Parallel Bridges Research Articles

Significant source harmonic reduction achieved using direct parallel connection of two 6-pulse converters

The paper shows that direct parallel connection of the DC outputs of two 6-pulse thyristor converters, fed from 30°-displaced transformer-derived electrical supplies, results in dramatically different operation to the traditionally used arrangements of series bridges or parallel bridges with DC-side inter-phase transformers or sharing reactors. By careful specification of the transformer reactance, very low harmonic content may be achieved at a specific operating point, whereas performance at all others is as good, or better, than the traditional bridge circuit arrangements. No additional equipment or circuitry is required to achieve these benefits. By including additional active/passive components within the DC circuit, ultra low harmonic benefits may be achieved for all operating conditions of voltage and current. This ‘harmonic compensator’ circuit adjusts automatically to ensure that minimum harmonic performance is achieved over the full voltage and current ranges, including inversion. Modelling results over the full motoring and regeneration speed and load range for these arrangements are presented. Very low propulsion noise and bus electrical distortion is a requirement for propulsion drives of some military and research ships. In particular, they are required to operate quietly under reduced generator capacity. Circuit analysis and circuit modelling results are presented for these ‘Silent running’ conditions for a fisheries research vessel (FRV), fitted with 12-pulse direct parallel-bridge converters with the transformer reactance optimised to give minimum harmonic conditions. Practical results from the vessel are presented which validate the modelling.

Testing of Active Current Balance in Parallel Thyristor Bridges

Test results of active current balance (ACB) at Georgia Power's McDonough Plant show the effect of ACB applied to parallel thyristor bridges in generator excitation applications. This paper presents test results of ACB, a new technique that adjusts the average current in parallel leg thyristors to facilitate current balance between the parallel operating bridges. The unbalance between parallel thyristors is typically caused by differences in inductance feeding the thyristors and by the conducting characteristics of the thyristors. Balance is accomplished by periodically inhibiting the firing of thyristors that carry more than the average current. The thyristors that are conducting carry more current during this interval and their average current is increased. Test results show that the current balance between the parallel operating thyristors in the bridges is significantly improved when ACB is enabled. The results of rapid changes in load while ACB is enabled are also shown.

Bridge Scour and Substructure Deterioration: Case Study

In 1988, the State of Mississippi set up its first statewide underwater bridge inspection program. During this inspection, serious damage was found on the substructure of two parallel bridges on I-10 near Biloxi. Significant scour had occurred, exposing the steel piling. These piles had severe corrosion with cross sections reduced by 50%. Because of the reduced cross section, the web and flanges of the piles had buckled locally. This damage was exacerbated by collisions associated with barge traffic on the waterway. This accumulation of problems resulted in a bridge on the verge of collapse. Two alternatives were used to repair the substructure. The more seriously damaged piles were encased in concrete, and the less seriously damaged ones were dewatered and a concrete seal was placed around the piles. After 10years, the piers with pile encasements showed no additional scour effects. However, the piers with the large concrete seals had scoured by as much as 3m(10ft). Described in this paper is the analysis of damage, design of the repair, and a review of the performance of the two types of repair over a 10-year period.

Tacoma Narrows 50 years later—wind engineering investigations for parallel bridges

Wind engineering investigation of the Parallel Tacoma Bridges was performed. Half a century after the historic collapse of the original Tacoma Narrows Bridge, aerodynamic instability was again the principal concern. Since it was proposed to build a n

