouring first layer silicon atom (more than three hydrogen atoms). The lowest calculated adatom desorption energy is found to be 2.40 eV and corresponds to the desorption of SiH 2 from above a neighbouring first layer silicon atom. In contrast to an adatom, a hydrogen-bonded rest atom is found to remain close to its original three-fold equilibrium position. The smallest rest atom desorption energy is 2.10 eV and corresponds to an SiH 3 surface complex.

Boron, hydrogen and silicon adatoms on the Si(111) surface: an ab initio Hartree - Fock/density functional cluster study

Silicon and boron adatoms on the Si(111) surface have been studied using atomic clusters and Hartree - Fock/density functional theory. The equilibrium geometries of the , and structures, and those formed by the chemisorption of a single hydrogen atom onto these structures, have been obtained. The binding energies of the B and Si adatoms, and the resultant SiH and BH complexes, have also been determined. The electronic structures of the clusters have been investigated via a Mulliken population analysis. The calculated results are shown to be in good agreement with both the available LEED and synchrotron x-ray diffraction data, and other simulations. It is found that the binding energy of the Si adatom of the structure is greater than that of the B adatom of the structure by 1.93 eV, and 1.15 eV larger than the binding energy of the Si adatom of the structure. The chemisorption of atomic hydrogen stabilizes the geometry through the formation of a silicon adatom - hydrogen (SiH) complex located at the threefold position. In contrast to this, the Si adatom in the structure, and the B adatom bonded with a chemisorbed hydrogen atom in the structure, are observed to move away from their positions to the adjacent bridge sites.

Nondestructive Determination of Response of Shear Keys to Environmental and Structural Cyclic Loading

Adjacent box girder bridges use grouted shear keys to share loads among beams. These keys tend to crack and leak. A full-scale portion of an adjacent box girder bridge is being used to test the performance of grouted shear keys under environmental and cyclic loads. The pulse velocity method is employed to find cracks in the shear keys. Two separate tests were conducted. In the first, shear keys that were grouted in late autumn cracked soon after casting, before any load had been applied. Data from instruments embedded in the beams and shear keys showed large discontinuities in strain caused by freezing temperatures. The strains were much larger than strains that occurred under loads corresponding to the weight of an HS20-44 truck. The beams were subjected to 41,000 cycles of loading, simulating HS20-44 wheel loads. No new cracking occurred from the loading, but cracks caused by temperature propagated under loads. In the second test, the keys were grouted in summer. Higher temperatures caused by the sun’s heat on the top of the beams again caused large thermal strains, which cracked the shear keys. These keys were subjected to 1,000,000 cycles of load corresponding to an HS20-44 wheel load. As in the first test, the load itself did not cause new cracks but the existing thermal cracks propagated under the load.

An ab initio Hartree-Fock/density functional study of the cluster simulated Si(111)7 × 7:Cl adsorption system

The first-principles all-electron Hartree-Fock/density functional method contained within the Gaussian92/DFT package has been employed to study the chemisorption of atomic chlorine onto small clusters modelling the adatom and restatom sites of the Si(111)7 × 7 surface. Both the equilibrium configurations and the binding energies of the various SiCl x complexes (1 ≤ x ≤ 3) produced by the adsorption of up to six chlorine atoms in the vicinity of an adatom and restatom site, have been determined. As progressively more chlorine atoms are adsorbed near these chemisorption sites, the surface topology is found to reconstruct to yield stable configurations for SiCl, SiCl 2 and SiCl 3. A chlorine bonded adatom is observed to move from its original three-fold T 4 site (one chlorine atom) to an adjacent bridge site (two and three chlorine atoms), and then on-top of a neighbouring first layer silicon atom (more than three chlorine atoms). Similar behaviour is exhibited by the restatom except that movement towards the on-top position occurs when three chlorine atoms are chemisorbed, rather than four as in the adatom case. The lowest binding energies are determined to be 0.74 eV for SiCl 3 desorption from a restatom site and 0.80 eV for the removal of SiCl 2 from an adatom site. Our results also suggest that a relatively low fluence of chlorine should produce a stable chlorosylil layer consisting of SiCl on the restatom sites and SiCl 3 on the adatom sites, and yielding a saturation coverage of 1.35 mL. Increased chlorine exposure should result in further uptake at the restatom sites to produce SiCl 3 complexes at both chemisorption sites and a maximum saturation coverage of 1.84 ML. Both of these predictions appear consistent with experiment.

High exposure hydrogen chemisorption on the surface: a semiempirical cluster study

The various equilibrium structures of the hydrogen chemisorbed surface are investigated using the semiempirical molecular orbital Austin 1 calculational method (AM1). Up to five hydrogen atoms are allowed to adsorb near the corner adatom, centre adatom and restatom adsorption sites of the surface. The results obtained from minimizing the energy of the system show very little difference between the chemisorption processes occurring at the two different adatom sites. In both cases, as progressively more hydrogen atoms are chemisorbed, the hydrogen bonded adatom is found to move from its original threefold (T4) site (one hydrogen atom), to an adjacent bridge site (two or three hydrogen atoms) and then on top of a neighbouring first-layer silicon atom (more than three hydrogen atoms). The lowest calculated adatom desorption energies are 1.59 and 1.62 eV, and correspond to the desorption of at a corner adatom and centre adatom site, respectively. In contrast to an adatom, a hydrogen bonded restatom is found to remain close to its original threefold equilibrium position. The smallest desorption energy for this site is 0.75 eV and also corresponds to an surface complex.

