Toshiyuki Ishikawa

Effects of zooplankton on nutrient availability and seston C : N : P stoichiometry in inshore waters of Lake Biwa, Japan

Abstract Forty-eight-hour experimental manipulations of zooplankton biomass were performed to examine the potential effects of zooplankton on nutrient availability and phytoplankton biomass (as measured by seston concentration) and C : N : P stoichiometry in eutrophic nearshore waters of Lake Biwa, Japan. Increasing zooplankton, both mixed-species communities and Daphnia alone, consistently reduced seston concentration, indicating that nearshore phytoplankton were generally edible. The zooplankton clearance rates of inshore phytoplankton were similar to rates measured previously for offshore phytoplankton. Increased zooplankton biomass led to increased concentrations of nutrients (NH4-N, soluble reactive phosphorus [SRP]). Net release rates were higher than those found in previous measurements made offshore, reflecting the nutrient-rich nature of inshore seston. Zooplankton nutrient recycling consistently decreased TIN : SRP ratios (TIN = NH4 + NO3 + NO2). This effect probably resulted from the low N : P ratios of nearshore seston, which were lower than those commonly found in crustacean zooplankton and thus resulted in low retention efficiency of P (relative to N) by the zooplankton. Thus, zooplankton grazing inshore may ameliorate algal blooms due to direct consumption but tends to create nutrient supply conditions with low N : P, potentially favoring cyanobacteria. In comparison with previous findings for offshore, it appears that potential zooplankton effects on phytoplankton and nutrient dynamics differ qualitatively in inshore and offshore regions of Lake Biwa.

Ontogenetic changes in vertical distribution of an endemic amphipod, Jesogammarus annandalei, in Lake Biwa, Japan

We examined the seasonal variation in vertical distribution, gut fullness and diet of Jesogammarus annandalei, an endemic amphipod in Lake Biwa. The vertical distribution changed ontogenetically throughout the year and was restricted to depths with a water temperature < 12 °C. Newborn amphipods remained in the pelagic zone throughout the day while larger amphipods exhibited marked diurnal vertical migrations: individuals stayed on the lake bottom during the day and ascended to the pelagic zone at night. Gut fullness and contents of the migrating amphipods indicated that they fed upon plankton in the pelagic zone at night. Our results indicate that these migration patterns probably reduce the amphipods predation risk from fish. As a result, J. annandalei may play an important role in the vertical translocation of material and energy in the Lake Biwa ecosystem.

Convergent Stresses at the Adhesive Ends of a Steel Member Strengthened with Carbon Fiber Reinforced Polymer Plates

Recently, repairing or strengthening steel members with externally bonded Carbon Fiber Reinforced Polymer (CFRP) plates has been widely reported. In these reports, some criteria for preventing the debonding of the CFRP plates were proposed. In Japan, although CFRP plates have been employed to enhance steel members that are deteriorating from corrosion attack or fatigue cracks or have been applied to upgrade the load-carrying capacity of steel members, there is no clear standard for preventing the debonding of CFRP plates from steel members. In this study, to assist in determining the design criterion for preventing debonding, we derived and solved dierential equations that calculate the stresses in steel members with CFRP plates under concentrated and uniformly-distributed loads. Further, this paper presents the convergent stresses in the adhesive at the ends of the CFRP plates that were strengthening steel members. The convergent stresses at the adhesive ends are sucient to allow CFRP plates to be used under any load conditions.

Reduction of Thermal Stress Induced in a Steel Plate Strengthened by Bonded Aluminum-CFRP Composite:

In steel members strengthened with carbon-fiber reinforced polymer (CFRP) plate, thermal stresses are introduced in the steel members, the CFRP plates and the adhesive layers when temperature changes because the linear thermal expansion coefficients of steel and CFRP are mismatched. Therefore, the thermal stress caused by temperature change has to be considered when designing the repair or strengthening of a steel member with CFRP plates. With this in mind, the authors proposed a technique to reduce thermal stress in steel members strengthened by CFRP plate on both side, which involves bonding aluminum alloy plates with CFRP plates. In this proposed method, the thermal stresses in steel member can be reduced so that there are negligible levels of stress when the cross sectional areas of CFRP and aluminum plates are designed to correspond the coefficient of thermal expansion of steel, even though the thermal stresses are introduced in the CFRP and aluminum plates. So far, a thermal bending moment was not considered in the proposed method, because the steel members strengthened by CFRP-aluminum laminated plate on top and bottom sides were assumed. However, if the proposed method is applied on one side of steel member, the thermal stress in the steel member might not be reduced completely by generated thermal bending moment. Therefore, to confirm the effectiveness of the proposed method for one-sided bonding, heat tests were conducted on a steel plate with a laminated plate bonded on one side. Additionally, to verify test results and calculate the shear and peel stresses in the adhesive layers, a numerical model that used Eigenvalue analysis was proposed and applied. The tests revealed that using the proposed method to create a two-layered laminated plate consisting of CFRP and aluminum plates could not reduce the thermal stress completely in a thin steel plate. However, it was found that the thermal stress in steel plate can be completely reduced, even in thin steel plate, when the proposed method is used to create a three-layered laminated plate consisting of one CFRP plate between two aluminum plates, which when composited has a thermal bending moment equal to zero.

Development of Aluminum Decks for Highway Bridges

In Japan, the amendment of the design vehicle load from 196 kN to 245 kN in 1994 urges concrete decks and girders of existing bridges to be reinforced. To cope with this issue, we put forward an idea of reducing the weight of the roadway by replacing concrete decks with aluminum ones. To realize this idea, we fabricated an aluminum deck by the friction stir welding. In this paper, we discuss the following topics: (1) Design of aluminum decks of open-hollow sections. (2) Connections of an aluminum deck to a steel girder. (3) Composite effects of an aluminum deck on a steel girder.

Effect of Shear Connector Provided at Steel-Concrete Interface on Bending Behavior of Concrete-Filled Double Skin Steel Tubular Member

For weight reduction of CFT (concrete-filled tube) members, concrete-filled double skin steel tubular members (CFDT) have been developed. To ensure the composite action of CFDT members, shear connectors should be installed at the interface between steel tubes and concrete. In this study, to investigate the effect of shear connector on the bending behavior of CFDT members, four-point bending tests were carried out on CFDT and CFT members with shear connectors. The locations of the shear connectors were different for each specimen. The shear connectors were found to prevent slip between steel tubes and concrete. The maximum load of CFDT members was improved by shear connectors, particularly when placed on the inner surface of the outer steel tube.コンクリート充填二重鋼管（CFDT）部材は，二重に配置された鋼管の間にコンクリートを充填した構造であり，従来のコンクリート充填（CFT）部材よりも軽量となる利点を有する。一方，CFDT部材の内外鋼管と充填コンクリートが一体となって挙動するためには，両鋼管とコンクリートの界面にずれ止めを設置する必要があると考えられる。本研究では，鋼管にずれ止めを設置したCFDTおよびCFT部材の4点曲げ載荷実験を行った。その結果，内鋼管の径が小さいCFDT部材では，ずれ止めを設置した鋼管とコンクリートとのずれが抑制され，ずれ止めの設置によって最大曲げモーメントが向上した。特に，外鋼管にずれ止めを設置した場合には，最大曲げモーメントの向上率が大きかった。一方，内鋼管の径が小さいCFDT部材では，ずれ止めを設置してもずれの抑制効果が小さく，最大曲げモーメントの向上率も小さかった。

Reliability Engineering. Fatigue Reliability Assessment of Highway Bridges Using New Parameters.

Recently there have been many reports on the stress measurement of highway bridges. However, a generalized method is not still provided on how to introduce the results of stress measurement into the fatigue reliability assessment. In this paper, two new parameters, called “fatigue resistance c” and “fatigue-damage-accumulation q”, are proposed to make it easy to introduce the results of stress measurement of a monitored bridge into the fatigue reliability assessment of highway bridges. Using a fatigue test database, the probability distribution of c is determined for various structural details. It is shown that for fatigue failure, the assumption of q=c is equivalent to the Miners linear damage rule. The characteristics of the fatigue-damage-accumulation q of highway bridges are discussed. The application of the advanced first-order second-moment method to the state function of g=c-q gives the fatigue reliability assessment of highway bridges. It is expressed in terms of the design equivalent stress range and the allowable fatigue stress range with partial safety factors. The relation between the safety index and the lapse of time from opening to traffic is formulated. The safety index is shown to decrease with the time in logarithm scale.