Experimental Studies on Large-Scale Timber Trussed Systems

Trusses have been widely used for roof and bridge construction, to provide mankind with a means to meet the most indispensable of needs. Despite the extensive use of trusses, it is still believed that the significant simplifications and assumptions made to facilitate design e.g. frictionless pin-joints, are far from reality, resulting in over designed, uneconomical members. This study is a sequel to previous work, and evaluates the realistic structural behaviour of prototype timber trussed systems viz-a-viz the empirical or semi-empirical design assumptions, with the goal of seeking to optimize the very scarce timber resource in Kenya. A prototype timber fink truss was subjected to nodal point loads gradually to failure. Results obtained suggest that internal web members of the truss may be regarded as axially loaded, while continuous top and bottom chord members are mainly loaded in bending in contrast to design assumptions. Extensive deflections were recorded, and according to limit state design requirements the truss is considered to have failed in serviceability limit state of deflections, much below the recorded ultimate capacity of the truss. Generally, the results are quite similar to previous work on parallel chord bridge truss, and confirm the much held opinion that conventional analysis of timber trusses is an oversimplification that is far unrealistic, too conservative and hence against the current global trend towards sustainability and resource efficiency. Journal of Civil Engineering Research and Practice Vol.1(2) 2004: 77-90

The ''parallel bridge'' and its relevance to biotechnology new ventures

A firm's competitive strategy and innovation processes are strongly influenced by, and must be responsive to, its competitive environment. This is nowhere more strongly evident than in the high technology industries. In the present work, case studies of biotechnology new ventures are presented. These studies illustrate how an initial market entry strategy of competition (through creative imitation) has enabled several biotechnology start-ups to reduce their mortality risk. We coin the term parallel to describe this strategy. The bridge provides early cash flows which support research and development and provide time for new ventures to develop core competencies, including a capacity to produce second and third horizon products that will sustain longer term competitiveness.

Design-Construction of Bridge Street Bridge — First CFRP Bridge in the United States

The design, fabrication, and erection - and the research leading up to this cutting-edge technological achievement in civil engineering -are presented, as well as the long-term monitoring program and ongoing research that will continue. The Bridge Street Bridge in Southfield, Michigan, is the first vehicular concrete bridge ever built in the United States that uses carbon fiber reinforced polymer (CFRP) material as the principal structural reinforcement. The project consists of two parallel bridges - Structures A and B - over the Rouge River in the City of Southfield. Both structures use three skewed spans, each over 62 m (204 ft) long, to carry vehicular traffic. Structure A consists of a new substructure as well as a new superstructure, and incorporates five equally spaced conventional AASHTO Type III girders in each of its three spans. Its cast-in-place concrete deck slab is placed continuously across the three spans. Structure B consists of 12 special double-tee (DT) girders (four per span) using pretensioned Leadline tendons and post-tensioned carbon fiber composite cable (CFCC) strands. This project recently won PCl's Harry H. Edwards Industry Advancement Award.

Active Current Balance between Parallel Thyristors in Multibridge AC-DC Rectifiers

New technology in digital excitation control provides a method to balance thyristor currents in parallel power rectifier bridges that provide current to a synchronous generator field. Balancing current between parallel bridges feeding a common load has traditionally been accomplished by using current balancing transformers in each bridge or adding inductance in series with the thyristors or silicon controlled rectifiers (SCRs). This paper introduces active current balance, a new technique that modifies the average current in parallel leg thyristors to facilitate current balance between the bridges. This is accomplished by periodically inhibiting the firing of thyristors that carry more than the average current. The thyristors that are fired carry more current during this interval, increasing their average current. The thyristors that are not fired do not carry current during this interval and thus their average current is reduced. Using this technique (referred to in this article as "skip firing"), the average current in parallel legs can be adjusted through feedback and control to equalize the average thyristor current loading. Additional feedback and control parameters are proposed, such as heatsink temperatures, which can be measured and utilized to program the "skip firing," in order to balance thyristor heat-sink temperatures.

Effect of unilateral denervation on maximum specific force in rat diaphragm muscle fibers.