Chemisorption of fluorine onto the cluster modelled Si(111)7 × 7 surface: an ab initio Hartree-Fock/DFT study

The chemisorption of fluorine on atomic clusters modelling an adatom and a rest-atom site of the Si(111)7 × 7 surface is studied using the first principles all-electron Hartree-Fock and Density Functional method embodied in the Gaussian92/DFT package. A number of equilibrium geometries corresponding to n fluorine atoms chemisorbing onto an adatom ( n ≤ 5) or rest-atom ( n ≤ 6) cluster are discussed. Our results show that, as progressively more fluorine is chemisorbed onto an adatom site, the fluorine bonded silicon adatom moves from its original 3-fold (T 4) site (one fluorine atom), to an adjacent bridge site (2 fluorine atoms) and then on top of a neighbouring first layer atom (3 fluorine atoms). For a fluorine bonded silicon rest-atom, a series of strained surface configurations are found as a result of the displacement of the rest-atom from its 3-fold equilibrium position. The lowest calculated desorption energy is 0.11 eV and corresponds to SiF 4 bonded to a rest-atom site. The smallest desorption energy for the adatom site is found to be 1.09 eV for the case of SiF 2. Extrapolating these results to the fluorinated Si(111)7 × 7 surface suggests that a low fluence of fluorine should produce a stable fluorosilyl layer which consists mainly of SiF and SiF 3 species, and corresponds to a fluorine coverage of 1.35 ML. The presented results may be of great importance for the understanding and modelling of the etching of silicon surfaces by fluorine.

Ab initio studies of S chemisorption on GaAs(100)

Ab initio molecular orbital calculations are used to study S chemisorption on reconstructed GaAs(100) surfaces. Three cluster models are developed to simulate the main characteristics of 4×2, 4×6, and 2×6 reconstructed GaAs(100) surfaces. S atoms chemisorb preferentially on bridge sites and dramatically change the surface geometry. If a single S is adsorbed on a bridge site by breaking a Ga—Ga dimer bond, the Ga–Ga separation increases to 4.10 Å (from the dimerized separation of 2.80 Å), with a concomitant lowering of the bond energy by 0.07 (hartree). When two S atoms are adsorbed on adjacent bridge sites, the two S atoms weakly repel each other (establishing themselves ∼10° off from the normal bridge position). At the equilibrium geometry, each S atom retains a charge of −0.6 (e) and increases the net dipole moment perpendicular to the surface by −1.12 (hartree). This dipole moment in turn drives holes (electrons) toward (away) from the surface and increases the photoluminescence efficiency. Calculated electronic structure and local density of states also reflect a large charge accumulation near adsorbed S atoms.

Semiempirical calculations of fluorine chemisorption on the Si(111)7*7 surface

The chemisorption and etching of fluorine on the Si(111)7*7 surface has been studied using Si90H74Fn (1<or=n<or=5) clusters and the atom superposition and electron delocalization molecular orbital (ASEDMO) method. At low coverage, the fluorine atoms are adsorbed onto the dangling bonds. As more fluorines are added, however, a fluorine-bonded adatom is found to move from its original threefold site (one fluorine atom), to an adjacent bridge site (two fluorine atoms) and then to a neighbouring on-top site (three or more fluorine atoms), while the rest atoms remain fixed near their original threefold equilibrium sites. The lowest etching energy is found to be 0.35 eV for SiF4 desorption from a rest atom site.

Semiempirical calculations of the chemisorption of chlorine on the Si(111)7*7 surface

The chemisorption of chlorine on the Si(111)7*7 surface has been studied using clusters and the well known AM1 semiempirical molecular orbital method. Both the equilibrium structures corresponding to n Cl atoms (1<or=n<or=7) chemisorbing onto the adatom and rest atom positions of the Si(111)7*7 reconstructed surface, and the etching energies for an adatom or rest atom bonded to x Cl atoms (0<or=x<or=4), have been obtained. We find that as successively more chlorine is chemisorbed, an adatom moves from its original threefold site (one chlorine) to an adjacent bridge site (two and three chlorines), and then to a neighbouring on top site (four or more chlorines). A rest atom bonded to up to four chlorine atoms, on the other hand, remains dose to its threefold clean surface position. Our results also predict, in agreement with experiment, that all of the SiClx species (1<or=x<or=4) will occur on an Si(111)7*7 surface exposed to atomic chlorine at low temperature. Increasing the temperature will first result in all of the SiCl3 and SiCl4, and most of the SiCl2, desorbing from the surface. This will be followed by the desorption of the remaining SiCl2, leaving only SiCl on the surface. The final stage of the desorption process involves the removal of the SiCl. Whilst the etching energies appropriate to a rest atom site are generally lower than the corresponding adatom values, the minimum etching energy is found to correspond to the desorption of an SiCl4 complex involving an adatom of the Si(111)7*7 reconstructed surface.

Multimedia Data Model for Video-Detection Research

Traffic monitoring and detection systems are important for constructing closed-loop traffic-signal control, freeway-control, and congestion-management systems. Most detection and monitoring systems depend on inductive loops. Recently, many agencies have been investigating video-image–based detection systems because cameras can collect data on several traffic lanes and can be installed quite rapidly on an adjacent pole or bridge. User consensus is that the basic hardware architecture of these systems is well defined. However, there is room for significant improvements in performance and capability of video-detection algorithms. Several researchers are currently working to develop new algorithms. However, due to technology limitations these algorithms have traditionally been evaluated at a macroscopic level by comparing counts obtained by loop detectors with the image-based detection system. This paper discusses the development of a multimedia database-management system for investigating the microscopic performance of video-detection algorithms. The paper describes the process of acquiring the proper data, a definition for a suitable database schema, and the peripheral database clients required to evaluate detection algorithms.