We hypothesize that 1) the effect of denervation (DNV) is more pronounced in fibers expressing fast myosin heavy chain (MHC) isoforms and 2) the effect of DNV on maximum specific force reflects a reduction in MHC content per half sarcomere or the number of cross bridges in parallel. Studies were performed on single Triton X-100-permeabilized fibers activated at a pCa (-log Ca2+ concentration) of 4.0. MHC content per half sarcomere was determined by densitometric analysis of SDS-PAGE gels and comparison to a standard curve of known MHC concentrations. After 2 of wk DNV, the maximum specific force of fibers expressing MHC2X was reduced by approximately 40% (MHC(2B) expression was absent), whereas the maximum specific force of fibers expressing MHC2A and MHC(slow) decreased by only approximately 20%. DNV also reduced the MHC content in fibers expressing MHC2X, with no effect on fibers expressing MHC2A and MHC(slow). When normalized for MHC content per half sarcomere, force generated by DNV fibers expressing MHC2X and MHC2A was decreased compared with control fibers. These results suggest the force per cross bridge is also affected by DNV.

Mechanisms underlying increased force generation by rat diaphragm muscle fibers during development.

It has been found that maximum specific force (F(max); force per cross-sectional area) of rat diaphragm muscle doubles from birth to 84 days (adult). We hypothesize that this developmental change in F(max) reflects an increase in myosin heavy chain (MHC) content per half-sarcomere (an estimate of the number of cross bridges in parallel) and/or a greater force per cross bridge in fibers expressing fast MHC isoforms compared with slow and neonatal MHC isoforms (MHC(slow) and MHC(neo), respectively). Single Triton 100-X-permeabilized fibers were activated at a pCa of 4.0. MHC isoform expression was determined by SDS-PAGE. MHC content per half-sarcomere was determined by densitometric analysis and comparison to a standard curve of known MHC concentrations. MHC content per half-sarcomere progressively increased during early postnatal development. When normalized for MHC content per half-sarcomere, fibers expressing MHC(slow) and coexpressing MHC(neo) produced less force than fibers expressing fast MHC isoforms. We conclude that lower force per cross bridge in fibers expressing MHC(slow) and MHC(neo) contributes to the lower F(max) seen in early postnatal development.

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Active current balance between parallel thyristors in multi-bridge AC-DC rectifiers

New technology in digital excitation control provides a method to balance thyristor currents in parallel power rectifier bridges that provide current to a synchronous generator field. Balancing current between parallel bridges feeding a common load has traditionally been accomplished by using current balancing transformers in each bridge or adding inductance in series with the thyristors or SCRs (silicon controlled rectifiers). This paper introduces Active Current Balance, a new technique which modifies the average current in parallel leg thyristors to facilitate current balance between the bridges. This is accomplished by periodically inhibiting the firing of thyristors that carry more than the average current. The thyristors that are fired carry more current during this interval, increasing their average current. The thyristors that are not fired do not carry current during this interval and thus their average current is reduced. Using this technique (referred to in this paper as "skip firing"), the average current in parallel legs can be adjusted through feedback and control to equalize the average thyristor current loading. Additional feedback and control parameters are proposed, such as heatsink temperatures, which can be measured and utilized to program the "skip firing," in order to balance thyristor heat-sink temperatures.

Survivability maximization for vulnerable multi-state systems with bridge topology

The multi-state system consisting of elements combined into a bridge structure is considered. Both the impact of external factors and internal causes can prevent the system from performing its task. When the system consists of elements with different reliability and performance (productivity or capacity) characteristics, its survivability, defined as the ability to meet a demand, strongly depends on the way the elements are distributed between components as a result of separation. In this paper we consider the problem of how to separate the elements of the same functionality between two parallel bridge components in order to achieve a maximal possible level of system survivability. The procedure developed to solve this problem is based on the use of a universal moment generating function (UMGF) for fast evaluation of multi-state system reliability and a genetic algorithm for optimization. Basic UMGF technique operators are developed to evaluate survivability of bridge structures for two different types of systems: capacity based and processing speed based. Examples of the optimal transformation of series–parallel structures into bridge structures are presented.

Series-to-parallel transition in the filament lattice of airway smooth muscle.

Force-velocity curves measured at different times during tetani of sheep trachealis muscle were analyzed to assess whether velocity slowing could be explained by thick-filament lengthening. Such lengthening increases force by placing more cross bridges in parallel on longer filaments and decreases velocity by reducing the number of filaments spanning muscle length. From 2 s after the onset of stimulation, when force had achieved 42% of it final value, to 28 s, when force had been at its tetanic plateau for approximately 15 s, velocity decreases were exactly matched by force increases when force was adjusted for changes in activation, as assessed from the maximum power value in the force-velocity curves. A twofold change in velocity could be quantitatively explained by a series-to-parallel change in the filament lattice without any need to postulate a change in cross-bridge cycling rate.

The Cable-Stayed Meiko Grand Bridges, Nagoya

The Ise Bay Coastal Highway links with other regional expressways and crosses three wide channels of Nagoya Port (Meiko). Three large three-span cable-stayed steel bridges - the West, the Central and the East bridges - have been built across these channels. From west to east, center span lengths of the bridges are 405, 590 and 440 m, respectively. At the time of completion, the Meiko Central Bridge ranks as the world’s third longest cable-stayed bridge. The highway has six lanes of traffic with a central reserve. The West Bridge is actually two parallel bridges of similar form, each carrying three lanes, because one bridge had already been built at this site in 1985. At the time, this was the world’s longest steel cable-stayed bridge span.

Interconnecting LANs and a FDDI Backbone Using Transparent Bridges: A Model and Solution Algorithms

In this article, we construct a mathematical model to design the topology of a network interconnecting a number of local area networks (LANs) and a fiber-distributed data interface (FDDI) backbone using IEEE 802.1 transparent bridges. The model allows parallel bridge links between subnets and yields a two-node-connected topology for survivability. The model determines the routing tree on a given topology according to the IEEE Standard protocol, which uses as input only user-supplied parameters such as bridge IDs, port IDs, and path costs. The model ensures that none of the network components (LANs, FDDI, and bridge links) are congested even in the failure of a LAN or a bridge link. We develop a path cost adjustment algorithm and a tabu search algorithm to find a low-cost feasible topology. Through computational experiments, we identify the characteristics of feasible topologies found under varying traffic conditions. Under light traffic conditions (i.e., when LANs' total slack capacity is large relative to the total intersubnet traffic demand), the topology is close to a ring which costs much less than the conventional star topology. Under heavy traffic, the topology tends to be a ring augmented by parallel bridge links between LANs and the FDDI. Finally, we discuss how effective various heuristic ideas—such as uncapacitated, two-connected topology design heuristics, the short-term tabu list and the long-term tabu list—are with respect to improving the solution quality. We also present a Lagrangian relaxation-based lower bounding procedure and report the gap between the best feasible value and the lower bound. The parsimonious property of uncapacitated, two-connected topology design problems is employed in this dual partitioning procedure.

Hydrogen Bonding of Water to Phosphatidylcholine in the Membrane As Studied by a Molecular Dynamics Simulation: Location, Geometry, and Lipid−Lipid Bridging via Hydrogen-Bonded Water

Hydrogen (H-) bonding between water and phosphatidylcholine was studied using a molecular dynamics simulation of a hydrated phosphatidylcholine bilayer membrane in the liquid crystalline phase. A membrane in the liquid-crystalline phase composed of 72 L-R-dimyristoylphosphatidylcholine (DMPC) and 1622 water molecules was generated, starting from the crystal structure of DMPC. At the beginning of the equilibration process, the temperature of the system was raised to 550 K for 20 ps, which was effective in breaking the initial crystalline structure. The thermodynamic and structural parameters became stable after the equilibration period of 1100 ps, and the trajectory of the system obtained during the following 500 ps agreed well with most of the published experimental data. Each DMPC molecule forms 5.3 H-bonds with water, while only 4.5 water molecules are H-bonded to DMPC. The primary targets of water for the formation of H-bonds are the non-ester phosphate oxygens (4.0 H-bonds) and the carbonyl oxygens (1.0 H-bonds). Of DMPC’s H-bonds, 1.7 are formed with water molecules that are simultaneously H-bonded to two different DMPC oxygens (bridging water). In effect, approximately 70% of the DMPC molecules are linked by water molecules and form clusters of two to seven DMPC molecules. Approximately 70% of the intermolecular water bridges are formed between non-ester phosphate oxygens. The rest are formed between non-ester phosphate and carbonyl oxygens. About half of the intermolecular water bridges are involved in formation of multiple bridges, where two DMPC molecules are linked by more than one parallel bridge. These results suggest a possibility that water bridges are involved in reducing head group mobility and in stabilizing the membrane structure. Non-ester phosphate oxygen of DMPC makes one, two, or three H-bonds with water, but two H-bonds are formed most often (60%). In the case where two H-bonds are formed on non-ester phosphate or carbonyl oxygens, the average geometry of H-bonding is planar trigonal (in the case of water oxygen with two H-bonds, geometry is steric tetragonal). When oxygen atoms form three H-bonds, the geometry of H-bonding is steric tetragonal both for non-ester phosphate and water oxygens. On average, H-bonds make nearly right angles with each other when two or three water molecules are bound to the same DMPC oxygen, but the distribution of the angle is broad.

Multiple converter performance and active filtering

The application of active power filtering to power systems in limited by the low switching rate of available high power inverter switches. In this paper, parallel and series connection of multiple voltage source inverter bridges are examined to increase their effective switching rate. The analysis of the inverter voltage spectrum for double edge modulation shows the modulation desired for the separate bridges in open loop. Intuitively i bridges in series or parallel increases the effective switch rate by a factor i. However the modulation process for each power converter maintains its pattern for its switching period T which gives a roll off at high frequencies and reduces effective bandwidth. The experimental system of two 10 kVA voltage source inverters demonstrated how the closed loop control strategies for the active filter can be applied for multiple bridges implementing periodic optimized error sawtooth feedback control. The double bridge closed loop system showed that the lowest switch frequency terms were double that of the separate bridges. The higher frequency switching lines that should have been cancelled in theory were still discernible due to the finite precision of the edge timing. >

Fabrication and Erection of Precast Concrete Segmental Boxes for Baldwin Bridge

The two parallel 11-span bridges over the Connecticut River between Old Saybrook and Old Lyme are segmental post-tensioned concrete structures using single cell precast concrete boxes. The total span of each bridge is 2522.5 ft (769 m). This article describes the precasting plant and the carefully engineered fabrication process, along with the erection system using a fully automated launching gantry to place the segmental boxes. The key to the project's success was knowledgeable and experienced firms in all aspects of design and construction of segmental concrete bridges. The successful bid for the bridge structures was 6 percent below the lowest steel alternate and the project waas completed 12 months earlier than the specified contract time of 4 years.

The Pitan Bridge, Taiwan

To alleviate traffic congestion near Taipei, in 1985 the Taiwan Area National Freeway Bureau planned the construction of the 108 km long Northern Taiwan Second Freeway. The majority of the Second Freeway’s alignments are on grade, on embankments, in tunnels, or on viaduct structures. At several locations however, obstacles were identified that required special bridges. The Pitan Bridge spans one of these areas. The Pitan Bridge, actually two nearly parallel bridge structures, is located about 15 km south of Taipei. It connects typical low-level viaducts to a tunnel. Both bridge units have total length of approximately 800 m, a 750 m radius curvature and a slope of approximately 1%. The structure as designed and built makes use of the well-established economy of a segmentally erected post-tensioned concrete box girder bridge, adapted for the site with the innovative use of delta-shaped piers of post-tensioned concrete construction.